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What’s actually under your sink? Reverse osmosis system diagram explained.

David
Last updated: January 20, 2026

When I go and change the under sink water filters, I get asked this all the time by homeowners: “There are so many tubes and canisters under there – what do they actually do?”

It can look complicated, but honestly, it’s pretty straightforward. Whether you have a basic setup or the premium Excalibur systems we install, they all follow the flow in the diagram below. Let me walk you through it, just like I would if I were standing in your kitchen.

reverse osmosis under sink water filter — Diagram of a 5-stage under sink reverse osmosis water filter system with labeled parts

The Bodyguards (Parts 1, 2, & 3)

See those three vertical filters at the bottom? Think of them as the security team.

#1 catches the visible dirt and rust.
#2 & #3 remove chlorine and odors.

Here’s a tip: Some “economy” systems you may find elsewhere will cut corners here and only give you two pre-filters instead of three. We don’t do that. We stick to the Excalibur 5-stage standard because cutting corners here just kills the expensive membrane later on.

The Main Event (Part 4)

The horizontal canister labeled #4 is the Reverse Osmosis (RO) Membrane. This is where the magic happens. It filters down to the microscopic level – blocking lead, bacteria, and fluoride. This is the difference between “filtered water” and “pure water.”

Why the big tank? (Part 6)

I know, nobody likes losing cabinet space to that big white tank (#6). But because the RO membrane filters so strictly, the water trickles through very slowly. The tank stores it up under pressure so that when you want a glass of water, you get a fast flow immediately.

The Final Polish (Part 5)

Before the water hits your glass, it runs through one last filter (#5). This ensures the water tastes crisp and fresh, removing any stale taste from sitting in the tank.

Why Go Professional?

Water is something you ingest every day, so we take it seriously. That’s why we partner with Excalibur and offer our professional installation services. It’s also why we back our work with an Unlimited Warranty for our service plan members – so you never have to worry about leaks or failures.

Short on space? If you hate the idea of that storage tank (Part 6), check out the Excalibur Tankless Smart RO System. It ditches the tank completely and uses a smart LCD screen to tell you exactly when to change filters.