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

The capillary rise characteristic of the sand used to fill the ET bed is
important since this mechanism is responsible for transporting the water
to the surface of the bed. Thus, the sand needs to have a capillary
rise potential at least as great as the depth of the bed, and yet should
not be so fine that it becomes clogged by solids in the applied
wastewater (33).
Significant seasonal fluctuations in the free water surface are normal,
necessitating the use of vegetation that is tolerant to moisture ex-
tremes. A variety of vegetation, including grasses, alfalfa, broad-leaf
trees, and evergreens, have been reported to increase the average annual
evaporation rate from an ET bed to above that for bare soil (35). How-
ever, grasses and alfalfa also result in nearly identical or reduced
evaporation rates as compared to bare soil in the winter and the spring
when evaporation rates are normally at a minimum (331134). Similarly,
top soil has been reported to reduce evaporation rates. Certain ever-
green shrubs, on the other hand, have been shown to produce slightly
greater evaporation rates than bare soil throughout the year (33).
Thus, there are conflicting views on the benefits of cover soil and
vegetation.
Although ET system performance is generally affected less by construc-
tion techniques than most subsurface disposal methods, some aspects of
ET construction can affect performance. Insuring the integrity of the
impermeable liner and selecting the sand to provide for maximum capil-
lary rise properties are typically the most important considerations.
For ETA systems, the effects of construction techniques are similar to
those discussed previously with reference to subsurface disposal systems
in slowly permeable soils.
Salt accumulation in ET disposal systems occurs as wastewater is evaporated.
Salt accumulation is particularly pronounced at the surface of
the bed during dry periods, although it is redistributed throughout the
bed by rainfall. Experience to date indicates that salt accumulation
does not interfere with the operation of nonvegetated ET systems (39)
(40) l For ET systems with surface vegetation, salt accumulation may
adversely affect performance after a long period of use, although observations
of ET systems that have been in operation for 5 years indicate
no significant problems (33). In order to minimize potential future
problems associated with salt accumulation, the ET or ETA piping system
may be designed to permit flushing of the bed.
Since ETA systems utilize seepage into the soil as well as evaporation
for wastewater disposal, soil permeability is also a factor in the per-
formance of these systems. Discussion of this factor relative to sub-
surface disposal systems (Section 7.2) applies here.
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