Civil Engineering Reference
In-Depth Information
a viable population of zebra mussels are: freshwater, a satisfactory temperature regime,
adequate pH, sufficient calcium levels, suitable substrate, suitable flow conditions and
an adequate food supply. Of these, temperature, calcium, and pH may be limiting in
some areas in the eastern United States.
Zebra mussels are small, often less than 1 inch in length, and can colonize virtually
any hard, nontoxic surface. All surfaces contacted by raw water up to the point of
primary disinfection or sand filtration are at risk. Structures typically fouled in water
treatment facilities include the intake crib, trashracks, the main intake line, the screen
wells, traveling screens, and any strainers and metering equipment that may exist.
11
They also attach to other zebra mussels creating dense colonies that may be 0.5 meters
thick.
The primary impact is reduction of flow. Even before colonization is enough to
appreciably reduce the inside pipe diameter, the increased roughness coefficient can
reduce flow. Complete, immediate blockages can result from the detachment of mussel
clumps that travel down the piping system until they reach a restriction they cannot
pass through.
Another indirect impact of zebra mussels is increased microbially induced corrosion
of metal pipes that results from the anaerobic conditions formed at the byssal attach-
ment. Zebra mussels can also increase taste and odor problems.
Methods for controlling zebra mussels are discussed in Chapter 16, ''Oxidation.''
Pump Discharge
Wash
Water
Discharge
Backwash Inlet
Bar Rack
1-2-In.
Spacing
Outer
Casing
Inner
Casing
Wet
Well
Pump
Column
Sectional
Plan of
Intake Cowl
Well
Screen
Fig. 8-6.
Intake with vertical pump and backwashed well-type screen (From Fair, Gordon M.,
Geyer, John C., and Okun, Daniel A.,
Water and Wastewater Engineering,
Vol. 1. Copyright
1966 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)
