Abstract
Anaerobic digestion (AD) is a process in which microorganisms, under oxygen-free conditions, convert organic matter into biogas and digestate. Normally, only 40–70% of biomass is converted into biogas; therefore, digestate still contains significant amounts of degradable organic matter and biogas potential. The recovery of this residual biogas potential could optimize substrate utilization and lower methane emissions during digestate storage and handling. Post-treatment methods have been studied with the aim of enhancing the recovery of biogas from digestate. This review summarizes the studies in which these methods have been applied to agricultural and wastewater digestate and gives a detailed overview of the existing scientific knowledge in the field. The current studies have shown large variation in outcomes, which reflects differences in treatment conditions and digestate compositions. While studies involving biological post-treatment of digestate are still limited, mechanical methods have been relatively more explored. In some cases, they could increase methane yields of digestate; however, the extra gain in methane has often not covered treatment energy inputs. Thermal and chemical methods have been studied the most and have yielded some promising results. Despite all the research conducted in the area, several knowledge gaps still should be addressed. For a more thorough insight of the pros and cons within post-treatment, more research where the effects of the treatments are tested in continuous AD systems, along with detailed economic analysis, should be performed.
Highlights
An inter-stage treatment configuration with thermal explosion between two anaerobic digesters increased the overall biogas yield by 44, 31 and 38% when compared to a single step Anaerobic digestion (AD), double step AD and single AD preceded by steam explosion pre-treatment of raw sludge, respectively
Zhang et al (2016) [45] conducted a 4-day AD batch test, where it was observed that methane production could increase by 38 and 22% when digestate was treated with HNO2 at concentrations of 0.77 and 1.54 mg N/l, respectively
For positive effects on methane yields to occur, lignin should be the main fraction consumed and the possible enhancement on biodegradability caused by delignification should offset any VS losses caused by the consumption of digestible fractions, such as cellulose and hemicellulose
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Biogas can be produced from many types of organic waste materials, including sludge from wastewater treatment plants (WWTP) and livestock and agricultural wastes, such as crop residues and manure [3]. These materials are interesting due to their abundance and availability worldwide. Post-treatment aims at enhancing the hydrolAD of agricultural residues,and hemicellulose degraded faster than cellulose, resulting in ysis of the recalcitrant fractions and[12]. This review summarizes studies in which thermal, mechanical, chemical, and biological treatment technologies have been applied to agricultural and sludge-based digestate aiming to recover additional biogas. The results reported refer to the increase in methane yields obtained from single batch AD tests of treated WD in comparison to an untreated control
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