Page 263 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

(Al-Ani et al., 1986), which is rapid-mix and filtration. A flow required. The method is the same as with any experi-
bench scale filter was developed for such cases to perform a mental program: the effect of one variable is investigated
more rapid screening of chemicals and dosages as compared while the others are maintained constant. Other guidelines
to the use of a pilot plant (Brink et al., 1988). The filters used are given in Chapter 12.
were six, 51 mm (2 in.) diameter lucite or clear PVC columns
about 762 mm (30 in.) long, packed with media to be used in 9.9 POLYMERS
the filtration process (or 0.9 mm anthracite or 0.5 mm sand).
The jar test may proceed to the point of rapid-mix completion Synthetic organic polymers were considered in the 1950s (see
at which point a portion of the contents of the jar are poured Johnson, 1956) and after about 1960 were used increasingly
through the media at a slow rate. As with the standard jar test, in drinking water treatment, industrial wastewater treatment,
the effluent turbidities from the respective filters should be and for conditioning of chemical and biological sludges. In
plotted against the dosage of coagulant. The procedure was 1967, the first synthetic cationic polymer was accepted by
adapted as a proprietary apparatus, with an integrated jar- the USPHS for use in drinking water treatment (AWWA
tester, c. 2000, by Phipps & Bird, Inc. B451-87); over one thousand polymer products were
approved by the USEPA through 1985 (Dentel et al., 1986).
They are used by virtually every water and wastewater utility
9.8.3 PILOT PLANTS
and industrial treatment facility.
A pilot plant is a physical model of the system as a whole, that Usually polymers are selected based on manufacturer’s
is, rapid-mix, flocculation and settling (if used), and rapid recommendation, word-of-mouth, jar-test data, or pilot plant
filtration, and therefore is the most accurate means to assess results. Important characteristics include: molecular weight,
the effects of coagulation. The jar test and the pilot plant are charge density, and chemical structure of the monomer.
complementary tools for operation of the filtration process, Although these characteristics have not been cataloged, manu-
that is, the jar test for initial screening and the pilot plant for facturers may provide limited information about their poly-
final assessment and ‘‘fine tuning.’’ mers, for example, whether it is cationic, anionic, or nonionic,
and a category of molecular weight.
9.8.3.1 Independent Variables The selection of a polymer, its dosage, point of addition,
The key independent variable in coagulation is coagulant mixing intensity, etc. remains a trial and error as opposed to a
dosage, although others may be important, such as pH and rational procedure (Ghosh et al., 1985). The tools for empir-
alkalinity. In addition, polymer selection and dosage may be ical testing are the jar test and a pilot plant (Dentel et al.,
assessed as a primary coagulant, as a coagulant aid, as a 1986). Dependent variables in coagulation include: floc size
flocculent, or as a filter aid. Also, mixing variables (turbulence and shear strength. In filtration the dependent variables are
intensity, circulation pattern, detention time) are relevant to rate of headloss increase and effluent turbidity and whether
coagulation performance. At a Colorado State University the filter media becomes clogged.
(CSU) pilot plant (Figures 3.4 and 3.5), the mixing variables
were taken into account to some extent by inclusion of three
9.9.1 DEFINITIONS
alternative rapid-mix basins (different detention times and
each with variable impeller speed), any one of which could A polymer is defined as a repeating monomer; if formed by a
be included in the treatment system by opening and closing single monomer, it is called a homopolymer. If two or more
the appropriate ball valves. monomers are involved, the product is called a copolymer.A
polymer that has ionized sites along its length is called a
9.8.3.2 Dependent Variables polyelectrolyte. With respect to charge, a polymer without
Coagulation determines the potential for effective flocculation, charge is nonionic; with positive charged sites its cationic;
settling efficiency, filtration effluent turbidity, rate of headloss with negatively charged sites its anionic. A polyelectrolyte
increase, and the mass flux of solids as waste. All of these may function as a primary coagulant, that is, if it is added to
effects may be evaluated by means of a pilot plant. If filtration the rapid-mix instead of alum or ferric iron (as a rule, not done
is ‘‘conventional,’’ this may be associated with effective floc- due to cost, among other considerations). As used here, the
culation (e.g., large, dense, tough floc), which in turn leads to term polymer is used without necessarily specifying whether
efficient settling. The next in priority is to minimize the waste it is a polyelectrolyte; but it is usually implicit that a polyelec-
solids flux while not diminishing the other gains. Finally, trolyte is the context.
another goal of filtration is to minimize TOC, which may If a polyelectrolyte is added along with alum or ferric iron
require ‘‘enhanced’’ coagulation, that is, higher dosage. to the rapid-mix and if the polyelectrolyte provides sites for
microflocs (or floc particles), it functions as a coagulant aid;
9.8.3.3 Pilot Plant Design bridging between microflocs. A coagulant aid may help to
The pilot plant should be constructed to replicate the pro- tailor size, settling characteristics, and shear strength of the
cesses of the full-scale plant. Its scale should be large enough resulting floc. The purpose is to improve settling or filtration
such that scale effects are not an important concern, for performance and=or to reduce the dosage of alum or ferric
example, Q 38 L=min (10 gpm), albeit there is no specific iron (with perhaps less sludge and associated lower cost).