Pretreatment and co-digestion of wastewater sludge for biogas production: Recent research advances and trends

Currently, sludge is not considered as a waste any more, since it is capable of producing valuable products. Besides land disposal and thermochemical processes (i.e. pyrolysis, combustion, gasification), biological processes appear as promising valorisation routes to treat wastewater sludge efficiently. Anaerobic digestion (AD) processes are already being applied at industrial scales for the effective disposal and valorisation of sludge. However, methane yields from sludge anaerobic digestion remain low compared to other types of organic waste. Thus, pretreatment and co-digestion contribute to improve the degradability of organic matter and methane potential of sludge, respectively. This paper reviews the recent achievements in sludge pretreatment and co-digestion with other substrates such as the organic fraction of municipal solid waste, fatty waste, lignocellulosic and algal biomass. Furthermore, recent studies combining co-digestion and pretreatment are examined. The paper also provides recommendations to better manage sludge recovery by taking into account multiple aspects such as techno-economic feasibility, the effect of pretreatment on both the physico-chemical properties of sludge and the quality of digestate. The socio-environmental and legislative aspects are also essential in order to ensure the sustainability of the process.