TM 5-814-3/AFM 88-11, Volume III
a.
Filter depth. Stone media trickling filters will be designed with depths of 5 to 7 feet for low-rate and
depths of 3 to 6 feet for high-rate applications. Synthetic media manufacturers recommend depths of 10 to
40 feet for columnar or stacked module media. Randomly placed polypropylene media filters are designed
within the depth ranges of the low and intermediate-rate filters. The deeper trickling filters can improve
nitrification potential and can be used as the second stage in two-stage biological system designs for
nitrification.
b. Recirculation. This is a recommended method of increasing the biochemical oxygen demand removal
efficiency of high-rate trickling filter processes. Figure 12-1 shows acceptable recirculation systems for single-
stage and two-stage trickling filters treating domestic wastewater. Table 12-2 lists recommended recirculation
rates for high-rate filters. Whether to use recirculation and the amount to be recycled when used are matters
of economics which may involve either first cost or annual costs of various designs providing equal treatment.
Unless other conditions control, recirculation should provide continuous dosing at a minimum surface
application rate of 10 million gallons per acre per day. In flow diagrams B, C and D (fig 12-1), fluctuations
in the organic loading applied to the filter are dampened. Filter sloughings are recycled to the filter in flow
diagram A but little, if any, dampening of variations in organic loading is provided. Flow diagram E may
include a low- rate filter for the second stage unit. Intermediate settling tanks will always be provided
between first and second stage filters. Flow diagrams G and H attempt to improve treatment by developing
greater biological activity on the second stage filter but are not acceptable for military installations because
there are no intermediate clarifiers. Flow diagrams E, F, G and H require inclusion of the recirculated flow
in the forward flow used for design of any tanks through which it passes.
12-3
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