Filtration
Which micron rating does your water need?
Quick answer
Finer is not better on sediment. For well water, some sand or grit the sensible staging is 50 micron, then 20 micron, then 5 micron, coarse first. A cartridge finer than the water needs plugs in days and throttles the house without catching anything useful.
Micron ratings are the most misread specification in filtration, for two reasons. The first is that finer sounds better, and on sediment it usually is not: a cartridge too fine for the water plugs in days, which is the commonest cause of a new filter apparently killing the water pressure.
The second is that the rating itself means two different things depending on one word that is frequently missing from the listing.
Work out your own staging
For well water, some sand or grit, run 50 micron, then 20 micron, then 5 micron.
| Figure | Value |
|---|---|
| Water source | Well water, some sand or grit |
| Ideal staging | 50 micron, then 20 micron, then 5 micron |
| Spin down first | Yes, 50 micron flushable screen |
| First cartridge | 20 micron |
| Final cartridge | 5 micron |
Shown with the example figures above. Change any field to run your own numbers.
The working
Rule of thumbstage coarse to fine, never one cartridge doing everything city water, clean -> 5 micron city water, particulate -> 20 then 5 well water, grit -> 50 then 20 then 5 well water, fine silt -> 50 then 20 then 5 then 1 cyst claim under NSF/ANSI 53 needs 1 micron ABSOLUTE
Staging is trade convention. The nominal against absolute distinction and the cyst rating are published definitions. Water treatment trade convention rather than a published standard. Taught everywhere, useful for planning, and not a specification. No standards body publishes it.
Spin down screens and cartridges
The coarse stage is the one that should be loading up, and a flushable spin down is the part that stops everything behind it being a consumable.
iSpring WSP50ARB Auto Flush Spin Down Sediment Filter, 50 Micron
$229.99 Published specsFlushes itself on a timer, which removes the one maintenance step people forget until the pressure drops.
Best for: A high sediment well, or a property nobody is at every week
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iSpring WSP-50SL Spin Down Sediment Filter with Siphon Flush, 50 Micron
$64.99 Published specsAdds a siphon scraper to the flush, which clears the screen more completely than a plain blowdown.
Best for: A well where the screen loads fast enough that flushing is a routine
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iSpring WSP-50 Spin Down Sediment Filter, 50 Micron
$59.99A flushable 50 micron stainless screen with a clear bowl, so sand is visible and can be blown down without opening anything.
Best for: The very first stage on any well with grit
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Waterdrop RPFK Spin Down Sediment Filter, 40 to 50 Micron
$52.99A lower cost flushable prefilter with a gauge, so the pressure drop across it can be watched rather than guessed.
Best for: A first spin down installation
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Pentair Pentek ECP20-20BB Big Blue Pleated Sediment Filter, 20 Inch, 20 Micron
$46.55 Published specsPleated media gives far more surface area than a spun cartridge, so it holds more dirt before the pressure drop climbs.
Best for: A high sediment load where cartridge life is the complaint
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Pentair Pentek DGD-2501-20 Big Blue Dual Gradient Sediment Filter, 20 Inch
$42.48 Published specsThe finer dual gradient, stepping 25 micron to 1 micron, for water that has already had the coarse material taken out.
Best for: A second stage after a coarse prefilter or a spin down
Check price on AmazonEvery figure shown is the manufacturer own published specification. None of this is a recommendation for water you have not tested: the arithmetic above is only as good as the numbers you put into it, and on a private well those numbers come from a certified laboratory rather than from a chart.
Nominal and absolute are not the same claim
An absolute rating is defined: the cartridge removes at least 99.9 percent of particles at or above the stated size. A nominal rating is not standardized at all. It generally implies something like 85 percent removal, but the threshold is chosen by the manufacturer, which is why two cartridges both marked "5 micron nominal" can perform very differently.
Where this matters most is cyst reduction. An NSF/ANSI 53 cyst claim requires 1 micron absolute. A 1 micron nominal cartridge does not carry that claim and must not be treated as though it does. If a listing gives a micron number without the word absolute, assume nominal.
Staging by water source
A single cartridge asked to do the whole job loads at the surface and blinds off, which is why dirty water gets staged filtration rather than a finer cartridge.
Rule of thumb Water treatment trade convention rather than a published standard. Taught everywhere, useful for planning, and not a specification. No standards body publishes it.
| Water source | Staging |
|---|---|
| City water, no visible sediment | 5 micron |
| City water, fine particulate or rust flakes | 20 micron, then 5 micron |
| Well water, some sand or grit | 50 micron, then 20 micron, then 5 micron |
| Well water, fine silt or turbidity | 50 micron, then 20 micron, then 5 micron, then 1 micron |
Staging is trade convention rather than a published standard. The principle underneath it is not: a coarse cartridge protects a fine one, and the coarse stage is the one that should be loading up.
What size things actually are
Fine sand is around 50 to 100 micron and a Cryptosporidium cyst is roughly 4 to 6 micron, which is why cyst claims sit at 1 micron absolute.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| What it is | Typical size |
|---|---|
| Fine sand and grit | roughly 50 to 100 micron |
| Human hair | roughly 50 to 70 micron |
| Silt | roughly 2 to 50 micron |
| Giardia cyst | roughly 8 to 12 micron |
| Cryptosporidium cyst | roughly 4 to 6 micron |
| Bacteria | roughly 0.2 to 2 micron |
| Dissolved minerals and metals | no size, they pass through every rating |
Dissolved contaminants have no size in this sense. Nitrate, arsenic, lead and hardness are in solution and pass through the finest sediment cartridge made, which is why a micron number never answers a dissolved contaminant question.
Frequently asked questions
What micron filter do I need for well water?
Stage it rather than picking one rating. On a well with grit, run 50 micron, then 20, then 5. On fine silt add a 1 micron final stage. A single cartridge asked to do the whole job loads at the surface and blinds off, which is why one fine cartridge plugs in days while the same cartridge behind a coarse prefilter lasts its rated life.
Is a lower micron rating always better?
No, and on sediment it is usually worse. A cartridge finer than the water needs plugs quickly, cuts flow and throttles the house without catching anything useful a coarser one missed. Dissolved contaminants are unaffected by any rating. Finer is only better when there is a specific certified claim behind it, such as 1 micron absolute for cysts.
What is the difference between nominal and absolute?
Absolute is defined as at least 99.9 percent removal at the stated size. Nominal is not standardized at all: it generally implies around 85 percent, but the manufacturer chooses the threshold, so two cartridges marked the same nominal rating can perform very differently. An NSF/ANSI 53 cyst claim requires 1 micron absolute specifically.
Will a sediment filter remove bacteria?
Not reliably, and it must not be relied on. Bacteria run roughly 0.2 to 2 micron, below most sediment ratings, and a cartridge that traps organisms becomes somewhere for them to grow. Microbiological treatment is ultraviolet at an adequate dose or a specific certified claim. A positive bacteria test needs a laboratory and usually a professional.
Do I need a spin down filter as well?
On a well carrying sand, yes, and it is the highest value part of the train. A spin down is a reusable screen flushed with a valve rather than replaced, so it catches the coarse material that would otherwise turn every cartridge behind it into a consumable. On city water it is rarely justified.
How do I know the rating is wrong for my water?
Pressure drop tells you. Fit a gauge either side of the housing and note the drop with a fresh cartridge: a pound or two is normal. If a brand new cartridge already drops more than that, or the drop reaches 10 to 15 psi within weeks, the rating is too fine or the housing is undersized.
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Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.