Characterization and prediction of organic nitrogen biodegradability during anaerobic digestion: A bioaccessibility approach

Retention time and organic loading rate as anaerobic co-digestion key-factors for better digestate valorization practices: C and N dynamics in soils

The growing use of anaerobic co-digestion (AcoD) in processing organic waste has led to a significant digestate production. To effectively recycle digestate back into soils, it is crucial to understand how operational variables in the AcoD process influence the conversion of organic matter (OM). To address this, a combination of biochemical fractionation and various soil incubation tests were employed to assess the stability of OM in digestates generated from anaerobic continuous reactors fed with a food waste-hay mixture and operating at different hydraulic retention times (HRT) and organic loading rates (OLR). This study revealed that digester performance and operating parameters impacted carbon dynamics in soils. A decrease in the carbon mineralization in soils when increasing the HRT was reported (48 ± 4 % for 70 days compared to 59 ± 1 % for 42 days). Specific HRT and OLR values were found to be linked to carbon accessibility and complexity, confirming that longer HRT lead to higher OM removal and increased complexity in soluble OM, despite minor discrepancies in relative carbon distribution. Furthermore, comparable rates of nitrogen mineralization in soils were observed for all digestates, consistent with the accessibility of nitrogen from the particulate OM. Nevertheless, AcoD converted substrates with the potential to immobilize nitrogen in soils into fast-acting fertilizers. In summary, this study underscores the importance of controlling the AcoD performances to evaluate the suitability of digestates for sustainable agricultural practices.

Study on the mechanism of action of methane production by co-fermentation of sludge and lignite

To improve the methanogenic efficiency of lignite anaerobic fermentation and explore innovative approaches to sludge utilization, a co-fermentation technique involving lignite and sludge was employed for converting biomass into biomethane. Volatile suspended solids were introduced as a native enrichment of the sludge and mixed with lignite for fermentation. The synergistic fermentation mechanism between sludge and lignite for biomethane production was analyzed through biochemical methane potential experiments, measurement of various parameters pre- and post-fermentation, observation of bacterial population changes during the peak of reaction, carbon migration assessment, and evaluation of rheological characteristics. The results showed that the addition of sludge in the anaerobic fermentation process improved the microorganisms' ability to degrade lignite and bolstered biomethane production. Notably, the maximum methane production recorded was 215.52 mL/g-volatile suspended solids, achieved at a sludge to coal ratio of 3:1, with a synergistic growth rate of 25.37%. Furthermore, the removal rates of total suspended solids, and total chemical oxygen demand exhibited an upward trend with an increasing percentage of sludge in the mixture. The relative abundance and activity of the methanogens population were found to increase with an appropriate ratio of sludge to lignite. This observation confirmed the migration of carbon between the solid-liquid-gas phases, promoting enhanced system affinity. Additionally, the changes in solid-liquid phase parameters before and after the reaction indicated that the addition of sludge improved the system's degradation capacity. The results of the study hold significant implications in realizing the resource utilization of sludge and lignite while contributing to environmental protection endeavors.

Bioelectrochemically-assisted ammonia recovery from dairy manure

The sustainability of direct land application of dairy manure is challenged by significant nutrient losses. Bioelectrochemical systems for ammonia recovery offer a manure management strategy that can recover both ammoniacal and organic nitrogen as a stable ammonia fertilizer. In this research, a microbial fuel cell (MFC) was used to treat two types of dairy manure under a variety of imposed anode compartment conditions. The system achieved a maximum coulombic efficiency of 20 ± 18 % and exhibited both COD and total nitrogen removals of approximately 60 %. Furthermore, the MFC showed a maximum organic nitrogen removal of 73.8 ± 12.1 %, and no differences in organic nitrogen (orgN) removal were detected among different conditions tested. Decreasing concentrations of anolyte ammonia nitrogen coupled with the observed orgN removal from the anolyte indicate that the MFC is effective at recovering orgN in dairy manure as ammoniacal nitrogen in the catholyte. Additionally, ion competition between NH4+ and other relevant cations (Na+, K+, and Mg2+) for transport across the CEM was investigated, with only K+ showing minor competitive effects. Based on the results of this research, we propose three key processes and two sub-processes that contribute to the successful operation of the MFC for nitrogen recovery from dairy manure. Bioelectrochemical systems for nitrogen recovery from dairy manure offer a novel, robust technology for producing a valuable ammonia nitrogen fertilizer, a thus far untapped resource in dairy manure streams.

Effect of operational parameters on the performance of an anaerobic sequencing batch reactor (AnSBR) treating protein-rich wastewater

Treating protein-rich wastewater using cost-effective and simple-structured single-stage reactors presents several challenges. In this study, we applied an anaerobic sequencing batch reactor (AnSBR) to treat protein-rich wastewater from a slaughterhouse. We focused on identifying the key factors influencing the removal of chemical oxygen demand (COD) and the settling performance of the sludge. The AnSBR achieved a maximum total COD removal of 90%, a protein degradation efficiency exceeding 80%, and a COD to methane conversion efficiency of over 70% at organic loading rates of up to 6.2 g COD L-1 d-1. We found that the variations in both the organic loading rate within the reactor and the hydraulic retention time in the buffer tank had a significant effect on COD removal. The hydraulic retention time in the buffer tank and the reactor, which determined the ammonification efficiencies and the residual carbohydrate concentrations in the reactor liquid, affected the sludge settleability. Furthermore, the genus Clostridium sensu stricto 1, known as protein- and lipids-degraders, was predominant in the reactor. Statistical analysis showed a significant correlation between the core microbiome and ammonification efficiency, highlighting the importance of protein degradation as the governing process in the treatment. Our results will provide valuable insights to optimise the design and operation of AnSBR for efficient treatment of protein-rich wastewater.

Predicting the Stability of Organic Matter Originating from Different Waste Treatment Procedures

Recycling organic wastes into farmland faces a double challenge: increasing the carbon storage of soil while mitigating CO₂ emission from soil. Predicting the stability of organic matter (OM) in wastes and treatment products can be helpful in dealing with this contradiction. This work proposed a modeling approach integrating an OM characterization protocol into partial least squares (PLS) regression. A total of 31 organic wastes, and their products issued from anaerobic digestion, composting, and digestion-composting treatment were characterized using sequential extraction and three-dimension (3D) fluorescence spectroscopy. The apportionment of carbon in different fractions and fluorescence spectra revealed that the OM became less accessible and biodegradable after treatments, especially the composting. This was proven by the decrease in CO₂ emission from soil incubation. The PLS model successfully predicted the stability of solid digestate, compost, and compost of solid digestate in the soil by using only the characterized variables of non-treated wastes. The results suggested that it would be possible to predict the stability of OM from organic wastes after different treatment procedures. It is helpful to choose the most suitable and economic treatment procedure to stabilize labile organic carbon in wastes and hence minimize CO₂ emission after the application of treatment products to the soil.

Evaluating the impact of substrate addition for anaerobic co-digestion on biogas production and digestate quality: The case of deinking sludge

To reduce greenhouse gas emissions from organic waste, anaerobic digestion has created new opportunities for energy and nutrient recovery from these wastes. However, the use of certain organic wastes in anaerobic digestion is limited due to their atypical physicochemical characteristics (e.g. unbalanced carbon to nitrogen ratio, high ash concentration). Deinking sludge is a residue from the paper recycling industry and is one of such substrates. This study aims at evaluating the impact of deinking sludge (DS) addition into a conventional co-digestion mixture on methane production and digestate quality. To this end, an integrated method was proposed, combining the analysis of physicochemical and biodegradability characteristics with parsimonious modeling using the SYS-Metha tool. The measured characteristics of the deinking sludge showed that its potential use in mono-digestion conditions is very limited. When co-digested with food waste and municipal sludge, no significant synergies or antagonisms were found. Based on these experiments, model simulations were executed to determine the optimal conditions for co-digestion with food waste and municipal sludge. A maximum of 22% of deinking sludge on a fresh mass basis can be added into a co-digestion mixture to achieve proper wet anaerobic digestion conditions. Regarding digestate quality, the addition of DS reduced nutrient and contaminants concentrations, which have an impact on digestate management, particularly for land application. Overall, the proposed methodology in this study allows determining optimal co-digestion mixtures and highlighted the limits needing further investigation under pilot/real conditions.

Simultaneous recovery of nutrients and improving the biodegradability of waste algae hydrothermal liquid

The ever-increasing algae biomass due to eutrophication brings an enormous destruction and potential threat to the ecosystem. Hydrothermal carbonization (HTC) is a potential means converting algae to value added products such as sustainable bioenergy and biomaterials. However, the waste aqueous phase (AP) produced during the HTC of algae biomass needs to be treated carefully in case of the second pollution to environment. In this study, a model microbe (E. coli) was adopted for the microbial pretreatment of AP, by which the bioavailability of AP could be improved, and the nutrients could be reclaimed though struvite precipitation. Three-dimensional fluorescence spectra and GC-MS results illustrated that E. coli pretreatment could convert a large number of organic nitrogenous compounds to ammonia nitrogen by degrading aromatic protein substances and deaminating nitrogenous heterocyclic compounds. Afterwards, a serious of characterizations confirmed that 81.13% of ammonia nitrogen could be recovered as struvite though precipitation. Life cycle assessment indicates the cost of the two-step treatment process was much lower than that of conventional wastewater treatment processes, and is beneficial to environment. This work provides an environment-friendly strategy for the comprehensive utilization of algae, which may contribute to alleviating the algal disasters and bring certain economic benefits though algal treatment.

Use of fermentation processes for improving the dissolution of phosphorus and its recovery from waste activated sludge

Recycling phosphorus from waste activated sludge has attracted a lot of interest to tackle the problem of phosphorus stocks depletion and the increase in food demand. In this study, the use of fermentation processes was investigated to enhance phosphorus dissolution from waste activated sludge to improve its recycling. Two fermentation processes, bioacidification and dark fermentation, were used on two different sludges fermented with wheat starch syrup in continuous operating conditions. Hydrogen yield from the co-substrate fermentation with waste activated sludge reached 3.9 mmolH₂.gCOD_cosubstrate^-1 yield during dark fermentation process and was negligible during bioacidification. Dissolved phosphorus in the waste activated sludge increased by 68% during bioacidification and by 43% during dark fermentation. In both processes, phosphorus dissolution was accompanied by iron, calcium and magnesium dissolution. Results show that fermentation enhances phosphorus dissolution in waste activated sludge to improve its recovery along with hydrogen and organic acids.

Prediction of organic matter accessibility and complexity in anaerobic digestates

Further characterization to properly assess the fate of organic matter quality during anaerobic digestion and organic carbon mineralization in soils is required. Organic matter quality based on its accessibility and complexity was employed to successfully classify 28 substrate/digestate pairs through principal components and hierarchical clustering analysis. The two first components explained 58.02% of the variability and four main groups were separated according to the feedstock type. A decrease in the accessibility (16-66%) and an increase in the complexity (34-98%) of the most accessible fractions was noticed. Besides, an increase of non-biodegradable compounds (17-66%) was globally observed after anaerobic digestion. The observed trends in the conversion of organic matter during anaerobic digestion have allowed to fill the gap in the modeling of the anaerobic digestion process chain. Indeed, partial least squares regressions have accurately predicted the organic matter quality of digestates from their inputs (R² = 0.831, Q² = 0.593) although the digester operational conditions (temperature and hydraulic retention time) were non-explicative enough. As a novel approach, the predicted digestate quality was used to feed a partial least squares regression model previously developed to predict organic carbon mineralization in soil. The combined models have predicted experimental organic carbon mineralization in soil (R² = 0.697) with a model quality similar to the model for organic carbon mineralization in soil (R² = 0.894). This is the first study that has successfully conceived an additional step in the prediction of organic matter fate from raw substrate before anaerobic digestion to soil carbon mineralization.

A simple mass balance tool to predict carbon and nitrogen fluxes in anaerobic digestion systems

The increase in anaerobic digestion systems has profoundly affected the waste management of territories, particularly for agricultural systems. Changes in cultural practices and imports of organic waste modify the carbon (C) and nitrogen (N) fluxes on territories where anaerobic digestion is implemented. Successful anaerobic digestion can increase the economic and ecological efficiency of the waste management system. Conversely, poor anaerobic digestion leads to low economic and environmental efficiency due to greenhouse gas emissions and nutrient loss. Modeling the impact of anaerobic digestion on the systems integrating anaerobic digestion can improve the efficiency of these practices. The aim of this study was to develop, analyze, and evaluate a simple mass balance tool able to predict carbon and nitrogen fluxes in anaerobic digestion systems. The tool is composed of an exhaustive substrate database used by three models: (i) an anaerobic digestion model that predicts C and N contents in biogas and digestate; (ii) a phase separation model that predicts C and N content in liquid and solid phase digestates; and (iii) a storage model that predicts C and N content in raw, liquid phase, and solid phase digestates, as well as C and N emissions during storage. Sensitivity analyses were performed on the tool to determine critical inputs. Sensitivity analysis showed that outputs were highly sensitive to their respective inputs and to total inputs of solids. Performance evaluation showed that the tool can provide good quality predictions with R² correlations between observation and prediction varying from 0.72 to 0.99 with the best predictions obtained for raw digestate.

Recovery and applications of ammoniacal nitrogen from nitrogen-loaded residual streams: A review

Total ammoniacal nitrogen (TAN) is considered to be a pollutant, but is also a versatile resource. This review presents an overview of the TAN recovery potentials from nitrogen (N)-loaded residual streams by discussing the sources, recovery technologies and potential applications. The first section of the review addresses the fate of TAN after its production. The second section describes the identification and categorisation of N-loaded (≥0.5 g L^-1 of reduced N) residual streams based on total suspended solids (TSS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), TAN, and TAN/TKN ratio. Category 1 represents streams with a low TAN/TKN ratio (<0.5) that need conversion of organic-N to TAN prior to TAN recovery, for example by anaerobic digestion (AD). Category 2 represents streams with a high TAN/TKN ratio (≥0.5) and high TSS (>1 g L^-1) that require a decrease of the TSS prior to TAN recovery, whereas category 3 represents streams with a high TAN/TKN ratio (≥0.5) and low TSS (≤1 g L^-1) that are suitable for direct TAN recovery. The third section focuses on the key processes and limitations of AD, which is identified as a suitable technology to increase the TAN/TKN ratio by converting organic-N to TAN. In the fourth section, TAN recovery technologies are evaluated in terms of the feed composition tolerance, the required inputs (energy, chemicals, etc.) and obtained outputs of TAN (chemical form, concentration, etc.). Finally, in the fifth section, the use of recovered TAN for three major potential applications (fertilizer, fuel, and resource for chemical and biochemical processes) is discussed. This review presents an overview of possible TAN recovery strategies based on the available technologies, but the choice of the recovery strategy shall ultimately depend on the product characteristics required by the application. The major challenges identified in this review are the lack of information on enhancing the conversion of organic-N into TAN by AD, the difficulties in comparing the performance and required input of the recovery technologies, and the deficiency of information on the required concentration and quality of the final TAN products for reuse.

Improving two-stage thermophilic-mesophilic anaerobic co-digestion of swine manure and rice straw by digestate recirculation

The anaerobic co-digestion (coAD) of swine manure (SM) and rice straw (RS) is appealing for renewable energy recovery and waste treatment worldwidely. Improving its performance is very important for its application. In this study, long-term semi-continuous experiments were conducted to evaluate the improving effects of digestate recirculation on the performance, energy recovery, and microbial community of two-stage thermophilic-mesophilic coAD of swine manure (SM) and rice straw (RS). The experimental results indicated that the coAD systems of SM and RS (mixing ratio of 3:1) with or without digestate recirculation could not realize phase separation. The reactors of both coAD systems were characterized by pH values ranging from 7.74 to 7.85, methane production as 0.41 ± 0.02 and 0.44 ± 0.03 L/L/d, and stable operation. Notably, digestate recirculation increased total methane production, organic matter removal, and reaction rate of the coAD system by 9.92 ± 5.08, 5.22 ± 1.94, and 9.73-12.60%, respectively. Digestate recirculation improved the performance of the coAD by significantly increasing the abundance of Methanosarcina (from 4.1% to 7.5%-10.7% and 35.7%) and decreasing that of Methanothermobacter (from 94.2% to 87.3%-83.6% and 56.8%). Thus, the main methanogenesis pathway of the coAD system was changed by digestate recirculation and the methane production was effectively improved. Although the energy input of the coAD system increased by 30.26%, digestate recirculation improved the energy balance of the total system by 6.83%.

Biogas residues in the battle for terrestrial carbon sequestration: A comparative decomposition study in the grassland soils of the Greater Region

The recycling of biogas residues resulting from the anaerobic digestion of organic waste on agricultural land is among the means to reduce chemical fertilizer use and combat climate change. This in sacco decomposition study investigates (1) the potential of the granulated biogas residue fraction to provide nutrients and enhance soil carbon sequestration when utilized as exogenous organic matter in grassland soils, and (2) the impact of different nitrogen fertilizers on the organic matter decomposition and nutrient release processes. The experiment was conducted in two permanent grasslands of the Greater Region over one management period using rooibos tea as a comparator material. The decomposition and chemical changes of the two materials after incubation in the soil were assessed by measuring the mass loss, total carbon and nitrogen status, and fibre composition in cellulose, hemicellulose and lignin. Overall, after the incubation period, granulated biogas residue maintained up to 68% of its total mass, organic matter and total carbon; increased its content in recalcitrant organic matter by up to 45% and released 45% of its total nitrogen. Granulated biogas residue demonstrated resilience and a higher response uniformity when exposed to different nitrogen fertilizers, as opposed to the comparator material of rooibos tea. However, the magnitude of fertilizer-type effect varied, with ammonium nitrate and the combinatorial treatment of raw biogas residue mixed with urea leading to the highest organic matter loss from the bags. Our findings suggest that granulated biogas residue is a biofertilizer with the potential to supply nutrients to soil biota over time, and promote carbon sequestration in grassland soils, and thereby advance agricultural sustainability while contributing to climate change mitigation.

Biochar improves heavy metal passivation during wet anaerobic digestion of pig manure

Anaerobic digestion (AD) is regarded as an effective treatment to stabilize organic materials and recycle the energy in pig manure. In this study, 0%, 3%, 5%, and 7% biochar (based on dry weight) were added to pig manure to investigate its influence on improving biogas production and reducing heavy metal bioavailability. The potential ecological risk of heavy metals (namely Mn, Zn, Cu, Ni, As, Cd, Pb, and Cr) in digestates was also assessed. Results show that the methane yield was significantly (P < 0.05) increased by 26.7%, 23.0%, and 26.4% following addition of 3%, 5%, and 7% biochar, respectively. Moreover, there was a significant change in the heavy metal speciation in amendment each group. The 5% biochar group showed the highest passivation rate of Ni, As, and Pb, while the highest passivation rate of Cd, Cr, Mn, and Zn was observed with 7% biochar. Although the anaerobic digestion process slightly increased the ecological risk of heavy metals, all tested digestates were still classified as a moderate risk. Results of this study can provide a reference for the treatment of heavy metal pollution in large- and medium-sized anaerobic digesters treating pig manure.

The impact of biogas digestate typology on nutrient recovery for plant growth: Accessibility indicators for first fertilization prediction

In recent years, anaerobic digestion of organic waste (OW) is rapidly appearing as a winning waste management strategy by producing energy and anaerobic digestates that can be used as fertilizers in agricultural soils. In this context, the management of the OW treatment process to maximize agro-system sustainability satisfying the crop nutrient demands represents the main goal. To investigate these traits, two protocols to assess the plant availability of digestate nitrogen (N) and phosphorus (P) were evaluated. With this aim, the N and P availability was determined on 8 digestates and 2 types of digestate-based compost from different OW via sequential chemical extractions (SCE). In addition, the digestates were tested in soil incubations and in plant pot tests with Italian ryegrass and compared with chemical fertilizer and a non-amended control soil. The N extracted from digestates via SCE was related to soil N mineralization and plant N recovery. The C: N ratio had negative impact on mineralized N and its recovery in shoots (Shoots_N = -0.0085.(C/N)+0.172, r² = 0.67), whereas water extractable mineral N was positevely related to the root N apparent recovery fraction (N-ARF) with (Roots_N = 5E^-5.N_solublemin+0.0138, r² = 0.53). The shoot P-ARF was positively correlated with the inorganic water extractable fraction of P (Shoots_P =0.1153.H₂O-P_i-0.2777.H₂O-P_o+0.0249, r² = 0.71) whereas the root P-ARF was positively correlated with the less accessible fractions (Roots_P = (b)   0.0955.NaHCO₃-P_o+0.0955.NaOH-P_o-0.0584NaHCO₃-P_i+0.0128, r² = 0.8641). Feedstock digestate typology impacted the N and P recovery results leading to a better description of the typology properties and a first nutrients ARF prediction.

Comprehensive determination of input state variables dataset required for anaerobic digestion modelling (ADM1) based on characterisation of organic substrates

This article contains the data of 11 organic substrates including physicochemical, biochemical and nutritional characterisations. Additionally, it includes for all substrates the data of organic matter fractionation into easily biodegradable, slowly biodegradable and inert fractions performed with anaerobic respirometry method. Finally, based on physicochemical characterisations and organic matter fractionation, a detailed methodology for the determination of input state variables required for the anaerobic digestion model N°1 (ADM1) was presented and the dataset for all substrates is provided. An example of calculation for one substrate illustrates the methodology for the determination of these variables. Data provided in this article could be useful to any person interested in modelling anaerobic digestion and particularly co-digestion. Data could be also used for implementation of a database for anaerobic digestion modelling.

Effects of temperature and inoculation ratio on methane production and nutrient solubility of swine manure anaerobic digestion

This study investigated the effects of temperature and volatile solids (VS) ratio of feedstock to inoculum (F/I ratio) on methane (CH₄) production and the solubility of nitrogen (N), phosphorus (P), cooper (Cu), and Zinc (Zn) after anaerobic digestion (AD) of swine manure. The highest cumulative CH₄ yield of 470 L/kgVS_feed was obtained with F/I ratios of 2.0 and 3.0 with mesophilic (37 °C) temperature, and methane production rate decreased with the increase of F/I ratio. As F/I ratio increased from 0.5 to 4.0, the lag phase for methane production increased from 1.02 days to 13.52 days, indicating an initial inhibition at high F/I ratios. AD increased the concentrations of ammonia, Cu and Zn in the AD effluent supernatant, while decreased total and water extractive P concentrations. The changes of ammonia, Cu, Zn, and P concentrations were more significant with the increase of F/I ratio.

Volatile Fatty Acids Production from Microalgae Biomass: Anaerobic Digester Performance and Population Dynamics during Stable Conditions, Starvation, and Process Recovery

Disturbances in anaerobic digestion (AD) negatively impact the overall reactor performance. These adverse effects have been widely investigated for methane generation. However, AD recently appeared as a potential technology to obtain volatile fatty acids (VFAs) and thus, the impact of process disturbances must be evaluated. In this sense, microbial response towards a starvation period of two weeks was investigated resulting in a conversion of organic matter into VFAs of 0.39 ± 0.03 COD-VFAs/CODin. However, the lack of feeding reduced the yield to 0.30 ± 0.02 COD-VFAs/CODin. Microbial analysis revealed that the starvation period favored the syntrophic acetate-oxidizing bacteria coupled with hydrogenotrophic methanogens. Finally, the system was fed at 9 g COD/Ld resulting in process recovery (0.39 ± 0.04 COD-VFAs/CODin). The different microbiome obtained at the end of the process was proved to be functionally redundant, highlighting the AD robustness for VFAs production.

The Use of Artificial Neural Networks and Decision Trees to Predict the Degree of Odor Nuisance of Post-Digestion Sludge in the Sewage Treatment Plant Process

This paper presents the application of artificial neural networks and decision trees for the prediction of odor properties of post-fermentation sludge from a biological-mechanical wastewater treatment plant. The input parameters were concentrations of popular compounds present in the sludge, such as toluene, p-xylene, and p-cresol, and process parameters including the concentration of volatile fatty acids, pH, and alkalinity in the fermentation sludge. The analyses revealed that the implementation of artificial neural networks allowed the prediction of the values of odor intensity and the hedonic tone of the post-fermentation sludge at the level of 30% mean absolute percentage error. Application of the decision tree made it possible to determine what input parameters the fermentation feed should have in order to arrive at the post-fermentation sludge with an odor intensity <2 and hedonic tone >−1. It was shown that the aforementioned phenomenon was influenced by the following factors: concentration of p-xylene, pH, concentration of volatile fatty acids, and concentration of p-cresol.

Modeling the fate of organic nitrogen during anaerobic digestion: Development of a bioaccessibility based ADM1

Simulating the fate of nitrogen during anaerobic digestion is required to predict the characteristics of digestates and to improve their exploitation for agricultural uses. The aim of this study was to develop a modified ADM1 model that includes bioaccessibility-based fractionation to accurately simulate the fate of nitrogen during anaerobic digestion. To this end, two complementary approaches were used: (i) changes in the bioaccessibility of protein and non-protein compounds were assessed on eight substrates during anaerobic digestion in batch experiments using the "EPS" fractionation method; (ii) experimental results were used to develop a bio-kinetic model based on anaerobic digestion model n°1. This new model incorporates bioaccessibility-based fractionation in its input state variables. The model was successfully calibrated and model evaluation showed that predicted methane production, ammonium production and changes in protein and non-protein bioaccessibility during anaerobic digestion were accurate.

Effects of different types of biochar on the anaerobic digestion of chicken manure

This study investigated the impact of different types of biochar on the anaerobic digestion (AD) of chicken manure. Wheat straw, discarded fruitwood, and air-dried chicken manure were pyrolysed at 350, 450, and 550 °C to generate biochar. A lab-scale batch anaerobic digestion experiment was conducted at 35 ± 1 °C. Substantial improvements in methane production were observed for all nine types of biochar. With the production of 294 mL CH₄/g VS_added, fruitwood char pyrolysed at 550 °C increased the methane yield by 69% from the control. Characteristic analysis indicated that fruitwood char pyrolysed at 550 °C exhibited the largest specific surface area and highest total ammonia nitrogen reduction capacity. The buffering capacity of the AD system was improved by the biochar through accelerating the transformation of macromolecular substances to dissolved substrates and reducing the contents of soluble salts, total ammonia nitrogen, and free ammonia.