Water Well Journal
Issue link: https://read.dmtmag.com/i/692787
Oxidation The majority of oxidation systems involve the removal of iron and manganese and employ the dual treatment processes of oxidation and filtration (Figure 3). The oxidant used, such as chlorine or ozone, chemically oxidizes and converts the state of the offending material (typically iron or manganese), forming a filterable particle, while at the same time it also kills iron bacteria and any other disease-causing bacteria that may happen to be present. The filter then removes the particles (precipitates) present in the bed through granular media or other types of filtration. Backflushing or backwashing of the filter is occasionally conducted to remove the filtered material from the filter. Generally, the backflushing procedure is initiated by a pressure or head differential between the inlet and outlet of the filter. Oxidation followed by filtration is a relatively simple process; however, the levels of contaminants in the source water must be monitored to determine and inject the proper oxidant dosage. The treated water must also be monitored to determine if the oxidation process was thorough and complete enough and therefore successful. Oxidant Selection Before iron or manganese can be removed through filtra- tion processes, they must be oxidized to the electrochemical state needed to form an insoluble (and filterable) compound. Oxidation involves the electrochemical transfer of electrons from the iron or manganese molecule to the oxidizing agent. For iron, ferrous iron (Fe 2+ ) is oxidized (or chemically stated, is reduced, shown as →) to ferric iron (Fe 3+ ), which then readily forms the insolu- ble complex of iron hydrox- ide: Fe(OH) 3 . For removal of manganese, reduced man- ganese (Mn 2+ ) is oxidized to manganic oxide (Mn 4+ ), which then forms the insoluble manganese dioxide (MnO 2 ) compound. The most common chemi- cal oxidants used in water treatment are air, chlorine, chlorine dioxide, potassium permanganate, and ozone, along with the use of special oxidizing coatings on media. Oxidation using chlorine or potassium permanganate is frequently applied for small- to mid-size groundwater sys- tems, as the dosing is rela- tively easy, requires simple and accurate dosing equip- ment, and is fairly inexpensive to use. Chlorination is widely used for the oxidation of iron and manganese, but the possible formation of trihalomethanes in waters with high levels of organics may cause a potential health problem. Chlorine feed rates and contact time require- ments can be determined by the use of theoretical calculations and confirmed by using either jar testing or trial-and-error methods for small systems. As an oxidant, potassium permanganate (KMnO 4 ) is nor- mally more expensive than chlorine and ozone but, for iron and manganese removal, it has been reported to be as efficient (often more efficient for manganese) and requires consider- ably less equipment and capital investment than ozone. How- ever, the required dose of potassium permanganate must be carefully controlled as too little permanganate will not fully oxidize all of the iron and manganese. Too much of an over- dose can allow the excess permanganate to enter the distribu- tion system, which can result in a slight pink color or tinge to the water. Excessive levels of permanganate can also form group pre- cipitates that can cause mudball formations to form in filters, which are heavier and therefore more difficult to remove. This can compromise filter performance by creating large voids or openings within the filter bed, resulting in possible "short- circuiting" of the water through the bed, which can often lead to incomplete or inadequate treatment of the raw water. Ozone may also be used in place of chlorine for iron or manganese oxidation, but it may not be as effective for oxida- Figure 3. Cross section of typical water treatment filter. ENGINEERING from page 43 waterwelljournal.com 44 July 2016 WWJ