Influence of the solids retention time on the formation of the microalgal-bacterial aggregates produced with municipal wastewater

Abstract

The easy separation of the microalgal-bacterial biomass is crucial during wastewater treatment. Aggregates present excellent settling properties to enhance biomass separation. This paper evaluated the influence of solids retention time (SRT) on the formation of microalgal-bacterial aggregates during the treatment of municipal wastewater in continuous high-rate algal ponds (HRAP). Three SRT (6, 12, and 18 days) were studied in two 80 L HRAP operated at a fixed hydraulic retention time of 6 days. The aggregates formation was characterized by evaluating size distribution, settling properties, and biochemical composition, while the performance of the HRAP was evaluated considering the organic matter, suspended solids, nitrogen, and phosphorous removal. It was evidenced that the increment in the SRT in continuous HRAP favored the formation of the granular structures and settling properties impacting, therefore, in the effective solids' separation. No significant differences were found for COD removal, and bromatological characteristics for the three SRT evaluated. However, the increment on the SRT significantly affected the removal of N-NH4+, P-PO43− and suspended solids in the effluent, obtaining the best performances for an SRT of 12 days. It was found that the SRT imposes a community selectivity, allowing the formation of the aggregates. The increase in solids retention time induced the change from floccular aggregates to granules with an increment in the settling velocity (up to 7.4 m h−1). Microbiological characterization showed that aggregation depends on filamentous algae, cyanobacteria, and diatoms developed when the HRAP is operated under an SRT higher than 12 day