Anaerobic co-digestion of waste activated sludge and greasy sludge from flotation process: Batch versus CSTR experiments to investigate optimal design

In this study, the maximum ratio of greasy sludge to incorporate with waste activated sludge was investigated in batch and CSTR experiments. In batch experiments, inhibition occurred with a greasy sludge ratio of more than 20–30% of the feed COD. In CSTR experiments, the optimal greasy sludge ratio was 60% of the feed COD and inhibition occurred above a ratio of 80%. Hence, batch experiments can predict the CSTR yield when the degradation phenomenon are additive but cannot be used to determine the maximum ratio to be used in a CSTR configuration. Additionally, when the ratio of greasy sludge increased from 0% to 60% of the feed COD, CSTR methane production increased by more than 60%. When the greasy sludge ratio increased from 60% to 90% of the feed COD, the reactor yield decreased by 75%.

► Greasy sludge is a suitable co-substrate to waste activated sludge digestion.
► Inhibitions occur with high greasy sludge ratio in batch and CSTR experiments.
► Inhibitions occur at a lower greasy sludge ratio in batch experiments than in CSTR.
► Batch experiments are not suitable to determine the maximal ratio of co-substrate.
► Performance drops for high greasy sludge ratios are induced by LCFA accumulations.