Page 337 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

added before flocculation is a ‘‘flocculent’’; often the literature 11.3 HISTORY
does not make this distinction, calling both ‘‘flocculents.’’
In The Quest for Pure Water, M.N. Baker (1949, p. 299)
noted that coagulation as an aid for the clarification of house-
11.2 APPLICATIONS
hold water supplies has been practiced since ancient times. At
Flocculation is applied to a variety of applications in water large industrial plants, coagulation was established sometime
treatment. Common ones are mentioned here. before 1830 (p. 304). Then in 1885, the Hyatt brothers, in
developing their proprietary ‘‘mechanical’’ filters, found that
the addition of alum improved performance; this was the
11.2.1 CONVENTIONAL FILTRATION
beginning of coagulation and the subsequent flocculation
Flocculation is a part of the treatment train in conventional in water treatment practice.
filtration, i.e., rapid mix, flocculation, settling, filtration. The
objective is to cause floc growth to a size such that a high
11.3.1 PRACTICE
settling velocity results.
The role of alum in causing removal of colloidal particles was
documented in 1869 by a commission in the Netherlands.
11.2.2 DIRECT FILTRATION
They described a ‘‘flocculent precipitate’’ which took up the
Flocculation is a part of the treatment train in direct filtration,
turbidity of water and left it perfectly clear (Baker, 1949,
i.e., rapid mix, flocculation, filtration. The objective is to
p. 308).
create a floc size that will penetrate the filter media, such as
a pinpoint floc. In general, the floc size should be less than 11.3.1.1 Quiescent Basins
what is visible, e.g., 30 mm, as in the case of water from
Initially, alum was used in fill-and-draw basins prior to filtra-
Deer Creek Reservoir in Utah for which Gu 42,000 was
tion. Such a basin can be seen at the Fort Collins Water
satisfactory (Treweek, 1979, p. 100). Higher Gu values
Treatment Plant #1 in the Cache La Poudre River Canyon
resulted in larger settleable flocs; the latter were not suitable
(see Figure 10.1). Alum was added to the raw water at a
for direct filtration since they did not penetrate the filter bed.
‘‘chemical house’’; the treated water then flowed by open
channel to an open basin that provided about 24 h detention
11.2.3 FLOTATION time. There was no designed ‘‘agitation’’; the coagulation
occurred passively in the open channel and the flocculation
In flotation, flocculation follows coagulation. The floc size is
was in the basin itself. Brownian motion and differential
important; Edzwald (1995, p. 12) suggested that 10 d(floc)
settling were the mechanisms for contacts. The basin was
100 mm, with median size 40 mm, the smaller sizes being more
built probably between 1910 and 1925 (the filters were con-
desired. There are no specificguidelines for G and detention
structed about 1910). In some cases, in the 1900s, coagulation
time, but Edzwald (1995, p. 16) found that for South African
practice, the range was 50 G 120 s 1 and 4 u 15 min. and flocculation were the only treatments with the latter
occurring in ‘‘settling reservoirs.’’ Examples included
Omaha, Nebraska in 1889; Chester, Pennsylvania in 1901;
11.2.4 ACTIVATED SLUDGE FLOC SETTLING Kansas City, Missouri in 1902; Nashville, Tennessee in 1908
(Baker, 1949, p. 311). In such cases, flocculation was a
The floc that forms in an activated sludge reactor should settle
‘‘passive’’ process.
readily. Practice has found that such floc needs no induced
flocculation and is ready to settle when it reaches the final
11.3.1.2 Langelier’s Paddle Wheels
settling basin, depending on the design of the basin (Section
In 1916, at the invitation of Professor Charles Gilman Hyde,
6.6.7). Filamentous floc, on the other hand, does not settle
Professor Wilfred F. Langelier moved from the Illinois State
readily, and is an issue in operation.
Water Survey as a staff chemist to the University of
California, Department of Civil Engineering. Professor Hyde
11.2.5 SOFTENING
was active as a consultant and in 1919 he asked Professor
In ‘‘softening,’’ i.e., the removal of calcium and magnesium Langelier to review preliminary drawings for a water filtration
hardness by lime precipitation, a solid precipitate is formed of plant for the City of Sacramento and suggest changes, with
Ca(OH) 2 or Mg(OH) 2 . The precipitate grows in size to form a particular attention to coagulation with alum prior to filtration
floc that will settle. (Chall, 1970, p. 23). Prior to this time, Professor Langelier
had observed that unsatisfactory performance of small filtra-
tion plants was traceable usually to poor coagulation. On one
11.2.6 TERTIARY TREATMENT
occasion, for example, he had collected a sample of clear filter
Removal of orthophosphate from secondary municipal effluent and observed clouding after stirring with a pencil.
wastewater effluent is formed by precipitation with Ca . Also, alum sludge was found in the water mains in large
2þ
The precipitate, Ca 3 (PO 4 ) 2 , is the basis for a floc that grows quantities, and ‘‘it was obvious that either insufficient mixing
in size for subsequent settling. or insufficient time had elapsed between the addition of