Type of Treatment
Normal Range of
Doses (mg/l)
Primary
3 -- 40
Trickling Filter
3 -- 15
Activated Sludge
2 -- 9
Sand Filter
1 --
6
Table 2-1. Chlorine doses for different types of treatment plant effluents.
b. Trickling Filter Unit.
(1) Construction and use. A trickling filter of one kind or another is the
most common secondary treatment unit at Army wastewater plants (see Figure 2-13).
The filter medium is a bed of broken stone or slag 3 to 8 feet deep. Its holding structure
is usually concrete, although it may be brick, masonry, or wood. The filter has a
distribution system for spreading the wastewater uniformly and an underdrain system to
remove filter effluent and circulate air through the filter. The underdrain system usually
has a sloping channeled floor with slotted or open-joint vitrified-clay half tiles or blocks
directly supporting the filter medium (see Figure 2-14). Wood-grid underdrains are also
used.
(2) Filtration process. The action of the trickling filter is not mechanical or
physical but is truly biochemical. The surfaces of the stones become coated with a
gelatinous growth or zoogleal film (more commonly known as the slime layer). This
layer of slime consists of bacteria, algae, fungi, and filth. It serves as a reservoir for an
enormous volume of wastewater and keeps the wastewater in intimate contact with air
and aerobic bacteria as it passes through the filter. Most of the filter consists of
bacteria, which decomposes most of the organic waste into nitrates and is used as fuel.
Protozoa feed on the bacteria, including the coliform bacteria Escherichia coli (E. coli).
The bacteria content of the wastewater is thereby reduced by 50 to 70 percent as the
wastewater passes through an efficient trickling filter bed. Fungi, although it treats the
waste, is not as functional. Algae, which grow on top of the filter, do a limited amount of
treatment. The waste flows over and partly through the slime. Colloidal, dissolved, and
suspended organic matter passes to the underdrains and is converted into nitrates and
used as fuel or food. For the filter to work, it must have enough oxygen. The oxygen is
received from the wastewater and the air that circulates through the spaces between
the media slime. These spaces or channels must be kept open to supply oxygen (rapid
oxidation) to the slime and allow the waste to trickle through the filter. While the outside
part of the biological slime is aerobic, there is a dark gray or black layer next to the
media that is anaerobic. This layer occurs because the slime becomes thick enough to
MD0161
2-19
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