Environmental Engineering Reference
In-Depth Information
slime found on rocks in a stream. As the wastewater passes over this
slime, the slime adsorbs the organic matter. This organic matter is used
for food by the microorganisms. At the same time, air moving through
the open spaces in the filter transfers oxygen to the wastewater. This
oxygen is then transferred to the slime to keep the outer layer aerobic.
As the microorganisms use the food and oxygen, they produce more
organisms, carbon dioxide, sulfates, nitrates, and other stable byprod-
ucts; these materials are then discarded from the slime back into the
wastewater flow and are carried out of the filter.
Organics + Organisms + O
2
= More Organisms + CO
2
+ Solid Wastes (7.9)
The growth of the microorganisms and the buildup of solid wastes
in the slime make it thicker and heavier. When this slime becomes too
thick, the wastewater flow breaks off parts of the slime. These must be
removed in the final settling tank. In some trickling filters, a portion of
the filter effluent is returned to the head of the trickling filter to level out
variations in flow and improve operations (recirculation).
7.3.5.1 Overview and Brief Summary of Trickling Filter Process
A trickling filter consists of a bed of coarse media, usually rocks or
plastic, covered with microorganisms.
Note:
Trickling filters that use ordinary rock are normally only about
10 ft in depth because of structural problems caused by the weight of
rocks—which also requires the construction of beds that are quite wide,
in many applications up to 60 ft in diameter. When synthetic media are
used, the bed can be much deeper.
•
The wastewater is applied to the media at a controlled rate, using
a rotating distributor arm or fixed nozzles. Organic material is
removed by contact with the microorganisms as the wastewater
trickles down through the media openings. The treated wastewater
is collected by an underdrain system.
Note:
To ensure sufficient air flow to the filter, the underdrains should
never be allowed to flow more than 50% full of wastewater.
•
The trickling filter is usually built into a tank that contains the
media. The filter may be square, rectangular, or circular.
•
The trickling filter does not provide any actual filtration. The filter
media provide a large amount of surface area that the microorgan-
isms can cling to and grow in a slime that forms on the media as
they feed on the organic material in the wastewater.
•
The slime growth on the trickling filter media periodically
sloughs
off and is settled and removed in a secondary clarifier that follows
the filter.
