Effects of two different nutrient loads on microalgal production, nutrient removal and photosynthetic efficiency in pilot-scale wastewater high rate algal ponds

• Microalgal photosynthetic efficiency responded positively to high nutrient loads.
• Microalgal productivity was greater in high nutrient loads than moderate nutrient loads.
• Microalgae were more efficient at assimilating NH4–N under higher nutrient loads.
• Higher effluent water quality was achieved at moderate, compared to high, nutrient loads.

When wastewater treatment high rate algal ponds (HRAP) are coupled with resource recovery processes, such as biofuel production, short hydraulic retention times (HRTs) are often favoured to increase the microalgal biomass productivity. However, short HRT can result in increased nutrient load to the HRAP which may negatively impact on the performance of the microalgae. This paper investigate the effects of high (NH4–N mean concentration 39.7 ± 17.9 g m−3) and moderate ((NH4–N mean concentration 19.9 ± 8.9 g m−3) nutrient loads and short HRT on the performance of microalgae with respect to light absorption, photosynthesis, biomass production and nutrient removal in pilot-scale (total volume 8 m3) wastewater treatment HRAPs. Microalgal biomass productivity was significantly higher under high nutrient loads, with a 133% and 126% increase in the chlorophyll-a and VSS areal productivities, respectively. Microalgae were more efficient at assimilating NH4–N from the wastewater under higher nutrient loads compared to moderate loads. Higher microalgal biomass with increased nutrient load resulted in increased light attenuation in the HRAP and lower light absorption efficiency by the microalgae. High nutrient loads also resulted in improved photosynthetic performance with significantly higher maximum rates of electron transport, oxygen production and quantum yield. This experiment demonstrated that microalgal productivity and nutrient removal efficiency were not inhibited by high nutrient loads, however, higher loads resulted in lower water quality in effluent discharge.