408 Organic waste reuse and recycling: technology and management
8.4.1 Irrigation or SR system
Wastewater application rate
Typical wastewater application rates for irrigation or slow rate systems range
from about 1.5 to 10 cm/week (Figure 8.1 and Table 8.2). The choice depends
on the climate, soil permeability, crop type and management practices, and
required quality of the treated water. Information on climate and soil
permeability can be obtained from meteorological data and site investigation,
respectively; this information is used in the water balance calculation as shown
in the next section on "Hydraulic loading rate". The required quality of the
treated water is usually evaluated by using the nitrogen balance. When crop
production is the priority, the irrigation requirements of the crop may limit the
application rate.
Example 8.1
For a clay-loam soil, which is irrigated periodically, the moisture content before
irrigation is 19% by weight. If the effective infiltrated irrigation water is 1,000
m^3 per hectare, estimate the soil moisture content after irrigation. Assume that
the soil's bulk density is 1.35, and the depth to which the soil is wetted is 90 cm.
What should be the period of irrigation if the evapotranspiration rate is 250
mm/month?
Infiltrated water, I = 1,000 m^3 /10,000 m^2 = 100 mm
The effect of irrigation on the soil moisture content can be estimated from the
following equation.
I = (P" - P') S.D/100 (8.2)Where,
I = effective infiltrated irrigated water, mm
P" = final moisture content, % by wt.
P' = initial moisture content, % by wt.
S = soil's bulk density
D = depth of soil to the root zone to be wetted, mmHence,
100 mm = (P" - 19%) 1.35 x 900/100
Thus, P" = 27.3%
Therefore, soil moisture content after irrigation = 27.3%