The optimization of microalgal culturing in liquid digestate after struvite precipitation using gray relational analysis.

Liquid digestate (LD) is highly turbid and contains ammonium (NH ₄ ⁺ -N), which negatively influences microalgal growth. Therefore, a method of reducing LD turbidity and NH ₄ ⁺ -N content is proposed, using struvite precipitation. To obtain struvite precipitation supernatant with an ideal UV transmittance, NH ₄ ⁺ -N concentration, and N/P ratio for microalgal growth, the effects of pH and the molar ratio of NH ₄ ⁺ /Mg ²⁺ /PO ₄ ^3- were studied. Results show that the optimal NH ₄ ⁺ /Mg ²⁺ /PO ₄ ^3- molar ratio was 1:1.5:1.5, with a pH value of 8.5, following NaOH addition. Gray relational analysis (GRA) was applied to obtain a relative gray scale for the evaluation of multiple outputs. Results show that Chlorella regularis FACHB-1068 was the optimal microalgae species to support growth in the struvite precipitation supernatant. Using struvite precipitation and treatment with cultured C. regularis FACHB-1068 for 7 days, the removal efficiencies of NH ₄ ⁺ -N, PO ₄ ^3- -P, and COD in LD were 96.52, 99.33, and 35.30%, respectively.