How Iron Filters Work – Part Two
This is part 2 of a 3 article series that was written to help explain how Iron Filters work.
Part One was a broad overview of the three basic stages of Iron Filtration:
1. Pre-Treatment aka Oxidation
In this article we’ll expand on each stage and explain several critical best practices.
Need Pre-Treatment? Choose Your Oxidation Wisely…
Dissolved iron, manganese, and hydrogen sulfide cannot be removed from water. They need to precipitate into large chunks so they can later be strained and removed by a filter. For that to happen, we need to pre-treat the water with an oxidant.
Air Injection is by far our most popular pre-treatment method for Iron Filter Systems. Air is free, you can’t run out (if you do run out of air, you’ll have more serious problems than iron stains!), and it doesn’t add any chemicals to the water.
Chlorine is a more powerful oxidant, and is our second choice. City Water Treatment Departments all use chlorine. It’s the most documented treatment chemical in our industry; it keeps water safe by killing bacteria, and it just plain works. If you run low on chlorine, you can run down to your local Target store and pick up a jug – it’s convenient.
Potassium Permanganate, aka PotPerm, is our third choice. This is another chemical used in water treatment plants as it’s even more powerful than chlorine. Unfortunately it’s also messy to handle, and somewhat expensive. We use GreensandPlus filter systems that regenerate with PotPerm in rare applications when the water chemistry won’t allow Air Injection to work, or if there’s not enough room for a Chlorination System.
Hydrogen Peroxide is another option, but we don’t use it on a regular basis. Its performance is not as well documented, it’s expensive, and it turns into water if it sits around too long. You can’t buy it off the shelf either, so you’re in trouble if you run out on a weekend!
Oxidation – It’s Just a Matter of Time
It takes time to make a good cup of coffee, and it takes time for oxidation to happen too.
The length of time that air/chlorine, etc. mixes with water is referred to as “Contact Time”. Here are several well documented guidelines for Contact Time:
2 Minutes Contact Time – Iron Removal Only
With 2 minutes of contact time, a 20 gallon retention tank would have enough storage volume to oxidize iron at a flow rate of 10 gallons per minute (gpm).
(20gal. / 2min. = 10gpm)
3 Minutes Contact Time – Iron, Manganese, and/or Hydrogen Sulfide Removal
With additional contaminants, that same 20 gallon tank would now only have enough retention volume to cause oxidation at 6.7 gallons per minute (gpm).
As you can see, Hydrogen Sulfide and Manganese take longer to oxidize. In fact, Hydrogen Sulfide consumes 7x more oxygen than iron.
Using a larger aeration tank(s) and recharging the aeration system more frequently in a Two Tank Iron Filter will dramatically improve results with nasty water.
The lack of flexibility with All-In-One Iron Filters make them better suited for Iron-Only application.
Without an accurate water analysis, designing an Iron Filter is a guessing game. Our FREE WATER TEST is a great way to get started.
The Most Important Filter Measurement
Iron Filters are measured in square feet (surface are) of filter media – NOT in cubic feet (volume) of resin like water softeners.
The square footage of a filter is determined by the diameter of the filter tank. For instance a 14” diameter tank has a surface area of 1.07 sq. ft.
If you need more filtered water, then you need more filter surface area. This means you would either need a larger tank, or multiple tanks run in parallel.
Iron Filter performance is measured in Gallons per Minute per Square Foot of Surface Area (gpm/sq. ft.).
The flow rate of water you can push through a filter will depend on both the water quality, and how long you plan on running the water.
Service Flow Rates (Iron-Free water till the cows come home)
Service Flow Rates refer to steady, continuous water flow for more than 10 minutes. Examples include irrigation, filling up a hot tub, etc. (This may also include your teenager’s shower!)
You can safely use either of these flow rates depending on your water quality:
5gpm/sq. ft. – Iron Removal Only
At 5 gallons per minute per square foot, a 14” diameter tank (1.07 sq. ft. surface area) could deliver 5.36gpm of iron-free water for hours!
(1.07sq. ft. x 5gpm/sq. ft. = 5.36gpm)
3gpm/sq. ft. – Iron, Manganese, Arsenic, and/or Hydrogen Sulfide Combined Removal
With more severe water problems, that same 14” filter could only deliver 3.21gpm of iron, manganese, and odor-free water.
Peak Flow Rates (Just for a little bit)
Peak Flow Rates refer to the quick (1-3 minute) bursts of water use that residential homes have. This could be that brief period when the dishwasher’s rinsing, the washing machine’s filling, you just flushed a toilet, and now you’re washing your hands.
7 – 10 gpm/sq.ft. – Peak flow rate
During a residential peak, a 14” tank could produce 10.7gpm of filtered water – depending on water quality of course!
**These flow rates should not be used for irrigation systems, filling up whirlpool type tubs, etc!**
Backwash Flow Rates and Filter Size
If contaminants are not properly oxidized they can’t be filtered.
If a filter is too small, the crud in the water won’t get removed.
Don’t go through all the effort of properly sizing your Pre-Treatment tank and Filter tank only to kill the whole system with a weak backwash. Improper backwash will quickly kill a filter. To work correctly, you really need dedicated Iron Filter hardware, not just re-purposed water softener components. Here’s why:
Water Softener resin is light and fluffy. You can backwash a huge 14” softener with 4.2gpm of water.
Iron Filter media is MUCH heavier, and requires a far more powerful flush. Compare these popular media options:
A water softener valve with a ½” drain could NEVER backwash a 14” Iron Filter (maybe a 10” filter if you’re lucky). You certainly can’t get 11gpm through a ½” line without a MASSIVE drop in pressure, and there’s no way it could deliver 30gpm!
Our 10” Iron Filter Systems sometimes use ¾” drain lines. All of our larger systems use 1”, 1.5”, 2”, or even 3” drain lines to ensure the backwash is never restricted.
At this point, you’re probably starting to understand that the severity of your water quality has a big impact on the treatment system you need.
We’ll say it again: If you haven’t had your water tested yet, sign up for our FREE WATER TEST. There’s no obligation, and you will see first hand how to design the perfect Iron Filter for your needs.
In Part Three of this series, we will wrap everything up and create a few sample systems. We’ll review a sample system we installed for an average residential family in Deepahven, MN, as well as a large irrigation system that was designed for an estate in Orono, MN.
Who We Are
Premier Water is a local, family-owned water treatment company based in Chanhassen, MN. Since 1978, Premier Water has successfully implemented 1000’s of Residential and Commercial Iron Filters to treat a variety of well water conditions. We offer a FREE WATER TEST to gather information and help our clients with the right solution for their needs. We can also be reached at (952) 479-4553 for more information.