Civil Engineering Reference
In-Depth Information
Two types of iron are found in water supplies, and methods for their removal are
quite different. Inorganic iron is generally associated with groundwaters low in dis-
solved oxygen. It will oxidize readily to its insoluble form and form a turbid water
that can be filtered to remove the iron. The second type of iron is organically com-
plexed iron, which may be found in both groundwaters and surface waters. The for-
mation of organic complexes and chelates may increase the solubility of iron in some
waters. Organically complexed iron can be significantly more difficult to oxidize than
inorganic iron and may require special treatment considerations. At pH values en-
countered in natural waters, it is possible that organically bound iron will be insoluble
but highly dispersed. This condition may require a coagulation step to adequately
remove the complexed iron. Natural color found in water is frequently due to such
highly stabilized colloidal dispersions of Fe(II).
Mn(IV) does not readily form complexes with organic or inorganic ligands in water
and, therefore, does not have the same treatment problems associated with organically
complexed iron. Inorganic manganese is readily oxidized and removed by filtration.
The sources and occurrence of iron and manganese are discussed in Chapter 2,
''Inorganic and Radionuclide Contaminants.''
FACTORS INFLUENCING THE TREATMENT OF IRON AND MANGANESE
It has been reported that, according to surveys, only 50 to 60 percent of the iron and
manganese removal plants in the United States and 67 percent of those in Europe
consistently meet the minimum standards for finished water iron and manganese con-
centrations.
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Several factors were identified as to the cause of this poor performance,
including:
Iron complexation by silica and humic substances
•
Too low oxidation pH
•
Negative effect of chlorination
•
Flocculation problems with the iron and manganese floc
•
Too large effective media size
•
Lack of analytical data at the time of design
•
Deterioration in raw water quality
•
Interference by nitrification
•
Insufficient detention time for metal oxidation
•
Unoptimized chemical dosing locations
•
All of these factors should be considered during pilot testing, design, and operation
of an iron and manganese removal plant to minimize operational problems. A thor-
ough, well-designed pilot study prior to design and construction is essential to the
success of a treatment system. A pilot column in an operating plant can also be used
to evaluate and optimize the process and provide the utility with valuable process
information.
Of the reasons listed for poor plant performance, the presence of total organic
carbon (TOC) in the water can provide some of the most significant treatment chal-
lenges. TOC will exert an oxidant demand in the system, making the required chemical
dosages higher than might be expected. This condition, if unknown during design,
