Global aquaculture production has increased greatly over the last 40 years and it represents the fastest growing food production sector. However, further expansion of this practice is greatly limited by the current approach to waste management. Waste management poses a significant problem for aquaculture since it internalizes all the costs. The removal of suspended and settleable solids from a RAS waste stream is an intensive and costly process in terms of capital investment, maintenance, operation, and storage and discharge of removed solids as per EPA standards. Discharged RAS waste is considerably more concentrated than effluent streams from other production methods due to low water usage and can serve as a major point source pollutant. A capture-and-reuse model must be developed to mitigate environmental impact and offset costs of waste treatment and disposal. Currently, the most suitable capture-and-reuse method for RAS couples with hydroponic crop production.
Integrating RAS with hydroponic cropping systems requires a systems approach, considering the needs of each component prior to integration. One important consideration is nutrient balance. RAS effluent sodium concentrations often far exceed the phytotoxic threshold of 50 mg/L requiring remediation prior to use as hydroponic fertilizer. To resolve this issue, many practitioners have replaced sodium salts used for pH balancing in RAS with potassium derivatives. Potassium is an essential nutrient for plants that is available at insufficient levels in RAS effluent. However, complete replacement of sodium with potassium salts is considerably more expensive and potentially poses health concerns with fish production, both lethal and sublethal. Sublethal effects include stunted growth and gill irritation. Practitioners have also noted rapid mortalities as a result of high potassium levels in a number of species including rainbow trout (Oncorrhynchus mykiss) and hybrid striped bass (Morone chrysops x M. saxatilis). It has also been suggested that sodium can mitigate some of these toxic effects when maintained at an electrochemical balance. Unfortunately, the toxic and sublethal thresholds for potassium have not been established in the literature for any commercially important fish species to date, nor have the mitigating effects of sodium.
Research is being conducted currently at a pilot-scale intensive RAS research facility at the University of New Hampshire in Durham, NH in the Anadromous Fish and Invertebrate Research (AFAIR) lab to assess the acute and chronic effects of potassium on rainbow trout (Oncorrhynchus mykiss) and the ability of sodium to offset acute toxic effects of potassium. Findings to date will be presented.