Sustainable Management and Advanced Nutrient Recovery from Biogas Energy Sector Effluents

Abstract

Anaerobic digestion (AD) is an effective technology for the sustainable management of organic agricultural waste, producing both biogas and nutrient-rich digestate. This study aims to review and evaluate different methods for obtaining valuable products from digestate, with a focus on innovative and sustainable approaches. The main objectives are to identify effective technologies for the recovery of nutrients and organic matter, assess their environmental and economic impact and outline the challenges and prospects in this area. The review covers established techniques (with a technology readiness level (TRL) of six to nine, indicating their maturity from pilot to full scale) such as struvite precipitation and ammonia stripping, which are very effective in recovering nitrogen and phosphorus from digestate and converting it into valuable biofertilizers. Struvite, for example, offers an option for slow-release fertilizers that reduces dependence on synthetic fertilizers. A comparative analysis shows that ammonia stripping can efficiently capture nitrogen and produce fertilizer without harming the environment. New methods, such as microalgae cultivation, use digestate as a nutrient source for the production of biofuels and bioplastics, contributing to renewable energy and sustainable material production. The study also examines composting and vermicomposting, where digestate is converted into nutrient-rich soil conditioners that significantly improve soil health and fertility. The production of biochar through pyrolysis is highlighted for its benefits in improving soil properties and sequestering carbon, providing a dual benefit for waste management and climate change mitigation. Membrane technologies, including ultrafiltration (UF) and reverse osmosis (RO), are being investigated for their effectiveness in nutrient recovery, despite challenges such as membrane fouling and high operating costs. The study highlights the potential of these valorization processes to improve the sustainability and economic viability of AD systems and to align with circular economy principles. The results suggest that the continuous optimization of these technologies and the integration of recycling processes are crucial to overcome existing challenges and realize their full potential.

1. Introduction

The estimated annual production of agricultural waste (crop residues, animal residues and waste from the distribution and processing of agricultural products) is around 998 million tons, of which 80% is organic waste [1]. The direct application of agricultural waste, including manure, as a soil improver and soil amendment is regulated and limited due to the risk of odor and water pollution [2]. Alternatively, agricultural waste can be used as substrates for the production of biofuel, which not only reduces greenhouse gas emissions, balances the carbon cycle and decreases the environmental impact of landfill disposal, but also improves energy security [3].

Among the processes leading to the production of biofuels, anaerobic digestion (AD), in which organic waste is converted into valuable products such as biogas, is of great importance given its widespread use in full-scale plants [2]. According to the European Biogas Association (EBA), around 20,000 units (total number of biogas and biomethane plants) were operated in Europe in 2020, of which over 70% used agricultural substrates [4].

In addition to biogas, which can be used as transport fuel, for space heating or to generate electricity, the residual organic material is digestate. Digestate, the main by-product of AD, contains stabilized organic matter (which cannot be further converted into gaseous products) and is rich in nutrients (nitrogen (N), phosphorus (P) and potassium (K)) that are crucial for intensive agriculture. This makes it a potential substrate for the production of bio-based fertilizers. On the other hand, the management of digestate can become a bottleneck for AD sustainability, if its use is restricted due to the excess of nutrients and its acceptance by the market is low. On the other hand, it has been estimated that the use of digestate to fertilize soils with low fertility near biogas plants increases the production of biomass suitable for biomethanation up to threefold [5].

The annual production of digestate in the European Union (EU) has reached 31 Mt (dry matter) (data from 2022) [6]. The composition and, thus, the management strategy of digestate is determined by the feedstock characteristics and the operating parameters of the digesters. The direct application of digestate after its separation into solid and liquid phases as soil conditioner and liquid fertilizer can induce some problems, including intensive transportation needs or the emission of greenhouse gasses, particularly NH₃ and N₂O, during digestate storage [7,8]. In the atmosphere, NH₃ can form microparticles of ammonium sulfate, which is one of the main reasons for air pollution in both rural and urban areas. On the one hand, an adequate amount of nutrients is necessary to ensure good crop yields. On the other hand, an excess of nutrients can lead to the leaching of nutrients and the contamination of surface water and groundwater. Controlling the amount of digestate applied directly to the fields is therefore a bottleneck in digestate management. Three criteria (environmental efficiency, agronomic efficiency and economic efficiency) are needed to optimize the balance between the amount of nutrients consumed by crops and the amount of fertilizer applied, including fertilizer from digestate. The overproduced digestate that cannot be applied directly to the soil must be stabilized. One of the stabilization approaches is nutrient and organic matter recovery to upgrade the digestate into biofertilizers and soil improvers. This requires the digestate to be separated into a liquid fraction (total solids (TS) of approx. 2% w/w) and a solid fraction (TS of approx. 20–30% w/w). The liquid fraction contains most of the N and K in the form of ammonium and potassium ions, whereas the solid fraction contains most of the N (87%) and P (71%) [9].

The recovery of nutrients and organic matter from digestate instead of direct applying untreated digestate to the land helps to control nutrient pollution, prevent the eutrophication of water bodies and reduce odor nuisance and the risk of pathogens. Keeping in view these environmental aspects and the fact that digestate is rich in nutrients and organic matter, digestates should be converted into valuable products, such as biofertilizers [10]. Biofertilizers are natural fertilizers derived from microbial inoculants that can fix atmospheric nitrogen, solubilize phosphorus or mobilize other nutrients to make them more available to plants [11]. Biofertilizers can be applied to agricultural lands to improve soil fertility and crop productivity. They offer a sustainable alternative to synthetic fertilizers and reduce the environmental impact associated with their production and use [12]. By recycling nutrients back into the soil, these bio-products contribute to the circular economy and sustainable agricultural practices. This is especially important as the production of synthetic fertilizers is unsustainable and P-based fertilizer reserves are depleted. The production of biofertilizers requires supplementary technologies to upgrade digestate to the desired quality. Despite the undoubted benefits of nutrient recovery for the development of the circular economy, issues related to digestate composition, economics and market are the bottlenecks limiting the process [13]. Therefore, the formulation of valuable and marketable biofertilizers still remains a knowledge gap [14]. To fill this gap, various by-products are mixed to produce multi-component organic–mineral biofertilizers [15]. In addition to biofertilizer production, the valorization of digestate by upgrading it or transforming into other high-value products, including biochar, makes the AD of agricultural waste more sustainable and economically efficient [16]. Similar profits can be achieved, for example, by using digestate for microalgae production or composting.

This work provides a comprehensive overview of the recovery of valuable products from AD digestate, with a particular focus on innovative and sustainable methods. The novelty of this work lies in its integrated approach to the valorization of digestate through advanced nutrient and organic matter recovery technologies. In contrast to previous studies, this work addresses not only established techniques such as struvite precipitation and ammonia stripping, but also emerging methods such as microalgae cultivation, biochar production, membrane technologies and various integrated systems. By examining the environmental and economic impacts of these methods and addressing their technological challenges, this paper provides a forward-looking perspective on improving the sustainability and profitability of AD systems. The integration of different recovery processes to maximize efficiency and the emphasis on aligning these processes with the principles of the circular economy underline the innovative approach of this study. Furthermore, the paper highlights the potential of these technologies to transform waste management practices and offers practical insights for future research and implementation in the biogas industry.

2. Digestate—A By-Product of AD

The composition and thus the management strategy of digestate is determined by the feedstock characteristics (

Table 1. Composition of AD digestates from various feedstocks.

Waste Type	pH (–)	TS (%)	VS (%)	VFA (mg/L)	N (%)	P (%)	K (%)	Ca (%)	Mg (%)	Ref.
Food waste	6.77–6.83	4.49	2.93	8581	6.9	7.2	–	–	–	[19]
Municipal solid waste	–	10.50	64.12	–	1.94	0.62	–	–	–	[20]
Chicken manure	8.40	11.80	8.20	–	7.9	1.6	6.9	4.2	0.75	[21]
Biowaste	8.31	24.40	63.00	–	4.32					[22]
Agricultural residue	–	–	70.20	–	3.2	1.2	1.6	1.3	0.7	[23]
Residual municipal solid waste	–	–	36.20	–	1.5	0.7	1.6	10.4	1.4	[23]
Sewage sludge	–	–	51.00	–	3.4	2.7	0.7	4.6	0.9	[23]
Vegetable, garden and fruit waste	–	–	42.70	–	2.0	2.6	0.7	4.3	0.8	[23]
Sewage sludge	–	1.78	0.95	–	–	–	298 *	146 *	26.5 *	[24]
Food waste	8.10	–	–	300	1.14	–	–	–	–	[25]
Food waste	–	76.31	66.10	–	1.92	18.77	2.76	14.93	2.77	[26]
Maize and grass silage and cattle manure	–	–	74.10	–	3.6	1.35	5.23	2.01	0.94	[27]

* mg/L.

). The pH value is usually slightly basic, ranging from 6.8 to 8.4, although this depends on the progress of the AD reactions; the pH increases in the presence of ammonia and after the degradation of volatile fatty acids (VFAs), and decreases after VFA accumulation or the precipitation of carbonate and phosphate [17]. The composition of digestate shows high fluctuations (mainly with regard to the TS and volatile solids (VS) contents) and depends on the feedstock material and the operating conditions of AD (e.g., dry or wet process). Due to the variability of the biodegradability of the feedstock, the content of TS ranges from 1 to 76%. A higher TS content indicates high amounts of lignocellulosic substrates of lower biodegradability [8]. Higher solids contents were observed in digestate from dry AD processes [17]. Lower values of organic matter (VS) indicate higher manure fractions, whereas higher VS values indicate a higher proportion of kitchen and garden waste. For total N, ammonia accounts for more than 80% of the digestate after the AD of manure and no more than 44–47% of the digestate after the AD of kitchen and garden waste. The contents of total P, K, Ca, Mg and heavy metals do not change during AD; they are regulated by the feedstock’s composition [18]. The co-digestion of different raw materials can lead to further variations in the digestate’s composition.

Digestates derived from biowaste, such as energy crops and manure, are classified into the component material categories (CMCs) CMC4 and CMC5, respectively [8]. The minimum amount of N, P (as P₂O₅) and K (as K₂O) in single-component fertilizers should be 2, 1 and 2% w/w, respectively (for liquids), and 2.5, 2 and 1% w/w, respectively (for solids), to be considered as fertilizers. For multi-component fertilizers (both liquid and solid), it should be minimum 1% by weight for each element. The maximum oxygen uptake rate is 25 mmol O₂/(kg VS·h), and the maximum biogas production is 0.25 L/g VS.

Against the backdrop of increasing environmental concerns and the urgent need for sustainable agricultural practices, the valorization of digestate has become an important issue. Recovering valuable products from digestate is not only a solution to the problem of waste disposal, but also contributes to the circular economy by converting waste into resources [28]. The recovery and reuse of N, P and K, which are important for plant growth and soil health, can reduce dependence on synthetic fertilizers, diminish greenhouse gas emissions and mitigate the environmental impacts associated with fertilizer production [29]. The recovery of nutrients and organic matter from digestate instead of direct application of untreated digestate to land helps to control nutrient pollution, prevent the eutrophication of water bodies and reduce odor nuisance and the risk of pathogens. Economically, the utilization of digestate can open up new revenue streams for waste management and biogas plants, reduce operating costs and contribute to the economic viability of anaerobic digestion projects.

To realize its potential, the extraction of valuable products from digestate must overcome various challenges, including technological limitations, economic feasibility and regulatory barriers [30]. However, ongoing research and technological innovations are improving the efficiency and cost-effectiveness of recovery processes. In addition, growing awareness of environmental sustainability and resource conservation is creating a favorable market for recovered products and opening up significant opportunities for progress in this area.

3. Recovery of Valuable Products from Digestate

Figure 1 shows the strategies for recovering valuable products from AD digestate. Techniques such as struvite precipitation and ammonia stripping are used to recover N and P and produce effective biofertilizers. Digestate also supports the cultivation of microalgae for the production of biofuels and bioplastics. Composting and vermicomposting convert digestate into nutrient-rich soil conditioners that improve soil health, while pyrolysis produces biochar, which improves soil fertility and sequesters carbon. Membrane technologies help recover nutrients, despite challenges such as membrane fouling. These recovery processes improve the sustainability and cost-effectiveness of AD systems and comply with circular economy principles by converting waste into valuable products.

3.1. Recovery of Nutrients—Production of Biofertilisers

The N and P concentrations in the liquid fraction of digestate are too high to meet the standards for wastewater discharge and are, relatively, too low to serve as fertilizers. This contradiction is intensified by the fact that the treatment of digestate is expensive, transportation costs are high and the increase in digestate production due to the expansion of biogas plants is at odds with land capacity [31]. Therefore, the simultaneous treatment of the liquid fraction of digestate with N and P recovery is recognized as the most sustainable method for its management.

Agricultural digestate is considered a potential candidate for the production of bio-based fertilizers, since it contains high concentrations of N (2–5 kg/m³), P (0.5–1.5 kg/m³) and K (1.05–5.48 kg/m³) [32,33]. Much of the N and K remaining in digestate after AD is in the soluble fraction, whereas P is mainly present in the particulate fraction. For example, food waste typically contains 1 kg N/ton and 0.25 kg P/ton [34], waste-activated sludge contains 2 kg N/ton and 0.5 kg P/ton [35] and cattle and chicken manure contain 5–15 kg N/ton and 0.1–1 kg P/ton [36]. These nutrients are easily recoverable; they can be recovered in a concentrated form for convenient transportation to different agricultural areas [32].

In granular biofertilizers, a uniform distribution of nutrients is important. In the production of biofertilizers from anaerobic digestate (liquid phase; nutrient source) and limestone powder (raw material), the improvement of nutrient homogeneity in a granule was achieved by increasing the liquid-to-solid ratio, the impeller speed and the processing time [37]. The biofertilizer produced contained less nutrients than synthetic fertilizer but more than raw limestone; the nutrients were effectively released into solution.

The recovery of nutrients from digestate has also been implemented by cultivating microalgae. Various algal strains can grow effectively on nutrient-rich agricultural digestate by sequestering N and P in their fixed form. Although the resulting microalgal biomass is suitable for biochemical and biofuel production in anaerobic digestion, it requires a low-cost nutrient source to ensure the sustainability and economic viability of production [7]. The abundance of N and P in digestate makes it a promising medium for microalgal production. Microalgae have a high growth rate and photosynthetic efficiency, store lipids, absorb CO₂ and do not compete with edible feedstocks [38]. The ability of microalgae to harvest solar energy and utilize nutrients to generate biomass, which can serve as a substrate for the production of biofuels, bioplastics or animal feed, enables an expansion of the possibilities for digestate valorization, especially since the produced biofuel is renewable and causes low CO and no SO_x emissions. At the laboratory scale, the cultivation of microalgae on agricultural digestate was carried out in photobioreactors, bottles and raceways, with various pretreatments such as settling, filtration, centrifugation or dilution [39,40]. For example, at pilot scale in the raceway pond, the average algal productivity (as total suspended solids (TSS)) from digestate from a pig farm was 32.4 ± 33.1 mg TSS/L·d [40]. Field pilot-scale tests are still scarce due to the wide variation in performance. These variations may result from different efficiencies of liquid–solid separation; separation is very important for controlling digestate turbidity, which affects photosynthetic algal growth [41]. The diverse effects of AD microorganisms on the growth rate and biomass composition (in terms of protein or starch content) of algal species are another reason for variations in algal productivity [42]. The growth of microalgae was inhibited by several components of digestate, such as ammonia, suspended materials, metals with toxic effects, the turbidity-limiting light transmission and competing biological contaminants, such as bacteria, viruses or foreign algae [43]. Therefore, pilot-scale systems of algae cultivation encounter problems with contaminants, excessive nutrient concentration and the unstable production of biomass [18]. To solve these problems, digestate dilution and pre-treatment, microalgae strain selection, extra organics addition, nitrification and desulfurization were used. Nitrification can reduce the ammonium toxicity of the digestate and stabilize the nitrogen [7]. In the experiments to improve nutrient recovery, digestate valorization was combined with biogas upgrading by using co-cultures of Chlorella sorokiniana and Methylococcus capsulatus [44]; methane and carbon dioxide were converted to microalgal biomass. When compared to the monoculture, the co-culture increased biomass production and N and P use by 120, 71 and 164%, respectively.

3.2. Nutrient Recovery Techniques

Techniques that have been used to recover nutrients from digestate with average yields of over 50% include drying with acidic recovery, struvite precipitation, stripping with acidic recovery, membrane separation, ion exchange and adsorption [32]. Struvite precipitation and ammonia stripping enable an 80–90% recovery yield of nutrients and are the most developed in full-scale facilities, resulting in relatively low operating costs for these processes (2–7 EUR/kg of nutrient recovered) [8]. Simultaneously, the low environmental impact of membrane techniques makes them increasingly popular in the EU nutrient recovery market [8].

3.2.1. Drying

Drying is based on the removal of water from digestate and concentrating the residual fraction with hot air to obtain a biologically stable powder that is reduced in volume (98% dried solid) [17,32]. For this process, part of the heat produced in AD plants with a heat and power generation unit can be used. During drying, the recoverable heat may be limited, which may result in less than 50% of the digestate being processed [32]. When using diluted feedstock in AD (with a digestate water content of ca. 90%), the recovered heat may not be sufficient to dry the entire volume of digestate [45,46]. In this case, the additional heat can be recovered from the exhaust gas using gas–water heat exchangers [32].

If digestate is acidified with mineral acids, ammonia nitrogen can be retained in the digestate. Alternatively, ammonia nitrogen can be removed by vaporization. In this case, N can be recovered by reverse osmosis (RO) as a concentrated ammonium solution or by acidic scrubbing as ammonium nitrate (if HNO₃ is used) or as ammonium sulfate (when H₂SO₄ is used) [32]. To increase the economic viability and reduce the environmental impact of digestate drying (due to emissions), the development of ammonia recovery processes is crucial [14].

Numerous drying full-scale digestate plants in Europe employ such systems as belt dryers, mechanical drum dryers and solar dryers [17]. These drying technologies are only economical if the heat is free, regardless of the volume-reducing benefits [47].

3.2.2. Struvite Precipitation: A Sustainable Approach to Nutrient Recovery

Agricultural digestate, especially when derived from livestock manure, is considered a potential candidate for the production of bio-based fertilizers [8]. Struvite, for example, is a bio-based fertilizer that can be obtained from fermentation residues and offers numerous environmental and economic benefits. Struvite crystals can be dried and packaged for use. Struvite serves as a slow-release fertilizer that increases the availability of nutrients for plants and reduces dependence on synthetic fertilizers. For these reasons, among others, struvite precipitation is considered a leading technology for the sustainable recovery of valuable nutrients from the liquid fraction of digestate, overcoming several environmental and agricultural challenges.

Struvite precipitation is a mature process that simultaneously recovers the dissolved N and P by dosing magnesium salts into the digestate under the prescribed ratio of N:Mg:P, yielding struvite as a product (magnesium ammonium phosphate, MgNH₄PO₄⋅6H₂O). Struvite formation through the reaction of magnesium, ammonium and phosphate ions in water under specific conditions involves steps such as pH adjustment, magnesium addition and the precipitation and recovery of struvite crystals. These steps are critical for creating an environment that favors struvite formation, promotes the release of ammonium and phosphate ions into solution and balances the often-insufficient magnesium content in the digestate. Tsaridou et al. [48] reported that, when valorizing liquid digestate from an industrial anaerobic digestion plant treating dairy-processing waste, the maximum removal of nutrients and production of struvite-rich precipitate was achieved at a molar N:Mg:P ratio of 1:1.5:1.5 and a pH of 10 in the retentate stream. Moreover, struvite was almost completely precipitated within ~30 min.

Muhmood et al. [49] investigated the effects of biochar incorporation (wheat straw biochar and rice husk biochar) on nutrient recovery through struvite formation and the size of struvite particles precipitated from the AD supernatant. In addition, the influence of the incorporated biochar on the accumulation of heavy metals and the removal of pathogens (total coliform bacteria and Escherichia coli) was evaluated under different operating conditions, e.g., pH, supersaturation, reaction time and incorporation rates. Compared to the process without inoculation (the maximum efficiencies of phosphate and ammonium recovery were 91% and 83%, respectively, at a particle size of 70 μm) and the process with struvite inoculation (the respective maximum efficiencies were 97% and 94% at a particle size of 100 μm), the biochar-inoculated process improved nutrient recovery by up to 7% and 11% for P and N, respectively, and increased struvite particle size by 43%, regardless of biochar type, compared to the non-inoculated process.

Struvite is considered a slow-release fertilizer [50]. Szymańska et al. [51] investigated the use of struvite, which is obtained from the liquid fraction of biorefinery digestate, as a fertilizer for agricultural applications. In the second year after fertilization, the average grass yields of silty loam and loamy sand soils were higher after treatment with struvite than after treatment with commercial ammonium phosphate (8% higher after treatment with struvite alone and 16.5% higher after treatment with struvite with ammonium sulfate). The associated N efficiency of the struvite and ammonium sulfate treatment on silty clay and loamy sandy soils exceeded that of the ammonium phosphate treatment by a factor of two and six, respectively. The associated P efficiency was about two times higher in the struvite treatments than in the ammonium phosphate-treated soils.

The process has been implemented in full-scale installations; the most established commercially available technologies include AirPrex (Germany, DE), ANPHOS (the Netherlands, NL), CAFR (DE) and Ceres (Belgium, BE), which allow for the recovery of 80–90% of soluble P and 10–40% of ammonia [18]. For example, in the struvite precipitation plant in Belgium (with a technology readiness level (TRL) of nine), struvite was formed by adding MgCl₂ [52]. Before crystallization, the digestate was minced to remove the biggest fibers. After crystallization, the precipitate was separated using a hydrocyclone. Finally, 5% of P recovery was obtained. Despite the promising potential of the world-wide implementation of this technology, struvite precipitation faces challenges, including high operating costs because of the large amount of chemicals used, the need for initial investment in equipment and the optimization of operating parameters to maximize recovery efficiency. It should also be noted that struvite precipitation requires a phosphorus concentration above 50 mg P/L to be economically viable [53]. Future research should aim to purify the struvite, improve the economics of the process, integrate struvite production into existing waste management systems, and explore new applications of struvite in environmental remediation. To achieve this, methods avoiding chemical addition (e.g., electrochemical or bio-electrochemical struvite recovery) are being developed [18]. The need for research to maximize struvite precipitation results from the composition of the digestate. While the ammonium concentration in digestate is sufficient to support struvite precipitation, only 10% of the P is available because of high pH value of the digestate. To increase the available P in the digestate, acidification, heating and microwave or ultrasonic treatment were investigated. In addition, the presence of Ca²⁺ makes struvite purification difficult. Although Ca²⁺ can be chelated by ethylenediaminetetraacetic acid (EDTA), the toxicity of EDTA is not yet fully understood [54]. Struvite precipitation is a sustainable approach to recover valuable nutrients from digestate. It makes an important contribution to nutrient recycling, environmental protection and the promotion of sustainable agriculture, which means that it can play a crucial role in modern waste management and resource recovery strategies.

3.2.3. Ammonia Stripping: An Efficient Nutrient Recovery Method

Ammonia stripping is a key technology for the recovery of ammonia from various waste streams, including the liquid fraction of digestate, and plays an essential role in nitrogen management in waste treatment processes [55]. As this method effectively converts a potential environmental pollutant into a valuable resource, it can play an important role in sustainable waste management and resource recovery. The ammonia stripping process is based on the principle of converting ammonium (NH₄⁺) in the digestate into ammonia gas (NH₃) by increasing the pH, which facilitates the removal of ammonia from solution by air or steam stripping [50]. Important steps in the process include pre-treatments for solid/liquid separation; pH adjustment, typically achieved by adding lime (Ca(OH)₂) or sodium hydroxide (NaOH) to shift the equilibrium towards ammonia gas formation; vigorous aeration or steam injection to strip the ammonia gas into the gas phase; and ammonia recovery, in which the ammonia-rich air or steam is passed through a recovery unit such as an acid scrubber to absorb the ammonia gas into an acidic solution and produce valuable nitrogenous fertilizers. Because ammonia can be recovered as variable products such as ammonium sulfate, ammonium hydroxide, ammonium bicarbonate or ammonium nitrate, stripping is the most viable technology for industrial applications [50]. For example, when using sulfuric acid, the production of ammonium sulfate is 25–35% [45,56]. The process concludes with the final treatment of the digestate, significantly reducing its ammonia content for further processing or use as a diluted fertilizer [57]. In stripping technology, N is recovered in pure form, whereas C, P and K remain in the liquid fraction, which can be applied to fields. When using this stream in agriculture, the salinity level should be of concern because it affects the cationic exchange capacity of the soil. This is particularly the case when using NaOH to increase pH, as relatively high Na⁺ levels can appear in the liquid phase, affecting the salinity [58]. In addition, the presence of suspended solids in the liquid fraction of the digestate can limit the efficiency of stripping [32].

To improve the efficiency of ammonia stripping, the composition of the stripping gas was optimized [59]. In air stripping, the oxygen inhibited the anaerobic microorganisms. Also, the efficiency of stripping was negatively correlated with the level of CO₂ in the stripping gas. As alternatives to air, biogas and flue gas were investigated. Flue gas was found to be a promising alternative to air due to its high thermal energy, which can be reused in the stripping plant. For biogas, due to its CO₂ content, a significant decrease in efficiency was observed, in contrast to flue gas. When biogas was used as a stripping agent for valorizing food waste digestate, effective ammonia removal was achieved at a pH value of 10 and a temperature of 70 °C [60].

The benefits of ammonia stripping include the efficient recovery of nutrients, the effective reduction of the environmental impact of directly applying digestate and the production of valuable fertilizer products [61]. By capturing ammonia, the process also reduces its release into the atmosphere, preventing air pollution and the eutrophication of water bodies. The process is also so versatile that it can be used at different scales in different types of waste treatment plants [62].

Although advanced technologies of biological ammonia removal are cheaper than stripping, stripping additionally produces a marketable end-product and removes odors and dust particles [18]. Therefore, it can be competitive in regions with high nitrogen demand. Despite its proven effectiveness in nitrogen recovery, ammonia stripping with the use of traditional technology faces some problems and challenges, such as the energy demands of aeration or steam injection and the need for careful pH management [63]. High operating costs result from the high requirement for alkaline chemicals used for pH increases and the high energy consumption when stripping with air or steam. These requirements result from the main bottleneck of stripping technology, namely the scaling and fouling of the packing material due to calcium carbonate formation [18]. The consumption of energy for air stripping (4.1 kWh/m³) was found to exceed that of chemical precipitation (0.8 kWh/m³) [50]. Additionally, the risk of environmental pollution (air and water) arises when the ammonia is not completely absorbed by the acidic solution. All these problems can be intensified by the complex and unstable composition of the digestate. Cost effectiveness also depends on the conditions of the local market, the availability of residual heat and the ammonia concentration. It has been generally demonstrated that the process is economically feasible at an ammonia concentration of at least 1000–1500 mg/L [50]. Current research is aimed at improving the energy efficiency of the process, integrating it into other waste treatment processes and finding new applications for the recovered nitrogen products. In addition, innovative technologies such as stripping under vacuum conditions or without the addition of alkali, the use of atomization, spraying, high gravity, microwave heating or electrochemical stripping to strengthen the process of mass transfer were developed. For example, in the electrodialysis (ED)-electrochemical process, the recovery of ammonia exceeded 90% [64]. Moreover, the sustainability of the technology can be improved by adding a softening step or a CO₂ stripper before ammonia stripping, developing a more effective liquid–solid separation stage to increase the solid fraction, or developing the stripping process without internal packing [18].

Full-scale ammonia stripping has been carried out in the USA and Europe, mainly producing ammonium sulfate [17]. Commercially available technologies include, among others, AMFER (NL) or ANAStrip (DE) [18], whose recovery efficiency may reach 98%, although 80–90% is usually reached to reduce the costs; it should be kept in mind that the technology of stripping is only economically viable if the objective of the process is to remove nitrogen [47]. As an example, a digestate processing plant with ammonia stripping, which produces ammonium sulfate through gypsum scrubbing without direct H₂SO₄ dosing, was rated with a TRL of nine [21]. In this full-scale plant, 57% ammonium was recovered from digestate.

3.2.4. Membrane Technology

The use of membrane technology makes it possible to combine the treatment process with the nutrient recovery from liquid digestate. This is a cutting-edge technology for separating and recovering nutrients and water from the liquid fraction of digestate, using semi-permeable membranes to selectively separate components based on their size or molecular weight [65]. This refined approach to resource recovery plays a significant role in sustainable waste management and environmental protection. There are several approaches to membrane technology, including pressure-driven technologies like microfiltration (MF) [66], ultrafiltration (UF) [67], nanofiltration (NF), RO [68] and forward osmosis (FO), as well as non-pressure technologies like membrane distillation (MD) and electrodialysis (ED) [50]. MF retains particles larger than 0.1–1.0 µm and UF removes particles of 0.01–0.20 µm; the other techniques remove salts. These processes are performed at different pressures and offer distinct separation capabilities to achieve the desired efficiency and specificity, depending on the digestate’s composition. The membrane separation efficiency reached 50% and the digestate volume was reduced [32].

Among the pressure-driven membrane techniques for the filtration of the liquid fraction after solids removal via centrifugation, UF and RO are the most frequently used, although they are not selective [8,32]. The concentrate produced in RO is rich in macro- and micronutrients, which are characteristic for fertilizer [69,70]. The removal efficiencies of N and P are about 75–95% and 85–99%, respectively. The management of the liquid fraction of digestate is urgent in these facilities in which huge volumes of digestate are produced. These facilities include livestock farms, where liquid digestate was found to contain large amounts of suspended solids, requiring advanced pretreatments before RO. A membrane bioreactor (MBR), which combines biological treatment with membrane filtration, effectively degrades organic matter and suspended solids and is considered a pretreatment method for RO. For digestate from the AD of swine wastes, the UF-MBR and RO were used [71]. The reduction of suspended solids and turbidity in the UF component protected the downstream RO membrane. In RO, a water recovery of 90% was achieved and higher nutrient concentrations were obtained in the RO concentrate.

To achieve a higher concentration of ammonium than with RO, ED was developed. This is an electrically driven membrane technology that enables the transport of ions from the feed to the product, using ion-exchange membranes. ED allows the concentration of ammonium to reach 16–21 g/L [54]. However, research into the application of ED for digestate valorization has not gone beyond the laboratory scale.

Membrane fouling is considered to be the main drawback of membrane technologies, which prevents their large dissemination in full-scale facilities. MD is recognized as a method with less fouling, a high flux, a high mass transfer coefficient and a low operating pressure [31,50]. In this process, the partial pressures of volatile organic matter and ammonia in the liquid are higher than those of water and are preferentially transferred across the membrane. Non-volatile components (such as P) are rejected and concentrated on the feed side. Only gaseous substances can be transported through the membrane. The addition of chemicals or post-treatment is not necessary. N can be condensed directly and recovered as ammonia nitrogen. As MD is a thermally driven separation method, the heat produced in the biogas plants can be used to force the process.

MD requires porous hydrophobic membranes that serve as a barrier, separating the feed (heat side) from the permeate (cold side). They are fabricated of polyvinylidene fluoride (PVDF), polypropylene, and polytetrafluoroethylene, which has the highest hydrophobicity, the lowest surface energy and good chemical stability [31]. In the search for higher ammonia transfer efficiency and lower membrane fouling, membrane modifications have been investigated [72]. Grafting PVDF membranes with FeOOH nanoparticles and fluorosilane led to the fabrication of membranes with superior hydrophobicity. This resulted in no accumulation of foulants for 6 h and an outstanding ammonia recovery of 95.2 ± 2.3% (flux of 3.89 ± 0.18 g/m²·h).

Another method to increase mass transfer in MD is a vacuum membrane distillation (VMD), which was developed for simultaneous N and P recovery from the liquid fraction of the digestate [31]. This resulted in a P rejection of 99% and a flux of polytetrafluoroethylene membrane of 6000 g/(m²·h) at 40 °C at a flowrate of 120 L/h. In the concentrate, the P concentration was five times higher than in the initial digestate. The process involved the partial condensation of the permeated vapor by vacuum on the membrane downstream, and most of the water was collected. The remaining vapor, rich in N, was condensed, and the separation factor for N increased to 114.

The advantages of membrane technology are numerous. These include the selective recovery of nutrients such as N and P, the reuse of water and, thus, the conservation of resources and the minimization of environmental impact, as well as the flexibility of integration into existing waste treatment systems and the lack of chemical additions [73]. The applications range from the production of nutrient-rich fertilizer and high-quality water for reuse to the improvement of digestate quality for soil improvement. Despite these benefits, there are still challenges, such as membrane fouling and clogging, high energy requirements, and relatively high operating costs (4–12 EUR/m³ of digestate), limiting its full-scale applications and driving research to develop more efficient, cost-effective and fouling-resistant membrane technologies [8,28,74]. To mitigate fouling, the membrane process is usually preceded by enhanced solid removal from the digestate. Precipitation/flocculation or more filtration steps are usually employed; however, this increases the costs of the process. Membrane technologies are economically viable in biogas plants, where transportation and distribution costs are high [47]. Because of excessive operating costs, full-scale applications for membrane technologies are available but still limited; however, some commercial pilots have been installed in biogas plants [8,18]. In these plants, vibrating shear has been used to reduce the cleaning frequency; however, there are little data available on the energy consumption of this technology. However, membrane filtration technology is the most established technology to date for the simultaneous recovery of both N and K [18] and is of increasing interest in the EU due to its low environmental impact [8].

To minimize the drawbacks of membrane technologies and enhance nutrient recovery, membrane techniques have been combined with other techniques. One example of this is the combination of ammonia stripping and membrane separation. In this process, ammonia separation by air or steam is replaced by a gas-permeable membrane. In membrane contactors, the ammonia is stripped directly in the membrane [8]. Although ammonia recovery exceeds 95%, this technology is still in the laboratory and pilot test stages.

The combination of membrane filtration and chemical precipitation enabled the simultaneous recovery of N and P. As a result, almost 100% P was recovered by precipitation at an optimum Fe:P molar ratio. Subsequently, N was recovered from P-free digestate by polyelectrolyte-modified NF membranes [75]. In the other combined system, N was recovered as ammonium sulfate with a gas-permeable membrane (first step), and ED recovered P to the solution from which P was precipitated as struvite (second step) [76]. The recovery efficiencies reached 94% (N) and 74% (P), demonstrating the great potential of integrated technologies.

3.2.5. Ion Exchange and Adsorption

The recovery of nutrients from the liquid fraction of digestate is conducted in columns filled with solid sorbents (resins, zeolites and clays), operating in batch or continuous mode. The average extraction recovery reaches 83% N and 100% P [17]. When using zeolites, the most extensively used adsorbent for nutrient recovery, N recovery reached between 71 and 91%, due to the adsorption capacity of 19 g NH₄⁺/kg (natural zeolite) and 21 g NH₄⁺/kg (zeolite modified with Na) [77]. Clinoptilolite was optimized for the recovery of N, P and K from the liquid fraction of digestate by adjusting the optimal nutrient–clinoptilolite ratio [78]. The removal efficiencies were 40–89% (N), 64–80% (P) and 37–78% (K). Increasing the ratio increased the nutrient concentration on the adsorbent, but decreased the removal efficiencies. When using acidic gel cation exchange resin for the N recovery from the digestate pre-treated with filtration and RO of the liquid fraction, a yield of 27.6 g NH₄⁺-N/L resin and an efficiency of 99% were obtained [79].

The advantage of these processes is that the produced biofertilizers are slowly released due to the porosity of the adsorbents, which prevents the fast release of the adsorbed nutrients. The limitations in the application of this technology arise from the competition of foreign ions (zeolites can adsorb metals along with NH₄⁺), the fouling of the pores of the adsorbent bed with the high content of suspended solids, the reduction in the exchange capacity after multiple recovery/regeneration cycles and the costs, which are higher than those of ammonia stripping and struvite precipitation [17,18,54]. As a result, despite the increasing interest in using natural zeolites for ammonia removal from the streams of high N content, the implementation of this technology in full-scale is scarce. An investigation of the process optimization, including recovery/regeneration cycles, at the pilot scale should be performed, because the adsorption of zeolites is currently the most cost-effective option available (depending on zeolite availability) [18].

3.3. Recovery of Organic Matter

3.3.1. Composting: Transforming Digestate into a Valuable Soil Amendment

Composting is a natural and efficient process for decomposing organic matter, including the solid fraction of digestate, into a stable, nutrient-rich soil amendment known as compost. This bio-oxidation process, facilitated by microorganisms such as bacteria and fungi, breaks down organic materials in the presence of oxygen, transforming digestate into not only a lower volume and environmental impact, but also enhancing its agricultural value. The composting of digestate involves preparation and mixing with bulking agents to optimize the carbon-to-nitrogen ratio (C:N ratio) and improve aeration and active composting, where microorganisms degrade organic material while generating heat, followed by a curing phase to stabilize and mature the compost, and finally screening and packaging for use or sale [80].

The advantages of composting are manifold. It adds value to waste by converting digestate into a high-value product, promotes waste reduction and supports circular economy initiatives. Compost significantly improves soil health by improving its structure, water retention and microbial activity, which in turn contributes to healthier soil ecosystems [81]. In addition, composting contributes to the recycling of nutrients, reducing dependence on synthetic fertilizers and promoting sustainable agricultural practices. By facilitating aerobic decomposition, composting effectively reduces methane emissions that would otherwise result from landfill disposal or anaerobic digestion processes.

Digestate from agricultural waste and distillery stillage was found to have a proper structure and C:N ratio for composting; compost was obtained after 51 days of the process [82]. Compost obtained from digestate is widely used in agriculture and horticulture. It serves as a valuable soil additive to improve soil fertility and structure, helps control erosion and suppresses soil-borne diseases through increased microbial activity. However, the process faces challenges such as space requirements, potential odor emissions and the need for effective management to ensure pathogen reduction and material stability. Advances in composting technologies, including in-vessel composting systems, offer controlled environmental conditions, shorter processing times and minimal odor emissions [83]. Taking all these factors into account, it has been estimated that recycling agricultural waste through composting results in about a 45% reduction in the overall cost of managing this waste and the cost of fertilizer use [84].

3.3.2. Vermicomposting: Enhancing Soil Fertility through Earthworms

Vermicomposting stands out as an eco-friendly and efficient method of converting the organic fraction of waste, including digestate, into vermicompost—a nutrient-rich, humus-like material [85]. By using different species of earthworms, this process decomposes organic material and improves its nutrient content and structure. It therefore offers an excellent solution for the management of organic waste and the production of a high-quality soil conditioner. The vermicomposting process involves various steps, such as the pre-treatment of the digestate to regulate moisture content and remove potential toxins, feeding the pre-treated material to earthworms for digestion, harvesting the processed vermicompost and further maturation to ensure stability before application. Earthworms promote aeration conditions and the fragmentation of the organic substrate, thus liquefying the substrate. Among other things, this leads to the release of nutrients such as N, P and K, enriching the digestate. It also increases the activity and diversity of microorganisms that can suppress plant pathogens, reducing the reliance on chemical treatments. Additionally, the humic substances produced improve the soil structure by enhancing water retention and aeration [86].

Vermicompost has a wide range of applications, from soil improvement in gardens and agriculture to a growing medium for seedlings and container gardens, demonstrating its ability to significantly improve soil health and plant growth. In addition, the microbial activity in vermicompost promotes plant resistance to diseases and pests [87]. Despite its many benefits, vermicomposting requires careful management of the environmental conditions to ensure the health of the earthworms and the quality of the compost produced. Challenges such as scaling up the process to handle larger volumes of waste and optimizing the system design for different earthworm species are areas of ongoing research aimed at improving the efficiency and applicability of the process [88].

3.3.3. Biochar Production: Converting Digestate into a Carbon-Rich Soil Enhancer

The production of biochar from digestate is an innovative approach to waste management that converts organic residues from anaerobic digestion into a stable, carbon-rich material that offers significant benefits for soil health and contributes to carbon sequestration [16]. This process, grounded in pyrolysis, not only addresses waste disposal challenges, but also bolsters environmental sustainability and agricultural productivity. In the biochar production, the digestate undergoes drying to minimize moisture content, enhancing its pyrolysis efficiency [89]. The dried digestate is then subjected to high temperatures in an oxygen-limited environment, resulting in the formation of biochar, syngas, and bio-oil [90]. The biochar is subsequently cooled and collected for use or further processing.

The advantages of biochar production are manifold. As a soil amendment, biochar augments soil fertility [91], water retention [92] and microbial activity [93], thereby facilitating improved plant growth and yields. It plays a crucial role in carbon sequestration by stabilizing organic carbon, thus mitigating climate change through reduced greenhouse gas emissions [94]. In addition, the production of biochar offers a sustainable method of managing digestate that reduces the need for landfill disposal and incineration, while limiting the leaching of nutrients and pollutants into groundwater [95].

Due to its special properties, biochar is suitable for various applications in agriculture and environmental management. It promotes soil health by improving the physical properties of the soil and the availability of nutrients [96]. In water treatment, biochar acts as a filtering medium, purifying water by removing impurities and contaminants [97]. When added to compost, biochar not only improves the composting process but also elevates the quality of the resulting compost [98]. Furthermore, the production and application of biochar serve as effective strategies for carbon offsetting, directly contributing to climate change mitigation.

Despite its numerous benefits, the biochar production process faces challenges, including the upfront costs of pyrolysis equipment, the necessity for specialized technical knowledge, and the variability of biochar quality [99]. Current research is directed towards refining production processes, elucidating the long-term impacts of biochar application on soil health and uncovering novel uses for biochar beyond soil amendment, such as in building materials and energy storage solutions [100].

4. Conclusions and Future Perspectives

The conversion of digestate into products such as nutrient-rich fertilizers or biochar is in line with circular economy objectives and provides a sustainable solution for waste management while contributing to soil health and carbon sequestration. These processes promote the valorization of waste and highlight the potential of digestate as a resource rather than a waste. The challenge is to balance technological advancement with economic feasibility and regulatory compliance to ensure that digestate valorization can be effectively integrated into sustainable waste management practices.

Table 2. Recovery methods for digestate management.

Recovery Method	Products Recovered	Benefits	Challenges
Struvite precipitation	Struvite (biofertilizer)	Slow-release fertilizer, reduces dependency on synthetic fertilizers	Initial investment, optimization of operating parameters
Ammonia stripping	Ammonia-based fertilizers	Efficient nitrogen recovery, reduces environmental impact	High energy consumption, pH management
Membrane technologies	Nutrient-rich concentrate, purified water	Efficient nutrient recovery, water reuse	Membrane fouling, high operational costs
Microalgae cultivation	Biofuels, bioplastics	Renewable energy production, sustainable materials	Digestate component inhibition, strain selection
Composting	Compost	Enhances soil health, reduces waste volume	Space requirements, odor emissions
Vermicomposting	Vermicompost	Improves soil fertility, increases microbial activity	Environmental condition management
Biochar production	Biochar	Soil enhancement, carbon sequestration	Upfront costs, quality variability

summarizes the different recovery methods for digestate and highlights the key products recovered, their benefits, and challenges.

The comprehensive study shows that established techniques such as struvite precipitation and ammonia stripping are effective for the recovery of N and P and transform digestate into valuable biofertilizers. Other methods, such as microalgae cultivation, show great promise for using digestate as a nutrient source for the production of biofuels and bioplastics. This innovative approach not only recovers important nutrients, but also contributes to the production of renewable energy and sustainable materials. Composting and vermicomposting have proven to be efficient processes for converting digestate into nutrient-rich soil conditioners that significantly improve soil health and fertility. The production of biochar through pyrolysis is recognized for its dual benefits of improving soil properties and sequestering carbon, helping to mitigate climate change. Pressure and non-pressure membrane technologies are effective in recovering nutrients and treating water from liquid digestate. However, to improve the feasibility of these technologies, issues such as membrane fouling and high operating costs need to be addressed. These findings have significant implications for future research. There is a need for continuous innovation and optimization of these recovery technologies to overcome existing challenges and improve their efficiency and cost-effectiveness. Future research should focus on the integration of different recovery processes to maximize resource recovery and minimize the amount of waste. In addition, research into the environmental and economic impacts of these technologies through life cycle analysis can provide deeper insights into their sustainability. The development of policy frameworks that support the adoption of digestate utilization technologies is also crucial to promote their widespread use. Overall, the study highlights the importance of transforming digestate from a waste product into valuable resources, aligning with circular economy principles and promoting sustainable waste management practices. These advances promise to significantly improve the sustainability of the biogas industry and contribute to greater environmental and economic benefits.