Water Well Journal
Issue link: https://read.dmtmag.com/i/692787
tion in the presence of organic acids in the raw water, such as humic, tannic, or fulvic materials. In addition, if not dosed carefully, ozone can oxidize any reduced manganese that may already be present and revert it to permanganate which can result in the same pink water condition previously cited. Manganese dioxide particles, also formed by the oxidation of reduced manganese, must be carefully coagulated to ensure their removal by the filter media. One low-cost method of providing oxidation is through use of oxygen, already present in atmospheric air, as the primary oxidizing agent (aeration). This can be performed through various methods, such as using an inline venturi or air eductor incorporated into the flowstream or through the injection of air between the well and filter with an oil-less air compressor. As the well pump operates, air is injected and mixed with the water, which oxidizes the iron and manganese to form a large enough and filterable floc. The principal advantage to this method is no chemical dos- ing is required, which allows for unattended and safe opera- tion. But this method is not as effective for water in which the iron is combined with any large organic molecules, since oxy- gen is not a strong enough oxidizing agent to break the strong attractions formed between iron and manganese and these large organic molecules. Furthermore, the amount of oxygen in dry air is generally around 21% by volume (23% by weight), so added air (four to five times the volume of required oxygen) must be introduced to inject the amount of oxygen needed for effective oxidation. The rate of reaction between atmospheric oxygen and iron and manganese is very slow below pH values of 6.5 and 9.5, respectively, so the application is also limited to those with long contact times and/or higher pH values of >8.2. Finally, the presence of any additional contaminants, such as hydrogen sulfide, that may increase the oxidant demand must also be accounted for when applying and dosing the oxi- dant. In general, manganese oxidation is more difficult than iron oxidation because the reaction rate is much slower and the process is much more pH-dependent. A longer detention time (up to 30-45 minutes) following the addition of the oxi- dant may be needed prior to filtration to fully allow the reac- tion to take place. Ion Exchange (Water Softening) The final process on the topic of chemical alteration and solids separation involves the well-established method of ion exchange, often referred to as water softening (Figure 4). Al- though various other methods exist for the so-called softening (sometimes referred to as conditioning) of water, the most common methods used for water well applications are known as ion or anion exchange. Ion or anion exchange depends on the existence and use of a special type of electrochemical filtration media (usually syn- thetic), called a resin, which is manufactured with inherent and permanent positive (ion) or negative (anion) electrical charges that are capable of attracting and holding onto ions with an opposite charge—positive to negative and vice versa. Organic ion exchange resins can be formulated with either anionic or cationic materials of many types, yielding resins with nearly every conceivable functionality and exchange strength and capacity. This means there are now resins formulated specifically to exchange nitrate or fluoride ions, preferentially, as well as for the more common calcium or magnesium ions, or even for the exchange of uranium and plutonium ions from nuclear wastes—and the potential list goes on. ENGINEERING continues on page 46 Figure 4. Typical water softener installation (in service). WWJ July 2016 45 Twitter @WaterWellJournl