A waste effluent treatment system based on microbial mats for black sea bass Centropristis striata recycled-water mariculture

Abstract

The objective of this project was to determine if a low-cost biosolar filter system, based on microbial mats and fluidized sand filters, could be used for a greenhouse recirculating black sea bass ( Centropristis striata) mariculture project. Microbial mats are stratified microbial communities, composed of a complex of bacteria and dominated by photoautotrophic cyanobacteria, which can transform nitrogenous wastes into cellular protein and rapidly metabolize other fish wastes. The microbial mats also provided an excess of oxygen for the nitrifying bacteria on the fluidized sand filters, providing favorable conditions for ammonia removal. The filter system was altered after it was found that microbial mat surfaces became covered with fish wastes, which resulted in oxygen-production decreases and ammonia-concentration increases. The modification included delivering the fish wastewater under the mats, rather than on the surface, thus depositing solids beneath the photosynthetic surfaces. Instead of pellets, live tilapia ( Oreochromis sp.) juveniles were used as food which decreased the amount of solid wastes produced. Water and sludge from the fish tank were drained by gravity from the bottom of the fish tanks into the microbial mat trays at a rate of 7 l/min. Stable functioning mats provided high oxygen levels, supported the nitrifying bacteria and removed wastes produced by the black sea bass. Oxygen concentrations ranged from 6 to 10 mg/l and total ammonia concentrations remained below 1 mg/l. During the 9th month there was a steady fish growth, little buildup of solids and the fish tank remained clear. The mean weights of the 15 fishes increased from 161±45 to 215±72 g, and to 337±99 g after 2, 4, and 9 months, respectively.