Wastewater Calculations - Mathematics and Statistics for the Environment - page 723

24.12.5 C entriFuge t hiCKening C alCulations
A centrifuge exerts a force on the biosolids thousands of times greater than gravity. Sometimes
polymer is added to the influent of the centrifuge to help thicken the solids. The two most important
factors that affect the centrifuge are the volume of the biosolids put into the unit (gpm) and the
pounds of solids put in. The water that is removed is the centrate . Normally, hydraulic loading is
measured as flow rate per unit of area; however, because of the variety of sizes and designs, hydrau-
lic loading to centrifuges does not include area considerations. It is expressed only as gallons per
hour. The equations to be used if the flow rate to the centrifuge is given as gallons per day or gallons
per minute are

Flow (gpd)
24 hr/day

Hydraulic loading(gph) = (24.132)

Flow (gpm)60min
hr

×

Hydraulic loading(gph) = (24.133)

■ EXAMPLE 24.101
Problem: A centrifuge receives a waste activated biosolids flow of 40 gpm. What is the hydraulic
loading on the unit in gal/hr?

Solution:

40 gpm × 60 min

Hydraulic loading = = 2400 gph

hr

■ EXAMPLE 24.102
Problem: A centrifuge receives 48,600 gal of biosolids daily. The biosolids concentration before
thickening is 0.9%. How many pounds of solids are received each day?

Solution:

48,600 gal
day

8.34 lb/gal
gal

09
100

.

× × = 3648 lb /day

24.13 STABILIZATION

24.13.1 b iosolids d igestion
A major problem in designing wastewater treatment plants is disposing of biosolids into the envi-
ronment without causing damage or nuisance. Untreated biosolids are even more difficult to dispose
of. Untreated raw biosolids must be stabilized to minimize disposal problems. In many cases, the
term stabilization is considered synonymous with digestion.

Note: The stabilization of organic matter is accomplished biologically using variety of organisms.
The microorganisms convert the colloidal and dissolved organic matter into various gases
and into protoplasm. Because protoplasm has a specific gravity slightly higher than that of
water, it can be removed from the treated liquid by gravity.

Biosolids digestion is a process in which biochemical decomposition of the organic solids occurs;
in the decomposition process, the organics are converted into simpler and more stable substances.
Digestion also reduces the total mass or weight of biosolids solids, destroys pathogens, and makes