Civil Engineering Reference
In-Depth Information
that the final water quality will protect the health of the consumer even with mechan-
ical failures. The chemical feeders and pumps should operate at no lower than 20
percent of the feed capacity. The chemical feed and handling design also should take
into account future expansions of the plant.
The materials chosen for conveyance, storage, and measurement of a chemical must
be compatible with the properties of that chemical. Corrosive chemicals must be han-
dled in such a manner as to minimize potential for corrosion.
The conveyance of chemicals from unloading to the application point must be done
in separate conduits for each chemical. Slurry-type chemicals, especially lime, should
be fed by gravity where practical. If at all possible, locate pneumatic conveyance
tubing for chemicals vertically, because horizontal tubing frequently plugs.
Liquid, Solid, or Gaseous Chemical Form
Once a chemical has been selected, it is necessary to decide whether to feed it in a
liquid, solid, or gaseous form. Some chemicals are available only in one form; how-
ever, many of the chemicals, as shown in Table 24-1, are available in a variety of
forms.
There are certain advantages to using the liquid form of chemicals. Liquid chem-
icals are generally easier to handle during loading, unloading, and feeding. The liquid
form also eliminates the dust problem associated with the use of solids. Because the
liquid form can be directly fed to the process, the feeding equipment is simplified, as
mixers and dissolvers associated with dry chemicals are not needed. Some of the
disadvantages of the liquid form are the additional cost of hauling the extra water and
the additional storage space required.
The solid form of chemicals generally requires less storage space, may be less
expensive, and provides a greater selection of chemicals. The disadvantages of using
a solid chemical include: dusting problems, caking and lumping problems during stor-
age and feeding, additional labor, and the need for more equipment to dissolve and
feed the chemical.
Chemicals fed in the gaseous form are not so widely available as those fed in the
liquid or solid form. The most common gaseous chemicals are chlorine, sulfur dioxide,
carbon dioxide, oxygen, and anhydrous ammonia. Ozone and chlorine dioxide are fed
as gases but must be generated on-site. When chemicals are available in the gaseous
form, that may simplify the feeding and control of the chemical as wall as its storage
and handling. With increased safety concerns and resulting regulations and code re-
quirements, means to contain, and treat, a gaseous chemical release must be provided.
Even with these facilities, some utilities are electing to feed liquid chemicals.
The selection of the chemical form will be dictated by such factors as cost, local
availability, method of feed or control anticipated, compatibility with the existing fa-
cilities, and the quantity of the chemical required. The larger the quantity used, the
more favorable the solid form of most chemicals becomes. The solid form may be the
more economical when large quantities are used because of the high cost of liquid
storage. Also, the cost of transporting the water associated with the liquid form be-
comes a more significant portion of the costs. However, although a liquid chemical
may require a greater storage volume and have higher freight costs than the solid, it
may still be more economical when used in small quantities, because the feeding
equipment is less complex.
