Anaerobic co-digestion of sewage sludge with other organic wastes: A comprehensive review focusing on selection criteria, operational conditions, and microbiology

Abstract

Mono-digestion of sewage sludge (SS) usually suffers from insufficient content of organic matter and low C/N ratio. In order to modify these parameters and enhance the process performance, many organic wastes have been used as co-substrates in conjunction with SS in anaerobic co-digestion (ACoD). In this review paper, the co-substrate selection criteria for ACoD with SS were surveyed. In addition, the effects of all examined co-substrates and operational conditions on process performance in terms of volatile solids (VS) reduction, methane production and microbial community were comprehensively reviewed. Accordingly, the reviewed waste materials were classified into three groups. The first group, including food waste and FOG (fat, oil and grease), can be used as co-substrate with high reliability due to their high organic content, high C/N ratio, negligible amount of toxic compounds, and as well, no evidence of adverse effects on the process in previous reports. The second group includes the wastes with moderate probability of disrupting the process due to low C/N ratio or low organic content, the presence of inhibitory factors, and as well, the observation of their adverse effects in some studies. Industrial waste, slaughterhouse waste, agricultural residue and animal manure can be classified as the second group. The third group, including microalgae, represents the wastes with a high risk of disrupting the process since a considerable number of studies have reported their adverse effects on the process. The issues that should be addressed in future studies include application of paper and cardboard waste in sewage sludge ACoD, sewage sludge ACoD under thermophilic conditions, microbiology of the process, and the dewaterability of the digested sludge resulting from ACoD of sewage sludge.