412 Organic waste reuse and recycling: technology and management
Application schedule
The application schedule for irrigation depends on the soil permeability, type of
crop, application technique, and the climate. Operator convenience should also
be considered. For permeable soils, the schedule should be to irrigate once a
week or more frequently. For less permeable soils using surface irrigation, the
application schedule should include a longer period of drying.
8.4.2 Rapid infiltration or RI system
Wastewater application rate
In rapid infiltration, evaporation and vegetation are relatively unimportant, but
the soil permeability is critical. It is therefore important to concentrate the
planning and design on determining the optimal infiltration rate to ensure that
the system will work hydraulically while providing the necessary wastewater
treatment. As given in Figure 8.1 and Table 8.2, typical application rates for RI
systems range from 10 to 250 cm/wk.
Hydraulic loading rate
Although the soil permeability can be related to the infiltration rate, as
previously described, it is recommended that the soil profile be evaluated and
that field measurements of infiltration rates be conducted. The preferred method
of determining the infiltration rate depends on the nature of the soil profile. If
the profile is generally homogeneous, a surface flooding basin 2 m or more in
diameter can be used. The basin is filled with clean water until the soil is
saturated, and then the rate of infiltration is measured. Clean water is generally
used unless the actual wastewater is available.
Treatment performance
In rapid infiltration systems, the required treatment performance is of primary
importance in determining the application rate. Lance and Gerba (1977) showed
that decreasing the application rate from the hydraulic limit could result in
increased removals of constituents, especially nitrogen. Because the chief
mechanism of nitrogen removal in rapid infiltration systems is denitrification,
the following requirements of biological denitrification must be met: adequate
detention time, anoxic conditions (or at least anaerobic micro-sites), and
adequate carbon to drive the reaction. The reduction in application rate increases
detention time and increases the potential for denitrification. The effect of