Water treatment of land-based fish farm effluents by outdoor culture of marine diatoms

The feasibility of using fish farm effluents was evaluated as a source of inorganic nutrients for mass production of marine diatoms. Batch cultures were conducted from May to July 1995 in 16-L outdoor rectangular tanks, homogenized by gentle aeration (0.2 L(air) L(-1) h(-1)). The effluents from the two fish farms studied were both characterized by high concentrations of inorganic materials (NH4-N, PO4-P, Si(OH)(4)-Si) and were shown to support production of marine diatoms. Moreover, periodic measurements of inorganic matter levels in the cultures showed that clearance was efficacious (90% in 3-5 days). Water purification efficiency and culture productivity were further increased through appropriate nutrient balancing. When effluents were limited in silicate, addition of Na2SiO3 induced a significant increase in both diatom biomass and nutrient removal efficiency. In this case, up to 720000 cell mL(-1) were produced dominated by Skeletonema costatum. By contrast, in effluents loaded with silicate, adjustment of the N:P:Si ratio by NH4-N and PO4-P supplementation then gave increased biomass production. In this case, the maximum cell density found was 450000 cell mL(-1), dominated by Chaetoceros spp.