Handbook of Civil Engineering Calculations

FIGURE 7. Cutaway view of a trickling filter. (Metcalf & Eddy, Wastewater Engineer-

ing: Treatment, Disposal, and Reuse, 3rd Ed., McGraw-Hill.)

liquid then washes the slime off the media, and a new slime layer starts to grow. The phe-
nomenon of losing the slime layer is called "sloughing" and is primarily a function of the
organic and hydraulic loading on the filter.
Two possible process flow schematics for a two-stage trickling filter system are
shown in Fig. 8.
The NRC equations for trickling filter performance are empirical equations which are
primarily applicable to single and multistage rock systems with recirculation.
The overall efficiency of the two-stage trickling filter is calculated using:

influent BOD 5 - effluent BOD 5

Overall efficiencyJ = — x 100

influent BOD 5

Using the influent and effluent BOD 5 values presented in the problem statement, the

overall efficiency is:

Overall efficiency =

24

° "ff ~ ™

mg/L

x I 00 = 91.7%

240 mg/L

Influent

Primary

Clarifier

First-Stage

Filter

Second-Stage

Filter

Recycle Recycle

Clarifier

Effluent

Recycle Recycle

Influent

Primary

Clarifier

First-Stage

Filter Clarifier

Second-Stage

Filter Clarifier

Effluent

FIGURE 8. Two-stage trickling filter process flow schematics.