Page 717 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

Q(avg) 2.2 and Q(min)=Q(avg) 0.35. Neglect vari-
BOX 21.1 DESIGN
ation in water quality.
In reviewing the chemistry of water treatment, Moffett Required
(1968) summarized the overall state of the art. Many of Determine the volume required for an equalization stor-
his comments are pertinent to understanding contem- age basin such that the average flow enters the treatment
porary design and operation, with two of the most train; let Q(max) occur at t 9 days; let Q(min) occur at
pertinent abstracted: t 21 days, with the variation being a continuous func-
‘‘For many years, both the design and the operation tion (appearing approximately sinusoidal).
of water purification plants were considered to be arts.
Hint: Construct a mass flow diagram and solve for DV
Little attention was given to the chemistry of water
(equalization) graphically as the sum of the maximum
treatment and to the effects of chemical variables upon
differences between the cumulative actual flow above
the engineering design or operation of a plant. Design
and below the cumulative average flow, that is, the sum
had focused on steel and concrete. The sizing of plant
of the maximum ordinate differences between the two.
facilities was based on millions of gallons of water to be
21.3 Cooling Basin Salt Accumulation
treated per day and detention time.
Approximately ten years ago, there appeared to be an Given
awakening to the fact that water treatment should be a A clay-lined, that is, essentially zero seepage, cooling
scientific discipline, . . . . In the last 5 years, the number basin for a coal-fired power plant with a volume,
3
and competency of published articles has increased until V ¼ 12,300 m (10 acre-ft). To avoid applying for
now we can delineate the chemical and engineering a national pollution discharge elimination system
variables in the process design and the operation of a (NPDES) permit, the management elected to have zero
water plant. Today, no water plant should be built discharge. The evaporation rate is about 250 mm=year.
without a complete process study prior to physical The incoming water, mostly snowmelt, has total dis-
design’’ (Moffett, 1968, p. 1255). solved solids concentration, TDS 50 mg=L.
Required
Estimate the number of years before TDS 1000 mg=L.
21.3.3.2 Cyanide Precipitation Hint: Do an annual mass balance, that is, Q C ¼ V dC=
dt and t(years) ¼ 1000=(dC=dt).
Removals of cyanides, free and complexed, at concentrations
about 30 mg=L total cyanide were investigated with respect to
mining wastes. In the first step, hydrogen peroxide and
ACKNOWLEDGMENT
sodium thiosulfate are added at 7 < pH < 9 in a 1:2 molar
ratio to convert free and weakly complexed cyanide to non- John H. Smith III, president, Sepco, Inc., Fort Collins, Color-
toxic thiocyanate. Then steryldimethylbenzylammonium ado, provided the photograph of the settling tank for heavy
chloride is added to precipitate ferro-cyanide and, finally, metals removal (Figure 21.3) and the material on the case
ferric sulfate is added as a sweep floc and sequestrant for study.
heavy metals. Final effluent concentrations were 0.02
mg=L (Schiller, 1983).
GLOSSARY
Amorphous solid: A solid that lacks molecular level order
PROBLEMS
(Silberberg, 1996, p. 428).
21.1 Precipitation Reactions Anion: Ion with one or more negative charges, for example,
Cl ,SO 4 .
2

Given
Cation: Ion with one or more positive charges, for example,
Phosphate in wastewater is considered a nutrient pollu-
Na ,Ca .
2þ
þ
tant and is removed by lime addition. Assume the con-
Colligative properties: Of the four important solution prop-
3þ
centration of phosphate as PO 4 is 30 mg=L, that is,
erties as affected by solutes, it is the number of solute
particles that makes the difference, not their chem-
3þ ¼ 30 mg=L
C PO 4
ical identity. The properties are vapor pressure low-
ering, boiling point elevation, freezing point
Required
depression, and osmotic pressure.
Determine the concentration of Ca 2þ needed to precipi-
Complex: A central metal cation bonded to molecules and=or
tate the phosphate.
anions called ligands. To maintain electroneutrality,
21.2 Equalization Basins the complex ion is typically associated with simple
Given ions, called counterions, for example, for the coord-
3
The flow from a mine with Q(avg) 0.32 m =s (5000 ination compound, [Co(NH 3 ) 6 ]Cl 3 , the complex ion
gpm) may vary from month to month with Q(max)= is [Co(NH 3 ) 6 ] , the three Cl ions are counterions,

3þ