Comparison of mesophilic and thermophilic anaerobic sludge digestion at Hammarby Sjöstadsverk MBR pilot plant for wastewater treatment

Abstract: Anaerobic digestion is a commonly used method for stabilisation of sewage sludge and production of biogas at municipal wastewater treatment plants (WWTPs). The hydraulic retention time for sludge in the digestion reactors is about 16 days at Henriksdal WWTP in Stockholm. As Henriksdal WWTP is being extended and rebuilt for increased capacity, the hydraulic retention time will be decreased to 14 days. The present study investigated to what extent further decrease in retention time is possible by evaluating data from stress tested anaerobic digestion processes. The study also investigated methods for analysis of volatile fatty acid (VFA) as well as estimation of methane production. The present pilot study, conducted at Hammarby Sjöstadsverk pilot facility, proves that anaerobic digestion can prevail at nine to four days retention time. A retention time of four days resulted, however, in a specific methane production which was 42-48% lower than at Henriksdal WWTP at thermophilic (55 °C) and mesophilic (37 °C) temperatures. The ratio between VFA and alkalinity stayed within normal levels during both stress tests, indicating stable processes. Unexpectedly, VFA did not increase substantially during the stress tests. This might be explained by low organic loading rate and low degradability of the substrate. Alkalinity, however, was one of the first parameters to decrease below normal levels, possibly due to lower nitrogen mineralisation. Although this study shows that it is possible to maintain a viable anaerobic digestion process at nine to four days retention time, the loss of buffering capacity and lower methane generation should discourage long term operation at short retention times. The present study also suggests spectrophotometric VFA analyses for detection of low VFA concentrations as well as methane production estimations based on reduction of fat, protein and carbohydrate.