Comparison of the wastewater treatment performance of continuously and discontinuously mixed high-rate algal ponds at Kingston on Murray.

High-rate algal ponds (HRAPs) incorporate shallow raceway designs and paddlewheel mixing. HRAPs use UV disinfection and the symbiotic environment between microalgal photosynthesis and heterotrophic bacteria for the assimilation of nutrients for efficient wastewater treatment. Mixing of a HRAP provides a homogenous environment and influences both the disinfection of pathogens and algal growth by exposing the wastewater to sunlight. Guidelines require continuous mixing of the HRAP. This study aimed to determine the effect of cessation of mixing for 10 days, on wastewater treatment by comparison with a continuously mixed pond operated over the same period. The period of 10 days was equivalent to the HRAP hydraulic retention time. Samples of inlet and HRAP-treated wastewater were collected from the HRAP at Kingston on Murray. Parameters measured were Escherichia coli, chlorophyll a, total suspended solids (TSS), NH-N, NO-N, NO-N, PO-P and biochemical oxygen demand (BOD). The discontinuously mixed and the continuously mixed HRAPs complied with the wastewater effluent guidelines, of an E. coli concentration ≤104 MPN100 mL and a BOD of <20 mg L. An E. coli log reduction value of >1 was also recorded. This study shows that cessation of mixing for 10 days had no significant effect on HRAP wastewater treatment performance.