Aquaculture faecal waste generates different products during anaerobic digestion depending on nutrient composition

The inclusion of carbohydrate-rich ingredients in aquafeeds has resulted in an increased fraction of undigested material, culminating in faecal waste enriched in unutilized nutrients containing carbon, nitrogen and phosphorus. This study explored the impact of faecal composition, as influenced by diet, on the products of anaerobic digestion with a focus on organic acids (OAs). The aim was to in vitro assess the potential of faeces as internal carbon for denitrification, promoting circularity in recirculating aquaculture systems. In this regard, settleable faeces originating from six diets (DDGS, Dried distillers' grains with solubles; HFM, Hydrolyzed feather meal; IM, Insect meal; SCP, Single-cell protein; SSM, Shrimp shell meal; SWP, Seaweed protein) fed to European seabass, were incubated for a 14-day period in anoxic batch reactors. Nutrient solubilization (chemical oxygen demand, total Kjeldhal nitrogen, total phosphorus) was measured over time, and the final yield of OA was studied in relation to prokaryotic community composition. Results showed that digestion of faecal waste with a high crude-protein-to-carbohydrate ratio leads to an increased amount of OA which is greatly dominated by acetate. Among them, SSM faeces exhibited the highest final OA yield, resulting from a continuous increase over time. Despite variations in OA quantity and profile, prokaryotic composition did not substantially differ among treatments at the end of the trial, with only the relative abundance of three genera varying significantly (Anaerostignum, Bythopirellula, Mycobacterium). Yet, lactate concentration positively correlated with several taxa (Trichococcus, Oleispira, Defluviitaleaceae, Anaerocolumna, and Carboxyliverga) and butyrate with Anaerostignum. Alongside, ammonia release was minimal for all treatments, while phosphorus dissolution did not correlate to the faecal phosphorus content but was rather a result of acidification due to OA production. Overall, considering that an optimal carbon source for denitrification should produce a high amount of end OAs (i.e. acetate) along with low dissolved nitrogen and phosphorus, this research suggests that faecal waste originating from certain carbohydrate-rich aquafeed ingredients can be suitable as internal carbon source for denitrification.