Modeling of nitrate and bromate in a seawater aquarium

Closed marine systems where there is no significant primary productivity and water changes are impractical exhibit increasing nitrate concentrations over time. At The Living Seas artificial seawater aquarium, nitrate concentration reached 9700 μM after 5 yr of operation. Concern about potential toxic effects associated with a further increase led to the development of a batch denitrification system, separate from the Main Tank of the aquarium. The system was installed in early 1991 and after 350 days of operation, nitrate concentration in the aquarium decreased to 3200 μM. Two models are applied to the empirical nitrate data — one designed specifically for nitrate and one originally developed for bromate. Both models have correlations greater than 0.95 with the empirical nitrate data over the period studied. The models were used to determine the steady state nitrate concentration in the aquarium: the first model predicts 850 μM, assuming no idle time between denitrification runs. The second model predicts 2300 μM, with the actual idle times taken into account. Potential toxicity and carcinogenicity of bromate has been of concern in recirculating seawater aquaria, where bromate is produced from oxidation of bromide during ozonation. By late 1990, bromate concentration in The Living Seas reached 4.6 μM. After the installation of the denitrification system, bromate concentration has been controlled by the balance between ozonation and denitrification processes. We developed a model based on the existing kinetic data regarding bromate formation during ozonation and an empirically determined rate of bromate loss during denitrification. The model predicts a bromate concentration of 3.6 μM by early 1994, in reasonable agreement with the observed value of 3.4 μM. The eventual steady state bromate concentration predicted by the model is 0.7 μM. Verification of the model with the observed bromate and nitrate data from The Living Seas shows that it is a useful tool for predicting concentrations of redox-reactive chemical species in a closed seawater system. In addition, empirical data from this study suggest that denitrification can be used to control bromate concentrations in seawater aquaria that employ ozone for disinfection.