A SURVEY OF CONSTRUCTED WETLANDS IN JAPAN

Constructed wetland (CW) generally requires large area which is limited in Japan; therefore, CW has not been well received in Japan. Recently, CW starts to be paid attention because it works not only for wastewater treatment but also for ecological enhancement. In Japan, CW is usually used to treat polluted rivers or lakes, and CW influent is characterized by less polluted water, high hydraulic loading rate (HLR), and high suspended solid (SS). In this study, a survey of CWs was carried out by reviewing available documents, inquiring by mail, and visiting CWs. Nine operating CWs and 4 experimental CWs are selected. These CWs are studied comparing 24 different parameters and operating conditions (e.g., nitrogen, phosphorus, HLR, size, and location). All these facilities are free water surface flow wetlands (FWS). Hydraulic retention time (HRT) of most Japanese facilities is less than 10 hours, which is considerably shorter than those found in other countries. It is found that the nutrient removal efficiencies for total phosphorus (T-P) and total nitrogen (T-N) increase proportionally as HRT increases. The removal efficiencies in the facilities, where HRT is over 10 hours, are approximately 40 % for T-N and 60 % for T-P. When the nutrient loading is low, its removal rate increases linearly with its corresponding loading rate. When the loading rate is high, the removal rate is nearly constant. The removal rate for T-N is 0.4 gm-2d-1 when the loading rate exceeds 1 gm-2d-1, and the removal rate for T-P is 0.08 gm-2d-1 when the loading rate is larger than 0.15 gm-2d-1