394 Organic waste reuse and recycling: technology and management
Reliability
As shown in Table 8.3, a high-quality effluent can be expected from SR process.
For a well-operated SR system, treatment efficiency is in the order of 99% for
BOD 5 , suspended solids, and fecal coliforms. As irrigation soils are loamy (see
Figure 8.1), considerable amounts of organic matter, heavy metals, phosphorus,
and microorganisms are retained in the soil by adsorption and other
mechanisms. It should be noted that plant uptake and/or adsorption of heavy
metals and pathogens are a potential problem in the reuse of these irrigated
plants. Nitrogen is up taken by plant growth and if the crop is harvested, the
removal rate can be in the order of 90%.
Site selection
Soils ranging from clay loams to sandy loams are suitable for irrigation (Figure
8.1). Soil depth should be at least 0.3 m of homogeneous material and
preferably 1.5 to 2 m throughout the site. The depth is needed for extensive root
development of some plants and for wastewater renovation. The minimum depth
to groundwater should be 0.6-1.0 m to avoid groundwater contamination (Table
8.4). If the site drainage is poor, control procedures such as under drains or
wells may be required. For crop irrigation, slopes should be limited to 20% or
less depending upon the type of farm equipment to be used. Forested hillsides
and non-cultivated land up to 40% in slope have been spray irrigated
successfully (Table 8.4).
8.2.2 Rapid infiltration process (RI)
RI is the controlled application of wastewater to earthen basins in rapidly
permeable soils (e.g. sandy loam, loamy sand and sand) at a high rate (Figure
8.1). Treatment is accomplished by biological, chemical and physical
interactions in the soil matrix with the near surface layer being the most active
zone. The design flow path requires infiltration, and typically lateral flow away
from the application site. A cyclic application is the typical mode of operation
with the flooding period followed by the drying period. This allows aerobic
restoration of the infiltration surface and drainage of the applied percolate. The
geo-hydrological aspects of the RI site are more critical than for other processes.
Proper subsurface conditions and the local groundwater system are essential for
design. Schematic views of the typical hydraulic pathways for RI process are
shown in Figure 8.3. A much greater portion of the applied wastewater
percolates to the groundwater than with slow rate land treatment. The percolated
water is collected for reuse by under drains or recovery wells (Figure 8.3 b & c).