Civil Engineering Reference
In-Depth Information
Algal Control
Algal control in reservoirs is usually accomplished by addition of
copper sulfate to the water. This topic is discussed in Chapter 8.
Many chemicals are not compatible with each other, and great care must be taken
to avoid the possibility of them coming into contact with each other. When chemicals
are not compatible, they should be separated either by locating them in another build-
ing, or at least in separate containment areas. Table 24-3 shows frequently used chem-
icals and their compatibility with one another.
Raw-Water Quality
There are many considerations in selecting the specific chemical to be used in a water
treatment process, one important item being the raw-water quality. The raw water must
first be analyzed to determine the level and type of treatment required. It is helpful to
consider the effectiveness of each chemical under consideration, based on experience
and operating results from full-scale plants at other locations with similar raw-water
quality. However, caution should be exercised when using data from other plants. The
designer should not assume that a particular chemical used at one plant will produce
identical results at a different plant. The effectiveness of each chemical varies in dif-
ferent applications, depending upon operating conditions. Laboratory and / or pilot tests
can be used to predict the effectiveness of a chemical for a given application. The
objectives of laboratory studies usually are: (1) to determine what chemical dosages
are needed to obtain the desired results, and (2) to obtain data for the design and
operation of a pilot or full-scale facility. After the chemicals that perform unsatisfac-
torily are eliminated, a preliminary cost comparison can be made for the remaining
chemicals to determine which should receive further consideration. Also, it should be
recognized that laboratory and pilot tests do not always accurately predict plant-scale
dosages and performance.
Availability and Cost
Other important considerations when selecting a chemical are its availability in a par-
ticular region, reliability of supply, and cost. There is little advantage in selecting a
chemical that satisfies all the requirements of a water treatment process if the chemical
is not readily available. Capital costs for handling and feeding various chemicals vary
considerably, depending on the characteristics of the chemical to be fed, the form
(liquid, solid, gas) in which the chemical is purchased, and the form in which the
chemical ultimately is used in the treatment process.
Transportation is a significant cost for some locations. The cost at the point of
origin usually is quoted by the manufacturer in cents or dollars per pound, per 100
pounds, or per ton, and varies according to the size of the order. It may be a price
''f.o.b. cars'' at the point of manufacture or at a regional stock point. When small lots
are purchased, the f.o.b. point is important because the manufacturer ships to the
regional stock point in bulk and at lower rates in order to give the customer the benefit
of this savings. The point of shipment origin should always be clearly stated, because
transportation costs on some chemicals may be more than the cost of the chemical,
especially if long hauls are involved.
Many manufacturers quote prices ''f.o.b.'' from a distribution point but also will
give the customer information on the expected cost of transportation by rail or truck
to the point of usage. Sometimes manufacturers will also quote ''freight allowed,''
