Low-grade heat recovery from wastewater

Problem statement

Heat is typically present in domestic wastewater, primarily from warm-temperature procedures including showering, washing clothing and dishwashing. This heat can be recovered in small-scale applications within homes, in medium-scale applications (150 kW – 700 kW) from the sewage, or in large-scale applications at wastewater treatment plants.

Executive summary

Focusing on the medium-scale application, it is possible to state that depending on the source and the outside temperature, wastewater entering the sewer system typically has a temperature of between 10 and 20 degrees Celsius or higher. Temperature and flowrate fluctuations cause variations in the available heat over time. A heat exchanger inside or next to the sewer is used to recover the heat.
Three different cases of heat recovery from wastewater and treated wastewater are given. In the first, low-grade heat is obtained straight from a sewer system's wastewater. The second one refers to the NextGen project, where heat is recovered from the effluent of a membrane bioreactor. The heat will have the function to temper a rapid composting unit. In the Ultimate project, various options to reuse the heat from an anaerobic membrane bioreactor (AnMBR) are considered.

Technology description

Heat from the wastewater is transferred to a closed heat exchange loop via the heat exchangers. Usually, water serves as the loop's heat medium. Heat exchangers with plates or shells and tubes are typically used. A heat pump is connected to the heat exchanger. By abstracting the thermal energy at a high temperature, the heat pump makes it possible to repurpose it for heating, for example, a neighbouring building.
During the summer, a building can be partially cooled by using a heat pump in reverse, serving as a partial replacement for a traditional air conditioner. The wastewater receives the heat that the heat pump extracts from the surrounding air. A heat pump is far more energy efficient than a boiler or air conditioner. However, a building's heating system must be modified to accommodate low-temperature heating systems in order to permit space heating.

Applications:
In the NextGen case study Athens, the heat obtention occurs in a sewer mining unit that includes a membrane bioreactor (Fig. 2). The treated wastewater effluent from a membrane bioreactor (MBR), which has a temperature between 15 and 20 °, is used for heat recovery. The contained heat is recovered in a tank and transferred from the wastewater to a circulating water by the heat exchanger.
In Ultimate, the effluent of an anaerobic membrane bioreactor (AnMBR), which treats distillery wastewater, has a temperature of 35-38 °C. The first option is to use the heat for the stripping process of the ammonia from the AnMBR effluent. The other option is to reuse the heat to increase the temperature of the nutrient-depleted liquor that will be post-treated via reverse osmosis. The benefit would be a lower dynamic viscosity due to the higher temperature. This results in a higher flux and hence, a higher water recovery efficiency.

Recovery of the heat and the demand for heat will not always occur at the same time. Often it is required to store the heat. For smaller systems, water tanks or phase transition heat storage systems can be used. For larger systems, aquifer theraml energy storage (ATES) systems can provide an adequate solution.

Market deployment considerations

Environmental considerations