Page 707 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

662 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

forming ‘‘insoluble sticky curds that tend to adhere to all Water Supply in an 1869 report, softening was not adopted.
things with which they come in contact.’’ Hard water also By 1900, only a few softening plants had been built in England.
causes scale, which reduces the effectiveness of pipes and About that time the residual calcium carbonate precipitant, that
boilers with ancillary economic damage. Hard water also is, after softening, was found to cause incrustation of filter
has an economic loss in that larger quantities of soap are sand, and clogging of service pipes and meters. This was
required for laundry purposes and clothes have shorter life. remedied by ‘‘re-carbonation,’’ that is, dissolving carbon diox-
ide to convert such residual CaCO 3 to Ca 2þ and HCO 3 , which

21.1.2.2 Toxic Metals Removal became common practice in the United States by about 1921.
The toxic ‘‘heavy’’ metals include arsenic, cadmium, chro- A 30 mgd plant was put on line in 1908 in Columbus, Ohio,
mium, lead, mercury, and a few others. They are found in which was the largest softening plant in the United States. The
waste flows from various industries and in drainage from plant was designed by George A. Johnson, then with Hering &
mines. In addition, zinc and copper are considered contamin- Fuller (Rudolph Hering and George W. Fuller). Just before
ants when found in such waste streams. rapid filtration, carbon dioxide gas was added to neutralize
Some examples of heavy metal sources include metal pro- excess lime, that is, converting carbonates to bicarbonates.
cessing and refining, metal plating, petroleum refining, chlor- Other cities that employed lime softening included New Orleans
oalkali production, battery manufacturing, steel production, in 1909, Cincinnati in 1938, Minneapolis in 1939, and St. Paul
pigment manufacturing, tanning, anodizing, photographic film in 1940 (Baker, 1948, p. 432). By the 1930s the softening
manufacturing, automotive production, etc. (Banerjee, 2002, process employed rapid mix (baffled mixing was common),
p. 181). Removals of metals are mostly by precipitation and paddle-wheel mixing, sedimentation, re-carbonation, and filtra-
ion exchange; other processes include oxidation=precipitation, tion. A similar plant was described by Gelston (1934) for
reduction=precipitation, coagulation=coprecipitation (Patterson, Quincy, Illinois to reduce hardness of water withdrawn from
1990, p. 28). To remove metal ions from solution requires the Mississippi River (typically 150 mg=L as CaCO 3 total
anions that when combined with a particular heavy metal cation, hardness). The clarifier was proprietary, that is, Dorrt,circular
have a low solubility product. The most common anions for type, 24 m (80 ft) diameter, with center feed and sludge removal
chemical precipitation are hydroxides, mostly because lime, at the bottom, later called a ‘‘reactor-clarifier.’’ Iron removal is a
the source, is cheaper than other chemicals, and because the by-product of softening and may be achieved by first oxidizing
solubility products, with most metals, are low. ferrous ion (common in ground water) to the ferric state
(Walker, 1934, p. 79).
21.1.3 HISTORY
21.1.3.2 Sewage Treatment
The traditional application of chemical precipitation as a unit The beginning of sewage treatment was in England after
process has been the softening of municipal water supplies, about the year 1850. Prior to this, the streams of England
with removal of heavy metals emerging as a concern during were regarded as public dumps for ashes, cinders, demolition
the 1970s. An application, not too well known after about wastes, dead animals, etc. The Second Royal Commission of
1900, was that, in England, precipitation was considered as a River Pollution, appointed in 1868, in its report on methods of
promising technology for treatment of municipal wastewaters. treatment in 1870, reviewed three principal methods: irriga-
This can be seen by adding alum to raw sewage; a visible tion, filtration, and chemical precipitation. Its fifth report in
heavy precipitate forms readily and settles leaving a clarified 1873 was on river pollution by mining and metal industries.
supernatant. The commission formulated standards of purity for British
rivers by forbidding discharge of various substances. A Sec-
21.1.3.1 Softening ond Royal Commission of Sewage Disposal was appointed in
The need for softening of ‘‘hard’’ water because of excessive 1898 (the first was 1857). Its fifth report in 1908 summarized
soap consumption was known in the 1730s in England. The the state-of-the-art in sewage treatment, which included sedi-
terms, ‘‘soft’’ water and ‘‘hard’’ water were common and the mentation, chemical precipitation, contact beds, trickling fil-
‘‘alkaline salts’’ were used to remove hardness (Baker, 1948, ters, and land treatment (Metcalf and Eddy, 1916).
pp. 415–420). The use of lime, that is, CaO, in softening, was The chemical precipitation method was well established by
described in 1841 by Thomas Clark, Professor of Chemistry, 1870. Lime was used in most cases but CaCl 2 , MgCl 2 , alum,
Aberdeen University, Scotland. Clark patented a method to and other chemicals were used also. Patented processes were
remove hardness, which involved the use of lime, followed by numerous. The effluent was reported as ‘‘clear and of good
‘‘subsidence.’’ quality.’’ The idea of chemical precipitation of treating sew-
The first attempted use of lime softening for a municipal age after primary settling was explored again in the late 1960s
water supply was in 1854 by the Plumstead, Woolrich & without reference to the early history of the process.
Charlton Consumer’s Water Co., to compete with the Kent
Waterworks Co., each operating in the London metropolitan 21.1.3.3 Heavy Metals
region. Both companies used wells as a source (Baker, 1948, Until about 1970, the main emphasis of wastewater treatment
p. 422). Although endorsed by the Royal Commission on was to reduce the biochemical oxygen demand (BOD) load on