Investigation of separate anaerobic stabilization potential of primary and secondary sewage sludges at upper mesophilic temperature

The main purpose of the study was to propose a suitable model for conventional anaerobic sludge digestion process at a higher performance and better quality of stabilized sludge in wastewater treatment plants (WWTPs). Upper mesophilic temperature (40℃) and separate system were investigated against combined system at increasing organic loading rate (OLR) values in paralel lab-scale semi-continuous reactors fed with primary sludge (PS), secondary sludge (SS) and mixed sludge (MS) to determine optimum methane conversion, volatile solid (VS) removal, hydraulic retention time (HRT) and stabilized sludge quality. OLR was applied for PS, SS and MS, respectively, at 1.5-3, 0.4-1.3 and 0.82-2.28 kg VS/m3 day. The maximum daily methane formation in the reactors was 1200±200, 580±110 and 1150±225 mL/day for PS, SS and MS, respectively, with methane conversion rates at 500±80, 700±30 and 700±50 mL CH4/g VSadded.d. VS removal were obtained at 63±2, 32±5 and 48±5% during PS, SS and MS digestion, respectively, at optimum hydraulic retention time (HRT) values determined as 20, 8 and 18 d. The pH values of all three reactors were in the range of 7-8.5. In the final stage of each OLR, FeCl3 was supplemented where neutral and promotive effects were obtained on total dissolved sulfide (TDS) concentration and VS solubilization, respectively. As the final sludge quality, the highest and lowest filtering ability was obtained for SS and PS, respectively. Similarly, the nutrient content of the stabilized sludge fractions occurred at the highest level for stabilized SS as total P (TP) at 18, 50 and 22 mg/g TS and total N (TN) 60, 200 and 70 mg/g TS for stabilized PS, SS and MS, respectively. The oil and grease (OG) content reached the highest level for stabilized SS whereas a range of 0.056-0.223 % OG/TS. Potential for higher methane yield and lower reactor volume was presented with separate sludge digestion model. Keywords: Anaerobic, Methane, Biomass, Digestion, Sewage sludge, Upper mesophilic.