Co-digestion approaches to organic fraction of municipal solid waste with primary sludge for a municipal treatment plant in Turkey

The comparison of wet anaerobic digesters fed with differently sorted organic fraction of municipal solid waste (OFMSW) was carried out by co-digestion approaches to the primary sludge taken from Kayseri municipal wastewater treatment plant (WWTP) located in the Central Anatolian Region of Turkey. The WWTP was designed in two stages. The second mesophilic anaerobic digester is planned to be constructed after 20 years in the second stage. In this study, the construction of the second digester and a pre-treatment unit for municipal solid waste were proposed in the current stage for co-digestion purposes. Two alternatives were proposed using different solid waste contents in the co-digesters. In order to provide the required solid content, some amount of the treated wastewater was recycled to each digester together with the primary sludge. Although the existing collection method was assumed to be characterized as mechanical sorted (MS-OFMSW) (Option 1), source sorted (SS-OFMSW) alternative was also evaluated in the study (Option 2). The total energy demand of Kayseri WWTP was reported as around 42,300 kWh per day. Utilizing the energy produced by the digester, only 30% of the total energy demand and all of the heat energy demand including administration building can be covered. Thus, the aim was to evaluate how energy production would be increased by the application of co-digestion of OFMSW together with the primary sludge in Kayseri WWTP. Results indicated that the best operational condition in the cases of organic loading rate (OLR), hydraulic retention time (HRT) and energy recovery could be provided using 10% solid content in each co-digester for both options. According to the approach in Option 1, almost 77% of the total energy demand could be covered by applying co-digestion of MS-OFMSW together with the primary sludge and 200m³ treated wastewater recycle. On the other hand, almost 100% energy recovery could be obtained when co-digestion approach (Option 2) was performed according to SS-OFMSW together with the primary sludge at 150 m³ treated wastewater recycle. Significant amounts of sludge cakes (anaerobic compost) were produced after co-digestion and aerobic composting could be considered as an appropriate post treatment alternative. Co-digestion of OFMSW and sewage sludge may be an attractive alternative for sustainable management of two separate waste streams produced at large amounts in the developing countries.