Experimental and numerical investigation of drain water heat recovery in a grease interceptor

The European Union declared wastewater as a renewable energy source in its Green Deal. To meet these targets, the scientific community has been proposing new technologies to improve energy conversion efficiency of renewable energy powered thermal systems. The present research examines heat recovery from commercial kitchen grease traps (GT) as a means of reducing the net energy requirement of commercial hot water use in food related activities. A GT was integrated with heat recovery unit and is referred to as a hybrid GT system. Two designs of thermal recovery units comprising a planar and rectangular coil type heat exchanger (HX) were assessed individually to compare their thermal recovery capabilities. The planar HX equipped GT was first experimentally investigated and subsequently examined in more detail using 3D numerical analysis. Thermal stratification was observed along the GT depth for which the rectangular HX showed greater potential for thermal recapture from the high temperature kitchen wastewater. The novel rectangular HX design was found to save 2.6 kW of thermal energy in the kitchen GT and was 48% more efficient compared to the planar HX. The proposed GT hybrid design enables more sustainable production of food via a clean auxiliary thermal back-up, reducing greenhouse gases emission released in water heating. Heat recovery from commercial kitchen GTs has been shown in this study to be technically viable with strong potential for economic viability and positive environmental impact through reducing the carbon footprint of water heating.