Sediment Filter Micron Ratings: 1, 5, 20 — What to Actually Fit

Sediment filters are one of those components that look identical on the shelf but perform very differently in your pipes. The micron rating stamped on the housing is the single number that determines what the filter stops, how quickly it loads up, and ultimately what it costs you to run. Get it wrong and you either end up with gritty water through a filter that is too coarse, or cartridges that block within days because you chose something too fine for your source water.

This article explains what micron ratings actually mean, how they differ in practice, and which combination tends to work for Johannesburg homes drawing from a borehole or the municipal supply.

What a micron rating actually measures

A micron (micrometre, µm) is one-thousandth of a millimetre. A human hair is roughly 70 µm in diameter, so even a 20-micron filter is trapping particles invisible to the naked eye. The rating describes the particle diameter the filter is designed to stop — a 5-micron filter is intended to block particles 5 µm and larger from passing through.

There is an important qualification here: nominal versus absolute ratings.

• Nominal rating — the filter stops most particles at that size, typically 80–90%. Some particles at the rated size still slip through.
• Absolute rating — tested to stop effectively 99.9% of particles at that size under defined conditions.

Most inexpensive spun-polypropylene cartridges you will find at a hardware store are nominally rated. Absolute-rated filters (common in medical or food-grade settings) cost more. For household sediment filtration, nominal-rated cartridges are generally adequate, but knowing the distinction matters when a supplier quotes a specification.

The three most common ratings — 1, 5, and 20 micron

20 micron is the coarsest of the three. It blocks sand, grit, rust flakes, and larger suspended solids. It flows freely, resists clogging, and is inexpensive to replace. It will not stop fine clay, silt, or anything smaller. On borehole water with visible turbidity or a sandy aquifer, a 20-micron stage is a sensible first barrier — it protects the finer filters downstream.

5 micron is the workhorse for most residential applications. It catches the fine silt, sediment, and rust particles that a 20-micron filter lets through, and it is fine enough to protect downstream components such as UV sterilisers and carbon blocks, which have small internal channels that silt up quickly. Municipal water in Johannesburg, which is treated to SANS 241 drinking-water quality standards before it reaches your meter, usually arrives with low gross turbidity — a 5-micron filter is typically sufficient as a standalone sediment stage on town supply.

1 micron catches fine clay, cysts such as Cryptosporidium and Giardia, and very fine colloidal particles. It is rarely used as a first stage because it clogs rapidly with any meaningful sediment load. Its place is as a final polishing stage in a multi-stage system, immediately before a UV lamp or a reverse-osmosis membrane, where it needs to see relatively clean water.

As a rough guide, the relationship looks like this:

Rating	Catches	Best used as	Clogs quickly if…
20 µm	Sand, grit, rust flakes	First / pre-filter stage	Source water is very fine and silty
5 µm	Fine silt, rust, sediment	Second stage or standalone on municipal	Raw turbidity is very high
1 µm	Fine clay, cysts, colloids	Final polishing before UV or RO	Used directly on raw borehole water

How borehole water changes the equation

Borehole water in Johannesburg varies considerably depending on the geology of your suburb. Some aquifers yield clear water with minimal suspended solids; others carry fine clay or iron-rich sediment. Johannesburg Water’s own figures note non-revenue water of 44.8% for 2024/25, which partly reflects infrastructure age — and that same ageing infrastructure upstream of your property can introduce rust and particulate into what starts as treated municipal water.

For a borehole source, a two-stage approach is standard: a 20-micron pre-filter knocks out the bulk of grit, and a 5-micron second stage catches what remains. If you are adding UV disinfection (which SANS 241 compliance for private supplies would practically require for potable use), a 5-micron — or sometimes 1-micron — stage immediately before the UV lamp ensures the UV dose is not blocked by turbidity.

The article on sediment filters as your first line of defence covers the role pre-filtration plays in protecting downstream equipment in more detail.

Flow rate and pressure drop

Finer micron ratings restrict flow more than coarser ones, and a partially loaded cartridge restricts it further. This shows up as reduced pressure at your taps, particularly at peak morning usage. If you have a 1-micron cartridge and notice pressure dropping noticeably within days of a new install, the filter is probably seeing more sediment than it is designed to handle at that stage.

A practical check: fit a pressure gauge before and after your filter housing. A pressure drop of more than roughly 30–50 kPa across a new cartridge suggests the rating is too fine for your source water at that position in the system.

Replacement intervals — and why they matter more than the rating

A filter that is left in place too long does one of two things: it either becomes so loaded that it restricts flow significantly, or — in some cases with bacterial contamination — it becomes a colonisation site. Neither outcome is acceptable in a potable water system.

Replacement frequency depends on your source water quality and usage volume, not on a fixed calendar. How often you should replace water filters explains the indicators to watch for and gives practical guidance on scheduling. As a rough guide, a 5-micron cartridge on municipal water in a typical Johannesburg household might last three to six months; on borehole water with visible turbidity, the 20-micron pre-filter might need replacing monthly until you establish a pattern.

If your sediment filter is clogging faster than it should, the rating or the system design may not suit your water source. Get 3 free filtration quotes — vetted installers, no obligation.

Nominal versus absolute: when it matters for Johannesburg homeowners

For most sediment-only applications — protecting a geyser, a washing machine, or a garden irrigation system — a nominal-rated cartridge is fine. Where it starts to matter is when the filter is doing double duty as a cyst barrier (in the absence of UV or chemical disinfection). A nominal 1-micron filter does not reliably remove Cryptosporidium oocysts, which are approximately 4–6 µm in diameter; an absolute-rated 1-micron filter is needed for that purpose.

The National Institute for Communicable Diseases (NICD) periodically issues guidance on waterborne pathogen risk, and Cryptosporidium features in South African water-quality literature. If you are on borehole water without a UV or chemical disinfection stage, do not rely on a nominal sediment filter to protect against biological contamination — it is not designed for that task.

For a broader look at how sediment filtration fits into a complete water treatment system, the water filtration guide covers the full range of technologies and where each sits in a treatment train.

Unsure whether your current filter setup suits your borehole or municipal supply? Compare quotes from local filtration specialists — vetted installers, no obligation.

Choosing the right rating for your situation

Here is the plain-English version:

• Municipal supply only, no visible issues: a single 5-micron nominal cartridge is a reasonable starting point.
• Municipal supply with old pipes or visible rust: 20 micron first, 5 micron second.
• Borehole with low turbidity: 20 micron pre-filter, 5 micron before UV.
• Borehole with visible silt or clay: 50 micron (if available) or 20 micron coarse stage, then 20 micron, then 5 micron before UV — a three-stage approach.
• Adding reverse osmosis: the RO membrane manufacturer’s specification takes precedence; typically a 5-micron or 1-micron stage immediately upstream.

Before specifying anything, get a basic water test from an accredited laboratory. Turbidity, iron, and total dissolved solids results will tell you far more than a generic recommendation — and they will save you money on cartridges that are either wrong for the job or failing faster than they should.