TM 5-814-3/AFM 88-11, Volume III
b. Design factors. Microstrainers will be designed on the basis of not exceeding 10.0 gallons per minute
per square foot of submerged screen area at design maximum flow. Multiple units will be provided and all
units will be protected against freezing. Typical opening sizes for microstraining fabrics are, 23, 35 and 60
microns (respective number of openings per square inch being 165,000, 80,000 and 60,000). With the 23-
micron screen fabric, the microstrainer can be credited for 75 percent solids removal; 60 precent removal is
achievable using the 35-micron fabric. Maximum solids loading for microstraining of activated sludge
effluent is 0.88 pounds per day per square foot at a hydraulic loading of 6.6 gallons per minute per square
foot. Table 15-3 presents typical power and space requirements for microscreens.
c. Advantage and efficiency. An advantage of microstraining is the relatively low head loss (between
12 and 18 inches, with a 6-inch limit across a single screen). The efficiency of a microstrainer is determined
by the hydraulic and solids loadings as well as by the filtering characteristics of the influent. Microstrainers
will not remove colloidal material or small (micron size) algae. Microstrainers are also adversely affected by
fluctuations in influent composition and quality.
d. Hydraulic control. Hydraulic control of microscreening units is effected by varying the drum speed
in proportion to the differential head across the screen. The controller is commonly set to give a peripheral
drum speed of 15 feet per minute at 3 inches differential and 125 to 150 feet per minute at 6 inches. In
addition, backwash flow rate and pressure may be increased when the differential reaches a given level. The
operating drum submergence is related to the effluent water level and head loss through the fabric. The
minimum drum submergence value for a given installation is the level of liquid inside the drum when there
is no flow over the effluent weir. The maximum drum submergence is fixed by a bypass weir, which permits
flows in excess of unit capacity to be bypassed; at maximum submergence, the maximum drum differential
should never exceed 15 inches. Effluent and bypass weirs should be designed as follows:
-- Select drum submergence level (70 to 75 percent of drum diameter) for no flow over the effluent weir;
-- Locate top of effluent weir at selected submergence level;
-- Determine maximum flow rate;
-- Size effluent weir to limit liquid depth in effluent chamber above the weir to 3 inches at the maximum
flow rate;
-- Position the bypass weir 9 to 11 inches above effluent weir (3 inch head on effluent weir maximum flow
plus 6 to 8 inch differential on drum at maximum drum speed and maximum flow);
-- Size bypass weir length to prevent the level above effluent wire flow exceeding 12 to 18 inches at peak
maximum flow or overflowing the top of the backwash collection hopper.
15-6
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