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 is the scenario that leads to the lowest treatment costs (below 1 c€/m 3 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/m 3 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.

Development of emerging technologies for a low carbon footprint wastewater treatment

“…Taken into consideration that there is no need for aeration in the CEPT process applied in the current study, energy requirement would be highly reduced by adopting such treatment strategy. This is also supported with the recent findings of Taboada-Santos et al [39], who reported that the electricity consumption for CEPT process is below 0.01 kWh/m 3 . Indeed, energy produced from primary sludge through anaerobic digestion could reduce the electricity cost and WWTP could become energy self-sufficient by generating renewable energy [18].…”

Agricultural and non-agricultural directions of bio-based sewage sludge valorization by chemical conditioning

“…From an economic point of view, it is energy and waste disposal costs that are important. The energy costs associated with achieving high temperatures are offset by savings resulting from no waste landfilling fees (Taboada-Santos et al 2019b ).…”

Thermal effects

“…Taboada‐Santos, Lema, and Carballa (2019) assessed the sludge THP in novel WWTP schemes from an energy and economic perspective. The contribution of THP to novel WWTP economy was mainly due to the sludge disposal saving rather than the increase of energy production, and a total solid content higher than 1%–2% for thermal hydrolysis of sewage sludge was necessary to achieve the reduction in operational cost.…”