by Bill Andresen, Sask Water
IIron is a major problem in many rural water supplies. It stains laundry, bathroom fixtures and water-using appliances a reddish- brown colour, and gives an objectionable taste to foods and beverages.
In well water, iron usually occurs in its ferrous state. The water is clear when freshly drawn, but once exposed to air the iron will oxidize and go into a ferric state. It will be seen as red discoloured water and forms insoluble rust particles. Concentrations above 0.3 mg/litre will cause staining.
Iron bacteria feed on iron from the water and on water system components, and add iron to the water. These living organisms can be identified as a rusty coloured slime in toilet tanks. They do not present a health hazard, but they can clog pipes and valves. These bacteria can usually be controlled with regular shock chlorination of the well and water system, but they cannot be eliminated.
For wells, a thorough shock chlorination treatment is an important first step in determining what portion of the staining is caused by iron bacteria, relative to that caused by dissolved (mineral) iron. Generally, if one or two shock chlorination treatments do not correct the staining problem, treatment equipment is required for iron (mineral) removal.
Iron can be removed by the following treatment equipment:
1.Water Softener A water softener will remove small amounts of iron, up to 2 mg/litre, providing the softener is backwashed regularly and a resin cleaner (sodium hydrosulfite or Res Up) is used to protect and prevent the build up of iron on the resin. While a softener can remove some iron from the water, a higher level will foul the resin bed.
2.Air Injection Iron Removal This system removes iron without the use of chemicals to backwash. It consists of an aspirator and a bed-type filter containing a special filter media. Different media can be used, depending on the pH and manganese content of the water. The aspirator (small brass venturi device) is installed on the water line between the well and the pressure tank. A Y strainer should be installed in the line before the aspirator, to trap sediment from the well. When the water passes through this device the velocity of the water is increased and air is drawn in to mix with the water. The introduction of the air starts the oxidation of the iron and causes the iron to change into a colloidal form (very small insoluble particles) which the filter removes by attracting the colloidal iron. These filters are backwashed to the drain at regular intervals, which rids it of iron. The backwash rate is less frequent than manganese greensand filters since the entire bed filters, not just the top 8-12 inches.
3.Manganese Greensand Filters These filters use an oxidizing media coated with manganese oxide. Iron is oxidized by the coating on manganese greensand. The bed requires frequent regeneration with potassium permanganate. Failure to do this often enough can ruin the greensand. The pH of the raw water should be above 7. These filters are also effective in removing manganese and hydrogen sulphide gas levels below 2 mg/litre.
4.Chlorination Filtration The iron is oxidized by the chlorine changing it to iron oxide. A sediment filter mechanically filters out the iron oxide. A carbon filter is then used to remove the taste and odour of the chlorine. The filters require regular backwashing and the carbon needs to be changed at least once per year. The pH of the raw water should be above 7. The success of this method of iron removal depends upon at least five minutes of contact time for the chlorine to fully react with the iron. A coil of plastic pipe or retention tank can be used.
5.Aeration and Settling Water that contains unoxidized iron is forced through a venturi spray nozzle into a cistern. The iron is oxidized when it comes in contact with the air and will settle to the bottom of the cistern. This type of cistern should be considered for removing large concentrations of iron. A second pressure system is needed. Sometimes a softener or iron filter is installed after the cistern to polish the water.
Before purchasing iron removal equipment it is important to have a proper water chemical analysis done and to properly select and size treatment equipment required. Backwash rates and the proper operating water pressures are important for the proper operation of the filters. Filter capacity is important to ensure a sufficient supply of iron-free water.
Your provincial or federal government agency involved with water, or a reputable water treatment company, can help you design an effective iron removal system.
