| This paper reviews the nutrient recycling potential of constructed wetlands and aquaponic greenhouses for filtering land-based aquacultural wastewater. Compared with conventional wastewater treatment technologies, constructed wetlands and greenhouse aquaponics may provide suitable, effective, cost-efficient, low-maintenance wastewater treatment facilities with the potential for polyculture. These technologies have now been proven in warm environments, and there is evidence that with some modification they have significant potential for temperate climates. Removal rates for BOD5, total nitrogen (TAN) and total phosphorus (P) averaged as high as 99, 95 and 84 percent respectively for Behrends et al., (2003) in a two cell wetland. Comparatively, Knight et al. (1994) determined the average contaminant removal efficiency, based on the North American Wetland Treatment System Database to be 68, 51, and 31 percent. Aquaponic greenhouses have proven excellent waste removal systems while polyculturing edible crops. Heinen et al. (1996) 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. Sweet basil and lettuce removed phosphorous at a rate greater than 60 mg P/m 2 · d while nitrate removal was 980 mg N/m 2 · d (Alder, 1998).; Although these systems can clearly work effectively on relatively small scales, the utility of a large scale, land-based operation is an area of ongoing research as some challenges remain to be overcome. Based on this review, it is apparent that: (1) physical and chemical parameters for optimum water purification must be developed; (2) the potential of these systems in cold climates must be further studied; (3) the most favourable combinations of fish and plants must be understood; (4) the potential to use these systems in year-round, large-scale, land-based operations must be examined; and (5) "other functions such as biodiversity, habitat, climatic, hydrological and public use functions...need to be developed, evaluated and weighed in relation to the water quality issues" (Bri, 1999). |
