The potential for secondary anaerobic treatment of digestates and associated capture of biogas

The effluent stream of anaerobic digestion (AD) processes, digestate, holds biogas production potential under further anaerobic conditions due to incomplete stabilization of raw feedstock in digesters [1]. Several studies have been employed to capture residual biogas (RB) of digestates in closed storage tanks [1], to evaluate the degradation efficiency of anaerobic digesters and the stability of the digestates [2-4], and to recover the biogas potential by recycling the digestate to the anaerobic digester with or without pre-treatment [5-6]. On the other hand, a secondary anaerobic digester for digestate treatment may have a potential to reduce the organic load of digestate as well as to capture RB associated with the decomposition of residual organics. In this context, six digestate samples obtained from anaerobic digesters operated with manures and their mixtures, mixture of manures with organic wastes and mixture of primary and secondary sewage sludges were further digested in batch anaerobic digesters. The results obtained were evaluated in terms of RB yields with specific emphasis on the anaerobic treatability of digestates. Further AD of the digestates resulted in the removal of total chemical oxygen demand (COD) by 25-64% from the digestate content. Nutrient supplementation improved COD removal to 35-84%. VS of the digestates could be degraded by 13-65%. RB yields obtained were in the range of 0.111-0.326 Lbiogas/g VS (Image 1). These yields were found to be comparable to many raw feedstocks of the digesters, such as grey waste (0.08-0.15 Lbiogas/g VS), dairy manure (0.076-0.470 Lbiogas/g VS), cattle manure (0.15-0.35 Lbiogas/g VS), horse manure (0.222 Lbiogas/g VS) and municipal secondary sludge (0.20-0.35 Lbiogas/g VS).