Paul Adler examines young lettuce seedlings which are grown for about 3 weeks in a separate hydroponic system before they are set in the “conveyor production system” to remove nutrients from the rainbow trout effluent. (Photo by Keith Weller, USDA-ARS).
Adler, P.R. 1998. Phytoremediation of aquaculture effluents. Aquaponics J. 4(4):10-15. From the abstract:
The study is on an integrated system for rainbow trout production, effluent treatment and production of lettuce. The objective was to reuse water by removal of the nutrients in a vegetable product. The microscreen filter removes about 80% of the P excreted by the fish with the biosolids, leaving about 20% of the P in the effluent. A mass balance of system nutrients was conducted and it was determined that it takes 7.5 – 10 heads of lettuce to remove the P excreted in the effluent by the production of 1 pound of trout or 13 – 18 lettuce heads for each kg of feed consumed. Greenhouse studies demonstrated that by using the conveyor production strategy (CPS), phosphorus could be removed to <0.01 mg/L by lettuce without an apparent reduction in production or quality.
Conventional thinking regarding the use of food crops to clean aquaculture effluents has been that plants cannot remove nutrients in water to low levels without a reduction in productivity and quality. If water is distributed in a horizontal plug-flow pattern, all nutrients will be luxury consumed at the inlet, making nutrients limiting at the outlet and significant greenhouse space will be dedicated to growing plants that have no market value.
Because greenhouse space is expensive, productivity is critical for a profitable operation. A unique production system for lettuce, called the conveyor production strategy (CPS), was developed using thin-film technology for plant production in dilute aquaculture effluents. With the CPS, young plants are positioned near the solution inlet and are moved progressively, like along a conveyor belt, towards the outlet as they grow. Luxury consumption by lettuce (Lactuca sativa L.cv. Ostinata) enabled them to store P in their tissues early in their growth cycle for use later as water P levels decreased and influx could no longer meet current demands.