standard, high-capacity, and super-high-capacity filters. The standard and high-
capacity filters are mentioned below.
(a) Standard (low-capacity) filters. These filters are usually rock
media and with depths varying from 6 to 8 feet. They are a one-time filter with no
recirculation and normally have high BOD removal (80 to 90 percent). Intermittent
dosing is done by means of either fixed nozzles or a rotary distribution mechanism (see
Figure 2-15. Trickling filter with rotary distributor.
(b) High-capacity trickling filters. High-capacity filter beds are made of
rock media and vary from 3 to 8 feet in depth. The rocks serving as filter media vary in
size from 3 to 4 1/2 inches, which is generally larger than those of a standard filter, thus
allowing a great wastewater flow. Continuous dosing is usually by means of rotating
distributors, although a few filters with special fixed nozzles have been constructed at
Army posts. The underdrain system usually is designed to flow only one-third full under
maximum operating conditions to ensure adequate ventilation. Recirculation almost
1 High-capacity versus low-capacity filtration. The high-capacity
filter has a continuous high-rate application that is well distributed over the bed surface.
The high rate is assured by recertifying wastewater already passed through the filter,
either continuously or during low flow (see Figure 2-16A and B). The heavy flow of
wastewater over the biological growth produces continuous sloughing instead of the