Sewer heat recovery represents a significant opportunity for the decarbonisation of heat in urban areas – particularly when combined with heat networks.

The stable temperature of sewage (as compared to ambient air), and the correlation between population density, heat demand, and the scale of the sewer system means its potential as a heat source for low carbon heat networks is unique.

By establishing an ‘Exclusion Zone’ upstream of a sewer source heat pump, it is possible to safeguard performance in relation to the quality of the heat source, giving operators the surety required to make their investment.

The size of the Exclusion Zone should not be fixed, however, since it depends on the scale of the proposed new heat abstraction (the required offtake) relative to the flow rate of the sewage (the available energy). It is necessary, therefore, to assess each case on its own characteristics.

How was this guidance developed?

This guidance has been developed using evidence from a detailed modelling exercise that simulates the rate of sewage temperature recovery downstream of a notional heat abstraction. The model uses sewer network topology and dry weather flow rate profiles from detailed sewer system hydraulic models provided by several UK water companies. It calculates the rate at which sewage temperature would recover to within a given percentage of the pre-abstraction sewage temperature from a range of heat pump capacities, taking into account the downstream catchment characteristics and flow rate profiles. By repeating this exercise for a large number of notional heat pump capacities across a large number of notional heat abstraction sites, it is possible to 6 statistically analyse the likelihood of sewage temperature recovery within a given downstream distance of a heat abstraction according to two key variables:

• Minimum (base) dry weather flow rate at the abstraction point 
• Heat pump capacity . 

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