Tropical treatment wetlands dominated by free-floating macrophytes for water quality improvement in Costa Rica

Five tropical treatment wetlands dominated by floating aquatic plants and constructed to deal with a variety of wastewaters were compared for their effectiveness in treating organic matter and nutrients in the Parismina River Basin in eastern Costa Rica. Wastewaters were from a dairy farm, a dairy processing plant, a banana paper plant, and a landfill. Four of the five wetland systems were effective in reducing nutrient levels of effluents before water was discharged into rivers. Ammonia levels in water entering most wetlands were considerably higher than ambient (i.e., riverine) levels; concentrations were reduced by as much as 92% in the wetlands and retained at a maximum rate of 166 g N m−2 year−1. Nitrate nitrogen removal was variable, but occurred in low concentrations in the inflows (less than 1 mg N L−1). Phosphate phosphorus was present in high levels but was effectively reduced through the wetlands (92 and 45% reductions through dairy farm wetlands, 83% reduction through banana paper wetlands, and 80% reduction through dairy processing wetlands). Retention of phosphate phosphorus ranged from 0.1 to 10.7 g P m−2 year−1 in the treatment wetlands. Dissolved oxygen in the wetland outflows were ≤2 mg L−1 in three of the sampled wetlands, most likely a result of the abundant free-floating macrophytes that sheltered the water from diffusion and shaded aquatic productivity. The efficacy of these created wetlands to treat effluents from different sources varied, and modified wetland designs or active management may be necessary to improve water quality even further. Recommendations on tropical wetland design and management are presented, as are suggestions for implementing this ecological engineering approach with farmers in Central America.