Polyhydroxyalkanoates, triacylglycerides and glycogen in a high rate activated sludge A-stage system

This paper quantifies the effect of varying solids retention time (SRT), hydraulic retention time (HRT) and DO concentrations on intracellular storage production and subsequently methane yield in a high rate activated sludge A-stage system treating domestic wastewater. Two 511 L pilot-scale A-stage reactors were set-up with HRTs of 30 (0.28 day SRT) and 60 (0.56 day SRT) minutes. Cascade DO control was used to maintain DO set-points of 0.5, 1.0 and 1.5 mg/L. Polyhydroxyalkanoates (PHA), triacylglycerides, and glycogen concentrations were measured. The shortest SRT and DO of 1.0 mg/L resulted in the highest fraction of soluble COD (sCOD) in the influent per day being captured as PHA per day (0.33 ± 0.02 kg PHA COD/ kg sCOD). At all scenarios, glycogen concentration decreased in the anaerobic sludge blanket in the clarifier and then increased in the aerobic reactors and the opposite trend was observed for PHA. When one of the aerated tanks was turned into an anaerobic selector, the same trend was observed in addition to uptake and release of orthophosphate (OP). Glyceride bound lipids increased in the anaerobic sludge blanket in the clarifier with the greatest increase at the 0.28 day SRT and 1.5 mg/L DO. This could have been due to microorganisms using the free fatty acids for growth or storing lipid droplets. Overall, the shorter SRT resulted in higher potential methane yield and this could be attributed to higher sludge yield and more intracellular storage products formation.