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

Data that,quantitatively describe performance are not available for ET
or ETA disposal. However, the technical feasibility of nondischarging
ET disposal has been demonstrated under experimental conditions (33)
(34). In addition, observations of functioning ET systems indicate that
adequate performance can be achieved at least in semiarid and arid
areas. The performance of ETA systems depends primarily on the rela-
tionship between climate and soil characteristics, and has not been
quantified. However, the technical feasibility of such systems is well
accepted.
7.3.2.5 Design
ET and ETA systems must be designed so that they are acceptable in per-
formance and operation. Requirements for acceptability vary. On one
hand, acceptable performance can be defined for an ET system as zero
discharge for a specified duration such as 10 years, based on the wea-
ther data for a similar period. Alternatively, occasional seepage or
surface overflow during periods of heavy rainfall or snowmelt may be al-
lowed. In addition, physical appearance requirements for specific types
of vegetation and/or a firm bed surface for normal yard use (necessita-
ting a maximum gravity water level approximately 10 in. [25 cm] below
the surface) may also be incorporated in the criteria.
Appropriate acceptance criteria vary with location. For example, occa-
sional discharge may be acceptable in low-density rural areas, whereas
completely nondischarging systems are more appropriate in higher density
suburban areas. Thus, acceptance criteria are usually defined by local
health officials to reflect local conditions (33).
Since the size (and thus the cost) of ET and ETA systems are dependent
on the design hydraulic loading rate, any reduction in flow to those
systems is beneficial. Therefore, flow reduction devices and techniques
should be considered an integral part of an ET or ETA system.
The design hydraulic loading rate is the principal design feature affec-
ted by the acceptance criteria. Where a total evaporation system is
required, the loading rate must be low enough to prevent the bed from
filling completely. Some discrepancy in acceptable loading rates has
been reported. Although reports of system designs based on higher load-
ing rates have been presented in the literature (35)(37), other data
obtained under controlled conditions indicate that pan evaporation must
exceed precipitation in all months of a wet year (based on at least 10
years of data) if a total, year-round evaporation system is usfd. Under
these canditions, loading rates between 0.03 and 0.08 gpd/ft (1.2 and
3.3 l/m /day) were found to be appropriate in western states (Colorado
and Arizona) (33)(34).
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