Land treatment of wastewater 415Wastewater application rates for OF systems are usually determined from
comparisons with existing systems and research projects (see also Figure 8.1
and Table 8.2). For raw wastewater, 7.5 to 10 cm/week should be considered.
For primary effluent, 10 to 20 cm/week could be used, depending on the level of
overall treatment required. For secondary effluent, either from waste
stabilization ponds or conventional secondary treatment facilities (e.g. activated
sludge or trickling filters), the need is either for polishing (further reductions for
BOD 5 and suspended solids) or for nutrient removal. For polishing, 20 to 40
cm/week could be considered. For nutrient removal, only 10 to 20 cm/week may
be possible because adequate detention time is necessary for denitrification.
Application schedule
Experience with existing systems suggests application schedules to be 6 to 8
hours on and 16 to 18 hours off over 5 to 6 days/week. At Melbourne, Australia,
however, the application has been continuous rather than intermittent. The
optimum cycle will depend on the climate and the BOD 5 loading. Other
considerations include the harvesting of grass and the potential for propagation
of insects.
8.4.4 Other design considerations
There are many factors that determine the area required for a wastewater land
treatment system. These factors are related to the characteristics of the soil,
wastewater, climate, crop, pre-application treatment, and should be evaluated
using site-specific information.
In determining the required land area, the land area for each potentially
limiting parameter (such as hydraulic, nitrogen, toxic compounds and salt
loadings) should be evaluated. The parameter that requires the largest land area
to avoid environmental problems becomes the limiting parameter. When the
land area determined for the limiting parameter is used for the design of a land
treatment system, there is an additional degree of safety in terms of the
application rates of the other constituents or parameters of concern.
It should be possible to determine application rates for land treatment
systems on a case-by-case basis using data on soil infiltration rates, climate,
wastewater characteristics, and required treatment performance. Unfortunately,
neither the relationship between the infiltration rate and loading rate nor the
relationship between the loading rate and treatment performance have been
developed to the point where they can be used in design without requiring
comparisons with other land treatment systems.