Oxidizing Filters
Oxidative filtration systems employ the technique of oxidation for water purification, effectively eliminating heavy metals. The process involves the complete oxidation of iron through the addition of oxidizing agents such as chlorine (found in household bleach) or potassium permanganate to the water source. This method is particularly effective in treating iron bound with organics, in the presence of iron bacteria, or when iron concentrations exceed the limits of other treatment methods.
It is important to note that both bleach and permanganate solutions must be prepared weekly due to their instability, which can lead to a decrease in concentration over time.
When calculating the strength of a bleach solution for water systems, factors including the well pump’s hourly capacity, the chemical pump’s hourly capacity, the desired chlorine dosage in parts per million (ppm), and the weekly water usage must be taken into account. A 5.25% sodium hypochlorite bleach solution contains approximately 50,000 ppm chlorine. The calculation for the required amount of bleach is as follows:
(Well pump capacity in gallons per hour × desired chlorine feed in ppm × 128 oz/gallon) / (Chemical pump capacity in gallons per hour × 50,000 ppm chlorine) = liquid ounces of 5.25% hypochlorite bleach per gallon of solution.
For instance, with a well pump capacity of 300 gal/hr, a chemical pump set at 15 gal/day (0.625 gal/hr), a desired chlorine feed of 4 ppm, and 2,000 gallons of water used weekly, the calculation would be:
(300 gal/hr × 4 ppm × 128 oz/gal) / (0.625 gal/hr × 50,000 ppm) = 4.91 oz bleach per gallon of solution.
To determine the solution volume:
(Weekly water consumption × chemical pump setting in gallons per hour) / Well pump capacity in gallons per hour = gallons of solution.
In this scenario, (2,000 gallons/week × 0.625 gal/hr) / 300 gal/hr = 4.16 gallons of solution. It is recommended to maintain a reserve, so prepare five gallons of solution by diluting 25 oz of bleach into five gallons of soft water. Adjust the chemical pump setting as necessary to achieve the desired concentration.
For permanganate solutions, the calculation is as follows:
(Well pump capacity in gallons per hour × 133 advoir oz/gallon × desired KMnO4 in ppm) / (Chemical pump capacity in gallons per hour × 1,000,000) = ounces of permanganate per gallon of solution.
To calculate the solution volume for water consumption:
(Weekly water consumption in gallons × chemical pump capacity in gallons per hour) / Well pump capacity in gallons per hour = gallons of solution for one week.
For example, with a well pump capacity of 300 gal/hr, a chemical pump set at 15 gal/day (0.625 gal/hr), a desired permanganate feed of 20 ppm, and 4,000 gallons of water used weekly, the calculation would be:
(300 gal/hr × 133 × 20 ppm) / (0.625 gal/hr × 1,000,000) = 1.28 oz KMnO4 per gallon of water. Therefore, dissolve 11.5 oz (1.28 oz × 9) of potassium permanganate in 9 gallons of water.
Sodium hypochlorite solutions remain stable in highly alkaline conditions. When added to water, alkalinity is neutralized, and chlorine is released, a process that occurs more rapidly in acidic conditions. Potassium permanganate is effective across different pH levels but should be considered based on the water’s pH.
For organic or chelated iron, longer contact times with chlorine and permanganate may be required, and these times cannot be precisely determined without adjustments based on experience.
Chemical feeders, such as positive displacement pumps and eductors, are utilized to administer the oxidizing agents, converting ferrous iron to ferric iron. The resulting precipitate is then removed through filtration. Chlorine not only removes iron but also disinfects the water when sufficient contact time is allowed. Subsequently, activated carbon filters are used to eliminate the precipitated iron and any excess chlorine. With potassium permanganate, an iron filter is commonly employed to remove the iron precipitate.
The oxidizing agent should be introduced before the pressure tank to ensure adequate mixing. For simple ferrous iron, filtration can occur immediately after the pressure tank. In the presence of iron bacteria, two methods are employed: one involving short contact times and high chlorine concentrations to kill the bacteria, relying on the filter to remove the bacteria and iron precipitate; the other involving longer contact times and lower concentrations for complete oxidation before filtration.
Treating organic iron is challenging and often necessitates the use of strong oxidizing agents and extended contact times. Potassium permanganate is typically more effective than chlorine. Ensuring sufficient contact time is crucial, with 20 to 30 minutes often required. Regular backwashing is necessary to maintain filter cleanliness, and sometimes retention tanks or settling basins are utilized to reduce the filter load. Further details regarding chemical feed pump operations will be provided later in the lesson.