Abstract
Abstract Wastewater heat recovery upstream of wastewater treatment plants (WWTPs) poses a risk to treatment performance, i.e. the biological processes. In order to perform a sustainability analysis, a detailed prediction of the temperature dynamics over the WWTP is needed. A comprehensive set of heat balance equations was included in a plant-wide process model and validated for the WWTP in Linköping, Sweden, to predict temperature variations over the whole year in a temperate climate. A detailed model for the excess heat generation of biological processes was developed. The annual average temperature change from influent to effluent was 0.78 °C with clear seasonal variations; 45% of the temperature change arises from processes other than the activated sludge unit. Hence, plant-wide energy modelling was necessary to predict in-tank temperature in the biological treatment steps. The energy processes with the largest energy gains were solar radiation and biological processes, while the largest losses were from conduction, convection and atmospheric radiation. Tanks with large surface areas have a significant impact on the heat balance regardless of biological processes. Simulating a 3 °C lower influent temperature, the temperature in the activated sludge unit dropped by 2.8 °C, which had a negative impact on nitrogen removal.
Highlights
As resource recovery from wastewater (WW) is widely applied to improve sustainability of wastewater treatment (WWT) systems (Verstraete et al 2009), wastewater heat recovery (WWHR) is gaining wider interest (Culha et al 2015; Cecconet et al 2020; Wärff et al 2020)
Heat balances have been implemented in a plant-wide wastewater treatment plant process model and, in contrast to previous work, validated for a wastewater treatment plant in a temperate climate for a full year of dynamic simulation
A novel set of dynamic expressions was applied for the consecutive equations and parameters of the heat balance model to allow for temperature predictions throughout the plant and over the wide temperature range at the location
Summary
As resource recovery from wastewater (WW) is widely applied to improve sustainability of wastewater treatment (WWT) systems (Verstraete et al 2009), wastewater heat recovery (WWHR) is gaining wider interest (Culha et al 2015; Cecconet et al 2020; Wärff et al 2020). According to several international studies, the energy use in the urban water cycle adds up to 10% or more of the total national energy use (Olsson 2012) Out of this only about 10% (corresponding to 1% of the total energy use) is used for withdrawal, treatment and distribution of tap water and collection, treatment and discharge of wastewater. The Swedish Energy Agency (2009) has estimated the heat requirement in households for domestic hot water (DHW) to be 1,150 kWh/cap/yr. This contributes to an elevated temperature of wastewater compared to tap water. It can be implemented either upstream of the wastewater treatment plant (WWTP), such as in showers, in building sewer stems, in sewer mains, or at the WWTP effluent after treatment
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