In a typical recirculating aquaculture system, the oxidation of ammonia in the biofilters will cause nitrate to build up to the point of being detrimental to animal health. The general solution to this problem is a water change, but regulations in locations affected by drought now limit the amount of water available. In addition, environmental regulations limit nitrate discharge concentrations under many conditions. Nitrate discharges help control nitrate concentrations in the water, but also remove any salts that have been added to the recirculating system water and any alkalinity added to neutralize the nitrate.
Enter the denitrification system. Denitrification is the process of reducing nitrates by converting them into nitrogen gas. Denitrification systems have been in use for several decades, primarily in wastewater treatment, but have not been traditionally used in aquaculture and aquatic filtration systems.
The denitrification system currently being developed by Aquaneering uses a specially-designed fluidized bed bioreactor. Like our fluidized bed biofilters, the bioreactor contains media on which bacteria grow. The bioreactor is designed to prevent oxygen from entering (anaerobic), forcing the bacteria to reduce the nitrate to nitrite and then to nitrogen gas. A closed packed column connected to the bioreactor removes the nitrogen produced, without adding oxygen to the water.
As some potentially toxic compounds and residual ammonia are often formed as byproduct of this dentrification process, the water will then be treated by a high-performance aerobic fluidized bed biofilter and “protein skimmer” to control colloidal solids and surfactants.
The overall system is tied into a SCADA system for alarming and remote notifications. This particular design is skid mounted for easy installation.
The system will be used to process and recycle the high nitrate wastewater for a striped bass hatchery in Ensenada, Mexico. The system will support brood stock and fingerlings for production, which will be released to pens in the Pacific at a certain age and size for grow-out. The unique design is a collaboration between Dallas Weaver, Ph.D., of Scientific Hatcheries, and Mark Francis of Aquaneering. The system will process 60 to 120 tons of water per day, providing nitrate-free return water and eliminating an equal volume of total water demand.
The Ensenada hatchery will have recirculating water and zero discharge in an environment with a very limited water supply. The end result: protecting precious resources and supporting fish hatcheries through technological innovation.