Page 269 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

polymer (Mangravite, 1983, p. 5). Solid polymers require from the container is associated with 1000 mL of raw water,
feeding equipment first to wet the particles and then to agitate that is, 10 mg product=L raw water.
them in solution for a specified period of time, with subse-
quent transfer to a holding tank. The solid form contains 9.9.5.6.2 Total Solids
75%–95% polymer, with the remainder being moisture or The ‘‘total solids’’ basis means that the fraction of solids in
salts such as sodium carbonate or sodium sulfate. the product is determined. For example, suppose the fraction
of total solids in the product is 0.40, and a 10 mg total
9.9.5.3 Specification Sheets
solids=L solution is desired. Since 400,000 mg solids is asso-
Each manufactured product has a specification sheet that
ciated with 1000 g product, then 10 mg solids is associated
states such items as principal uses, typical properties (physical 6 6
with x g product. Then, x ¼ [10 =(0.4 10 )] 10 ¼ 25 mg
form, ionic nature, density, viscosity, charge, freezing point),
product. Therefore, 25 mg product is metered into 1000 mL
preparation and feeding, typical dosages, materials for storage
raw water.
and feed, handling and storage, safety information, shipping
information. These are available from manufacturer’s repre-
9.9.5.6.3 Active Polymer
sentatives, brochures, or Web sites. Some manufacturers
The ‘‘active polymer’’ basis means that the fraction of
state the kind of structure used, for example, polyacrylamide,
solids in the product is determined. For example, suppose
polyamine, etc.
the active polymer is 0.50 and the total solids in the product
9.9.5.4 Prepared Batches is 0.40, and a 10 mg active polymer=L solution is desired.
Since 0.50 400,000 ¼ 200,000 mg active polymer is associ-
Prepared batches of polymer are used normally within 24–48 h
ated with 1000 g product, then 10 mg solids is associated
to ensure use before loss of activity. Stock solutions are made 6 6
with x g product. Then, x ¼ [10 =(0.2 10 )] 10 ¼ 50 mg
usually with 0.1%–0.5% (1000–5000 mg=L), as a compromise
product. Therefore, 50 mg product is metered into 1000 mL
between storage volume, batch life, and viscosity. Polymers
raw water.
are considered more effective when fed as dilute solutions
because they are easier to disperse (i.e., to distribute uni- 9.9.5.6.4 Discussion
formly). Typically, the feed strength is between 0.01%
The ‘‘as sold’’ basis is most common convention for stating
and 0.05% (100–1000 mg polymer=L solution), but should be
concentration since it is straightforward and easy to deter-
based on the recommendations of the manufacturer.
mine. In expressing concentration, the basis should be speci-
9.9.5.5 Feed of Polymer fied, for example, 10 mg product=L raw water.
In feeding low=medium molecular weight polymers, the neat
solution, as provided by the manufacturer, may be pumped to
Example 9.8 Metering a Neat Polymer Solution
a dilution line, preferably just prior to a static mixer, and then
pumped (by metering pump) to the point of application.
Given
Alternatively, the neat solution may be pumped to intermedi- As an example of a concentration calculation, suppose
ate batch mixing and then to the point of application. The that the flow of raw water is 1.0 m =s (22.8 mgd)
3
concentration at the point of application may be 1%–5% and that the concentration of polymer is to be 0.40 mg=L
(10,000–50,000 mg polymer=L solution). Mixing at the on an ‘‘as sold’’ basis. The polymer density (as supplied) ¼
point of application is the next step, which is critical to 1.064 kg polymer emulsion=L emulsion.
effective coagulation. Example 9.8 illustrates how to calculate
Required
the metering rate of a polymer, given the concentration Determine the rate of feed flow of neat polymer solution.
desired and the raw water flow. An emulsion polymer requires
Solution
intermediate mixing before being metered into the raw water
1. Specification sheet data. Suppose that the polymer
flow. Example 9.9 shows how such a polymer is prepared
is Clarifloc A-210P. The specification sheet dated
prior to metering.
1991 (polydyneinc.com) states the polymer has a
medium charge, is a polyacrylamide, is in emulsion
9.9.5.6 Concentration: Convention (Adapted
form, and is approved by the National Sanitation
from AWWA B453-96)
Foundation (NSF) for clarification of potable water
In feeding low=medium molecular weight polymers, the poly- at dosages 1.0 mg=L, with density ¼ 8.88 lb=gal
mer may be fed as neat solution. In designating the concen- (1.064 kg=L).
tration of a polymer, three possible forms are as follows: (1) 2. Calculation
as-sold, (2) total solids, and (3) active polymer (AWWA,
Mass flux of polymer in raw water flow
1996, p. 11).
¼ Q C(polymer)
9.9.5.6.1 As Sold
¼ 1000 L=s 0:40 mg=L
The ‘‘as sold’’ basis means as it comes out of the container.
For example, a 10 mg=L solution means that 10 mg of product ¼ 400 mg polymer=s