Environmental Engineering Reference
In-Depth Information
DID YOU KNOW?
The primary purpose of a secondary digester is to allow for solids separation.
In operation, process residual (thickened or unthickened biosolids/sludge) is
pumped into the sealed digester. The organic matter digests anaerobically by a two-
stage process. Sugars, starches, and carbohydrates are converted to volatile acids,
carbon dioxide, and hydrogen sulfide. The volatile acids are then converted to meth-
ane gas. This operation can occur in a single tank (one stage) or in two tanks (two
stages). In a single-stage system, supernatant and digested solids must be removed
whenever flow is added. In a two-stage operation, solids and liquids from the first
stage flow into the second stage each time fresh solids are added. Supernatant is
withdrawn from the second stage to provide additional treatment space. Periodically,
solids are withdrawn for dewatering or disposal. The methane gas produced in the
process may be used for many plant activities.
Various performance factors affect the operation of the anaerobic digester. The
percent volatile matter in raw sludge, digester temperature, mixing, volatile acids-
to-alkalinity ratio, feed rate, percent solids in raw biosolids, and pH are all impor-
tant operational parameters that the operator must monitor (see Table 5.12). Along
with being able to recognize normal and abnormal anaerobic digester performance
parameters, digester operators must also know and understand normal operating
procedures. Normal operating procedures include biosolids additions, superna-
tant withdrawal, sludge withdrawal, pH control, temperature control, mixing, and
safety requirements.
Caution:
Keep in mind that in fixed-cover operations additions must be balanced
by withdrawals. If not, structural damage occurs.
Sludge must be pumped (in small amounts) several times each day to achieve
the desired organic loading and optimum performance, and supernatant withdrawal
must be controlled for maximum sludge retention time. All drawoff points are sam-
pled, and the level with the best quality is selected. Digested sludge is withdrawn
only when necessary; at least 25% seed remains. A pH of 6.8 to 7.2 is maintained by
TABLE 5.12
Sludge Parameters for Anaerobic Digesters
Raw Biosolids (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
