Investigating the Impact of Anaerobic Retention Time and Phosphorus-to-Carbon Influent Ratio on Phosphorus Accumulating Organisms’ Kinetics

Abstract

Existing phosphorus removal technologies face challenges in achieving recent, more stringent regulatory discharge limits on effluent phosphorus levels, necessitating further process optimizations. While anaerobic-phase metabolic activities drive Enhanced Biological Phosphorus Removal (EBPR)’s core mechanism, the design of the aerobic phase dominates the system sizing. This thesis aims to investigate the influence of anaerobic retention time and phosphorus loading on the EBPR’s performance. EBPR performance in bench-scale reactors was compared under baseline conditions and test conditions, which include varying Phosphorus-to-Carbon (P/C = 3:80, 1:80 and 1:40) ratios and anaerobic retention times (60, 90, 120 minutes). The polyphosphate content, release/uptake rates, and maintenance energy demands are evaluated to analyze performance. Polyphosphate content in the biomass was shown to influence P/C ratios (R2= 0.673), directly impacting phosphorus release and uptake rates. Meanwhile, reducing the retention time from 90 mins to 60 mins preserved the specific release rates but significantly lowered uptake and removal efficiency. Extending the anaerobic phase to 120 minutes decreased release rates without affecting uptake. Secondary phosphorus release, indicative of maintenance energy demands, varied across anaerobic retention times. This variation highlights how PAOs adapt polyphosphate hydrolysis rates to sustain baseline metabolism during shortened or extended anaerobic phases, beyond initial substrate-driven release. Results confirm polyphosphate reserves in biomass govern release/uptake dynamics which varies with anaerobic retention time.