TM 5-814-3/AFM 88-11, Volume III
e. Backwashing. Backwash jets are directed against the outside of the microscreen drum as it passes the
highest point in its rotation. About half the flow penetrates the fabric, dislodging the mat of solids formed
on the inside. A hopper inside the drum receives the flushed-off solids. The hopper is positioned to
compensate for the trajectory that the solids follow at normal drum peripheral velocities. Microscreen effluent
is usually used for Backwashing. Straining is required to avoid clogging of backwash nozzles. The inline
strainers used for this purpose will require periodic cleaning; the frequency of cleaning will be determined
by the quality of the backwash water.
(1) Sytems. The backwash system used by Zurn employs two header pipes; one operates continuously
at 20 pounds per square inch, while the other operates at 40-55 pounds per square inch. Under normal
operating conditions, these jets operate at 35 pounds per square inch. Once a day they are operated at 50
pounds per square inch for hour to keep the jets free of slime buildup. Should this procedure fail to keep
the jets clean, the pressure is raised to 55 pounds per square inch. At this pressure the spring-loaded jet
mouth widens to allow for more effective cleaning.
(2) High pressure. Backwash pressure can be increased to compensate for heavy solids loadings which
require higher pressure for thorough cleaning. The superiority of the higher-pressure system is manifested
by the following:
(a) Operation at 50 pounds per square inch, as opposed to 15 pounds per square inch, increases the
process flow capacity 30 percent.
(b) Suspended solids concentration in the backwash can increase from 260 milligrams per liter at 15
pounds per square inch to 425 milligrams per liter at 50 pounds per square inch.
(c) Water consumption of the jets as a percent of process effluent decreases from 5 percent at 15
pounds per square inch to 2 percent at 50 pounds per square inch. In general, backwash systems are operated
at as low a pressure as possible consistent with successful cleaning. High-pressure operation incurs added
system maintenance, particularly jet replacement, and is used only as needed.
f. Supplemental cleaning. Over a period of time, screen fabrics may become clogged with algal and
slime growths, oil, and grease. To prevent clogging, cleaning methods in addition to Backwashing are
necessary.
(1) Ultraviolet lamps. Th reduce clogging from algal and slime growth, the use of ultraviolet lamps
placed in close proximity to the screening fabric and monthly removal of units from service to permit screen
cleaning with a mild chlorine solution is recommended. While most literature sources say ultraviolet lamps
are of value, one authority feels these lamps are uneconomical because they require frequent replacement.
Zurn Industries claims that, because their screening fabric is completely bonded to the supporting material,
crevices where algae become lodged are eliminated and Backwashing alone is sufficient to remove algal and
associated slime growths.
(2) Hot water. Where oil and grease are present, hot water and/or steam treatment can be used to
remove these materials from the microscreens. Plastic screens with grease problems are cleaned monthly with
hot water at 1200 Fahrenheit to prevent damage to the screen material. Downtime for cleaning may be up
to 8 hours.
g. Operation. In starting a microscreening unit, care should be taken to limit differential water levels
across the fabric to normal design ranges of 2 to 3 inches. For example, while the drum is being filled, it
should be kept rotating and the backwash water should be turned on as soon as possible. This is done to limit
the formation of excessive differential heads across the screen which would stress the fabric during tank fill-
up. Leaving the drum standing in dirty water should be avoided because suspended matter on the inside
screen face which is above the water level may dry and prove difficult to remove. For this reason, introducing
unscreened waters, such as plant overloads, into the microscreen effluent compartment should also be
avoided. If the unit is to be left standing for any length of time, the tank should be drained and the fabric
cleaned to prevent clogging from drying solids.
15-7
Previous Page
