Improved energy balance at a municipal wastewater treatment plant through waste activated sludge low-temperature alkaline pretreatment

Abstract

Sludge management has a significant impact on operating costs in municipal wastewater treatment plants (WWTPs). The current trend is to optimize the sludge management in order to enhance the energy independence from external energy suppliers, while reducing the operating costs of the WWTP. One of the methods is through the pretreatment of waste activated sludge (WAS). In this work, low-temperature alkaline pretreatment of WAS was examined as a method of improving the economic balance of the WWTP. In the initial pretreatment tests, the impacts of NaOH dose, temperature and reaction time on COD release as well as the degree of sludge disintegration were examined. Semi-continuous anaerobic digestion (AD) trials were then performed for pretreated WAS in selected conditions with low NaOH dosage for which no pH adjustment was required (16 g of NaOH/kgTS of WAS, 60 °C, 60 min). The tests showed an increase in methane yield from 210.4 ± 10.9 mL CH4/gVS to 248.8 ± 13.1 mL CH4/gVS and 267.1 ± 14.3 mL CH4/gVS for HRT 15 d and HRT 21.5 d, respectively. Energy balance and economic calculations for a full-scale WWTP were conducted based on the obtained results. It was concluded that it would be possible to increase electricity production from methane originated from AD of pretreated WAS up to 27% as well as to reduce the amount of dewatered sludge up to 8.7%. Energy balance and cost-benefit analyses indicated a possibility to reduce the operating cost by over 300,000 EUR/year in the case of a 1,200,000 people-equivalent WWTP.