Civil Engineering Reference
In-Depth Information
greater than about 500 mg / l. Because the upper limit of this solution strength should
not exceed 3,500 mg / l to prevent breakout of molecular chlorine at the point of ap-
plication, the effective range of chlorine dioxide production is about 7:1. Chemical
feed devices can handle ranges up to 20:1 on a flow proportional basis and 100:1 on
a compound loop control system.
Ferric Chloride
Ferric chloride is always fed as a liquid, and is normally obtained
in liquid form containing 20 to 45 percent FeCl
3
. When iron salts such as ferric
chloride are used for water coagulation in soft waters, a small amount of base (such
as sodium hydroxide or lime) is needed to neutralize the acidity of these strong acid
salts.
Dilution of ferric chloride solution from its shipping concentration to a weaker feed
solution should be avoided, because of a potential to hydrolyze. Ferric chloride solu-
tions may be transferred from underground storage to day tanks with rubber-line self-
priming centrifugal pumps having Teflon rotary and stationary seals. Because liquid
ferric chloride can stain or leave deposits, glass-tube rotameters are not used for me-
tering. Instead, rotary-wheel feeders and diaphragm metering pumps made of rubber-
lined steel and plastic are often used for feeding ferric chloride.
Ferric Sulfate
Feed solutions are usually made up at a water-to-chemical ratio of
2:1 to 8:1 (on a weight basis). The usual ratio is 4:1, and the feed solution is made
up in a 20-minute detention tank. Care must be taken not to dilute ferric sulfate
solutions to less than 1 percent, in order to prevent hydrolysis and deposition of ferric
hydroxide.
Dry feeding requirements are similar to those for dry alum except that belt-type
feeders are rarely used because of their open type of construction. Closed construction,
as found in the volumetric- and loss-in-weight-type feeders, generally exposes a min-
imum of operating components to the vapor, and thereby minimizes maintenance. A
water-jet vapor remover should be provided at the dissolver to protect both the ma-
chinery and the operator.
Ferrous Sulfate
The granular form of ferrous sulfate has the best feeding charac-
teristics, and gravimetric or volumetric feeding equipment may be used. The optimum
chemical-to-water ratio for continuous dissolving is 0.5 lb / gallon, or 6 percent, with
a detention time of 5 minutes in the dissolvers. Mechanical agitation should be pro-
vided in the dissolver to ensure complete solution.
Hydrogen Peroxide
Hydrogen peroxide is obtained from commercial suppliers and
is available in 35 percent, 50 percent, and 75 percent strengths. Hydrogen peroxide is
supplied in either drums or in bulk by tankcar. It can be stored on-site but deteriorates
rapidly if it is contaminated. Storage is typically in polyethylene drums or tanks.
Hydrogen peroxide is added to water using metering pumps, and the pump heads
should be constructed of hydrogen peroxide-resistant materials. It has a specific grav-
ity of 1.39 for 50 percent solution. Acceptable pipe materials include 316 stainless
steel, polyethylene, CPVC, and Teflon. Pipe gaskets should be Teflon.
Lime
Although lime comes in many forms, quicklime and hydrated lime are used
most often for water coagulation or softening. Quicklime is almost all calcium oxide
(70 to 96 percent CaO). High-calcium quicklime contains more than 88 percent CaO
