Nitrogen availability influences phosphorus removal in microalgae-based wastewater treatment

• N removal was independent of P supply, P removal improved with increased N supply.
• Biomass N and P concentration was adjusted to the nutrient supply.
• Biomass concentration ranged only a factor 2.
• Chlorella has a higher N concentration and lower P concentration compared to Scenedesmus.

Microalgae offer a promising technology to remove and re-use the nutrients N and P from wastewater. For effective removal of both N and P, it is important that microalgae can adjust the N and P concentration in their biomass to the N and P supply in the wastewater. The aim of this study was to evaluate to what extent microalgae can adjust the N and P concentrations in their biomass to the N and P supply in the wastewater, and to what extent supply of one nutrient influences the removal of the other nutrient. Using Chlorella and Scenedesmus as model organisms, we quantified growth and biomass composition in medium with different initial N and P concentrations in all possible combinations. Nutrient supply marginally affected biomass yield of both microalgae but had a strong influence on the composition of the biomass. The nutrient concentrations in the biomass ranged 5.0–10.1 % for N and 0.5–1.3 % for P in Chlorella and 2.9–8.4 % for N and 0.5–1.7 % for P in Scenedesmus. The concentrations of P in the biomass remained low and were relatively constant (0.6–0.8 % P) when the N concentration in the biomass was low. As a result, removal of P from the wastewater was influenced by the concentration of N in the wastewater. When the initial N concentration in the wastewater was above 40 mg L−1 the microalgae could remove up to 6 mg P L−1, but this removal was only 2 mg P L−1 when the initial N concentration was below 20 mg L−1. A lower N supply increased the carbohydrate concentration to about 40% and lipid concentration to about 30% for both species, compared to around 15% and 10% respectively at high N supply. Our results show that sufficiently high N concentrations are needed to ensure effective P removal from wastewater due to the positive effect of N on the accumulation of P.

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