Sediment filters for well water

The tradeoff hiding in the micron rating

A finer filter is a different point on a tradeoff rather than a better filter.

A filter’s rating is the particle size it stops. Finer ratings catch more, and in catching more they block sooner. Every particle retained is also a small reduction in flow, and a filter approaching the end of its life is a restriction in the main line to the whole house.

On a well producing real sand, a 5 micron cartridge can go from new to noticeably restricting pressure in under a week. The owner’s experience is mysteriously poor showers, and the cause is the thing installed to help.

So the rule is to be as coarse as the downstream equipment allows. Softeners and oxidising filters want grit kept out and do not care about fine silt. Ultraviolet disinfection is the exception, because it needs low turbidity, and it is usually the reason to fit something finer.

Which rating, by what the well produces

North Dakota State University Extension’s guidance turns on how the well was built rather than on how dirty the water looks:

• Drilled wells tend to produce relatively large particles alongside fine iron, and NDSU puts a 10 to 15 micron rating as the right range.
• Dug or bored wells often carry fine clay, which calls for the finest filter available, around 2 microns.
• Rust and iron as sediment on a treated supply is handled at about 15 microns.

The other number is 5 microns, which is the rating the Water Quality Association recommends as pretreatment ahead of any sub-micron filter. Fine cartridges load out quickly without something coarser in front of them.

Nominal and absolute, and why the difference matters

Two cartridges can both say “1 micron” and behave differently, because the rating can be measured two ways.

An absolute rating means essentially every pore is at or below the stated size. The Water Quality Association defines it as trapping 99.9% of particles larger than the rating.

A nominal rating is an average, and the WQA’s definition puts retention at about 85% of particles at the rated size. Some pores are larger, so some particles pass.

For ordinary sediment that distinction is academic. For anything health-related it decides the purchase. The CDC states that some nominal 1 micron filters allow 20% to 30% of 1 micron particles, the size of cryptosporidium, to pass through. The CDC’s guidance for removing parasites asks for an absolute pore size of 1 micron or smaller, or certification to NSF/ANSI Standard 53 or 58 for cyst reduction, and it explicitly lists “nominal pore size of 1 micron or smaller” among the labels that are not sufficient.

If the reason for filtering is grit, a nominal rating is fine. If the reason is an organism, the label has to say absolute or carry the certification.

When the well cannot backwash

A backwashing filter needs a sustained flow rate to lift and rinse its bed, and that requirement is a property of the equipment rather than of the water. A common residential control valve wants around 7 gallons per minute to backwash a filter tank, and larger configurations want considerably more.

Plenty of drilled wells cannot sustain that. The Water Quality Association states the consequence: inadequate backwash flow rates mean the media will foul prematurely, resulting in system failure.

Sediment has a way out that the media stages behind it do not. A cartridge filter has no backwash cycle at all. Water is shut off, the cartridge is swapped by hand, and the well is never asked for flow it does not have.

Change it on a pressure gauge rather than a calendar. Public water works standards put the replacement point at a 15 psi rise in pressure drop across the filter, or whatever the manufacturer specifies. A clean cartridge starts well under 5 psi even at its rated flow, so the gauge tells you far more than the date does.

What it cannot do

A sediment filter removes particles. That is the entire mechanism, and it sets a hard boundary.

Dissolved iron is not a particle. Water carrying 3 ppm of ferrous iron passes through a 1 micron filter completely unchanged, because at that point the iron is in solution. It becomes filterable only after it has been oxidised, which is what an iron filter does and a sediment filter does not.

The same applies to hardness, manganese in solution, nitrate and every other dissolved constituent. If the problem is chemistry rather than debris, this is not the stage that solves it.