New opportunities for the European Biogas industry: A review on current installation development, production potentials and yield improvements for manure and agricultural waste mixtures

Despite most biogas-plants worldwide being within the EU-country territory, the potential of converting existing waste streams is not exploited sufficiently in the current industry landscape. Through analysing EU databases, it was determined that only 2.65% of biogas is produced to what is possible. The biogas generation potential is quoted as 2.46 x 10(7) TJ in energetic value, which is juxtaposed by the current production estimates of 6.51 x 10(5) TJ in 2021. We summarise the current legislative landscape and biogas outlook, primarily showing that many waste systems become uneconomical due to high transport distances to industrial-scale, high-efficiency biogas plants. To reduce environmental impact and profit from waste-to-energy mechanisms, individual farms are to focus on small-scale local biogas plants to boost their self-sufficiency objectives. The main feedstocks present in agriculture have been analysed to evaluate the best performance parameter to provide insight into process optimisation. Through statistical analysis of 65 independent co-digestion studies of manure and ligno-cellulosic biomass systems, the carbon-nitrogen ratio (C/N) is the most important factor to produce a high biomethane proportion in biogas. The ratio is equally crucial in the production quantity of biogas, as slight deviances may cause pH changes to either volatile fatty acid or ammonia accumulation. Modelling tasks showed that especially systems containing less than 50% manure show several correlations between pH, reduced volatile solids and C/N. Any inflection in pH yields inhibition as microbial performance reduces due to unfavourable conditions in the biogas reactor. It is apparent that anaerobic co-digestion of feedstocks is advantageous and using mixing regimes, larger implementation of low-cost pilot-scale digesters is advised in rural settlements. The behaviour of C/N, pH, solids retention time and manure proportion are discussed, showing that low C/N ratios of 15-25 are vital for good performance, whilst especially manure-heavy digestions perform better under higher pH conditions than comparative studies with more lignocellulosic biomass. The potential to degrade volatile solids (VS) is decreased substantially after more than 50% of manure have been added to the co-digestion.