Wastewater Calculations - Mathematics and Statistics for the Environment - page 676

■ EXAMPLE 24.20
Problem: If 120 mg/L suspended solids are removed by a primary clarifier, how many lb/day sus-
pended solids are removed when the flow is 6,250,000 gpd?

Solution:

SS removed = 120 mg/L × 6.25 MGD × 8.34 lb/gal = 6255 lb/day

■ EXAMPLE 24.21
Problem: The flow to a secondary clarifier is 1.6 MGD. If the influent BOD concentration is 200
mg/L and the effluent BOD concentration is 70 mg/L, how many pounds of BOD are removed
daily?

Solution:

BOD removed = 200 mg/L - 70 mg/L = 130 mg/L

After calculating mg/L BOD removed, calculate lb/day BOD removed:

BOD removed = 130 mg/L × 1.6 MGD × 8.34 lb/gal = 1735 lb/day

24.5 TRICKLING FILTER
The trickling filter process (Figure 24.3) is one of the oldest forms of dependable biological treat-
ment for wastewater. By its very nature, the trickling filter has its advantages over other unit pro-
cesses; for example, it is a very economical and dependable process for treatment of wastewater
prior to discharge. Capable of withstanding periodic shock loading, process energy demands are
low because aeration is a natural process.
As shown in Figure 24.4, trickling filter operation involves spraying wastewater over a solid
media such as rock, plastic, or redwood slats (or laths). As the wastewater trickles over the surface
of the media, a growth of microorganisms (bacteria, protozoans, fungi, algae, helminths or worms,
and larvae) develops. This growth is visible as a shiny slime very similar to the slime found on rocks
in a stream. As wastewater passes over this slime, the slime adsorbs the organic (food) matter. This
organic matter is used for food by the microorganisms. At the same time, air moving through the
open spaces in the filter transfers oxygen to the wastewater. This oxygen is then transferred to the
slime to keep the outer layer aerobic. As the microorganisms use the food and oxygen, they produce
more organisms, carbon dioxide, sulfates, nitrates, and other stable byproducts; these materials are
then discarded from the slime back into the wastewater flow and are carried out of the filter.

Waste sludge

Cl 2 or NaOCl

Chlorine
contact tank

Grit
Chamber

Primary
sedimentation

Trickling
filter

Settling
tank

Euent

Influent Bar Racks

Return euent

FIGURE 24.3 Simplified flow diagram of trickling filter used for wastewater treatment.