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

7.3.2 Evapotranspiration and Evapotranspiration/Absorption Beds
7.3.2.1 Introduction
Evapotranspiration (ET) beds can be used to dispose of wastewater to the
atmosphere so that no discharge to surface or groundwater is required.
Evapotranspiration/absorption (ETA) is a modification of the ET concept
in which discharges to both the atmosphere and to the groundwater are
incorporated. Both ET and ETA have been utilized for onsite wastewater
disposal to the extent that several thousand of these systems are in use
in the United States (33).
7.3.2.2 Description
Onsite ET disposal normally consists of a sand bed with an impermeable
liner and wastewater distribution piping (see Figure 7-35). The surface
of the sand bed may be planted with vegetation. Wastewater entering the
bed is normally pretreated to remove settleable and floatable solids.
An ET bed functions by raising the wastewater to the upper portion of
the bed by capillary action in the sand, and then evaporating it to the
atmosphere. In addition, vegetation transports water from the root zone
to the leaves, where it is transpired. In ETA systems, the impervious
liner is omitted, and a portion of the wastewater is disposed of by
seepage into the soil.
Various theoretical approaches are used to describe the evaporation
process. This suggests that there may be some uncertainty associated
with a precise quantitative description of the process. However, cur-
rent practice is to limit the uncertainties by basing designs on a
correlation between available pan evaporation data and observed ET
rates, thereby minimizing assumptions and eliminating the need to aver-
age long-term climatic data, References (33)(34)(35) and (36) provide a
more detailed discussion of the correlation method.
7.3.2.3 Application
Onsite systems utilizing ET disposal are primarily used where geological
limitations prevent the use of subsurface disposal, and where discharge
to surface waters is not permitted or feasible. The geological condi-
tions that tend to favor the use of ET systems include very shallow soil
mantle, high groundwater, relatively impermeable soils, or fractured
bedrock. ETA systems are generally used where slowly permeable soils
are encountered.
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