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11.16 CHAPTER ELEVEN

mum percentage of solids of 1% by weight is recommended by the Recommended Stan-
dards for Water Works (commonly called the 10 State Standards).
Solids contact units offer a smaller footprint because of the combined, higher-rate pro-
cess. They also provide a high degree of residuals recirculation and contact to complete
the softening reaction and are typically used instead of conventional basins for softening.
However, solids contact units can require greater operator attention to monitor and con-
trol the solids inventory in the basin, and these units are more prone to upset from vary-
ing inlet water temperatures, flows, and turbidity conditions.
Helical flow solids contact units are an alternative to conventional solids contact units.
Helical flow solids contact units are typically constructed of steel and have an inverted
conical shape. These units are designed to induce a circular flow pattern within the clar-
ifier, with the incoming water flowing upward through a blanket of previously formed
precipitates. Lime (and soda ash if necessary) is added to the incoming water at the base
of the clarifier. The conical shape of the clarifier and the helical flow pattern provide the
required influent/solids mixing and contact without the need for mechanical mixing equip-
ment within the clarifier.

Recarbonation Basins. For single-stage softening processes, a recarbonation basin with
a contact time of at least 20 min is typically provided to stabilize water before filtration.
The contact basin should be designed for plug flow, and mixing should be provided ahead
of the basin to introduce carbon dioxide.

SPECIAL LIME SOFTENING DESIGN CONSIDERATIONS

Many special design and operations and maintenance considerations are associated with
lime softening processes, including problems with encrustation and removing, handling,
and disposal of the large quantities of residuals resulting from treatment processes.

Encrustation Problems

A significant problem in operations and maintenance of lime softening processes is en-
crustation of calcium carbonate on the process and hydraulic components. Encrustation
can occur anywhere but is generally greater and harder near application points of lime
and soda ash and where water velocity is increased, such as on mixing equipment. En-
crustation can also occur in flumes and pipelines after lime or carbon dioxide addition or
where split-treatment flow is introduced before a secondary process.
The burden of designing for encrustation on equipment is usually placed on the basin
equipment manufacturer. An allowance should also be made in designing pipelines and
flumes, with additional hydraulic capacity and provisions for ease of cleaning. Open flumes
are generally preferred over pipelines for this reason. Calcium carbonate buildup may be
removed with high-pressure jet equipment or by mechanical means. Basins and pipelines
are usually cleaned once per year during seasonal low-demand periods. If the plant has
no low-demand periods, additional treatment capacity in the form of more treatment units
will have to be provided.
Recirculating residuals to the point of addition of lime or, for second-stage processes,
to the point of addition of carbon dioxide or split-treatment flow provides nuclei for cal-
cium carbonate deposition and may reduce encrustation problems.
Additional information on sludge thickening, dewatering, and handling is provided in
Chapter 17.