Environmental Engineering Reference
In-Depth Information
much activated sludge is removed from the system, the solids become
very light and will not settle quickly enough to be removed in the sec-
ondary clarifier.
8.9.7 Temperature
Because temperature directly affects the activity of the microor-
ganisms, accurate monitoring of temperature can be helpful in identi-
fying the causes of significant changes in organization populations or
process performance.
8.9.8 sludge blanket depth
The separation of solids and liquid in the secondary clarifier results
in a blanket of solids. If solids are not removed from the clarifier at the
same rate they enter, the blanket will increase in depth. If this occurs,
the solids may carry over into the process effluent. The sludge blanket
depth may be affected by other conditions, such as temperature varia-
tion, toxic wastes, or sludge bulking. The best sludge blanket depth is
dependent on such factors as hydraulic load, clarifier design, and sludge
characteristics. The best blanket depth must be determined on an indi-
vidual basis by experimentation.
Note:
When measuring sludge blanket depth, it is general practice to
use a 15- to 20-ft long clear plastic pipe marked at 6-inch intervals; the
pipe is equipped with a ball valve at the bottom.
8.10 oPeraTional ConTrol levels
*
The operator has two methods available to operate an activated
sludge system. The operator can wait until the process performance
deteriorates and make drastic changes, or the operator can establish
normal
operational levels and make minor adjustments to keep the pro-
cess within the established operational levels.
Note:
Control levels can be defined as the upper and lower values for
a process control variable that can be expected to produce the desired
effluent quality.
Although the first method could maintain plant performance within
the effluent limitations, the second method has a much higher probabil-
ity of achieving this objective. This section discusses methods used to
establish normal
control levels for the activated sludge process. Several
major factors should be considered when establishing control levels for
the activated sludge system, including:
•
Influent characteristics
•
Industrial contributions
*
Much of the information in this section is based on
activated Sludge Process Control
,
Part II, 2nd ed., Virginia Water Control Board, Richmond, VA, 1990.
