Land treatment of wastewater 425Figure 8.12 TOC fraction remaining versus distance down-slope; application rate study
with primary effluent (from Smith and Schroeder 1985; reproduced by permission of the
Water Pollution Control Federation, USA)
Design application rate (q)
The design q value for an OF should generally match with the infiltration rate of
the soil or vegetated surface of the soil, to prevent excessive run-off and
impairment of effluent quality. The value of q in Equation 8.14 should be less
than the value used to determine z to provide a safety factor in process
performance and to allow operating flexibility by permitting a range of
application rates to be used. A factor of 1.5 is suggested for design. Thus, if a
design value for q of 0.37 m^3 /(hr-m) is used to determine z, then a design value
for q of 0.25 m^3 /(hr-m) should be used in Equation 8.13 to calculate the required
area.
Design application period (Pd)
Design application periods of 8-12 hr/day are recommended. For small systems
it may be more convenient or cost-effective to operate only during one working
Distance down slope (m)TOC fraction remaining (C-7/C)o