Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | MAY 2016 | 25 In the know Research Notes: sewage heat Sewer heat: recovering a forgotten energy loss Recovering heat from sewers is a potentially valuable energy source, but the amount of energy available varies according to usage patterns. Researchers at the University of Bath are modelling these patterns in order to explore the viability of heat recovery installations PROFESSOR JAN HOFMAN DIRECTOR, WATER INNOVATION AND RESEARCH CENTRE UNIVERSITY OF BATH E nergy is everywhere in the urban water cycle. Water utilities use energy to produce drinking water and pump it to your tap at home. And a er use the water is discharged into the sewer and puri ed at the sewage treatment works. And drainage and waste water treatment again use energy. For sustainability reasons water companies strive to reduce their energy demand and climate footprint as much as possible. Nevertheless, the energy demand of the water cycle is still increasing. This is due to population growth or more stringent water quality regulations requiring more advanced treatment systems. For a sustainable water cycle, many water companies across the globe strive toward an energy neutral water cycle. However this requires more radical changes than just optimisation. It is important to change the water cycle into a net energy factory, for instance by production of biogas from the waste load. Further analysis of energy consumption in the water cycle reveals that the energy for producing hot tap water is much larger than for pumping and treating drinking and waste water. In fact the energy used to produce hot tap water is ve to ten times higher than the operational energy of the water utilities. And at your home it is responsible for about 15-30 % of your energy bill. A er use, all the hot tap water goes down the drain, still containing most of the thermal energy. This means that wastewater still contains a fair amount of energy and has a relatively high temperature. Thus, recovering this heat can improve the overall energy e… ciency of the water cycle and reduce the carbon footprint signi cantly. The important questions are then of course: how much energy can e‡ ectively be recovered? How can this be done? And where in the sewer network is the best place to do this? To answer these questions we could do ‰ ow and temperature measurements in the sewer system. Abstraction of thermal energy will reduce the temperature of the wastewater. The recoverable heat is proportional to the ‰ ow and to the temperature di‡ erence caused by abstracting the heat. But these measurements are extremely di… cult in a sewer system, because of rapid fouling of measuring devices (it is not only water ‰ owing in the sewer) and risk of blockages by installing measuring equipment in the water ‰ ow. Furthermore, temperature and ‰ ow in a sewer are very dynamic. It's also necessary to understand the ‰ ow and temperature in every place in the entire sewer network, which of course needs measurements in many locations. To overcome the di… culties of these measurements, we have developed a modelling framework that can predict the heat availability in the sewer network. The model framework consist of two basic models. The rst one is a model that describes water use patterns in homes. The model, Simdeum, uses statistics on frequency of use and ‰ ow of di‡ erent appliances in the home and the occupancy of the home to predict the water use. It delivers a time series of total water ‰ ow and hot water ‰ ow in one home. The time series also contains information on the water temperature. Figure 1 shows a typical result of this model. By assuming that all water use in the house will go into the sewer, Simdeum can be used to estimate the water ‰ ow and temperature of the discharges into the sewer. The second model, Sobek, is used to describe the water ‰ ow in the sewer, but it also contains a module to calculate a heat balance. This heat balance predicts the temperature change of the water in the sewer mains, due to heat loss through the pipe wall into the surrounding soil. Sobek is a commercial code that is used by many municipalities and water utilities to design and monitor The Quads (centre), student accommodation in Bath, was used to study heat recovery potential