Bioenergy Production from Agro-Industrial Wastewater Using Advanced Oxidation Processes as Pre-Treatment

Abstract

Agro-industrial activities generate large volumes of wastewater. When this wastewater is discharged to the environment without proper treatment, it represents a serious problem. Bioenergy production can be conducted using wastewater, but the presence of some recalcitrant compounds may require a pre-treatment step. Advanced oxidation processes (AOPs) were traditionally used to treat hazardous materials but have recently been applied in various bioenergy production processes. AOPs are highly competitive water/wastewater treatment technologies and their application in the bioenergy sector is increasing as a pre-treatment process. Despite the increasing interest in using AOPs to enhance biofuel production, there is a lack of comprehensive documentation on their integration into biofuel production operations. This critical review highlights the application of AOPs as pre-treatment for agro-industrial wastewater (AIW) to enhance bioenergy production. It was noted that AOP applications can reduce the COD, VS, TS and total polyphenols, resulting in an improvement in their biodegradability. Moreover, these processes help remove hemicellulose and lignin contents, increasing the production of biogas, biodiesel and bioethanol. Among the different AOPs presented in this work, wet air oxidation showed promise for pre-treating lignocellulosic biomass to produce various energy types, while sonolysis and ozonation proved effective as a biosolid pre-treatment. Ozonolysis, Fenton reagents and photocatalysis are commonly used to selectively remove phenolic compounds and colorants from organic effluents. The high energy requirements and chemicals reagents costs are identified as obstacles to the application of AOPs in bioenergy production. Further studies should investigate the integration of AOPs with other treatment processes to improve the cost-effectiveness.

1. Introduction

In the last half-century, the world population has doubled and, consequently, the energy demand has increased [1,2]. Fossil fuels still constitute the primary energy source worldwide. However, the constant depletion of natural resources has encouraged research on methods that provide alternative energy strategies [2,3]. A suitable alternative could be bioenergy synthesized from organic substrates such as agro-industrial biomass and other waste materials [3,4,5].

Agro-industrial activities represent a significant economic sector in the Mediterranean region [6]. The wine and olive oil production industries are among this region’s most extensive and valuable activities, where approximately 22.6 million tons of grapes and 11.1 million tons of olives were produced in 2020 [7]. Several other agro-industries, including vegetable and fruit processing, meat production and pulp mill industries, have become significant industrial activities in various countries. The increasing development of agro-industrial activities has resulted in large volumes of wastewater and solid waste materials generated along different production stages [8]. These wastewaters and solid waste materials are occasionally inappropriately discarded into natural water courses or deposited in landfills, causing detrimental effects to the environment [6,9]. This biowaste can be treated with appropriate technologies and recovered through the production of energy. Nevertheless, the presence of substantial amounts of hemicellulose, lignin and polyphenols in agro-industrial wastewaters poses a challenge for bioenergy production. These components tend to hinder or completely disrupt the effectiveness of biological wastewater treatment processes, rendering bioenergy generation unviable [10]. As a result, implementing a pre-treatment step prior to the biological treatment processes is essential. This pre-treatment process aims to enhance the biodegradability of wastewaters in order to enable more efficient and effective treatments.

Advanced oxidation processes (AOPs) are considered highly effective technologies for treating water and wastewater, particularly to remove bio-recalcitrant pollutants and inactivate pathogenic microorganisms that are untreatable by traditional techniques [11,12]. The concept was initially established in 1987 for water treatments near ambient temperatures and pressures involving the generation of highly reactive radicals, such as hydroxyl radicals (HO^•) [13]. Various advanced oxidation processes (AOPs) are frequently utilized, such as ozonation, Fenton reagent application, photocatalysis, wet air oxidation, microwave treatment, electrochemical oxidation and ultraviolet radiation. Recently, researchers have extensively examined AOPs as effective oxidation methods in diverse areas, including water and wastewater treatment [14,15], soil remediation [16], the removal of hazardous substances [17], medicine [18], organic synthesis [19] and energy production [4,20,21]. The application of AOPs is increasing in the bioenergy sector as a pre-treatment step for biogas production from lignocellulose biomass [22], biohydrogen [4] and bioethanol substrates [23], sludge biodigestion improvement [21], a transesterification enhancement for biodiesel production [24], the improvement of bioenergy efficiency [20,21,22] and the treatment of the effluents produced during bioenergy production [25].

Given the growing interest and importance that AOPs provide for bioenergy production improvement and the lack of recent detailed reviews on this topic, the present work concentrates on the application of AOPs as a pre-treatment for agro-industrial wastewater (AIW) to enhance bioenergy production. Initially, a literature review was composed of the typical characteristics of wastewater generated from different agro-industrial processes, specifically, olive mills, winery, livestock and paper and pulp mills. These agro-industrial activities were selected based on their economic relevance and environmental impact in the Mediterranean region. The main bioenergy sources, the pre-treatment of the substrates, the principal limitations of traditional treatments (physical, biological and chemical) and the different applications of AOPs in bioenergy production are then reported. The review was conducted in January/February 2023 through a bibliographic search using the Google Scholar, Web of Science and Scopus databases. The bibliography used in this review was identified by searching for the following keywords: “olive mill industry + production + wastewater”; “winery industry + production + wastewater”; “livestock industry + production + wastewater”; “paper and pulp mill industry + production + wastewater”; “pre-treatment + bioenergy”; “advanced oxidation processes + bioenergy”; “agro-industrial wastewater + bioenergy”; “advanced oxidation processes + agro-industrial wastewater pre-treatment”; and “advanced oxidation processes + traditional methods + agro-industrial wastewater pre-treatment”. Overall, 166 documents were selected to support this review.

This work is organized as follows. Section 2 provides an overview of the main agro-industrial effluents in the Mediterranean region. Section 3 discusses the advanced oxidation processes operation and performance. Section 4 highlights the bioenergy sources, production and pre-treatments. Section 5 explores the AOP applications in bioenergy substrate pre-treatment. Section 6 provides the conclusions and future research suggestions are summarized by presenting the challenges, limitations and opportunities.

2. Agro-Industrial Effluents

Agro-industries can be defined as a set of economic activities, including the production, processing and commercialization of agricultural and forestry products for food or non-food purposes [26,27]. Agro-industrial activities require significant amounts of soil, energy and water, generating solid wastes and wastewaters with significant polluting characteristics. These wastewaters have high levels of organic matter with refractory and/or toxic compounds (such as polyphenols) and even heavy metals that can impact in the quality of water, soil and air [28,29,30].

2.1. Olive Mill Industry

Olive oil production represents a significant sector of agro-industrial activities in the Mediterranean Basin. The European Union (EU) produces approx. 68.1% of the world’s olive oil, with Spain, Greece, Italy and Portugal being responsible for 99% and 67.6% of the EU and worldwide production, respectively [31,32].

Despite its economic importance and constant growth in the Mediterranean region, olive oil production generates a significant amount of liquid and solid waste within a relatively short period. Therefore, the proper treatment of olive mill wastewater (OMW) is necessary before discharge to reduce the impact of these effluents on the environment [33].

There are three types (Figure 1) of extraction systems for olive oil production: (1) a traditional discontinuous press process, (2) a two-phase centrifugal extraction system and (3) a three-phase centrifugal extraction system [34].

• The traditional discontinuous press process involves the combined use of a crusher and hydraulic press. This process does not require the addition of water, thus the OMW generation produces between 0.5 and 0.8 m³ of wastewater per ton of olives [35]. On the other hand, the lower production efficiency and greater labor costs have led to its replacement by other extraction systems [36].
• The two-phase centrifugal system separates pomace in two stages: oil and solid waste. It requires a small amount of water for the oil extraction, reducing the processing costs and the quantity of produced solid residues. However, the wet pomace that is produced tends to have high moisture contents and is more difficult and expensive to treat [35,37].
• The three-phase centrifugal system separates pomace into three stages: oil, residual water and pomace. This system produces a high volume of dark-colored wastewater between 1.18 and 1.68 m³ per ton of olives, representing a significant environmental problem [36,38].

Figure 1. Olive oil extraction processes [39,40].

Figure 1. Olive oil extraction processes [39,40].

The different olive oil production processes produce wastewater and sludge that are highly toxic. Thus, adequate treatments are needed to allow for the reduction in the organic load and the valorization of these effluents.

Olive Mill Wastewater

Olive oil production generates two main waste products: (1) olive mill solid waste (OMSW), which includes the olive pulp and olive pomace, and (2) OMW, which comprises the water used for washing and processing the olives and the water released from the olives themselves during pressing [38].

OMW is characterized by its dark color, high turbidity, strong smell, acidity and high suspended solids content. The organic component includes carbohydrates, polysaccharides, sugars, polyalcohols, proteins, organic acids, long-chain fatty acids and oil. Additionally, this wastewater is rich in phenolic compounds which are highly toxic to microorganisms and plants [41]. The physical and chemical characterizations of OMW are reported in

Table 1. Olive mill wastewater characteristics. BOD₅—biochemical oxygen demand; COD—chemical oxygen demand; TOC—total organic carbon.

Parameter	Range Values	Reference
pH	4.6–5.5	[6,42,43,44,45]
Conductivity (mS/cm)	6.5–43.0	[43,45]
BOD5 (g O2/L)	8.0–30.0	[44,46]
COD (g O2/L)	51.0–243.0	[6,42,43,44,45]
TOC (g C/L)	14.0–31.0	[45]
Total suspended solids (g/L)	24.7–215.0	[6,42,43,44,45]
Total polyphenols (g/L)	4.9–9.8	[6,43,45]
Total nitrogen (g/L)	0.4–2.5	[6,42,43]
Total phosphorus (g/L)	0.2–0.4	[6,43]

.

Considering the high organic content and low biodegradability of OMW due to the presence of refractory substances, it is important to identify a sustainable treatment approach, preferably in a circular economy perspective.

2.2. Winery Industry

The worldwide production of wine has increased dramatically, with Italy, France and Spain being responsible for 47% of the world’s wine production in 2021 [47]. In 2021, wine production was approx. 154 MhL in the EU, marking a decrease of 8% compared to the previous year. This downfall recorded in the EU was primarily explained by adverse weather conditions throughout the year [47].

Wine production represents an important sector of the food processing industry. However, the intensive use of agrochemicals and natural resources increases the generation and toxicity of organic waste. Therefore, is important to treat the winery wastes properly to allow for its valorization or proper disposal.

The wine production process (Figure 2), for either white or red, includes different steps. The grapes are separated from their stalk (i.e., stemming), crushed, fermented and the wine is separated from the suspended solids. Different treatments are used to clarify and stabilize the wine before it is stored. Additionally, ageing processes may be applied to enhance the wine quality. Finally, the wine is filtered and bottled [48].

Throughout each step of the wine production process, significant quantities of water are used whereby wastewater and organic wastes that contain high pollutant loads are produced. In fact, the winery industry generates between 1.3 and 2.5 liters of wastewater for each liter of wine produced [49].

Winery Wastewater

The generation of winery waste streams occurs primarily during the harvesting and wine production processes. This waste includes solid organic waste (e.g., grape marc, skins, pips, etc.), wastewater, greenhouse gases (e.g., CO₂, volatile organic compounds, etc.) and packaging waste [50]. Winery wastewater (WW) mainly originates from washing operations during the crushing and pressing of the grapes, and the rinsing of the fermentation tanks, barrels and other equipment or surfaces [51].

WW (

Table 2. Characteristics of winery wastewater.

Parameter	Range Values	Reference
pH	3.6–4.6	[53,54,55,56,57]
Conductivity (mS/cm)	0.06–0.48	[53,54,55,57]
BOD5 (g O2/L)	0.55–6.5	[53,54,57]
COD (g O2/L)	0.5–38.4	[53,54,55,56,57]
TOC (g C/L)	0.14–1.96	[53,54,55,57]
Total suspended solids (g/L)	0.75–7.66	[53,54,56,57]
Total polyphenols (g/L)	0.03–1.35	[53,54,55,56,57]
Total nitrogen (g/L)	0.01–0.38	[56,57]
Total phosphorus (g/L)	0.04–0.05	[56]

) is characterized by its acidity, high content of suspended solids and an organic load, which includes polyphenols, organic acids, alcohols, sugars (maltose, glucose, fructose), aldehydes, soaps and detergents, nitrogen compounds, inorganics and traces of heavy metals [52].

The different winemaking processes and technologies may represent a difficulty in the characterization, management and treatment of the pollution load of WW [58]. Thus, it is important to establish strict criteria that allow for its proper discharge into the environment or, when possible, its reuse.

2.3. Livestock Industry

The dramatic growth of the world population signifies increased human protein requirements. The livestock industry is a significant source of livelihood in nearly all developing countries, with Asia being responsible for over 40% of the total meat production. Moreover, this activity uses one-third of all croplands for feed crop production and one-quarter of ice-free land for pastures [59]. It is expected that meat and dairy diets will intensify, placing considerable pressure on food systems in the coming decades [60]. In addition, inadequate livestock management provides several environmental consequences at different levels which have not been sufficiently addressed in developing countries.

The livestock industry includes: (1) slaughterhouses, (2) tanneries and (3) dairy industry (Figure 3) [59].

• The animals are reared, fattened and transported to the slaughterhouses. The meat is processed and stored before its transport to retail outlets. This process demands energy and water and produces high quantities of wastewater and manure [59,61].
• Tanning processes are applied to the hides produced at slaughterhouses. These processes include several steps, where large amounts of wastewater and solid waste are produced [59].
• Milk is produced and stored at the farm before its collection and transportation to a processing plant. This consumes significant energy and generates wastewater and solid waste [59].

Agriculture is the most water-demanding activity, claiming more than 69% of all water withdrawals and reaching 95% in some developing countries [59]. The livestock industry represents 56% of the water consumed by agriculture [62,63]. The values of the average annual water footprint per animal and the corresponding percentages are reported in

Table 3. Average annual water footprint per animal category [63].

Animal Products	Average Annual Water Footprint (m3/Year/Animal)	Water Footprint (%)
Cattle	2686	52
Swine	520	19
Poultry	59	18
Horse	1599	7
Sheep	68	3
Goat	32	1

.

The high water consumption in animal production increases the volume of generated livestock wastewater (LW) and manure. The continuous production of livestock wastewater in large amounts is unavoidable and has become a global environmental issue, especially since LW is rich in organic content and microorganisms [64].

Livestock Wastewater

LW primarily consists of urine, feces, feed leftovers, washing wastewater and wastewater generated during the life and production process of workers on livestock farms. This wastewater (

Table 4. Livestock wastewater (cattle, swine and poultry) main characteristics. TSS—total suspended solids.

Parameter	Swine Wastewater		Cattle Wastewater		Poultry Wastewater
pH	7.9–8.6	[66,69]	7.7–7.8	[67,70]	6.6–8.0	[71,72]
Conductivity (mS/cm)	12.0–25.4	[66,69]	1.80–3.92	[70,73]	0.8–2.8	[72,74]
BOD5 (g O2/L)	3.0–5.4	[69,75]	0.2–1.49	[70,73]	0.2–0.9	[71,74]
COD (g O2/L)	4.8–15.0	[65,66,69,75]	1.5–9.9	[67,70]	0.3–0.9	[71,72]
TSS (g/L)	0.4–0.6	[69,75]	1.22–4.97	[70,73]	0.002–0.05	[71,72]
Total nitrogen (g/L)	0.15–2.10	[65,75]	0.11–0.23	[70,73]	0.05–0.15	[71,74]
Total phosphorus (g/L)	0.02–0.25	[65,75]	0.04–0.08	[67,70]	0.008–0.05	[71,74]

) contains a high quantity of suspended solids content, organic matter (BOD₅ and COD), nutrients and fecal coliforms due to washing wastewater [65]. All these factors contribute to environmental degradation. For example, the dissolved oxygen is consumed by the organic matter leading to a hypoxia zone in water bodies [66]. Additionally, the presence of nitrogen and phosphorus nutrients may result in the eutrophication of aquatic environments [67]. The use of antibiotics, parasiticides and steroid hormones in livestock feed to prevent diseases and enhance feed efficiency and growth represents an additional environmental problem [68]. These substances are excreted through feces and urine. If they are discharged without proper treatment, they can contaminate water bodies.

2.4. Pulp and Paper Mill Industry

The pulp and paper mill industry is an important consumer of energy and natural resources [76]. These activities produced an estimated 404 million tons of paper and paperboard and 190 million tons of virgin pulp in 2019 [77]. The consumption of water depends on the raw material used, the paper grade produced and the plant structure [78]. Despite the significant reduction in water consumption due to the improvement of operational techniques, the accumulation of progressive pollutants in water remains an environmental problem.

The pulp and paper mill industry produces different paper products using several types of raw materials, e.g., virgin and recycled fibers, and manufacturing processes [79,80]. The manufacturing process depends on each industry, but a simplified diagram can be presented highlighting the principal steps, such as raw material preparation and handling, pulp manufacturing, washing and screening, chemical recovery, bleaching, stock preparation and papermaking (Figure 4) [79].

Pulp and Paper Mill Wastewater

The pulp and paper mill industry uses significant quantities of water, producing considerable amounts of wastewater. Among the different stages of the manufacturing process, wood preparation, pulping, screening, washing, bleaching, papermaking and coating operations are the main sources of contaminants [82].

Pulp and paper mill wastewater (PPW) has the potential to degrade receiving water bodies. This effluent is characterized by an intense smell, a high quantity of organic matter (COD), a low biodegradability, chlorinated compounds, fatty acids, total suspended solids, tannins and lignin (

Table 5. Characteristics of pulp and paper mill wastewater.

Parameter	Range of Values	Reference
pH	5.0–8.5	[20,84,85,86,87]
Conductivity (mS/cm)	2.3–3.6	[85,87]
BOD5 (g O2/L)	0.1–0.9	[84,85,86,87]
COD (g O2/L)	0.6–2.7	[20,84,85,86,87]
TOC (g C/L)	0.2–0.4	[86,87]
Total suspended solids (g/L)	0.02–0.84	[20,84,85,87]
Total polyphenols (g/L)	0.20–0.22	[84,87]
Total nitrogen (g/L)	0.004–0.013	[20,84,85,87]
Total phosphorus (g/L)	0.0009–0.014	[84,85,87]

) [83]. Additionally, its characteristics may depend on the manufacturing process used, the raw material, the quantity of water used and the selected management practices leading to a high range of values [82].

PPW contains several active compounds composed of harmful chemicals that pose a high environmental risk when discharged without proper treatment. The use of technologies to treat PPW is important to accomplish the required legislation for its discharge, to decrease water pollution and to reuse PPW in different functions, promoting a circular economy.

3. Advanced Oxidation Processes

Advanced oxidation processes (AOPs) are an emerging and promising methodology for degrading highly persistent organic pollutants. The efficiency of these processes is based on the production of highly reactive species, such as hydroxyl radicals (HO^•). These highly oxidizing and non-selective reactive radicals react with recalcitrant and toxic compounds, resulting in their mineralization or an increase in their biodegradability [51].

These technologies have been used in several studies to pre-treat wastewater biomass and enhance bioenergy production (Figure 5), including ozonation, Fenton reagent [5], ultraviolet radiation [88], photocatalysis [89], ultrasound [90], wet air oxidation [91], microwave [92] and electrochemical oxidation [93]. An overview of their operating principles is provided to better understand their potential for different applications.

3.1. Ozonation

Ozone (O₃) is a powerful oxidant that has potential for disinfecting and degrading organic pollutants. Ozone can react directly with an organic substrate through a slow and selective reaction (Equation (1)). This reaction is important for unsaturated compounds and compounds containing amine groups or acid groups under acidic conditions. However, this pathway results in a limited mineralization of the organic compounds [94].

O₃ + M → M_ox

(1)

An important fraction of ozone is decomposed and catalyzed by HO⁻ ions. It leads to the formation of several highly reactive species with a strong oxidant character, such as a hydroxyl radical and a hydroperoxyl radical (HO₂^•). These radicals can act as chain propagators, leading to a higher rate of ozone decomposition [94].

Ozone can react rapidly and non-selectively, producing hydroxyl radicals at alkaline conditions (Equation (2)) [94].

2O₃ + 2H₂O → 2HO^• + O₂ + 2HO₂^•

(2)

Ozone decomposition leads to the formation of hydroxyl radicals, especially when it is started by hydroxyl radicals under alkaline conditions (Equations (3)–(9)) [94].

O₃ + HO⁻ → O₂ + HO₂⁻

(3)

HO₂⁻ + O₃ → O₃^−• + HO₂^•

(4)

HO₂^• ↔ O₂^−• + H⁺

(5)

O₂^−• + O₃ → O₃^−• + O₂

(6)

O₃^−• + H⁺ → HO₃^•

(7)

HO₃^• → HO^• + O₂

(8)

O₃ + HO^• + O₂ ↔ HO₂^•

(9)

Almomani et al. (2019) demonstrated that the application of ozonation before anaerobic digestion for three mixed agricultural solid wastes had a significant effect on the organic matter solubilization. Moreover, the biogas production increased in the pre-treated agricultural waste samples. The authors suggested that ozonation could have disintegrated the solids, producing compounds that were more viable for methane production [5].

Despite that O₃ reacts with different contaminants, its selectivity decreases the oxidation efficiency, leading to the incomplete mineralization of the refractory compounds or the production of hazardous intermediate products that cannot react with O₃ [95,96].

A small dose of hydrogen peroxide can be added to improve the hydroxyl radical formation. This process is known as peroxone (Equations (10)–(13)) and it enhances the velocity of the reaction and the efficiency of the organic pollutant mineralization.

H₂O₂ → HO₂⁻ + H⁺

(10)

HO₂⁻ + O₃ → HO₂^• + O₃^−•

(11)

HO₂⁻ + 2O₃ → 2HO^• + 3O₂

(12)

O₃ + H₂O₂ → HO^• + O₂ + HO₂^•

(13)

The peroxone process was used to pre-treat an agricultural mixed waste before anaerobic digestion, which resulted in an additional reduction in the total solids (TS), COD, total phenols, sugar and total humic acids concentrations. Furthermore, the anaerobic digestion efficiency increased in the pre-treated agricultural mixed waste [5].

3.2. Fenton Reagent

A traditional Fenton reaction occurs when H₂O₂ decomposes in the presence of iron ions, producing hydroxyl radicals at a pH of 3 that oxidize the organic or inorganic compounds (Equation (14)) [97].

Fe²⁺ + H₂O₂ → Fe³⁺ + HO⁻ + HO^•

(14)

The Fenton substrate treatment decreases the organic solid concentrations and increases the soluble content of the substrates. This leads to a decrease in the total suspended solids and an increase in the soluble chemical oxygen demand in the substrate [98].

In Fenton’s reagent, in addition to the fundamental reaction mentioned above which constitutes the initiation step, several competitive reactions involving Fe²⁺, Fe³⁺ e H₂O₂ are possible (Equations (15)–(19)). Therefore, the decomposition process efficiency decreases since other reactive species with lower reduction potentials can be formed, such as HO₂^• (Equation (16)) [97].

Fe²⁺ + HO^• → Fe³⁺ + HO⁻

(15)

H₂O₂ + HO^• → H₂O + HO₂^•

(16)

Fe²⁺ + HO₂^• → Fe³⁺ + HO₂⁻

(17)

Fe³⁺ + HO₂^• → Fe²⁺ + O₂ + H⁺

(18)

Fe³⁺ + H₂O₂ → Fe²⁺ + H⁺ + HO₂^•

(19)

One recent development in the Fenton reagent process includes the use of UV radiation (from lamps or the sun) to promote the generation of radicals from reagents [99,100]. In this method, the use of chemicals is reduced and the oxidation efficiency is increased. Another way to generate radicals using the Fenton reagent and reduce the formation of iron sludge is employing heterogeneous sources of iron, called heterogeneous Fenton, where the iron is embedded on a solid catalyst. This process ensures that the catalyst can be recovered for reuse and allows for operations to occur at higher pH values.

3.3. Ultraviolet Radiation

Ultraviolet (UV) radiation is usually applied in combination with other treatments, such as hydrogen peroxide, ozone or photocatalysts for bioenergy production [88]. The photolytic degradation of H₂O₂ initially occurs by producing two hydroxyl radicals (Equation (20)).

H₂O₂ + hν → 2HO^•  (λ = 254 nm)

(20)

Regarding the photolytic ozonation processes (Equations (21) and (22)), UV radiation promotes the formation of hydroxyl radicals through the acceleration of the O₃ decomposition. Then, the hydroxyl radical is formed by peroxide hydrogen photolysis, initiating a radical reaction chain.

O₃ + H₂O + hν → O₂ + H₂O₂  (λ = 254 nm)

(21)

2O₃ + H₂O₂ + hν → 2HO^• + 3O₂ (λ = 254 nm)

(22)

The application of photolytic ozonation processes in the pre-treatment of vinasse before anaerobic digestion decreased both the COD and TOC and improved the methane yield [88].

The combination of Fenton-based techniques with irradiation improves the hydroxyl radical’s production from the decomposition of H₂O₂ (Equation (20)) and the photocatalytic conversion of Fe³⁺ to Fe²⁺ (Equation (23)). Therefore, this can enhance the degradation of the organic compounds.

Fe³⁺ + H₂O + hν → HO^• + Fe²⁺ + H⁺

(23)

The application of UV radiation during AOPs improves the hydroxyl radical formation, organic compounds degradation and cost-benefit of the processes [101].

3.4. Photocatalysis

Interest in photocatalysis processes has been increasing. They do not involve high temperatures or pressures, require a relatively small amount of energy and are environmentally friendly processes [102]. Heterogeneous photocatalysis (Figure 6) depends on the catalytic properties of excited semiconductors, such as TiO₂, ZnO, WO₃, CdS, Fe₂O₃ or GaP, and the activator factor, such as solar or artificial UV radiation (Equation (24)) [103].

$$\mathrm{Organic} \mathrm{pollutants}+{\mathrm{O}}_2 \stackrel{\mathrm{s}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{u}\mathrm{t}\mathrm{o}\mathrm{r}+\mathrm{U}\mathrm{V} \mathrm{r}\mathrm{a}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{\to }{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{inorganic}\mathrm{salts}$$

(24)

Among the different semiconductors, TiO₂ is the most widely studied primarily due to its non-toxicity, high oxidizing power, relatively low cost, photostability and chemical stability in a wide range of pH values. When it is irradiated with UV radiation, TiO₂ promotes the transfer of electrons from the valence band to the conduction band, which creates holes in the valence band (Equation (25)). The high concentration of H₂O and HO⁻ adsorbed to the surface of the semiconductor promotes the production of hydroxyl radicals, and thus, the degradation of nearly all the chemicals [102].

TiO₂ + hν → TiO₂ (e⁻_bc + h⁺_bν)

(25)

A study carried out by Corro et al. (2014) demonstrated that the use of a heterogeneous photocatalysis process as a pre-treatment increased the biogas production by almost three times from a mixture of coffee pulp and cattle manure [89].

3.5. Wet Air Oxidation

The wet air oxidation (WAO) or thermal liquid-phase oxidation process oxidizes the organic compounds to carbon dioxide and water (Equations (26) and (27)) [104]. This process requires high temperature conditions (150–325 °C) and high oxygen pressure (10–200 bars) [105].

RH + O₂ → R* + HO₂⁻

(26)

RH + HO₂⁻ → R* + H₂O₂

(27)

The radicals produced can react with additional substrates and oxygen to extend the chain or generate different catalyst species (Equations (28) and (29)). The WAO process also produces low molecular weight organic compounds, including carboxylic acids, acetaldehydes and alcohols [105].

R* + O₂ → ROO*

(28)

R₁OO* + R₂H → R₁OOH + R₂*

(29)

The WAO can improve COD reduction and methane production when used as a pre-treatment for distillery effluent during bioenergy production [91].

3.6. Ultrasonication

Ultrasonication is ultrasound radiation with a frequency exceeding 20 kHz. This process is characterized by the generation of acoustic cavities, with the subsequent growth and collapse of high-energy cavitation bubbles that produce hydrogen radicals (Figure 7) [106]. Ultrasonication has been widely used as a green technology for environmental remediation to degrade organic pollutant matter in wastewater [106].

Organic matter degradation using ultrasound occurs through three processes: supercritical water reactions, direct pyrolysis and reactions with radicals generated by the thermal reaction (Equation (30)) or by Equations (31)–(33) in the presence of oxygen.

H₂O + US → H• + HO•

(30)

O₂ + US → 2O•

(31)

H• + O₂ + US → HO₂•

(32)

H• + O₂ + US→ HO• + O•

(33)

The ultrasound process has been used as a pre-treatment in different substrates for subsequent bioenergy production [20,90,107]. The principal limitation of the ultrasounds process is the high energy demand that minimizes its cost-effectiveness.

Hydrodynamic cavitation (HC) is also a promising technology for the oxidation of dissolved organic chemicals. In this process, high energy is generated in a flowing liquid upon bubble implosion due to a decrease and a subsequent increase in the local pressure (~1000 bar), temperature (~4727 °C) and reduction (hydroxyl radicals)/oxidation (hydrogen radicals). Recent studies have demonstrated the ability of HC to be used as an intensification technology and to generate high quantities of highly reactive species [108,109].

3.7. Microwave Radiation

Microwave (MW) radiation is electromagnetic radiation with wavelengths between 1 mm and 1 m and frequencies between 300 MHz and 300 GHz. This process combines two different effects known as (1) thermal and (2) non-thermal [110,111].

• For the thermal effects, the organic compounds are degraded due to the rotation of dipole molecules under an oscillating electromagnetic field, resulting in the rapid and energy-efficient heating of water to the boiling point [110,111].
• For the non-thermal effects, the degradation of toxic compounds is achieved through the rapid oscillation of polar and polarizable molecules or polarized side chains of macromolecules as they try to orient themselves to the electric field, resulting in the breaking of hydrogen bonds [110,111].

MW radiation offers a distinct advantage, i.e., rapid and efficient heating in a controlled environment. However, the addition of catalysts or oxidants such as hydrogen peroxide (Equation (34)), a Fenton reagent and persulfate may be necessary to increase the removal of persistent recalcitrants (Equations (35) and (36)) [112].

H₂O₂ + MW → 2HO^•

(34)

S₂O₈²⁻ + MW → 2SO₄^−•

(35)

2SO₄^−• + HO⁻ → 2SO₄²⁻ + HO^•

(36)

MW-assisted heating has gained significant interest as a pre-treatment process since its application before anaerobic digestion could enhance methane production and COD solubilization, increasing the biodegradability of biomass materials [94].

3.8. Electrochemical Oxidation

An electrochemical process occurs between an electrode and an ion conductor (solution). The reaction occurs at the electrode interface when an external potential is applied. In an electrochemical cell, the cathode is the electrode at which reductions occur [113]. Two primary processes are possible for the electrochemical oxidation of pollutants (Figure 8), namely (1) direct anodic oxidation on the anode surface or (2) indirect oxidation caused by electrogenerated oxidants on the anode surface [114].

• In direct oxidation reactions, the pollutants are adsorbed and oxidized on the anode surface, producing lower kinetic rates and a high anode electrocatalytic activity dependence with a low potential [114].
• In indirect oxidation, the intervention of precursors or oxidizing species in the pollutant oxidation occurs. These precursors are electrogenerated on the anode surface and only oxidize the pollutants in the solution media [114].

Figure 8. Mechanism of electrochemical oxidation [115].

Figure 8. Mechanism of electrochemical oxidation [115].

The previous studies indicate that electrochemical oxidation can be used as a pre-treatment process for biogas production through the reformation of agro-industrial effluents [98]. This method improves methane production and the degradation efficiencies of the COD solubility.

4. Bioenergy

Bioenergy is the energy that results from any fuel derived from a biomass substrate. Any waste containing biomass can be used to generate bioenergy, and thus, it is considered a renewable resource and an alternative feedstock for producing energy. Several substrates can be used to produce bioenergy such as food wastes, plant crops, cellulosic biomass, anaerobic sludge and algae biomass. The main biomass constituents include carbohydrates, lipids, proteins, lignin and water. Since these constituents are present in agricultural wastes, palm and paper mill effluent and animal waste in good amounts, these wastes can be considered promising sources of bioenergy [116].

4.1. Bioenergy Sources

The bioenergy industry can be split into three distinct sectors, biogas, bioethanol and biodiesel, with each one relying on different feedstocks. These sources of energy are environmentally friendly alternatives to fossil fuels due to their lower pollutant emissions and renewable character.

Biogas is produced through the anaerobic digestion or gasification of agricultural wastes, municipal and industrial wastes, sewage sludge and cellulosic biomass. Depending on the substrate used, a pre-treatment may be necessary to enhance the biogas production [43,117]. The traditional methods, e.g., biological and physical–chemical, have a reduced capability for breaking the complex structure of the hemicelluloses, which leads to a lower productivity and yields [118]. Therefore, the use of AOPs may be a potential alternative for pre-treating biogas substrates.

Bioethanol is produced by the fermentation of sucrose from sugarcane or starch primarily from corn [119]. This energy can also be produced from lignocellulosic agro-industrial wastes that do not compete with food markets and enhance wastewater valorization [120]. These substrates require a pre-treatment step to break the recalcitrant structure of hemicelluloses, reduce the size and solubilize the substrate [121]. AOPs can be used to enhance the degradation of biomass substrates in bioethanol production [23].

Biodiesel is produced through a transesterification process, which converts fatty acid esters in a short period of time [122]. Despite the traditional methods such as alkali, acid and enzyme catalyzed processes to improve the transesterification, the use of AOPs has demonstrated higher biodiesel yields [123]. The use of microalgae for biodiesel production has drawn considerable attention due to its rapid growth, fast accumulation of lipids and possible use of lipid-extracted cell residues in the production of animal feeds or other kinds of energy when protein or carbohydrate contents are high [124]. Some methods, such as milling and homogenization, are used for cell lysis and oil extraction from microalgae for biodiesel production [125]. AOPs can be also applied to improve algae cell lysis for oil extraction during biodiesel production.

4.2. Pre-Treatment of Bioenergy Substrate

Bioenergy has several advantages over traditional fossil fuels in terms of its large quantity, renewability, lower pollutant load and environmental risk. However, this source of energy presents a lower efficiency and calorific value. The use of bioenergy as a pre-treatment step can be considered for improving the efficiency of the overall bioconversion. There are various pre-treatment methods that include physical, chemical, biological and advanced oxidation processes.

The use of agro-industrial waste composed of cellulose, hemicellulose, starch, protein and lipids can serve as a cost-effective feedstock for biotechnological energy production without competing with the increasing demand for food and by reducing the environmental impact caused by the disposal of such waste. Energy can be produced from this material through biological processes, e.g., esterification, fermentation and anaerobic digestion, and physicochemical processes, e.g., pyrolysis, gasification and incineration. Several studies have been conducted to improve the cost-effectiveness of these biological methods. Therefore, treating this biowaste is important for decreasing the pollutant load and generating energy.

Biowastes originating from the agro-industry are composed of complex carbohydrate polymers (cellulose and hemicellulose), aromatic polymers (lignin), proteins and lipids [126]. A pre-treatment is required to solubilize and hydrolyze the recalcitrant composition of this biomass.

The physical methods used in bioenergy substrate treatment include steam explosion, hydrothermal (liquid hot water), communion and extrusion [127]. The objective of physical pre-treatment is to increase the surface area and reduce the particle size of the polymer particles. Meanwhile, the major disadvantages of this method are the high-power consumption and the usually low efficiency [128]. The chemical methods are divided into acid, alkaline and ionic liquids [126]. The principal objective of acid pre-treatment is to increase the degradation of hemicellulose using different acids such as H₂SO₄, HNO₃, acetic acid and HCl [129]. Nonetheless, the high cost of acid recovery and the formation of inhibitors are the main drawbacks of this method [126]. The alkaline method is one of the most reliable pre-treatments due to its strong effect and relatively simple process. However, the reaction time is considerable [130]. The high solubility of biomass in ionic liquid methods resulted in a high bioenergy yield. However, the high energy requirement, high cost and high waste generation with a difficult recovery were the main disadvantages of this method [131]. The biological pre-treatment includes the use of fungi [132], bacterial strains [133] or enzymes [134]. Even though chemical recycling is not required in this method after the pre-treatment, low costs, low energy consumption, minimum inhibitor formation and simple operating comprise the most important benefits. On the other hand, the extremely low effectiveness provides the primary obstacle [126]. In this study, the use of AOPs as a potential pre-treatment for biowaste substrates was evaluated.

5. Application of AOPs in Bioenergy Substrate Pre-Treatment

The application of advanced treatment technologies is important for fulfilling the legal requirements for direct disposal or improving the bioenergy production. Advanced oxidation processes (AOPs) exhibit a significant potential for breaking the complex structure of hemicelluloses, which act as protective barriers for lignocellulosic compounds, impeding their reactivity. By doing so, AOPs increase the solubility of the substrates, facilitating their biodigestion and thereby enhancing their biodegradability [17]. This biodegradability leads to higher bioenergy production yields. Furthermore, AOPs exhibit remarkable speed compared to the physical–chemical or biological methods. AOPs can also be integrated with other processes for bioenergy production.

Table 6 presents some studies using AOPs as a pre-treatment substrate and summarizes the principal and most relevant results. Most of these focus on ozone-based AOPs [5,88], Fenton processes [44,135,136], ultrasounds [90,137] and MW radiation [92,138]. However, other oxidation processes have also been investigated.

A study conducted by Bampalioutas et al. (2018) investigated the use of the Fenton reagent process before anaerobic digestion to treat OMW obtained from olive oil production using the three-phase centrifugal extraction. The OMW pre-treatment with the Fenton reagent resulted in a total phenolic content (TPC) removal of 88.8% and a TOC degradation of 35.4%. Additionally, a maximum methane production of 20.1 L CH₄/L of pre-treated OMW was achieved [136]. Maamir et al. (2017) also investigated the Fenton reagent efficiency as a pre-treatment for OMW. In this case, the pre-treated effluents presented a lower volume of cumulative methane (168 and 224 mL/g VS) than the effluent without treatment (332 mL/g VS). An explanation could stem from the phenolic content produced during the Fenton process, which inhibits the microorganisms that reduce the efficiency of anaerobic digestion and biogas production [44]. On the other hand, the pre-treated OMW resulted in an increase of 24% in the methane yield, which could have resulted from the removal of hemicellulose and lignin. The degradation of these compounds resulted in a disruption of the recalcitrant structures of the cellulosic biomass, improving the accessibility of cellulose to the enzymes that convert carbohydrate polymers into fermentable sugars [139].

The application of the electro-Fenton process using a current density of 7.5 A/dm² was also investigated as an OMW treatment before anaerobic digestion [135]. This process proved to be highly efficient for the mineralization and detoxification of real OMW. Khoufi et al. (2006) achieved 66% of the TPC, 68% of the COD and 78% of the color removals and the production of methane improved from 0.3 to 0.35 L CH₄/g for the COD_introduced. The authors concluded that the electro-Fenton process for OMW decreased the toxic effect of this wastewater during anaerobic digestion [135].

Akbay et al. (2022) compared the electro-oxidation and the Fenton reagent performance for pre-treating a mixture of juice–puree industry waste and municipal sewage sludge. Regarding COD removal, the Fenton reagent presented a better degradation efficiency (48%) compared to electro-oxidation (37%). In addition, the Fenton reagent presented a greater improvement for the cumulative biogas production (20%), methane content (62%) and methane yield (39%) than electro-oxidation (12%, 60% and 28%, respectively). On the other hand, the Fenton process used more energy than the energy that was produced by the pre-treatment process, while electro-oxidation exhibited a positive cost-benefit [98].

The effects of the Fenton reagent, ozone and ozone combined with Fe(II) and H₂O₂ on the disintegration/solubilization of a mixture of wood dust, sheep dung and cow dung with wastewater and the subsequent anaerobic digestion were studied [5]. The highest solubilization was observed for O₃/Fe(II) with a 9.6%, 37.4%, 27.2%, 23% and 1.3% reduction in the TS, COD, TPC, total sugar and total humic acids, respectively. The ferrous ions acted as the initiator of the radical chain reactions to yield additional hydroxyl radicals, improving the ozone efficiency [140]. The substrates treated with the Fenton reagent and O₃/Fe(II) showed an additional 36.6 and 32.3 L/kg VS of cumulative methane production compared to the untreated substrates [5].

The efficiency of ozone and ozone combined with UV radiation and TiO₂ were also investigated for the vinasse treatment before anaerobic digestion [98]. The O₃/TiO₂/UV treatment of the vinasse samples decreased the COD from 109,200 mg O₂/L to 33,650 mg O₂/L as well as the TOC from 36,100 mg O₂/L to 33,650 mg O₂/L. The O₃ and O₃/UV treatments decreased the COD and TOC values. However, the results were not as remarkable as the previous findings. Vinasse treated with O₃/TiO₂/UV possessed an increased yield coefficient and an increased mean specific rate of anaerobic digestion (by 25%) relative to untreated vinasse. The association between ozone and heterogeneous photocatalysis has emerged as a promising alternative for improving the degradation efficiency and avoiding photocatalyst deactivation during refractory compound oxidation. Additionally, the use of semiconductors, such as TiO₂, during the ozonation process promotes hydroxyl radical production [141].

The combination of Cu and TiO₂ during a photocatalysis process for enhancing biogas production was also studied [89]. Coffee pulp was treated with the two catalysts before being used as the nutrient matter during the biogas process from cattle manure. A decrease in the TS (8.8%), volatile solids (VS) (81.4%), lignin (2.8%), cellulose (14.5%) and hemicellulose (15.9%) was accomplished. In addition, the biogas generated in this reaction was almost three times (2.78) higher than the total biogas generated without the catalysts.

A high efficiency in terms of the TPC and COD removal was also reached when H₂O₂ was used as an olive mill waste pre-treatment [43]. The oxidation process was found to be more efficient under alkaline conditions (pH 7), ensuring satisfactory polyphenols abatement (72%) with a relatively low dosage and mild operating conditions. Moreover, the basic environment caused a partial conversion of recalcitrant organic matter into biodegradable compounds, resulting in a COD reduction (28–37%). Methane production improved from 0.08 to 0.328 L CH₄/g of the COD_removed [43].

Oz et al. (2015) investigated the treatment of OMW obtained from olive oil production by using ultrasound processes, which resulted in an increase of approximately 23% in the COD solubilization [90]. Furthermore, the ultrasound pre-treatment applied to diluted OMW increased the biogas and methane production by 20% when compared to untreated OMW.

The application of the ultrasound processes combined with photofermentation was also investigated for pre-treating palm oil and pulp and paper mill waste [20,135]. In both studies, a combined industrial effluent (25%, v/v palm oil mill effluent and 75%, v/v pulp and paper mill effluent) with a similar pollutant load was used. Budiman et al. (2016) achieved a 37% total COD removal and an 87% biohydrogen production improvement [20]. In 2017, they achieved a 52% COD removal and a 45% biohydrogen production improvement [137]. The difference between the results was primarily due to the parameters chosen for each pre-treatment, respectively, 775 J/mL [20] and 306 J/mL [137]. The author concluded that the higher values of ultrasonication pre-treatment did not improve the organic removal and incurred extra operating costs due to the electricity consumption.

Ormaechea et al. (2018) also investigated the ultrasound efficiency using cattle manure and cattle manure with glycerin as the effluents. In both cases, the ultrasounds were able to effectively oxidize the resistant compounds, reaching a 97% COD and 92% VS removal for cattle manure and an 88% COD and 73% VS removal for cattle manure combined with glycerin. Moreover, methane production increased from 0.29 to 0.46 m³ CH₄/kg VS for cattle manure and from 0.44 to 0.59 m³ CH₄/kg VS for cattle manure combined with glycerin [107].

The efficiency of the ultrasound processes was also investigated for pre-treating fruit and vegetable waste before anaerobic digestion [142]. The highest methane yield occurred in the biodigester with pre-treated fruit and vegetables (237 mL CH₄/g of VS_in, 80% higher than control) and the total biogas yield increased from 249 to 396 mL biogas/g of VS_in.

Table 6. Pre-treatment of agro-industrial wastewaters using advanced oxidation processes for bioenergy production.

Substrates	Pre-Treatment Conditions	Product Target	Effect of Pre-Treatment	Effect on Output	Reference
Olive mill wastewater	Fenton: 1 to 5 g/L concentrations of H2O2 and FeSO4.7H2O at 20–35 °C for 20 min	Biogas, methane	88.8% of the total phenolic content and a 35.4% TOC removal	20.1 L CH4/L pre-treated olive mill wastewater	[136]
Olive mill waste	Microwave: 10 °C/min, 5 min	Biogas, methane	26.2% CODsoluble and 30.3% biodegradability	Methane yields increased from 201.9 mL CH4, STP/g VSRAW to 244.5 mL CH4, STP/g VSP	[92]
Olive mill wastewater and olive mill solid waste	Fenton: [H2O2]/[Fe2+] = 1000:1, [Fe2+] = 1.5 mM, 120 min and pH 3	Biogas, methane	>43.11% delignification, 32.31% hemicellulose and 82% CODsoluble degradation	Methane content increased by 24% and improved the methane yield by 15%	[44]
Wet olive mill wastes	0.05 g H2O2/g COD	Biogas, methane	28–37% COD and a 72% total phenol reduction	Methane production improve from 0.08 L CH4/g of CODremoved to 0.328 L CH4/g of CODremoved	[43]
Olive mill wastewater	Ultrasound: 20 kHz, 10 min	Biogas, methane	>23% CODsoluble degradation	Methane production increased by 20%	[90]
Olive mill wastewater	Electro-Fenton: [H2O2] = 1 g/L, 7.5 A/dm2	Biogas, methane	68% COD and 65.8% total polyphenols removal	Methane production improved from 0.3 L CH4/g CODintroduced to 0.35 L CH4/g CODintroduced	[135]
Winery waste	Microwave: 10 °C/min, 5 min	Biogas, methane	49.7% CODsoluble and 37.1% biodegradability	Methane yields increased from 154.0 mL CH4, STP/g VSRAW to 256.8 mL CH4, STP/g VSP	[92]
Distillery effluent	Wet air oxidation: 6–12 bar, 150–200 °C, 15–120 min	Biogas, methane	16–60% of COD reduction and >0.2–0.88 biodegradability	Methane increased from 4.8% to 57%	[91]
Vinasse	Ozonation: [O3] = 34 g/m3, [H2O2] = 4 × 10−3 mol/L, [TiO2] = 2 g/L UV light: mercury vapor lamp emitting at 200–280 nm	Biogas, methane	COD and TOC decreased from 109 g/L and 36 g/L to 74 g/L and 34 g/L, respectively	Methane yields increased from 0.87 mL CH4 /mg TOC to 1.09 mL CH4/mg TOC	[88]
Wood dust, sheep and cow dung and wastewater	Fenton: 0.07 g Fe2+/g H2O2, 50 g H2O2/Kg-DS Ozonation: 15.8 mg O3/g-TSS Ozonation combined with H2O2/Fe2+: 50 g H2O2 or 3.5 g Fe2+/Kg-DS	Biogas, methane	>15.2–29.5% TS and 33.6–37.5% COD removal	Cumulative methane production increased by 23–30%	[5]
Cattle manure	Ultrasounds: 520 kJ/kg TS	Biogas, methane	97% COD and 92% VS removal	Methane yields increased from 0.29 m3 CH4/kg VS to 0.46 m3 CH4/kg VS and biogas production increased from 60.1% to 62.2%	[107]
Cattle manure and coffee pulp	Heterogeneous photocatalysis: 100 g of 10% Cu/TiO2 catalyst	Biogas	TS, VS, lignin, cellulose and hemicellulose decreased by 8.8%, 81.4%, 2.8%, 14.5% and 15.9%, respectively	Biogas production was 2.78 times higher	[89]
Palm oil, pulp and paper mill wastewater	Ultrasounds: 20 kHz, amplitudes 70% and 90%, 45 min	Biohydrogen	A70: 34.2% CODsoluble and 36.9% CODtotal removal A90: 34.8% CODsoluble and 36.7% CODtotal removal	A70: Biohydrogen production improved by 86.8% A90: Biohydrogen production improved by 88.4%	[20]
Palm oil, pulp and paper mill wastewater	Ultrasounds: 20 kHz, amplitude 20%, 10 min	Biohydrogen	51.1% CODsoluble and 52.2% CODtotal removal	Biohydrogen production improved by 44.6%	[137]
Pulp and paper mill waste	Electrochemical: 15 V, 45 min	Biogas, methane	-	Methane yields increased from 0.264 mL CH4/g VS to 0.301 mL CH4/g VS	[93]
Juice industry waste	Microwave: 10 °C/min, 5 min	Biogas, methane	71.4% CODsoluble and 82.0% biodegradability	Methane yields increased from 166.0 mL CH4, STP/g VSRAW to 451.5 mL CH4, STP/g VSP	[94]
Juice–puree industry waste and municipal sewage sludge	Electro-oxidation: 15 mA/cm2 (15 V for 120 min), pH 4 Fenton: [H2O2]/[Fe2+]:1000, [FeSO4.7H2O] = 1.5 mM, [H2O2] = 30% (v/v), pH 4.	Biogas, methane	CODsoluble removal increased from 9600 mg/L to 13,990 mg/L and 20,290 mg/L for electro-oxidation and Fenton, respectively	Biogas production, methane content and methane yield increased by 12%, 60% and 28% for electro-oxidation and 20%, 62% and 39% for Fenton.	[98]
Mixture of fruits, vegetables, potatoes and paper wastes	Ultrasounds: 20 kHz, during three sonication times (9, 18 and 27 min)	Biogas, methane	58% TS removal	Biogas yield increased from 249 mL/g VSin to 396 mL/g VSin >80% methane yield	[142]
Fruit and vegetable waste and activated sludge	Microwave combined with H2O2: 660 W, 3 min and 80 °C, [H2O2] = 1% w/w H2O2/TS	Biogas, methane	>7.2% COD removal and 33% CODsoluble; <6% VS and 5.2% TS	Methane yield increased from 127 mL/g VS to 276 mL/g VS	[138]

Table 6. Pre-treatment of agro-industrial wastewaters using advanced oxidation processes for bioenergy production.

Substrates	Pre-Treatment Conditions	Product Target	Effect of Pre-Treatment	Effect on Output	Reference
Olive mill wastewater	Fenton: 1 to 5 g/L concentrations of H2O2 and FeSO4.7H2O at 20–35 °C for 20 min	Biogas, methane	88.8% of the total phenolic content and a 35.4% TOC removal	20.1 L CH4/L pre-treated olive mill wastewater	[136]
Olive mill waste	Microwave: 10 °C/min, 5 min	Biogas, methane	26.2% CODsoluble and 30.3% biodegradability	Methane yields increased from 201.9 mL CH4, STP/g VSRAW to 244.5 mL CH4, STP/g VSP	[92]
Olive mill wastewater and olive mill solid waste	Fenton: [H2O2]/[Fe2+] = 1000:1, [Fe2+] = 1.5 mM, 120 min and pH 3	Biogas, methane	>43.11% delignification, 32.31% hemicellulose and 82% CODsoluble degradation	Methane content increased by 24% and improved the methane yield by 15%	[44]
Wet olive mill wastes	0.05 g H2O2/g COD	Biogas, methane	28–37% COD and a 72% total phenol reduction	Methane production improve from 0.08 L CH4/g of CODremoved to 0.328 L CH4/g of CODremoved	[43]
Olive mill wastewater	Ultrasound: 20 kHz, 10 min	Biogas, methane	>23% CODsoluble degradation	Methane production increased by 20%	[90]
Olive mill wastewater	Electro-Fenton: [H2O2] = 1 g/L, 7.5 A/dm2	Biogas, methane	68% COD and 65.8% total polyphenols removal	Methane production improved from 0.3 L CH4/g CODintroduced to 0.35 L CH4/g CODintroduced	[135]
Winery waste	Microwave: 10 °C/min, 5 min	Biogas, methane	49.7% CODsoluble and 37.1% biodegradability	Methane yields increased from 154.0 mL CH4, STP/g VSRAW to 256.8 mL CH4, STP/g VSP	[92]
Distillery effluent	Wet air oxidation: 6–12 bar, 150–200 °C, 15–120 min	Biogas, methane	16–60% of COD reduction and >0.2–0.88 biodegradability	Methane increased from 4.8% to 57%	[91]
Vinasse	Ozonation: [O3] = 34 g/m3, [H2O2] = 4 × 10−3 mol/L, [TiO2] = 2 g/L UV light: mercury vapor lamp emitting at 200–280 nm	Biogas, methane	COD and TOC decreased from 109 g/L and 36 g/L to 74 g/L and 34 g/L, respectively	Methane yields increased from 0.87 mL CH4 /mg TOC to 1.09 mL CH4/mg TOC	[88]
Wood dust, sheep and cow dung and wastewater	Fenton: 0.07 g Fe2+/g H2O2, 50 g H2O2/Kg-DS Ozonation: 15.8 mg O3/g-TSS Ozonation combined with H2O2/Fe2+: 50 g H2O2 or 3.5 g Fe2+/Kg-DS	Biogas, methane	>15.2–29.5% TS and 33.6–37.5% COD removal	Cumulative methane production increased by 23–30%	[5]
Cattle manure	Ultrasounds: 520 kJ/kg TS	Biogas, methane	97% COD and 92% VS removal	Methane yields increased from 0.29 m3 CH4/kg VS to 0.46 m3 CH4/kg VS and biogas production increased from 60.1% to 62.2%	[107]
Cattle manure and coffee pulp	Heterogeneous photocatalysis: 100 g of 10% Cu/TiO2 catalyst	Biogas	TS, VS, lignin, cellulose and hemicellulose decreased by 8.8%, 81.4%, 2.8%, 14.5% and 15.9%, respectively	Biogas production was 2.78 times higher	[89]
Palm oil, pulp and paper mill wastewater	Ultrasounds: 20 kHz, amplitudes 70% and 90%, 45 min	Biohydrogen	A70: 34.2% CODsoluble and 36.9% CODtotal removal A90: 34.8% CODsoluble and 36.7% CODtotal removal	A70: Biohydrogen production improved by 86.8% A90: Biohydrogen production improved by 88.4%	[20]
Palm oil, pulp and paper mill wastewater	Ultrasounds: 20 kHz, amplitude 20%, 10 min	Biohydrogen	51.1% CODsoluble and 52.2% CODtotal removal	Biohydrogen production improved by 44.6%	[137]
Pulp and paper mill waste	Electrochemical: 15 V, 45 min	Biogas, methane	-	Methane yields increased from 0.264 mL CH4/g VS to 0.301 mL CH4/g VS	[93]
Juice industry waste	Microwave: 10 °C/min, 5 min	Biogas, methane	71.4% CODsoluble and 82.0% biodegradability	Methane yields increased from 166.0 mL CH4, STP/g VSRAW to 451.5 mL CH4, STP/g VSP	[94]
Juice–puree industry waste and municipal sewage sludge	Electro-oxidation: 15 mA/cm2 (15 V for 120 min), pH 4 Fenton: [H2O2]/[Fe2+]:1000, [FeSO4.7H2O] = 1.5 mM, [H2O2] = 30% (v/v), pH 4.	Biogas, methane	CODsoluble removal increased from 9600 mg/L to 13,990 mg/L and 20,290 mg/L for electro-oxidation and Fenton, respectively	Biogas production, methane content and methane yield increased by 12%, 60% and 28% for electro-oxidation and 20%, 62% and 39% for Fenton.	[98]
Mixture of fruits, vegetables, potatoes and paper wastes	Ultrasounds: 20 kHz, during three sonication times (9, 18 and 27 min)	Biogas, methane	58% TS removal	Biogas yield increased from 249 mL/g VSin to 396 mL/g VSin >80% methane yield	[142]
Fruit and vegetable waste and activated sludge	Microwave combined with H2O2: 660 W, 3 min and 80 °C, [H2O2] = 1% w/w H2O2/TS	Biogas, methane	>7.2% COD removal and 33% CODsoluble; <6% VS and 5.2% TS	Methane yield increased from 127 mL/g VS to 276 mL/g VS	[138]

Pellera and Gidarakos [92] investigated the MW efficiency of olive mill waste, winery waste and juice industry waste treatment before anaerobic digestion. The experimental conditions were the same for the substrates, with a heating rate of 10 °C/min and a holding time of 5 min. Nevertheless, the results showed different MW heating effects on the solubilization and methane potential of the studied substrates. The difference in the composition of the substrates in combination with the pre-treatment conditions led to different disruptions/decompositions of lignocelluloses, and thus, different results [92]. The pre-treatment resulted in relatively high COD solubilization levels for winery waste (49.7% at 175 °C) and juice industry waste (71.4% at 200 °C), and increased methane production by 102.8 mL_{CH4, STP}/g_VSP at 125 °C for winery waste and 285.5 mL_{CH4, STP}/g_VSP at 150 °C for juice industry waste [92].

The effects of MWs combined with H₂O₂ on the solubilization of a mixture of fruit and vegetable waste with activated sludge waste and the subsequent anaerobic co-digestion were studied [138]. The COD, VS and TS removal increased by 7%, 6% and 5%, respectively, facilitating the availability of soluble organic content for subsequent utilization by the acidogenic microbial population. Furthermore, the methane yield increased from 127 to 276 mL/g of VS. The authors concluded that the MW-enhanced oxidation process exhibited oxidative stress on anaerobic digestion from the generation of the reactive oxygen species oxidizing the organic microbial fractions and causing a breakdown of the cell wall.

A high efficiency in terms of COD reduction was also reached when the WAO process was investigated as a pre-treatment for distillery effluent [91]. This process resulted in a 16–60% COD reduction and a biodegradability enhancement in the range of 0.2–0.88 at various experimental conditions. Furthermore, methane production increased by 52% when the pre-treatment was applied. The WAO process could improve the substrate metabolic value, reducing the molecular weight of the compounds and making them suitable for achieving better degradation values.

Veluchamy et al. (2018) used electrohydrolysis to treat pulp and paper mill waste. In this study, the paper and pulp mill waste contained a high solids content, so it was mixed with distilled water. The pre-treatment process fulfilled this study’s main objective by increasing the methane yield by 14% [93]. In addition, the process produced 1.5 times more energy than was spent, presenting a positive energy balance.

The choice of the AOP for the enhancement of the bioenergy potential is primarily determined by the substrate and operational conditions. The optimal oxidation is sometimes dependent on the pH, reaction time, pressure and temperature focusing on the target biofuels.

The primary obstacle to fully utilizing biofuel substrates for bioenergy production lies in the presence of the inhibitors and recalcitrant compounds. In the case of lignocellulosic biomass, the process of biodegradation is impeded by various compounds such as phenolics, furfural, 5-hydroxymethyl furfural, polyphenols and other volatile substances. These substances act as barriers, making it challenging to efficiently extract energy from the biomass for biofuel purposes [143,144]. Olive mill wastewater and similar effluents are primarily hindered by the presence of polyphenols and phenolic compounds [145]. To enhance the energy yields and eliminate these phenolic compounds from the substrates, ozonation is a widely employed pre-treatment process [146,147]. The advantage of this process lies in its ability to selectively target and remove toxic phenolics while leaving other organic compounds largely unaffected. As a result, the subsequent conversion of these treated substrates into bioenergy becomes more efficient. Most of these compounds have a negative impact on the fermentation process and are not effectively removed using standard treatment methods. Nevertheless, AOPs offers distinct advantages for the pre-treatment of these substrates. AOPs are capable of effectively eliminating inhibitors and toxic substances, leading to significantly higher bioenergy yields [148]. This approach is beneficial in enhancing the overall efficiency of bioenergy production from such challenging substrates. Nonetheless, it is essential to conduct comparative studies to assess the effectiveness of AOPs in comparison to other chemical technologies for removing inhibitors and improving bioenergy substrates [149]. These studies should not only evaluate the technical performance but also consider the cost-effectiveness of these processes, providing insights into the most suitable approaches.

For instance, a comparison of various oxidation processes for lignocellulosic biomass pre-treatment revealed that hydrogen peroxide outperformed ozone and showed comparable yields to alkaline and acid treatments [150]. However, it is crucial to exercise caution when making direct comparisons between AOPs and other processes for biofuel substrate pre-treatment or effluent final treatment. Each process operates optimally under specific conditions [151], and the results can be influenced by the type of substrate used, making it challenging to draw straightforward conclusions from the literature values. To enable more meaningful comparisons, future studies should focus on using the same substrates under controlled conditions. This approach would help establish clearer insights into the relative performance of the different treatment methods, thus providing valuable guidance for effective bioenergy production.

A significant benefit of employing AOPs in bioenergy pre-treatment lies in their ability to enhance biodegradability with minimal chemical oxygen demand (COD) loss or mineralization into smaller compounds [126]. Moreover, the potential for further improving bioenergy production using AOPs can be achieved by integrating two or more processes during the pre-treatment stage. This synergistic approach can lead to even more efficient outcomes in bioenergy generation.

5.1. Combination of AOPs with Traditional Methods in Bioenergy Substrate Pre-Treatment

The combination of different treatment processes focuses on improving the cost-effectiveness of bioenergy production. The use of different treatment processes for different applications is dependent on the substrate and its pollutant load. A simple method for establishing the most appropriate process should involve a comparison of the costs involved and the outputs in terms of the pollutant load and bioenergy production. Although there is ample literature on the applications of oxidation processes, there is limited documentation on combining AOPs with traditional treatment methods for enhancing the cost-effectiveness using agro-industrial waste as biomass.

Different authors have investigated the use of AOPs in combination with physical–chemical processes. The coagulation–flocculation integration with Fenton oxidation removed 58% of the COD, 95% of the TS and 80% of the TPC and increased the biogas production from 45.7 mL to 65.5 mL [152]. On the other hand, a study coupling the coagulants CaCO₃ and FeCl₃ with ultrasound resulted in a decrease in the TS and VS removal and bioenergy production when compared to the substrate pre-treatment using only the coagulants or the ultrasounds [153]. The literature reported conflicting results on the effects of ultrasound as a pre-treatment process. Park et al. (2012) and Tyagi et al. (2014) combined NaOH and ultrasounds to pre-treat pulp and paper mill waste [154,155]. In both studies, the COD, TS and VS removal increased, as well as the biogas production. Therefore, more studies are required particularly on optimizing the combined processes.

5.2. Scale-Up of AOPs in Bioenergy

The application of AOPs in the treatment of AIW at a pilot scale has been demonstrated. Amaral-Silva et al. (2017) applied the Fenton process in an estimated treated volume of 450 m³ of OMW at an industrial scale and achieved an 85% COD removal [156]. In another study, the application of TiO₂/H₂O₂/UV and Fe²⁺/H₂O₂/UV as WW treatment using a 45 and 100 L pilot-scale plant was explored. The solar photo-Fenton reaction was also studied and showed the highest efficiency, achieving a COD degradation of 85% [157].

Although the efficient treatment of wastewater was extensively observed, no studies were identified on the scale-up of AOPs in bioenergy production. Laboratory-scale studies have shown a positive balance both economically and energetically [93,98,142]. On the other hand, previous studies have also shown the opposite. For example, when the applied processes were energetically demanding such as microwaves, the final balance could be negative [92]. Thus, more efforts are still required in the studies using real AIW, and considerable work needs to be undertaken in terms of the design strategies for the scale-up.

5.3. Other Applications of AOPs in Bioenergy

The number of studies that use microalgae-based technology for wastewater treatment have increased. This technology has several advantages, such as being driven by solar energy, eco-friendliness, efficient fixation of CO₂ and high O₂ production [158]. Furthermore, microalgae can produce proteins, lipids and carbohydrates in high quantities, which can be used as feedstock for bioenergy production or other high value-added products [159]. However, this technology presents some disadvantages such as complex pre-treatment steps and a low phytoremediation efficiency and biomass production rate, which prevent its use in industrial wastewater treatments.

One possible method could stem from the combination between AOPs and microalgae-based technology. Different recalcitrant compounds that are not immediately degraded for microalgae could be oxidized by AOPs, improving the biodegradability of diverse wastewaters. Some combined AOPs and microalgae processes have been applied to municipal solid waste [160], landfill leachate [161] and artificial pharmaceutical effluents [162]. The number of studies regarding AIW treatment are few [163,164] as well as the studies that were applied for subsequent bioenergy production [165]. Considering these facts, further investigations are required to understand the potential of these effluents on bioenergy production once they have been treated by combined AOPs and microalgae processes.

The quality of the effluents generated during bioenergy processes depends on the biomass substrate and the production processes that are used. Generally, it is considered a highly toxic effluent since it presents significant concentrations of TOC, COD, oils and greases [25]. Treating these effluents is necessary before it can be discharged into the environment. AOPs can mineralize the recalcitrant compounds thereby reducing the organic content and toxicity of the final residue [25,166].

6. Challenges, Limitations and Opportunities

Using AOPs as a pre-treatment for bioenergy production is a promising and potential technology. These processes have the advantages of high mineralization, the biodegradation of highly refractory pollutants, applications with other treatment processes and no post-treatment requirements. However, there are certain limitations associated with using AOPs until these processes can be implemented in industries. The application of AOPs in AIW treatments for subsequent bioenergy production may represent additional costs to the industries. The costs associated with the application of the oxidation processes are dependent on the wastewater contamination; the higher the pollutant load and the extent of pollution removal required, the more severe the treatment conditions, and thus, the higher the application costs. Additionally, every AOP has an optimal pH value that requires an initial adjustment that needs to be monitored throughout the treatment process and a final adjustment depending on the generated effluent characteristics and intermediate products. The increase in the dose or the exposure time may also represent additional costs when AOPs are used as pre-treatment.

Different treatment processes can be combined to improve the performance and costs of bioenergy production. The simultaneous use of different AOPs such as UV/H₂O₂, ozone/TiO₂/UV, UV/the Fenton reagent, MW/H₂O₂ etc., may increase the generation of reactive species compared to the respective individual processes.

The sequential application of various AOPs could be also a sustainable treatment strategy for agro-industrial effluents containing different classes of organic compounds and levels of oxidation. The application of a separation treatment, e.g., coagulation–flocculation, before the use of AOPs can remove colloidal particles that interfere with the oxidative processes, and thus, easily treat the wastewater. Complex treatments comprising various physical processes, AOPs and biological processes could enhance the cost-effectiveness of bioenergy production. Moreover, the use of AOPs for oxidizing recalcitrant compounds of wastewater and enhancing the subsequent wastewater purification using microalgae could also be a sustainable approach.

The application of AOPs as a pre-treatment step in bioenergy production lack proper scientific studies. AIW must be previously characterized, and treatment tests must be realized on a laboratory scale to select the most appropriate method in terms of economics and efficiency. The establishment of the limiting step of the reaction, the ideal dose and exposure time, the comparison with other treatments and the possible combination with other processes should be considered before implementing any of the treatment processes.

7. Conclusions Remarks

This work evaluated the possible utilization of AOPs for generating bioenergy. The proper treatment of wastewater generated from agro-industrial activities can enhance the bioenergy process and mitigate the impact of the emerging demand for water. This will have positive repercussions on freshwater resources, environmental conservation, income generation and poverty alleviation. Furthermore, the increased interest in renewable energies that can replace fossil fuels has led to an intensification in the number of studies that improve bioenergy production, and thus, the pre-treatment of biomass substrates.

Given the above facts, this review showed that AOPs have great potential in AIW treatment for subsequent bioenergy production. In treating wastewater containing a highly resistant COD, AOPs can be employed as intermediate processes before biological digestion. Their role is to enhance the recovery of biofuels and reduce the COD levels in the final effluent to meet the required standards for disposal. The application of AOPs helps to remove hemicellulose and lignin contents thereby increasing the production of biogas, biodiesel and bioethanol. The degradation of these compounds results in a disruption of the recalcitrant structures of cellulosic biomass, making cellulose more accessible to the enzymes that convert carbohydrate polymers into fermentable sugars.

Apart from traditional biofuels, processes such as WAO can be utilized to maximize the production of other valuable products, such as volatile acids. However, WAO has drawbacks such as corrosion and high energy requirements due to the high operating pressure and temperature. Nevertheless, researchers are exploring the use of suitable catalysts to enable milder operations and overcome these challenges.

Ozonation and the Fenton reagent are AOPs that can be utilized to enhance the biodegradability, resulting in increased bioenergy yields. These processes can remove inhibitory compounds in organic effluents, leading to a higher production of methane. When dealing with complex effluents, AOPs play a crucial role in improving biodegradability using selectively refractory substances. This selectivity ensures that the energy-rich bulk COD is not removed as sludge or mineralized with the recalcitrant components. Among the AOPs, ozonation stands out for its advantageous selectivity towards phenolics, which aids in enhancing biodegradability and its capability for targeting colorants, which is essential for decolorizing the colored substrates. Moreover, the combination of AOPs with other treatment processes, such as biological, physical or microalgae, can be also applied to enhance bioenergy production. The amount of bioenergy that is produced depends on several factors, including the type of substrate that is used and the specific AOP that is employed. Additional comparative research is required to determine which AOPs are best suited for different bioenergy substrates. By applying AOPs as pre-treatment, the biodegradability of the substrates can be improved.

Further investigations into the scalability of AOPs in bioenergy production will expand their potential applications. The future of biofuel production lies in the biorefinery concept, where bioenergy substrates are utilized to produce multiple biofuels and bio-products. The approach involves maximizing high-value products first and then utilizing the remaining substrates to produce bulk products with improved performances by incorporating AOPs. Further research should focus on the application of these processes in biorefinery production.

Despite the advantages of AOP applications, these processes require continuous improvements to optimize the dose and exposure time, employ solar radiation instead of UV light, synthesize stable and efficient catalysts and reuse catalysts. These developments will facilitate the implementation of AOPs for scale-up and operations using real AIW.