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LIME SOPTENING 11.19

let reactors should not be considered for systems high in magnesium content because of
magnesium hydroxide fouling of the reactor.
Pellet reactor systems should be designed cautiously. Where the application is not
proved, pilot testing is advisable. Design should be carefully coordinated with the equip-
ment manufacturer.

Chemical Feed and Layout Considerations

The design of lime softening plants is influenced, to a large degree, by the need to handle
and feed large quantities of lime. Lime solution readily encrusts solution pumps, pipelines,
and troughs and presents major maintenance considerations. Pumping lime solutions should
be avoided if possible; gravity flow in open troughs or hoses readily accessible for cleaning
are preferable. Soda ash feed systems also have encrustation problems.
The lime and soda ash feed systems should be located as close to the point of feed as
possible. On the other hand, feed systems for these chemicals, particularly lime, require
frequent attention and should be located as close as practical to the operator's station. Be-
cause of these factors, it is generally preferable to bring the water to be treated as close
as practicable to central chemical feed facilities readily accessible to the operator.
Provisions should be made to feed alum or ferric coagulants and coagulant polymer
at the mixing facilities ahead of softening basins. It is often necessary to use coagulants
to enhance clarification within softening units. Provisions should also be made to feed
polyphosphate and filter aid polymer ahead of the filters.

FUTURE TRENDS IN SOFTENING
With the technological advances in membrane process manufacturing, it is anticipated that
lime softening may gradually be replaced by membrane processes for some applications. This
option appears to be particularly applicable for smaller installations where the potential for
reduced operator attention and remote monitoring of membrane processes has a significant
cost impact. In addition, membrane processes produce residuals containing only the con-
stituents removed from the source water, which may make membranes more attractive to reg-
ulatory agencies where residuals disposal to source waters is a consideration. Membrane pro-
cesses may also remove regulated contaminants in conjunction with softening.
Lime softening uses large amounts of chemicals (primarily lime) and produces large
amounts of residuals in comparison to coagulation treatment only. Because of the cost of
lime, other chemicals, and residuals disposal problems, some utilities have considered re-
ducing the degree of hardness removal This idea has raised the question as to what de-
gree of hardness removal for a particular utility is beneficial and cost-effective to the con-
sumers. Greater attention to this area is expected in the future.

BIBLIOGRAPHY

Aieta, E. M., E. J. Singley, A. R. Trussell, K. W. Thorbjarnarson, and M. J. McGuire. "Radionu-
clides in Drinking Water: An Overview." Journal AWWA 79(4):144, 1987.
American Water Works Association Trace Inorganic Substances Research Committee. "A Review
of Solid-Solution Interactions and Implications for the Control of Trace Inorganic Materials in Wa-
ter Treatment." Journal AWWA 80(10):56, 1988.