A control system to simulate diel pH fluctuation in eutrophic aquaculture ponds

The toxicity of substances that vary in toxicity through time (daily) as a function of pH are not effectively assessed with static renewal test method s. A control system to simulate diel pH fluctuation in eutrophic aquacultur e ponds was established in a flow-through, wet-laboratory setting. Water (1 00 L/ min) was conditioned in a packed column of 3.81-cm Pall rings prior t o distribution to 48 189-L fish tanks. Control system components included a distributed data acquisition and control system, a personal computer, and various pH control devices, pH values greater than that of influent water ( pH 8.0) were achieved by additions of concentrated sodium hydroxide solutio ns: pH values less than that of influent water were achieved by additions o f carbon dioxide gas. Two data acquisition and control software strategies were developed to control pH: one from pH 7.4 to 8.0 and the other from pH 8.0 to 9.6. The basic control strategy involved comparison of a projected p H value to a set point. A moving linear regression was used to forecast the pH value that would result following base or gas addition, and the rate of chemical addition was adjusted accordingly. The system has resulted in acc eptable pH control (deviation +/- 0.1 pH unit), indicating the potential ut ility of a relatively simple control system for evaluation of fish physiolo gical response to the characteristically dynamic environment of eutrophic a quaculture ponds.