Environmental Engineering Reference
In-Depth Information
Table 10.4 anaerobiC digesTer
sl
udge ParaMeTers
raw sludge solids
impact
<4% solids
Loss of alkalinity
Decreased sludge retention time
Increased heating requirements
Decreased volatile acids-to-alkalinity ratio
4-8% solids
Normal operation
>8% solids
Poor mixing
Organic overloading
Decreased volatile acids-to-alkalinity ratio
and safety equipment (e.g., vacuum relief, pressure relief, flame traps,
explosion proof electrical equipment). Various performance factors
affect the operation of the anaerobic digester; for example, the percent
volatile matter in raw sludge, digester tempera-
ture, mixing, volatile acids-to-alkalinity ratio,
feed rate, percent solids in raw sludge, and pH
are all important operational parameters that
the operator must monitor.
Key Point:
In an anaerobic digester,
the entrance of air should be prevented
because of the potential for an explo-
sive mixture resulting from air mixing
with gas produced in the digester.
Note:
The primary purpose of a secondary digester is to allow for solids
separation.
Along with being able to recognize normal and abnormal anaero-
bic digester performance parameters, wastewater operators must also
know and understand normal operating procedures. Normal operating
procedures include sludge additions, supernatant withdrawal, sludge
withdrawal, pH control, temperature control, mixing, and safety require-
ments. Important performance parameters are listed in Table 10.4.
10.6.2.1 Sludge Additions
Sludge must be pumped (in small amounts) several times each day
to achieve the desired organic loading and optimum performance.
Note:
Keep in mind that in fixed-cover operations additions must be bal-
anced by withdrawals; if not, structural damage occurs.
10.6.2.2 Sludge Withdrawal
Digested sludge is withdrawn only when necessary—always leave
at least 25% seed.
10.6.2.3 Supernatant Withdrawal
Supernatant withdrawal must be controlled for maximum sludge
retention time. When sampling, sample all drawoff points and select the
level with the best quality.
