1 History of Technical Developments of Soil Percolation System for Wastewater and River Water Treatment in Japan
Y. Fujikawa,

Research Reactor Institute,
Kyoto University,

1. Introduction
Soil percolation system in this text is a form of land treatment similar to soil aquifer treatment, i.e., a soil-based natural treatment system for renovation of water. The concept of the treatment (infiltration of water to soil layer and removal of the organic matter, nutrients and suspended solid) is relatively simple but the physical, chemical and biological processes involved in the removal of pollutants are very complex. Full understanding of the purification mechanisms is necessary to design and operate the treatment plants for optimum performance.
Soil-based water treatment technologies such as slow rate (SR), subsurface infiltration, rapid infiltration (RI), and overland flow (OF) systems have been applied extensively in the North and the South America (USEPA, 1982). In SR systems (Fig. 1), water is treated by filtration, adsorption, ion exchange, precipitation, microbial action, and plant uptake as it passes through the soil. In OF systems (Fig. 2), treatment is achieved primarily through sedimentation, filtration, and biochemical activity as the wastewater flows across the vegetated surface of the terraced slope. Water infiltration rate is typically 0.1- 2 cm/day for OF systems, 1- 5 cm/day for SR systems, and 2 ? 34 cm/day for RI systems (Reed et al., 1995). Vegetation is an important component of OF and SR systems, but not for RI systems.

Fig1 Slow rate System

Fig2 Overland flow  system

2. Soil - percolation systems used in Japan
2-1. Trench system

Trench system in Japan is a modified version of septic-tank and soil-absorption system, an on-site wastewater treatment system described in USEPA report (1980). Trenches consist of a layer of clean gravel with a perforated percolation line installed in each of them to discharge wastewater in the soil (Fig. 3). The system was placed on a layer of filter medium, and sometimes impermeable lining was installed below the trench to use the capillary phenomena for gradual discharge of wastewater into soil. The system is used with appropriate resting period, but in order to prevent the clogging, aeration system was installed to enhance the biological degradation of organic solid trapped in soil. Soil mixed with polyaluminum chloride, charcoal, humus, sand, etc. was often used.
The trench system was used to treat household wastewater, and for advanced treatment of secondary effluent of municipal wastewater. It was also used after septic tanks to reuse it to flush toilets in a public park.

Fig3 Trench system

2-2 Multi-media-layering system
The multi-media-layering system is designed as an on-site wastewater treatment system that can remove nitrogen, phosphorus, BOD and COD at the hydraulic loading rate of 0.1 to 0.85 m/day. Soil was mixed with organic materials and iron particles, processed into block, and arranged in zeolite layer with high hydraulic conductivity (Fig. 4). Intermittent aeration is necessary to facilitate the removal of BOD5, COD and soluble reactive phosphorus while excessive aeration caused the decline in the removal of nitrogen. The problem with this system is the relatively high cost of complex construction. Optimization of soil block processing process is also needed because if the hydraulic conductivity of the soil layer was too low compared to the zeolite layer, most of the wastewater may bypass the soil layer. 

Fig4 Multi-media-layering system

2-3 RI system using granulated soil
Traditional RI systems inject water through sandy aquifer via infiltration basin at the flow rate of 2 - 34 cm/day and the treated water is directly discharged into groundwater or surface water body. In Japan, Akadama soil from Tochigi Prefecture that naturally forms relatively hard granules is often used in place of natural aquifer to achieve the very high hydraulic loading rate of a few meters per day. As shown in Table 2, performance of such system can be good because compared to sand used in conventional RI systems, soil granules can be more effective in treatment through sorption, latitudinal diffusion and hosting of biological communities within their internal pores. Problems associated with usage of soil granule are the availability, durability and cost of the materials.

Fig5 RI system using granulanted soil

2-4 Other systems
SR system using sludge from water treatment plant was used for removal of phosphorus from Onga River, Fukuoka, Japan. As sludge from WTP often have high sorption ability of phosphate ion, its usage was considered to be effective. The system, however, suffered from clogging by fine particles discharged from the sludge itself and resultant significant decline in hydraulic loading rate.
A small-scale overland flow system to treat blackwater from individual household was developed. The system uses 48 L volume of Kanuma soil in stacked sloped layers and treats the water in aerobic-anaerobic environment by sedimentation, filtration, sorption and biological reactions.

3. Conclusions
The soil-percolation systems applied for treatment of water in Japan are relatively small in number and scale of the plants because of the limitation in space available for treatment. Research and development, however, has been conducted to develop some new systems, i.e., systems with increased hydraulic loading with unique structure to treat not only wastewater but river water, sometimes using materials other than natural soil. Further research and experience in operation of the plants are expected to lead to a more efficient design guide and broader application of the natural treatment technology in Japan.

Water Remediation Using the Soil Percolation System
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2 Trench system using composite soil for advanced treatment of municipal wastewater