*Article* **Twin-Screw Extrusion Mechanical Pretreatment for Enhancing Biomethane Production from Agro-Industrial, Agricultural and Catch Crop Biomasses**

**Arthur Chevalier 1,2, Philippe Evon 1, Florian Monlau 3, Virginie Vandenbossche <sup>1</sup> and Cecilia Sambusiti 2,\***


**Abstract:** This study aimed to evaluate the effects of mechanical treatment through twin-screw extrusion for the enhancement of biomethane production. Four lignocellulosic biomasses (i.e., sweetcorn by-products, whole triticale, corn stover and wheat straw) were evaluated, and two different shear stress screw profiles were tested. Chemical composition, particle size reduction, tapped density and cellulose crystallinity were assessed to show the effect of extrusion pretreatment on substrate physico-chemical properties and their biochemical methane production (BMP) capacities. Both mechanical pretreatments allowed an increase in the proportion of particles with a diameter size less than 1 mm (from 3.7% to 72.7%). The most restrictive profile also allowed a significant solubilization of water soluble coumpounds, from 5.5% to 13%. This high-shear extrusion also revealed a reduction in cellulose crystallinity for corn stover (i.e., 8.6% reduction). Sweetcorn by-products revealed the highest BMP values (338–345 NmL/gVS), followed by corn stover (264–286 NmL/gVS), wheat straw (247–270 NmL/gVS) and whole triticale (233–247 NmL/gVS). However, no statistical improvement in maximal BMP production was provided by twin-screw extrusion. Nevertheless, BMP kinetic analysis proved that both extrusion pretreatments were able to increase the specific rate constant (from 13% to 56% for soft extrusion and from 66% to 107% for the high-shear one).

**Keywords:** anaerobic digestion; kinetics; lignocellulosic biomass; mechanical pretreatment; methane potential; twin-screw extrusion

#### **1. Introduction**

The increase in the world's population correlated with the increase in energy demand has raised concerns about the problem of the supply of fossil resources such as oil, gas or coal, and their harmful effects, contributing strongly to global warming. Renewable and alternative energy sources may be able to solve these problems, but they require major investments and innovative technologies to do so [1].

One of the most promising and profitable biotechnologies for replacing fossil energy with renewable energy, such as bioethanol or biodiesel production, is anaerobic digestion (AD) as methane produced by the bioconversion of numerous sources of organic materials can be used to generate heat, electricity and fuel, while the digestate generated in the process may be disposed of as an organic amendment for agricultural soils [2]. AD is a biological process characterized by four successive metabolic pathways, i.e., hydrolysis, acidogenesis, acetogenesis and methanogenesis, and involves the gradual conversion of high molecular weight compounds such as carbohydrates, proteins and lipids into biomethane [3].

Lignocellulosic biomass represents one of the most attractive sources of organic matter for biogas production since it is widely available through by-products or wastes generated

**Citation:** Chevalier, A.; Evon, P.; Monlau, F.; Vandenbossche, V.; Sambusiti, C. Twin-Screw Extrusion Mechanical Pretreatment for Enhancing Biomethane Production from Agro-Industrial, Agricultural and Catch Crop Biomasses. *Waste* **2023**, *1*, 497–514. https://doi.org/ 10.3390/waste1020030

Academic Editors: Catherine N. Mulligan, Dimitris P. Makris and Vassilis Athanasiadis

Received: 17 March 2023 Revised: 4 May 2023 Accepted: 12 May 2023 Published: 22 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

by activities such as agriculture or agro-industry [4]. Lignocellulose consists of three main biopolymers, which are associated with each other to form a complex network. Cellulose, the most abundant natural polymer, is a polysaccharidic homopolymer made of glucose units linked by β-1,4 glycosidic bonds. Hemicelluloses are also polysaccharides, but these heteropolymers are made from different monomers such a pentoses (e.g., xylose and arabinose), hexoses (e.g., mannose, glucose and galactose) and uronic acids. Finally, lignin is, unlike the previous biopolymers, an amorphous polyphenolic heteropolymer composed of three different monolignols (i.e., coumaryl, coniferyl and sinapyl alcohols). [5].

Among lignocellulosic biomasses, agro-industrial or agricultural by-products are key carbon resources available in vast quantities since the four most farmed plants in the world, namely sugar cane, corn, rice and wheat, already generate more than 2.55 billion tons of plant fiber annually [6]. Catch crops such as grass, clover, hemp or triticale also represent an appealing answer to the competitive use of agricultural land, and they can be efficiently valorized through biogas production [7].

However, as the complex structure and physico-chemical properties of cell walls inside lignocellulosic substrates constitute a natural barrier against pests such as insects or microorganisms, they also limit the hydrolysis stage of AD, and therefore the substrate's natural digestibility, which is the main limiting factor for their utilization [8].

Therefore, to break this complex structure of lignocellulose and access the monomeric sugars forming cellulose and hemicelluloses, which are consumed by microorganisms during AD, a pretreatment step is essential [9]. Biological, mechanical, physical and/or chemical pretreatments are the main pretreatments that can be applied [10–12]. For the mechanical pretreatments, such as grinding, milling or shredding, the main issue is their energy requirement, especially when coupled with thermal pretreatment [13]. However, these are able to efficiently disrupt the cell wall structure, thus improving the accessible contact surface area between anaerobic microorganisms and the substrate. Physical pretreatments, mainly ultrasound or microwave, or physico-chemical pretreatments such as steam explosion, are also able to disrupt the cell wall structure, and they are more cost-effective than mechanical pretreatments. However, the former lack their industrial maturity, whereas the latter may generate toxic compounds [10,14,15]. Chemical pretreatments, mainly alkaline and acid pretreatments, while often very efficient in increasing biogas production through the removal of either hemicelluloses or lignin, have the main disadvantages of being pretty expensive and not very environmentally friendly, and they can also generate AD inhibitors in the process such as furans and phenols compounds [16]. On the other hand, biological pretreatments, which mainly use enzymes or fungi, are environmentally friendly, and do not generate inhibitors, but they are expensive and constitute pretty slow processes [17].

Twin-screw extrusion is a continuous mechanical process allowing biomass disruption with strong mixing and shearing forces generated at least by the intermeshing of the two rotating screws [18] and, in most cases, by the use of specific shearing screw elements along the screw profile [19]. Twin-screw extrusion is considered a promising process to simultaneously apply mechanical and thermal pretreatments for lignocellulosic substrates, explaining why it was investigated since the 1990s [20]. This technology presents key parameters that can be set according to the required purposes, i.e., configuration parameters such as screw elements and module types, and operational parameters such as screw rotation speed, temperature profile, solid-to-liquid ratio and feeding rates [21]. Twin-screw extrusion processes can be implemented from ambient to hot temperatures, and they can adapt to many types of biomasses. Twin-screw extrusion also has the possibility of combining mechanical pretreatment with other categories of pretreatment, either thanks to the addition of chemicals in the case of reactive extrusion [22] or enzymes in the case of bioextrusion [23,24]. Finally, twin-screw extrusion is an easy scalable technology with excellent repeatability results when transferred from a laboratory scale to pilot and industrial ones [25,26].

Over the past decade, several studies have especially focused their efforts on assessing the effect of twin-screw extrusion pretreatment for AD. Several biomasses including rice, wheat or corn straw, grass, sprout stem, vine shoot or miscanthus were proven to generate

better methane yields (from a 16% to 72% increase) after mechanical twin-screw extrusion pretreatment [27–30]. Triticale, when harvested fresh at the milky stage as a catch crop, is another biomass of interest for AD as it presents a lower lignin content than other cereal straws harvested at the fully ripe stage [31] and, to the best of our knowledge, has not yet been pretreated through twin-screw extrusion. Furthermore, for all the references listed above, a particle size reduction up to a 2–40 mm range has been conducted beforehand, which may minimize the real efficiency of mechanical pretreatment through twin-screw extrusion. Lastly, a 60–100 ◦C temperature range has also been set inside the extruder barrel, which could add thermal pretreatment in addition to the mechanical one.

The aim of this study was to assess the effect of twin-screw extrusion mechanical pretreatment on biomethane production from different agro-industrial, agricultural and catch crop biomasses. Two different screw profiles were tested on four biomasses (i.e., sweetcorn by-product, triticale, corn stover, wheat straw), and twin-screw extrusion treatment was conducted at ambient temperature while avoiding particle size reduction before extrusion. Then, the influence of twin-screw extrusion pretreatment on their physico-chemical properties and their performances in AD were evaluated.

#### **2. Materials and Methods**

#### *2.1. Feedstocks and Inoculum*

Four feedstocks were selected among agro-industrial wastes, agricultural by-products or energy catch crops. These were chosen from among lignocellulosic biomasses available in the south-west part of France, and in sufficient quantities for potential large-scale exploitation through biogas plants. Sweetcorn by-product (i.e., husk and cob) recovered from Soleal–Bonduelle factory (Bordères-et-Lamensans, France) is referred to as "SB". Whole fresh triticale harvested in a plot located in the city of Pavie (France) is referred to as "WT". Corn stover harvested in the city of Cescau (France) is referred to as "CS". Lastly, wheat straw, also harvested in the city of Cescau, is referred to as "WS". All four biomasses were comb milled using a hammer mill (Goulu N, Electra, Poudenas, France). Then, the only WS was also processed through a crushing mill (Bro140, Electra, Poudenas, France) equipped with a 15 mm grid. Comb or crush milled samples before extrusion are referred to as "CM". WT was then dried at 50 ◦C during 24 h for conservation purposes, and then rehydrated at initial humidity (i.e., 69% moisture content) by mixing it with water in a concrete mixer before extrusion.

Anaerobic digester inoculum was recovered from an industrial biogas plant from TotalEnergies located in the city of Bénesse-Maremne (France). Inoculum was maintained in a 5 L glass bioreactor at 38 ◦C under stirring and anaerobic conditions before performing Biochemical Methane Potential (BMP) tests. Main inoculum parameters were 3.9 ± 0.1 TS, 2.7 ± 0.1 VS, 7.8 ± 0.0 pH, 1.5 ± 0.0 gCH3COOH/L, 4.1 ± 1.4 gN-NH4+/L and 0.17 ± 0.0 FOS/TAC ratio.

#### *2.2. Extrusion Pretreatment*

A BC 45 twin-screw extruder (Clextral, Firminy, France) was used for extrusion pretreatment (Figure 1). The machine is made of 7 consecutive modules, each 200 mm in length, and its screw elements have a diameter of 55 mm and lengths of either 50 or 100 mm. Three different screw profiles were investigated (Figure 2). The first screw profile (A) is referred to as soft extrusion (SE), and it was tested on all four biomasses. It consists of trapezoidal double-thread screw elements (T2F) on module 1, followed by conveying double-thread screw elements (C2F) on modules 2 to 6, and then 5 kneading (i.e., bilobal) elements (BL22) with a −45◦ angle at the early beginning of module 7, followed by C2F elements up to the extruder outlet. The second screw profile (B) is referred to as high-shear extrusion (HE), and it was only used on SB and WT. It also starts with T2F elements on module 1, followed by C2F elements on modules 2 to 4, then conveying single-thread screw elements (C1F) in modules 5 and 6, and lastly reverse single-thread screw elements with grooves (CF1C) and C1F elements on module 7. For this screw profile, a filter section

was positioned at the level of module 6 to allow the collection of a liquid separately from the solid and thus avoid waterlogging the machine over time. It consisted of six semicylindrical grids with eight per square centimeter conical holes with 1 mm inside diameter and 2 mm outside diameter. The third and last screw profile (C), referred to as high-shear extrusion with rehydration (HER), is actually only a variation of the second one. It was needed as CS and WS required rehydration at the moment of high-shear extrusion to prevent the machine from blocking. It only differs from profile B by the addition of 5 BL22 elements with a +45◦ angle on module 4 to favor intimate mixing between the solid and water. The latter was injected thanks to a piston pump at the beginning of module 3. HE and HER screw profiles are expected to generate the same mechanical pretreatment as the intense mechanical shear zone in module 7, made of CF1C elements, and are perfectly identical.

**Figure 1.** Photograph of the Clextral BC 45 twin-screw extruder that has been used in this study.

**Figure 2.** Schematic representation of the three screw profiles tested in the Clextral BC 45 twin-screw machine. (**A**) Soft extrusion, (**B**) high-shear extrusion and (**C**) high-shear extrusion with rehydration.

During extrusion tests, neither heating nor temperature regulation were applied but material temperature was measured at the level of the stress zone (i.e., module 7). Screw rotation speed was set at 60 rpm. Biomass supply was fulfilled either manually in the case of SB, WT, CS and WS for SE, and in the case of SB and WT for HE, or with a volumetric twin-screw feeder in the specific case of CS and WS for HER. CS and WS were slightly rehydrated by mixing them with water in a concrete mixer up to a 70% total solid content (TS) and 75% TS, respectively, before SE as they were too dry to be extruded as such. Extrudate samples and filtrate ones for the HE and HER configurations were collected after reaching the extruder stability. For each test, three samplings were conducted during 4 min, and this enabled the calculation of mean outlet flow rates. The latter were then used through a material balance to calculate the raw material inlet flow rate, based on the dry matter contents of raw and extruded solid samples plus that of the filtrate for HE and HER.

#### *2.3. Biochemical Methane Potential (BMP)*

Extruded samples were dried at 40 ◦C for 24 h before BMP. Batch BMP values were assessed in 569 mL glass flask with a working volume 269 mL at 37 ◦C with a stirring speed of 75 RPM for a duration of 32 days corresponding to time needed to reach a daily biomethane production <1%. An 18 mL macro element solution (Na2HPO4 22.4 g/L; NH4Cl 10.6 g/L; KH2PO4 5.4 g/L; MgCl2 2 g/L; CaCl2 1.5 g/L; FeCl2 0.4 g/L), 0.3 mL oligoelement solution (CoCl2 0.1 g/L; MnCl2 0.05 g/L: NiCl2 0.01 g/L; ZnCl2 0.005 g/L; H3BO3 0.005 g/L; Na2SeO3 0.005 g/L; CuCl2 0.003 g/L; NaMoO4 0.001 g/L) and 15 mL bicarbonate buffer solution (NaHCO3 50 g/L) were added the medium to ensure good bacterial development. Carboxymethylcellulose was used as positive control and sole inoculum as negative control (BMP produced was then subtracted from results). Extrudate and filtrate samples from HE/HER were reassembled as one sample keeping the same proportions as at the extrusion outlet. A 0.5 gVS substrate/gVS inoculum ratio was set, and 7–8.5 pH range check were realized to avoid initial medium acidification. Flasks were flushed with nitrogen at the beginning of the trials to ensure anaerobic condition, then were sealed with impermeable red butyl rubber septum-type stoppers. BMP tests were performed in duplicate.

Biogas analyses were performed by using an Agilent 990 Micro GC with two columns: one at 80 ◦C and 200 kPa with argon as carrier phase for H2, O2, N2 and CO2, one at 60 ◦C and 150 kPa with helium as carrier phase CH4 and one for CO2 and H2S. Injector temperature was 80 ◦C. Biogas production was calculated from pressure increase measured with a manometer.

Elemental analyses (CaHbNcSdOe) of dried samples were performed to calculate their maximal theorical BMP (BMPth) using Buswell equation [32,33]:

$$\text{BMP}\_{\text{th}} \left[ \frac{\text{NmL}}{\text{gVS}} \right] = \frac{4\mathbf{a} + \mathbf{b} - 2\mathbf{c} - 3\mathbf{d} - 3\mathbf{e}}{12\mathbf{a} + 2 + 16\mathbf{c} + 14\mathbf{d} + 32\mathbf{e}} \times 1000 \tag{1}$$

Khongchamnan et al.'s [34] elemental analysis of lignin was also used to calculate lignin maximal theorical BMP (621 NmLCH4/gVS), which was then used to determine samples' adjusted theorical maximal BMP (BMPth adjusted) using the following formula:

$$\text{BMP}\_{\text{th adjusted}} \left[ \frac{\text{NmL}}{\text{gVS}} \right] = \text{BMP}\_{\text{th substrate}} - \text{BMP}\_{\text{th lignin}} \times \text{\textdegree\%light}\_{\text{substrate}} \tag{2}$$

The biodegradability index (BI) was finally calculated using the following equation:

$$\text{BI } \left( \% \right) = \frac{\text{BMP}\_{\text{experimental}}}{\text{BMP}\_{\text{th adjusted}}} \tag{3}$$

#### *2.4. Analytical Methods*

#### 2.4.1. Sample Preparation for Analysis

Parts of raw and extruded samples were dried at 105 ◦C until constant weight for total solid content (TS) determination, and they were then mineralized at 550 ◦C during 8 h for ash and volatile solid content (VS) determination according to the National Renewable Energy Laboratory (NREL) procedures [35]. The rest of raw and extruded samples were dried at 40 ◦C during 24 h for further characterizations. Parts of dried samples were used as such for granulometry and tapped density measurements, and the remaining dried samples were milled with a 1 mm grid on a microfine grinder drive (MF 10 basic, IKA Werke, Staufen im Breisgau, Germany). Two mechanical sieves (aperture sizes of 0.8 and 0.18 mm, respectively) and a bottom plate were used on a vibratory sieve shaker (AS 200, Retsch, Hann, Germany) during 10 min at a 3 mm amplitude to recover sample fractions depending on their particle sizes. Sample fractions between 0.8 and 0.18 mm were used for lignocellulosic composition analysis, whereas fractions under 0.18 mm were used for cellulose crystallinity assessment.

#### 2.4.2. Fiber Composition

Fiber composition of lignocellulosic samples was determined in triplicate according to an adapted protocol [36] from the National Renewable Energy Laboratory (NREL) procedures [37]. Water and then 96% (*v*/*v*) ethanol extractions were performed on an extraction system (Fibertec FT 122, Foss, Hillerød, Denmark) using 1 g of dry sample and 100 mL of boiled solvent at 100 ◦C and atmospheric pressure for 1 h. Cellulose, hemicelluloses and lignin contents were assessed with a two-step hydrolysis using a 72% (*w*/*w*) sulphuric acid at 30 ◦C for 1 h and then a 4% (*w*/*w*) solution after dilution during 1 h at 121 ◦C, followed by filtration. Acid-insoluble lignin in residues was determined by weight loss after calcination during 8 h at 450 ◦C. Part of the extracts was then used for acid-soluble lignin on UV spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) at 320 nm for CS samples and at 240 nm for the other ones, and the rest of the extracts were neutralized with calcium carbonate until reaching pH 5–7 and then filtered using a 0.2 μm cellulose acetate filter. Analysis of sugar monomers (i.e., arabinose, glucose, galactose, xylose and mannose) was performed on a Dionex (Sunnyvale, CA, USA) ICS-3000 type ion chromatography HPLIC system with a pumping device, an auto-injector, an electrochemical detector with a gold electrode and an Ag/AgCl reference electrode. A pre-column (4 × 50 mm, Dionex) connected to a Carbopac PA1 column (4 × 250 nm, Dionex) was used for the stationary phase with a 1 mM sodium hydroxide solution as an eluent. A total of 25 μL of samples were injected automatically with separation of sugars carried out at a flow rate of 1 mL/min at 25 ◦C. A range of standards was made from 1 to 100 mg/L to undertake external calibration for the quantification of sugar monomers. Chromeleon analysis was conducted with the 6.8 version of Dionex processing software.

#### 2.4.3. Granulometry and Tapped Density

Seven mechanical sieves (aperture sizes of 4.0, 2.0, 1.0, 0.8, 0.5, 0.25 and 0.125 mm) and a bottom plate were used to measure, in triplicate, the particle size distribution using the Retsch AS 200 vibratory sieve shaker during 10 min at a 3 mm amplitude.

A bulk density tapping instrument (Densi-Tap, Ma.Tec, Novara, Italy), modified to support a 1000 mL graduated cylinder, with a cam shaft speed of 250 rpm and a stroke travel of 3.2 mm, was used to determine tapped density. A total of 1000 taps were repeated until the tapped volume did not change between two consecutive cycles. The final tapped volume was read on the graduated cylinder. All determinations were conducted in triplicate.

#### 2.4.4. Cellulose Crystallinity

Cellulose crystallinity was determined using an X-ray diffraction (XRD) instrument (D8 Advance, Brucker, San Jose, CA, USA) with a 0.154 nm wavelength, Cu/Kα radiation at 40 kV and 40 mA tube current. Samples were implemented with a speed of 1◦/min, in a range of 2θ varying from 6◦ to 30◦, and a step size of 0.0303◦ at room temperature.

The crystallinity index (Cr) was determined using the following equation [38]:

$$\text{Cr} = \frac{\text{I}\_{002} - \text{I}\_{\text{amorphous}}}{\text{I}\_{002}} \times 100 \tag{4}$$

where:

I002 is the intensity of the crystalline portion of the biomass (cellulose) at 2θ = 22◦; Iamorphous is the peak of the amorphous portion at 2θ = 16◦. Analyses were conducted in triplicate.

#### *2.5. Data Analyses*

Kinetic study of the biomethane production was assessed by applying a model based on the modified Gompertz equation [39]:

$$\mathbf{B} = \mathbf{B} \mathbf{M} \mathbf{P}\_{\infty} \exp\{-\exp[\frac{\mathbf{R}\_{\mathrm{m}} \times \mathbf{e}}{\mathbf{B} \mathbf{M} \mathbf{P}\_{\infty}} \times (\boldsymbol{\lambda} - \ \mathbf{t}) + \mathbf{1}]\}\tag{5}$$

where:

B: cumulative biomethane production (NmL/gVS); BMP∞: maximal biomethane production (NmL/gVS); Rm: specific biomethane production rate (NmL/gVS.day); λ: lag phase time (day).

Kinetic parameters were determined by minimizing the sum of the least squares between the observed and predicted values.

#### *2.6. Statistical Analyses*

For statistical analyses made on chemical composition, tapped density, cellulose crystallinity and experimental BMP, Student's tests were conducted with statistical significance level of *p* < 0.05 on the Microsoft Office Excel software (Microsoft, Albuquerque, NM, USA). Data are expressed as means ± standard deviations (s.d.).

#### **3. Results and Discussion**

#### *3.1. Description of the Twin-Screw Extrusion Pretreatments*

Usually, when working on laboratory scale extruders, biomasses are first reduced to smaller particle sizes (within a 1 mm to 2 cm range) before extrusion to enable stable device feeding [25,40,41]. In this study, in order to be as close as possible to pilot or industrial reality, biomasses were extruded fresh and they were just comb milled to avoid any influence of additional mechanical pretreatment before extrusion [4]. Measured parameters during samplings and material balances are given in Table 1.

Dry inlet flow rates were intended to be set to 5 kg/h. However, from a practical point of view, it was not possible to achieve precisely this 5 kg/h dry inlet flow rate as substrates were manually fed into the machine for the SE and HE configurations. In the case of SE, dry inlet flow rates were 5.3 kg/h, 5.6 kg/h and 5.1 kg/h, respectively, for SB, WT and WS. For CS, it was only 3.7 kg/h due to there being coarser and less flexible solid particles in the starting material. When using the HE configuration, trials conducted from SB and WT revealed rather different inlet flow rates, i.e., 7.2 kg/h and 3.7 kg/h, respectively. Regarding HER configuration, which was applied to CS and WS only, rehydration should have been set to a 75/25 water-to-dry matter ratio. However, this could not be performed as such because when this rehydration rate was applied, it generated non-homogeneous rehydration in the extruder and a too low moisture content, resulting in the blocking of the machine. CS and WS were instead rehydrated to higher water-to-dry matter ratios, which were respectively 85/15 and 82/18.


**Table 1.** Measured parameters and material balances of the different twin-screw extrusion pretreatments.

As no temperature regulation was applied during the extrusion pretreatment, the temperature in the stress zone (i.e., module 7) was probably too low to allow good rehydration according to the literature [42,43]. However, this solution has the advantage of being less energy-consuming and therefore less expensive. Extrudate-to-filtrate ratios expressed in terms of dry mass for the HE/HER configurations were 80/20 for SB, 84/16 for WT, 90/10 for CS and 92/8 for WS.

The decrease in the proportion of dry matter in the filtrate in the case of the HER configuration in need of rehydration at the moment of twin-screw extrusion seems to correlate with the hypothesis of superficial rehydration. SB, WT, CS and WS initial dry matter contents increased by 2.8%, 2.6%, 6.0% and 4.6%, respectively, using the SE configuration, and they increased much more, i.e., by 15.4%, 42.3%, 67.0% and 59.0%, respectively, using the HE/HER ones. It has to be noted here that for the HER configuration, water injected in the extruder at the moment of the pretreatment was taken into account for calculating the above-mentioned increases in dry matter content for the CS and WS solid materials.

The high compression action of the CF1C reverse screw elements used in the HE and HER screw profiles reflects the increase in mechanical shear applied to biomasses in comparison with the less restrictive bilobal elements used during soft extrusion. This compression action appeared to be more intense for CS and WS as illustrated by the higher values of the extrudate's dry matter contents.

Moreover, when comparing the SE and HE/HER configurations with each other, temperature ranges in the stress zone increased from 18–22 ◦C to 31–36 ◦C for SB, from 26–27 ◦C to 30–34 ◦C for WT, from 32–34 ◦C to 28–42 ◦C for CS and from 27–32 ◦C up to 58–63 ◦C for WS, which illustrates once again the mechanical stress increase using the high-shear extrusion conditions, especially with WS and, to a lesser extent, with CS. In the same way, the motor current range increased between SE and HE/HER from 14–15 A to 16–25 A for SB, from 27–35 A to 41–47 A for WT, from 11–19 A to 27–37 A for CS and from 12–21 A to 27–30 A for WS. These amperage increases were further proof of the increase in mechanical shear applied to the biomass using CF1C reverse screws instead of BL22 kneading elements.

These results are consistent with those found in the literature where the reverse screw elements used in HE/HER are presented as stronger flow-restricting elements than the kneading elements mounted with reverse pitch used in SE [19,44].

#### *3.2. Effect of the Twin-Screw Extrusion Mechanical Pretreatment on the Chemical Composition of Biomasses*

The chemical compositions of biomasses before and after extrusion are shown in Table 2. SB is characterized by 30%TS of cellulose, 17%TS of hemicelluloses and 15.5%TS of Klason lignin, which is perfectly in accordance with the literature data as Lallement et al. [45] showed 27–30% content for cellulose, 16–21% for hemicelluloses and 13–16% for Klason lignin for maize residues. WT is characterized by 27%TS of cellulose, 24.5%TS of hemicelluloses and 18%TS of Klason lignin, which is similar to the already published data for lignin and hemicelluloses but a bit lower for cellulose: e.g., 36%TS, 25%TS and 16%TS for cellulose, hemicelluloses and lignin, respectively, in Pronyk et al. [46], and 35%TS, 23%TS and 17.5%TS in Tamaki and Mazza [47].

As triticale was harvested fresh, its lignocellulosic composition may differ from the literature data in which it was harvested at a late stage [48]. In addition, triticale is not a species that is as documented as others such as wheat or maize. Thus, its composition could also differ from those of other varieties as triticale includes over 320 species in the European catalogue [49]. CS contained 32%TS of cellulose, 19%TS of hemicelluloses and 17%TS of Klason lignin, which is similar to the already published data, i.e., 28–44% TS, 13–25%TS and 14–26%TS, respectively, for cellulose, hemicelluloses and lignin [50]. WS contained 34%TS of cellulose, 19%TS of hemicelluloses and 22.5%TS of Klason lignin, which is also quite similar to the already published data, i.e., 23–36%, 11–31% and 10–23%, respectively, for cellulose, hemicelluloses and lignin [51], although presenting high cellulose and Klason lignin contents.

Overall, there are slight variations regarding SB before and after extrusion even if a small but statistically significant reduction in the content of extractables was observed, especially after HE, due to their partial solubilization and then their removal by filtration in the case of HE. Regarding WT, for all families of molecules quantified, the differences in composition between CM, SE and HE are systematically significant from a statistical point of view. Only a small solubilization of extractables (5%), leading to an increase in cellulose (5%) and in lignin (1.5%), simultaneously with a small decrease in hemicelluloses (3%), is observed after SE. However, a much more significant solubilization of extractables (15%) is observed after HE, also leading to the increases in both cellulose (8%) and lignin (6%) and a very small decrease in hemicelluloses (1%).

As shown earlier with the motor's amperage, HE was more intense for WT than for SB, which led to a much higher proportion of extractables removed by filtration and thus to a much more significant reduction in the content of extractables inside the WT-based extrudate. For CS, there is a moderate difference in the biomass chemical composition after SE. However, there is again an important solubilization of extractables (13.5%) after HER, leading as well to statistically significant increases in cellulose (9%), hemicelluloses (3.5%) and lignin (3%). Finally, WS shows similar trend as CS with no drastic modification in chemical composition after SE but a solubilization of extractables (5.5%), leading, at the same time, to the increases in cellulose (2%), hemicelluloses (1.5%) and lignin (2%) after HER. Moreover, for each biomass, the lower extractable content measured after HE/HER was always statistically different compared to SE.

Even if Menardo et al. [26] used a mixture of rice straw silage, maize silage and triticale silage in their study, no literature was found about whole triticale or triticale straw extrusion. However, as a rye and wheat hybrid [52], it might be approximated as such. Vandenbossche et al. [53] reported slight composition variations as well when extruding wheat straw with reverse single-thread screw elements despite an internal heating temperature of 80 ◦C. Moreover, Zheng et al. [54] showed no difference in corn cob composition after using conveying, kneading or reverse elements through twin-screw extrusion. Finally, in the case of rehydrated corn stover, which was mechanically treated with a twin-screw extruder, Wang et al. [55] showed a slight increase for cellulose (1%), and slight decreases for hemicelluloses (1%) and, especially, lignin (0.2%) contents. Lastly, in their very recent study, Elalami et al. [56] showed no statistical difference in the chemical composition of corn stover before and after extrusion.


**Table 2.** Chemical compositions of solid samples before and after twin-screw extrusion pretreatment.

In conclusion, although HE/HER configuration allowed partial solubilization of extractables, which significantly impacted the extrudate's composition in lignocellulosic compounds from a statistical point of view, mechanical extrusion only slightly altered the biomass lignocellulosic composition.

#### *3.3. Effect of the Twin-Screw Extrusion Mechanical Pretreatment on Granulometry*

The samples' cumulated granulometries before and after extrusion treatments are shown in Figure 3 and expressed in % of total particle weight (%wt). Overall, SE and HE had quite similar effects regarding SB: only 2.3%wt of particles were smaller than 0.25 mm before extrusion instead of 5.7%wt after SE and 4.3%wt after HE. In the same way, for bigger particles, 6.4% of them were smaller than 0.8 mm before extrusion instead of 24.3% after SE and 30.4%wt after HE, and 16.8%wt of particles were smaller than 2 mm before extrusion instead of 59.3%wt after SE and 65.9%wt after HE.

**Figure 3.** Cumulated granulometries of solid samples before and after twin-screw extrusion pretreatment: (**A**) SB, (**B**) WT, (**C**) CS and (**D**) WS.

For WT, the results obtained are really different: only 0.8%wt of particles were smaller than 0.25 mm before extrusion instead of 2.8%wt after SE and 16.7%wt after HE. In the same way, 5.8%wt of particles were smaller than 0.8 mm before extrusion instead of 16.7%wt after SE and 51.2%wt after HE, and 27.6% of particles were smaller than 2 mm before extrusion instead of 51.4%wt after SE and 85.4%wt after HE. In the case of WT, the SE effect in particle size reduction was less important than for SB. However, the HE one was much more important, with the high-shear extrusion process contributing to a significant additional reduction in particle size in comparison with the soft one.

Concerning CS, only 0.3%wt of particles were smaller than 0.25 mm before extrusion, and this mass content increased to 1%wt after SE and to 42.1%wt after HER. Identically, 4.4%wt of particles were smaller than 0.8 mm before extrusion instead of 6.9%wt after SE and 71.1%wt after HER. For particles smaller than 2 mm, their mass content was 12.2%wt before extrusion, 19.6%wt after SE and 92%wt after HER. The important reduction in the particle size after HER in comparison with SE is therefore even more pronounced for CS than for WT. Oppositely, the SE effect in particle size reduction was less important than for SB and WT.

Quite the same conclusions can be made for WS. Only 1.1%wt of particles were smaller than 0.25 mm before extrusion instead of 1.7%wt after SE and 37.4%wt after HER. In the same way, 9.5%wt of particles were smaller than 0.8 mm before extrusion instead of 12.9%wt after SE and 77.7%wt after HER, and 35.5%wt of particles were smaller than 2 mm before extrusion instead of 52.7%wt after SE and 96%wt after HE. SE and HER effects in

particle size reduction are thus similar for WS and CS samples. However, in contrast to the other substrates, one should be aware that WS had to be ground once more through a 15 mm grid before extrusion, which must have already reduced its particle size.

In their study, Duque et al. [57] showed a small (i.e., 20%) reduction of particles with a size that was more important than the 3.14 mm on extruded barley straw processed with reverse screws as samples were crush milled with a 5 mm mesh before extrusion, while Zheng et al. [54] showed a clear reduction in particle size after extrusion in the case of corn cob but no difference depending on the type of screws used along the profile (i.e., conveying, kneading or reverse elements). However, in that study, corn cob particles were already ground to particle sizes between 0.6 and 0.76 cm before extrusion, meaning that the effect of reducing the size of large particles more or less at the moment of the extrusion pretreatment depending on the screw elements used may probably not have been illustrated properly. In contrast, Garuti et al. [58] showed a net reduction of 7.1% of particles with a size of more than 5 mm, simultaneously with a 19.6% increase in particles less than 0.3 mm in diameter after extrusion conducted on an agricultural waste mix.

In the present study, the absence of fine milling before extrusion, although being more restrictive for the application of the mechanical pretreatment in the extruder as shown earlier, showed more realistic particle size reduction at the moment of the only pretreatment. This reduction in particle size was evidenced for the four biomasses treated, and, even if both SE and HE pretreatments resulted in the same size reduction for SB, the high-shear extrusion pretreatment was much more restrictive for WT, and especially for CS and WS. This important reduction in size with the HER configuration (i.e., CS and WS feedstocks) is probably the consequence of superficial rehydration inside the extruder. With cell walls within the particles being less moist and therefore more rigid, a reduction in size was probably favored.

#### *3.4. Effect of the Twin-Screw Extrusion Mechanical Pretreatment on Tapped Density*

The evolutions of tapped densities before and after extrusion are shown in Table 3, and the results are in general agreement with those of particle size distribution presented previously (Figure 3). After SE, the tapped densities of all substrates increased significantly, with increases of 2.7, 4.8, 2.4 and 2.6 times for SB, WT, CS and WS, respectively, indicating particle refining even with soft extrusion. After HE/HER, tapped densities increased even more with increases of 5.3, 5.4, 7.0 and 8.5 times, respectively, for SB, WT, CS and WS. This increase in tapped densities was thus consistent with the particle size reduction discussed earlier, as smaller particle sizes led to a better stacking of the particles between them after compaction, and therefore a reduction in the inter-particle voids and, as a result, higher densities. This was also confirmed by Chen et al. [27] who showed that the bulk density of rice straw increased by 2.2-fold after extrusion. However, when looking at SB results, and taking into account that particle sizes were quite similar after SE and HE pretreatments (Figure 3), it would have been expected for values of tapped densities to be closer to each other. Likewise, WT tapped density after HE would have been expected to be much higher than that after SE. In this case, one possible explanation would be the formation of aggregates during sample drying after extrusion that would have been separated after particle size distribution measurements but not in the case of the tapped density ones.

**Table 3.** Tapped densities and cellulose crystallinities of solid samples before and after twin-screw extrusion pretreatment.


For each biomass, means ± s.d. followed by different letters are significantly different (*p* < 0.05).

#### *3.5. Effect of the Twin-Screw Extrusion Mechanical Pretreatment on Cellulose Crystallinity*

As shown in Table 3, no cellulose crystallinity change was observed after extrusion for SB, WT and WS. In contrast, the 8.6% decrease in cellulose crystallinity of CS after HER pretreatment was statistically significant. Regarding the results for CS, the most logical explanation would be that HER pretreatment was more impactful on it than on the other biomasses as it was previously illustrated by its high decrease in particle size (Figure 3) and its much higher tapped density value (Table 3). In the literature, mechanical pretreatments are generally known to be efficient in the decrystallization of cellulose [59]. However, it is not always the case with extrusion pretreatments. While Zhang et al. [60] achieved an impressive 48.4% cellulose crystallinity reduction on rice straw with only extrusion, Zhang et al. [22] instead did not observe a decrease in cellulose crystallinity for corn stover after reactive alkali extrusion. Here, the most likely reason for the cellulose crystallinity reduction of CS after HER is that its more rigid morphological structure compared to other biomasses was more affected by the high-shear twin-screw extrusion mechanical pretreatment.

#### *3.6. Effect of the Twin-Screw Extrusion Mechanical Pretreatment on BMP Results*

Figure 4 shows experimental BMP and the BMP kinetics using the modified Gompertz equation model, while their parameters are gathered in Table 4. The experimental BMP tests showed methane production ranges of 338–345 NmL/gVS for SB, 233–247 NmL/gVS for WT, 264–286 NmL/gVS for CS and 247–270 NmL/gVS for WS, which is in accordance with standard values for lignocellulosic biomasses [61,62]. SB shows the highest methanogenic potential with a 40–48% increase compared to WT, a 20–28% increase compared to CS and a 27–37% increase compared to WS, which definitely makes it the best candidate for biomethane production followed by CS.

However, no BMP increase or BMP decrease provided by SE or HE/HER pretreatments was observed from a statistical point of view, and this for all the biomasses tested. Therefore, extrusion had no impact on the experimental maximal BMP. Wahid et al. [63] and Victorin et al. [64] also reported no statistical increase or statistical decrease in final BMP after the extrusion of wheat straw. Hjorth et al. [18] also reported no statistical increase in BMP after 90 days on extruded barley straw. This confirms that the mechanical treatment provided by extrusion does not generate or allow a better degradation of the compounds usually non-valorized during AD, which also correlates with the lack of cellulose crystallinity variation shown earlier.

Adjusted maximal theorical BMPs are proposed in this study as a way to calculate a more realistic estimation of the real biodegradability of the substrates as the monolignols constituting the native lignin are not consumed during AD [65]. SB, WT, CS and WS achieved around 89%, 66%, 76% and 74%, respectively, of adjusted theoretical biodegradability after CM, SE and HE/HER pretreatments. Therefore, a BMP improvement from 11% to 34% is still accessible for these biomasses according to the adjusted theorical biodegradability. The higher biodegradability shown by SB compared to the other biomass is in agreement with the 132 AD feedstock database established by Lallement et al. [45], who reported maximal theoretical biodegradabilities of 72% for agro-industrial residues and 56% for lignocellulosic matter, and this correlates with its higher experimental CH4 production.

BMP kinetics showed that most of the biomethane was produced within the first 10–20 days of BMP tests for all biomasses, which is usual according to the literature data [63,64]. A modified Gompertz model was used to model BMP kinetics as it is one of the most accurate models for anaerobic digestion processes [66]. The predicted BMP showed methane production ranges of 330–339 NmL/gVS for SB, 229–244 NmL/gVS for WT, 259–284 NmL/gVS for CS and 242–265 NmL/gVS for WS. These results are slightly lower to the experimental ones but nonetheless are very close as proven by an R2 correlation coefficient of more than 0.99 for all kinetics, and, this is expected when using a modified Gompertz model [67–69].

Lag phases were very short with an initial 0.2–1.3 day range, and they varied slightly with a 0.2–1.3 day range after SE and with a 0.01–1.4 day range after HE while retaining their short durations, which shows excellent adequacy between used inocula and substrates, and illustrates the easily digested biomass. This result is also correlated with the higher biodegradability of SB compared to WT, which, respectively, had the shortest and longest initial lag phases. The impact of pretreatment seems more impactful in the case of a longer initial lag phase as demonstrated by Tsapekos et al. [70] who managed to reduce the grass lag phase from 3.4 to 2.7 days thanks to a mechanical pretreatment.

The specific rate constant increased for all biomasses after SE and even more after HE or HER. For SB, it increased by 26% after SE and by 81% after HE. For WT, it increased by 56% after SE and by 67% after HE. For CS, it increased by 13% after SE and by 107% after HER. Lastly, for WS, it increased by 21% after SE and by 72% after HER. This demonstrates that the morphological modifications of the biomasses induced by the twin-screw extrusion mechanical pretreatments, especially the high-shear one, allowed better accessibility and degradability of methanizable compounds during digestion by the anaerobic microorganisms, which correlates with the particle size reduction and increase in tapped densities discussed earlier. Chen et al. [27] also reported a decrease in anaerobic digestion time after rice straw extrusion as well as Pérez-Rodrígez et al. [71] after the extrusion of corn cob. This improvement in the specific rate constant is a great indicator of how a substrate would react in the case of continuous biogas production [26], and the impact of SE and especially HE/HER pretreatments on the four biomasses evidenced in this study indicates promising perspectives in reducing their hydraulic residence times [72]. As a direct consequence, a smaller sizing of new biogas plant bioreactors could be considered, thus allowing a diminution in their capital expenditures (CAPEX) and their operating expenditures (OPEX) [73].

**Figure 4.** Experimental BMP (square symbols) and BMP kinetics using the modified Gompertz equation model (lines) before and after twin-screw extrusion pretreatment: (**A**) SB, (**B**) WT, (**C**) CS and (**D**) WS. BMPs from HE/HER samples were assessed after mixing extrudate and filtrate samples in the same proportions as at the extrusion outlet.

**Table 4.** Experimental BMP, maximal and adjusted maximal theorical BMP and kinetics parameters of modified Gompertz equation model before and after twin-screw extrusion pretreatment.


**Table 4.** *Cont.*


For each biomass, means ± s.d. followed by different letters are significantly different (*p* < 0.05).

#### **4. Conclusions**

The effect of the mechanical pretreatment induced through twin-screw extrusion was able to greatly impact the physico-chemical properties of all four biomasses tested, at different levels depending on the selected screw profile as illustrated by the increase in the proportion of smaller size particles correlated with the impressive increase in tapped densities. The more intense mechanical shear effect of the high-shear extrusion pretreatments also allowed a significant solubilization of water-soluble compounds just as the reduction in the corn stover cellulose crystallinity by 8.6% showed, without affecting those of other biomasses. With soft extrusion, no modification of chemical composition and cellulose crystallinity was observed. Sweetcorn by-products revealed the highest BMP values (i.e., 338–345 NmL/gVS), followed by corn stover (264–286 NmL/gVS), wheat straw (247–270 NmL/gVS), and lastly, whole triticale (233–247 NmL/gVS), thereby illustrating their great potential as inputs for biogas production. Even if none of the physico-chemical modifications induced by the two applied extrusion pretreatments were proven to statistically improve the maximal BMP production, kinetic analysis revealed that both extrusion pretreatments, and especially the high-shear one, were able to increase the specific rate constant by up to 56% for soft extrusion and even 106% for high-shear extrusion. Further studies on either bioextrusion (i.e., addition of enzymes) and/or reactive extrusion (i.e., addition of chemicals) as additional pretreatments, known to improve biomethane production, are recommended to attain a synergistic effect with the only mechanical pretreatment.

**Author Contributions:** Conceptualization, A.C., P.E., F.M., V.V. and C.S.; methodology, A.C., P.E., F.M., V.V. and C.S.; validation, P.E., F.M., V.V. and C.S.; formal analysis, A.C.; investigation, A.C.; resources, P.E. and F.M.; writing—original draft preparation, A.C.; writing—review and editing, P.E., F.M., V.V. and C.S.; supervision, P.E., F.M., V.V. and C.S.; project administration, P.E. and C.S.; funding acquisition, P.E. and C.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by ANRT (National Association for Research and Technology in France) under grant number 2021/0737.

**Data Availability Statement:** All the data are described in the Figures and Tables.

**Acknowledgments:** The authors would like to express their sincere gratitude to Ovalie Innovation (Auch, France) for supplying the batch of whole triticale used for the purpose of this study. They would also like to deeply thank the PERL and CIRIMAT laboratory teams for their respective help with BMP and cellulose crystallinity assessments.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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## *Review* **Value Chain Analysis of Rice Industry by Products in a Circular Economy Context: A Review**

**W. A. M. A. N. Illankoon 1,\*, Chiara Milanese 2, Maria Cristina Collivignarelli <sup>3</sup> and Sabrina Sorlini <sup>1</sup>**


**Abstract:** The quantity of organic waste generated by agricultural sectors is continually increasing due to population growth and rising food demand. Rice is the primary consumable food in Asia. However, many stakeholders follow a linear economic model such as the "take–make–waste" concept. This linear model leads to a substantial environmental burden and the destruction of valuable resources without gaining their actual value. Because these by-products can be converted into energy generating and storage materials, and into bio-based products by cascading transformation processes within the circular economy concept, waste should be considered a central material. This review examines the composition of rice straw, bran, and husks, and the procedures involved in manufacturing valueadded goods, from these wastes. Moreover, starting with an extensive literature analysis on the rice value chains, this work systematizes and displays a variety of strategies for using these by-products. The future development of agricultural waste management is desirable to capitalize on the multifunctional product by circulating all the by-products in the economy. According to the analysis of relevant research, rice straw has considerable potential as a renewable energy source. However, there is a significant research gap in using rice bran as an energy storage material. Additionally, modified rice husk has increased its promise as an adsorbent in the bio-based water treatment industry. Furthermore, the case study of Sri Lanka revealed that developing countries have a huge potential to value these by-products in various sectors of the economy. Finally, this paper provides suggestions for researchers and policymakers to improve the current agriculture waste management system with the best option and integrated approach for economic sustainability and eco- and environmental solution, considering some case studies to develop sustainable waste management processes.

**Keywords:** rice straw; rice bran; rice husk; agricultural waste; valorization; circular economy; biomass; bioeconomy

#### **1. Introduction**

Rice is an annual plant crop mainly cultivated in areas with high rainfall and rice is a primary edible food and crop in Asia [1,2]. Many South Asian countries have an agrarian economy and have been producing significant amounts of agricultural waste related to the rice industry. As a food crop, rice ranks first in consumption, second in total output, and third in total cultivation [1]. Most (around 90%) of the world's rice supply is grown in Asia; the world's two major rice producers, China and India, provide more than half of the world's rice supply [1,3]. The world increase in cultivated area from 120 million to 163 million ha (0.5%) each year and the increase in paddy yield from 1.8 to 4.6 t/ha (1.6%) between 1960 and 2018 caused a more than threefold increase in global rice output, from 221 million to 745 million metric tons (2.1% per year) [4] (Figure 1). The Green Revolution dramatically increased rice production, which helped stave off famines, feed millions of

**Citation:** Illankoon, W.A.M.A.N.; Milanese, C.; Collivignarelli, M.C.; Sorlini, S. Value Chain Analysis of Rice Industry by Products in a Circular Economy Context: A Review. *Waste* **2023**, *1*, 333–369. https: //doi.org/ 10.3390/waste1020022

Academic Editors: Catherine N. Mulligan, Dimitris P. Makris and Vassilis Athanasiadis

Received: 15 December 2022 Revised: 10 January 2023 Accepted: 8 February 2023 Published: 4 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

people, alleviate poverty and hunger, and improve the lives of countless people throughout Asia [1,5]. Since there is a limit on how much land may be devoted to rice cultivation, any future increases in output will have to come from higher yields. Most of the world's population relies on rice for sustenance. With a global per capita consumption of 64 kg per year, milled rice accounts for 19% of global daily calorie intake [1,3,4,6].

Agricultural wastes are biomass residues that can be grouped into two categories, i.e., crop residues and agro-industrial residues [7]. However, traditional approaches handling agricultural waste have been linked to environmental damage and financial losses. Many farmers and other agro-industry stakeholders engage in open field burning or open dumping to clear their land for future cultivation rather than extracting their total value. In developing countries, burning crop straws and other agricultural wastes in the open air or in the kitchen is a significant contributor to dangerously high levels of air pollution. According to estimates provided by the World Bank and the Institute for Health Metrics and Evaluation (2016), the welfare losses caused by exposure to air pollution cost the global economy around \$5.11 trillion in 2013 [8]. For sake of example and comparison, the size of welfare losses in Belarus, Bulgaria, India, Romania, Kazakhstan, and Bangladesh as a share of regional gross domestic product (GDP) are as follows: 9.25%, 8.85%, 7.69%, 7.21%, and 6.81%. The cost of air pollution caused by open-air straw burning in mainland China in 2004 was estimated to be over 19.65 billion CNY or around 0.14% of the country's GDP [9]. Thus, it is possible to minimize these adverse effects by considering the valorization process for these potential feedstocks and using them as a valuable material or source for the national economy [10].

Considering their waste quantities and physical and chemical properties, rice industry by-products have a high potential for generating energy and extracting nutrients, minerals, and biochemicals through different valorization processes [10]. However, their potential still needs to be explored due to issues relating to their supply chain, appropriate technologies for pretreatment, and cost-effective methods. Therefore, it is necessary to combine them with feasible business models to facilitate the valorization methods for the rice industry by-products in many developed and developing countries [7,10].

**Figure 1.** Rice production and area harvested, adopted from [11].

However, if wastes are valorized as raw materials to complete parts of equipment through disassembly and remanufacturing, they can create value for entire supply chains, allowing additional markets for used components beyond the raw materials market and developing new specialized professions. These openings are crucial because they establish an indirect market where resources, energy, and components from various waste streams in the rice value chain produce income for the communities [12].

Therefore, the primary purpose of this study is to consider current waste management practices for the main types of waste in the rice industry, namely rice straws, rice bran, and rice husk, including rice husk ash that remains from processes involving energy generation. Additionally, this review presents the strategies and potentialities of these by-products for developing full utilization in the economy. Therefore, the study is focused on the potentiality of all types of products, such as high-value low-volume products, high-volume low-value products, and value-added products (Figure 2). However, the implementation of these valorization options will be determined by practical feasibility. In addition, these options will be analyzed for compatibility with other factors such as economic feasibility and social acceptance.

**Figure 2.** Focused valorization options.

#### **2. Rice Industry Value Chain**

Agricultural food processing consists of a variety of value chains and generates different types of agricultural waste through the value chain from farm to fork [13,14] (Figure 3). Large quantities of valuable wastes are produced during the harvesting and processing stages, and these wastes should be studied and analyzed to extract their valuable parts sustainably. For example, paddy straws are produced in large quantities during paddy harvesting. These straws are commonly found in the field and are often used as fodder for animals and as bedding for livestock [15]. The remaining part is burned by the farmers when ready for subsequent cultivation. Additionally, rice husk frequently becomes a material for burning and is discarded in landfills [16]. The destruction of this valuable biomass without proper use will cause irreversible damage to the environment and all living beings on the planet. Bran is another by-product of the rice processing value chain (Figure 3). After harvesting and milling, the by-products of the rice industry can be subjected to industrial symbiosis to exploit their full value [2,13].

Rice farming is integrated with geography, soil type, water availability, harvesting and processing techniques, and market behavior [17,18]. Farmers grow different types of rice in different regions because each grows best in specific soil types and climates. Additionally, access to water and proper irrigation systems boost agricultural production [19,20]. Many small and medium rice farmers still depend on labor [14]. Traditional techniques are slow and dependent on workability and experience. Modern technology and equipment are helpful in rice-producing areas but expensive [13,14,21,22]. Consequently, different production conditions affect the rice industry's waste [21,22]. Therefore, waste calculation should include unused rice and other waste due to harvesting and processing errors [2,13]. Consumer behavior and attitudes will determine how much rice is wasted at the end of the food value chain [23–25]. Edible rice wastage is due to bad taste, rotting, and leftovers [23–25].


**Figure 3.** Conceptual model of the rice waste generated all along the food chain (red arrows represent the interaction points of each stage throughout the value chain) adopted from [14,26–29].

#### *Waste Production throughout the Value Chain*

After completion of the harvesting process, the paddy is transferred to the rice mills to be processed into white or brown rice. The paddy is subjected to a series of operational procedures during the rice milling process to remove straw particles, half-filled seeds, husks, bran, and germ. Several milling processes exist, such as one-stage milling and multi-stage milling [2,30]. Compared to multi-stage mills, the one-stage milling technique produces fewer by-products. The large-scale industrial milling process has several steps, such as cleaning (removing chaff, dead seeds, seeds that are only half full, and stones), parboiling, de-husking, peeling, polishing, and grading [2,30,31]. In addition, specific

varieties of rice will be washed in hot water for a certain amount of time to remove the husk, enhance its size, and obtain a better shape of the grains [31].

There are several ways to remove rice husks from rice seeds. The germ particles and outer bran are removed after the husking in a series of huller reels and pearling cones, where the waxy cuticle is sheared off by friction between the high-speed abrasive cone and its casing [30–32]. As a result, rice bran is generated as a by-product [33]. The milling gap between the cone and the cover can be changed. Therefore, the grinding ratio can be changed by raising or lowering the cone [30,31]. Typically, in most rice mills, the rice passes through several cones, each with a higher milling rate than the previous one [30]. Since the milling space between the cone and the casing is adjustable [31], the milling rate can be varied by raising or lowering the cone [29]. The bran from the different stages is usually quantified as one product [33]. Next, rice from the pearlier is passed through polishers to get a finer appearance to the rice grains [2,26]. In this process, some parts of the starchy kernel are removed. This by-product is called fine bran if it is included within the inner bran layer. Finally, the mixture of whole and broken rice from the polishers are subjected to the sieving process and graded according to the standard at which the rice is sold [27].

According to previous research, the ratio of useable products to by-products is shown in Figure 4. Pollards are often a blend of polishings and bran. However, all these byproducts are generated during the rice milling process, and their amount is roughly 60% rice husk, 35% rice bran, and 5% polishing from the whole rice mill waste stream [16,34–39].

**Figure 4.** Quantity of rice processing by-products throughout the value chain adopted from [16,34–39].

Total rice consumption worldwide from 2008/2009 to 2021/2022 (in 1000 metric tons) is shown in Figure 5. The FAO Agricultural Outlook predicts that paddy production will rise to 52603 metric tons by 2027 compared to 2018 [40]. Due to factors including the increase in agriculturally usable land, technological advancements, and faster population growth in recent decades, global agricultural output has expanded dramatically [40].

**Figure 5.** Total rice consumption worldwide from 2008/2009 to 2021/2022 (in 1000 metric tons) adopted from [41]. \*\* estimate as of January 2022.

As agricultural waste generates economic benefits, agricultural waste recycling is not meant to degrade value like other industrial waste recycling does [42]. Due to the nature of systemically implemented operations, recycling must be compared to materials that remain the same or lose performance when recycled. Due to their inherent propensity for rapid spoiling, agri-food supply chain management may need to be more sustainable and efficient [43]. Having a systemic vision and viewpoint that prioritizes the concepts of complexity and networks is essential for solving this challenge [13,42–44]. According to this method of thinking, a system is a collection of interconnected individuals whose behaviors are determined by their connections. When all of these elements are considered, they form a holistic system with more worth than just the sum of its individual parts. From this point of view, designing the agri-food scenario using a systemic approach is a viable method to begin a paradigm shift that entails switching from linear to circular structures.

#### **3. Analysis of Rice Supply Chain Waste**

#### *3.1. Rice Straw*

Rice straw is the vegetative part of the rice plant (*Oryza sativa* L.). Rice straw consists of the plant's stem, leaves, and pods and is generated after being cut off during harvest. Rice straw comprises cellulose, lignin, waxes, silicates, and minerals. In general, animals are often fed with rice straw, and rice straw can be utilized for creating compost, paper, cow bedding, and crafts; it also offers energy to specific industries and covers agricultural areas [45,46]. The rice straw of the current year is usually burned before the subsequent plowing to prepare the field.

#### Composition of Rice Straw

Variety, cultivated area, seasons, nitrogen fertilizer, plant maturity, plant health, and several other environmental and human variables significantly impact the chemical composition of any biomass [15,47]. Changes in chemical and physical parameters affect the yield and quality of the final product. Heterogeneity is thus seen as detrimental to the manufacturing process. Additionally, this impacts how by-products are used at the end of their life cycle. Therefore, compositional analysis and structural characterization should be considered to enhance the effectiveness of the valorization options. Tables 1–3 provide the average values of various important parameters describing raw and processed rice straw (based on the energy, nutritional, and fertilizer properties, respectively) as obtained by the

chemical analyses. Rice straw has a greater silica concentration but less lignin than the straws of other cereals [48]. In order to maximize silica amount in the stem ratio, it is advised that the rice straw be shortened as much as possible [2]. Cell walls may contain silica, or silica may be soluble in water. They are eliminated with urine, where they sometimes crystallize. Since rice straw has a high oxalate content (1–2% of dry matter) and is known to lower Ca concentrations, adding supplemental Ca is often recommended [49]. Variety, time duration between harvest and storage, amount of nitrogen fertilizer used, plant maturity (lignin content increases with maturity), plant health, and environmental conditions affect the quality of rice straw [15]. Although rice straw is a rich energy source, it contains only 2–7% protein and is indigestible due to its high silica content. Therefore, it is considered a coarse and low-quality food source [50]. Minerals such as sulfur may be a limiting factor when considering it as fodder [51]. Other conditions usually involve:



**Table 1.** Energy characteristics of rice straw [2,47,55–59].

**Table 2.** Fodder characteristics of rice straw [2,47,55–59].



**Table 3.** Fertilizer characteristics of rice straw [2,47,55–59].

#### *3.2. Rice Bran*

A significant waste product in the value chain of rice processing is rice bran. It is mainly used as animal feed and is regarded as a healthy source of fiber for pets because of its high nutritional content. Additionally, farmers may get it at a significant discount because of its availability. Due to the high fat and fiber content of rice bran, up to 40% of it is added to the diets of cattle, dogs, pigs, and chickens [60–62]. Additionally, rice bran is a valuable feed for many animals since it contains 14–18% oil. Therefore, dehulled rice bran may be utilized in more value-added processes than ordinary rice bran [2].

#### Composition of Rice Bran

The composition of rice bran has a significant role in defining its possible valorization options. Rice bran's physical and chemical properties are influenced by several aspects concerning the grain and the milling procedure [63]. Rice variety, environmental circumstances, grain size and form, distribution, chemical components, strength of the outermost layer, and breaking resistance are the primary elements affecting rice grain [64]. Additionally, the type of grinding machine is the main factor related to the processing conditions, and the grinding process of different layers of rice grain at different depths shows different chemical compositions [63,64].

Rice bran contains various nutrients, including carbohydrates, proteins, minerals, and lipids. It has a high carbohydrate (cellulose and hemicellulose) content and is simple to employ to create microbial products with added value [65]. As a result, before the valuation procedure, it is required to assess the composition. Because rice bran is employed in valueadded goods as a microbial product or as a food additive, it is generated during several phases of the rice milling process, which are eventually combined and discharged as rice bran. As a consequence, the chemical composition varies significantly [2]. In addition, the chemical composition of raw rice bran and de-oiled rice bran varies in fiber concentration [55]. Tables 4–7 reflect the chemical analyses regarding its energy-related parameters, fertilizerrelated features, feed-related parameters, and bioactive-component qualities.

Rice bran stands out compared to other cereal grains due to the tocotrienol, tocopherol, γ-oryzanol, and β-sitosterol contents [66]. This is significant since there is mounting evidence that these substances may help to lower levels of total plasma cholesterol, triglycerides, and low-density lipoprotein while raising levels of high-density lipoprotein [66]. In addition, ferulic acid and soluble fiber (including β-glucan, pectin, and gums) are found in the indigestible cell walls of rice bran. While the United States Department of Agriculture (USDA) nutritional database values for crude rice bran are often utilized in animal diet formulation [67], caution must be taken since they may not account for changes across rice cultivars [68].

The variety of rice bran utilized determines the chemical content and quality of the end product. According to Hong and his co-workers [69], the fatty-acid content of rice bran oil varies based on the type of rice bran utilized. According to the same paper, rice bran oil, which includes a high concentration of free fatty acids, has several drawbacks when used as fuel in diesel engines in the winter season.


**Table 4.** Energy characteristics of rice bran [2,55,56,63–65,70–74].

#### **Table 5.** Fodder characteristics of rice bran [2,55,56,63–65,70–74].



**Table 6.** Fertilizer characteristics of rice bran [2,55,56,63–65,70–74].

**Table 7.** Biochemical characteristics of rice bran [2,55,56,63–65,70–75].


#### *3.3. Rice Husk*

Rice husk is the outer covering of the rice grain and is produced as a by-product of the rice milling process. It is also called hull and chaff [39,76]. In agricultural nations, this is the most prevalent agricultural by-product. In particular, rice husk is utilized as the primary source of energy in rice mills, poultry farming, and silica-rich cement [56,77,78]. Additionally, small quantities are used as construction materials and fertilizers [79]. However, most rice husks eventually wind up in landfills or are burned in the open air, significantly polluting the environment. The calorific value of rice husk is considerably high, roughly 16,720 kJ/kg [80]. As previously stated, many millers directly burn or gasify rice husk as their primary energy source [16]. Rice husk ash is another type of waste produced during this burning procedure. This additional waste, which makes up around 25% of the original volume of rice husks, has a significant adverse effect on the environment [38].

Composition of Rice Husk

Due to photosynthesis and biochemical interactions, silica and a barrier layer are formed on the rice plant's stem and husk surfaces [81]. These layers have developed to shield the rice plant and its grains from environmental changes such as temperature variations, excessive water evaporation, and microbial assault [81]. Approximately 20–30% of the rice husk is made up of mineral components, including silica and metallic residues containing magnesium (Mg), iron (Fe), and sodium (Na). Calcium (Ca), manganese (Mn), and potassium (K) are further examples of trace elements [82]. Rice husk mainly comprises organic compounds, including cellulose, lignin, and hemicellulose, making up around 70–80% of the total weight [37,83]. Rice husk is maturing into a raw material prospective in the manufacturing sector. However, when rice husk accumulates to the point that it poses a severe threat to the local ecosystem, it is classified as agro-waste. As a result, these adverse effects on the environment must be softened via a process of valorization or value addition. Therefore, it is crucial to conduct a physicochemical investigation and determine the composition of the material. Tables 8–11 show the characteristics of rice husks in terms of energy, fodder, fertilizer, and biochemical properties. The chemical components of rice husk ash are shown in Table 12.


**Table 8.** Energy characteristics of rice husks [2,16,35,55,56,84–86].

**Table 9.** Fodder characteristics of rice husks [2,16,35,55,56,84–86].



**Table 10.** Fertilizer characteristics of rice husks [2,16,35,55,56,84–86].

**Table 11.** Biochemical characteristics of rice husk in different rice varieties [2,55,56].


**Table 12.** Chemical composition of rice husk ash [87–89].


#### **4. Valorization Potential of Rice Industry By-Products**

#### *4.1. Valorization of Rice Straw*

Rice straw could also be valorized for four different purposes: energy production, animal feed, fertilizer, and other uses. By pyrolyzing rice straw, bio-oil, biochar, and syngas may be generated. Numerous chemical substances are found in rice straw bio-oil, including alcohols, acids, furans, aromatics, ketones, phenols, and pyranoglucose [47]. Alcohol and pyranoglucose are created as a consequence of the pyrolysis of cellulose, while hemicelluloses are used to create ketones [47]. The metabolic process through which carbohydrates are changed into alcohols or acids is known as fermentation, as shown in Equations (1) and (2). Second-generation biofuels are made from cellulose feedstock (Equation (1)). Physical, chemical, or biological pretreatment and fermentation are all viable routes to their production. While its lack of competition from other feedstock substrates is an advantage, its need for highly efficient lignohemicellulose enzymatic breakdown is a drawback. Although the commercialization of second-generation ethanol facilities

shows promise, the longevity of these plants will primarily rely on the market availability of the feedstocks at affordable costs [90]. Bacteria convert carbohydrates into lactic acid (Equation (2)). Numerous chemical or physical pretreatments are required, followed by enzymatic hydrolysis to convert fermentable sugars from lignocellulosic materials into ethanol or lactic acid. In addition to its many uses in the food and beverage industries, lactic acid and its derivatives also have a wide range of applications in the pharmaceutical, cosmetic, and manufacturing industries [91,92]. Numerous studies have demonstrated that rice straw can be utilized to make second-generation biofuels [47,93–96]. Typically, bacteria and yeast turn carbohydrates into lactic acid and sugars into alcohol. *Trichoderma reesei*, which was derived from decaying rice straw waste, produces cellulases that break down cellulose in the rice straw to glucose, which is then fermented with yeasts such as *S. cerevisiae* to make ethanol [93,97,98].

$$\rm{C}\_6\rm{H}\_{12}\rm{O}\_6 \rightarrow 2\rm{C}\_2\rm{H}\_5\rm{OH} + 2\rm{CO}\_2\tag{1}$$

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 \rightarrow 2\text{CH}\_3\text{CHOH}\text{HCOOH} \text{ (lactic acid)}\tag{2}$$

The anaerobic digestion process may convert rice straw into biogas [75,99,100]. Anaerobic digestion is a sustainable process that converts organic waste into usable energy. Generating green energy from rice straw is an effective way to lessen the effects of global warming [75]. Around the globe, rice straw is utilized directly as an energy source for heating rooms by direct burning, firing clay pots, and cooking [101]. Additionally, small grids in certain nations such as Nigeria have a higher potential for using rice husks and straw as a source of rural power [102,103]. Umar and co-workers [102,103] claimed that rice straws have the potential to generate 1.3 million MWhy-1 energy in a country such as Nigeria. A 36 MW power plant in Sutton, Ely, Cambridgeshire, was constructed in 2000, producing more than 270 GWh annually while using 200,000 tons of rice straw [104]. Another work shows that Sri Lanka has a total energy capacity of 2129.24 ktoe/year of primary energy from rice straw and rice husk and a capacity of 977 Mwe, allowing it to produce 5.65 TWh of electricity per year [16].

According to literature, rice straw may be used efficiently for composite preparation [105–111]. Furthermore, rice straw microfibrils at 5% increase the characteristics of rice straw polypropylene composites [107]. Another study highlights many uses of rice-straw cement bricks for load-bearing walls [106]. Rice straw can also be used to lower the price of cement bricks with sufficient thermal insulation, appropriate mechanical qualities, and fire resistance [112–115]. Furthermore, rice straw-based composites with adhesives generated from starch can be used as ceiling panels and bulletin boards [109]. Finally, following proper pretreatment, rice straw could also be utilized to produce fiberboard [116].

Rice straw may be used to produce many kinds of enzymes in an industrial setting [47,57,58]. *Trichoderma harsanum* SNRS3 can generate cellulase and xylanase using alkali-pretreated rice straw [58]. According to this research, rice straw is a more effective inducer of the formation of cellulase and xylanase and does not need the inclusion of other chemicals. Lactic acid can be produced by using pretreated rice straw [117,118]. A Naviglio extractor and trifluoroacetic acid could transform rice straw into a unique bioplastic that can be used as shrink films, sheets, or for shape memory effects. Its mechanical characteristics are equivalent to polystyrene in the dry state, while in the wet state, the cast bioplastic performs equal to plasticized poly(vinyl chloride) [119].

Rice straw has a low value as a feeding material, despite its use as bedding for cattle [15]. In contrast to ruminants, which depend on symbiotic bacteria to break down cellulose in the gastrointestinal system, all vertebrates lack the enzymes necessary to dissolve β-acetyl bonds [52]. Additionally, dried rice straw contains low nutrient value owing to its low amount of protein and high amounts of lignin and silica. However, this may be addressed by pretreating it with ammonia or urea [48]. To increase the nutrient availability of rice straw, it can be converted into silage. Therefore, some researchers have focused on improving rice straw harvesting technologies for silage production [120]. Other studies have explored several practical examples of silage processing, including using different additives to enhance fermentation quality and adding yeasts such as *Candida tropicalis* [121–123]. Feed intake, digestibility, rumen fermentation, and microbial N synthesis efficiency are improved after urea treatment of rice straw [124].

Rice straw has been proposed as a low-cost adsorbent for purifying contaminated water [125]. However, straw surface composition and metal speciation significantly impact the adsorption capacity, which changes with metal ions and water pH [126,127]. On the adsorbent surface of rice straw, methyl/methylene, hydroxyl, quaternary ammonium, ether, and carbonyl groups predominate; adding additional quaternary ammonium or incorporating carboxyl groups enhances its adsorption capability [128]. The existence of these groups is supported by the ATR–FTIR spectrum shown in Figure 6. However, competing cations and chelators in the solution are likely to result in decreased sorption capacities [129]. Furthermore, most heavy metal ions exhibit maximal adsorption capacities around pH 5. In contrast, very acidic circumstances promote Cr adsorption [130], which could be the result of the reduction of Cr(VI) to Cr(III). Moreover, cellulose phosphate derived from rice straw that has been treated with NaOH and then reacted with phosphoric acid in the presence of urea has a more remarkable ability to absorb heavy metals. This ability is increased when microwave heating is used to produce it [131]. The addition of epoxy and amino compounds to rice straw by reacting with epichlorohydrin and trimethylamine results in a high sulfate adsorption efficiency, demonstrating the material's anion exchangeability [132]. Like rice husk, straw can be used as an adsorbent for different water contaminants, such as alkali and phenolic chemicals, that can usually be recovered using anionic species [133]. Various adsorbents from rice straws have also been developed to remove dyes from wastewater. An example of cationic dye application is rice straw treated with citric acid, which increases the specific surface area and pore size. These treated straws have been used to absorb crystal violet or methylene blue from an aqueous phase [134]. It has been observed that the addition of activated rice straw causes a significant reduction in microalgae in water, which has been attributed to the synergistic effects of humic chemicals and H2O2 created by the straw breakdown [135]. According to a different investigation, water and methanol extracts from rice straw controlled the cyanobacterium *Anabaena* sp. but promoted *Chlorella* sp. To prevent the development of *Anabaena* sp., rice straw extraction is an economical and ecologically beneficial option, but it may not work as well on other cyanobacteria and microalgae [136].

Rice straw is utilized as organic fertilizer for various crops in many places throughout the globe. It can also be used as a soil conditioner to replace the organic matter in the soil [112]. In addition, rice straw is also a growth medium for mushrooms [137]. Adding biochar derived from rice straw to the soil makes it possible to enhance the characteristics of the soil by lowering its pH, cation exchange capacity (CEC), nutrient availability, and nitrate leaching [138–140]. Figure 7 displays an overview of all rice straw value-adding possibilities [2,47,53,57,58,75,100,106,110,130,137–139,141–144].

**Figure 6.** ATR–FTIR spectrum of Sri Lankan rice straw (own source) as-received sample acquired at room temperature using a Nicolet FTIR iS10 spectrometer (Nicolet, Madison, WI, USA) equipped with a Smart iTR with diamond plate. Straw was dried at 45 ◦C, milled, and sieved with a 1000 μm mesh sieve before analysis. Thirty-two scans in the 4000–600 cm−<sup>1</sup> range at 4 cm−<sup>1</sup> resolutions were co-added.

**Figure 7.** Summary of possible valorization options for rice straw.

#### *4.2. Valorization of Rice Bran*

Considering the concept of circular economy and green product technology, the biorefinery plan would be the best option for managing and utilizing rice bran [145]. In addition to providing more nutrients than other cereal grains, rice bran has more lipids, protein, and calories [146–148] (Table 5). Rice bran is vulnerable to oxidative rancidity; thus, heat stabilization is necessary to avoid spoilage and rancidity [149]. Rice bran oil is widely recommended around the world due to the presence of several beneficial natural and healthy bioactive ingredients. Companies have been encouraged to manufacture stabilized rice bran and rice bran products to improve the health of organisms because of the unique mix of lipids, minerals, and nutrients found in rice bran, including calcium, phosphorus, and magnesium [65]. In addition, several researchers have found that the manufacturing of de-oiled rice bran and rice bran oil is in great demand worldwide [62,150,151].

The production of biodiesel from rice bran is actively marketed all over the globe. However, rice bran oil must be removed from the rice bran to produce biodiesel via a transesterification process [152]. Several methods have purportedly been utilized to produce biodiesel from rice bran oil, including acid-catalyzed and base-catalyzed transesterification and lipase-catalyzed transesterification. However, each technique has different environmental effects as well as technological and economic benefits and drawbacks [153–156].

Some researchers have examined bioethanol synthesis from rice by-products such as rice bran, defatted rice bran, and rice washing drainage [157–160]. After pretreating stripped rice bran with diluted acid and detoxifying it, the *Pichia stipitis* NCIM 3499 strain generated an ethanol concentration of 12.47 g/L [161]. Additionally, another study found that biological pretreatment with the fungus *Aspergillus niger* increased ethanol output [162]. Numerous scientists have attempted to manufacture lactic acid from dehulled rice bran using various microbes [163–166]. Another study discovered that many *Bacillus coagulans* isolates could grow in denatured rice bran enzymatic hydrolysates without adequate nutrients, with the majority producing concentrations of lactic acid more significant than 65 g/L and yields greater than 0.85 g/g [163]. They stressed in the same paper that manufacturing lactic acid from dehulled rice bran might be economically viable.

Due to its numerous similarities to gasoline, biobutanol is the most ecologically benign substitute for traditional fossil fuels. Additionally, when HCl and enzyme treatments are used together, they can remove 41.18 g/L of sugar from dehulled rice bran and 36.2 g/L of sugar from rice bran [167]. Another study reported that both defatted rice bran hydrolysates and rice bran hydrolysates could be fermented in bioreactors with nutrients to make butanol at a rate of 12.24 g/L and 11.4 g/L, respectively [163].

Rice bran can be used as an adsorbent for polluting substances because it has a granular shape, is chemically stable, does not dissolve in water, and is easy to get. Its surface has several active sites that can remove pollutants [39]. How well these sites work depends on the chemical nature of the solution and whether or not there are other ions in the solution besides the ones to be trapped. Additionally, various functional groups on the surface of rice bran, such as hydroxyl and carbonyl groups, are responsible for its high adsorption effectiveness [39]. The existence of these groups is supported by the ATR–FTIR spectrum shown in Figure 8, which exhibits rice-straw-like peaks. Some researchers have tried to figure out the best way to remove arsenic from water using a fixed-bed column system made of rice bran. The objective of this study was to look at how different design parameters, such as flow rate, bed height, and initial concentration affected the adsorption process. The uptake capacities of As(III) and As(V) were found to be 66.95 μg/g and 78.95 μg/g, respectively [168].

**Figure 8.** ATR–FTIR spectrum of Sri Lankan rice bran (own source) as-received sample acquired at room temperature using a Nicolet FTIR iS10 spectrometer (Nicolet, Madison, WI, USA) equipped with a Smart iTR with diamond plate. Bran was dried at 45 ◦C, milled, and sieved with a 1000 μm mesh sieve before analysis. Thirty-two scans in the 4000–600 cm−<sup>1</sup> range at 4 cm−<sup>1</sup> resolutions were co-added.

Another excellent substitute for conventional fossil fuels is hydrogen, which, when oxidized, merely produces water vapor (H2O). Additionally, hydrogen has a higher energy content for mass units than traditional fuels, ranging from 112 to 142 kJ/g [168,169]. Photo and dark fermentation and their combination are all capable of producing biohydrogen [170]. Some studies have investigated hydrogen generation from rice bran and defatted rice bran using isolated bacteria from the same substrates. They identified *E.ludwigli* IF2SW-B4 as the most promising strain. When rice bran was utilized as a substrate, 545 mL/L of bio-hydrogen was produced [171]. The whole biotechnology process will be more economical once enzymes are produced utilizing low-cost ingredients. For the environmentally friendly and more effective release of fermentable sugars from different affordable and sustainable biomasses such as rice bran, enzymatic hydrolysis is used [171]. Researchers have conducted several investigations to synthesize enzymes from defatted rice bran and rice bran [172–174]. Figure 9 displays an overview of all rice bran value-adding possibilities.

**Figure 9.** Possible valorization options for rice bran [39,145,146,150–153,157–174].

#### *4.3. Valorization of Rice Husk*

South Asian countries such as India, Pakistan, Bangladesh, and Sri Lanka were among the best in the world in utilizing rice husk from 1970 to 1985 [175]. In addition, governments and other organizations engaged in rice farming and the post-harvest process have provided essential direction and strong support for rice husk management. Rice husk differs from other agricultural wastes in several important physicochemical aspects, including high silica concentration, low density, high porosity, and a significant outer surface area [176]. Because of these qualities, rice husk is more valuable than other waste materials. As a result, it covers a range of industrial applications.

In water treatment, using activated carbon for the adsorption process to remove heavy metals from industrial effluents is appealing. Numerous functional groups, including hydroxyl, methyl/methylene, ether, and carbonyl are present on the rice husk's adsorbent surface, contributing to the material's enhanced adsorbent efficiency (Figure 10) [39]. The existence of these groups is supported by the ATR–FTIR spectrum shown in Figure 10, which mainly exhibits bands at 3307 cm<sup>−</sup>1, 2921 cm−1, 2000–2500 cm−1, 1654 cm−1, 1034 cm−1, and 788 cm−<sup>1</sup> representing hydroxyl groups, C–H groups, C≡C or C≡N bonds, C=O groups, C–O and C–H bonds, and Si–O bonds, respectively.

**Figure 10.** ATR–FTIR spectra of Sri Lankan rice husk (own source) as-received sample acquired at room temperature using a Nicolet FTIR iS10 spectrometer (Nicolet, Madison, WI, USA) equipped with a Smart iTR with diamond plate. Husk was dried at 45 ◦C, milled, and sieved with a 1000 μm mesh sieve before analysis. Thirty-two scans in the 4000–600 cm−<sup>1</sup> range at 4 cm−<sup>1</sup> resolutions were co-added.

The presence of several types of polar groups on the surface of rice husks results in a significant cation exchange capacity, indicating a potential efficacy in physisorption mode [177]. Rice husk treated with H3PO4 showed enhanced copper absorption capacity [178]. Some studies found that chemically treated rice husk can absorb cationic dyes such as methylene blue [179,180] and malachite green [181]. To study the absorption of fluoride from aqueous solutions, some researchers produced rice husk by chemically impregnating it with nitric acid, followed by physical activation [182]. According to their findings, the highest absorption of fluoride was 75% at a pH of 2, and the ability to absorb fluoride decreased as the pH rose from 2 to 10. Ahmaruzzaman and Gupta [183] confirmed this conclusion. When modified rice husk is cross-linked with poly(methyl methacrylateco-maleic anhydride), nanoparticles are formed that can be used to absorb heavy metal ions (such as Pb(II)) and dyes (such as crystal violet) [184]. Researchers have discovered that novel green ceramic hollow fiber membranes made from rice husk ash can act as an adsorber and separator to remove heavy metals from water effectively [185]. Treating rice husk with H2SO4 and NaOH prior to heating enhances the product's capacity to absorb phenol [186].

Biomass derived from agricultural waste has been identified as a rich source of feedstock for biochar production; however, at present, farmers, and other stakeholders such as millers, practice open field burning or open dumping to dispose of these by-products. Compared to low-cost traditional treatment procedures (boiling, chlorination, sand filtration, and solar disinfection), biochar adsorbent offers various advantages. It is also suitable for low-income countries because of its availability, cheap cost, and accessible technology. Low-cost conventional approaches mainly destroy pathogens, while biochar can remove a wide range of pollutants from drinking water. Existing processes, such as chlorination, emit carcinogenic by-products, and boiling concentrates chemical contaminants. Pyrolysis temperature, vapor residence time, and other chemical and physical alteration variables influence the properties of biochar (Figure 11). Compared to conventionally activated biochar, rice husk biochar activated by a single phase of KOH-catalyzed pyrolysis under CO2 has a larger surface area and greater capacity for phenol adsorption [187]. The gold-thiourea complex can be effectively adsorbed using biochar derived from rice husks that have been heated to 300 ◦C and have particular silanol groups and oxygen functional groups [188]. Rice husk-activated carbons are effective in removing phenol [189], chlorophenols [190], basic dyes [191,192], and acidic dyes [193,194] from water, as well as heavy metal ions such as Cr(VI) at low pH [195,196], Cu(II), and Pb(II) [197].

**Figure 11.** Strategic schematic diagram for biochar production, modification/engineering, characteristics, and water treatment applications adopted from [198–201].

Rice husk pellets provide an alternative to diesel oil and coal for energy generation in small-scale power plants. Through pyrolysis and gasification processes, they may also be used to produce biodiesel [202]. Rice husk, subjected to a thermochemical conversion process, may provide an inexhaustible supply of gaseous and liquid fuel. Thermochemical and biochemical processes are shown in Figure 12 as the two ways rice husk can be converted to energy. Thermochemical processes such as combustion, gasification, and pyrolysis are often regarded as the primary means of producing secondary energy substances. Fermentation and transesterification are also critical biochemical steps in ethanol and biodiesel production [175,203,204]. The briquettes made from rice husks with starch or gum arabic as binders burn stronger and more efficiently than timbers [205]. Another study describes a reactor that uses rice husk combined with sawdust or charcoal to generate high-grade fuel [206]. In order to obtain charcoal, which has a comparatively high calorific content, rice husk is subjected to carbonation using starch as a binder and either ferrous sulfate or sodium hypophosphite, which promote ignition [135]. Economically viable primary pyrolysis oil, suitable as boiler fuel oil and for the manufacture of catalytically treated, upgraded liquid products, can be obtained by fluidized-bed rapid pyrolysis with the catalytic treatment of rice husk [207].

Materials derived from rice husks have been used in the world's most advanced technical equipment and industries. For example, the Indian space agency has figured out how to extract high-quality silica from rice husk ash. This high-purity silica might also increase its use in the information technology sector [175]. In addition, the same publication has stated that other scientists have discovered how to extract and purify silica from rice husk ash to produce semiconductors. In addition, several researchers have pointed out the prospects of using silicon-based compounds extracted from rice husk and ash in various industries [208,209].

**Figure 12.** Energy conversion process of rice husk, adopted from [176,203,204].

Li et al. [210] stated that KOH-activated rice husk char could make porous carbons for CO2 capture at low pressures. Low activation temperature and a small KOH/char ratio favor high CO2 absorption and CO2-over-N2 selectivity. This is presumed to be due to the micropores' narrow size distribution. A similar investigation has been conducted using KOH-activated rice husk biochar for hydrogen storage. It revealed that 77 K/6 bars have a hydrogen storage capacity of 2.3%wt [211].

Due to its microscopic particle size, high solution pH, and low supportive electrolyte content, rice husk ash is an effective adsorbent for heavy metals, including lead and mercury [212,213]. In addition, the fluoride-absorption capacity of rice husk ash treated with aluminum hydroxide is enormous [214]. Both the effective removal of phenol from aqueous solutions and the adsorption of various dyes, including indigo carmine, Congo red, and methylene blue, has been accomplished using rice husk ash [180,215–218]. Due to its high silica concentration and the existence of mesopores and macropores, rice husk ash is a promising adsorbent for removing contaminants from biodiesel [219]. Zou and Yang [220] examined different approaches for generating silica and silica aerogel from rice husk ash. Epoxy paints can use rice husk ash as a filler, and the inclusion of rice husk ash can improve a variety of qualities, including wear resistance, elongation, and scratch resistance [221]. In addition, a paper's printing quality might be enhanced by using rice husk ash. Because rice husk ash contains more silica, it can improve the paper's surface quality, and the coating layer it generates reduces the quantity of ink penetrating the paper [222]. Additionally, some researchers have studied pigments made from rice husk and ash [175].

Rice husk ash can improve the properties of cementitious materials such as concrete in resistance to corrosion. With the addition of rice husk ash, the cement particles are encased in a calcium silicate hydrate gel, making the cement denser and less porous. These characteristics are also helpful in protecting concrete against cracking, corrosion, and chemical breakdown caused by leaching agents [89,223–226]. The use of powdered rice husk ash derived under controlled burning conditions as a reinforcing filler for different rubbers has been researched. The authors discovered that substantially reinforced rice husk ash had no adverse effect on the vulcanization or aging behavior of certain rubber types, such as natural rubber, styrene–butadiene rubber, and ethylene–propylene–diene elastomers [227]. Moreover, the use of rice husk ash as a raw material in cement production has the potential to reduce production costs. Figure 13 displays an overview of all rice husk and rice husk ash value-adding possibilities.

**Figure 13.** Possible valorization options for rice husk and its ash [176–202,205–210,212–214,216–222, 224–226,228,229].

#### **5. Circular Economy**

The circular economy (CE) has gained attention as a way to overcome the current production and consumption model of "take, make and throw away" or the linear model based on continuous progress and increasing resource output [230]. CE aims to optimize resource usage and achieve an equilibrium between economy, environment, and society by supporting closed manufacturing processes [231,232]. Numerous studies have focused on political, environmental [233], economic, and corporate issues [13]. The "reduce, reuse and recycle" (3R) concept has nine steps from recovery to recovery (Figure 14). Industrial ecology, environmental economics, and environmental policy have influenced CE [13,232,233]. Some authors have claimed that broad systems theory is where CE first emerged [13,232]. Modern concepts including "sustainable design", "performance economy", "cradle-to-grave", "biomimicry", and "blue economy" are associated with developing CE [234,235]. CE was first introduced to Europe in 1976 with Germany's Waste Disposal Act [13,232]. Later, the European Union promoted CE through the Waste Directive 2008/98/EC and the Circular Economy Package [236]. "Reduce, reuse and recycle" is part of the European Waste Directive 2008/98/EC and has been part of the US Solid Waste Agenda since 1989 [236,237]. "CE includes corporate-level sustainable production practices, increasing producer and consumer awareness and responsibility, using renewable technology and materials (where possible), and adopting appropriate, consistent and clear policies and systems" [238].

**Figure 14.** Priority order for circularity techniques within the production chain, adopted from [230–232].

A systemic perspective requires new solutions focusing on environmental processes and stakeholders in the relevant sectors. Agri-food waste management aims to increase resource efficiency and protect the environment. Innovative waste management solutions are needed to reduce waste or transform it into new raw materials. These management practices are part of the CE system, an industrial framework meant to be restorative or regenerative (producing no waste or pollution). Several studies have demonstrated the potential to produce bioenergy, biodegradable polymers, alcohols, and antioxidants from the food supply chain to manage agricultural wastes effectively. Thus, agricultural waste is a source of macronutrients, including proteins, carbohydrates, and fats, as well as micronutrients and bioactive chemicals used to generate new products. Biorefineries use agricultural waste to produce value-added energy and industrial goods. The scientific community considers this concept a sustainable alternative.

If a circular economy-based waste management system is successfully implemented, waste can become a source of wealth for a community or country. Sweden is a prime instance of this. They have made significant investments in infrastructure and imported a large percentage of Norway's waste to convert it into energy (electricity and heat). Thanks to this decision, Sweden can turn Norway's waste into money for its people. Consequently, it charges Norway for waste treatment, generates sufficient energy (electricity and heat) from Norway's waste to meet demand, and recycles or sells the metals it extracts from the bottom ash. In addition, the remaining bottom ash is taken to use in public infrastructure and precast concrete products, drastically reducing the need for mining operations. Because of its innovative approach to waste management within the context of the circular economy, Sweden is a standout among countries [12].

#### **6. Case Study: Sri Lanka**

#### *6.1. Paddy and Rice Value Chain in Sri Lanka*

Since 800 BC, rice has been grown in Sri Lanka [239]. Rice agriculture has grown throughout the nation due to ideal climatic conditions and geographic locations for paddy production (Figure 15). The low country dry zone has the highest rice production as this zone has had a well-planned irrigation system since ancient times. According to the Sri Lanka Rice Research Center, rice consumption per capita in 2019 was close to 107 kg. The Yala season (March to August) and the Maha season (September to December) are the two primary rice harvesting seasons in Sri Lanka. The paddy and rice value chain in Sri Lanka comprises public and private partners connecting rice producers such as small, medium, and industrial-scale farmers, millers (cooperative millers, rice marketing boards, and private millers), food processors, and consumers. In specific Sri Lankan mills, just one step of milling is performed [14]. Figure 16 represents the paddy and rice value chain in Sri Lanka.

Small-scale farmers in villages and semi-urban areas produce sufficient rice for personal use and store it throughout the season or the year. Mid-level farmers store for their consumption and sell excess paddy. Farmers sell these quantities directly to millers, and paddy collectors act as intermediaries in this buying and selling process. After collecting a substantial amount of paddy, they will sell it to co-operative or industrial-scale mills. Large rice farmers who grow rice on an industrial scale sell their crops directly to rice processing mills or process them in their mills. In most cases, small-scale private millers process only a small amount of waste in a particular area throughout the year or during a specific period.

**Figure 15.** Map of Sri Lanka district distribution by climate zone and geographic regions adopted from [240].

**Figure 16.** Paddy and rice value chain of Sri Lanka adopted from [14].

#### *6.2. Rice Waste Availability in Sri Lanka*

The vegetative portion of the rice plant is called rice straw (*Oryza sativa* L.). After the grain has been harvested or sliced, rice straw is produced. According to several researchers, the ratio of rice straw to grain is between 1.0 and 1.5 kg [2,16,39]. Previous studies have shown that 0.1 kg of rice bran is produced for every kilogram of rice [2,78]. Various scholars have revealed that approximately 20–28% wt of rice husk generates greater grain weight [2,35,37,38]. Abbas and Ansumali [79] noted that around 50% of the country's rice mill husk is burned to fulfill its energy needs for steam production. According to some literature [38], approximately 25% of rice husk ash is generated during the burning process. According to the data gathered from Sri Lanka Rice Research Center, in the 2019 Yala season, Sri Lanka produced 1,519,475 metric tons of rice, 1,899,343.75 metric tons of rice straw, 151,947.5 metric tons of rice bran, 364,674 metric tons of rice husk, and 45,584.25 metric tons of rice husk ash as shown in Figure 17. Because of varying weather conditions and several other variables, rice production differs among districts.

**Figure 17.** Rice straw, Rice bran, rice husk, and rice husk ash availability in each district of Sri Lanka (Source: Sri Lanka Rice Research Center).

#### *6.3. Potentiality of Rice Waste Valorization in Sri Lanka*

Currently, rice industry by-products are resold and recycled but have yet to be fully utilized. They are high in nutrients and have chemical–physical properties, making them useful in various sectors of the economy (food, fertilizer, building materials, energy, etc.). Therefore, sustainable and efficient agricultural waste management has emerged as a critical concern for all parties involved in the agricultural value chain in Sri Lanka. According to Figure 17, massive volumes of waste are discharged into the environment at every point along the value chain, permanently harming the air, water, and land. Proper management and implementation of a sustainable valorization system is a complex transformation for developing countries. It needs more funding, laws, and regulations to enhance the capabilities and techniques for agricultural waste management and coordinated efforts by local, regional, and global stakeholders. Therefore, any waste management system that applies to this industry should be founded on value principles and promote the circular economy throughout the process. Figure 18 displays the challenges that should be overcome to implement a sustainable valorization waste management system. The authors have found some feasible valorization techniques for rice processing by-products in the Sri Lankan context:


**Figure 18.** Challenges of valorization of rice industry by-products.

#### **7. Management Issues for Handling Rice Industry By-Products**

Lastly, the authors would like to stress the importance of taking precautions around rice husk remnants since they always include small dust particles created during processing. Breathing filters should be worn while working with these substances since they may irritate the upper respiratory tract and trigger allergic responses, including rhinitis, asthma, bronchitis, COPD, and extrinsic allergic alveolitis. Moreover, the dust from rice husk and straw may be readily ignited due to the small size of the dust particles. They can produce explosive concentrations in the air and may smolder when exposed to heat [241,242]. As a result, when working with rice wastes the same precautions should be followed as when working with other flammable dust.

#### **8. Conclusions and Outlook**

Rice husk, bran, and straw are often considered low-value waste. However, agriculture, energy generation and storage, pollution control and water treatment, construction materials, and many other vital valorization processes have already been adopted for them. Consequently, to implement these options, the legal requirements governing their disposal methods should be considered. Figures 7, 9 and 13 illustrate rice straw, bran, and husk valorization options. These lignocellulosic materials do, however, have further potential applications. As science and technology improve, identification of many more applications is anticipated as the scientific community and societies become more concerned with sustainability. According to the analysis of relevant research that has already been carried out, rice straw has considerable potential as a renewable energy source. However, there is a significant research gap in using rice bran biochar as an energy storage material. Additionally, modified rice husk biochar has a high promise as an adsorbent in the bio-based water treatment industry. Therefore, further research and development are needed to fill these gaps permanently. In the future, these by-products are expected to be used in fields such as the pharmaceutical industry, space science, etc. Most approaches are anticipated to take place in very small-scale operations, particularly if governments adopt a rural development strategy to halt urbanization, and non-scientific variables might judge the usefulness of research. Therefore, the present review mainly discusses the economics of different procedures. However, in the end, the conclusions about specific applications must be made by politics. For example, taxes or laws can be used to encourage each type of activity, while traditional activities such as burning straw and husk are intended to be discontinued. This will lead to laws that make alternative ways of doing things more fascinating.

**Author Contributions:** Conceptualization, W.A.M.A.N.I., C.M. and S.S.; methodology, W.A.M.A.N.I., C.M. and S.S.; software, W.A.M.A.N.I.; validation, W.A.M.A.N.I. and C.M.; formal analysis, W.A.M.A.N.I.; investigation, W.A.M.A.N.I.; data curation, W.A.M.A.N.I.; writing—original draft preparation, W.A.M.A.N.I.; writing—review and editing, W.A.M.A.N.I., C.M., M.C.C. and S.S.; supervision, C.M., M.C.C. and S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** Darusha Witharana of the Sri Lanka Rice Research Center deserves our deepest thanks for supplying important data that helped make this research a success. Hashani Ruwanthika Padmasiri is acknowledged for her invaluable assistance in making this research a success.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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