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4.30 CHAPTER FOUR

previous installation in the same area. During the study phase, geotechnical data permit
a more accurate comparison of the cost of viable alternatives to identify the most eco-
nomical solution for the project. Geotechnical investigations should cover a sufficient area
to account for adjustments in the intake location during final design. The cost of a few
additional borings during the initial study is much less than the cost of mobilizing a barge
for additional borings, should they be deemed necessary later in the design. Provisions
should also be included in construction specifications for diver inspections to verify that
the subgrades are clean of loose and soft material before the placing of tremie concrete.

Mechanical Considerations
The mechanical design for an intake structure includes many of the routine considerations
found in other, similar structures. Special mechanical design considerations for intakes
include adequate provisions for ventilation, insects, and flooding impacts.
Ventilation systems must be designed to accommodate heat generated by electric mo-
tors, motor control centers, and solar gain. Ventilation systems must also consider odor-
ous gases that may be present, especially if screening material is temporarily stored in the
building. With the trend toward more chemical treatment at the intake, ventilation must
provide airflow required by applicable building codes for each stored chemical system.
Ventilation of below-grade spaces that may occasionally be occupied for inspection or
maintenance should also be provided.
Insects can be a serious problem at some intake sites, and insect screens on the air in-
take and exhaust louvers are one solution. However, large quantities of insects can block
these screens, requiring increased maintenance by facility personnel. Another solution is
to provide air filters with a large surface area on air handling units. Facility personnel
may prefer the ease of changing a filter to the difficulty of cleaning louver screens.
Flooding conditions can affect the design of sanitary and storm drainage systems. If
flooding is expected, backwater or knife gate valves should be provided.

Chemical Treatment Considerations

The intake facility provides the first opportunity in a water treatment system to impact,
remove, or alter contaminants in the source water before subsequent treatment. A num-
ber of chemical treatment choices that may be provided at the intake should be consid-
ered during the intake facilities design (Table 4.13).

TABLE 4.13 Chemical Treatment Alternatives
Contaminant or criterion Suggested treatment chemical
Debris and screenings No treatment; discharge to water source or landfill
Taste and odor Chlorine
Chlorine dioxide
Potassium permanganate
Carbon
Coagulation Alum or ferric chloride
Cationic polymer
Zebra mussels Chlorine and other oxidants