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Characteristics of Substrates Used for Biogas Production in Terms of Water Content
New technologies based on the anaerobic digestion process make it possible to manage problematic waste. Methane efficiency depends largely on the level of the hydration of the substrates used for biogas production and their ability to decompose easily. The aim of this study was to present the current state of knowledge and practices in substrate hydration characteristics, focusing on pretreatment methods as the preferred method for improving efficiency. The paper discusses issues related to the degree of hydration of substrates in the context of their use in biogas plants. Reference was also made to topics related to the transportation and logistics of raw material supply regarding environmental impact. Biogas plant projects should be expanded to include an element related to assessing the impact of raw material deliveries on the immediate environment. Previous papers have not sufficiently analyzed the aspect related to the hydration of substrates used in anaerobic digestion processes. The presented and discussed research results can be implemented to optimize biogas plant water management processes. By replacing standard feedstock transportation methods with a pipeline, the environmental impact can be reduced by nearly ten times.
Digestate as a Source of Nutrients: Nitrogen and Its Fractions
Due to fossil-fuel-limitation constraints, new energy sources are being sought. On the other hand, organic fertilizers that can be used in agriculture are increasingly being sought. One of the renewable energy sources is biogas produced from substrates large in organic matter. Apart from biogas, the product of the anaerobic digestion process is digestate. Due to the high content of nutrients, mainly nitrogen, this product can be successfully used as a fertilizer. This study aims to determine the content of total nitrogen (Ntot) and its selected fractions in the raw and processed digestate from agricultural biogas plants. The nitrogen fractions included N-NH4, N-NO3, and Norganic. The total nitrogen content (Ntot) and its fraction in raw digestate were determined. Samples used for the research came from five agricultural biogas plants. Separation into liquid and solid fractions is one of the methods for digestate management. The nitrogen content in selected samples obtained after separation of digestate in a biogas plant and on a laboratory scale was also checked. The obtained results show that digestate from agricultural biogas plants is a nitrogen-rich fertilizer. The content of Ntot in the tested samples ranged from 1.63 g∙kg−1 to 13.22 g∙kg−1 FM. The N-NH4 content in the analyzed material ranged from 0.75 to 4.75 g∙kg−1 FM. The determined physical and chemical properties confirm that the raw and processed digestate is characterized by appropriate fertilization properties, with particular emphasis on the content of Ntot and the share of its mineral forms. Based on the chemical composition, digestate from agricultural biogas plants can be considered a multi-component fertilizer.
Research on a New Method of Water Recovery from Biogas Plant Digestate
Digestate is a product with valuable fertilizing properties, remaining after the anaerobic fermentation process. An essential feature of the substance in question is its high water content of up to 97%. To improve the fertilizer value of the digestate, it is necessary to dehydrate it to produce a concentrated product. This paper determined the possibility of dewatering the digestate using an innovative reactor design. The study, conducted on a laboratory scale, used digestate from a Polish biogas plant. The dewatering technique described in the paper is based on the evaporation and condensation of water vapor on the inner surface of the reactor dome. The condensate accumulated on the leach trough and was directed to a storage tank. During the weeks of testing, 11.5 kg of condensate was separated from the initial weight of the digestate (32 kg), with a dry weight of 6.11%. The resulting condensate from dehydration had an average pH value of 9.0 and an average ammonium nitrogen content of 2.07 g∙kg−1. The economic calculations made in the paper allowed for estimating the expected savings associated with the management of digestate in Poland. The research showed the proposed technology’s high potential for dewatering digestate under laboratory conditions.
Food Waste Bioconversion Features Depending on the Regime of Anaerobic Digestion
Approximately one-third of global food production is wasted annually, which contributes significantly to greenhouse gas emissions and economic costs. Anaerobic digestion (AD) is an effective method for converting food waste into biogas, but its efficiency depends on factors such as temperature and substrate composition. This study compared mesophilic and thermophilic AD of selectively collected fruit and vegetable waste, quantifying process efficiency and identifying factors leading to collapse. Studies were performed in 1 dm3 reactors with gradually increasing organic loading rates until process collapse. Process dynamics, stability, and gas yields were assessed through daily biogas measurements and analyses of pH, FOS/TAC ratio, sCOD, ammonia, volatile fatty acids, alcohols, total and volatile solids, and C/N ratio. Research has shown that peak methane yields occurred at OLR = 0.5–1.0 kg VS·m−3·d−1, with thermophilic systems producing 0.63–5.48% more methane during stable phases. Collapse occurred at OLR = 3.0 in thermophilic and 4.0 in mesophilic reactors, accompanied by sharp increases in methanol, acetic acid, butyric acid, propionic acid, and FOS/TAC. The pH dropped to 5.49 and 6.09. While thermophilic conditions offered higher methane yields, they were more susceptible to rapid process destabilization due to intermediate metabolite accumulation.