Comparison of disinfection in intermittently mixed (6 am-6 pm) and continuously mixed high rate algal ponds treating domestic wastewater in winter

High rate algal ponds (HRAPs) used for wastewater treatment typically are mixed continuously. This increases capital and operating costs, particularly for remote communities considering utilising solar power with battery storage. This study investigated the effects of intermittent mixing (6 am-6 pm) a full-scale HRAP treating septic tank effluent from a township in South Australia. The study was conducted in winter to be consistent with the validation guidelines for determination of logarithmic (log₁₀) removal values (LRVs) of bacterial and viral indicator organisms by natural wastewater treatment systems. The mean LRVs in the daytime only mixed HRAP were 1.95 for E. coli, 2.06 and 2.58 for F-RNA bacteriophage and somatic coliphage, respectively. A concurrently operated, continuously mixed HRAP recorded LRVs for E. coli of 1.81, 2.18 and 2.55 for F-RNA bacteriophage and somatic coliphage, respectively. The E. coli count in the treated effluent from both HRAPs was also <4.0 most probable number (MPN) 100 mL⁻¹ compliant for irrigation of non-food crops. The LRV 5th percentile for F-RNA phage was also >1.0 for both HRAPs. Results from this study demonstrated that LRVs from an intermittently mixed HRAP (6 am-6 pm) were equivalent to those recorded for the continuously mixed HRAP. Sulphate-reducing anaerobes (Clostridia) were shown as unreliable surrogate indicators for pathogenic protozoa for an open pond system. Intermittent mixing reduces energy consumption and storage capacity. Consequently, reducing the capital costs of installation of solar PV and batteries facilitating the adoption of HRAPs in rural and remote communities.