Anaerobic stabilisation of sludge produced during municipal wastewater treatment by electrocoagulation.

Anaerobic digestion of sludge from small electrocoagulation wastewater treatment plant (SEWWTP) is described. The sludge for digestion (SEWWTP sludge) was taken from pilot-scale SEWWTP with the capacity of about 200-population equivalent (25 m3 of municipal wastewater per day). Due to the technology of wastewater treatment, the characteristics of SEWWTP sludge was different from sludge produced in conventional mechanical-biological wastewater treatment plant. Therefore, experiments were focused on possibilities of anaerobic sludge digestion and determination of conditions and parameters (amount and quality of the sludge, biogas production, etc.). Average COD removal efficiency in the pilot-scale SEWWTP exceeded 80%. Organic content of excess sludge (volatile suspended solids (VSS)) was in the range of 52.1-59.2% (these values are much lower compared to VSS content in raw sludge from conventional municipal wastewater treatment plant, where VSS is about 75%). Biogas production from anaerobic digestion of SEWWTP sludge was approximately three times lower compared to standard production in conventional municipal wastewater treatment plant. Low pH (6.5-6.7), high concentration of iron (up to 1400 mg/L) and aluminium (up to 1300 mg/L) and very low (almost zero) concentration of dissolved phosphorus in sludge water were the main factors limiting the rate of anaerobic processes. Based on these results, anaerobic digestion of SEWWTP sludge was not recommended as an appropriate stabilisation method.