Managing anaerobic wastewater for benign algal culture: a bio-circular economy approach

Inorganic N and P were introduced into municipal wastewater from anaerobic ponds to manipulate the N:P ratio thereby promoting the growth of exogenously inoculated green algae (Chlorella sp.) that outcompeted blue-green algae (Microcystis sp.). Alternate nutrient limitation and enrichment were used to manipulate the N:P ratio. An N:P of 1:60 was used to culture both green algae and cyanobacteria either separately or in combination. These ratios were then reversed to compare growth in order to develop effective N:P manipulation. The correlation between phosphate concentration and Chlorella sp. abundance showed that both Chlorella sp. and mixed algal culture can grow well within the range of 0.05–0.45 mg l‒1 of phosphate with N:P = 1:60. However, the relationship between growth of Chlorella sp. and nitrate-N indicated that green algae were almost independent of water nitrate-N. But the overall relationship between N:P and Chlorella sp. showed that more input of phosphorus than nitrogen in the N:P = 1:60 treatment was conducive to growth of Chlorella sp. This relative N:P manipulation will help to outcompete blue-green algae which generally grow even at low levels of nitrate-N with the help of their attached microbiome. Therefore, simple inorganic phosphate fertilizer addition to anaerobic wastewater with high ammonium-N, can prove to be a less challenging, cost-effective wastewater nutrient management and re-use strategy using a bio-circular economy approach for culture of benign green algae in the aquatic food chain as well as for aquaculture production.