Can CO2 addition improve the tertiary treatment of anaerobically digested abattoir effluent (ADAE) by Scenedesmus sp. (Chlorophyta)?

Microalgae offer a cost-effective option for wastewater treatment due to their natural ability to uptake and assimilate various forms of nutrients for the production of valuable biomass. Here, we assessed the potential of Scenedesmus sp. culture for treating anaerobically digested abattoir effluent (ADAE). Scenedesmus sp. was cultured at different pH (5.5, 6.5, 7.5 and 8.5) regulated by the addition of CO2 (pH-stat systems) under outdoor climatic conditions in paddle wheel driven raceway ponds. pCO2 was highest in cultures operated at pH 5.5 followed by 6.5, 7.5, 8.5 and uncontrolled pH. The highest biomass productivity was obtained for the Scenedesmus culture grown at pH 6.5 (19.24 g m- 2d-1) which was 2.1 times greater than uncontrolled pH culture (9.13 g m- 2d-1). In general, the growth was pH 6.5 > pH 7.5 = pH 8.5 > uncontrolled pH > pH 5.5. The percentage of ammoniacal nitrogen (NH3-N) fixed into biomass was highest (about 86%) for the cultures at pH 6.5. In overall, percentage of NH3-N assimilation was pH 6.5 > pH 7.5 > pH 8.5 > uncontrolled pH > pH 5.5. More than 83% of phosphorus was assimilated into algal biomass when Scenedesmus was grown at pH 6.5, 7.5 and 8.5. These results clearly showed that the addition of CO2 (keeping pH at 6.5) did not only remarkably increased biomass productivity of microalgae grown in ADAE but also significantly increased the removal of nutrients from the wastewater.

Automated summary

This study assessed the potential of Scenedesmus sp. for treating anaerobically digested abattoir effluent under different pH conditions, with the highest biomass productivity observed at pH 6.5. The assimilation rate of ammoniacal nitrogen and phosphorus into algal biomass was also highest at pH 6.5, indicating the effectiveness of CO2 addition in increasing both biomass production and nutrient removal from wastewater.