Assessment of the degradation efficiency of full-scale biogas plants: A comparative study of degradation indicators

Exploring increased hydraulic retention time as a cost-efficient way of valorizing residual biogas potential

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.

Effect of fruits and vegetables in the anaerobic digestion of food waste from university restaurant

The aim of this study was to evaluate the theoretical potential of methane production of the food waste generated by a university restaurant, as well as to verify the influence of the fruit and vegetable waste in the feeding composition of an anaerobic bioreactor treating this type of waste. Four feeding compositions combining three fractions of the food waste (fruit and vegetable fraction, soy protein and beans fraction, and rice fraction) at different concentrations were tested in anaerobic processes lasting 10 and 30 days. Additionally, a study of the theoretical potential of methane production from each fraction that composes the food waste was carried out, as well as the evaluation of the specific methanogenic activity of the anaerobic sludge. Despite its low theoretical potential of methane production (0.037 L_CH4/g), the presence of the fruit and vegetable mixture in three of the feeding compositions led to greater organic matter degradation (above 69%) and CH₄ yields (above 0.20 L_CH4/g_VS) in both periods tested, in comparison with the achieved by the feeding composition lacking this fraction. The results suggest that the presence of the fruit and vegetable mixture contributed with the supplementation of micro- and macroelements to the anaerobic sludge during the digestion of food waste.

Impact of Operational Factors, Inoculum Origin, and Feedstock Preservation on the Biochemical Methane Potential

Anaerobic digestion for the valorization of organic wastes into biogas is gaining worldwide interest. Nonetheless, the sizing of the biogas plant units require knowledge of the quantity of feedstock, and their associated methane potentials, estimated widely by Biochemical Methane Potential (BMP) tests. Discrepancies exist among laboratories due to variability of protocols adopted and operational factors used. The aim of this study is to verify the influence of some operational factors (e.g., analysis frequency, trace elements and vitamins solution addition and flushing gas), feedstock conservation and the source of inoculum on BMP. Among the operational parameters tested on cellulose degradation, only the type of gas used for flushing headspace of BMP assays had shown a significant influence on methane yields from cellulose. Methane yields of 344 ± 6 NL CH₄ kg⁻¹ VS and 321 ± 10 NL CH₄ kg⁻¹ VS obtained from assays flushed with pure N₂ and N₂/CO₂ (60/40 v/v). The origin of inoculum (fed in co-digestion) only significantly affected the methane yields for straw, 253 ± 3 and 333 ± 3 NL CH₄ kg⁻¹ VS. Finally, freezing/thawing cycle effect depended of the substrate (tested on biowaste, manure, straw and WWTP sludge) with a possible effect of water content substrate.

Operating Performance of Full-Scale Agricultural Biogas Plants in Germany and China: Results of a Year-Round Monitoring Program

Germany (DE) and China (CN) have different political approaches in supporting the biogas sector. Three German and three Chinese large-scale biogas plants (BGPs) were evaluated as part of a year-round monitoring program. Laboratory methods were utilized to analyze the chemical indicators. Results showed a stable anaerobic digestion process without system failures in all BGPs. The methane yield had a range of 0.23–0.35 m3CH4/kgODM for DE BGPs and 0.11–0.22 m3CH4/kgODM for CN BGPs, due to different substrates and working temperatures. Financial analyses indicated that DE BGPs are viable under their current feed-in tariffs contracts. Their financial internal rate of return (IRR) ranged between 8 and 22%. However, all CN BGPs had negative IRRs, indicating that they are financially unfeasible. Risk analyses illustrated that DE BGPs will face financial nonviability if benefits decrease by 9–33% or costs increase by 10–49%, or if a combined worse case (benefit decrease and cost increase) of 5–20% occurs. Incentives to BGP operations are particularly important in China, where the government should consider switching the construction-based subsidy to a performance-based subsidy system to motivate the operators. BGP monitoring is necessary to understand the performance, in addition to briefing policymakers in case a policy reform is needed.

Influence of Aerobic Pretreatment of Poultry Manure on the Biogas Production Process

Anaerobic digestion of poultry manure is a potentially-sustainable means of stabilizing this waste while generating biogas. However, technical, and environmental protection challenges remain, including high concentrations of ammonia, low C/N ratios, limited digestibility of bedding, and questions about transformation of nutrients during digestion. This study evaluated the effect of primary biological treatment of poultry manure on the biogas production process and reduction of ammonia emissions. Biogas yield from organic matter content in the aerobic pretreatment groups was 13.96% higher than that of the control group. Biogas production analysis showed that aerobic pretreatment of poultry manure has a positive effect on biogas composition; methane concentration increases by 6.94–7.97% after pretreatment. In comparison with the control group, NH3 emissions after aerobic pretreatment decreased from 3.37% (aerobic pretreatment without biological additives) to 33.89% (aerobic pretreatment with biological additives), depending on treatment method.

Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP)

Biochemical methane potential (BMP) tests used to determine the ultimate methane yield of organic substrates are not sufficiently standardized to ensure reproducibility among laboratories. In this contribution, a standardized BMP protocol was tested in a large inter-laboratory project, and results were used to quantify sources of variability and to refine validation criteria designed to improve BMP reproducibility. Three sets of BMP tests were carried out by more than thirty laboratories from fourteen countries, using multiple measurement methods, resulting in more than 400 BMP values. Four complex but homogenous substrates were tested, and additionally, microcrystalline cellulose was used as a positive control. Inter-laboratory variability in reported BMP values was moderate. Relative standard deviation among laboratories (RSDR) was 7.5 to 24%, but relative range (RR) was 31 to 130%. Systematic biases were associated with both laboratories and tests within laboratories. Substrate volatile solids (VS) measurement and inoculum origin did not make major contributions to variability, but errors in data processing or data entry were important. There was evidence of negative biases in manual manometric and manual volumetric measurement methods. Still, much of the observed variation in BMP values was not clearly related to any of these factors and is probably the result of particular practices that vary among laboratories or even technicians. Based on analysis of calculated BMP values, a set of recommendations was developed, considering measurement, data processing, validation, and reporting. Recommended validation criteria are: (i) test duration at least 1% net 3 d, (ii) relative standard deviation for cellulose BMP not higher than 6%, and (iii) mean cellulose BMP between 340 and 395 NmLCH4 gVS−1. Evidence from this large dataset shows that following the recommendations—in particular, application of validation criteria—can substantially improve reproducibility, with RSDR < 8% and RR < 25% for all substrates. The cellulose BMP criterion was particularly important. Results show that is possible to measure very similar BMP values with different measurement methods, but to meet the recommended validation criteria, some laboratories must make changes to their BMP methods. To help improve the practice of BMP measurement, a new website with detailed, up-to-date guidance on BMP measurement and data processing was established.

Planning methodology for anaerobic digestion systems on animal production facilities under uncertainty

Anaerobic digestion (AD) reduces GHG emission and facilitates renewable energy generation. The slow rate of adoption of this technology is often attributed to economic and technical considerations. Collaboration of two or more dairy farms into a centralized AD system can improve the process economics through economies of scale. However, uncertainties related to the process parameters and the scope/scale of the collaborative implementation impede its adoption. This study presents techno-economic optimization model as a design aid to determine ideal location, capacity, and participation level (cluster size) that maximize economic return on a cooperative digester. This study employs a probabilistic approach to overcome uncertainty regarding project parameters such as manure biomethane potential (BMP), project capital, and electricity sale price. Two case studies based on dairy production regions in Wisconsin were developed to test the model and demonstrate its capabilities. Herd sizes and spatial distribution in a given region were found to be critical factors in determining the viability of digestion projects in general, and collaborative digestion systems in particular. The number of simulation runs needed to capture the probability of profitable AD facility establishment was less than 1000 for both case studies assessed. Electricity sale price and biomethane potential of feedstock utilized were found to be the most restrictive to the feasibility of AD adoption. Changing the optimization objective function, to adopting maximization, favored the formation of collaborative AD facilities for both case studies evaluated.

Legal requirements for digestate as fertilizer in Austria and the European Union compared to actual technical parameters

Biogas is mainly discussed in the context of renewable energy, digestate is playing a minor role. However, in the upcoming EU Fertilizer Product Regulation digestate as eligible component material is in line with the circular economy action plan. This article discusses the quality of raw digestate based on analysis data from Austrian biogas plants regarding to the current Austrian as well as the European fertilizer regulatory framework. Therefore, a survey among Austrian biogas plant operators for results of digestate analysis and substrate input and energy output is used. The results show, that the legislative framework applies to a minimum of 1.5 million tonnes of digestate. Austrian digestate is of high quality with regards to the nutrient content, required limit values for heavy metals and is also safe with respect to their hygienic parameters. The following European fertilizer product function categories are available for international trade of digestate: "organic soil improver", "growing medium", and "organic, non-microbial plant biostimulant". Establishing legal criteria determining End-of-Waste and subsequently the product status for digestate is another important step in the European circular economy.

Biochemical Methane Potential (BMP) Assay Method for Anaerobic Digestion Research

Biochemical methane potential (BMP) tests are widely used for characterizing a substrate’s influence on the anaerobic digestion process. As of 2018, there continues to be a lack of standardization of units and techniques, which impacts the comparability and validity of BMP results. However, BMP methods continue to evolve, and key aspects are studied to further eliminate systematic errors. This paper aims to update these key aspects with the latest research progress both to introduce the importance of each variable to those new to BMP measurements and to show the complexity required to design an accurate BMP test.