Selective enrichment of auto-floating microalgae for wastewater bioremediation and biofuel/bioproduct production

Integrating microalgal cultivation with wastewater treatment is a promising option for sustainable resource recovery and eco-friendly wastewater bioremediation. However, harvesting and culture control are major hurdles hindering its application. This study provided an effective solution to these challenges by selective enrichment of auto-floating microalgae in mixed cultures. This was achieved by repeatedly retaining the floe layer and discharging the subnatant in a semi-batch photo-bioreactor. The enriched auto-floating microalgae were dominated by Tribonema sp., which maintained its dominance (84.8 ± 9.2 %) over 180 days under non-sterile conditions. Effective autoflotation (>90 %) was achieved at a separation time of 30 min without coagulation, which enabled cost-effective microalgae harvesting. The autoflotation mechanism was in situ gas flotation, induced by photosynthetic micro‑oxygen bubbles. Cell surface hydrophobicity played a pivotal role in effective bubble-cell adhesion. This autoflotation mechanism inherently led to excellent dewaterability (SRF = 2.7 × 1011 m/kg). The enriched auto-floating microalgae demonstrated excellent nutrient removal efficiency (N and P > 98 %), and high biodiesel production potential (TFA content: 27–29 %; biodiesel productivity: 48–76 mg/L/d). It also had a high content (29–31 %) of β-(1–3/1–6)-glucans, which are well-known biological response modifiers and have wide applications in pharmaceuticals, nutraceuticals, food, feed, and cosmetics sectors. Overall, the present work demonstrates that selective enrichment of auto-floating microalgae offers great opportunities for a microalgae-based biorefinery in sustainable wastewater bioremediation.