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Hot stuff: making the most of sewer heat

Technology now allows heat from sewage to be extracted and utilised. But what is the true potential of the innovation? Martin Osborne explores the issues at stake

by Martin Osborne, Technical Director, WSP

Extracting heat from sewage is a current “hot” topic. Every week a new technology or a potential scheme is announced. But what is the real opportunity?

At WSP we have a lot of experience in both sewerage and heat management, so we have put the two together to consider three issues for managing heat using sewage: how much, where and how?

How much?
Renewable heat sources are becoming increasingly popular, partly driven by the Renewable Heat Incentive scheme. They are now often used for public buildings and office blocks, and even sometimes for domestic properties. The most common source of heat is the ground. To avoid causing long term changes in the ground temperature, there is a need to balance the heat extracted from the ground for heating in winter with the heat returned to the ground for cooling in summer. Using sewage as the source has the advantage that there is a constant flow bringing in and taking away energy, so that balance is less important.

Sewage is normally above ambient temperature, as much of the water that we use is heated; schemes currently being planned in the UK are focussed on extracting that energy for heating. Future schemes are already considering both heating and cooling.

However, one drawback is that the temperature of the sewage is important for the effectiveness of wastewater treatment. At lower temperatures, plants are less able to treat ammonia; some plants already struggle to do this in winter. Even a one degree reduction could be significant. So extracting heat from sewage might mean that we need 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.

The location of a sewage energy management systems influences its capacity. This is affected by the quantity of wastewater flow, which increases down the system; the temperature of the flow, which reduces down the system due to heat loss; and the consistency 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 contaminated 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 greywater for toilet flushing, so it should be done anyway.
A location downstream, at the outfall from the wastewater 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 treatment process; although it may still affect the receiving 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 middling 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 temperature. 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 may have reducing margins of operation.

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 individual 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 wastewater 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 approaches.

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-processing imposes an additional maintenance requirement and the way in which rags are discharged back to the sewer could cause problems downstream. 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 operate 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 building 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 process to increase its efficiency.

-This article appeared in the May 2018 issue of WWT magazine.


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