Water Softening - Public Water Systems - page 424

Mg Magnesium hardness removed, mg / L as CaCO 3
Alum Alum dose, mg / L
Fe Iron dose, mg / L
SS Suspended solids in the influent water, mg / L
P Polymer (or other coagulant aid) dose, mg / L
S Sludge produced, lb dry solids per day
Lime Recalcining and Reuse Lime recalcining has been used to some extent in
water works practice. Long used in industrial applications, it has also been employed
in water and wastewater treatment facilities that use lime treatment. The recalcin-
ing process is simple, and consists of heating the calcium carbonate sludge to about
1,850 F (1,000 C). This drives off water and carbon dioxide, leaving only the calcium
oxide (quicklime) plus some inert material. The off-gases can be used in recarbonation.
Recalcining can be done by multiple-hearth furnace, fluid-bed rectors, or rotary kilns.
Rotary kilns are used only in very large recalcining operations. Before sludge is re-
calcined, some pretreatment is required to reduce the water content. This normally
includes gravity thickening and dewatering by centrifuge, filter press, or some other
means that will produce a cake of satisfactory dryness.
Buildup of inerts can reduce the available lime in the recycled product to a point
where it is no longer useful as a softening chemical. To minimize this problem, some
lime sludge may be wasted periodically. Removal of lighter solids such as metal hy-
droxides prior to recalcining will also help. This can be done by pretreating the sludge
prior to dewatering with a centrifuge designed for classification (rather than concen-
tration), which will separate the lighter solids from the sludge stream.
In plants treating waters high in calcium hardness, an excess of recalcined lime
may be produced. In small water treatment plants (5 to 10 mgd), the cost of reclaimed
lime may not be less than the cost of new lime, but when the costs associated with
sludge disposal are included in the calculation, recalcining may be economical. As
sludge disposal costs rise, recalcining may become an attractive option.
UNIT PROCESS DESIGN CRITERIA FOR LIME SOFTENING
Mixing, Flocculation, and Sedimentation
Two types of softening process trains are described below: conventional mixing, floc-
culation sedimentation; and solids contact. In addition, laboratory requirements are
presented.
In lime-soda softening the applicable design standards are similar to those for
conventional clarification. These criteria are outlined below, and are described in more
detail in other chapters.
Mixing
Rapid Mixing Rapid mixing provides the rapid dispersal of chemicals throughout
the water to be treated, by violent agitation. Mechanical mixers, hydraulic jump, and
Parshall flume are recommended methods. Baffled mixing chambers are acceptable
only when provisions are made for proper mixing under anticipated variations in flow.