Wastewater Calculations - Mathematics and Statistics for the Environment - page 724

it easier to dry or dewater the biosolids. Well-digested biosolids have the appearance and charac-
teristics of a rich potting soil. Biosolids may be digested under aerobic or anaerobic conditions.
Most large municipal wastewater treatment plants use anaerobic digestion. Aerobic digestion finds
application primarily in small activated biosolids treatment systems.

24.13.2 a erobiC d igestion p roCess C ontrol C alCulations
The purpose of aerobic digestion is to stabilize organic matter, to reduce volume, and to eliminate
pathogenic organisms. Aerobic digestion is similar to the activated biosolids process. Biosolids are
aerated for 20 days or more. Volatile solids are reduced by biological activity.

24.13.2.1 Volatile Solids Loading (lb/ft 3 /day)
Volatile solids (organic matter) loading for the aerobic digester is expressed in pounds of volatile
solids entering the digester per day per cubic foot of digester capacity:

Volati le solids added(lb/day)

Digester volume(ftt)

3 =

Volatile solids loading(lb/day/ft) (24.134)

3

■ EXAMPLE 24.103
Problem: An aerobic digester is 20 ft in diameter and has an operating depth of 20 ft. The biosolids
that are added to the digester daily contain 1500 lb of volatile solids. What is the volatile solids
loading in pounds per day per cubic foot?

Solution:

1500 lb/day

0.24 lb/day/ft 3

Volatile solids loading = =

0.785 × 20 ft × 20 ft × 20 ft

24.13.2.2 Digestion Time (Days)
The theoretical time the biosolids remain in the aerobic digester is calculated by

Digester volume(gal )

Biosolidsadded(gpd)

Digestiontime(days) = (24.135)

■ EXAMPLE 24.104
Problem: The digester volume is 240,000 gal. Biosolids are added to the digester at the rate of
15,000 gpd. What is the digestion time in days?

Solution:

240,000 gal
15,000 gpd

Digestiontime = = 16 day s

24.13.2.3 pH Adjustment
In many instances, the pH of the aerobic digester will fall below the levels required for good biologi-
cal activity. When this occurs, the operator must perform a laboratory test to determine the amount
of alkalinity required to raise the pH to the desired level. The results of the lab test must then be
converted to the actual quantity required by the digester: