Methane is a greenhouse gas that is formed during, among other things, digestion processes. Methane has a GWP value of 72 for a period of 20 years. The sewage sector accounts for 5–8% of total methane emissions caused by humans, with sludge management dominating emissions in the sewage sector. By digesting the sludge, biogas is produced that consists largely of methane. Biogas as a renewable energy source has multiple applications, for example as fuel or a heat source through combustion. The resulting digestion residue can continue to be binding methane and give rise to methane emissions in subsequent stages. Methane emissions in subsequent stages increase exponentially with sludge temperature, as methane emissions increase sharply at sludge temperatures higher than 35°C, this is a critical level to fall below. At sludge temperatures lower than 30°C methane emissions decrease and below 20°C emissions are almost zero. 

This is a case study of a sewage treatment plant, Sjöstadverket, in Karlstad. Sjöstadverket produces heat from its biogas and sells it to the municipal district heating network. At the same time, district heating is purchased when the digesters need heating. The digested sludge ends up in a storage tower after digestion where methane is released. 

The purpose of the study is to investigate the possibilities of optimizing the heating system for the digesters with regard to a reduced district heating requirement and reduced methane release from stored sludge. For a reduced district heating requirement, the focus is on investigating internal use of heat and for a reduced methane release from sludge storage, the focus is on lowering the sludge temperature prior to storage by installing extra heat exchanger stages. The work includes a literature study, energy and temperature calculations and a cost analysis. The calculations are based on operating data for one year. 

The results show that the biogas generates more heat than the digesters require, which means that Sjöstadverket's digesters can become self-sufficient in heat. In order for the existing system, where heat is sold and district heating is purchased, to be economically profitable, the sales price must be at least 1936 SEK/MWh, which is just over three times as high as Karlstad Energi's current average market price for district heating energy. 

An analysis of heat exchangers shows that an extra installed heat exchanger stage reduces both the district heating requirement for the digesters and the temperature of the sludge prior to storage, however, the temperature does not drop enough to be able to determine a significant reduction in methane emissions. To get the average temperature below 35°C, 30°C and 25°C, four, six and eight heat exchanger stages are needed in total, respectively, which is not a reasonable number to get a sufficient temperature reduction in the sludge. 

The study shows that internal use of heat from biogas is environmentally and economically beneficial. Further investigations are recommended to effectively lower the sludge temperature prior to storage and reduce methane emissions.