On-Site Wastewater Treatment and Disposal Systems - Forced ...

7.2.2.2 Application
Site criteria for trench and bed systems are summarized in Table 7-1.
They are based upon factors necessary to maintain reasonable infiltra-
tion rates and adequate treatment performance over many years of con-
tinuous service. Chapter 3 should be consulted for proper site eval-
uation procedures.
The wastewater entering the trench or bed should be nearly free from
settleable solids, greases, and fats. Large quantities of these
wastewater constituents hasten the clogging of the soil (9). The
organic strength of the wastewater has not been well correlated with the
clogging mat resistance except in granular soils (4)(5). Water softener
wastes have not been found to be harmful to the system even when signi-
ficant amounts of clay are present (9)(10). However, the use of water
softeners can add a significant hydraulic load to the absorption system
and should be taken into account. The normal use of other household
chemicals and detergents have also been shown to have no ill effects on
the system (9).
7.2.2.3 Design
a. Sizing the Infiltrative Surface
The design of soil absorption systems begins at the infiltrative surface
where the wastewater enters the soil. With continued application of
wastewater, this surface clogs and the rate of wastewater infiltration
is reduced below the percolative capacity of the surrounding soil.
Therefore, the infiltrative surface must be sized on the basis of the
expected hydraulic conductivity of the clogging mat and the estimated
daily wastewater flow (see Chapter 41.
Direct measurement of the expected wastewater infiltration rate through
a mature clogging mat in a specific soil cannot be done prior to design.
However, experience with operating subsurface soil absorption systems
has shown that design loadings can sometimes be correlated with soil
texture (31(4)(111(12). Recommended rates of application versus soil
textures and percolation rates are presented in Table 7-2. This table
is meant only as a guide. Soil texture and measured percolation rates
will not always be correlated as indicated, due to differences in struc-
ture, clay mineral content, bulk densities, and other factors in various
areas of the country (see Chapter 3).
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