Anaerobic digestion of municipal sludges with high silt content using granular seed

The performance of anaerobic digestion of municipal sludge having high inorganic solid/silt in Tuzla Wastewater Treatment Plant (TWWTP) in Istanbul was evaluated using granular seed. High silt/solid content is the main problem related to wastewater collection system in TWWTP. Due to high TSS concentration of the influent, inorganic solid ratio reaches to 60-70% in the primary sludge that makes sludge digestion very difficult. The aim of this study was to evaluate the detrimental effects of inorganic silt/clay on anaerobic sludge solubilization of the primary sludge (PS), of the biological sludge (BS), and of the mixed sludge (50%PS+50%BS). Anaerobic solubilization was carried out in continuously stirred anaerobic reactors at mesophilic temperature (35°C). Results indicated significant total biogas productions in all reactors during digestion period. The biogas production of the primary sludge was higher than the biogas productions of the mixed sludge and the biological sludge. The methane contents of the primary, biological, and mixed sludges were found as around 72, 78, and 75%, respectively. When all types of sludges were used as substrates, the reactors removed VSS with a corresponding production of Soluble COD (SCOD). The greatest degree of hydrolysis/acidogenesis was observed in all types of sludges in about 10 days of operation and became almost stable which indicated that the reactors were still in the hydrolysis/acidogenesis phase. VFAs measurements were well matched with SCOD results. In all reactors, observation of the conversion into methanogenic phase in about 1.5 month was not possible which might be attributed to high inorganic silt/clay and salinity content of TWWTP. High inorganic silt/clay and salinity might have decreased the biodegradability of all sludges and methanogenesis could not operate in early periods of digestion. Higher SCOD decreases might have been possible if longer treatment time and more optimal conditions for anaerobic solubilization were applied. The pH did not fall below the unfavorable range in all reactors during the study.