Purification performance of a shallow free-water-surface constructed wetland receiving secondary effluent for about 5 years

The effluent from combined household wastewater treatment facilities used in unsewered areas of Japanis generally high in nitrogen (N) and phosphorus (P). In Japan, environmental quality standards for zinc( Zn) pollution were enacted recently because of the toxicity of Zn to aquatic ecosystems. In 2004 a fallowpaddy field at the Koibuchi College of Agriculture and Nutrition in central Japan was converted into a shallow, 500-m2, free-water-surface constructed wetland to polish the secondary effluent from a dormitory. The wetland was planted with Zizania latifolia. The efficiencies of removal of N, P, and Zn by the wetlandwere evaluated for about 5 years. The average total N and total P concentrations in the wetland influent were 20.0 mg/L and 1.95 mg/L, respectively; the corresponding concentrations in the effluent were10.3 mg/L and 0.95 mg/L, respectively. The average acid-soluble Zn concentration decreased from 0.048in the influent to 0.023 mg/L after passing through the wetland. Inorganic N, PO4-P, and dissolved Zn werethe principal forms of N, P, and Zn removed by the constructed wetland system. Very little particulateZn, and very little particulate plus organic N or P was removed. The mean first-order rate constants (k) for removal of inorganic-N, PO4-P, and acid-soluble Zn were 10.3 m/yr, 18.8 m/yr, and 16.5 m/yr, respectively. The wetland effectively removed not only nutrient salts but also Zn from the secondary effluentthroughout the 5-year study period. The k values for inorganic-N and PO4-P were significantly influencedby temperature, although the k value for acid-soluble Zn was not. These results and mass balance datasuggest that N was removed mainly by denitrification and secondarily by adsorption to soil and by plantuptake; P was removed mainly by adsorption to soil and secondarily by plant uptake; and Zn was removedmainly by adsorption to soil. There was little sedimentation of particulates in the shallow constructedwetland. (C) 2014 Elsevier B. V. All rights reserved.