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MASTER PLANNING AND TREATMENT PROCESS SELECTION 2.9

TREATMENT OPTIONS

As illustrated in Table 2.4, a summary of more common drinking water treatment pro-
cesses, there are many treatment options available to the designer to achieve the desired
water quality results.

Principal Process Alternatives and Issues

Where pertinent, detailed unit process tests and comparisons should be conducted under
the guidelines of the companion volume to this text, Water Quality and Treatment, to de-
termine the more effective options. Bench and pilot-scale treatment operations would also
be conducted in the water quality and treatment phase to determine the more critical pro-
cess design and operating parameters.
Under the current regulations, some form of filtration must be provided for effective
particulate solids removal. In general, filtration would be required for all source water
supplies except perhaps for some groundwater sources. The particular type of filtration
required would depend upon the particulate solids loading contributed by that in the source
water in addition to that which may be generated in treatment such as in precipitation (i.e.,
cold lime softening, manganese dioxide) and coagulation. Filtration types and solids load-
ing capabilities may be categorized in general terms as follows:

• Direct, slow sand, and diatomaceous earth filtration may be feasible for waters low in
turbidity (5 ntu or less) and organic matter. Potential supply sources must also be sta-
ble, of high quality, and not subject to significant algal blooms or other major water
quality changes.
• Conventional clarification and rapid sand filter plants would be needed for source wa-
ters higher in turbidity and organic matter content, and those where higher coagulant
dosages may be required.
• The limitations of membrane filtration are more difficult to define. Closed-vessel ap-
plications would more likely be limited to source waters of higher quality. Immersed
membranes can handle higher solids loading including that generated in precipitation
and coagulation to improve solids removal.
In considering conventional clarification/filtration, the type of clarification selected
would also be influenced by the type of source water solids. Where suspended and/or dis-
solved organic matter predominates, high-rate clarifiers including dissolved air flotation
could be the more effective and more economic application. Where seasonal algal blooms
occur, dissolved-air flotation may be the preferred alternative. Plain settling with greater
detention would be desired where more profuse, denser solids predominate, such as in
many river sources.
With few exceptions, most U.S. public surface water supplies are currently provided
treatment the preponderance of which is some form of filtration. For this reason, a major
water treatment plant design effort now and in the future will be devoted to retrofit rather
than new design. New designs for the most part will be for new area development, ma-
jor capacity expansion, or changing rules, such as for groundwater.
Another feature of water treatment plant process selection and design that must be ob-
served is potential regulatory conflict. Scrutiny of Tables 2A and 2.3 will show that there
are two groups of rules that currently apply or soon will apply to all public drinking wa-
ter treatment. These are the control of microbiological contaminants and the formation of