Abstract
Urban water systems and, in particular, wastewater treatment facilities are among the major energy consumers at municipal level worldwide. Estimates indicate that on average these facilities alone may require about 1% to 3% of the total electric energy output of a country, representing a significant fraction of municipal energy bills. Specific power consumption of state-of-the-art facilities should range between 20 and 45 kWh per population-equivalent served, per year, even though older plants may have even higher demands. This figure does not include wastewater conveyance (pumping) and residues post-processing. On the other hand, wastewater and its byproducts contain energy in different forms: chemical, thermal and potential. Until very recently, the only form of energy recovery from most facilities consisted of anaerobic post-digestion of process residuals (waste sludge), by which chemical energy methane is obtained as biogas, in amounts generally sufficient to cover about half of plant requirements. Implementation of new technologies may allow more efficient strategies of energy savings and recovery from sewage treatment. Besides wastewater valorization by exploitation of its chemical and thermal energy contents, closure of the wastewater cycle by recovery of the energy content of process residuals could allow significant additional energy recovery and increased greenhouse emissions abatement.
Highlights
The water–energy nexus has been long neglected but became recently a high-priority issue from which better understanding and development of new paradigms of water cycle sustainability may arise [1,2]
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 byproducts
The most efficient and technologically robust solutions in term of energy demand reduction in wastewater treatment plants (WWTPs) seem to lie in AGS processes, as far as aerobic technologies are concerned, or, as an alternative, Upflow Anaerobic Sludge Blanket (UASB)/Expanded Granular Sludge Bed (EGSB) anaerobic treatment of domestic wastewater
Summary
The water–energy nexus has been long neglected but became recently a high-priority issue from which better understanding and development of new paradigms of water cycle sustainability may arise [1,2]. The cited values do not include the upstream water supply cycle, wastewater conveyance (mostly energy for pumping) and residues post-processing and disposal (chemical and biological sludges). These facilities may be considered as highly energy demanding, with high operational cost, and significant sources of greenhouse gas (GHG) emissions, whose reduction has been recently mandated by European Union and other countries’ policies. The almost unique form of energy recovery from these facilities consisted of the anaerobic post-digestion of process residuals (waste activated sludge, WAS) by which chemical energy in the form of biogas (mostly methane) is obtained This is still one of the main energy recovery options applied, generally sufficient to cover about half of total plants requirements, but with low (30% to 60%) conversion efficiency of the organically embedded chemical energy into a readily usable source [16]. 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 byproducts
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