Wastewater Calculations - Mathematics and Statistics for the Environment - page 726

cleaning equipment, and safety equipment such as vacuum relief and pressure relief devices, flame
traps, and explosion-proof electrical equipment. In the anaerobic process, biosolids enter the sealed
digester where organic matter decomposes anaerobically. Anaerobic digestion is a two-stage process:

1. Sugars, starches, and carbohydrates are converted to volatile acids, carbon dioxide, and
hydrogen sulfide.

2. Volatile acids are converted to methane gas.

Key anaerobic digestion process control calculations are covered in the sections that follow.

24.13.4.1 Required Seed Volume in Gallons

Seed volume (gal) = Digester volume (gal) × % Seed (24.138)

■ EXAMPLE 24.107
Problem: The new digester requires 25% seed to achieve normal operation within the allotted time.
If the digester volume is 280,000 gal, how many gallons of seed material will be required?

Solution:

Seed volume (gal) = 280,000 × 0.25 = 70,000 gal

24.13.4.2 Volatile Acids to Alkalinity Ratio
The ratio of volatile acids to alkalinity can be used to control an anaerobic digester:

Volatile acids concentration
Alkalinit

Ratio = (24.139)

yyconcentration

■ EXAMPLE 24.108
Problem: The digester contains 240 mg/L volatile acids and 1840 mg/alkalinity. What is the volatile
acids/alkalinity ratio?

Solution:

240 mg/L
184

Volatile acids/alkalinityratio = = 0.13

00mg/L

Key Point: Increases in the ratio normally indicate a potential change in the operating condition of
the digester.

24.13.4.3 Biosolids Retention Time
The length of time the biosolids remain in the digester is calculated as

Digester volume(gal)
Biosoli

Retentiontime = (24.140)

ds volumeadded(gpd)

■ EXAMPLE 24.109
Problem: Biosolids are added to a 520,000-gal digester at the rate of 12,600 gal/day. What is the
biosolids retention time?