Heat Recovery from a Wastewater Treatment Process—Case Study

This article presents the potential of heat recovery from wastewater with an example of a wastewater treatment plant (WWTP) in Mokrawica, which is located in the West Pomeranian region of Poland. A thorough literature review discusses the relevance of the topic and shows examples of heat recovery conducted with heat pumps. Raw and treated wastewater are mostly used as heat sources, with the latter achieving higher thermal capacities. Heat recovery from a biological treatment process is rarely implemented and requires more detailed studies on this subject. The proposed methodology for estimating possible heat recovered from wastewater, requiring heating and cooling capacities, as well as the coefficient of performance (COP) of a heat pump, is based on only three parameters: wastewater volumetric flow, wastewater temperature, and the required temperature for heating or air-conditioning. The heat recovery potential was determined for different parts of WWTP processes, i.e., the sand box, aeration chamber, secondary sedimentation tank, and treated sewage disposal. The average values of 309–451 kW and a minimum of 58–68 kW in winter were determined. The results also indicate that, depending on the location of the heat recovery, it is possible to obtain from wastewater between 57.9 kW and 93.8 kW of heat or transfer to wastewater from 185.9 to 228.2 kW. To improve biological treatment processes in the winter season, wastewater should be preheated with a minimum of 349–356 kW that can be recovered from the treated wastewater. The heat transferred to the wastewater from the air-conditioning system amounts to 138–141 kW. By comparing the required cooling and heating capacities with the available resources, it is possible to fully recover or transfer the heat for central heating, hot water, and air conditioning of the building. Partial preheating of wastewater during the treatment process requires further analysis.