Biogas production, sludge accumulation and mass balance of carbon in anaerobic ponds

This work concerned the application of anaerobic ponds for the primary treatment of urban wastewater in a Mediterranean climate. It was carried out on anaerobic ponds at large scale in Mèze (France). The anaerobic ponds constitute a good primary treatment with the removal of 55% of SS and 30% of BOD₅, with a small surface area. The accumulation rate of sludge was only 0.017 m³/capita.year, due to their intensive anaerobic degradation. The anaerobic digestion reached equilibrium after one year of operation. The accumulation of sludge then showed seasonal variations with a substantial accumulation in winter and the digestion of the stock in summer. This change can be related to the influence of the temperature on methanogenesis. The production of biogas (83% CH₄) was measured by gas collectors especially developed for this study and was also strongly dependent on temperature. The mass balance of carbon showed that 74% of the removed organic carbon was converted into CH₄, 13% into dissolved inorganic carbon and 15% was stored in sludge. However, the anaerobic ponds presented a risk of creating odor nuisances with the emission of H₂S.