Cow manure stabilizes anaerobic digestion of cocoa waste

Anaerobic digestion of a mono-feedstock often causes low methane yields and process instability. An effective strategy to overcome these barriers is co-digestion with animal manure. The obtained process improvement is often attributed to buffer capacity, nutrients, vitamins and trace metals, and microorgan-isms present in manure, but it remains unknown which factor plays the key role in digester performance. Here, we investigated anaerobic digestion of cocoa waste in four different treatments: mono-digestion, addition of synthetic nutrients, co-digestion with sterile cow manure, and co-digestion with raw cow manure. Co-digestion with raw manure resulted in the highest methane yield of 181 +/- 39 L kg(-1) VS (vola-tile solids), similar to the co-digestion with sterile manure, i.e., 162 +/- 52 L kg(-1) VS. The supplementation of synthetic nutrients to the anaerobic digestion of cocoa waste only temporarily increased methane yield, indicating that this will tackle a lack of nutrients in the short term, but has a limited long-term con-tribution to the stabilization of the process. Hence, because of the inability of synthetic nutrients to sta-bilize the digestion process and the similarity between the digesters fed sterile and raw manure, both at the physico-chemical and microbial level, the key contribution of manure co-digestion with cocoa seems to be the provision of buffering capacity. (C) 2021 Elsevier Ltd. All rights reserved.