Water & Wastewater Treatment Magazine
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26 | MAY 2016 | WWT | www.wwtonline.co.uk In the know Research Notes: sewage heat predicted data in the manholes near The Quads coincide very well with the measured temperatures. The temperatures and flows were used to calculate the available heat potential. Due to the strong diurnal pattern in flow, the available heat varies quite dynamically. Power is mainly available during two periods, the morning peak and evening peak. In between these peaks, during the day and during the night, the available power is low. Nevertheless, the total amount of energy is considerable. Figure 3 shows the available heat power. The strong variations and the fact that heat demand and availability in sewers don't occur simultaneously, the system can only be operated if heat storage is applied or if the heat is fed into a larger heating system. But although the available energy in The Quads is significant, the recovery at such a relatively small scale is technically not feasible, because the heat exchangers for these small systems are not available (yet). A case study on a larger scale residential area in the city of Almere in The Netherlands showed that heat recovery from sewers becomes attractive if applied at larger sewers, collecting waste water from 5,000 Person Equivalents (PE) or Figure 2. Average and Simulated water consumption at The Quads. the sewer network. This gives the advantage that existing sewer models can be used. In a recent study we used this modelling framework to predict the water and potential for heat recovery on the University of Bath campus. In this study we focussed on five relatively new students' accommodations, called The Quads. A questionnaire among the students was used to find their daily water use patterns. This information was used to feed into Simdeum. A‰er a number of further improvements it proved to be possible to accurately predict the water use in the students' flats. Figure 2 shows the average weekly flow pattern for the water flow in the sewer of one of the blocks of The Quads. The data follow a clear diurnal pattern with a very high morning peak, caused by many students taking a shower. During the day the flow rate drops to a low value and increases again in the late a‰ernoon and evening hours. During the night the flow drops to almost zero. It can also be clearly seen that the weekend days have their own distinctive pattern with a lower and later morning peak. Using these data, a very accurate prediction of the sewage temperature can be achieved as well. Again the Mon00:00 Tue00:00 Wed00:00 Thu00:00 Fri00:00 Sat00:00 Sun00:00 Mon00:00 Tue00:00 Wed00:00 Thu00:00 Fri00:00 Sat00:00 Sun00:00 Time (h) Time (h) October November ***** Block C - simulation Block C - Water use ***** Block C - simulation Block C - Water use Flow rate (m3/min) 0.04 0.01 0.015 0.02 0.025 0.03 0.035 0.005 0 0.04 0.01 0.015 0.02 0.03 0.035 0.005 0.025 0 Figure 3. Potential heating power from waste water from The Quads. Figure 1. Typical water demand pattern in a house on a working day. more. This is typically the size of a neighbourhood or residential area. In the case study in Almere it was estimated that for 5,000 PE a total energy recovery potential of 8.3 GJ is possible. Concluding remarks The results shown here indicate that large amounts of energy are just flushed away with our waste water and lost in the environment. Recovering this energy can be an efficient way of improving the energy efficiency of our buildings and homes and reducing the carbon footprint. Nevertheless, the available energy is low grade heat. And availability and demand o‰en occur not simultaneously, so heat storage may be required. Although the technology is available and applied in Switzerland, Germany and Scandinavia, further knowledge is required to find where and when heat recovery is a viable option. • About the author: Professor Jan Hofman is Director of the Water Innovation & Research Centre at the University of Bath. Additional contributor: Laura Piccinini, Visiting Researcher & MSc student in the Department of Chemical Engineering, University of Bath