Struvite Precipitation Research Articles

OPTIMIZATION OF ALKALINE–THERMAL HYDROLYSIS TO OBTAIN STRUVITE FROM DIGESTED SLUDGE USING A BOX–BEHNKEN DESIGN: SOLUBILIZATION OF NUTRIENTS AND METALS

In this study, we investigated ways by which to optimize metals and nutrients solubilization from sewage sludge using alkaline–thermal hydrolysis and the Box–Behnken design. We also examined through struvite crystallization the recovery of solubilized nutrients from hydrolyzed liquid and determined the effect of NaOH concentration, the liquid/solid ratio, and temperature on the hydrolysis process. Nutrients solubilization was positively affected by decreasing liquid/sludge ratio and increasing NaOH concentration. Ca, Al, and Zn solubilization increased with increasing temperature. The optimum condition for solubilization of nutrients and metals was 0.7 M NaOH and a 5/1 mL/g liquid/solid ratio at 35 °C. EDS analyses of hydrolyzed sludge obtained under optimum conditions showed that the mass percentage of C, P, Fe, Al, and K decreased compared to that of the digested sludge. Under optimum conditions, the removal efficiencies of NH4+ and PO43- from hydrolyzed liquid by struvite precipitation were 57.43 and 79.22% at a N:Mg:P molar ratio of 1:1:1, and 73.31 and 99.02% at a N:Mg:P molar ratio of 1:1.5:1, respectively. XRD analyses of the dry precipitate showed hazenite in addition to struvite formation at a molar ratio of N:Mg:P of 1:1:1.

Microbial phosphorus removal and recovery by struvite biomineralisation in comparison to chemical struvite precipitation in municipal wastewater

Microbial biomineralisation is attracting significant interest as an innovative process to recover nutrients from wastes. Nevertheless, little is understood about the requirements to form struvite using biological pathways in wastewater and how does this compare with conventional chemical processes. To address this gap, Halobacterium salinarum, Bacillus pumilus, Brevibacterium antiquum, Myxococcus xanthus and Idiomarina loihiensis were grown in wastewater to explore the relationships between cell growth, nutrients levels and properties of the recovered precipitates. The microorganisms were capable of removing ortho-phosphate (PO4-P) from municipal wastewater at concentrations ranging from 5.4 to 62.4 mg PO4-P/L. Visible crystals of biological struvite (bio-struvite) (identified by morphology XRD and elemental analysis), were observed at PO4-P ≥ 19.7 mg/L, compared to chemical struvite precipitation at 62.4 mg/L PO4-P (with pH adjustment). The initial nutrient concentrations presented a strong positive correlation with bio-struvite production yields and crystal size distribution. B. antiquum distinguished itself by relatively stable PO4-P removal (68–97%) independent of the initial nutrient concentration, with effluents containing as low as 1 mg PO4-P/L. The recovered bio-struvite presented high purity with low heavy metal contents, meeting regulations for inorganic fertiliser. The microbial processes for phosphorus (P) recovery as bio-struvite presented several key advantages: bio-struvite crystals were released to the wastewater and recoverable by filtration at PO4-P ≥ 19.7 mg/L, no need to adjust pH, bio-struvite crystals had purity equivalent to 11.8–12.3% P and low heavy metal content, which was similar or better than that of chemical struvite (12.6% P). This study validates bio-struvite’s relevance for low nutrient concentrations.

Treatment of Liquid Fraction of Digestate by Integrated Process Struvite Precipitation—Forward Osmosis

The research undertaken in this paper was aimed at determining the effect of struvite precipitation, one of the potential products that can be obtained during digestate management, on the performance of the non-pressurized membrane process—forward osmosis (FO). The effect of using an integrated struvite precipitation—forward osmosis process to treat the digestate liquid on the changes in the properties of organic substances present in the treated solution (particle size distribution, ζ-potential) was analysed as well. The study was conducted for the liquid fraction of municipal waste biogas plant digestate. The obtained results demonstrate the suitability of this process for recovering water from liquid digestate. It was found that forward osmosis conducted for a digestate pre-treated by chemical struvite precipitation leads to higher water flux values and increased salt concentration in the receiving solution (from 0.5 to 3 mol/dm3 NaCl). There is practically no concomitant infiltration of organic substances into the receiving solution. Therefore, the use of 3 mol/dm3 NaCl as a draw solution results in the recovery of the highest volume of water per unit of time. An analysis of the particle size distribution shows that the removal of the macromolecular fraction of organic compounds from the digestate mainly takes place simultaneously with the chemical precipitation of struvite. It was found that an increase in the concentration of the draw solution, which allows for greater water recovery, resulted in the aggregation of the concentrated organic molecules.

Pressure-driven membrane nutrient preconcentration for down-stream electrochemical struvite recovery

Nutrient recovery from wastewater is a sustainable solution to combat the harmful release of nutrients to the environment. Here, we investigate nutrient recovery from wastewater by pre-concentration of wastewater nutrients using pressure-driven membranes prior to downstream electrochemical nutrient precipitation. When using electrochemical struvite precipitation, a higher nutrient concentration leads to a higher precipitation efficiency. Therefore, we investigate the performance of commercial nanofiltration (NF) and reverse osmosis (RO) membranes with different polymer chemistry and molecular weight cut-offs (MWCO) and discuss the membrane selection based on design goals and wastewater compositions for maximum nutrient recovery efficiency. Our results indicated that the Alfa membrane, a polyamide thin film composite (PA-TFC) NF membrane with 300 Da MWCO has the highest concentration factor in phosphorus (P) preconcentration among all the membranes studied due to the high flux and removal efficiency. BW30LE (PA-TFC, 100 Da), Synder (PA-TFC, 100–250 Da) and NF90 (PA-TFC, 200–400 Da) achieved a high concentration factor for nitrogen (N) recovery. NF90 and Synder NF membranes are able to achieve a nitrogen concentration factor similar to BW30LE RO membrane at much lower energy consumption. A multistage membrane system design is suggested using the selected membranes for effective nutrient recovery. Life cycle assessment (LCA) was conducted to characterize the environmental impact of the multistage membrane nutrient recovery system. Among the different process configurations of the selected membranes, key trends included: 1) environmental impacts across most categories increased as the number of membranes increased, and 2) as the amount of fertilizer substitute that could be produced in a given configuration increased, total environmental impacts decreased with some exceptions.

Ammonium-nitrogen recovery as struvite from swine wastewater using various magnesium sources

Struvite (MgNH4PO4·6H2O) precipitation is widely used for nutrient recovery from swine wastewater. Additional magnesium source is required to produce struvite for nitrogen recovery due to the limited magnesium content in swine wastewater. The present study investigated the optimized condition for treating swine wastewater and producing struvite crystals using swine slurry, commercial MgCl2, and seawater as magnesium sources. The results showed that swine slurry as an alternative source is feasible both technically and economically. The struvite precipitation process using swine slurry at pH 9.0 and [P]0/ [Mg]0/ [N]0 M ratio = 1.2/1.0/1.0 achieved removal efficiencies of 82.7 %, 83.1 %, and 92.7 % for nitrogen, phosphorus, and magnesium, respectively. X-ray diffraction analysis validated the solid product consisted of struvite. Further investigations revealed the harvested precipitate had a struvite content of 72.7 % with low heavy metal impurity, which could be used as fertilizer. Nutrient recovery from swine manure is a future driver for the economic and environmental sustainability of agricultural and wastewater treatment processes.

Precipitation of struvite using MgSO4 solution prepared from sidestream dolomite or fly ash

Struvite (NH4MgPO4∗6H2O) is a slow-release fertilizer produced from phosphorus and nitrogen-containing wastewater in the presence of Mg salts. Commercial Mg salts are the single most significant cost of struvite precipitation. In this study, H2SO4 formed as an industrial sidestream was used to prepare MgSO4 solution from waste dolomite (DOL) and fly ash (FA). MgSO4 solution was then used to precipitate struvite from a synthetic (NH4)2HPO4 solution and from actual industrial process waters. The best results were obtained with real process waters where over 99% of phosphate and about 80% ammonium removals were achieved with both MgSO4 solutions after 30 min of reaction time. A higher molar ratio between Mg and P improved the phosphate removal efficiency, especially with DOL-based MgSO4 solutions; however, it had no practical effect on ammonium removal. The struvite content of precipitates was 75.49% with an FA-based chemical and 60.93% with a DOL-based chemical; other valuable nutrients (Ca, K, S, Fe, Mn, and Cl) were captured in the precipitates. The results indicate that both sidestream-based reagents perform well in struvite precipitation and that the formed precipitates could be used as fertilizers.

Improvement of biogas production and quality by addition of struvite precipitates derived from liquid anaerobic digestion effluents of palm oil wastes

When biogas is produced, the liquid anaerobic digestion effluents (LADE) still contain pollutants. Struvite precipitation is a technique for reducing pollutants as well as recovering useful elements such as nitrogen, phosphorus, and magnesium. This study aims to precipitate struvite from LADE and use it as an additive to palm oil mill effluent (POME) feedstocks in subsequent biogas production systems (batch and continuous operational modes). Struvite precipitation was simply performed using NaOH at a pH range of 8–10 without external sources of magnesium and phosphorus. The result showed that the optimum pH for struvite precipitation was 9, giving the highest yield and purity of struvite. The addition of 5% struvite to POME feedstock improved the performance of biogas production in the continuous stirred tank reactor (CSTR) in terms of stability, high COD removal efficiencies, increased methane yields, and decreased H2S concentrations. The greatest methane yield of 509.8 mL/g-VS and the methane concentration of 65.10% were obtained in the CSTR. The significant decrease in H2S concentration in biogas can increase the energy content of biogas, save on the cost of H2S removal, reduce corrosion, and extend the operating time of gas turbines used for electricity production. The results of this study emphasize the role of integrating struvite precipitation with continuous biogas production in improving the process sustainability and economy. Struvite precipitation from LADE diminishes the polluted wastewater, decreases the long-term process problems of fouling and clogging caused by struvite, and improves the performance of the biogas production.

Struvite Production from Dairy Processing Waste

Food security depends on sustainable phosphorus (P) fertilisers, which at present are mostly supplied from a finite rock phosphate source. Phosphate (PO43−) and ammonium (NH4+) in dairy processing wastewater can be recovered as struvite (Mg + NH4+ + PO43− 6H20), a nutrient rich mineral for fertiliser application. The objectives of this study were to (1) quantify the effects of, pH, temperature and Mg: PO43− dosing rates on nutrient (PO43− and NH4+) removal and struvite precipitation from post anaerobic digested dairy processing wastewater, and (2) co-blend different dairy processing wastewaters to improve the reactant stoichiometry of NH4+ and PO43− for optimal struvite recovery and NH4+ removal. Phosphate removal (>90%) and struvite production (>60%) was achieved across a range of synthesis conditions, and was significantly impacted by pH as determined by response surface modelling. A combination of disproportionate molar ratios of PO43− and NH4+, presence of calcium and the apparent mineralisation of organic N, resulted in co-precipitation of hydroxyapatite and elevated levels of residual aqueous NH4+. In the second phase of this study, struvite was successfully precipitated and NH4+ removal was improved (~17%) however, higher concentrations of calcium in the wastewater blends resulted in greater hydroxyapatite co-precipitation (up to 30%). While struvite was the desired product in this study the formation of multiple heterogenous P-rich products (struvite and hydroxyapatite) has the potential to improve P recovery from dairy processing wastewaters and produce a fertiliser blend with amenity and value in agricultural systems.

Estimating environmental and societal impacts from scaling up urine concentration technologies

There is a growing trend for nutrient recovery from wastewater as part of the transition to a circular economy. Most nutrients in household wastewater originate from urine and one way to facilitate reuse of these nutrients is to concentrate the urine into fertiliser products. Urine concentration technologies are still in the development phase and not implemented at scale. The aim of this study was to provide guidance to technology developers and policymakers by assessing the environmental and societal impacts of urine concentration technologies. In particular, it includes practical aspects such as worker safety, space availability and local fertiliser needs that have not been included in previous studies. Future scenarios on implementing three different urine concentration technologies (alkaline dehydration, nitrification-distillation, ion-exchange with struvite precipitation) in a planned residential area in Malmö, Sweden, were developed. The technologies were evaluated using multi-criteria assessment (MCA), with environment, technical, economic and health sustainability criteria derived from the Sustainable Development Goals (SDGs). It was found that all urine concentration technologies performed well against many of the sustainability criteria examined and can contribute to achieving SDGs, especially regarding nitrogen recovery. Specific areas for further development were identified for each technology. An impact assessment on scaling up demonstrated that nitrogen emissions to surface water were significantly reduced when more than 60% of urine in Malmö city was subjected to urine concentration. Nitrogen and phosphorus recovered from recycling only 15–30% of urine in Malmö could supply 50% of Malmö municipality's fertiliser demand.

Phosphorus recovery alternatives for sludge from chemical phosphorus removal processes – Technology comparison and system limitations

Several approaches have recently been proposed in the literature for P recovery after metal precipitation, but a robust comparison of these approaches in a techno-economic framework is still lacking. Five phosphorus recovery methods using sewage sludge or sludge ash as feed material were compared based on their specific operational recovery cost by scaling the processes to unified size and operational conditions. The selected technologies were (1) wet leaching + struvite precipitation, (2) magnetic vivianite separation, (3) sludge melt gasification, (4) the thermochemical sodium sulfate process, and (5) white phosphorus recovery. The analyses were based on the literature values in addition to a plant-wide model used to estimate chemical consumption and phosphorus and metal-related sludge properties. The technologies were assessed by operational cost, end-product quality, recovery efficiency, and technology maturity. The choice of the recovery process is dependent on the precipitant used in the wastewater treatment processes. Technologies using sewage sludge ash, (4) and (5), had the highest recovery efficiency, technical maturity, and product quality, but they require mono-incineration. Technology (3) had better recovery efficiency than (1), but the end-product had much lower P content. Technology (2) had the lowest recovery efficiency among all the compared technologies, however, it produced an end-product with the second highest P content. The operational costs were calculated for energy and chemical costs for same scale and operational conditions. The specific recovery cost ranges from 6 to 38 €/kgPrecovered.

Phosphorus harvesting from fresh human urine: A strategy of precisely recovering high-purity calcium phosphate and insights into the precipitation conversion mechanism

Phosphorus (P) harvesting from source-separated urine to optimize the overall nutrient loop is one of the most appealing benefits and is a global research interest in wastewater management and treatment. However, current P precipitation is mainly oriented to struvite, which is limited by the issues such as relatively low product purity and high cost of Mg source. Distinguished from previous conventional struvite precipitation, the strategy of precisely harvesting P from fresh human urine as high-purity calcium phosphate was first proposed in this study. This enhanced strategy can optimize P harvesting performance and product purity by simply regulating the consumption of calcium-based materials via model simulation and experimental validation. The thermodynamic model was constructed to probe the precipitation conversion mechanism, and visually predict the component and yield for products under various operating conditions. Batch experiments were conducted to investigate P recovery performance as a function of initial Mg2+ concentration, initial pH level, as well as degree of urine hydrolysis. Moreover, the alternative dosing scheme with different calcium salts and alkali was presented, diversifying the options for efficient P recovery. The results showed that, from the perspective of acidic storage for fresh urine, P recovery can be boosted along with eliminating urine hydrolysis. In urine with an initial pH=2.0, P can be completely recovered and purity for calcium phosphate can be optimized to 100% within a Ca/P ratio range of 1.67–2.3. Overall, this work is of great significance for precisely and efficiently harvesting P from urine and provides an integrated strategy for P resource recovery from urine.

Prospective Life Cycle Assessment and Cost Analysis of Novel Electrochemical Struvite Recovery in a U.S. Wastewater Treatment Plant

Nutrient recovery in domestic wastewater treatment has increasingly become an important area of study as the supply of non-renewable phosphorus decreases. Recent bench-scale trials indicate that co-generation of struvite and hydrogen using electrochemical methods may offer an alternative to existing recovery options utilized by municipal wastewater treatment facilities. However, implementation has yet to be explored at plant-scale. In the development of novel nutrient recovery processes, both economic and environmental assessments are necessary to guide research and their design. The aim of this study was to conduct a prospective life cycle assessment and cost analysis of a new electrochemical struvite recovery technology that utilizes a sacrificial magnesium anode to precipitate struvite and generate hydrogen gas. This technology was modeled using process simulation software GPS-X and CapdetWorks assuming its integration in a full-scale existing wastewater treatment plant with and without anaerobic digestion. Struvite recoveries of 18–33% were achieved when anaerobic digestion was included, with a break-even price of $6.03/kg struvite and $15.58/kg of hydrogen required to offset increased costs for recovery. Struvite recovery reduced aquatic eutrophication impacts as well as terrestrial acidification impacts. Tradeoffs between benefits from struvite and burdens from electrode manufacturing were found for several impact categories.

Bittern-impregnated sisal: An alternative magnesium source for phosphorus recovery through struvite precipitation?

Looking for cheaper and more transportable alternative magnesium(Mg2+) sources to envisage the full-scale application of downstream transfer of P recovery through struvite precipitation, sisal fibres were impregnated with bittern:water (B:W) ratio solutions 1:4; 1:5; 1:7 and 1:10. Two methods were attempted: the one-step method with a simultaneous Mg2+ release and struvite precipitation, and the two-step method with release and precipitation made separately. Sisal1:5 showed the highest recovery yield regardless of the method applied. The issue with the one-step method is that part of the crystals formed directly on the fibre surface, making their recovery harder. The two-step method using the sample impregnated with a 1:5 ratio resulted in highly pure struvite crystals, which are larger than those obtained with MgCl2·6H2O. Moreover, this method allows the retention of part of Cl− in the fibres instead of releasing it into the solution, and crystals do not form over the fibres. This work shows that the use of Mg2+-impregnated materials is a poor alternative for urine with a high concentration of P (>600 mg P L−1) because it shows a poorer performance, 50 % P recovery, when compared to the direct use of bittern (>99 % P recovery yield). However, the higher struvite purity and the retention of part of the Cl− make Mg2+-impregnated materials a potential solution to test in lower P concentrations (<300 mg P L−1). This study also opens several research opportunities regarding the application of Mg2+-impregnated material in P recovery through struvite precipitation.

Electrochemical Struvite Precipitation Enhanced by an Amelogenin Peptide for Nutrient Recovery

Electrochemical Struvite Precipitation Enhanced by an Amelogenin Peptide for Nutrient Recovery

Phosphorus retention and agronomic efficiency of refined manure-based digestate—A review

Digestate, a by-product from anaerobic digestion of organic materials such as animal manure, is considered a suitable plant fertilizer. However, due to its bulkiness and low economic value, it is costly to transport over long distances and store for long periods. Refinement processes to valorize digestate and facilitate its handling as a fertilizer include precipitation of phosphorus-rich mineral compounds, such as struvite and calcium phosphates, membrane filtration methods that concentrate plant nutrients in organic products, and carbonization processes. However, phosphorus retention efficiency in output products from these processes can vary considerably depending on technological settings and characteristics of the digestate feedstock. The effects of phosphorus in plant fertilizers (including those analogous or comparable to refined digestate products) on agronomic productivity have been evaluated in multiple experiments. In this review, we synthesized knowledge about different refinement methods for manure-based digestate as a means to produce phosphorus fertilizers, thereby providing the potential to increase phosphorus retention in the food production chain, by combining information about phosphorus flows in digestate refinement studies and agronomic fertilizer studies. It was also sought to identify the range, uncertainty, and potential retention efficiency by agricultural crops of the original phosphorus amount in manure-based digestate. Refinement chains with solid/wet phase separation followed by struvite or calcium phosphate precipitation or membrane filtration of the wet phase and carbonization treatments of the solid phase were included. Several methods with high potential to extract phosphorus from manure-based wet phase digestate in such a way that it could be used as an efficient plant fertilizer were identified, with struvite precipitation being the most promising method. Synthesis of results from digestate refinement studies and agronomic fertilizer experiments did not support the hypothesis that solid/wet separation followed by struvite precipitation, or any other refinement combination, results in higher phosphorus retention than found for unrefined digestate. Further studies are needed on the use of the phosphorus in the solid phase digestate, primarily on phosphorus-rich soils representative of animal-dense regions, to increase understanding of the role of digestate refinement (particularly struvite precipitation) in phosphorus recycling in agricultural systems.

Combined Electrodialysis and Peptide-Directed Struvite Recovery System

Recovery of N and P as struvite (MgNH4PO4 6H2O) has gained interest in recent years as both a flow assurance measure and as a sustainable P source for fertilizer. Electrochemically precipitated struvite offers advantages compared to conventional precipitation methods by reducing the need for expensive additives and has the potential for coupling with renewable energy sources. However, high energy inputs to overcome the solution resistance of most wastewaters limit the feasibility of the technology. In this work, a coupled electrodialysis-struvite recovery system is presented to concentrate the wastewater (producing a separate desalinated stream) while producing struvite in a single unit operation. Furthermore, a common dental peptide was explored as a method to control and increase growth of struvite crystals at neutral pH. Cysteine-capped peptide was attached to a gold mesh substrate in a range of loadings (5-27ug/cm2). Chronoamperometry was conducted by first placing the peptide-loaded mesh in the struvite reactor filled with synthetic wastewater at pH 7.1 and then applying a range of potentials (0.5-0.99V) to the magnesium anode. White precipitates were observed and characterized as struvite by XRD. Struvite crystal morphology was investigated by SEM. We report that a peptide loading of 19 ug/cm2 led to a nearly 14% increase in struvite precipitated compared to bare Au with no peptide attached. Furthermore, longer, dendritic struvite crystals formed in the presence of peptide compared to orthorhombic crystals which formed without peptide. These results indicate that the peptide alters local supersaturation, leading to an increase in directional crystal growth. By demonstrating the utility of peptides for increasing struvite precipitation at neutral pH, our findings improve the viability of electrochemical struvite precipitation for a wider range of wastewater compositions and highlights how peptides can modulate crystal growth.

Application of Pulsed Potential Waveform in Electrochemical Phosphate Recovery As Struvite from Wastewater

Phosphate production is apprehended to peak as early as 2030, and post-peak, phosphate production, and thus food supply, will be limited by dwindling resources, strained economics, and reduced rock quality, with remaining resources expected to last 50-300 years. The limited supply of phosphate demands P conservation by changing the food production system, from fertilization to wastewater treatment. Stuvite (MgNH4PO4.6H2O) has been recognized as a high-value, slow-release, effective fertilizer that can be obtained from both solid (e.g., food, animal, and human waste) and liquid wastes (e.g., wastewater from industrial, municipal, and agricultural operations). The goal of our research is to recover struvite from wastewater using an alkaline electrochemical reactor with a magnesium alloy anode and a cathode constructed of standard catalysts for reducing water to hydrogen gas and hydroxide. Over time with successive reactor batches, the magnesium electrode used to generate magnesium cations in solution and precipitate struvite becomes severely fouled, which in turn increases energy demand and decrease struvite fertilizer recovery. A dynamic, pulsed voltage waveform, as opposed to a constant voltage, causes a stress-strain-like effect at the magnesium electrode surface, thereby preventing or reducing the amount of struvite precipitate that would attach to the electrode. In an aqueous solution of ammonium dihydrogen phosphate (NH4H2PO4, 10 mM) at neutral pH, a significant 50% increase in magnesium release is observed compared to constant voltage applications, resulting in a 30% increase in phosphate recovery. Studies of the recovered precipitate using X-ray diffraction and scanning electron microscopy (SEM) indicate the presence of struvite.

Sewage Sludge Management at District Level: Reduction and Nutrients Recovery via Hydrothermal Carbonization

In this study, two scenarios of a municipal wastewater treatment plant (WWTP) are presented, which include the integration of the hydrothermal carbonization (HTC) process into the sludge line as a post-treatment of the anaerobic digestion (AD) process. The objective of the simulation is to investigate the performances of AD + HTC treatment to reduce sludge production and improve nutrient and energy recovery. For this purpose, the scheme of an under-construction WWTP was considered, named Trento 3 (Trento, Italy) and with a treatment capacity of 300,000 PE. In the first scenario, the HTC process was fed with thickened sludge from the Trento 3 WWTP, while in the second scenario, dewatered sludge from other local WWTPs was also used as feedstock for the HTC process. Both scenarios allowed to obtain a considerable sludge reduction ranging from 70 to 75% with a notably increase in the biogas production up to 47%, due to the recycling of HTC liquor (HTCL) to the anaerobic digester. Considering nutrients recovery, all the phosphorus and nitrogen present in the HTCL could be used for struvite precipitation with an average yearly gain of 1 million euros. Moreover, the introduction of HTC in the Trento 3 WWTP could allow a reduction in the sludge management costs of up to 2 M€/year.Graphical

Ammonia recovery from anaerobic digestate: State of the art, challenges and prospects

Nitrogen-containing wastewater and organic wastes are inevitably produced during human activities. To reduce nitrogen pollution, much energy has been used to convert ammonia nitrogen into nitrogen gas through biological nitrogen removal method. However, it needs to consume high energy again during industrial nitrogen fixation, which give rise to massive greenhouse gas (GHG) emissions. Therefore, ammonia recovery from organic wastes has attracted much attention in recent years. In this review, the advantages and disadvantages of ammonia stripping, membrane separation and struvite precipitation are discussed firstly. The ammonia stripping mechanisms, influencing factors, mass transfer process, and the latest innovative ammonia stripping techniques from the anaerobic digestate of organic wastes are critically reviewed. Additionally, a comprehensive economic analysis of different ammonia removal or recovery processes is carried out. The challenges and prospects of ammonia recovery are suggested. Ammonia recovery is of great significance for promoting nitrogen cycle, energy saving and GHG emission reduction.

Digestate Management and Processing Practices: A Review

The implementation of sustainable agro-energy systems that integrate crop, livestock, and bioenergy production is attracting increasing interest from farmers. Livestock produces large amounts of animal manure which can serve as organic fertilizer for crops and pasture growth. However, the nutrients contained in manure can adversely affect air, water, and soil quality and pose a public health risk if not handled properly. Existing manure management practices vary widely on a global scale. Researchers are striving to identify appropriate manure management practices with the aim of environmental protection. Anaerobic digestion of manure and subsequent digestate (DG) processing technologies have been proposed to stabilize manure so that it can be safely used for land applications. DG, which represents digested substrate removed from the anaerobic reactor after recovery of biogas, is a rich source of N, P, K, and S, various micronutrients, and organic matter, the addition of which to the soil can stimulate soil microbial biomass metabolic activities thus improving soil ecosystem function. However, the optimal fertilization properties of DG can be lost if it is neither fully stabilized nor contains biodegradable materials. To overcome these problems, various processing technologies can be used to convert DG into value-added by-products. Composting has been proposed as one such preferred post-treatment that can convert DG into mature, stable, safe, humus- and nutrient-rich compost. Other processing technologies such as thermal drying, gasification, hydrothermal carbonization, pyrolysis, membrane filtration, struvite precipitation, ammonia stripping, and evaporation have also been proposed for DG processing and nutrient recovery from DG. The objective of this review paper was to provide an overview of the current state of the art in DG management regulations and practices and to provide an update on the various processes that have been developed to meet DG stabilization requirements, with a focus on composting as one of the preferred solutions.