Treatment of eutrophic pond water by using plant bed filter ditches of garden type throughout the year and effect of burying organic matter within the beds in the midstream

Abstract

Eutrophication associated with ponds for agricultural use has begun to adversely affect rice production and residential living environments in Japan. We evaluated the effectiveness of plant bed filter ditches on the treatment of eutrophic pond water containing particulate N and P throughout the year and the effect of the addition of organic matter to the bed filters in order to improve N treatment efficiency in winter. The experimental ditch was 0.4 m wide, 0.3 m deep, and 11.2 m long, and was filled with zeolite as bed filter material (particle size: 6-8 mm). The ditch was planted with 8 plant species which were effective in removing N and P from artificial wastewater and could be used by rural communities for the production of handicracts and ornamental purposes. The bed filter surface was about 0.1 m higher than the water level for the terrestrial species and the same as the water level for the aquatic species. Pond water was continuously supplied to the ditches at a loading rate of 0.4-1.1 g m-2 d-1 for N and 0.01-0.035 g m-2 d-1 for P. The plant-free ditch was effective in removing particulate P, but the PO4-P was dissolved from particulate P accumulated in the bed filter. By the addition of plants to the ditches, PO4-P was removed successfully, Plant uptake and filtration by the bed filters played an important role in P removal in the ditches containing plants. The plant-free ditch removed particulate N completely and N03-N to some extent. The ditch containing plants removed NO3-N and particulate N quite well in summer. The N balance indicated that N was removed mainly by denitrification and plant absorption. It was suggested that denitrification proceeded by using the suspended solids accumulated in the bed filter as hydrogen donors. The planted ditches purified eutrophic pond water resulting in a T-N concentration below 0.3 mg L-1 and T-P less than 0.02 mg L-1 which are the minimum Nand P concentrations required for water bloom formation. The addition of organic matter to the ditch was effective in improving the N removal efficiency by 0.10.2 g m-2 d-1 in winter and spring.