Page 706 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

21 Precipitation

Chemical precipitation has been applied traditionally to soft- 21.1.1.3 Process Description
ening of domestic water. Since removal of heavy metals The precipitation process has several steps between the ions in
became a major concern, during and after the 1970s, precipi- solution and the solid precipitate being settled and removed.
tation has been a ‘‘workhorse’’ process in the treatment of The steps are as follows:
industrial wastes and mine drainage waters. This practice has
evolved to a large extent by empirical designs and operation, 1. (a) Introduce an anion that will form a precipitate
with theory providing a rationale, that is, to explain rather with the target cation (or that will cause a change in
than to lead. Nevertheless, theory provides the foundation for water chemistry, for example, to form carbonates or
modern practice. hydroxides that have an affinity for the target cation).
(b) Introduce a cation that will remove the target
anion.
21.1 DESCRIPTION
2. Cause sufficient mixing, for example, by rapid mix,
In water treatment, precipitation is the addition of a chemical to to cause a high rate of contacts between the anions
cause an ion, usually a cation, to be removed as a solid. The and the target cations in order to effect formation of
‘‘target’’ ions are those that may impair the use of water. precipitant crystals.
Cations that are most likely to be an issue include calcium, 3. Apply moderate turbulence, for example, by paddle-
magnesium, iron, manganese, and various heavy metals. Other wheel flocculation, to cause floc formation and growth
cations may cause problems, depending on the situation and or, alternatively, cause flow through a floc blanket of
the concentration. Fewer anions are amenable to precipitation. previously formed floc to cause floc growth.
Those that are amenable include phosphates, silica, sulfides, 4. Permit settling by traditional continuous flow basin,
and cyanide. Nitrate, a ‘‘contaminant’’ in drinking water, is not by tube settlers, or by plate settlers with vacuum
amenable to removal by precipitation; neither are the innocu- pickup of sludge or scraping into a pocket.
ous anions such as chloride and sulfate. The exact nature of 5. Apply filtration to remove residual floc particles.
þ 6. Dewater sludge and transport to disposal site.
silica is complex. For example, SiO 2 , SiO , SiOH 2 are
included in the silica group by Letterman et al. (1999, p. 6.9). 7. Solids disposal=recovery may be reclamation, dis-
posal in a municipal landfill, or disposal in a secured
Stumm (1992, p. 175) gives a dissolution reaction as, SiO 2 (s) þ
H 2 O ! H 4 SiO 4 (ag). The latter is orthosilicic acid, which may landfill. The latter, used for heavy metals and toxic
dissociate to H 3 SiO 4 (and there are other derivatives). wastes, are usually constructed for a hypothetical

1000-year life (based upon EPA regulations).
21.1.1 PRECIPITATION IN-A-NUTSHELL Alternatively, the entire process may be done in a ‘‘reactor-
clarifier,’’ a single unit, that has a center-well chemical feed
Chemical precipitation occurs when the solubility product is
and mixing, a sludge blanket flocculation, an upflow settling.
exceeded for the anions and cations of any given salt. The
Units of this type are probably most common for small muni-
ions comprising the salt are thus removed from solution.
cipalities, industrial wastes, and mining.
21.1.1.1 Definitions
Precipitation has not evolved to become a specialized field in 21.1.2 APPLICATIONS
which a unique vernacular, for example, acronyms and terms, Chemical precipitation is used commonly for softening of
has evolved. Many of the terms come from the field of chem- municipal or industrial water supplies, for removal of toxic
istry, however, such that glossary may be useful for reference. metals (heavy metals) from mining wastes or industrial waste-
waters, or for removal of certain anions, for example, phos-
21.1.1.2 Comparison with Other Processes phates, silica, or cyanide.
Along with oxidation, precipitation involves a chemical reac-
tion as the central issue of the process. It is characterized by 21.1.2.1 Softening
the formation of a solid precipitate that is settled and=or As described by Walker (1934, p. 77), the calcium and
filtered. magnesium comprising ‘‘hard’’ water combines with soap,

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