Ajeet Singh
Figure 1: The Rediscovery Centre.
As part of our work aimed at developing a grease trap fitted with a heat exchanger to recover heat from wastewater in commercial kitchens, we monitored the grease trap at the café of the Rediscovery Centre (RDC) in Ballymun, Dublin (Ireland).
The centre is the National Centre for the Circular Economy in Ireland: a circular economy model aims at minimising waste, so why not also minimising the heat that is wasted from their commercial kitchen by reusing it?
The interest to explore opportunities to highlight the circularity of the heat embedded in wastewater arose after staff of RDC took part in one of our Sustainability Webinars on drain water heat recovery in March last year. The onsite café, equipped with a grease trap, serves food and drinks to the visitors and customers of the facility.
Figure 1: Thermal sensor
To assess how much heat could be recovered from the wastewater passing through the grease trap, it is necessary to know the temperature of the water over time and how much water flows through the grease trap. To monitor the water temperature, a small heat sensor (Figure 2) was placed inside the grease trap for three weeks in December 2021. The hotter the water, the greater the potential. The sensor was recovered and data was analysed to see the range of temperatures occurring within the grease trap. The maximum temperature recorded was 35.1 and the average was about 32 ; moreover, a consistent thermal gradient (difference in temperature between the wastewater entering the grease trap and the cold freshwater of the supply lines) of more than 15 was maintained during the kitchen operational hours. The number of customers served during the monitoring period has been used to estimate the amount of water that flowed through the grease trap.
The data will be used to predict the feasibility of exploiting the thermal gradient to preheat the freshwater by installing a heat exchanger, also called thermal recovery unit, in the grease trap. Next steps will include assessing the necessary pipeline arrangements to carry the pre-heated freshwater to the hot water storage tank as well as a cost-benefit analysis. If installed, the system would reduce the primary energy consumption for hot water production, saving costs and emissions (where replacing fossil fuels).