Environmental Engineering Reference
In-Depth Information
10.6 sludge sTabilizaTion
The purpose of sludge stabilization is to reduce volume, stabilize
the organic matter, and eliminate pathogenic organisms to permit reuse
or disposal. The equipment required for stabilization depends on the
specific process used. Sludge stabilization processes include:
Aerobic digestion
• Anaerobic digestion
• Composting
• Lime stabilization
• Wet air oxidation (heat treatment)
• Chemical oxidation (chlorine oxidation)
• Incineration
10.6.1 aerobic digestion
Equipment used for aerobic digestion includes an aeration tank
(digester), which is similar in design to the aeration tank used for the
activated sludge process. Either diffused or mechanical aeration equip-
ment is necessary to maintain the aerobic conditions in the tank. Solids
and supernatant removal equipment is also required. In operation, pro-
cess residuals (sludge) are added to the digester and aerated to main-
tain a dissolved oxygen (DO) concentration of 1.0 mg/L. Aeration also
ensures that the tank contents are well mixed. Generally, aeration con-
tinues for approximately 20 days of retention time. Periodically, aeration
is stopped, and the solids are allowed to settle. Sludge and the clear
liquid supernatant are withdrawn as needed to provide more room in the
digester. When no additional volume is available, mixing is stopped for
12 to 24 hours before the solids are withdrawn for disposal.
Process control testing should include alkalinity, pH, percent sol-
ids, and percent volatile solids for influent sludge, supernatant, digested
sludge, and digester contents. Normal operating levels for an aerobic
digester are listed in Table 10.3. A typical operational problem associ-
ated with an aerobic digester is pH control. When pH drops, for example,
this may indicate normal biological activity or low influent alkalinity.
This problem is corrected by adding alkalinity (e.g., lime, bicarbonate).
Table 10.3 aerobiC digesTer
norMal oPeraTing levels
Parameter
normal levels
Detention time (days)
10-20
Volatile solids loading (lb/ft 3 /day)
0.1-0.3
Dissolved oxygen (mg/L)
1.0
pH
5.9-7.7
Volatile solids reduction
40-50%
Search WWH ::




Custom Search