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Introduction: Anaerobic 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. Methods: Digestate 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 discussion: The 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. [ABSTRACT FROM AUTHOR]

The enzymatic hydrolysis of lignocellulosic polymers is generally considered the rate-limiting step to methane production in anaerobic digestion of lignocellulosic biomass. The present study aimed to investigate how the hydrolytic microbial communities of three different types of anaerobic digesters adapted to lignocellulose-rich wheat straw in continuous stirred tank reactors operated for 134 days. Cellulase and xylanase activities were monitored weekly using fluorescently-labeled model substrates and the enzymatic profiles were correlated with changes in microbial community compositions based on 16S rRNA gene amplicon sequencing to identify key species involved in lignocellulose degradation. The enzymatic activity profiles and microbial community changes revealed reactor-specific adaption of phylogenetically different hydrolytic communities. The enzymatic activities correlated significantly with changes in specific taxonomic groups, including representatives of Ruminiclostridium , Caldicoprobacter , Ruminofilibacter , Ruminococcaceae , Treponema , and Clostridia order MBA03, all of which have been linked to cellulolytic and xylanolytic activity in the literature. By identifying microorganisms with similar development as the cellulase and xylanase activities, the proposed correlation method constitutes a promising approach for deciphering essential cellulolytic and xylanolytic microbial groups for anaerobic digestion of lignocellulosic biomass. [ABSTRACT FROM AUTHOR]

Abstract In this work, ensiling of Festuca arundinacea (Tall Fescue, TF) prior to biogas production was studied and the effect of storage time, additives (pilot-scale trial) and mechanical pretreatment (full-scale trials) were investigated. In the pilot-scale test, TF was ensiled by adding two biological additives and stored for up to 9 months. In the full-scale test, two 7 ton TF piles (unchopped and chopped) were ensiled for 3 months. Both the ultimate CH 4 potential and silage products were measured to evaluate the effect of ensiling. Results show that ensiling of TF resulted in well-preserved silages with low pH for most of the variants stored for up to nine months. The biological additives contained both hetero- and homo-fermentative lactic acid bacteria and enzymes showed a positive effect on both organic dry matter (ODM) preservation and biogas production. Full-scale ensiling presented silage quality with higher ODM losses if the grass is not cut prior to the ensiling process. In general, ensiling is an appropriate storage method for TF, allowing it to be used as a long-term feedstock for biogas plants. Highlights • The study investigated the ensiling time, additive and pretreatment on biogas yield. • Both pilot- and full-scale ensiling trials were conducted. • Biological additive showed positive effects on biomass preservation. • Mechanical pretreatment reduced the ensiling losses significantly. [ABSTRACT FROM AUTHOR]

This study investigates the effect of applying a mixture of enzymes (ME) to dairy cattle manure (DCM) as substrate in anaerobic digestion (AD). The aims of this study were to evaluate different methods of ME application to DCM at different temperatures and to investigate the effect of adding ME during the pre-treatment of the solid fractions of dairy cattle manure (SFDCM). The results showed that there was no positive effect of direct ME addition to substrate at either mesophilic (35 °C) or thermophilic (50 °C) process temperatures, but there was a significant 4.44% increase in methane yield when DCM, which had been incubated with ME addition at 50 °C for three days, was fed to a digester when compared to a control digester operating at the same retention time. Methane production was detected during the pre-treatment incubation, and the total sum methane yield during pre-treatment and digestion was found to be 8.33% higher than in the control. The addition of ME to the SFDCM in a pre-incubation stage of 20 h at 35 °C gave a significant increase in methane yield by 4.15% in a digester treating a mixed substrate (30% liquid fractions DCM and 70% enzyme-treated SFDCM) when compared with the control digester treating a similar mixed substrate with inactivated enzyme addition. The results indicate that direct physical contact of enzyme molecules and organic material in DCM prior to AD, without the intervention of extracellular enzymes from the indigenous microorganism population, was needed in order to increase methane yields. [ABSTRACT FROM PUBLISHER]

Cattle manure, dewatered pig manure and chicken manure were pre-treated in a high-temperature reactor under isochoric conditions for 15 min at temperatures between 100 and 225°C with 25°C intervals to study the effect on their methane yield. After 27 days of batch incubation, cattle manure showed a significant improvement in its biochemical methane potential (BMP) of 13% at 175°C and 21% at 200°C. Pig manure showed improvements at temperatures of 125°C and above, with a maximum 29% increase in yield at 200°C. The BMP of chicken manure was reduced by 18% at 225°C, but at lower temperatures there were no significant changes. It was found that this method of pre-treatment could be feasible if sufficient surplus energy was available or if the energy used in the pre-treatment could be recovered. [ABSTRACT FROM PUBLISHER]

Here, we demonstrate the applicability of national strategies towards massive biogas deployment, through a case study Denmark. First, a variety of sustainable agricultural intensification measures to produce additional biomass resources were investigated; as a result, it was found that the biomass currently used in Denmark's biorefineries (including biogas) could be tripled without compromising soil carbon and inducing little to no land use changes. The degree to which these resources could be mobilized for the biogas sector was analysed through examining the extremes, here labelled as LOW and HIGH biomass-to-biogas scenarios. The resulting biomethane production was calculated considering three combinations of biogas production and upgrading technologies: (i) conventional biogas production and upgrading technologies; (ii) plants with prolonged retention time and conventional upgrading technologies and (iii) as in (ii), but upgrading via biological methanation of carbon dioxide in the biogas, using renewable hydrogen. These scenarios revealed a biomethane potential of 24–111 PJ y−1. The key finding of our study is that only the extreme deployment measures, in terms of biomass and technology, allowed to fulfill the emerging gas demands, namely buffering the deficits from fluctuating power and transport (light- and heavy-duty vehicles, urban buses, coaches), quantified at 95 PJ y−1. Yet, just harnessing the full sustainable potential of animal manure, straw and perennial grass allows to supply half of this demand. In the LOW and HIGH biomass scenarios, doubling the retention time brought an increased methane production of 20% (energy-wise), while this increase was 87% when methanation was added. Image 1 • A methodology is presented to uncover the limits for future reliance on biomethane. • Denmark was used as a national case study to quantify the limits of massive biogas deployment. • Covering 85% of today's national gas demand by biogas requires methanation technologies. • 80% of the biogas potential is ensured by animal manure, straw and perennial grasses. • More grass via less rape & cereal areas while supplying the nutritional demand is possible. [ABSTRACT FROM AUTHOR]

[Display omitted] • Agricultural digestates were characterized and re-digested. • Significant contents of fermentable fractions were observed. • The residual methane potential depended negatively on the hydraulic retention time. • An extension of the hydraulic retention time can be economically viable. 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 €/m3, with gains up to 40 €/year/m3 of newly installed reactor capacity. The results show the importance of operating at sufficient hydraulic retention time. [ABSTRACT FROM AUTHOR]

This study investigated the potentials of forbs; caraway, chicory, red clover and ribwort plantain as substrates for biogas production. One-, two- and four-cut systems were implemented and the influence on dry matter yields, chemical compositions and methane yields were examined. The two- and four-cut systems resulted in higher dry matter yields (kg [total solid, TS] ha −1 ) compared to the one-cut system. The effect of plant compositions on biogas potentials was not evident. Cumulative methane yields (LCH 4 kg −1 [volatile solid, VS]) were varied from 279 to 321 (chicory), 279 to 323 (caraway), 273 to 296 (ribwort plantain), 263 to 328 (red clover) and 320 to 352 (grass-clover mixture), respectively. Methane yield was modelled by modified Gompertz equation for comparison of methane production rate. Near infrared spectroscopy showed potential as a tool for biogas and chemical composition prediction. The best prediction models were obtained for methane yield at 29 days (99 samples), cellulose, acid detergent fibre, neutral detergent fibre and crude protein, ( R 2 > 0.9). [ABSTRACT FROM AUTHOR]

A combination of briquetting wheat straw (BWS) and additives is presented in this work. Two experiments were designed: firstly the mold size of the briquetting equipment and the additive addition were evaluated as an integrated pre-treatment promoting methane (CH 4 ) yield and hydrolysis rate ( k ). Secondly, two combinations from the first experiment were used in continuous stirred tank reactors (CSTR) in co-digestion with cattle manure (CM). In this second experiment, CH 4 yield, reactor performance and residual CH 4 production were measured. The addition of alkali to BWS had a more significant effect on k than on the CH 4 promotion. The CSTR results showed that the addition of 3% KOH to BWS had a positive effect on VS degradation, CH 4 production and energy and economic balances. In addition, the potential for CH 4 emissions during the subsequent storage was reduced. [ABSTRACT FROM AUTHOR]

Abstract: During biogas production from various types of substrates such as animal manure, fats and proteins, bacterial growth and biogas production can be inhibited by excessive ammonia (NH3) concentrations. If NH3 is removed from the biogas digester without damaging the digestion process, inhibition of the methane (CH4) producing bacteria will diminish. This study shows that it is possible to remove a significant quantity of NH3 from the biogas digester headspace and liquid phase by a simple gas circulation method where gas bubbles free of NH3 is forced through the upper 30 cm of the liquid phase in the biogas digester, into the headspace and out of the digester. The suggested method improves conditions for anaerobic bacteria exposed to high concentrations of NH3 by simply removing NH3 from the digester. In full-scale biogas production the system presented in this study can be improved by circulating headspace gas through an ammonia absorber and returning the NH3 depleted biogas into the biogas digester. This method can also replace the need for mixing in biogas digesters. [Copyright &y& Elsevier]

Abstract: Batch assays investigating the ultimate methane yields (B0 ) of acidified slurry fractions produced with different solid–liquid slurry separation techniques were done. The result showed that the anaerobic digestion (AD) process was inhibited when raw and liquid fractions of sow, pig and dairy cow acidified slurry are digested, but AD treating solid fractions (SF) acidified slurry showed no sulphide inhibition. The B0 of SF acidified sow slurry increased significantly with increasing screen size in the screw press. No significant effect of acidification processes on B0 of SF dairy cow slurry (DCS) was observed. The ultimate methane yields of SF acidified DCS and SF non acidified DCS were 278±13 and 289±1LkgVS−1, while in term of fresh weigh substrate were 59±2.8 and 59±0.3Lkgsubstrate−1, respectively. [Copyright &y& Elsevier]

Abstract: This study examined the potential for partly substituting dairy cow manure (DCM) with solids from solid to liquid separation of acidified dairy cow manure (SFDCM) during thermophilic anaerobic digestion. Three different substituting levels with a maximum of 30% substitution were tested. All digesters substituting DCM with SFDCM showed a stable biogas production with low volatile fatty acid concentrations after a short transition period. An increased methane yield in terms of digester volume compared to DCM alone was obtained with increasing amount of SFDCM and about 50% more methane was achieved when 30% of DCM was substituted with SFDCM. The digestates were subsequently digested in a post digestion, during which the methane yield increased proportionally with increasing amounts of SFDCM. It can be concluded that SFDCM is a suitable biomass for co-digestion and can be used to increase methane yield in terms of digester volume at ratios up to at least 30%. [Copyright &y& Elsevier]

The present study investigated the feasibility of using pilot-scale anaerobic filter to treat by-product liquid (brown juice, BJ) from grass protein extraction in a green biorefinery, and produce biogas via anaerobic digestion without co-digestion substrate. Prior to feed into the un-heated anaerobic filter reactor, the BJ was warmed up to 55−60 °C in order to maintain the digestion temperature. The influence of retention time and feeding frequencies were tested in order to optimize the overall performance. The study demonstrated that anaerobic filter is feasible for converting BJ to biogas. The specific CH production reached 230 mL gCOD−1 d −1 while up to 80 % of influent COD was removed within a hydraulic retention time of 5.5 days. In addition, higher feeding frequencies enhanced the process stability as it narrowed the temperature variation and decreased the instant impact on buffer capacity. [ABSTRACT FROM AUTHOR]

Abstract Anaerobic co-digestion of agricultural by-products or wastes with complementarity characteristics is commonly used to enhance methane yield. This study firstly explores the possibility of co-digesting grass and forb species (white clover, chicory and plantain) differing in nutrient composition in enhancing methane yield. This was examined with two inocula (a cattle manure-based inoculum and a grass-based inoculum) in a batch assay. Results showed that co-digesting grass and forbs synergistically enhanced methane yield potential on average by 31 L kg−1 volatile solids (+11%) and reduced lag phase time by 0.8 day in the grass-based inoculum, but not in the cattle manure-based inoculum. Mixtures containing plantain showed more consistent synergistic effect than chicory. Synergistic effects were attributed to more balanced nutrient composition (especially C/N ratio) in grass-forb mixtures. We demonstrate that anaerobic co-digestion of grass and forbs is feasible for enhancing methane yield, which promotes the utilization of multi-species grasslands for bioenergy production. Highlights • Co-digestion of grass and forbs was examined with two inocula in a batch assay. • Co-digestion enhanced bio-methane potential in grass-based inoculum. • Co-digestion reduced the lag phase time in grass-based inoculum. • Synergistic effect was attributed to better nutrient balance in grass-forb mixtures. • No effect of co-digestion occurred in cattle manure-based inoculum. [ABSTRACT FROM AUTHOR]

One of the main obstacles for a more widespread use of wheat straw for renewable energy production is the absence of a low-cost technology which promotes its biodegradation. The aim of this study was to determine whether methane yield from wheat straw could be improved by using alkali at low concentration and ambient temperature if the fiber fraction had previously undergone mechanical pretreatment. After combining mechanical and alkali pretreatment, a linear correlation between alkali concentration and hydrolysis of the fiber content, mainly hemicelluloses, was found. A positive effect by combining briquetting of wheat straw with sodium hydroxide addition was found with the hemicellulose content decreasing by up to 66%. In general, alkali improved the ultimate methane yield and the methane yield obtained after 30 days. Briquetting reduced the optimal alkali concentration needed to reach the maximal improvement in methane yield compared to merely shredding. The results proved that it is not necessary to work with the high alkali concentrations traditionally used to improve methane production if the straw is mechanically pretreated prior to alkali addition. Methane yield obtained after 30 days as well as net energy yield rose by around 40% when briquetting was combined with 2% sodium hydroxide. Therefore, a low concentration of alkali, especially sodium hydroxide, applied at ambient temperature can improve the energy and the economic balance of pretreatment. [ABSTRACT FROM AUTHOR]

Antibiotics are frequently used in animals to treat sickness and prevent infection especially in industrial meat production. Some of the antibiotics cannot be completely metabolized and, as an unavoidable result, are excreted and thus end up in manure which is then spread in the environment. Currently increasing amounts of manure is used in biogas production before spreading the residuals on agricultural fields. In this study, the removal patterns of sulfonamides (sulfadiazine, sulfamethizole, sulfamethoxazole) and macrolides (clarithromycin, erythromycin), as well as trimethoprim, were investigated during the anaerobic digestion of pig manure. Batch kinetic tests were conducted both at thermophilic and psychrophilic condition for 40 days. Some of the antibiotics (clarithromycin, sulfadiazine, sulfamethizole) were persistent in all experiments. Thus, no biodegradation was found for sulfadiazine and sulfamethizole in this study. From the studied compounds, only erythromycin was clearly removed and probably degraded during anaerobic digestion with 99% and 20% removal under thermophilic and psychrophilic condition. The removal of erythromycin was fitted to a single first-order kinetic reaction function, giving reaction rate constant of 0.29 day − 1 and 0.005 day − 1 , respectively. [ABSTRACT FROM AUTHOR]

Alkaline pretreatment of lignocellulosic biomass has been intensively investigated but heavy water usage and environmental pollution from wastewater limits its industrial application. This study presents a pretreatment technique by in-situ injection of potassium hydroxide concentrations ranging from 0.8% to 10% (w/w) into the briquetting process of wheat straw and meadow grass. Results show that the biomethane yield and hydrolysis rate was improved significantly with a higher impact on wheat straw compared to meadow grass. The highest biomethane yield from wheat straw briquettes of 353 mL.g −1 VS was obtained with 6.27% (w/w) potassium hydroxide injection, which was 14% higher than from untreated wheat straw. The hydrolysis rates of wheat straw and meadow grass increased from 4.27 × 10 −2 to 5.32 × 10 −2 d −1 and 4.19 × 10 −2 to 6.00 × 10 −2 d −1 , respectively. The low water usage and no wastewater production make this a promising technology. [ABSTRACT FROM AUTHOR]

The interest in sustainable bioenergy solutions has gained great importance in Europe due to the need to reduce GHG emissions and to meet environmental policy targets, not least for the protection of groundwater and surface water quality. In the Municipality of Solrød in Denmark, a novel bioenergy concept for anaerobic co-digestion of food industry residues, manure and beach-cast seaweed has been developed and tested in order to quantify the potential for synergies between climate change mitigation and coastal eutrophication management in the Køge Bay catchment. The biogas plant, currently under construction, was designed to handle an annual input of up to 200,000 t of biomass based on four main fractions: pectin wastes, carrageenan wastes, manure and beach-cast seaweed. This paper describes how this bioenergy concept can contribute to strengthening the linkages between climate change mitigation strategies and Water Framework Directive (WFD) action planning. Our assessments of the projected biogas plant indicate an annual reduction of GHG emissions of approx. 40,000 t CO 2 equivalents, corresponding to approx. 1/3 of current total GHG emissions in the Municipality of Solrød. In addition, nitrogen and phosphorous loads to Køge Bay are estimated to be reduced by approx. 63 t yr. − 1 and 9 t yr. − 1 , respectively, contributing to the achievement of more than 70% of the nutrient reduction target set for Køge Bay in the first WFD river basin management plan. This study shows that anaerobic co-digestion of the specific food industry residues, pig manure and beach-cast seaweed is feasible and that there is a very significant, cost-effective GHG and nutrient loading mitigation potential for this bioenergy concept. Our research demonstrates how an integrated planning process where considerations about the total environment are integrated into the design and decision processes can support the development of this kind of holistic bioenergy solutions. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates near infra-red spectroscopy (NIRS) as an indirect and rapid method to assess the biochemical methane potential (BMP) of meadow grasses. Additionally analytical methods usually associated with forage analysis, namely, the neutral detergent fibre assay (NDF), and the in-vitro organic matter digestibility assay (IVOMD), were also tested on the meadow grass samples and the applicability of the models in predicting the BMP was studied. Based on these, regression models were obtained using the partial least squares (PLS) method. Various data pre-treatments were also applied to improve the models. Compared to the models based on the NDF and IVOMD predictions of BMP, the model based on the NIRS prediction of BMP gave the best results. This model, with data pre-processed by the mean normalisation method, had an R 2 value of 0.69, a root mean square error of prediction (RMSEP) of 37.4 and a residual prediction deviation (RPD) of 1.75. [Copyright &y& Elsevier]