Aquaculture wastewater treatment and reuse by wind-driven reverse osmosis membrane technology: a pilot study on Coconut Island, Hawaii

Nitrogen in aquaculture wastewater may cause many environmental problems to the receiving water. To protect its pristine coastal water, the State of Hawaii established stringent water quality limits for aquaculture wastewater. Effluents from aquaculture facilities in Hawaii generally exceed these limits-sometimes by one to two orders of magnitude. Development of cost-effective treatment technology would be one of the most important factors for a profitable aquaculture industry in Hawaii. Furthermore, recirculating of aquaculture wastewater is highly desirable for environmental protection and resource conservation. To achieve these goals, a wind-driven reverse osmosis (RO) technology was developed and applied for the removal of nitrogenous wastes from the culture water of tilapia on Coconut Island, the home of the Hawaii Institute of Marine Biology, University of Hawaii at Manoa. A conventional multi-blade windmill is used to convert wind energy directly to hydraulic pressure for RO membrane operation. Aquaculture wastewater passing through the RO membrane is separated into permeate (freshwater) and brine (concentrated wastewater). The permeate is recirculated to the fish tanks, while the brine is collected for possible treatment or reuse. As a result, no wastewater discharge is made to the ambient coastal water. Testing results indicated that the prototype wind-driven RO system can process and recycle freshwater at a flux of 228-366 L/h, depending on wind speed. The nitrogen removal rate ranges from 90% to 97%, and the recovery rate of the RO membrane is about 40-56%. A preliminary cost analysis shows that the production of 1.0 m3 permeate from aquaculture wastewater would cost US$ 4.00. Further study will focus on the reuse of concentrates and on further enhancement of cost-effectiveness.