A description of ammonium content of output waters from trout farms in relation to stocking density and flow rates

Daily levels of ammonium content in the outflow water of an intensive trout farm and of a mountain pond stocked with rainbow trout were monitored. Output water was sampled for 30 days and analysed every 30 min by an automatic ion selective electrode system (Applikon ADI 2013); input water was also monitored. The ammonium output level was influenced more by atmospheric events than nitrogen excretion by fish. Observed data confirmed an overall ammonium excretion model previously estimated in both laboratory and field conditions. The high water how, that characterises the intensive trout farm where the observations were made induced a high dilution of metabolites. Consequently. if the peaks of ammonium output did not reach values of 0.35-0.40 mg l(-1) the environmental impact was limited and not easily detected. Our results allow us to affirm that the optimal level of water flow rate for effective zeolite-operated filtration is around 10.3 l t(-1) s(-1). (C) 2001 Elsevier Science B.V. All rights reserved.