Environmental Engineering Reference
In-Depth Information
trapped on the bars for removal. The bar screen may be coarse (2- to 4-in.
openings) or fine (0.75- to 2.0-in. openings). The bar screen may be manu-
ally cleaned (bars or screens are placed at an angle of 30° for easier sol-
ids removal; see Figure 5.2) or mechanically cleaned (bars are placed at a
45° to 60° angle to improve mechanical cleaning
operations). The screening method employed
depends on the design of the plant, the amount
of solids expected, and whether the screen is for
constant or emergency use only.
Key Point:
The screenings must be
removed frequently enough to prevent
accumulation that will block the screen
and cause the water level in front of the
screen to build up.
5.3.1.1 Manually Cleaned Screens
Manually cleaned screens are cleaned at least once per shift (or
often enough to prevent buildup that may cause reduced flow into the
plant) using a long tooth rake. Solids are manually pulled to the drain
platform and allowed to drain before storage in a covered container.
The area around the screen should be cleaned frequently to prevent a
buildup of grease or other materials, which can cause odors, slippery
conditions, and insect and rodent problems. Because screenings may
contain organic matter as well as large amounts of grease, they should be
stored in a covered container. Screenings can be disposed of by burial in
approved landfills or by incineration. Some treatment facilities grind the
screenings into small particles, which are then returned to the wastewa-
ter flow for further processing and removal later in the process.
5.3.1.1.1 operational Considerations
Manually cleaned screens require a certain amount of operator
attention to maintain optimum operation. Failure to clean the screen
frequently can lead to septic wastes entering the primary, surge flows
after cleaning, or low flows before cleaning. On occasion, when such
operational problems occur, it becomes necessary to increase the fre-
quency of the cleaning cycle. Another operational problem is excessive
grit in the bar screen channel. Improper design or construction or insuf-
ficient cleaning may cause this problem. The corrective action required
is either to modify the design or to increase cleaning frequency and
flush the channel regularly. Another common problem with manually
cleaned bar screens is their tendency to clog frequently. This may be
caused by excessive debris in the wastewater or the screen being too
fine for its current application. The operator should locate the source
of the excessive debris and eliminate it. If the screen is the problem, a
coarser screen may have to be installed. If the bar screen area is filled
with obnoxious odors, flies, and other insects, it may be necessary to
dispose of screenings more frequently.
5.3.1.2 Mechanically Cleaned Screens
Mechanically cleaned screens use a mechanized rake assembly to
collect the solids and move them out of the wastewater flow for dis-
charge to a storage hopper. The screen may be continuously cleaned
