Control of Bacterial Growths in Rapid Sand Filters

Nitrification has been observed in the rapid sand filters of many iron removal plants in Illinois. The biological oxidation of ammonia to nitrate by nitrifying organisms often results in the total depletion of oxygen within the filter. The resulting anaerobic conditions have been associated with the breakthrough of iron. Because the growth of bacteria, particularly nitrifying organisms, is a slow process, taking up to a few weeks to reach nuisance levels, periodic filter disinfection can often be practiced at lower cost than continuous control. Potassium permanganate was used in a pilot plant study in an effort to oxidize accumulated organic matter within a rapid sand filter. Since the oxidation of organic matter by potassium permanganate proceeds rapidly at pH about 10.5, sodium carbonate was used for pH adjustment. The application of 5,000 mg per l of KMnO₄ for 3 hr was sufficient to prevent further oxygen and ammonia depletion in the filters. However, the bacterial growth was reestablished by the end of the fourth week indicating that this treatment must be repeated whenever bacterial growth reaches a nuisance level.