Handbook of Civil Engineering Calculations

3. Find the quantity of neat polymer required
   The quantity of neat polymer required is calculated as follows:

, , „ N active polymer, gal/h

Neat polymer (gal/h) = — : ^- -7 TT-T

% active polymer in emulsion as supplied

Using values obtained above:

Neat polymer =^0 '^083 gal/h = 0 .277 gal/h (1.05 L/h)

This quantity of neat polymer represents the amount of polymer used in its "as supplied"
form. Therefore, if polymer is supplied in a 55 gallon (208.2 L) drum, the time required to
use one drum of polymer (assuming polymer is used 24 h/d, 7 d/wk) is:

55 gal

Time required to use one drum of polymer = ————— = 200 h = 8 days

0.277 gal/h

DESIGN OFA TRICKLING FILTER USING

THE NRC EQUATIONS

A municipal wastewater with a flow rate of 1.0 Mgd (3,785 m
3
/d) and a BOD 5 of 240
mg/L is to be treated by a two stage trickling filter system. The effluent wastewater is to
have a BOD 5 of 20 mg/L. Both filters are to have a depth of 7 feet (2.1 m) and a recircu-
lation ratio of 2. Filter media will consist of rock. Size both stages of the trickling filter
assuming the efficiency (E) of each stage is the same.

Calculation Procedure:

1. Find the efficiency of the trickling filters
   The modern trickling filter, shown in Fig. 7, consists of a bed of highly permeable medi-
   um to which microorganisms are attached and through which wastewater is percolated or
   trickled. The filter media usually consists of either rock or a variety of plastic packing ma-
   terials. The depth of rock varies but usually ranges from 3 to 8 feet (0.91 to 244 m). Trick-
   ling filters are generally circular, and the wastewater is distributed over the top of the bed
   by a rotary distributor.
   Filters are constructed with an underdrain system for collecting the treated wastewater
   and any biological solids that have become detached from the media. This underdrain sys-
   tem is important both as a collection unit and as a porous structure through which air can
   circulate. The collected liquid is passed to a settling tank where the solids are separated
   from the treated wastewater. In practice, a portion of the treated wastewater is recycled to
   dilute the strength of the incoming wastewater and to maintain the biological slime layer
   in a moist condition.
   The organic material present in the wastewater is degraded by a population of mi-
   croorganisms attached to the filter media. Organic material from the wastewater is ab-
   sorbed onto the biological slime layer. As the slime layer increases in thickness, the mi-
   croorganisms near the media face lose their ability to cling to the media surface. The