Land treatment of wastewater 409100 mm
Required irrigation period = = 12 days
250 mm/monthSelect 5 days for irrigation period and 7 days for resting or drying period.
Wastewater application rate is 100 mm/5 days or 20 mm/day during the
irrigation period.
Hydraulic loading rate
In the water balance, the inputs are wastewater application and precipitation and
the outputs are evapotranspiration, percolation and runoff. The precipitation and
evapotranspiration values should be determined for a design year that is wetter
than the normal climatic condition. The percolation rate can be either estimated
from the soil permeability or measured in the field. Recommended field
measurement techniques for soil percolation rates are basin flooding or the
sprinkler infiltrometer, the details of which can be found in U.S. EPA (1981) or
other handbooks on groundwater and hydrology.
Using water balance, the following is obtained:
Lh + Pp = ET + W + R (8.3)Where:
Lh = wastewater hydraulic loading rate, cm/year
Pp = design precipitation, cm/year
ET = evapotranspiration, cm/year
W = percolation, cm/year
R = net runoff, cm/yearFor slow rate systems, the design is such that there is no surface runoff.
Therefore net runoff, R, is usually negligible in the calculation.
Nitrogen loading rate
Because of the ability of nitrate nitrogen to move with the percolating water,
nitrogen is often the limiting water quality parameter in SR and RI systems. In
slow rate systems, the input nitrogen is balanced against crop uptake,
denitrification, and the nitrogen that percolates through the root zone. The
climate can also influence the nitrogen balance. For example, in a humid
climate, the water from precipitation (in excess of evapotranspiration) can dilute
the percolating nitrogen concentration, as shown in Example 8.2 and Table 8.8
for a percolate containing a maximum of 10 mg/L nitrogen. Thus, under a