Denitrification in aquaculture systems: an example of a fuzzy logic control problem

Nitrification in commercial aquaculture systems has been accomplished using many different technologies (e.g. trickling filters, fluidized beds and ro tating biological contactors) but commercial aquaculture systems have been slow to adopt denitrification. Denitrification (conversion of nitrate, NO3- to nitrogen gas, N-2) is essential to the development of commercial, close d, recirculating aquaculture systems (<1 water turnover 100 day(-1)). The p roblems associated with manually operated denitrification systems have been incomplete denitrification (oxidation-reduction potential, ORP > - 200 mV) with the production of nitrite (NO2-), nitric oxide (NO) and nitrous oxide (N2O) or over-reduction (ORP < - 400 mV), resulting in the production of h ydrogen sulfide (H2S). The need for an anoxic or anaerobic environment for the denitrifying bacteria can also result in lowered dissolved oxygen (DO) concentrations in the rearing tanks. These problems have now been overcome by the development of a computer automated denitrifying bioreactor specific ally designed for aquaculture. The prototype bioreactor (process control ve rsion) has been in operation for 4 years and commercial versions of the bio reactor are now in continuous use; these bioreactors can be operated in eit her batch or continuous on-line modes, maintaining NO3- concentrations belo w 5 ppm. The bioreactor monitors DO, ORP, pH and water flow rate and contro ls water pump rate and carbon feed rate. A fuzzy logic-based expert system replaced the classical process control system for operation of the bioreact or, continuing to optimize denitrification rates and eliminate discharge of toxic by-products (i.e. NO2-, NO, N2O or H2S). The fuzzy logic rule base w as composed of > 40 fuzzy rules; it took into account the slow response tim e of the system. The fuzzy logic-based expert system maintained nitrate-nit rogen concentration < 5 ppm while avoiding any increase in NO2- or H2S conc entrations. (C) 2000 Elsevier Science B.V. All rights reserved.