Enrichment of highly settleable microalgal consortia in mixed cultures for effluent polishing and low-cost biomass production

Microalgae cultivation is a promising technology for integrated effluent polishing and biofuel production, but poor separability of microalgal cells hinders its industrial application. This study intended to selectively enrich settleable microalgal consortia in mixed culture by applying “wash-out” pressure, which was realized by controlling settling time (S_T) and volume exchange ratio (VER) in photo-SBRs. The results demonstrated that highly settleable microalgal consortia (settling efficiency>97%; SVI = 17–50 mL/g) could be enriched from indigenous algal cultures developed in WWTP's effluent. High VER was the key factor for the fast development of settleable microalgae. VER was also a controlling factor of the algal community structure. High VERs (0.5 and 0.7) resulted in the dominance of diatom, while low VER (0.2) facilitated the dominance of cyanobacteria. The settleable microalgal consortia were very efficient in phosphorus removal (effluent PO₄ ^3--P<0.1 mg/L; removal efficiency>99%), which was largely attributed to intensive chemical precipitation of phosphate induced by high pH (8.5–10). However, the high pH decreased the bioavailable inorganic carbon, resulting in incomplete nitrate removal (effluent NO₃ ⁻-N = 2.2–4 mg/L; removal efficiency = 61–79%) under high VERs and low lipid content (up to 10%) in the settleable microalgae. This problem could be resolved by sparging CO₂ or controlling pH. Overall, this study demonstrated a simple and effective method to overcome the separation challenge in scale-up of microalgae biotechnology for advanced wastewater purification and biofuel production.