Anaerobic Co-Digestion of Fish Sludge Originating from a Recirculating Aquaculture System

Anaerobic co-digestion (AcoD) of fish sludge (FS) with food waste (FW), and fruit and vegetable waste (FVW) for biogas and methane production was optimised in small-scale bioreactors, and batch and semi-continuous pilot-scale digesters, under mesophilic (37 ℃) conditions. An experimental mixture design was first applied to small-scale biomethane potential (BMP) tests, to determine the optimal mixture proportions of the AcoD of FS, FW, and FVW that maximise the specific methane yield (SMY in NmLCH₄ gVS⁻¹). The optimal mixture proportion was 67%FS:18%FW:19%FVW (w/w), producing 401 mLCH₄ gVS⁻¹, which was 8 times higher than the SMY when FS was mono-digested (48 mLCH₄ gVS⁻¹). The SMY achieved in batch pilot-scale digesters were 70–82% of methane yields obtained in BMP tests under the same operating conditions, with stable biogas production and no apparent inhibition during the batch run. Semi-continuous operation of the pilot-scale digester was undertaken with organic loading rates (OLRs) of 1, 2, and 3 \(gVS{L}^{-1}{d}^{-1}\) $gVS{L}^{-1}{d}^{-1}$ , provided intermittently. However, the digester did not achieve stable biogas production at all of the evaluated OLRs, due to the intermittent feeding and accumulation of volatile fatty acids (VFAs): Improved process stability was achieved at an OLR of 2 \(gVS{L}^{-1}{d}^{-1}\) $gVS{L}^{-1}{d}^{-1}$ , compared to OLRs of 1 and 3 \(gVS{L}^{-1}{d}^{-1}\) $gVS{L}^{-1}{d}^{-1}$ . Optimisation of the AcoD process resulted in attractive biomethane yields from FS with FW and FVW co-feeds, indicating that producing biogas from co-digestion of FS with relevant substrates is a valuable managing tool for FS, while simultaneously providing renewable energy. The work provides novel data that elucidated optimal proportions in which to combine FS, FW and FVW to obtain optimal biogas production, and provided important new information relevant for the scale up and continuous operation of an AD process for treating FS.