Removal of Nitrogenous Aquaculture Wastes by a Wind Powered Reverse Osmosis System

Removal of Nitrogenous Aquaculture Wastes by a Wind Powered Reverse Osmosis System

U.S. Geological Survey

Year 1: 03/01/01 - 02/28/02
Year 2: 03/02/02 - 02/28/03

Tilapia fish are a major source of protein around the world. In the United States, tilapia farming is the fastest growing segment of aquaculture, and over the last year, domestic tilapia production reached 15-20 million pounds. Rapid accumulation of nitrogenous wastes is one of the major operational problems of aquaculture operation of fresh water tilapia. As a result, water in a fishpond of tilapia culture mush be replaced regularly to maintain its water quality. In the mean time, wastewater from the fishpond must be treated to meet the stringent effluent water quality standards. The objective of this project was to develop and test a water circulation system for tilapia aquaculture. This system used the wind-powered reverse osmosis process to remove nitrogenous wastes produced in a fish tank and reuse the treated wastewater as a fresh water supply.

It has lead to a cost-effective and environmental-friendly aquaculture operation. In the first year (March 2001 to February 2002) research of this project, a tilapia fish tank with flow control and water quality sampling devices was installed, and the water quality sampling devices was installed, and the water quality variation in tilapia culture, especially the accumulation of nitrogenous wastes was characterized. Modification of the existing wind-powered reverse osmosis system was conducted and was completed by the end of February 2002. In the year 2 research (March 2002 to February 2003), the following research tasks were conducted: (1) constructing and test a closed aquaculture system of tilapia production with water circulation and reuse, (2) conducting an economic feasibility analysis of this aquaculture system, and (3) developing a simulation and optimization model of this wind-powered reverse osmosis nitrogen removal system.

Goal and Design of Study:

In this two-year project, we studied the characteristics of the tilapia culture and investigated the feasibility of a closed system tilapia culture, which uses a wind-powered reverse osmosis system to treat and to re-circulate wastewater. Following results and benefits were derived by this project:

  1. This research project has provided a better understanding of nitrogen in tilapia culture effluent. By monitoring the variation of nitrogen series of tilapia culture, including ammonia, nitrite, nitrate and organic nitrogen, the data of the accumulation of nitrogen at different stage of tilapia growth was obtained.
  2. This project studied the nitrogen removal by using reverse osmosis (RO) membrane. RO membrane has long been thought of as an effective methods for water desalination. Recently, researchers have had great interests on its application in nitrogen removal. The project gave us a chance to find a relatively new pollutant control method in aquaculture waste.
  3. Another major research task of this project was the use of wind energy. Since RO membrane is an energy-intensive process, its application is confined to a limited area. While wind energy is called a renewable or "alternative" energy source, it is inexhaustible and free, and most of all creates no pollution. We used a pressure stabilizer to connect existing windmill and a RO module. In this way, the treatment system was cost-effective and environmental-friendly.
  4. A water re-circulating system that allows the effluent reuses, and thus maximizes the water reuse and minimize the waste effluent.

This project, with effluent reuse and the use of natural energy, has made the Hawaii Institute of Marine Biology on the Coconut Island a showcase of sustainable development.

Specific Objectives:

Year 1

The first year research investigation was to study the characteristic of aquaculture waste, especially the concentration of nitrogen at different stage of fish (tilapia) growth.

  • Task 1(a)
    An experimental system, including a fish tank of tilapia culture and monitoring/sampling devices were constructed in the Hawaii Institute of Marine Biology research facilities on the Coconut Island.
  • Task 1(b)
    Water samples were collected from the fish tank of tilapia culture. Samples were then transported to and analyzed in the water quality laboratory of the Water Resources Research Center in the University of Hawaii at Manoa.
  • Task 1(c)
    Water with different nitrogen concentrations were prepared and sued as the fee water into the reverse osmosis module. Performance of the reverse osmosis module to remove nitrogen was evaluated by studying nitrogen concentrations in the feed water and product water (permeate), and the operating flow rate and feed water pressure.

Year 2

The second year research was conducted to investigate the performance of nitrogen removal by the reverse osmosis process and to develop a water re-circulating system for tilapia production.

  • Task 2(a)
    A closed system of aquaculture production, which was formed by integrating together the tilapia fish tank and the wind-powered reverse osmosis (RO) nitrogen removal system was constructed and tested.
  • Task 2(b)
    An economic feasibility analysis of this closed system of tilapia production was performed by comparing its cost with alternative method, which requires the replacement of the tank water periodically and the treatment of its effluent before discharging into the receiving water.
  • Task 2(c)
    Mathematical modeling and optimization of the wind-powered osmosis nitrogen removal system.

Research Objectives:

  • Investigating the nitrogen build-up in fresh water aquaculture of tilapia.
  • Developing a wind-powered reverse osmosis nitrogen removal system.
  • Evaluating the economic feasibility of the wind-powered reverse osmosis system for removing nitrogen from aquaculture wastes.