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11.18 CHAPTER ELEVEN

where Mg is magnesium in milligrams per liter as magnesium (calcium carbonate equiv-
alent divided by 4.1). Impurities in lime are also a source of residuals produced in the
softening process. The amount can be estimated as follows:
Dry weight lime impurities (lb/mil gal) = lime dose (lb/mg)
× (100 - percent purity)/100 (11.24)
Total dry weight solids of softening residuals for the excess lime treatment process are
usually about 2.5 times the hardness removed, expressed in milligrams per liter or pounds
per milligram. The straight lime process produces total dry weight solids for softening
residuals approaching 2.0 times the hardness removed.
Residuals are also produced by source water turbidity removed in the process, by
the precipitation of iron and manganese that may be removed in the process, and by co-
agulants and powdered activated carbon that may be used in the process. Guidelines for
determining the quantities of residuals produced by these items may be found in
Chapter 17.

Filtration
The design of filters for softening plants is similar to that for conventional clarification
plants. Many modem softening plants use dual-media filters with bed depths of 36 (0.91
m) or more, although other types of media have also been used successfully. Design load-
ing rates typically range from 3 to 5 gpm/ft 2 (7.3 to 12.2 m/h). Available head should be
8 ft (2.4 m) or more at design rate, measured from the water surface in the filter to the
filter effluent line as it leaves the filter.
Particulate loading on softening plant filters consists, to a large extent, of fine calcium
carbonate that makes softening plant filters behave somewhat differently than coagulation
plant filters. Softening plant filters are generally characterized by long filter runs, on the
order of 36 to 48 h or more, and are often washed based on time rather than head loss or
turbidity breakthrough. However, effluent turbidity must be carefully monitored and con-
trolled because it is not unusual for the fine calcium carbonate to pass through the filter.
Filter aid polymer should be provided to assist in controlling turbidity breakthrough.
Water is typically under a calcium carbonate plating condition before filtration to main-
tain stability and to reduce corrosion in the distribution system. Therefore a polyphos-
phate is usually fed before the filters at a dosage of 0.25 to 1.0 mg/L to control encrus-
tation of the filter media. Annual inspection of the media for cementation and inspection
of the underdrain every 4 to 5 years through careful excavation of the media may also be
performed to monitor encmstation.
Surface wash or air-water backwash is essential to assist filter backwashing. The sup-
plemental agitation breaks up cementation of filter media and helps remove solids in the
filter. The design of backwash systems is similar to that for conventional plants.

Pellet Reactors
Pellet reactors have been used for softening in the Netherlands for many years and have
been installed at a number of locations in North America. A pellet reactor consists of an
inverted conical tank where calcium carbonate crystallizes on a suspended bed of fine
sand.
Advantages of the pellet reactor are its small size and low installation cost. Residuals
consist of small pellets that dewater readily, minimizing residuals volume. However, pel-