Investigation on high-solids anaerobic digestion: Rheological properties and organic matter content

High-solids waste activated sludge has been an increasingly attractive substrate for anaerobic digestion because of its high organic matter content. This work investigates the effects of thickening waste activated sludge on anaerobic digestion performance. Mesophilic batch anaerobic digesters of 2.10%, 3.40%, 4.10%, and 5.02%TS digested media (waste activated sludge mixed with inoculum), with the same ratio of food to inoculum, temperature, and mixing were set up while operating below ammonia and pH inhibition. The results showed that thickening the sludge increases its rheological properties more quickly than solubilized organic matter content. For example, initial solubilized oxygen demand, yield stress, and consistency index for the digester of 4.01%TS were 1.4, 1.8, and 3.3 times greater than those in the control (digester of 2.10%TS), respectively. In contrast, the properties for the digester of 5.02%TS were 1.5, 2.5, 4.9 times higher than those in the control. It also decreased the rates of organic matter degradation in the digester of 5.02%TS compared to the digesters of 4.10%, 3.40%, 2.10%. Therefore, flocs structure in the digester of 4.10%TS, which showed higher organic matter degradation during the first three days compared to the digester of 5.02%TS, was weaker; and hence had lower flow properties towards the end of digestion. Finally, biogas production yield from the digester of 4.10%TS increased 35.45%, while that of the digester of 5.02%TS increased 25.33% compared to the control, which indicates that higher organic matter content would be beneficial to biogas production provided that it does not restrict fluidity of digested media.