6-26. Intermittent sand filter system
basin is considered part of primary treatment
because it provides a quiescent zone for settling of
This system, depicted in figures 6-9 and 6-10, consists of three
fine, suspended solids that have been carried over
from the sediment basin. This function improves the
intermittent sand filter. All three units are located downstream
overall suspended solids removal efficiency of the
of the sediment basin in the order listed.
treatment process. Unfiltered water from the
equalization basin should be used as a source of
Equalization basin. The equalization basin (figs 6-
water for filling the bath prewash and for trench
11 and 6-12) serves two main functions. First, it is
drain flushing, thereby conserving the water supply.
considered a part of secondary treatment because it
provides storage and flow dampening of the sedi-
separate delivery pumps and piping are required,
ment basin effluent so that this water can be deliv-
but the size of intermittent sand filters and water
ered at a controlled rate to the intermittent sand
supply basins are usually reduced.
filters for final treatment. The basin buffers the
fluctuations in wastewater flow and quality caused
Siting. The equalization basin is located at a
by variable use of the wash facilities. It also
lower elevation than the sediment basin and
compensates for the difference between basin in-
the wastewater should flow by gravity from
flow and intermittent sand filter application rates.
the sediment basin to the equalization basin.
By regulating the dose of water to the intermittent
sand filter at designated intervals and rates, the filter
size can be minimized. Second, the equalization
be based on several factors, including the