Oxidizing Filter Filtration
These units are usually installed as Point-or-Entry (POE) and are placed at
the incoming water supply.
These filters often referred to
as iron filters or red water filters and use a "greensand" resin bed (coated
with manganese) to oxidize iron and manganese that are in solution. Oxidizing
filters are most effective in water with a pH of 7 or above. If water is acidic
(pH below 7), a chemical feed pump may also be needed.
As the water flows through the resin bed, the
iron and manganese are oxidized and changed from their soluble to an insoluble
form. The media also acts as a filter and traps iron and
manganese precipitates that have been oxidized prior to reaching the filter.
When iron and manganese are
above 1 mg/L, or if the water is pH below 7 (acidic), a strong oxidizing substance
should be applied prior to greensand filtration.
Super chlorination is commonly used to oxidize iron and
manganese in order to adjust the pH. At a pH of 7 or above, iron changes more readily
from its soluble to its insoluble form, which can then be removed by filtration.
This process also kills any iron bacteria present, however, the excess chlorine
will need to be removed prior to use. Oxidizing Filters (greensand) is effective at filtering or
reducing:
- sulfur (hydrogen sulphide)
concentrations up to ~5 mg/L.
- combined iron/manganese
concentrations up to 10 mg/L.
Considerations: Greensand filter do not tolerate iron
bacteria because the slimy material produced fouls the greensand by
coating it. Consider testing before investing.
Oxidation occurs as water flows through the filter bed,
because much of the
precipitate is filtered out near the discharge side of the greensand bed. it
requires thorough backwashing. Insufficient backwashing can cause the precipitated iron and manganese
to be expelled
from the filter in a disgusting discharge from a faucet
or it can ruin a washer load of clothes. Consider maintenance.
The backwash requirement rate
for a greensand filter is considerably more than for a Ion Exchange water
softener due to the media weight and density. Consider whether the water
supply is sufficient to backwash the filter.
The condition of the water, the size
of the unit, and the amount of water consumed all affect how quickly the
oxygen is depleted. The oxygen oxidizing filters must be regenerated with a solution of potassium
permanganate. As intimated above, backwashing these filters takes a
considerable amount of water. Consider ongoing costs and disposal of the
discharge water.
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