Removal of nutrients in agricultural runoff by using planted systems.

Abstract

Nutrient pollution refers to the contamination of water from excessive nutrient inputs. Nutrient pollution may lead to the eutrophication of water bodies. Agricultural runoff in Sri Lanka is rich from nutrient due to excessive use of fertilizer. Agricultural runoff ends up in the tanks allocated fofi irrigation. Hence, the agricultural runoff may be a source of pollution of water bodies. Aquatic plants are highly capable of absorbing nutrients. Their ability to absorb nutrients can be enhanced by using more competitive combinations of selected species as well as by controlling their density. It is necessary to evaluate the nutrient removal efficiency of different plant combinations in the design of constructed wetlands and floating wetlands. In our study, we chose an additive factorial design for developing different mixed cultures of different plant combination. Two plant combinations of Limnocharis flava and one species from Monochoria vagina/is, Ipomea aquatica and Pistia stratiotes were used in each individual experiment. The experiments were carried out in outdoor concrete tanks (volume -850 L, diameter - 1.2 m). A commercial fertilizer (Albert's Solution, Summer Field Chemicals (PVT) Ltd, total nitrogen (TN): 50.22 mg/L, total phosphorus (TP): 2. 84 mg/L) was used weekly for adjusting nutrient quantity in each experimental tank. Crushed granite rock was used as a substrate in all experiments. The study revealed that a combination of Limnocharis flava and Ipomea aquatica was capable of removing the highest percentage of TN and TP available in the system. Further, the initial density was also influential on the nutrient removal efficiency and the nutrient removal efficiency was maximum when both plants had a density of 5 plants/ m2 In the optimum conditions, the system was capable of removing 52% of TN and 90% of TP. Further increase and the decrease of the initial planting density caused a reduction in nutrient removal efficiency.