Renewable energy recovery from sewage sludge derived from chemically enhanced precipitation

To achieve energy neutrality in wastewater treatment, reduction of energy consumption and renewable energy production has been proposed as a sustainable solution within circular economy. The performance of an anaerobic Continuous Stirred Tank Reactor (CSTR) digesting sewage sludge from a Chemically Enhanced Primary Treatment (CEPT) unit regarding organic carbon removal and renewable biogas production was evaluated under various Hydraulic Retention Times (HRTs). Anaerobic digestion of CEPT instead of primary sludge resulted in higher Chemical Oxygen Demand (COD) and Volatile Suspended Solids (VSS) removal efficiencies, i.e. 46-51% and 28–35% for CEPT sludge compared to 33% and 20% for primary sludge, respectively. Biogas production from CEPT sludge was constant and stable (0.15–0.16 L/L.d) at all HRTs examined, while biogas production was lower during digestion of primary sludge (0.06 L/L.d). The combination of CEPT, which concentrates organic matter derived from wastewater and minimizes oxygen consumption, and the anaerobic digestion of the CEPT sludge, which reduces energy consumption and generates renewable energy form non-conventional resources, provides a sustainable wastewater treatment practice towards circular economy. In a study performed for the proposed CEPT + CSTR scheme for the treatment of wastewater in a city with 50,000 population equivalent, the annual energy gain was estimated as 1,314,000 kWh.