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Research Notes: Recovering heat from sewage

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

The Quads, the accommodation at the University of Bath where the study took placeThe Quads, the accommodation at the University of Bath where the study took place

by Professor Jan Hofman, University of Bath

Energy is everywhere in the urban water cycle. Water utilities use energy to produce drinking water and pump it to your tap at home. And after use the water is discharged into the sewer and purified 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 five 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.

After 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 efficiency of the water cycle and reduce the carbon footprint significantly. The important questions are then of course: how much energy can effectively 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 flow 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 flow and to the temperature difference caused by abstracting the heat. But these measurements are extremely difficult in a sewer system, because of rapid fouling of measuring devices (it is not only water flowing in the sewer) and risk of blockages by installing measuring equipment in the water flow. Furthermore, temperature and flow in a sewer are very dynamic. It’s also necessary to understand the flow and temperature in every place in the entire sewer network, which of course needs measurements in many locations.

To overcome the difficulties 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 first one is a model that describes water use patterns in homes. The model, Simdeum, uses statistics on frequency of use and flow of different appliances in the home and the occupancy of the home to predict the water use. It delivers a time series of total water flow and hot water flow in one home. The time series also contains information on the water temperature. By assuming that all water use in the house will go into the sewer, Simdeum can be used to estimate the water flow and temperature of the discharges into the sewer.

The second model, Sobek, is used to describe the water flow 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 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. After a number of further improvements it proved to be possible to accurately predict the water use in the students’ flats.

Looking at the average weekly flow pattern for the water flow in the sewer of The Quads, the data follows 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 afternoon 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 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. 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 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 often 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

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