Science of Water : Concepts and Applications

Water Treatment 301
packing materials in an aeration tower. The more fi nely divided the drops of water, the
more oxygen comes in contact with the water and the dissolved iron and manganese.
Chlorine—This is one of the most popular oxidants for iron and manganese control because
it is also widely used as a disinfectant; iron and manganese control by prechlorination can
be as simple as adding a new chlorine feed point in a facility already feeding chlorine.
It also provides a predisinfecting step that can help control bacterial growth through the
rest of the treatment system. The downside to chorine use, however, is that when chlorine
reacts with the organic materials found in surface water and some groundwaters, it forms
TTHMs. This process also requires the pH of the water to be in the range 6.5–7; because
many groundwaters are more acidic than this, pH adjustment with lime, soda ash, or caustic
soda may be necessary when oxidizing with chlorine.
Potassium permanganate—This is the best oxidizing chemical to use for manganese control
removal. As an extremely strong oxidant, it has the additional benefi t of producing manganese
dioxide during the oxidation reaction. Manganese dioxide acts as an adsorbent
for soluble manganese ions. This attraction for soluble manganese provides removal to
extremely low levels.
The oxidized compounds form precipitates that are removed by a fi lter. Note that suffi cient time
should be allowed from the addition of the oxidant to the fi ltration step. Otherwise, the oxidation
process will be completed after fi ltration, creating insoluble iron and manganese precipitates in the
distribution system.
Ion Exchange
While the ion exchange process is used mostly to soften hard waters, it will also remove soluble
iron and manganese. The water passes through a bed of resin that adsorbs undesirable ions from the
water, replacing them with less troublesome ions. When the resin has given up all its donor ions, it
is regenerated with strong salt brine (sodium chloride); the sodium ions from the brine replace the
adsorbed ions and restore the ion exchange capabilities.
Sequestering
Sequestering or stabilization may be used when the water contains mainly low concentration of
iron, and the volumes needed are relatively small. This process does not actually remove the iron or
manganese from the water but complexes (binds it chemically) it with other ions in a soluble form
that is not likely to come out of solution (i.e., not likely oxidized).
Aeration
The primary physical process uses air to oxidize the iron and manganese. The water is either pumped
up into the air or allowed to fall over an aeration device. The air oxidizes the iron and manganese
that is then removed by using a fi lter. The addition of lime to raise the pH is often added to the process.
While this is called a physical process, removal is accomplished by chemical oxidation.
Potassium Permanganate Oxidation and Manganese Greensand
The continuous regeneration potassium greensand fi lter process is another commonly used fi ltration
technique for iron and manganese control. Manganese greensand is a mineral (gluconite) that has
been treated with alternating solutions manganous chloride and potassium permanganate.
The result is a sand-like (zeolite) material coated with a layer of manganese dioxide—an
adsorbent for soluble iron and manganese. Manganese greensand has the ability to capture (adsorb)
soluble iron and manganese that may have escaped oxidation as well as the capability of physically
fi ltering out the particles of oxidized iron and manganese. Manganese greensand fi lters are generally
set up as pressure fi lters, totally enclosed tanks containing the greensand.
The process of adsorbing soluble iron and manganese “uses up” the greensand by converting
the manganese dioxide coating into manganic oxide, which does not have the adsorption property.