Soil Percolation System for River Water Treatment in Japan — History of Technical Developments —

摘要

Soil percolation system derived from wastewater land treatment, which is one of the water treatment technologies using the purifying capacity of nature. A common treatment process involves spraying wastewater over the soil surface to allow it to infiltrate into the soil and subjecting the pollutants in the wastewater to physical filtration, chemical sorption and biochemical decomposition by soil constituent particles, thus removing them through separation and decomposition. Removable pollutants include suspended solids, organic matter and phosphorus, but inorganic suspended solids are unlikely to be the target of purification and can cause clogging. Wastewater targeted for wastewater land treatment is diverse, ranging from effluents from miscellaneous household wastewater, raw sewage and livestock wastewater to environmental water, such as river and lake water, and rainwater. The major targets that wastewater land treatment is expected to remove are organic matter and nutrients. Soil types used in Japan are Masa soil, Andosls and Akadama soil (volcanic ash soil), and activated carbon, wood-charcoal and coagulants may be added to them to improve their ability to separate and remove nutrients such as organic matter and phosphorus. Hydraulic loads (filtration speed) are between several centimeters per day and several meters per day, and wastewater land treatment may be classed as slow filtration if compared with clarifying filtration such as sand filtration.