Storage-Tank Reverse Osmosis Systems: Complete UK Guide

What Does a Water Tank Do in an RO System?

The reverse osmosis membrane normally produces water more slowly than a kitchen tap can dispense it. A pressurised water tank for an RO system stores that purified water, then pushes it towards the drinking-water tap when required.

Inside the tank, a flexible diaphragm separates the water from a pre-charged air chamber. As purified water enters, it compresses the air. Opening the tap allows that stored pressure to move the water out at a useful rate.

How Does a Reverse Osmosis Storage Tank Work?

1. Mains water passes through the pre-filters and RO membrane.
2. Purified water enters the storage tank while rejected water goes to the drain.
3. Tank pressure increases as the tank fills.
4. An automatic shut-off valve stops production when the system reaches its designed pressure.
5. Opening the filtered-water tap releases stored water for drinking or cooking.

This arrangement allows many traditional systems to operate without electricity, provided the incoming mains pressure is within the manufacturer’s specified range.

Tank vs Tankless RO System: Which Is Better?

A storage-tank system is not outdated simply because it uses a tank. It can be the more sensible choice where simplicity, lower initial cost or non-electric operation matters. A tankless model is stronger where cupboard space, fast continuous dispensing or smart monitoring takes priority.

Read our tankless reverse osmosis guide for the other side of the comparison.

Advantages and Disadvantages of Storage-Tank RO

RO Tank Size and Usable Water Capacity

Do not assume a tank’s stated external volume equals the amount of water available at the tap. Part of the tank contains compressed air, and usable capacity changes with incoming water pressure, tank pre-charge and shut-off pressure.

Measure the complete under-sink area before buying, including pipework, waste fittings and stored items. For frequent cooking or larger households, compare usable drawdown and membrane production rate—not tank size alone.

Does a Storage-Tank RO System Need Electricity?

Many traditional storage-tank systems work from mains water pressure and require no electrical connection. This can simplify installation where no suitable socket is available beneath the sink. However, some tank-based systems use a booster pump or electronic monitoring, so always check the individual specification.

Reverse Osmosis Tank Pressure Explained

The air pre-charge helps push stored water out of the tank. Too little pressure can produce weak flow; too much reduces the amount of water the tank can accept. Pressure must be checked with the tank empty of water and set to the manufacturer’s figure. Many domestic tanks specify a pre-charge around 5–7 psi, but the system manual takes priority.

Why Is My RO System Not Filling the Tank?

An RO system not filling its tank does not automatically mean the tank has failed. Common causes include a closed supply or tank valve, low incoming pressure, exhausted pre-filters, restricted tubing, incorrect tank pre-charge or a problem with the automatic shut-off valve.

If water quality, pressure or component checks are required, follow the manufacturer’s troubleshooting procedure. Do not alter pressure while the tank still contains water.

Storage-Tank RO Maintenance

Replace filters on schedule, inspect the tubing and valve for leaks, and sanitise the system at the interval stated by the manufacturer. If dispensing slows noticeably, check filter condition and tank pressure before assuming a larger tank is required.

Who Should Choose a Storage-Tank RO System?

A tank-based system is worth considering when you want reliable drinking water without powering a pump, have enough cupboard space and prefer a straightforward design. Compact options such as the Aquaphor RO-101S suit many everyday kitchens, while the EcoWater ERO385 offers another traditional tank-based route.