Civil Engineering Reference
In-Depth Information
ability to hold the metals on them, thus removing the metals from the water with no
chemical addition. Catalytic media speed the oxidation reaction to completion. Several
different types of adsorptive and catalytic media have been developed and are com-
monly used in iron and manganese removal plants. Generally, all of these media use
some form of manganese dioxide ore or coating, and the chemistry of removal is
similar. Many of the process mechanisms are the same for each medium described. If
a mechanism applies to one medium, it more than likely applies to them all and should
be considered during process evaluation. These special materials will be discussed in
detail in the following paragraphs.
A drawback to using these special media is that they are generally susceptible to
fouling when the TOC concentration is greater than 1.0 mg / L. The presence of TOC
in water may also be an indication that a portion of the iron is organically complexed.
Adsorptive or catalytic media are ineffective in helping to remove organically com-
plexed iron. Even in the absence of organically complexed metals, as the TOC levels
increase in the feedwater, there is a tendency for the organic material to coat the
medium, seal off the adsorptive sites, and significantly reduce or even eliminate the
medium's ability to function. In the presence of significant TOC concentrations, the
medium must continually be cleaned with a strong oxidizing agent or even a detergent.
For some oxide-coated media, this organic fouling can be permanent and would ulti-
mately require that the medium be replaced to restore its adsorptive capacities. Use of
adsorptive or catalytic media in the presence of TOC greater than 1.0 mg / L is not
recommended.
Manganese Dioxide Ore
The ability of manganese dioxide ore to aid in the removal
of iron and manganese from water has been known since the 1920s. In the 1930s,
naturally occurring manganese dioxide ore (known as pyrolusite) was substituted for
the filter media in several water treatment plants in an attempt to take advantage of
this ability. These attempts were generally not successful because of problems in ex-
panding the media during backwashing and effectively cleaning the bed. Manganese
dioxide ore is very heavy, weighing in excess of 120 lb / ft
3
(1,900 kg / m
3
), and requires
significantly higher backwash rates than more conventional filter media for effective
cleaning of the filter bed. The filter beds where pyrolusite was tried did not have the
hydraulic capacity to allow proper backwashing of the filters and media cleaning. As
a consequence, the media soon fouled and became ineffective.
11
Small package treatment systems of proper hydraulic design have successfully used
manganese dioxide ore media for the removal of iron and manganese. The backwash
systems in these units are designed for the media and are capable of very high back-
wash flow rates of 25 to 30 gpm / ft
2
(61 to 73 m / h). This high backwash flow rate
will expand and clean the media and remove the oxidized metals. However, the in-
herent problems associated with these high backwash rates, as well as the resulting
volumes of backwash water, have prevented the use of MnO
2
ore media for large-scale
systems.
Manganese dioxide ore will act as a catalyst in the oxidation of iron. The iron is
adsorbed onto the surface of the media and permits the oxidant to operate more ef-
fectively and efficiently. Since iron oxidation is relatively fast in the presence of com-
mon oxidants at neutral pH, manganese dioxide ore is not considered when iron is
found alone. If manganese dioxide ore is installed to aid in the oxidation of dissolved
manganese, it will provide a mutual benefit in the oxidation of the soluble iron in the
water.
