Where Are Scale Inhibitors and Reducing Agents Typically Added in RO Systems?| Insights by AQUALITEK
Learn where scale inhibitors and reducing agents are typically dosed in RO systems, why injection location matters, and how proper dosing protects membranes and improves system performance.
- Introduction
- 1. Quick Answer: Typical Injection Locations
- 1.1 Scale Inhibitors (Antiscalants)
- 1.2 Reducing Agents (Dechlorination Chemicals)
- 2. Why Injection Location Is Critical
- 3. Typical RO Pretreatment Chemical Injection Diagram
- 4. Why Scale Inhibitors Are Injected Before Security Filters
- 4.1 Ensure Full Mixing and Reaction Time
- 4.2 Prevent Membrane Surface Scaling
- 4.3 Protect High-Pressure Pump and Piping
- 5. Why Reducing Agents Are Added Before Security Filters
- 5.1 Complete Chlorine Removal Before Membrane Contact
- 5.2 Protect Cartridge Filters from Oxidative Degradation
- 6. Typical Reducing Agents and Injection Practices
- 7. Engineering Best Practices for Dosing Point Design
- 7.1 Injection Pipe Configuration
- 7.2 Injection Quill Installation
- 8. Common Chemical Dosing Mistakes to Avoid
- 9. Monitoring Key Control Parameters
- 10. Best Engineering Configuration Summary
- Conclusion
Introduction
In reverse osmosis (RO) and seawater desalination (SWRO) systems, chemical dosing plays a critical role in preventing membrane scaling, fouling, and oxidative damage.
Among all chemicals used, scale inhibitors (antiscalants) and reducing agents (dechlorination chemicals) are two of the most essential. However, their injection points must be carefully designed to ensure:
•Maximum effectiveness
•Complete mixing
•Zero membrane damage
•Stable long-term operation
This article provides a complete engineering explanation of:
•Where antiscalants and reducing agents are typically added
•Why these locations are selected
•Best engineering dosing practices
•Common dosing mistakes to avoid
1. Quick Answer: Typical Injection Locations
1.1 Scale Inhibitors (Antiscalants)
Scale inhibitors are typically injected:
After pretreatment and before the security (cartridge) filter — upstream of the high-pressure pump and RO membranes.
Typical location:
✔ Between multimedia filter / UF outlet → security filter inlet
1.2 Reducing Agents (Dechlorination Chemicals)
When feed water contains chlorine or oxidants:
Reducing agents are typically injected:
After chlorination contact tank and before the RO membranes — usually before the security filter.
Typical location:
✔ After oxidation/disinfection → before cartridge filter
2. Why Injection Location Is Critical
Correct injection point ensures:
|
Factor |
Importance |
|
Full chemical mixing |
Uniform membrane protection |
|
Accurate dosing |
Stable performance |
|
Chemical reaction completeness |
Avoid oxidant breakthrough |
|
Equipment protection |
Prevent corrosion & fouling |
Incorrect injection location can result in:
•Localized scaling
•Incomplete dechlorination
•Membrane oxidation damage
•High CIP frequency
3. Typical RO Pretreatment Chemical Injection Diagram
Raw Water → Pretreatment → Chemical Dosing → Security Filter → High-Pressure Pump → RO Membranes
Chemical injection order:
1.Oxidant (chlorine / hypochlorite) — if used
2.Coagulant / flocculant
3.Scale inhibitor
4.Reducing agent
4. Why Scale Inhibitors Are Injected Before Security Filters
4.1 Ensure Full Mixing and Reaction Time
Injecting antiscalant upstream of the cartridge filter allows:
•Complete dispersion
•Sufficient reaction time
•Uniform chemical distribution
4.2 Prevent Membrane Surface Scaling
Scale inhibitors prevent precipitation of:
•CaCO₃
•CaSO₄
•BaSO₄
•SrSO₄
•Silica
Proper dosing point ensures scale control before concentration polarization occurs inside membranes.
4.3 Protect High-Pressure Pump and Piping
Antiscalant reduces:
•Crystal formation
•Hard deposits
•Pipe wall scaling
5. Why Reducing Agents Are Added Before Security Filters
5.1 Complete Chlorine Removal Before Membrane Contact
RO membranes are extremely sensitive to oxidants:
|
Oxidant |
Damage Potential |
|
Free chlorine |
Severe oxidation |
|
Chloramine |
Gradual degradation |
|
Ozone |
Instant membrane destruction |
Therefore, reducing agents must fully neutralize oxidants before membranes.
5.2 Protect Cartridge Filters from Oxidative Degradation
Dechlorination before security filters:
•Prevents PP fiber degradation
•Extends cartridge lifespan
•Improves filtration stability
6. Typical Reducing Agents and Injection Practices
|
Chemical |
Application |
|
Sodium metabisulfite (SMBS) |
Most common |
|
Sodium bisulfite (SBS) |
Fast reaction |
|
Activated carbon |
Physical dechlorination |
|
Ascorbic acid |
Special applications |
Injection requirement:
Residual chlorine at RO inlet: <0.02 mg/L (ideally 0)
7. Engineering Best Practices for Dosing Point Design
7.1 Injection Pipe Configuration
•Use static mixers or turbulence zones
•Minimum 5–10 seconds mixing time
•Avoid dead zones
7.2 Injection Quill Installation
•Install at pipe centerline
•Anti-backflow check valves
•Corrosion-resistant materials
8. Common Chemical Dosing Mistakes to Avoid
|
Mistake |
Consequence |
|
Dosing after cartridge filters |
Poor mixing, uneven protection |
|
Insufficient reaction time |
Oxidant breakthrough |
|
Overdosing SMBS |
Biofouling, sulfate formation |
|
No online ORP monitoring |
Chlorine leakage risk |
9. Monitoring Key Control Parameters
|
Parameter |
Target |
|
Free chlorine |
<0.02 mg/L |
|
ORP |
<200 mV |
|
SDI |
≤3–5 |
|
Cartridge filter ΔP |
<1 bar |
10. Best Engineering Configuration Summary
|
Chemical |
Typical Injection Point |
|
Coagulant |
Before coagulation tank |
|
Flocculant |
Before flocculation tank |
|
Scale inhibitor |
Before cartridge filter |
|
Reducing agent |
Before cartridge filter |
|
pH adjustment |
Before antiscalant dosing |
Conclusion
In RO and seawater desalination systems:
•Scale inhibitors are typically dosed after pretreatment and before the security filter to ensure uniform scale control and membrane protection.
•Reducing agents are injected before cartridge filtration to guarantee complete dechlorination and eliminate membrane oxidation risk.
Correct injection point design is essential for system reliability, membrane lifespan, and energy efficiency.
Request More Information or Expert Advice
Share a few details, and we’ll provide deeper insights, tailored suggestions, or product support.
Our 500 LPH Reverse Osmosis (RO) System is engineered to provide high-quality purified water for commercial applications. Designed with advanced RO technology, durable components, and a user-friendly interface, this system ensures consistent performance, low maintenance, and long-term reliability.
With its compact design and robust skid-mounted frame, it’s an excellent choice for businesses that demand efficiency and quality in water purification.
TWV series Reverse Osmosis (RO) systems are pre-engineered and pre-assembled units with 2.5”/4” membrane housings（single element type）for tap water(lower TDS).They are designed for overall superior performance, high recovery rates and offer great savings with low maintenance and operation costs.
TWF series Reverse Osmosis (RO) systems are pre-engineered and pre-assembled units with 4” membrane housings（multiple elements type) for tap water(lower TDS) .The medium large volumes can help meet your a variety of commercial and industrial applications. They are designed for overall superior performance, high recovery rates and offer great savings with low maintenance and operation costs.
TWE series Reverse Osmosis (RO) systems are pre-engineered and pre-assembled units with 8” membrane housings for tap water (lower TDS). The large volumes can help meet your a variety of industrial applications. They are designed for overall superior performance, high recovery rates and offer great savings with low maintenance and operation costs.
© 2026 AQUALITEK. All rights reserved.
AQUALITEK- Aimee Hoo
AQUALITEK - Aimee Hoo