Page 138 - Water and wastewater engineering

INTAKE STRUCTURES 3-35

Hints from the Field. Operation and maintenance personnel who have to live with the results
of the engineer’s design have offered the following suggestions:
• The overhead crane supplied to facilitate maintenance of the pumps and motors should also
provide a means to move the equipment outside of the building to facilitate replacement
of equipment that has reached its life expectancy. Alternatively, a hatch in the roof with a
means of positioning the equipment to be removed below the hatch should be provided. In
the second instance, a crane can be rented to remove the old equipment and transfer the new
equipment into the building. The hatch should be sized to consider the likelihood that the
replacement equipment will be larger than the equipment being removed.

• For zebra mussel control, a chlorine feed line should be supplied to the screen/bar rack of
the intake. The feed line is commonly located inside the conduit carrying water from the
intake structure to the low-lift pump station. Sodium hypochlorite solution is used to sup-
ply the chlorine. Care should be taken in selecting the pipe material and joining cements
because sodium hypochlorite may adversely affect the material and/or the cement. This
includes polyvinyl chloride (PVC) cements.
• In hard water, with high concentrations of calcium and magnesium carbonate, the addition
of commercial sodium hypochlorite may cause precipitation of carbonates on the interior
of the pipe. Operators call this “plating out” of the hardness. Over time this will limit the
capacity of the supply pipe. The addition of sodium hexametaphosphate to the hypochlorite
feed will help to mitigate the precipitation.

• The conduit from the intake to the low-lift pump station should be inspected regularly (pref-
erably annually). For large conduits (1.5 m or large in diameter) a diver is employed to
“swim” the conduit and make the inspection. For small pipes, a TV camera inspection may
be used. The inspection should include such items as structural integrity of the pipe, build-
up of zebra mussels and slime layers, and the condition of the chlorine supply line (both the
pipe and the hangers).
• Because the air supply used by divers will be released inside of the conduit during the inspec-
tion, some means must be provided to release the gas; otherwise the gas “bubble” that accu-
mulates in the pipe will limit the flow rate. This is particularly true if the discharge into the wet
well is through a downward facing bell. One solution is to provide a small (5 cm diameter) tap
at the terminus to the pipe just above the bell and use it to release the accumulated air.

Visit the text website at www.mhprofessional.com/wwe for supplementary materials
and a gallery of photos.

3-6 CHAPTER REVIEW

When you have completed studying this chapter, you should be able to do the following without
the aid of your textbook or notes:

1. For a client, sketch a diagram of a lake intake structure and a river intake structure.
Identify the following features: bar screen, sluice gate, fine screen, pump, operating
floor, water level in surface water supply relative to the intake structure vertical profile.
2. List and discuss the key requirements of intake structures.