The municipality, of Gatukontoret in Local government of Borås has in the long run, a required need to look after the current heating devices (the gas production plant) in sewage treatment plant, called Gässlösa. The practical work of the examination is about, trying to find a better way to take care of the current heat recovery. That may be one way or another, rather easy to implement in the existent construction. Through the work to increase the efficiency of the gas engine, implement of some external cooling system by heating exchange device. That gives an opportunity to heat (primary) the digester, (but even all internal public areas) with its own superfluous heating. The report shows that if the management group may invest 422 704 crown to implement a heat exchanging device, it gives more space to reduce the need to buy expensive external energy. Gatukontoret´s pay off time, for buying machines, is 20 years, with reservation to calculate with 4 percent internal interest, leads to the yearly mount of 31103 crowns. If Gässlösa increases the gas engine time in (effective) duty approximately (correspondingly) 48 hours a month the entire investment will generate a profit in January in year seven after the implementations. The amount of profit should therefore be 1 033 337 crowns after 20 years.