Effects of solids concentration and thermal pretreatment on continuous digestion of undigested dewatered sludge

In this study, semi-dry anaerobic digestion was conducted to treat undigested, dewatered sludge with total solids (TS) values ranging from 10.3 to 15.1 wt% in a horizontal tubular bioreactor with a working volume of 20 L and a solids retention time of 16 days. The low-thermal pretreatment at 60 °C for 3 h enhanced the continuous methane production from 18.8% to 25.0%. The highest biogas production of 193.8 mL CH4/g VS and 1.10 L CH4/L·d was achieved at a feed TS of 13.3 wt% and an organic loading rate (OLR) of 5.69 g VS/L·d. The obtained methane yield was equivalent to 88.8% of the biochemical methane potential of the pretreated feed sludge. The maximum specific methanogenic activity and the population of methanogenic archaea showed a close correlation with methane production. Methanogenesis shifted from the acetoclastic pathway to the hydrogenotrophic pathway as the OLR increased. The pretreatment of dewatered sludge not only improved methane production but also affected the methanogens. A tracer study confirmed that the bioreactor was properly mixed at high TS conditions. However, the digester performance temporarily declined due to the inhibition of the tracer, iodide. It was recommended to lower the iodide concentration below 20 mM to prevent inhibition of methanogenesis.