Civil Engineering Reference
In-Depth Information
Inexpensive to install
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Suitable for wellhead treatment
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Operator friendly
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Produces no sludge that would require handling and disposal
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Iron and manganese must be in the ionic state for this process to be effective. Gen-
erally, sequestering is appropriate for groundwaters containing sufficient carbon di-
oxide to ensure that iron and manganese will be present as bicarbonates. The
sequestering agent is added directly into the water as the water is pumped from the
well. Commonly used sequestering agents include sodium hexametaphosphate, tri-
sodiumphosphate, and sodium silicates. The addition of sequestering agents, especially
phosphates, can provide a source of nutrients and, therefore, promote bacterial re-
growth in the system. To minimize this possibility, a disinfectant residual must be
maintained in the system. If the disinfectant has oxidation capabilities (i.e., chlorine),
it must be added after the sequesterant to avoid oxidation of the metals.
Sequestering should be considered only for waters in which the sum of the iron
and manganese is less than 1.0 mg / L. Sodium silicates, while effective in sequestering
iron, have not been shown to be effective for manganese sequestering.
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Required chem-
ical dosages can vary significantly from product to product and from water to water.
Polyphosphate dosages should be limited to less than 10 mg / L, because phosphorus
can stimulate biological regrowth in the distribution system. Calcium hardness has
been found to interfere with treatment by both polyphosphates and orthophosphates.
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Given the variability of the chemicals and their effectiveness, it is advisable to pilot-
test several chemicals prior to investing in a sequestering system. Features of the
sequestering process (as well as other iron and manganese removal processes) are
described in Table 14-1; advantages and disadvantages are listed in Table 14-2.
Following the addition of a polyphosphate, an adequate disinfectant residual must
be maintained to control bacterial slime regrowth in the distribution system. The phos-
phates in the system can serve as a nutrient source for bacteria. Failure to control the
biological slimes could result in Total Coliform Rule (TCR) testing problems for the
utility. For best results, the sequestering agent is added first and the disinfectant second.
The reaction of the sequestering agent with the metals is very rapid, and it is recom-
mended that the disinfectant be added within a few seconds of adding the sequesterant.
Sequestering agents are generally effective in cold water systems; when the water is
heated or boiled, polyphosphates lose their dispersing properties and the metal may
oxidize and come out of solution. Sequestering agents are typically effective for only
48 to 72 hours. Systems that have large storage reservoirs with long detention times
or distribution systems with long dead-end pipelines where water could sit for signif-
icant periods of time may have problems using sequestering agents. When the se-
questering agents cease to be effective, the iron and manganese could oxidize in the
distribution system, leading to complaints of rusty or black water and staining from
consumers. Sequestering agents also have a limited effective shelf life. As the agents
age, they lose their effectiveness and will require higher dosages. The utility must
monitor the system to ensure continued long-term success of the sequestering feed
system.
Sequestering is an attractive treatment process because of its low capital costs and
simple operation. All it requires is a chemical feed pump and a dosing location. For
small community systems, seasonal systems, or wellhead treatment, sequestering may
