Reuse strategy of wastewater in prawn nursery by microbial remediation

A strategy of reusing the prawn nursery wastewater was developed by the previous remediation with Bacillus subtilis and nutrients addition. The suggested method was preliminarily verified in rearing prawn larval. Bacteria assimilation is proved as a main and powerful mechanism for removing dissolved organic matter (DOM) and total ammonia nitrogen (TAN) in the experiments. The process of microbial remediation could be featured as two sequential stages: DOM degradation and TAN reduction. In the first stage (from day 0 to day 2), DOM was degraded directly as the bacterial carbon and nitrogen sources. The 48-h COD removal efficiencies (R_COD) in the treatment were 57.7±5.5%, showing significantly different from 12.2±4.1% in control. In the second phase (from day 2 to day 5), the deficiencies of C and P source relative to N source might limit bacteria proliferation and TAN removal. The glucose and/or phosphate addition significantly influenced the TAN removal performance, while the addition of vitamin mixture and/or microelement solution was not. When the initial TAN level was nearly 5 mg N/l, the optimal TAN removal efficiencies from day 2 to day 5 were above 85% in the treatments with both glucose and phosphate additions, where the white microbial floccules were observed to suspend. The C/N or N/P weight ratio of 5.4:1or 5–7:1 was suggested for remediating the nursery effluents. The situation that the maximum bacterial levels did not exceed 10⁸ CFU/ml was estimated to correlate with the formation of microbial floccules. After 5 days of microbial remediation by these adequate methods, the wastewater had been became the “microbially matured” water suitable to reuse. In the practical operations, the nutrient supplements were directed by Glucose/TAN weight ratio of 13:1 and KH₂PO₄/TAN weight ratio of 0.6–0.9:1. The results of the application trial pointed out that the remediated water reclaimed to the larvae culture tanks did not produce observable deleterious effects on the water quality and on the mortality of the prawn larvae. The method of reusing the remediated wastewater by microbial agents and nutrients additions will be advantageous to reduce both production cost and environmental pollution in the inland hatchery and nursery for prawn.