Biological denitrification in a closed seawater system

Area of Science:

• Marine Biology
• Environmental Science
• Aquaculture Engineering

Background:

• High nitrate concentrations pose challenges in closed marine systems, particularly in aquariums where water changes are limited.
• The Ocean Tank at the New Jersey State Aquarium experienced nitrate buildup, reaching 9500 microM after six years.

Purpose of the Study:

• To assess the performance of a newly installed biological denitrification system in reducing nitrate levels.
• To develop predictive models for system operation and optimization.
• To evaluate additional benefits of the denitrification system on water chemistry.

Main Methods:

• Installation and monitoring of a biological denitrification system using methanol as an electron donor.
• Development of two semi-empirical models: one based on methanol addition and another on nitrogen gas removal.
• Empirical measurement of nitrate decrease and nitrogen gas removal rates.

Main Results:

• Nitrate concentration decreased from 9500 microM to 7000 microM within 100 days of system operation.
• The denitrification system positively impacted pH and alkalinity, creating a beneficial reducing environment.
• A model based on nitrogen gas removal accurately predicted the observed nitrate decrease (25 microM/day vs. empirical 23 microM/day).

Conclusions:

• Biological denitrification is an effective method for managing high nitrate levels in closed marine aquarium systems.
• Nitrogen gas removal is the critical factor controlling denitrification efficiency in this system.
• The developed model based on nitrogen gas removal can be utilized for predicting and optimizing denitrification system performance.