The effects of CO2 addition along a pH gradient on wastewater microalgal photo-physiology, biomass production and nutrient removal

• Addition of CO2 increased total microalgal biovolume and biomass.
• Highest biomass yield was achieved with the highest CO2/lowest pH combination.
• Light absorption efficiency per unit chlorophyll-a increased with CO2 addition.
• Both photosynthesis and light saturating intensity increased with CO2 addition.

Carbon limitation in domestic wastewater high rate algal ponds is thought to constrain microalgal photo-physiology and productivity, particularly in summer. This paper investigates the effects of CO2 addition along a pH gradient on the performance of wastewater microalgae in high rate algal mesocosms. Performance was measured in terms of light absorption, electron transport rate, photosynthetic efficiency, biomass production and nutrient removal efficiency. Light absorption by the microalgae increased by up to 128% with increasing CO2 supply, while a reduction in the package effect meant that there was less internal self-shading thereby increasing the efficiency of light absorption. CO2 augmentation increased the maximum rate of both electron transport and photosynthesis by up to 256%. This led to increased biomass, with the highest yield occurring at the highest dissolved inorganic carbon/lowest pH combination tested (pH 6.5), with a doubling of chlorophyll-a (Chl-a) biomass while total microalgal biovolume increased by 660% in Micractinium bornhemiense and by 260% in Pediastrum boryanum dominated cultures. Increased microalgal biomass did not off-set the reduction in ammonia volatilisation in the control and overall nutrient removal was lower with CO2 than without. Microalgal nutrient removal efficiency decreased as pH decreased and may have been related to decreased Chl-a per cell. This experiment demonstrated that CO2 augmentation increased microalgal biomass in two distinct communities, however, care must be taken when interpreting results from standard biomass measurements with respect to CO2 augmentation.

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