Improved microalgal productivity and nutrient removal through operating wastewater high rate algal ponds in series

Abstract

High rate algal ponds are recognised as a cost effective and efficient upgrade to conventional wastewater ponds for the treatment of a wide range of wastewaters. Their design allows microalgae to proliferate, which, in turn, results in high levels of nutrient removal, via algal assimilation. Furthermore, these ponds offer the opportunity to recover resources, in the form of algal biomass, for beneficial re-use, thus creating a circular bio-economy using wastewater. However, both increased microalgal biomass and nutrient removal is required to make coupled full-scale systems commercially viable. The performance of high rate algal ponds operated in series, on short hydraulic retention time of 4 days, versus in parallel, on a longer retention time of 8 days, was assessed with respect to nutrient removal and microalgal production. For microalgal productivity, the combined total volatile suspended solids (organic matter) and chlorophyll-a biomass were significantly higher (p < 0.01) under Series (191 ± 41 kg per day for volatile suspended solids) than Parallel (127 ± 18 kg per day) operation. The combined total dissolved inorganic nitrogen removed per day was significantly higher (p < 0.01) under Series (23 ± 4 kg of nitrogen per day) than Parallel (17 ± 4 kg) operation. The total amount of phosphorus removed per day was unaffected by mode of operation. Higher biomass production under short retention times came at the expense of nitrogen removal but treatment of the harvested effluent through a second pond in series, resulted in overall higher daily nitrogen removal and biomass production than ponds in parallel, for the same volume of wastewater treated. This study has demonstrated that with simple modifications to pond operation higher microalgal yields and improved effluent water quality without increased capital or operational costs.