Wastewater - Turning Problem to Solution

Energy recovery

Wastewater is a carrier of carbon energy (chemical energy) and heat (thermal energy). Energy recovery and energy generation from wastewater have the potential to contribute to energy requirements and climate mitigation. There is about five times more energy in wastewater than is required for its treatment (Tarallo et al. 2015). Wastewater typically has significant amounts of thermal energy. For example, in the Netherlands, householders heat 60 per cent of their water – water for showering is approximately 38°C and washing machines operate at 40–60°C (SenterNovem 2006). The wastewater is discharged into the sewer at a temperature between 10–25°C (Nagpal et al. 2021). Up to 90 per cent of this heat can be recovered and used for various domestic, urban and industrial heating and cooling purposes, through heat exchangers and heat pumps (Nagpal et al. 2021). Heat recovery from wastewater can occur at four levels within a wastewater management system (Nagpal et al. 2021): • at a component level within a building, e.g. at a shower or from cooking activities in a kitchen • at a building level, especially for residential and non-residential buildings, with high volumes of wastewater flow • at the sewer level • at the wastewater treatment plant level, from the influent, partially treated wastewater or the effluent There are several examples of heat recovery from wastewater at the various locations within a wastewater management system operating at full scale in various cities around the world. This includes recovery of energy for cooling and district heating at large wastewater treatment plants in Canada, China, Japan, the Russian Federation, Sweden, Switzerland and the Netherlands, among others (Funamizu et al. 2001; Mikkonen et al. 2013; Petersen and Grøn Energi 2018; Shen et al. 2018; Hao et al. 2019). An example is the use of wastewater for the year-round heating and cooling of the 28 storey high-rise office building Wintower in Switzerland, which exhibits about 600 kilowatts of heating energy extracted from wastewater directly from the sewer in winter months, and uses wastewater to absorb energy from the building for cooling it in summer months (Zandaryaa and Jimenez-Cisneros 2017).

more energy than the more conventional approach of energy recovery via biogas (Kehrein et al. 2020). Thermal energy derived from wastewater can be used for its own treatment (Hao et al. 2019). Chemical energy (the remaining 10 per cent of energy) that derives from organic substances present in wastewater can be recovered by digestion to produce methane or can be converted into biogas, but also into organic products. Methane is a powerful GHG; capturing it as biogas can contribute to climate mitigation (Fredenslund et al. 2023). With an estimated global caloric value of 1 908 × 109 MJ (531 × 109 kWh), assuming full energy recovery and anticipating the average household electricity needs of 3 350 kWh, Qadir et al. (2020) reported that the energy embedded in the global volume of wastewater would be enough to provide electricity to 158 million households (that is between 474 million to 632 million people, estimating 3 or 4 people per household, respectively), or to fulfil the energy requirements of the facilities equipped with wastewater treatment systems.

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It has been estimated that thermal energy recovery from wastewater effluent could result in up to 10 times

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