Co-digestion of sewage sludge and food waste in anaerobic membrane bioreactors: Process-level and plant-wide assessments

The co-digestion of sewage sludge (SS) and food waste (FW) in an anaerobic membrane bioreactor (AnMBR) was investigated in a lab-scale system. The AnMBR was operated at an organic loading rate of 1.2 kg chemical oxygen demand (COD)/m3·d during the first stage (mono-digestion) and 2.2 kg COD/m3·d during the second stage (co-digestion). Methane yield increased from 106 to 137 NmL CH₄/g COD_fed under co-digestion conditions, and the model predictions showed good agreement with the experimental results (adjusted R2 = 0.96). COD mass balance analysis showed that the fraction of influent COD converted to methane increased from 30.8% during mono-digestion to 41.3% during co-digestion, corresponding to a 34% relative increase, while the COD fractions ending up in the permeate and waste sludge decreased by 58% and 14%, respectively. From a plant-wide perspective, replacing a conventional anaerobic digester (CAD) with an AnMBR and implementing co-digestion of SS and FW led to a deterioration in effluent quality due to increased nutrient release, particularly phosphorus. Nevertheless, compared to mono-digestion in a CAD, the AnMBR reduced the operational cost index (OCI) by 22.4%, while FW co-digestion in the AnMBR further decreased OCI by 62.4%. Overall, these results demonstrate the potential of SS and FW co-digestion in AnMBRs to improve plant-wide operational cost performance, while also revealing the need for post-treatment of the nutrient-rich permeate to mitigate the deterioration in plant effluent quality and reduce the nutrient load on the main biological system.