FUNDAMENTALS OF MEDIA FILTRATION FOR IRON AND MANGANESE REMOVAL
Iron and manganese cause staining. The water may look clear, but staining appears as the water is used. Although there are no known health-related implications, the federal EPA considers iron and manganese aesthetically objectionable, so it has listed them under the Secondary Drinking Water Standards. The maximum contaminant levels (MCL) for iron and manganese are 3.0 mg/L and 0.05 mg/L, respectively.
Filters with Cullsorb manganese greensand media reduce iron and manganese through a combination of oxidation, ion exchange and particle entrapment. There are two common processes: one requires continuous potassium permanganate feed ahead of the filter, the other uses potassium permanganate as a regenerant on a batch treatment basis.
Continuous feed systems inject potassium permanganate into the raw water to oxidize iron and manganese. When iron is present in concentrations greater than a few milligrams per liter it is often more cost effective to pretreat with chlorine to reduce the amount of permanganate required. Feeding a cationic polymer also aids the filtration process.
The filter bed contains Cullsorb media topped with a coarse, light material called Culicate® filter medium. The top layer traps larger particles to reduce the load on the underlying Cullsorb media. The finer Cullsorb media removes remaining traces of manganese and iron. It also removes any residual potassium permanganate. When the pressure drop increases 5 to 10 psi (34 to 68 kPa) the filter must be backwashed. An upflow rate of 12 gpm/ft (29 m/hr) for 10 minutes is usually sufficient to clean the dual media bed. A 5-minute downflow rinse settles the bed before returning the filter to service.
Batch systems are suited to applications with small amounts of iron or manganese, or where the system will operate largely unattended. The filter uses a deep bed of Culisorb media activated by passing a potassium permanganate solution through it (a dosage of a few ounces per cubic foot is sufficient). As raw water passes through the filter, iron is removed and manganese may be reduced. Each cubic foot of media will remove about 6000 ppm-gallons (about 0.8 ounce by weight or 23 grams) of iron before exhaustion.
Regeneration consists of a 10 minute backwash at 12 gpm/ft (29 m/hr), injection of potassium permanganate, a slow rinse to ensure proper contact time and to displace residual permanganate from the tank, and a 5 minute fast rinse. The entire regeneration takes less than an hour.
Cullsorb media has some limitations. For example, the pH of the raw water should be at least 6.5 - higher if possible - in the presence of sufficient alkalinity so that iron precipitation occurs. Culligan engineers can help in proper application of this equipment.