pH treatments in continuous cultivation to maximize microalgal production and nutrient removal from anaerobic digestion effluent of aquatic macrophytes

Green algae have great potential for removing inorganic nutrients from anaerobic digestion effluent (ADE), but there is insufficient available magnesium (Mg) in the ADE from aquatic macrophytes for effective algal growth. In this study, we determined suitable pH and hydraulic retention time (HRT) for maximizing growth of the alga Chlorella sorokiniana and nutrient removal efficiency with a flow-through continuous cultivation system. This used ADE from aquatic macrophytes, adjusted to pH 5.0, 6.0, 6.5 and 7.0 at two HRTs of 6 and 8 days. The highest C. sorokiniana biomass concentration was obtained in the pH 6.5 treatment, being 0.50 and 0.67 g L−1 at 6 and 8 days of HRT, respectively, without Mg enrichment. Both were equivalent to 83.6 mg L−1 day−1 of biomass productivity. Removal rates of both ammonium nitrogen and phosphate phosphorus were almost 100% in pH 6.5 and 7.0 treatments and declined to below 60% in pH 5.0 and 6.0 treatments during the study period. The results demonstrated that a source medium pH of 6.5 using a continuous cultivation system (irrespective of the two HRTs), and increasing available Mg, was suitable for maximizing C. sorokiniana biomass productivity and nutrient removal from the ADE.