Wastewater Calculations - Mathematics and Statistics for the Environment - page 677

Rotating arm

Influent spray

Rock Bed

Influent

Underdrain
System

Euent

FIGURE 24.4 Schematic of cross-section of a trickling filter.

24.5.1 t riCKling F filter. p roCess C alCulations
Several calculations are useful in the operation of trickling filters; these include hydraulic loading,
organic loading, and BOD and SS removal. Each type of trickling filter is designed to operate with
specific loading levels. These levels vary greatly depending on the filter classification. To operate
the filter properly, filter loading must be within the specified levels. The main three loading param-
eters for the trickling filter are hydraulic loading, organic loading, and recirculation ratio.

24.5.1.1 Hydraulic Loading Rate
Calculating the hydraulic loading rate is important in accounting for both the primary effluent as
well as the recirculated trickling filter effluent. These are combined before being applied to the filter
surface. The hydraulic loading rate is calculated based on filter surface area. The normal hydraulic
loading rate ranges for standard rate and high rate trickling filters are

Standard rate—25 to 100 gpd/ft 2 or 1 to 40 MGD/ac
High rate—100 to 1000 gpd/ft 2 or 4 to 40 MGD/ac

Key Point: If the hydraulic loading rate for a particular trickling filter is too low, septic conditions
will begin to develop.

■ EXAMPLE 24.22
Problem: A trickling filter 80 ft in diameter is operated with a primary effluent of 0.588 MGD and
a recirculated effluent flow rate of 0.660 MGD. Calculate the hydraulic loading rate on the filter in
units gpd/ft 2 .

Solution: The primary effluent and recirculated trickling filter effluent are applied together across
the surface of the filter; therefore, 0.588 MGD + 0.660 MGD = 1.248 MGD = 1,248,000 gpd:

2 2 2

Circular surface area = 0.785 × (diameter) = 0.7 85 × (80ft) = 5024 ft

1,248,000 gpd
5024 ft

2

Hydraulic loadingrate = = 248.4 gpd/ft

2