Page 131 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

86 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

TABLE 5.3 TABLE 5.4
Typical Design Data for Wedge-Wire Screens Microscreens—Sizes and Manufacturers
Dimensions Low Nominal High Density of Mesh
Opening Size
2
2
v(screen) a m=s 0.25 0.53 (Microns) (No.=in. ) (No.=cm ) Manufacturer
Velocity down m=s 2–3
23 144,000 22,320 Crane Co., King of Prussia,
screen face
Pennsylvania
F 2 30
25 Walker Equipment Co.,
Total head drop across M 1.2–1.5
Chicago, Illinois
structure
35 80,000 12,400 Crane Co., King of Prussia,
w mm 1.52
Pennsylvania
s b mm 0.3 1.0 13
35 120,000 18,600 Zurn Industries, Inc., Erie,
f 8 3 5 6
Pennsylvania
u 8 60
40 Walker Equipment Co.,
Chicago, Illinois
Sources: Wahl, T.L., Hydraulic testing of static self-cleaning inclined screens,
60 58,500 9,067 Crane Co., King of Prussia,
in: 1st International Conference on Water Resources Engineering,
Pennsylvania
American Society of Civil Engineering, San Antonio, TX, August
14–18, 1995, available from Web site: http:==www.usbr.
Source: Burns and Roe, Inc., Process Design Manual for Suspended Solids
gov=pmts=hydraulics_lab=twahl=coanda=; Wahl, T.L., J. Hydraul.
Removal, US Environmental Protection Agency, Washington, DC,
Eng. Am. Soc. Civil Eng., 127(6), June, 2001, available from Web
October, 1971.
site:http:==www.usbr.gov=pmts=hydraulics_lab=twahl=coanda=,16pp.
a
Calculated by upper and lower limits of experimental plot of Wahl (1995)
from which Equation 5.6 was derived.
b
Higher and lower values from Hydroscreen (2002).
removal in both water treatment and in effluent from wastewater
stabilizationponds.Inconventionalwastewatertreatmentmicro-
screens have been used following secondary treatment, espe-
5.5 MICROSCREENS
cially to help ensure that effluent standards are met.
Microscreens are a special category of fine screens
which have fabric openings of microns size. The removal
5.5.3 PERFORMANCE
mechanism is straining based upon the size of opening in
the fabric. But like many screens, the retained material The effectiveness of a microscreen in removal of solids
which forms a mat functions to strain particles, perhaps smal- depends upon the mesh size and on the material being removed
ler than the microscreen openings. that forms the filter mat. Removals of suspended solids and
Table 5.4 lists some examples of microfabrics which are BOD are given in Table 5.5 for tertiary treatment applications
commercially available. A variety of metals and plastic are for two mesh sizes (Burns and Roe, 1971). The BOD removals
used to make the fabrics; carbon steel is common (Burns and shown are associated with the suspended solids removal. The
Roe, 1971). mat of previously trapped solids provides a finer filtration or
straining capability; undoubtedly this accounts for the high
suspended-solids removal. Table 5.5 is indicative of perform-
5.5.1 EQUIPMENT AND INSTALLATION
ance for tertiary treatment. Removals, for a given mesh size and
A proprietary microstrainer is constructed with a fabric cov- suspension, can be assessed by bench scale tests.
ering a steel drum support frame, which rotates. In operation, In addition to effectiveness in removal of particles, perform-
raw water enters the interior of the drum and passes through ance is based also on operating factors such as hydraulic
the fabric with a loss of head; a weir at the end of the basin on loading rate (HLR), requirements for cleaning, and mainten-
the outer side maintains the effluent-side water level. The ance factors. An evaluation of such performance factors as
influent-side water level may rise to the level required in cleaning and maintenance must be done by means of records
order that the flow will pass, i.e., so that there is sufficient examination of full-scale installations. For HLR determination,
headloss. As the unit rotates, trapping suspended matter from pilot plant testing is recommended at the site of the installation.
the feed stream, the fabric is backwashed by a jet above the
drum on the outer side. The weave and shape of the individual 5.5.4 OPERATION
fabric wires permit the water from the jets to penetrate and
detach the solids mat, which forms on the inside of the screen. Some operating problems are, depending on the application,
screen clogging by slimes, iron or magnesium buildups,
and perhaps oil and grease (Burns and Roe, 1971). Units
5.5.2 APPLICATIONS
must be taken out of service on a regular basis for cleaning
Microscreenshavebeeninusesinceabouttheearly1950s(Burns when clogging occurs. Cleaning may be done by a chlorine
and Roe, 1971). Frequent applications have been for algae solution for slimes, and acid solution for iron or magnesium,