Biological denitrification in a closed seawater system

Build-up of high nitrate concentrations in closed seawater systems where primary productivity is undesirable and water changes are impractical presents unique problems. Nitrate concentration in Ocean Tank at the New Jersey State Aquarium reached 9500 μM after 6 years of operation. A biological denitrification system was installed in 1998 and nitrate concentration in the aquarium decreased to 7000 μM within the first 100 days of operation. The system offers additional benefits by increasing the pH and alkalinity of seawater and providing a reducing environment to balance the oxidizing disinfection environment in the aquarium. The initial performance of the denitrification system was monitored and two semi-empirical models were developed: one based on the actual methanol additions, and another based on the daily amounts of nitrogen gas removed. The first model predicts a net nitrate decrease of 39 μM/day in the aquarium. The second model predicts a net decrease of 25 μM/day, in good agreement with the empirical value of 23 μM/day. This indicates that nitrogen gas removal is the controlling factor during denitrification in this facility, and the second model can be used to predict and optimize the operation of the system.