b. Sand Filter Trenches and Subsurface Sand Filters. In soil that is relatively
impermeable, neither absorption trenches, seepage beds, nor seepage pits are
satisfactory. When absorption systems are impracticable, the possibility of treating the
tank effluent in subsurface sand filters or filter trenches may be considered. (Soil
testing is a mandatory prerequisite for any subsurface disposal of waste.) These
systems are similar to soil absorption systems except that they are deeper, generally
somewhat wider, contain an intermediate layer of sand as filtering material, and are
provided with underdrains for carrying off the filtered effluent. For this reason, effluent
from a properly designed system can sometimes be disposed of without further
treatment. In some jurisdictions, chlorination of the effluent is required.
(1) Sand filter trenches. Sand filter trenches (see Figure 1-18) are
essentially wide absorption trenches (30 to 60 inches wide) underdrained with at least
24 inches of filter and below which open-joint tile lines laid in gravel collect and carry
away the filtered effluent.
(2) Subsurface sand filters. These systems (see Figure 1-19) are
essentially the same as seepage beds underdrained by 24 to 30 inches of filter sand
and open-joint tile lines similar to those used in filter trenches. As a rough guide,
subsurface sand filters should be used instead of sand filter trenches when the filter
area or length of tile is enough to require the use of dosing siphons (over 1,800 square
feet or over 300 feet of tile). However, clogging and installation costs are significant
disadvantages for these filters.