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

FIGURE 7-35
CROSS SECTION OF TYPICAL ET BED
Pipe (4”)
Topsoil (Varies O-4")
Although ET systems may be used where the application of subsurface dis-
posal systems is limited, they are not without limitations. As with
other disposal methods that require area-intensive construction, the use
of ET systems can be constrained by limited land availability and site
topography. Based on experience to date with ET disposal for year-gound
single-family homes, approximately 4,000 to 6,000 ft (370 to 560 m 1 of
available land is typically required. The maximum slope at which an ET
system is applicable has not been established, but use on slopes greater
than 15% may be possible if terracing, serial distribution, and other
appropriate design features are incorporated.
By far the most significant constraint on the use of ET systems is cli-
matic conditions. The evaporation rate is controlled primarily by cli-
matic factors such as precipitation, wind speed, humidity, solar radia-
tion, and temperature. Recent studies indicate that essentially all of
the precipitation that falls on an ET bed infiltrates into the bed and
becomes part of the hydraulic load that requires evaporation (33)(34)
(37). Provisions for long-term storage of effluent and precipitation in
ET systems during periods of negative net evaporation, and for subse-
quent evaporation during periods of positive net evaporation, are expen-
sive. Thus, the year-around use of nondischarging ET systems appears to
be feasible only in the arid and semiarid portions of the western and
southwestern United States where evaporation exceeds precipitation dur-
ing every month of operation, so that long-term storage capacity is not
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