High-rate algal ponds operated as sequencing batch reactors: Towards wastewater treatment with filamentous algae

Many rural communities across the globe face the SDG6 challenge of treating their wastewater, while experiencing restricted budgets and an inability to access or manage advanced technology. Furthermore, there is an increasing need to recycle the treated wastewater to address water insecurity. High-rate algal ponds (HRAPs) are an appropriate technology for regional communities, however the discharged, treated wastewater is high in microalgal suspended solids, the removal of which increases capital and operating costs. This study reframed the approach by using larger native filamentous algal strains, and operating HRAPs as sequencing batch reactors (SBRs). Judicious, independent management of solid and hydraulic retention times enabled selective enrichment of larger filamentous algae over microalgal populations. Cessation of mixing, followed by decantation of the supernatant from the SBRs, resulted in the discharge of a treated effluent that was low in suspended solids. HRAPs, of 4L volume, were operated in the laboratory and a glasshouse, to treat secondary effluent from septic tanks from a South Australian regional town. The SBR HRAPs operated at hydraulic retention times as short as 2–2.5 days, predominated by the filamentous green alga Stigeoclonium sp., produced a treated effluent with suspended solids < 70 mg L ⁻¹, biological oxygen demand < 10 mg BOD ₅ L ⁻¹ and with ammonium removals > 75%. These results are a major improvement of the currently validated HRAPs and herald a future for smaller and more efficient filamentous algal ponds delivering better quality treated wastewater for regional, rural and remote communities.