Science of Water : Concepts and Applications

Water Treatment 295
water before it enters the plant. River water, for example, typically contains suspended and fl oating
debris varying in size from small rags to logs. Removing these solids is important not only because
these items have no place in potable water, but also because this river trash may cause damage
to downstream equipment (clogging and damaging pumps, etc.), increase chemical requirements,
impede hydraulic fl ow in open channels or pipes, or hinder the treatment process. The most important
criteria used in the selection of a particular screening system for water treatment technology
are the screen opening size and fl ow rate. Other important criteria include costs related to operation
and equipment; plant hydraulics; debris handling requirements; and operator qualifi cations and
availability.
Large surface water treatment plants may employ a variety of screening devices including trash
screens (or trash rakes), traveling water screens, drum screens, bar screens, or passive screens.
CHEMICAL ADDITION
Two of the major chemical pretreatment processes used in treating water for potable use are iron
and manganese and hardness removal. Another chemical treatment process that is not necessarily
part of the pretreatment process, but is also discussed in this section, is corrosion control. Corrosion
prevention is accomplished through chemical treatment—not only in the treatment process but also
in the distribution process. Before discussing each of these treatment methods in detail, however, it
is important to describe chemical addition, chemical feeders, and chemical feeder calibration.
When chemicals are used in the pretreatment process, they must be the proper ones, fed in the
proper concentration and introduced to the water at the proper locations. Determining the proper
amount of chemical to be used is accomplished by testing. The operator must test the raw water
periodically to determine if the chemical dosage should be adjusted. For surface supplies, this
checking must be done more frequently than for groundwater (remember, surface water supplies
are subject to change on short notice, while groundwaters generally remain stable). The operator
must be aware of the potential for interactions between various chemicals and how to determine the
optimum dosage (e.g., adding both chlorine and activated carbon at the same point will minimize
the effectiveness of both processes, as the adsorptive power of the carbon will be used to remove
the chlorine from the water).
√ Note: Sometimes using too many chemicals can be worse than not using enough.
Prechlorination (distinguished from chlorination used in disinfection at the end of treatment)
is often used as an oxidant to help with the removal of iron and manganese. However, currently,
concern for systems that prechlorinate is prevalent because of the potential for the formation of
total trihalomethanes (TTHMs), which form as a by-product of the reaction between chlorine and
naturally occurring compounds in raw water.
√ Note: TTHMs, such as chloroform, are known or suspected to be carcinogenic and are limited
by water and state regulations.
The USEPA’s TTHM standard does not apply to water systems that serve less than 10,000
people, but operators should be aware of the impact and causes of TTHMs. Chlorine dosage or
application point may be changed to reduce problems with TTHMs.
√ Note: To be effective, pretreatment chemicals must be thoroughly mixed with the water. Shortcircuiting
or slug fl ows of chemicals that do not come in contact with most of the water will not
provide proper treatment.
All chemicals intended for use in drinking water must meet certain standards. Thus, when
ordering water treatment chemicals, the operator must be assured that chemicals meet all appropriate
standards for drinking water use.