Energetic and economic assessment of sludge thermal hydrolysis in novel wastewater treatment plant configurations

Abstract

Novel wastewater treatment plants (WWTPs) are aimed to be more energetically efficient than conventional ones. Their first step is a chemical oxygen demand (COD) preconcentration stage with different alternatives, such as rotating belt filters (RBF), chemically enhanced primary treatment (CEPT), high-rate activated sludge (HRAS), or combinations thereof, in which energy requirements are substantially reduced. The COD recovered as sludge allows a noticeable increase of biogas production in anaerobic digestion (AD). In conventional WWTPs, sludge anaerobic biodegradability can be significantly enhanced by applying sludge pretreatment methods, such as thermal hydrolysis (TH), before AD. However, considering that novel-sludges are more anaerobically biodegradable than conventional ones, the impact of TH on their methane production is expected to result significantly lower. In this study, an energetic and economic assessment of applying TH in novel WWTPs was performed. We found that TH is only justified to reduce operational costs as long as sludge TS concentration in the feeding to the TH unit is higher than 1–2%. The HRAS-based WWTP is the scenario that leads to the lowest treatment costs (below 1c €/ m3 wastewater if sludge is thickened over 10% of TS). However, the WWTP based on CEPT for COD preconcentration leads to the lowest electricity consumption (below 0.01 kWh/m3 of wastewater), but even in the most favourable conditions the energy autarky was not achievable. Results show that the main impact of TH is mainly due to sludge disposal savings (270,000–430,000 €/year for a 500,000 inhabitants WWTP) rather than the increase of energy production (achieves maximum savings of 35,000–60,000 €/year). Payback time is very dependent on the WWTP size, ranging from 15 to 30 years for a 100,000 inhabitants WWTP and from 2 to 4 years for a 1,000,000 inhabitants WWTP.