The use of high rate algal ponds for the treatment of marine effluent froma recirculating fish rearing system

A high rate algal pond (HRAP) system was used to treat effluent from a reci rculating sea water aquaculture system in southern France. Dicentrarchus la brax L. were farmed at a high density, with effluents containing an average of 10 mg L-1 dissolved inorganic nitrogen (DIN) and 1.3 mg L-1 reactive ph osphorus (RP). On a yearly basis, the algal pond removed 59% of the dissolv ed nitrogen and 56% of the phosphorus input, which was converted into 3.3 k g DW m(-2) algae. Green macroalgae were dominant throughout the year and th e algal biomass mirrored the seasonal changes in daily irradiance and tempe rature. This first year study supports the possibility of treating marine a quaculture wastes using HPAPs, although conditions will have to be found to mitigate the strong influence of climate on the algal community during win ter. During the more temperate season, only 150 m(2) of treatment ponds wou ld be necessary to remove the nutrients produced by 1 ton of fish. Treated water was characterized by a high pH, elevated levels of dissolved oxygen ( midday value) and low concentrations of nutrients and suspended solids. The absence of toxic phytoplankton meant that the water could be recycled thro ugh the farm tanks.