Where Are Scale Inhibitor and Reducing Agent Dosing Points Located in RO Systems?| Insights by AQUALITEK

Overview of Chemical Dosing in RO Systems

In industrial RO systems, chemical dosing is used to:

•Prevent membrane scaling

•Remove residual oxidants

•Protect membrane performance and lifespan

Among all chemicals used, scale inhibitors and reducing agents are the most critical for membrane protection, and incorrect dosing location can lead to membrane fouling, chemical wastage, or irreversible damage.

Typical Dosing Point for Scale Inhibitors

Standard Location

After pretreatment and cartridge filtration, but before the high-pressure pump

More specifically, scale inhibitors are typically dosed:

•After multimedia filters / softeners

•After activated carbon filters (if present)

•After cartridge (security) filters

•Immediately upstream of the RO high-pressure pump

Why This Location Is Ideal

1. Clean and Stable Water Quality

After pretreatment:

•Suspended solids are minimized

•Turbidity is stable

•Chemical consumption is predictable

This ensures the inhibitor works effectively on dissolved salts rather than being wasted on particulates.

2. Sufficient Mixing and Contact Time

Injecting before the high-pressure pump allows:

•Thorough mixing due to pump turbulence

•Uniform distribution across all membrane elements

3. Maximum Protection at the Membrane Surface

Scale formation occurs:

•On the membrane surface

•Inside the concentrate channels

Dosing upstream ensures protection before concentration polarization occurs.

4. Avoidance of Pretreatment Interference

Dosing scale inhibitor upstream of filters may:

•Be partially adsorbed by filter media

•Reduce chemical effectiveness

Typical Dosing Point for Reducing Agents (Dechlorination Chemicals)

Standard Location

After activated carbon filtration and before the RO membrane

Common reducing agents include:

•Sodium metabisulfite (SMBS)

•Sodium bisulfite

•Sodium sulfite

Why Reducing Agents Are Dosed Here

1. Final Protection Against Residual Oxidants

Even after carbon filters:

•Trace free chlorine or chloramine may remain

•Breakthrough events can occur

Reducing agents serve as a chemical safety barrier.

2. Preventing Irreversible Oxidative Damage

RO polyamide membranes are extremely sensitive to:

•Free chlorine

•Chloramines

•Other oxidants

Once oxidized, membrane damage is permanent and non-recoverable.

3. Avoiding Chemical Consumption by Pretreatment Media

If dosed too early:

•Activated carbon will consume the reducing agent

•Chemical cost increases

•Dechlorination control becomes unreliable

Recommended Injection Position

✔ After activated carbon
✔ After cartridge filter (or immediately before)
✔ Before the high-pressure pump
✔ With sufficient mixing length before membranes

Key Differences Between the Two Dosing Points

Best Practices for Chemical Dosing Layout

✔ Use dedicated injection quills
✔ Install non-return (check) valves
✔ Ensure adequate mixing length
✔ Monitor residual chlorine downstream
✔ Calibrate dosing pumps regularly

Common Mistakes to Avoid

❌ Dosing chemicals before multimedia filters
❌ Combining scale inhibitor and reducing agent injection points
❌ Injecting too close to membrane inlet without mixing
❌ Relying solely on carbon filtration for dechlorination

Conclusion

In a properly designed RO system, scale inhibitors are typically dosed after cartridge filtration and before the high-pressure pump, while reducing agents are dosed after activated carbon filtration as a final barrier against oxidants. Correct placement ensures maximum membrane protection, chemical efficiency, and long-term system stability.