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Other Treatment Methods

Bacteriostatic Filters
KDF Filters
Oxidizing Filters
Acid-neutralizing Filters
Ion Exchange Applied to the Removal of Other Ions
Aeration
Flocculation and Sedimentation

Bacteriostatic Filters
Bacteriostatic filters are activated carbon filters that also contain silver particles to help control bacterial growth inside the filter. However, their effectiveness is controversial. Silver may help contain but not necessarily reduce bacterial growth in activated carbon filters. The National Sanitation Foundation (NSF) lists and certifies some filter devices (and manufacturers) with “bacteriostatic effects.” However, their efficiency at controlling bacteria in tap water is not stated.

KDF Filters
KDF (redox) filters are a new type of home water filtration device that may work as intended to reduce already low levels of bacteria, chlorine, some metals, and some types of organic pollutants from water. The effectiveness of this type of filter is also controversial. The NSF lists KDF filter media in its website. These filters should not be used for any other reason than to (possibly) improve water aesthetics (control taste, odors, or residual chlorine).

Oxidizing Filters
Also known as iron filters, oxidizing filters reduce both ferric (yellow cloudy) and ferrous (green clear) iron, manganese, and hydrogen sulfide gas from well waters by oxidizing ferrous iron witih manganese green sand, converting it to the suspended ferric state, so that it can be filtered out. These filters require periodic backwashing to flush particulates and restore flow and regeneration with potassium permanganate to restore oxidizing properties. They should not be used on water supplies that have a pH of 6.8 or less, sulphur in excess of 2.0 ppm, or iron exceeding 10 ppm.

Acid-neutralizing Filters
Acid-neutralizing filters are used to reduce water acidity. They add crushed calcite or some other carbonate-based mineral at controlled rates to raise the water's pH and decrease corrosivity. The water to be treated must be low in tannins and free of oil. The filter media must be replaced periodically. Backwashing is recommended to remove trapped particles and oxidized metals unless a sediment filter is installed ahead of the unit.

 

Laboratory grade water deionizer system that uses four mix bed resins, and activated carbon and particle filters.

photo by: Janick Artiola

click to enlarge

Ion Exchange Applied to the Removal of Other Ions
Organic resins can also be used to remove from water any type of ion besides calcium and magnesium (see also section on Ion Exchange). Ion exchange resins are commonly used as POU treatment devices to produce ultra-pure (near completely demineralized) water in commercial and industrial laboratories. Usually, a water source with a very low TDS (less than 5 mg/L) is used. This usually requires pretreatment of the water source using an RO system. Typically, a series of mixed bed [anionic (-) and cationic(+)] resins followed by activated carbon filtration (packed in cartridges) are used to “polish” the water to strict purity standards. However this approach is not practical, cost-effective, or even necessary for home water treatment.

Ion Exchange Considerations
• Mixed bed resins are quickly exhausted when tap water is used because ions like sodium, calcium, chloride, and sulfate (among others) quicky overwhelm and saturate the resin sites.
• Unlike water softening resins, mixed bed resins cannot be regenerated at home and must be purchased new when exhausted or regenerated commercially. The cost of each cartridge starts at over $100 and varies upward depending on size.
• The efficiency of removal of trace levels of pollutants like cadmium, chromium, lead and many other metal ions varies greatly and depends mostly on the TDS of the water source. The higher the TDS, the lower the efficiency of removal.
• To maintain strict water quality, commercial laboratories regularly test the purity of their water source with sophisticated instruments.

Aeration
A process (also known as air stripping) in which small bubbles of air are passed through a solution. Air is injected, usually either through an open-ended tube or a diffuser, near the water intake. This process can be used to remove some substances from water that preferentially move into the air phase, such as volatile organics.

Flocculation and Sedimentation
Flocculation and sedimentation (also known as coagulation) are used globally in water treatment facilities to remove dirt and other large particles in water. This process adds alum, iron salts, or synthetic organic polymers (flocculants and coagulants) to the water in holding tanks to form sticky particles that attract the fine solids supsended in water so they can settle quickly or be filtered faster. The filtration process is complex and expensive, and is consequently mainly used to treat large volumes of surface water that is high in sediments and soil particles like silt and clay.


Go to other treatment methods:
Particle and Microfiltration Filters
Activated Carbon Filters
Reverse Osmosis
Distillation
Ion Exchange Water Softening
Disinfection of Drinking Water


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