Environmental Engineering Reference
In-Depth Information
Table 10.5 norMal oPeraTing ranges
for anaerobiC digesTers
Parameter
normal range
Sludge retention time
Heated
30-60 days
Unheated
180+ days
Volatile solids loading
0.04-0.1 lb VM/day/ft
3
Operating temperature
Heated
90-95°F
Unheated
Varies with season
Mixing
Heated, primary
Yes
Unheated, secondary
No
% Methane in gas
60-72%
% Carbon dioxide in gas
28-40%
pH
6.8-7.2
Volatile acids-to-alkalinity ratio
≤
0.1
Volatile solids reduction
40-60%
Moisture reduction
40-60%
•
symptom 1
. Digester gas production is reduced, pH drops below
6.8, or the volatile acids-to-alkalinity ratio increases.
Causal factors:
Digester souring; organic overloading; inadequate
mixing; low alkalinity; hydraulic overloading; toxicity; loss of diges-
tion capacity
Corrective actions:
Add alkalinity (e.g., digested sludge, lime);
improve temperature control; improve mixing; eliminate toxic-
ity; clean the digester.
•
symptom 2.
Gray foam is oozing from the digester.
Causal factors:
Rapid gasification; foam-producing organisms;
foam-producing chemical
Corrective actions:
Reduce mixing; reduce feed rate; mix slowly
by hand; clean all contaminated equipment.
10.6.2.9 Anaerobic Digester Process Control Calculations
Process control calculations involved with anaerobic digester oper-
ation include determining the required seed volume, volatile acids-to-
alkalinity ratio, sludge retention time, estimated gas production, volatile
matter reduction, and percent moisture reduction in digester sludge.
Examples on how to make these calculations are provided in the follow-
ing sections.
