Evaluating AGS efficiency in PHA synthesis and extraction integrated with nutrient removal: The impact of COD concentrations

Abstract

As natural and biodegradable biopolymers, Polyhydroxyalkanoates (PHA) were synthetized by aerobic granules sludge (AGS) in a sequential batch reactor in this study. The effect of different COD concentrations on PHA accumulation and nutrients removal were investigated. At the same time, different pretreatment methods for PHA extraction, including NaClO pretreatment for extracellular polymeric substances (EPS) removal, Na₂CO₃ pretreatment for EPS recovery, and grinding pretreatment to reduce particle size and augment the surface area available for interaction with the extraction solvent, were compared. The results showed that the PHA yield increased more than 2 times (from 91.1 to 233.3 mgPHA/gCDW (cell dry weight)) when COD concentration increased from 800 to 1600 mg/L. Polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV) both accounted for half of the total, while PHB fraction rose to 71% when COD concentration went up to 1600 mg/L. The PHB can be consumed 3 times faster than PHV. High COD concentration (1600 mg/L) adversely impacted the structure stability of AGS and the phosphorus removal efficiency, while the system consistently exhibited robust nitrogen removal capabilities, with ammonium and TN removal efficiencies exceeding >90%. The dominant bacteria shifted from Flavobacterium to Halomona and Hydrogenophaga as the COD concentration increased. In terms of PHA extraction, Na₂CO₃ pretreatment, which was used for EPS recovery, had the best PHA recovery with nearly 100% purity and EPS removal efficiency compared with NaClO and grinding pretreatments.