How to Size Commercial Filtration Systems for Iron and Manganese Treatment
How To Size Commercial Filtration Systems for Iron and Manganese Removal
Iron and manganese are some of the most common elements in the Earth’s crust and are widely found in groundwater worldwide.
These metallic contaminants are commonly found dissolved in water in a clear state. The water may appear clear but later turn to rust when exposed to air. Careful guidelines should be followed to size commercial filtration systems for iron and manganese removal.
Iron and manganese treatment systems use aeration and oxidation with air, chlorine, or ozone injection to convert the metals' dissolved forms to solid forms, followed by a filtration process.
When combined iron and manganese concentrations in the source water are high (above 8 to 10 PPM), a clarification sequence is typically required prior to filtration. However, a simple oxidation step using chlorination or air injection followed by filtration can be highly effective for lower levels.
The type of filtration used in this treatment approach may be a conventional anthracite-and-sand filter, a single media sand or anthracite filter, or a catalytic type filter containing oxidizing filter media.
For conventional sand/anthracite filters, the iron must be carefully oxidized and floc formed of the correct size for the filter to work properly. The oxidizing catalytic filters use manganese greensand, manganese dioxide-coated media, or manganese dioxide ore (such as Pro-OX NSF media) in a catalytic process.
The catalytic media can accommodate higher service flow rates with a smaller footprint, and unlike conventional sand filters, no contact tank is typically required. However, for best results, the oxidant should be injected into the water 20 to 30 seconds ahead of the filter.
Introduction to Iron and Manganese Contamination
Iron and manganese contamination is common in household water supplies, significantly affecting water quality and taste. Elevated levels of these metals can lead to unpleasant odors, tastes, and colors, as well as staining on fixtures and laundry.
One of the most noticeable by-products of iron and manganese bacteria is hydrogen sulfide, which produces a distinctive “rotten egg” odor. To effectively remove these contaminants, it’s essential to understand the types of contamination and how to identify problems in your water supply.
Whole house iron filters and water treatment systems are designed to improve water quality by reducing the presence of iron, manganese, and hydrogen sulfide, ensuring cleaner, better-tasting water for your home.
Types of Contamination
Several types of contamination can affect household water supplies, including iron, manganese, and hydrogen sulfide. Iron and manganese can exist in both soluble and insoluble forms, with soluble iron being the most common in domestic water sources. Manganese often accompanies iron and can cause brownish-black staining. Hydrogen sulfide, a gas released from the soil, produces a strong, unpleasant odor.
Filtration media such as catalysts and oxidizing agents like chlorine, hydrogen peroxide, or ozone are commonly used to remove these contaminants. Iron filters and water treatment systems also effectively remove iron, manganese, and hydrogen sulfide from water supplies, ensuring high-quality water for household use.
Identifying Contamination Problems
If you suspect a problem with iron or manganese in your household water, it’s crucial to have it tested. Test results will indicate the extent of the iron problem, the types and quantities of contaminants present, the pH, alkalinity, and hardness.
Ideally, iron levels should be below 0.3 ppm (or mg/l), and manganese levels should be below 0.05 ppm (or mg/l). To identify contamination problems, look for signs such as discolored water, bad tastes, or staining on fixtures and laundry. You can also detect the presence of hydrogen sulfide by smelling for a “rotten egg” odor.
Whole house iron filters and water treatment systems can help remove iron, manganese, and hydrogen sulfide from your water supply, significantly improving the quality and taste of your water.
Service Flow Rate and Filter Sizing
Whichever type of filter media is used, each type has a maximum flow rate (how fast the water flows through the media) that can be realized. This velocity of the water through the filter media (the Service Flow Rate) is commonly listed as Cubic Meters per Hour per Square Meter of Filter Area (M3/M2/Hour) or Gallons per Minute per Square Foot of Filter Area (GPM/Ft2).
The filter bed depth is typically 2 to 3 feet or no more than approximately 1 meter deep over a gravel underbed.
The filter surface area is the filter tank’s square footage or square meter. For example, a 4’ diameter filter tank would have a filter surface area of 12.5 Ft2. Multiply the radius by itself to square the number (2 x 2 = 4). Multiply the result by pi (3.14159): 4 x 3.14159 = 12.56 square feet, or 1.16 square meters.
A conservative service flow rate for many filter media is 3 to 5 gallons per minute per square foot of surface area. Depending on the type of filter media, water chemistry, and temperature, a maximum flow rate of 15 to 20 gallons per square foot of surface area is possible.
Factors affecting the Service Flow Rate include the levels of contaminants (iron, manganese, hydrogen sulfide, etc.) present, water chemistry (such as pH and ORP), and water temperature. The system's capacity to handle low concentrations of these contaminants is crucial for maintaining efficiency and functionality.
A pilot plant and pre-testing are recommended for large commercial and municipal iron treatment systems. High-capacity systems are particularly effective in these scenarios, as they can handle low concentrations of contaminants without significant changes to the filtration media. A conservative approach using properly sized filter systems works well for smaller systems.
Backwash Flow Rate Critical
In normal service mode, the water flows from the top of the tank down through the filter media from top to bottom, through the filter gravel under-bed and out to the distribution system. After a filter run (often 24 hours or less for commercial and municipal systems), the filter must be back-washed and rinsed.
Backwash involves diverting the water flow so the water flows into the bottom and lifts the media up, expanding the filter media and causing the accumulated iron floc to be rinsed out to drain. This process is crucial for the reduction of contaminants such as iron, manganese, and hydrogen sulfide, ensuring the efficiency of the filtration system.
A typical backwash length is 5 to 10 minutes. During the backwash, the media must be lifted and expanded by 15% to 30%.
After the backwash sequence, a short rinse-to-drain rinses the media before being returned to service. This is accomplished with an automatic top-mounted control valve or a series of hydraulic valves that control the flow for backwashing and rinse.
The warmer the water, the higher the flow rate is required to lift and expand the filter media.
A typical backwash flow rate is 10 to 15 GPM for coated filter media and 15 to 25 GPM for solid manganese dioxide ore media.
Metric to Gallons/Feet Conversions:
1 m2 = 10.7 square feet
5 m3/m2/hour = 22/10.7= 2.05 GPM/FT2
10 m3/m2/hour = 44/10.7= 4.1 GPM/FT2
20 m3/m2/hour = 88/10.7= 8.2 GPM/FT2
25 m3/m2/hour = 110/10.7 = 10.3 GPM/FT2
30 m3/m2/hour = 132/10.7 = 12.33 GPM/Ft2
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Further Reading
U.S. Environmental Protection Agency. (2006). Drinking water treatment for small water systems: Iron and manganese removal (EPA/625/R-06/003). National Risk Management Research Laboratory. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/7847688
