Energy use and recovery in wastewater treatment facilities

Abstract

Wastewater treatment facilities (WTFs) may require over 20% of the energy consumption of municipal utilities, and up to 3% of the total energy output of a country, also representing a significant source of GHGs emissions. On the other hand, wastewater contains different types of energy: chemical, thermal and kinetic. Until very recently, they went mostly untapped, the only form of recovery usually consisting of anaerobic digestion of process residuals (waste sludge), by which embedded wastewater chemical energy in extracted in form of methane. This could be sufficient to cover about half of total plant requirements, even though wastewater chemical energy alone may represent close to tenfold the energy requirements for its treatment. Implementation of new technologies is making more efficient strategies of energy recovery from sewage possible. Besides valorisation of its chemical and thermal energy contents, closure of the wastewater cycle by exploitation of the energy contents of process residuals could allow significant additional energy recovery and reduced GHG emissions. Wastewater and its residual products can therefore be considered renewable energy sources, if addressed by proper technological solutions. This paper summarizes the main items of energy consumption in the wastewater treatment cycle, and discusses the most promising state-of-the-art technologies currently available for energy recovery from both wastewater and its residual by-products.