Civil Engineering Reference
In-Depth Information
complete the reaction. The metal oxides are then removed from the filter by back-
washing. The free chlorine residual in the filter feedwater provides a continuous re-
generation of the media, allowing the process to work successfully.
Chlorine is an inexpensive chemical that is easily applied and provides the benefit
of primary disinfection; its use is, therefore, attractive for iron and manganese removal
systems. Care should be used in developing the proper dosing facilities for waters
containing ammonia or other nitrogenous compounds that interfere with the formation
of a free chlorine residual. The presence of TOC in the water, in addition to causing
complexing of the iron, can lead to objectionable levels of trihalomethane (THM)
formation with the use of chlorine and must be evaluated during the pilot study phase
of the project.
Chlorination or Chlorine Dioxide Oxidation, Precipitation, and Filtration
Chlorine dioxide is a powerful oxidizing agent that is used in the removal of iron and
manganese. It may be chosen over chlorine if trihalomethane formation is a problem.
Chlorine dioxide reacts to oxidize ferrous iron as illustrated in Table 14-3.
The oxidation of iron by chlorine dioxide takes place over the wide pH range of 4
to 10. At the optimum pH of 7.0, the reaction with iron is rapid. Organically complexed
iron is difficult to oxidize with chlorine dioxide.
Chlorine dioxide reacts more quickly with manganous compounds than does free
chlorine; therefore, it is more successful for manganese removal. The reaction of chlo-
rine dioxide and manganese is listed in Table 14-3. However, the high cost of chlorine
dioxide treatment limits its use to applications where manganese concentrations are
less than 1.0 mg / L.
Potassium Permanganate Oxidation, Precipitation, and Filtration
The use of potassium permanganate in iron and manganese removal systems is very
common throughout North America. KMnO
4
is simple to apply and regulate. Its use
for removal of both iron and manganese is attractive because reactions are rapid and
complete. The reactions and required dosages are illustrated in Table 14-3. Generally,
because of the catalytic effect of precipitated manganese dioxide on the filter medium,
less KMnO
4
is needed to complete the reactions than is theoretically required.
Iron oxidation by KMnO
4
is essentially instantaneous and requires no special pro-
visions for detention time. Sufficient time is provided in the piping and the filter above
the medium to allow the reaction to go to completion. The reaction of KMnO
4
in
oxidizing ferrous iron is further enhanced by the formation of MnO
2
on the medium,
which acts as a catalyst. However, because of its high cost and the availability of more
economical oxidants, KMnO
4
is not generally used solely for iron removal.
KMnO
4
is a popular means of oxidizing manganese because it is easy to use and
offers a fast reaction time. It will oxidize manganous ions to manganese dioxide rapidly
and over a wide pH range. The reaction time can be as short as 5 minutes,
7
and the
amount of KMnO
4
required for oxidation decreases with increasing pH. Generally, no
special provisions are provided for detention time for the reaction of KMnO
4
and
manganese. Sufficient time is provided in the piping and in the filter above the medium.
As the manganese is oxidized in the process and removed on the filter, manganese
oxides coat the medium and aid in manganese removal by chemical adsorption. This
phenomenon reduces the required KMnO
4
dosage.
