Reverse Osmosis or a Flow-Through Filter: How They Actually Differ

A flow-through filter and a reverse osmosis system look similar: a housing under the sink, a separate faucet, replaceable cartridges. They work on different physical principles and remove different groups of substances. Hence the different areas of application.

Two Different Principles

A flow-through filter works by mechanical retention and sorption. Suspended solids remain on a fibrous or wound cartridge. Chlorine, organics and some of the impurities that cause odor are bound to the surface of the activated carbon. Dissolved salts pass straight through, because they are of no interest to carbon sorption.

Reverse osmosis works on a semipermeable membrane. Pressure forces water through a dense polymer layer, while dissolved ions are rejected and leave with the concentrate stream to the drain. The membrane does not retain substances on its surface; it splits the flow into two.

The difference is fundamental. A flow-through filter accumulates contaminants inside itself. A membrane removes them from the system continuously.

What Carbon Actually Removes

Activated carbon works well on chlorine, organochlorine compounds, tastes and odors. These are exactly the parameters covered by the NSF/ANSI 42 standard. Some carbon cartridges are additionally certified under NSF/ANSI 53 for health-related parameters, including lead and cysts.

What carbon does not do:

• it does not reduce total hardness
• it does not reduce total dissolved solids
• it does not remove nitrates
• it does not remove sodium, chlorides and sulfates

The capacity of carbon is finite. After its service life is used up, the cartridge stops retaining substances, and during a long idle period it becomes a breeding ground for microflora. Service life is counted in liters, not in months.

What a Membrane Actually Removes

A membrane reduces the concentration of dissolved salts. In the data sheet for residential elements, manufacturers state the salt rejection. For example, for the FilmTec TW30-1812-100HR element, DuPont states a nominal salt rejection of 98 percent and a minimum of 96 percent. The test conditions are also stated: pressure 3.4 bar, 250 ppm solution, temperature 25 degrees, permeate recovery 15 percent.

These conditions matter. Rejection is not a constant value; it depends on pressure, temperature and water composition. When the pressure in the water mains drops, permeate quality deteriorates.

A membrane reduces hardness, total dissolved solids, nitrates, chlorides and sulfates. For nitrates it is the only available residential method. WHO sets a guideline value of 50 mg/L for nitrate and 3 mg/L for nitrite; the main risk is methemoglobinemia in bottle-fed infants. A carbon cartridge does not work on nitrates.

Drain, Pressure, Temperature

Reverse osmosis requires operating conditions that a flow-through filter does not.

• Part of the water goes to the drain. At a permeate recovery of 15 percent, as in the data sheet test conditions, about five to six liters of reject water are discharged for every liter of purified water. Systems with a booster pump operate at a higher recovery.
• Pressure is needed. Residential membranes are designed to operate from a few bar. If the pressure in the riser is low, a booster pump is installed.
• Temperature is limited. For the element in question, the maximum feed water temperature in continuous operation is 45 degrees. Connecting to a hot water line destroys the membrane.
• Pretreatment is needed. DuPont requires removing residual free chlorine before the membrane inlet and limits the SDI. Free chlorine destroys the polyamide layer.

That is why a carbon cartridge is still present in a reverse osmosis system. It is installed ahead of the membrane and protects it.

Maintenance and Service Life

With a flow-through filter, all cartridges are replaced according to their service life. Maintenance is simple but frequent.

A reverse osmosis system has a two-level scheme. Prefilters are replaced according to their service life; the membrane is replaced when quality actually drops. Monitoring is done with a TDS meter: compare the readings at the inlet and at the outlet. A rise in the outlet reading means the membrane is worn or fouled.

Hard water shortens the service life of the membrane. Calcium carbonate deposits on its surface and reduces output. A telling detail: in its data sheet tests, DuPont uses softened tap water. On hard water the membrane lasts noticeably less.

How to Choose for the Task

• The water is soft, and the only concern is chlorine, taste and odor. A flow-through filter with a carbon stage under Standard 42 is enough.
• The water is hard, total dissolved solids are above 700-1000 mg/L, and nitrates are present. Reverse osmosis under Standard 58 is needed.
• There are mechanical impurities from an old pipe. A mechanical stage is needed in both cases, as a separate stage ahead of everything else.
• The pressure in the riser is below 3 bar. Plan a system with a pump or abandon the membrane.
• There is no drain connection under the sink. There is nowhere to install reverse osmosis; a drain is mandatory.

What this means for a project in Tashkent

• The water in the city is hard. In our practice, in approximately 85 percent of Tashkent and in all other regions of the republic, the water is hard and of low quality. For a drinking water point this usually means reverse osmosis, not just carbon.
• In districts where the water at the source has an acceptable composition, the old mains are the deciding factor. Secondary contamination with iron and suspended solids makes a mechanical stage mandatory in any case.
• Before buying a membrane, measure the pressure in the riser and check for a drain connection. These two points most often derail an installation.
• Keep a TDS meter and record the readings once a quarter. This is the only simple way to tell whether the membrane is still working.