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that forces air into the trench. 4n the other end is a vent
that allows air to escape. Both vents may be used by the
operator to inspect each trench for the occurrence and
magnitude of any ponding.
The drop boxes control loading to the cells. The drop boxes
contain an inlet pipe, two 4 inch outlet pipes to the
distribution piping, and a 4 inch supply pipe to the next drop
box. The first drop box in each trench is fed by a 3 inch
forcemain from the trench dosing station.
3.9.3 Operation
During operation, the trench dosing pump delivers sand filter
effluent to the trench valve box. The valves are set so that
two cells are active and one is resting. Each cell should be
active for two years and then rest for one year as shown in
Table 3-7a. Cells are rotated by adjusting the manual control
valves.
Each pump feeds into the first drop box of one of the active
trenches. Effluent enters the drop box and flows through the
distribution piping. The supply line to the subsequent trench
is higher than the distribution piping, so initially, all of
the effluent goes into the first trench. As the first trench
begins to pond, the water level in the drop box rises and
eventually reaches the level of the supply line to the
subsequent trench. At this time, both the first and second
fu
trenches will be receiving effluent. This process continues
for all ten trenches.
3-60