Water treatment systems filtration (I)

 

Filtration represents a broad category of treatment systems used to remove particles, taste, odor, some organics and minerals, and some bacteria from the water. Filtration systems fall into several categories:

• Mechanical or Sedimentation Filtration
• Oxidation Filtration
• Neutralizing Filtration
• Reverse Osmosis or Membrane Filtration

In many cases, these filtration systems are an integral component of other water treatment systems. To remain effective, all filtration systems must be regularly inspected and maintained.

Mechanical or sedimentation filters simply retain debris as water passes through the filter unit. Mechanical filters are most effective for removing particles such as sand, silt, ferric iron, algae and some bacteria. Their effectiveness will depend on the particle size and the exit clearance of the filter.

Activated carbon filtration is a common treatment to remove offensive tastes and odor, color, chlorine and volatile organic chemicals, pesticides and trihalomethanes (a group of suspected carcinogens). Activated carbon will not remove bacteria, dissolved metals such as iron, lead, manganese and copper, or chlorides, nitrates and fluorides.

Activated carbon filters, usually made up of granulated, powdered or block carbon, act like a sponge with a large surface area to absorb contaminants in the water. Activated carbon, made from coal and nutshells, has a tremendous surface area - as much as 125 acres per pound of carbon. The efficiency of an activated carbon filter is dependent on the amount of carbon in it and the flowrate of water through the filter. The typical carbon filter is about 10 inches high and 3 inches in diameter - enough charcoal to treat about 1,000 gallons of water.

Home-use activated carbon filters are most commonly sold as faucet-mounted, stationary and line bypass systems. The stationary system is connected directly to the cold water faucet. The line bypass system has a separate faucet, but is tapped into the cold water pipe for its water supply. These systems are typically installed under the sink and can be purchased for $50 to $375. Pour-through carbon filters are also on the market. However, due to the small amount of carbon and minimal water contact time with the carbon, this system has limited effectiveness. This is also true of the faucet-mounted carbon filter costing about $25. A carbon unit for radon removal resembles a water softener tank and costs about $1,500 plus installation. Without periodic replacement, carbon filters may provide a breeding ground for bacteria. To assure maximum effectiveness, replace any carbon filter frequently, following the manufacturer's recommendations.

Oxidizing filters are used mainly to remove iron, manganese and hydrogen sulfide. Manganese greensand is a common chemically reactive medium designed to remove iron and manganese that is in solution. The greensand will also act as a filter and catch iron and manganese precipitates that have been oxidized. The unit works by providing oxygen to the iron and manganese from the greensand bed. As a result, these minerals change from their soluble to insoluble form. The precipitated minerals become trapped as rust particles within the greensand filter bed. To remain effective, manganese greensand filters must be periodically backwashed to thoroughly remove iron precipitate, and regenerated when the oxygen is depleted.