Automatic RO permeate diversion using a conductivity controller

Short answer: Using the relay on a conductivity controller you can make an RO plant look after itself — if permeate TDS rises, the plant stops or the water goes to drain, so poor quality water never reaches the tank. You need a CT 650 controller (₹5,880) and a solenoid valve.

Alternatively, take a ROMATIC controller (₹17,850), which has all of this built in.

The problem this solves

An RO membrane weakens gradually. One day permeate TDS starts rising — but the plant carries on running and the poor quality water goes straight into the tank.

You only find out when the whole tank is spoiled, or a customer complains. On larger plants this is a serious issue, particularly in packaged water and pharma.

The solution — automatic diversion

The controller continuously monitors permeate TDS. If it exceeds the set limit, the relay operates, and that relay can either:

• Stop the plant — the simplest option
• Send permeate to drain — the plant keeps running and the membrane keeps flushing (better)
• Sound an alarm — so somebody comes to look

What you need

Alternatively the ROMATIC controller (₹17,850) provides all of it in a single unit, with no wiring complications.

What setpoint to use

Set it somewhat above your normal reading — not too close, or the plant will keep diverting unnecessarily.

Where to fit the sensor

• On the permeate line, after the membrane and before the diverter valve
• Where flow is continuous
• Permanently immersed
• Easy to remove for cleaning

A meter on the feed line as well is better still, since that gives you rejection. The formula is in our TDS guide.

Startup flush — another useful benefit

For the first few minutes after an RO plant starts, permeate TDS is high, because standing water is being flushed out of the membrane. Automatic control handles this too:

1. Plant starts — TDS high, valve to drain
2. Within 2 to 3 minutes TDS falls
3. The valve switches automatically to the tank

So only good water ever enters the tank, without anyone having to think about it.

Wiring points

1. Check the relay rating — a small solenoid valve can be connected directly, but a larger load needs a contactor
2. Keep the sensor cable away from power cables — otherwise the reading becomes unstable
3. Normally open or normally closed — choose the valve so that on power failure the water goes to drain, not to the tank. This is a safety point

Where this setup is essential

• Packaged drinking water — quality cannot be compromised
• Pharma and food — validation and audit
• Large plants with big tanks — one bad batch spoils the whole tank
• Unmanned plants — nobody is watching

Where it is not needed

• Small plants where TDS is checked daily
• Where the end use is not sensitive to a modest rise in TDS
• Domestic RO systems

There a simple TDS meter (₹2,730) is sufficient — provided somebody makes a habit of looking at it.

Frequently asked questions