Reverse Osmosis Water Filter System Parts & Components Explained

This page may contain affiliate links. If you buy a product or service through such a link we earn a commission at no extra cost to you. Learn more.

A reverse osmosis system consists of many parts and components.

If you’ve ever wondered what these parts do, this article is for you.

Here, we talk about all the different reverse osmosis water filter components and their functions. Keep reading to learn all about them!

Key Takeaways

The key parts of a reverse osmosis system are:

  • Feed water valve
  • Pre-filters
  • Reverse osmosis membrane
  • Water storage tank
  • Carbon post-filter
  • ASO valve
  • Check valve
  • Flow restrictor
  • Drain saddle
  • RO faucet
  • Tubing

Optional components for a reverse osmosis system include:

  • Water pressure pump
  • Permeate pump
  • Remineralization post-filter
  • UV post-filter
  • TDS monitor
  • Pressure regulator

Reverse Osmosis Water Filter System Parts and Components

There are two types of reverse osmosis system components; the standard/essential parts and the optional components. The standard parts are required in all RO systems, while the optional components offer additional but not strictly necessary features.

Note: This article explains the parts and components that apply to under sink RO systems.

Standard Reverse Osmosis Parts Explained

Below are the standard components and parts of the classic under sink reverse osmosis system:

Feed Water Valve

The feed water valve is the part of the RO system that connects it to the sink’s cold water supply. The water supplied to the system flows through the feed water valve before it proceeds to the first filtration stage.


The RO system passes water through two or more pre-filter stages before pushing it through the semipermeable membrane. The pre-filters remove suspended sediments, chlorine, and other chemicals that are harmful to said RO membrane.

Sediment Pre-Filters

Sediment pre-filters remove silt, dust, sand, rust, and other dirt from the water before it reaches the semipermeable membrane. This means the membrane is exposed to fewer contaminant particles and protected from harm.

Sediment pre-filters are also used to protect water softeners and UV disinfectants.

Carbon Pre-Filters

Like sediment pre-filters, carbon pre-filters are designed to eliminate as many contaminants as possible before the water reaches the reverse osmosis membrane. Activated carbon protects the RO membrane from harmful chlorine and provides thorough filtration results.

Different Water Filter Cartridges and Membranes

Reverse Osmosis Membrane

This is the main component in any reverse osmosis system and removes the most contaminants from water.

The RO membrane contains really small pores (0.0001 microns in size). These super tiny pores reject contaminants, allowing only pure water molecules to pass through them. The filtered or rejected contaminants are flushed out of the membrane as waste water.

Water Storage Tank

After purifying water, the RO system stores it in a pressurized water storage tank. Reverse osmosis is a slow process, so a storage tank ensures water is available whenever you need it.

RO systems continuously filter water until the storage tank is full — that way, there’s always water available, and you don’t have to wait hours for a glass of water.

Carbon Post-Filter

In a standard RO system, water is filtered through a carbon post-filter after it has been stored in the tank.

The carbon post-filter is usually the last stage of water purification, and it helps to further improve the quality of water. It removes residual chlorine and other contaminants from water, giving it that finishing touch it needs to be completely contaminant free. It also improves water taste.

ASO (Automatic Shut-Off) Valve

The automatic shut-off valve (ASO valve) stops the RO system from endlessly filtering water when the storage tank is full. The valve is automatically activated when the tank is full, preventing the system from sending more water to the RO membrane and down the drain.

When you open your faucet and use water from the storage tank, the pressure drops, deactivating the ASO valve and allowing water to resume its flow.

Check Valve

The check valve is designed to prevent water flow from the storage tank back to the RO membrane. The ASO and check valves only work when they’re used together.

Flow Restrictor

The function of the flow restrictor is to restrict the flow of waste water in the RO system. It keeps the pressure at a level that allows the system to work efficiently, preventing excessive waste water production.

Drain Saddle

The drain saddle connects the system’s waste line to the under sink drain pipe. It’s important for sending waste water out of the system.

RO Faucet

The sole function of the RO faucet is to dispense filtered water. RO faucets are typically made of high-quality plastic and/or non-corrosive metal. They feature a small lever or button that is pulled to dispense water.

Filter Housings and Brackets

The filter housings hold the filter elements in a reverse osmosis system. They allow you to replace the filter cartridges conveniently.

Brackets are helpful because they provide a stable and secure positioning for the RO system. Using a bracket ensures the system is secure and less likely to get damaged and manhandled. Having the RO system on a bracket also allows you to access the system’s components easily during maintenance.


RO quick-connect fittings are used to attach tubing quickly and securely. The fittings are usually color-coded to tell you how the tubes should be connected.


The tubings are what link all the parts of the RO system together.

Mounting Clips

RO mounting clips are small clips designed to secure RO filters membranes. They fit tightly around the filter or membrane housing and hold them in place.

Mounting clips come in different shapes, sizes, and styles, depending on the type of RO model.

Under Sink Reverse Osmosis System

Optional RO System Components

These optional RO components provide extra features like boosting system efficiency.

Water Pressure Pump

A water pressure pump or booster pump is recommended for RO systems that don’t have enough feed water pressure. RO systems work best with pressure at around 60 psi. You must install a booster pump if your feed water pressure falls below 40 psi.

Permeate Pump

Permeate pumps are installed into RO systems to improve efficiency and reduce waste water production. Permeate pumps work by storing the waste water in a chamber and then using it as energy to push purified water into the storage tank.

This process stops the pressure in the tank from pushing against the RO membrane, making the system more efficient.

Remineralization Post-Filter

Reverse osmosis filters strip water of any foreign bodies. Unfortunately, this means beneficial minerals are also removed from the water. To solve this problem, remineralization post-filters add trace amounts of minerals like calcium, zinc, sodium, and magnesium into the filtered water, greatly improving its taste and nutritional benefits.

UV Post-Filter

An RO membrane is not designed to remove microbes from water. This is because the microorganisms can multiply and accumulate on the membrane eventually slipping through.

UV post-filters are placed after the other filter stages to remove bacteria etc. from the water. They kill microbes by attacking their DNA with UV light, effectively stopping them from functioning.

TDS Monitor

A TDS monitor is a less common optional feature in RO under sink systems. They are installed on a smart faucet and provide a reading of the TDS level in purified RO water. TDS monitors tell you if your system is getting rid of TDS just fine or if you need to change the reverse osmosis membrane.

Pressure Regulator

RO systems work under high pressure, but too much pressure can damage the filter cartridges and their housings. A pressure regulator protects the system from sudden rises in pressure.

Under Sink Reverse Osmosis System Diagram

Under sink reverse osmosis systems save countless people from drinking contaminated water. Below is a diagram of such a system showing the different parts and components and how they work.

under sink reverse osmosis system diagram

If you have any questions about reverse osmosis water filter components please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
Learn more about .


Information provided on BOS is for educational purposes only. The products and services we review may not be right for your individual circumstances.
We adhere to strict editorial guidelines. Rest assured, the opinions expressed have not been provided, reviewed, or otherwise endorsed by our partners – they are unbiased, independent, and the author’s alone. Our licensed experts fact-check all content for accuracy. It is accurate as of the date posted and to the best of our knowledge.

Leave a Comment: