# Filter filtration water

> How water filtration differs from softening and treatment, which filter types handle which contaminants, and how to match a filter to a lab report.

Canonical: https://watertreatmentpro.com/filter-filtration-water/

Last updated: 2026-11-09

Filtration is the physical or adsorptive removal of material from water. It is not softening and it is not disinfection. A filter catches particles, adsorbs chlorine and organic compounds, or blocks organisms, depending on the medium. Matching the medium to the contaminant is the entire skill.

## The Filter Types

**Sediment filters.** Pleated polyester, spun polypropylene, or wound string. They remove particles by size, measured in microns. Cheap, frequent, and essential upstream of everything else.

**Carbon block.** Extruded carbon with a binder, rated for chlorine, taste, and some organic compounds. Dense enough to also act as a fine particulate filter.

**Granular activated carbon.** Loose carbon in a tank or cartridge. Good contact time, but it can channel and it sheds fines.

**Specialty media.** Catalytic carbon for chloramine, iron-removal media, arsenic media, and nitrate-selective resin. Each targets a specific contaminant.

**Membranes.** Reverse osmosis and ultrafiltration. RO removes dissolved ions; ultrafiltration removes particles and organisms but not dissolved salts.

| Filter | Removes | Does not remove |
|---|---|---|
| Sediment | Sand, rust, grit | Chlorine, dissolved salts |
| Carbon block | Chlorine, taste, some organics | Nitrate, hardness, bacteria |
| GAC | Chlorine, taste | Fine particles, dissolved salts |
| Catalytic carbon | Chloramine | Nitrate, hardness |
| RO membrane | Dissolved ions, larger molecules | Chlorine without carbon first |
| UV | Inactivates microorganisms | Anything physical or chemical |

## Filtration vs Softening vs Treatment

Three terms get used interchangeably and should not be.

| Process | Mechanism | Targets |
|---|---|---|
| Filtration | Physical or adsorptive removal | Particles, chlorine, organics |
| Softening | Ion exchange | Calcium and magnesium |
| Disinfection | UV or chemical | Microorganisms |

A household that buys a carbon filter expecting soft water will be disappointed. A household that buys a softener expecting better taste will be disappointed too. The two solve different problems, and most homes with both hardness and a chlorine taste need both.

## Matching Filter to Report

The lab report is the specification. Read it and assign a filter to each finding.

| Finding | Filter |
|---|---|
| Sediment or turbidity | Sediment cartridge, 10 - 20 micron |
| Chlorine or chloramine | Carbon block or catalytic carbon |
| Taste and odour | Carbon block |
| Iron above 3 ppm | Iron filter, not a cartridge |
| Hardness above 7 GPG | Softener |
| Nitrate | Reverse osmosis |
| Bacteria | UV after pre-filtration |

The iron row is where cartridge filters fail. A standard carbon or sediment cartridge is not an iron filter; it will load with iron and blind in weeks. Iron needs an oxidation and backwash stage.

## Housing Size and Flow

The cartridge is the media; the housing is the pressure vessel, and its size sets how much water can pass.

| Housing | Cartridge | Flow | Fits |
|---|---|---|---|
| 2.5 x 10 | Standard | 4 - 6 gpm | Under sink, fridge |
| 4.5 x 10 | Big blue | 8 - 12 gpm | Whole house, mid |
| 4.5 x 20 | Jumbo | 12 - 20 gpm | Whole house, large |

Putting a standard housing on a whole-house line is the most common installation error in filtration. The cartridge cannot pass the flow, velocity through the media rises, and pressure drop climbs. The fix is a larger housing, not a looser cartridge.

## Micron Ratings

A micron is a thousandth of a millimetre. Ratings describe the particle size a filter is designed to capture, not a guarantee that nothing larger passes. Some cartridges are nominal, meaning they capture a percentage of particles at that size, while absolute cartridges capture essentially all of them, which is why the two are not interchangeable in critical applications.

| Rating | Captures | Pressure cost | Typical use |
|---|---|---|---|
| 20 micron | Sand, grit | Low | Coarse pre-filter |
| 10 micron | Fine sediment | Low | Whole-house pre-filter |
| 5 micron | Fine sediment, UV protection | Moderate | Pre-UV, pre-RO |
| 1 micron | Very fine particles | Higher | Polishing |
| 0.5 micron | Cysts, fine turbidity | Highest | Absolute filtration |

Tighter is not better. A 1 micron cartridge on turbid well water clogs in weeks and drops pressure. Stage the filtration: coarse first, fine second.

## What It Costs

Whole-house two-stage installed runs $500 to $1,800, with cartridges at $80 to $300 per year. Under-sink carbon runs $150 to $500 installed with $40 to $120 per year.

A ten-year figure for a $700 whole-house system with $140 per year in cartridges is $700 + $1,400 = $2,100, or $210 per year.

Those are estimates from published ranges. The consumable line dominates, which is why cartridge availability and price matter more than the housing brand.

## Five Steps to Choose

1. **Test the water** and list the contaminants.
2. **Assign a filter type** to each one.
3. **Order the stages** coarse to fine.
4. **Size the housing** to the flow, not the price.
5. **Set a change schedule** and write it on the housing.

## Maintenance

Change on the interval, not on the symptom. A carbon block that has passed its life may already be releasing adsorbed material, and a sediment cartridge that has loaded up is costing pressure daily.

If flow drops early, the upstream stage is undersized. If taste returns early, the carbon is exhausted sooner than expected, which usually means the water carries more chlorine or organics than the rating assumed.

There is a practical test for an exhausted carbon block. Fill a glass and let it stand for an hour, then smell it. Chlorine dissipates from untreated water as it stands, so that test is unreliable. A better check is a chlorine test strip before and after the filter: a reading above zero downstream of a carbon block means the media is spent. Those strips cost a few dollars and remove the guesswork from the change interval.

For the cartridge format, see [the 10-inch cartridge standard](https://watertreatmentpro.com/water-filter-cartridge-standards/). For the full-house version, [whole house water filtration](https://watertreatmentpro.com/whole-house-water-filtration-filter/) covers installation, and [water filter system](https://watertreatmentpro.com/water-filter-system/) covers staging.