Evaluating the impact of substrate addition for anaerobic co-digestion on biogas production and digestate quality: The case of deinking sludge.

To reduce greenhouse gas emissions from organic waste, anaerobic digestion has created new opportunities for energy and nutrient recovery from these wastes. However, the use of certain organic wastes in anaerobic digestion is limited due to their atypical physicochemical characteristics (e.g. unbalanced carbon to nitrogen ratio, high ash concentration). Deinking sludge is a residue from the paper recycling industry and is one of such substrates. This study aims at evaluating the impact of deinking sludge (DS) addition into a conventional co-digestion mixture on methane production and digestate quality. To this end, an integrated method was proposed, combining the analysis of physicochemical and biodegradability characteristics with parsimonious modeling using the SYS-Metha tool. The measured characteristics of the deinking sludge showed that its potential use in mono-digestion conditions is very limited. When co-digested with food waste and municipal sludge, no significant synergies or antagonisms were found. Based on these experiments, model simulations were executed to determine the optimal conditions for co-digestion with food waste and municipal sludge. A maximum of 22% of deinking sludge on a fresh mass basis can be added into a co-digestion mixture to achieve proper wet anaerobic digestion conditions. Regarding digestate quality, the addition of DS reduced nutrient and contaminants concentrations, which have an impact on digestate management, particularly for land application. Overall, the proposed methodology in this study allows determining optimal co-digestion mixtures and highlighted the limits needing further investigation under pilot/real conditions.