Water & Wastewater Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/971076
26 | MAY 2018 | WWT | www.wwtonline.co.uk treatment process; although it may still affect the receiv- ing environment. Systems in these locations are common in Switzerland and Germany. However they do depend on a nearby use for the available energy. In between, there are mid- dling locations which offer a balance between reduction in temperature and increase in flow. The further downstream, the more even the flow and temperature, but also the lower the average tempera- ture. Here, systems have the challenge of dealing with foul sewage in locations with poor access, confined spaces and resulting safety implications. These middle locations are caught between upstream and downstream constraints. If heat is managed upstream at households but temperature change at the treatment works is limited, then these systems to upgrade treatment plants to maintain performance. Increasing the temperature with rejected heat from cooling in summer risks reducing the efficiency of oxygen transfer in treatment and also increasing the temperature impact in the receiving water. So there is a limit to how much heat we can take out of, or put into, sewage. Where? The location of a sewage energy management systems influences its capacity. This is affected by the quantity of wastewater flow, which in- creases down the system; the temperature of the flow, which reduces down the system due to heat loss; and the consist- ency of flow and temperature, which is important for system efficiency. Locating it upstream at a development site or even an individual property would give lower flow, but higher temperatures. Potentially we can extract the heat from hot greywater before it is contami- nated with cold foul sewage. This would require that new houses were plumbed so that greywater and foul sewage are kept separate; but that would be good for reusing the grey- water for toilet flushing, so it should be done anyway. A location downstream, at the outfall from the wastewa- ter treatment works, gives the largest flow, but the lowest temperature. It has relatively clean water that is easy to deal with and it does not affect the The Knowledge Heat from sewage may have reducing margins of operation. How? Household scale systems are available for retrofit or new build that take heat from greywater and transfer it into the cold water going to the hot water tank. These range from a heat exchanger in an individu- al shower tray, to one fitted in an inspection chamber taking greywater from the whole house. Widespread adoption of these systems, especially in high density developments, could lead to a significant reduction in average wastewa- ter temperatures. As these are working with greywater they do not need pre-treatment of the flow and only occasional cleaning. For installation further downstream the system has to deal with foul sewage with the problems of grit, rags and fat. There are two main ap- proaches. One option is to pump some of the sewage out of the sewer to pass it through a heat exchanger. The grit and rags are removed upstream of the heat exchanger and then returned to the sewer with the returned flow. The pre-pro- cessing imposes an additional maintenance requirement and the way in which rags are discharged back to the sewer could cause problems down- stream. These systems are currently favoured in the UK. The alternative, which is popular in Germany, is to leave the sewage in the sewer and put the heat exchanger in or around the sewer. The heat exchanger can be either a flat plate in the invert of the sewer or a coil wrapped around the outside of the sewer. These are obviously simpler to oper- ate with little impact on the sewer flow. They also have the advantage that by using the buffering capacity of the sewer structure and the soil around it, they can extract heat even if there is no flow or cold flow in the sewer for part of the day. The future So what is the long term future for using sewage for energy management? I expect household- or development-scale systems to take off; initially for new build but also for retrofit. This will reduce the opportunity for downstream systems as the energy capacity has already been used. Systems on trunk sewer or main pumping stations will still be used, but with a move to systems that leave the sewage in the sewer. Allowing for such systems when build- ing new sewers or pumping stations by pre-installing heat transfer coils will make future installation much cheaper. I also expect extracting heat from treated effluent to become more common, either for external use or to cycle heat back to the treatment pro- cess to increase its efficiency. A key dilemma is where in the network to install the heat-harnessing technology District heating could make the most of sewage heat in future