| Summary: | This study investigated nutrient removal from anaerobic digestion effluent by cultivating mixed-culture microalgae enriched from anaerobic sludge under different pH conditions: R<sub>UC</sub> (uncontrolled), R<sub>7</sub><sub>–8</sub> (maintained at 7–8), and R<sub><8</sub> (maintained below 8). Significant amounts of NH<sub>4</sub><sup>+</sup>-N were lost by volatilization in R<sub>UC</sub> cultures due to increased pH values (≤8.6) during the early period of cultivation. The pH control strategies significantly affected the biological NH<sub>4</sub><sup>+</sup>-N removal (highest in R<sub>7</sub><sub>–8</sub>), microalgal growth (highest in R<sub>7</sub><sub>–8</sub>), biomass settleability (highest in R<sub><8</sub>), and microalgal growth relative to bacteria (highest in R<sub><8</sub>) in the cultures. <i>Parachlorella</i> completely dominated the microalgal communities in the inoculum and all of the cultures, and grew well at highly acidic pH (<3) induced by culture acidification with microalgal growth. Microalgae-associated bacterial community structure developed very differently among the cultures. The findings call for more attention to the influence and control of pH changes during cultivation in microalgal treatment of anaerobic digestion effluent.
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