Page 136 - Water and wastewater engineering

INTAKE STRUCTURES 3-33

Heating the water at the intake port to 0.1 C also appears to work, but the energy requirements
are extremely large. Using smooth materials such as solid plastic and plastic-coated metal also
helps (Foellmi, 2005, Bosserrman et al., 2006). Wood is commonly used because of its low ther-
mal conductivity.
Anchor ice is differentiated from frazil ice in that it forms in sheets and grows by attachment
of frazil ice. The methods of control are the same as those for frazil ice.

Zebra Mussels
The zebra mussel ( Dreissena polymorpha ) is a small bivalve mollusk that has alternating dark
and light bands on its shell and averages about 2.5 cm in length. The mussel grows filamentous
threads that allow it to attach to hard surfaces. Zebra mussels reproduce prolifically and as a con-
sequence clog intakes and pumps. They are well established in the Great Lakes and are forecast
to infest all freshwater in two-thirds of the United States and all of southern Canada.
The control techniques currently in use are listed in Table 3-10 . In the Great Lakes area,
chlorine and potassium permanganate feed systems piped to the inlet port have been successful
in mitigating the problem. Intermittent chlorination is used to kill the juveniles (velligers) before
shells develop and attachment becomes virtually permanent. This dosing scheme minimizes the
use of chlorine. Periodic cleaning of the screens is required to remove adult and dead mussels as
the chemical treatment does not remove them. Care should be used in the application of chlorine
as the presence of naturally occurring organic matter may lead to the formation of trihalometh-
anes, a group of compounds that are regulated (see Chapter 2 for details).

Fish Protection
The need to provide a means to prevent fish from entering the intake structure is addressed by
one of three approaches:
• Physical barrier screens,
• Behavioral guidance systems, and
• Capture and release systems.

The highest standard of protection is for juvenile, endangered species. The National Oceanic and
Atmospheric Administration (NOAA) has developed guidelines for inlets to protect these fish.

TABLE 3-10
Zebra mussel control
Treatment technique Remarks
Thermal 35 C for 2 h
100% effective
Repeat 2 to 3 times per year
Chemical Oxidizing chemicals such as chlorine, potassium permanganate, and ozone
Continue for 2 or 3 weeks; continuous application may be required
Mechanical Shovel or scrape, high-pressure hose, sandblasting
Other Ultrasound, electrocution, oxygen depletion, UV light