Immobilized Chlorella salina for removing nitrogenous and phosphorous compounds from mariculture tail water

The rapid development of the maricultural industry has brought both economic benefits and environmental challenges. One such challenge is the purification of mariculture tail water (MTW), which has become a key issue of marine environmental protection. Meanwhile, immobilized algal beads (IAB), has received growing attention as cost-effective wastewater treatment technology. However, most of the studies on the treatment of aquaculture tail water by IAB focus on the application of freshwater, and the use of seawater remains relatively rare. This study investigated the screening of algal species and embedding particle size suitable for the treatment of artificial MTWI, the optimization of algal beads preparation using the response surface method, and their application in treating artificial MTW-II. The results showed that immobilized Chlorella salina harbouring particle size of 3.2 mm were most effective for removing N from artificial MTWI. IAB fabricated under the optimized conditions of 4.2 % (w/v) sodium alginate (SA), 1 × 10⁷ cells·mL⁻¹ initial density of C. salina and 130 mL algal cells volume achieved the highest NH₄-N removal efficiency (75.780 %) during the 72 h experiment. Although immobilized cultivation inhibited algal cell photosynthesis, it also promoted their growth and NH₄-N removal ability. Groups with 10 % algal beads exhibited a better purification effect and could treat 5 batches of artificial MTW-II containing 815.75 mg of inorganic nitrogen and 71.80 mg of phosphate. Furthermore, the addition of 2 % algal beads was the most cost-effective scheme, resulting in a treatment cost of approximately 3.50 RMB per ton of tail water and 0.33 RMB per gram of inorganic nitrogen. These findings provide unique insight into marine environmental protection and suggest that IAB application offers a promising method for integrated MTW treatment.