Demonstration of Water Purification of Agricultural Drainage using Constructed Wetlands Pre-Treated with a Sedimentation Basin

Abstract

 To maintain the water quality of Lake Inawashiro, which has become neutral pH and is suspected to be water pollution, a demonstration test facility for water purification was constructed near Akai River flowing into Lake Inawashiro by the Fukushima Prefectural Centre for Environmental Creation. The facility is composed of a pretreatment tank （“Pretreatment”）, two surface flow constructed wetlands （“1st CW”）, and two vertical flow constructed wetlands （“2nd CW”）. The purpose of the demonstration test is to reduce the pollutants introduced to Lake Inawashiro from paddy fields. This study reports the design of the demonstration test facility and the removal effects during an irrigation period. The suspended solids （SS） and particulate phosphorus （PP） concentration in raw water （Akai River） dramatically increased because of soil puddling. During periods when the paddy field was not under cultivation and flooding （i.e. the ordinary period）, the following results were obtained. When removal efficiency （%） was selected as the performance indicator, it was productive for the best water purification to put into full-scale operation after an investigation of the relationship between a hydraulic load and a removal efficiency. When removal rate （g/m²/d） was selected as the performance indicator, it was productive for the best water purification to operate the facility at a higher loading rate. During soil puddling, the facility exhibited an effective inflow removal effect because the removal rate and loading rate of SS, total organic carbon （TOC）, total phosphorus （TP）, PP, total nitrogen （TN）, and particulate nitrogen （PN） were positively correlated （p ＜ 0.01） and the inclination of regression lines in all water quality was larger than the inclination during the ordinary period. Turbidity in the 1st CW treated water varied with changes in the raw water, while turbidity in 2nd CW treated water was low and only changed slightly. Therefore, the vertical flow constructed wetland was necessary to effectively remove high concentrations of particulate pollutants during soil puddling.