Final Effluent

FIG. 7.21.6 Contact stabilization plant flow sheet.

and organic loading applied. Filters are categorized as follows: the low rate is 2 to 4 million gal per acre per day (mgad), the intermediate rate is 4 to 10 mgad, the high rate is 10 to 30 mgad, and super-rate units are greater than 30 mgad.

Only a few fixed-nozzle trickling filters are in operation because the design requires extensive piping and nozzle heads that permit dosing the total filter bed. A pumpdischarge, head-driven, rotary distributor and a dosing chamber with siphon or a constant head-box design is more common. Single-stage and multistage filter arrangements are both used because many recirculation schemes and options of intermediate settling between multistage fil ters are available. The recirculation method is much less a factor in plant performance than the recirculation ratio.

Figure 7.21.7 shows the more common single-stage filter recirculation flow diagrams. Sludge from the final clarifier is usually recirculated to a point before the primary settling tank. The recirculation flow is also taken from in front of and behind the final clarifier to a point either before or after the primary settling. All units must be designed for total hydraulic flow and organic loading.

Figure 7.21.8 shows several flow routings used for multistage filters. All sludge returned from the intermediate and final clarifiers that is not wasted (excess) is returned to a point before the primary settling tanks.

FIG. 7.21.7 Typical single-stage, trickling filter, recirculation flow sheets.

FIG. 7.21.8 Typical multistage, trickling-filter, recirculation flow sheets.

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