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The demand for biogas is continuously growing and the biogas substrate, such as food waste, may soon become limited and it is therefore important for biogas producers to expand the range of substrates. One way can be to use microalgae in co-digestion with sewage sludge.The present study explores the possibilities to use harvested microalgae from Lake Mälaren, as a co-substrate to sewage sludge in biogas production under mesophilic and thermophilic conditions. The aim is to investigate if codigestion of microalgae and sewage sludge is more efficient for biogas production compared using the sludge alone. The study has been carried out as a BMP-experiment (Biochemical Methane Potential) in batch fermentation bottles. The substrate was undigested sludge where 0%, 12 %, 25 % and 37 % were replaced with the cultivated microalgae. The results showed that the use of an algae/bacteria community, cultivated in prior to digestion, can serve as a biomass substrate for biogas production together with municipal wastewater sludge. Co-digestion of microalgae and sewage sludge can be more efficient for biogas production compared to using the sludge alone under mesophilic conditions. It can also be concluded that thermophilic co-digestion between the microalgae and sludge give lower biochemical methane potential.

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