Effect of mixing ratio on sewage sludge and septage co-digestion

Septage, which is made up of liquid and solid materials pumped from septic tanks, can be a potential co-digestion resource for sewage sludge. This study was conducted to evaluate possibility of co-digestion of sewage sludge and septage. Biochemical methane potential tests were performed using primary sludge, secondary sludge and septage. The mixing ratio of the septage ranged from 30% to 62% (by volatile solid wt.) based on the typical ratio of the septage transported to municipal WWTP plants and the sewage sludge generated in the plants. The co-digestion of sewage sludge and septage showed synergistic effects on the methane potential due to the enhanced C/N ratio. The observed methane potential of co-digestion substrate was 8-16% higher than the predicted value based on weighted average of mono-digestion substrate's methane potentials. The maximum synergistic effect on methane potential was observed when 42% (by volatile solid wt.) of the septage was co-digested. In conclusion, the septage can be a suitable resource for co-digestion with sewage sludge.