What is sewage heat recovery?
Sewage heat recovery is the extraction of heat — or cooling — from the wastewater running through a sewer. It is a form of water-source thermal energy; in Dutch it goes by the name riothermie. Showering, washing up and laundry send large volumes of warmed water into the sewer every day. That heat normally disappears towards the treatment works; sewage heat recovery claws part of it back.
How it works
Sewage is relatively warm almost all year and, more importantly, fairly constant in temperature — even when it is freezing outside. That makes it a favourable source for a heat pump.
- Heat exchanger — a heat exchanger sits in the invert of a sewer, in a chamber, or alongside a pumping station. Wastewater flows over it and gives up a few degrees to a closed circuit. The wastewater itself never enters the building.
- Transport — the warmed fluid runs to the building’s plant room or to a district installation.
- Heat pump — this lifts the low-grade heat to a usable flow temperature for underfloor heating, low-temperature radiators or hot water.
In summer the same system can often be reversed to provide cooling.
Where it stacks up
- Enough flow — there must be a steady volume passing; trunk sewers and rising mains perform far better than a quiet residential street drain.
- Enough diameter — a heat exchanger will not fit in a small pipe without obstructing flow.
- A short distance to the user — moving heat costs energy and pipework costs money. A swimming pool, school, apartment block or terrace within a few hundred metres is the classic case.
- A low-temperature emitter system — underfloor heating or generously sized radiators make a substantial difference to the seasonal performance.
For a single detached house it rarely pays. This is a neighbourhood- or building-scale solution, often combined with a district heating network. The sewer is normally owned by the local authority or water company, so cooperation is essential.
Points to watch
A heat exchanger in a sewer needs maintenance: fat, fibres and fouling reduce heat transfer. The wastewater must also not arrive too cold at the treatment works, because the biological treatment stage then performs worse. Operators therefore set limits on how much heat may be taken.
Related terms
- Sewer
- Aquathermal energy
- Ground heat exchanger
- Low-temperature heating
- Geothermal energy
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