Figure 2-14. Trickling filter under construction.
use up all of the dissolved oxygen before it reaches the inner (anaerobic) layer. The
anaerobic layer does not cling tightly to the media, so, when it gets thick, its outer
portions die, break loose, and allow patches of slime to slough off the media. The
amount of sloughing on a filter depends on the reactions taking place on the filter and
upon certain conditions (the quality of the applied wastewater; rate and continuity of
application; air supply; temperature; changes in the season, more so in the spring and
fall; and other factors). The slime layer sloughs off from the filter (again, more so in the
spring and fall) and is removed in the secondary clarifier. This forms a sludge which is
called the trickling filter humus. The trickling filter removes solids by biological action (or
a better name, "biological oxidation bed"), not by filtration. The sludge is fairly stable,
but if it is not removed from the filter effluent by sedimentation, it may form objectionable
sludge banks in the receiving stream. Therefore, a trickling filter is always followed by a
settling tank where the slime is removed by sedimentation. These reactions differ in the
two general types of filters (the standard low-capacity filter and the high-capacity filter)
discussed a little later.
(a) Biological action. In deep, well-ventilated standard filters, two
zones of activity are apparent. Organisms in the top of the filter break down proteins
and amino acids into ammonia and simpler sulfur compounds and decompose
carbohydrates to acids. At the lower levels, organisms oxidize ammonia and sulfur
compounds to nitrites, stable nitrates, and sulfates.
(b) Unloading. A relatively thick growth develops in the standard filter
during normal operation until a temperature change or flow of influent through the bed
causes a large portion to slough off. This slough, usually occurring in the fall and
spring, makes the filter effluent quite turbid and frequently contains large masses of
worms normally present in the filter. These solids are removed in the final settling tank.
(3) Types of trickling filters. Trickling filters need food, oxygen, and
biological forms to function and are labeled in different classes according to their
amount of food (BOD) and flow. The four basic trickling filters are the roughing,