396 Organic waste reuse and recycling: technology and management
The main objective of a rapid infiltration system is wastewater treatment, and
the system design and operating criteria are developed to achieve this goal.
However, there are other objectives with respect to the utilization or final
disposal of the treated water. They are:
- Groundwater recharge
- Recovery of treated water for subsequent reuse or discharge
- Recharge of surface streams
- Seasonal storage of treated water beneath the site with recovery and
agricultural reuse during the growing season.
The wastewater is applied to rapidly permeable soils by spreading in basins
or by sprinkling, and is treated as it travels through the soil matrix. Vegetation is
not usually used, but there are some exceptions.
Advantages of this process are: it is a treatment system with nearly complete
recovery of renovated water; and it is a method of repelling salt-water intrusion
into the aquifers. Where ground water quality is being degraded by salinity
intrusion, groundwater recharge can be used to reverse the hydraulic gradient
and protect the groundwater.
The limitations of this process are in connection with groundwater effects:
influent nitrogen may be converted to the nitrate form, which is leached to the
groundwater; if the zone becomes anaerobic or anoxic, conversion of sulphates
to hydrogen sulfide may be a problem; and phosphorus retention in the soil
matrix may be neither complete nor of long duration.
Reliability
Removal of constituents by the filtering and straining action of the soil is
effective. Suspended solids, fecal coliforms, and BOD 5 are almost completely
removed in most cases. Nitrogen removal is generally about 50%, unless
specific operating procedures are established to maximize denitrification.
Phosphorus removals range from 70 to 95% depending on the physical and
chemical characteristics of the soil that influence retention of phosphorus.
Site selection
Acceptable soil types include sand, sandy loams, and loamy sand (Figure 8.1).
Very coarse sand and gravel is not ideal because it allows wastewater to pass too
rapidly through the upper few cm of soil where biological and chemical actions
take place. Other factors include percolation rate, depth, movement and quality
of groundwater, topography and underlying geological formations.