A comprehensive understanding of digestate viscosity can benefit anaerobic digestion by aiding the selection of pre- or post-treatment methods and supporting the prediction of energy consumption in biogas plants and the impacts of digestate post-processing. Viscosities and chemical composition of 43 full-scale agricultural digestate samples and 12 samples of the liquid fraction of the separated digestate were examined, with eight manure samples included for comparison. The effects of full-scale agricultural digestate separation were evaluated for the first time. Viscosity was described using the power-law model, including the consistency coefficient and flow behaviour index. Shear-thinning behaviour was found in all except three samples. Viscosities of digestates were similar to those of cattle manure, whereas liquid fractions resembled pig manure. Total solids content and content of solids larger than 2 mm correlated (p < 0.001) with the consistency coefficient (r of 0.57 and 0.70) and flow behaviour index (r of -0.56 and -0.69), indicating that the content of solids larger than 2 mm is a better predictor of viscosity. This was further confirmed by substantial viscosity reductions in the liquid fraction of digestate once larger particles were removed, even at inefficient overall total solids removal. Organic components correlated (p < 0.001) positively with the consistency coefficient and negatively with the flow behaviour index; however, they also correlated among themselves and with total solids content, impeding the isolation of effects. Conductivity, pH, density, and some metals did not influence viscosity (p > 0.05), while some other metals did (p < 0.01).

Recirculating the solid fraction of digestate could be an easy-to-implement strategy to increase methane production of biogas plants and further exploit expensive or scarce substrates. Solid-state alkaline treatment (at total solids content of 20 %) with 1-12 % KOH at 20 degrees C and a length of 1-8 days was explored as a method to enhance methane production from four solid digestate samples with the additional benefit of yielding a more valuable final digestate due to its enrichment in potassium. In comparison, the treatments were applied to a fresh commercial substrate mix. The ultimate methane yields of the solid digestate untreated samples varied between 122 and 162 ml/g of volatile solids, while that of the substrate mix was 294 ml/g of volatile solids. Post-treatments of the digestate samples resulted in ultimate methane yield changes between a 27 % decrease and a 41 % increase. These changes varied between a 4 % decrease and a 22 % increase when the substrate mix was pre-treated. In most cases, there was no evident correlation of KOH concentration and treatment duration on ultimate methane yields. However, increasing KOH concentration increased maximum methane production rates and shortened lag phase durations, while longer treatments reduced lag phase durations for solid digestate samples. Changes in maximum methane production rates and lag phase durations did not follow a clear trend for the substrate mix, and were, generally, less evident than for solid digestate. The economic viability of the process would be restricted by the high cost of KOH and low solid-liquid separation efficiency of digestate.

Anaerobic digestion harnesses microbial processes to convert organic wastes into renewable biogas, offering a sustainable pathway for energy production. In agricultural settings, biogas plants often co-digest livestock manure with crop residues, yet seasonal variations in feedstock quality introduce fluctuations that challenge process stability and yield optimization. Mechanistic models such as the Anaerobic Digestion Model No. 1 (ADM1) provide detailed biochemical simulations but require extensive substrate characterization, limiting their practicality for full-scale operations. Here we show that a simplified ADM1, alongside machine learning approaches—random forest and long short-term memory (LSTM) networks—achieves comparable accuracy in predicting daily biogas and methane production from a full-scale plant over 2023–2024. All models yielded Nash-Sutcliffe efficiencies above 0.78, with random forest excelling when incorporating feedstock quantities and maize silage volatile solids. While LSTM proved effective even with minimal inputs, it incurred a training time 141 times greater than ADM1, highlighting critical trade-offs in computational efficiency. These findings advance hybrid modelling strategies for real-time monitoring, enabling operators to balance predictive precision with data requirements to enhance renewable energy integration and agricultural sustainability.

Post-treatment of digestate benefits biogas plants by reducing gas emissions and recovering residual methane potential, but the substrate often lacks nutrients for soil application. Adding trace elements (TEs) can enhance energy production and agricultural value. This study examines whether copper (Cu) and zinc (Zn) solutions can improve residual methane potential as effectively as a commercial TEs mixture, supplied at a higher cost. Digestate from a hybrid full-scale biogas plant underwent batch anaerobic digestion assays. Cu and Zn from reagents were tested at 0, 0.1, 1, 2, and 5 mg L- 1 in laboratory-scale reactors (LBRs), while a commercial mixture also containing Mn, Co, Ni, Mo, Fe, and B was tested in both LBRs and bench-scale reactors (BBRs). Potential energy and net present value (NPV) were estimated across 5000 scenarios. Methane production rate was maximised with 0.1 mg L- 1 Cu and Zn and additional TEs due to synergistic effects. Residual methane increased by 39 % and 25.5 % with 2 mg L- 1 Cu and Zn and 1 mg L- 1 Cu, respectively, compared to the commercial mixture. Adding 1 mg L- 1 Cu resulted in a 30 % higher probability of achieving an NPV of 15,200-38,000 compared to 2 mg L- 1 Cu and Zn.

The objective of this study was to evaluate the effect on the in vitro methane yield from dairy cow faeces, when the diet is manipulated for either source or concentration of dietary fat (rapeseed or palm kernel fatty acids; 20 to 50 g/kg DM), or a source of carbohydrate (grass-clover vs maize silage, and barley vs dried beet pulp). The faecal samples from two nutrition trials were used in an in vitro system for anaerobic digestion for measuring gas volume and methane concentration at given times, from 5 to 90 d. The ultimate methane yield measured at d 90, and the coefficients estimated from a modified Gompertz model (maximum methane yield, methane production rate, and lag time) were analysed in a linear mixed model. Increasing dietary concentration of fat increased the maximum methane yield, the methane production rate, and the lag time when rapeseed was used as fat source, but use of palm kernel fatty acids in the diet did not result in an increase of any parameter. The effect of the carbohydrate inclusion from forage and concentrate was additive, so no interaction was observed, and maximum methane yield increased with increasing dietary starch concentration. Ultimate and maximum methane yield were highly correlated, with the latter underestimating ultimate methane yield by 3-4%. Estimated coefficients from a modified Gompertz model are useful to understand the effect of diet on biogas yield and production rate from faeces, but slight under-estimation of maximum methane yield was observed at incubation time of three months.

Benzalkonium compounds (BACs) are quaternary ammonium biocides widely used in healthcare and industrial animal production, resulting in high concentrations in manure and wastewater. While BAC biodegradation has been demonstrated in wastewater and soil, little is known about the relevant biodegradation pathways in multi-species biofilm systems. This study investigated BAC degradation pathways in three aerobic moving bed biofilm reactor (MBBR) systems: (1) a fungi (Trametes versicolor) inoculated reactor, and heterotrophic biofilm in (2) water, and (3) manure. We characterised transformation products and elucidated their fragmentation mechanisms to explain the spectra. ω-oxidation and β/α-oxidation dominated in the water-MBBR, whereas formation of benzyldimethylamine (BDMA), was observed only in the manure-MBBR, which was the only system hosting Pseudomonas and Aeromonas species. The water-MBBR microbial community showed a significant increase in abundance of Rhodococcus, known for ω- and β-oxidation, suggesting a role in BAC degradation. None of the five ARGs (sul1, sul2, qnrD, tetM, tetA) tested in this study were elevated in the systems by exposure to BAC. These results demonstrate that BACs are degradable in different aerobic biofilm systems via diverse pathways, influenced by microbial composition and matrix complexity. However, the different metabolites indicate towards complex risk assessments and bioremediation strategies.

Abstract Background There is an increasing interest in using lignocellulosic feedstocks for biogas production. Treatment of these feedstocks prior to anaerobic digestion (AD) can enhance their accessibility to microorganisms involved in the process. To improve the digestion of recalcitrant feedstocks and boost biogas yields, many biogas plants now employ two-step AD systems, extending substrate residence times. However, the combined effect of feedstock treatment and two-step AD on methane yield and fertiliser value of digestates are underexplored. This study, therefore, evaluated the effectiveness of thermal treatment (TT) of pre-digested agricultural feedstocks before a secondary AD step on the carbon (C) and nitrogen (N) dynamics of digestates following application to soil. It also investigated the effects of TT on methane yields. Pre-digested feedstock (PDF) was treated at three different temperatures (70 °C, 120 °C and 180 °C) for 60 min, followed by parallel secondary AD steps using lab-scale continuous stirred-tank reactors (CSTR) and a batch test. Thermally treated feedstocks with and without a secondary AD step were applied to soil to study C and N dynamics and turnover for 2 months. Results TT at 180 °C increased ultimate CH4 yields by 7.2%; however, it decreased the net mineral N release in soil from 42 to 34% (of N input). Adding a secondary AD step increased the net mineral N release in soil from an average of 39% to 47% (of N input), with the effect of TT levelling off. Moreover, the secondary AD step significantly reduced C mineralisation rates from an average of 37% to 26% (of C applied). Conclusions Overall, TT at 120–180 °C can improve biogas yields of recalcitrant feedstocks, but it may lead to the formation of refractory nitrogen compounds resistant to further degradation during AD, potentially resulting in a lower N fertiliser value of digestates. Graphical Abstract

IntroductionAnaerobic digestion of manure, together with other biowastes, produces biogas that can substitute fossil energy and thereby reduce CO2 emissions and post- digestion greenhouse gas emissions. The final digestate of the process is an organic fertilizer rich in plant nutrients and recalcitrant organic constituents. The digestate characteristics and quality depend on several parameters, such as input feedstocks and operational conditions of the biogas plants. In Denmark, the rapid expansion of the biogas sector in recent years has resulted in a great variety of feedstocks used in the plants. The first generation of biogas plants mainly treated manure, industrial wastes, and energy crops with short retention times, while the new generation of biogas plants are co-digesting manure with higher amounts of lignocellulosic feedstocks and operating with longer retention times. This study evaluated whether this shift in feedstock composition could impact the fertilizer quality and post-digestion greenhouse gases and ammonia emissions during storage and application of digestate.MethodsDigestate samples from 2015 to 2023 were collected and analyzed for composition and residual methane yields. The efficiencies of solid–liquid separation applied to several digestate samples from the new generation of biogas plants were investigated and the nutrients contents of the liquid and solid fractions of digestate were evaluated.Results and discussionThe most evident change caused by the feedstock transition was an average increase of 52% in the total solids content of digestate, which can negatively impact ammonia emissions during digestate application. In contrast, similar average residual methane yields on a fresh matter basis of approximately 5 L/kg indicated comparable risks of methane emissions during storage. The liquid fraction of industrially separated digestate presented, on average, nutrient concentrations similar to those of unseparated digestate, while the solid fraction presented similar K, lower total ammoniacal nitrogen, and higher organic N and P contents than unseparated digestate on a fresh matter basis. The average residual methane yield of the industrially separated solid fraction of digestate was 101 L/kg volatile solids, while the average calorific value was 21 MJ/kg volatile solids, indicating its potential for additional energy generation.

Source Segregation (SS) is a novel strategy in dairy housing that can reduce emissions and separate organic matter and nutrients more efficiently than traditional slurry solid-liquid separation. The anaerobic digestion (AD) yield of the SS fractions, however, is unknown. We aimed at unveiling the biomethane yield of these fractions by conducting AD experiments under different configurations: batch, continuous feeding, and fed-batch. In the batch test, the solid (SF) and liquid fraction (LF) from the SS system, a slurry collected from the pit (CS), and a self-made slurry (MF) were used as substrates. The results showed that the specific CH4 yields of the SF and MF were in same range and both higher than the CS. We concluded that SS can increase the CH4 yield of dairy excreta mainly by reducing losses in the animal house. The SF and MF were then compared in a continuously-fed thermophilic test, where SF had a higher specific (174 compared to 105 NL kg-1 VS) and volumetric (12.2 compared to 9.9 NL CH4 kg-1 excreta) yields. We concluded that the SF can effectively substitute slurry in AD without compromising the yield, possibly increasing economic viability by reducing transport costs and reactor size. Further, SF produced 356 NL CH4 kg-1 VS and a digestate with 1.8% lower dry matter in the fed-batch as compared to continuous feeding. Continuously stirred fed-batch can thus increase the CH4 yield of the SF and reduce the DM of its digestate potentially contributing to lower emissions in storage and field application.

Acidification through plasma treatment technology presents a potential solution to mitigate nitrogen losses throughout the manure management chain, offering a dual benefit of atmospheric nitrogen incorporation into manures while inducing acidification. This study evaluated the use and effectiveness of plasma treatment technology as a one-time acidification strategy for digested slurry during long-term storage, its effects on slurry nitrogen (N) availability and subsequent release in the soil. A digestate from a full-scale anaerobic digester was separated using a decanter centrifuge, and the resultant liquid fraction (LF) was plasma treated to achieve three different initial pH levels (4.27, 5.03 and 5.42). After that, a storage experiment was set up for 180 days to monitor the evolution in pH and changes in NH4+, NO3- and NO2- . Concurrently, a soil incubation experiment was conducted to study the N turnover for 80 days after incorporating the organic materials (Liquid fraction, plasma-treated liquid fractions, digestate and raw slurry). Plasma treatment significantly increased total N and the proportion of inorganic N in the slurries by fixing atmospheric N as NO3- and NO2- and reduced the slurry pH depending on the treatment duration. After six months of storage, pH increased by 0.91 pH units in the plasma-treated liquid fraction with the highest initial pH (LFpH-5.42). Plasma treatment significantly increased net inorganic N release in soil by 5-14 % compared to a non-treated liquid fraction. Unexpected prolonged inhibition of NH4+ nitrification in soil for over 80 days after applying plasma-treated slurries was observed. In conclusion, plasma treatment emerges as a promising alternative to acid- and bioacidification strategies, offering prolonged low slurry pH stabilisation, improved fertiliser value and delayed NH4+-N nitrification in soil.

Cereal straw is a large biomass resource which may potentially be pretreated for biogas production by combined ensiling and storage. Five pilot-scale ensiling experiments were performed with straw of wheat, barley and rye, baled in midi-bales (0.9*1.2*2.0 m) with various water contents (17-68 %), ensiling additives and ensiling periods (173-230 days). The water content in the bales differed depending on previous precipitation but also the time of day for baling. Temperature in wet straw increased to 30 and 63 degrees C during storage for bales with and without wrapping, respectively, and mean dry weight loss was 5.1 +/- 5.9 % and 10.5 +/- 7.3 %, respectively. During storage, pH dropped significantly in wrapped straw bales but not in bales without wrapping. In wrapped straw without additives, pH after ensiling decreased significantly with increasing initial water content, indicating the importance of water content above 30-40 % for ensiling of straw. Besides a more rapid pH reduction when applying acetic acid, there was no significant effect of acetic and formic acid, lactic acid bacteria and brown juice on pH after storage. Ensiling of wet straw had a significant effect on biochemical methane potential (BMP) in some cases, with up to 32 % increase. There was limited effect of ensiling additives on BMP, whereas increasing water content in the straw significantly increased BMP. Improper wrapping of straw during storage reduced BMP with 17-30 %. Overall, proper wrapping and good ensiling of wet straw may result in up to 25 % extra methane yield whereas poor storage may result in up to 37 % loss.

Cereal straw is a large bioresource but requires pretreatment before use in biogas production. We investigated ensiling of wet straw as a biological pretreatment for enhancing the biochemical methane potential (BMP). Lab-scale ensiling experiments with wheat and barley straw showed that pH reduction during ensiling was enhanced by high water content (up to 50 %), chopping and the ensiling additives lactic acid bacteria, formic acid, acetic acid and brown juice. Compared to dry, non-ensiled wheat straw, BMP was significantly increased after ensiling, with up to 19, 24 and 24 % increase after three, six and ten months of ensiling, respectively. BMP increased significantly with increasing water content in wheat straw during ensiling. There was only a limited additional effect on BMP from chopping and ensiling additives. For barley straw, BMP was increased up to 28 % after ensiling for six months. The results demonstrate ensiling of wet straw as a potential pretreatment method.

Effective substrate utilization with low residual methane yield in the digestate is crucial for the economy and sustainability of biogas plants. The composition and residual methane potential of 29 digestate samples from plants operating at hydraulic retention times of 13-130 days were determined to evaluate the economic viability of extended digestion. Considerable contents of fermentable fractions, such as cellulose (8-23%), hemicellulose (1-18%), and protein (13-22%), were present in the digestate dry matter. The ultimate residual methane yields varied between 55 and 236 ml/g of volatile solids and correlated negatively with the logarithm of the hydraulic retention time (r = -0.64, p < 0.05). Economic analysis showed that extending the retention time in 20 days would be viable for 18 systems if methane were sold for 1.00 euro/m(3), with gains up to 40 euro/year/m(3) of newly installed reactor capacity. The results show the importance of operating at sufficient hydraulic retention time.

Biogas operation parameters are usually optimised to enhance biogas yields while ignoring digestate quality. This study evaluated the effects of varying operation parameters and input feedstocks used in co-digestion with manure on digestate properties and N turnover in soil for 80 days. Net inorganic N release (% of N input) from digestates varied significantly (p < 0.001) due to their contrasting properties. Temperature, hydraulic retention time (HRT), manure type and input feedstocks significantly influenced soil inorganic N release. Animal slurry co-digested with energy crops had the highest average net soil inorganic N release at 67 %, while co-digestion with straw-based feedstock had the lowest at 56 %. Feedstock type greatly influenced the choice of digesting temperature and HRT, with most energy crop-based feedstocks digested at <50 days HRT. Digestate properties NH4+ -N/total N, C/N and total carbon concentration significantly influenced net inorganic N release in soil. A better understanding of the effects of biogas operation variables on nutrient availability and digestate fertilising properties could help design and optimise the anaerobic digestion process to increase biogas yields and N fertiliser value of the digestates.

Biogas production from anaerobic digestion (AD) of biowastes is restricted by the recalcitrant nature of many substrates, and this may also reduce the fertiliser value of the produced digestate. The degradability of substrates can potentially be enhanced by physico-chemical pre-treatments before AD, and/or the degradation can be increased by a longer digestion time. In this study, we evaluated the effects of electrokinetic (high voltage) and ultrasonication pre-treatments of biowastes in a two-step AD process on nitrogen fertiliser replacement value (NFRV) of digestates obtained from two biogas plants with contrasting hydraulic retention time (HRT) in the primary AD step. The fertiliser value was tested by direct injection to spring barley and surface-banding to winter wheat, and the ammonium N was 15N-labelled to evaluate ammonia losses. The electrokinetic pre-treatment step significantly (p < 0.05) increased the NH4+-N/total N in the digestates before the second AD step but had an insignificant effect on the fertiliser value in winter wheat and spring barley. Ultrasonication pre-treatment had also no significant effect on the fertiliser value. The two-step AD significantly (p < 0.001) increased 15N recoveries and mineral fertiliser equivalence of labelled ammonium-N in winter wheat and reduced ammonia losses, with a significant effect (p < 0.001) observed in digestates sourced from a shorter HRT biogas reactor. The fertiliser equivalence of labelled ammonium-N in the digestates was 80-88% after injection, indicating relatively low N immobilisation with all the digestates. NFRV in the crops was mainly explained by the NH4+-N/total N ratio, C/N ratio and dry matter content of the digestates. The findings suggest that electrokinetic and ultrasonication pre-treatments combined with a second AD step have no considerable impact on the fertiliser value of digestates, whereas a second AD step significantly reduced ammonia losses after application by surface-banding in winter wheat.

The study investigated the effects of ensiling and brown juice-assisted ensiling of wheat straw and press cake from a green biorefinery plant prior to anaerobic co-digestion (Aco-D) with cattle manure on methane yield and nutrient availability in the digestates. Straw was ensiled with and without brown juice at field-scale and lab-scale levels, followed by Aco-D with manure using continuous stirred-tank reactors (CSTR) and batch tests. The digestates from CSTR were incorporated into the soil to study nitrogen and sulphur dynamics and availability over 80 days. Results show that ensiling of straw increased net inorganic nitrogen release in soil and reduced net inorganic sulphur immobilisation compared to the non-ensiled straw. Brown juice-assisted ensiling of straw at the lab-scale level significantly increased methane production by 14 % and increased hydrolysis and methane production rates compared to non-ensiled straw. These results demonstrate that ensiling of straw has the potential as a pre-treatment technique for enhanced biogas production and higher nutrient availability in digestates for crop production and provides an opportunity to valorise brown juice as an ensiling additive.

The impact of post-treatment of digestate prior to its recirculation to the digester has been evaluated with industrial-scale ultrasonication and electrokinetic treatment units. Residual methane yields of untreated digestate samples from four biogas plants varied between 99 and 134 ml/g of volatile solids (after 97 days of digestion). At the tested conditions (1.90-7.60 kWh/t for ultrasonication and 4.37-6.06 kWh/t for electrokinetic disintegration), no significant increase in methane yields were obtained. Increases in maximum methane production rates of up to 42% were observed in samples from one of the biogas plants, resulting in net energy gains between 1.06 and 7.04 kWh/t (after 26 and 66 days of digestion). Both treatments significantly reduced digestate viscosities, especially of highly viscous samples. Lower viscosities could reduce the energy demand for pumping and stirring in the digester, enabling the potential for increased organic loading and increased methane production.

Bangladesh is in the midst of a severe and worsening energy crisis. The continuous depletion of fossil fuels is sticking the concern into the search for new energy sources which are renewable as well as sustainable. Among the renewable energy sources, biogas from the poultry and dairy industries is an increasingly growing and promising sector. A survey was conducted to understand the present status of biogas technology in Bangladesh. Data were collected by interviewing farm owners and major stakeholders. The problems and barriers to the implementation and dissemination of biogas plants in Bangladesh were also identified through SWOT analysis. A total of sixteen farms were selected to collect data regarding current operational conditions. The economic viability was also evaluated. Most of the selected farms were found to be under-fed in spite of having huge potential. Though the H2S concentration of biogas produced from different plants was high, most of them had no purification unit. The results of the economic analysis showed that the biogas plants operating in Bangladesh are at a loss as currently no subsidies are provided. The outcome of the SWOT analysis identified high initial investment, lack of technical knowledge, lack of local technology, poor management, and proper bio-slurry handling technology as the main barriers to disseminating biogas technology in Bangladesh.

Anaerobic digestion (AD) is an important tool for reducing greenhouse gas emissions from agricultural production. A prolonged retention time by adding an extra anaerobic digestion step can be utilized to further degrade the digestates, contributing to increased nitrogen mineralisation and reducing decomposable organic matter. These modifications could influence the potential N fertiliser value of the digestate and soil carbon sequestration after field application. This study investigated the effects of prolonging retention time by implementing an additional anaerobic digestion step on carbon and nitrogen dynamics in the soil and soil carbon sequestration. Two digestates obtained from two biogas plants operating at contrasting hydraulic retention times, with and without an additional digestion step, were applied to a loamy sand soil. N mineralisation dynamics were measured during 80 days and C mineralisation during 212 days. After 80 days of incubation, the net inorganic N release from digestates obtained from a secondary AD step increased by 9-17 % (% of the N input) compared to corresponding digestates obtained from a primary AD step. A kinetic four-pool carbon model was used to fit C mineralisation data to estimate carbon sequestration in the soil. After 212 days of incubation, the net C mineralisation was highest in undigested solid biomass (68 %) and digestates obtained from the primary AD step (59-65 %). The model predicted that 26-54 % of C applied is seques-tered in the soil in the long-term. The long-term soil C retention related to the C present before digestion was similar for one-and two-step AD at 12-16 %. We conclude that optimizing the anaerobic digestion configurations by including a secondary AD step could potentially replace more mineral N fertiliser due to an improved N fertiliser value of the resultant digestate without affecting carbon sequestration negatively.