Civil Engineering Reference
In-Depth Information
could result in undersized pumps and other chemical handling or storage facilities.
Under the right conditions, iron can become complexed with organic material and
silica. Complexed iron is much more difficult to remove than simple dissolved iron in
that it is resistant to oxidation and is not readily removed by filtration. Iron will
complex with TOC material in the water at a rate of 0.1 mg / L of Fe for every 1.0
mg / L of TOC in the water;
this
will make iron removal very difficult without a
coagulation step prior to filtration. Oxidant alone will usually not remove complexed
iron.
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No significant organic complexation of manganese has been reported in the research
literature. As far as the oxidation of manganese in the presence of dissolved organic
carbon (DOC) is concerned, the main issue is to provide sufficient oxidant chemical
to complete the oxidation reaction. The TOC in the water will impart an oxidant
demand on the water that must be satisfied in addition to the oxidant demand of the
manganese.
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Raw water quality must be adequately characterized during the predesign
and pilot study phases for manganese treatment facilities to be properly designed and
constructed.
TOC has been shown to foul oxide-coated media. Therefore, the presence of TOC
in a raw-water source will negatively impact the ability of oxide-coated or catalytic
media to work successfully. The surfaces of the oxide-coated or catalytic media grains
must be clean to work properly. They must have direct contact with the water in order
to adsorb the metals and remove them from the water. The TOC in the water will coat
the medium, fouling it and preventing it from working properly. Simple routine back-
washing will not generally remove this coating, and special cleaning techniques may
be required to restore the medium's effectiveness. In extreme cases, the medium will
be fouled beyond repair and will have to be removed and replaced. When the use of
an oxide-coated or catalytic medium is being considered, the presence or absence of
TOC in the raw water must be known.
In systems using ozone as the oxidant for iron and manganese removal, the presence
of TOC in the water can result in significant bacterial regrowth in the system. It has
been widely reported in the research literature that ozonating organic material in water
will increase the biologically assimilable organic carbon (AOC). This will result in
increased biological activity in the system and potentially the buildup of biological
slimes in the filters and the distribution system. Disinfectant demand will increase in
these systems because of this biological activity. The control of biological slime growth
in the distribution system is discussed in Chapter 22, ''Water Quality Control in Dis-
tribution Systems.'' Periodically disinfecting the filter media and backwashing will help
control growths in the treatment system. Careful consideration is required when pro-
posing the use of ozone for iron and manganese oxidation in the presence of TOC.
Water quality analyses, as well as evaluation and pilot studies, should be conducted
to develop design criteria for each of the proposed systems. The following sections
outline and describe many of the systems that are typically used to treat iron and
manganese in drinking water.
STABILIZATION OF SEQUESTERING METHODS
A low-cost alternative to removing iron and manganese is to hold the metals in solution
by stabilization or sequestering. Sequestration has several advantages over conventional
treatment:
