Environmental Engineering Reference
In-Depth Information
Process control calculations involved with anaerobic digester operation include
determining the required seed volume, volatile acids-to-alkalinity ratio, sludge reten-
tion time, estimated gas production, volatile matter reduction, and percent moisture
reduction in digester sludge. Examples on how to make these calculations are pro-
vided in the following sections.
Required Seed Volume in Gallons
Seed volume (gal) = Digester volume × %Se e d
(5.1)
■
Example 5.1
Problem:
A new digester requires a 25% seed to achieve normal operation within
the allotted time. If the digester volume is 266,000 gal, how many gallons of seed
material will be required?
Solution:
Seed volume = 266,000 × 0.25 = 66,500 gal
Volatile Acids-to-Alkalinity Ratio
The volatile acids-to-alkalinity ratio can be used to control operation of an anaerobic
digester:
Volatile acids concentration
Alkalinit
Ratio
=
(5.2)
yyconcentration
■
Example 5.2
Problem:
The digester contains 240 mg/L volatile acids and 1860 mg/L alkalinity.
What is the volatile acids-to-alkalinity ratio?
Solution:
Volatile acids concentration
Alkalinit
240 mg/L
186 mg/L
Ratio
=
=
= 013
.
yyconcentration
Note:
Increases in the ratio normally indicate a potential change in the operation
condition of the digester, as shown in Table 5.14.
TABLE 5.14
Volatile Acid-to-Alkalinity Ratios
Operating Condition
Volatile Acids-to-Alkalinity Ratio
Optimum
≤0.1
Acceptable range
0.1-0.3
Increase in % carbon dioxide in gas
≥0.5
Decrease in pH
≥0.8
