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

equired. ET systems for summer homes may be feasible in the more tem-
perate parts of the country. For ETA systems, the range of applicabil-
ity is less well defined, but the soils must be capable of accepting all
of the influent wastewater if net evaporation is zero for any signifi-
cant periods of the year.
In addition to climate and site conditions, the characteristics of
wastewater discharged to an onsite disposal system may affect its appli-
cation. For ET disposal, pretreatment to remove settleable and float-
able solids is necessary to prevent physical clogging of the wastewater
distribution piping. The relative advantages of aerobic versus septic
tank pretreatment for ET and ETA systems have been discussed in the
literature (33)(35)(37)(38). Although each method has been supported by
some researchers, reports of well-documented, controlled studies indii
cate that septic tank pretreatment is adequate (33)(34)(37).
7.3.2.4 Factors Affecting Performance
The following factors affect the performance of ET and ETA systems:
1. Climate
2. Hydraulic loading
3. Sand capillary rise characteristics
4. Depth of free water surface in the bed
5. Cover soil and vegetation
6. Construction techniques
7. Salt accumulation (ET only)
8. Soil permeability (ETA only)
As noted previously, climate has a significant effect on the application
and performance of ET and ETA systems. Solar radiation, temperature,
humidity, wind speed, and precipitation all influence performance. Since
these parameters fluctuate from day to day, season to season, and year
to year, evaporation rates also vary substantially. To insure adequate
overall performance, these fluctuations must be considered in the
design.
The hydraulic loading rate of an ET bed affects performance. Too high a
loading rate results in discharge from the bed; too low a loading rate
results in a lower gravity (standing) water level in the bed and ineffi-
cient utilization. Several researchers noted decreased evaporation
rates with decreased water levels (33)(34)(35). This problem can be.
overcome by sectional construction in level areas to maximize the water
level in a portion of the bed, and by serial distribution for sloping
sites.
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