Co-digestion of cultivated microalgae and sewage sludge from municipal waste water treatment

• Microalgae were co-digested with a sewage sludge mixture in batch studies.
• Highest CH4-yield, of 408 ± 16 Ncm3 g VS−1 was reached in 37 °C with 37% microalgae.
• In 55 °C the same increase in methane yield with microalgae added could not be seen.
• The microalgae seems to be easily degraded with a short lag phase in the batches.

In this study two wet microalgae cultures and one dried microalgae culture were co-digested in different proportions with sewage sludge in mesophilic and thermophilic conditions. The aim was to evaluate if the co-digestion could lead to an increased efficiency of methane production compared to digestion of sewage sludge alone. The results showed that co-digestion with both wet and dried microalgae, in certain proportions, increased the biochemical methane potential (BMP) compared with digestion of sewage sludge alone in mesophilic conditions. The BMP was significantly higher than the calculated BMP in many of the mixtures. This synergetic effect was statistically significant in a mixture containing 63% (w/w VS based) undigested sewage sludge and 37% (w/w VS based) wet algae slurry, which produced 23% more methane than observed with undigested sewage sludge alone. The trend was that thermophilic co-digestion of microalgae and undigested sewage sludge did not give the same synergy.

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