Evaluation of anaerobic sludge digestion in combination with thermal alkaline process as pre- and post-treatment: Impacts on energy, economic, and CO2 footprint balances

To determine whether wastewater treatment plants (WWTPs) should use the thermal hydrolysis process (THP) or the thermal alkaline process (TAP) as sludge pretreatment or posttreatment methods, this study was conducted on five WWTPs implementing THP/TAP under seven different scenarios. The evaluation focused on impacts on energy, economic, and CO_2eq. balances compared to conventional anaerobic sludge digestion. In this study, THP/TAP as a posttreatment method yielded better results in energy, economic, and CO_2eq. balances than pretreatment. TAP of digested sludge (DS) at pH 9 achieved the highest net electricity generation of 344.7-472.3 kWh/t total solids (TS). An economic profit of 29.9-171-3 US$/tTS can be obtained from digesting the liquid fraction of THP-treated DS, and in this scenario, CO_2eq. emissions can be further reduced by 149.8 ± 42 kg/tTS. However, additional space is required for the construction of a digester or an upflow anaerobic sludge blanket in the DS posttreatment.