RO Pretreatment Practical Handbook: Detailed Analysis of Four Major Raw Water Processes

RO Pretreatment Practical Handbook: Detailed Analysis of Four Major Raw Water Processes

Reverse osmosis (RO), as the core process for deep desalination, has become the “standard configuration” in industrial water treatment and seawater desalination. However, RO systems are as delicate as “Lin Daiyu,” demanding high-quality feedwater. The pretreatment stage serves as the “moat” safeguarding the safe and stable operation of RO systems. The success or failure of pretreatment directly determines the service life and operating costs of RO membranes. Today we delve into pre-treatment processes for four typical feedwaters: groundwater, surface water, seawater, and circulating cooling water discharge. This guide empowers you to build an impenetrable RO defense system!

01. 🌊 Groundwater

“Iron/Manganese Removal” and “Softening” are Core

Characteristics: Stable water quality with typically low turbidity and suspended solids, but rich in dissolved divalent ions like iron and manganese.

Core Challenges: Iron/manganese overshoot causes membrane oxidation; high hardness and alkalinity pose scaling risks.

→ Recommended Process Flow

This water treatment system begins with raw water undergoing pretreatment via aeration or oxidation to remove certain metal ions and organic matter. The water then passes through a multi-media filter to further remove suspended particles and colloids. It proceeds to a softener to reduce water hardness, followed by a security filter to ensure water quality meets feed requirements for the subsequent RO system. Finally, the RO reverse osmosis system completes deep desalination and purification.

Process Analysis Oxidation: Utilizes air aeration or sodium hypochlorite addition to oxidize Fe²⁺ and Mn²⁺ into insoluble Fe(OH)₃ and MnO₂ precipitates.

Filtration: The multi-media filter effectively removes these precipitates and other minute particles. Softening: Employs ion exchange softeners to remove calcium and magnesium ions, preventing calcium carbonate and calcium sulfate scaling.

Security Filtration: 5μm filter cartridges serve as the final safeguard, intercepting any potential particulate leakage.

✓ Critical Control Points

Oxidation-Reduction Potential (ORP): Ensures complete iron and manganese oxidation.

Effluent Hardness: Monitor softener effluent hardness; recommended <1 mg/L.

SDI Value: Incoming water SDI15 < 5 prior to RO.

02. 🏞️ Surface Water

“Turbidity Removal” and “Microbial Control” are Key

Water Characteristics: Significant fluctuations in water quality influenced by season and environment, containing high turbidity and colloidal matter.

Core Challenges: Severe organic and microbial contamination prone to forming biological slime, leading to membrane fouling.

→ Recommended Process Flow

Raw water undergoes chemical dosing for coagulation/flocculation to aggregate minute impurities; Then, it enters a clarification or dissolved air flotation (DAF) unit for solid-liquid separation, removing most suspended solids and colloids; The effluent undergoes deep filtration through a multimedia filter, followed by adsorption of residual chlorine and organic matter via an activated carbon filter; Subsequently, UV disinfection effectively sterilizes and inhibits microbial growth; Finally, a pre-filter ensures water quality before entering the core RO reverse osmosis system.

Process Analysis

Coagulation/Flocculation: Adds coagulants to destabilize colloids, forming settleable flocs to prepare for subsequent filtration.

Clarification/Air Flotation: Separates flocs via sedimentation or air flotation, significantly reducing turbidity and suspended solids.

Activated Carbon Adsorption: Removes organic compounds and residual chlorine, protecting RO membranes from oxidation and organic fouling.

UV Sterilization: Eliminates bacteria and algae, inhibits biological contamination, and prevents biofilm formation.

✓ Key Control Points

Flocculation Effectiveness: Determine chemical dosage via jar tests—a prerequisite for subsequent filtration efficiency.

SDI Value: This core metric requires SDI15 < 5 at the multimedia filter outlet, ideally < 3.

Activated Carbon Management: Perform regular thermal disinfection or replacement to prevent microbial breeding.

03. 🌊 Seawater

“Corrosion resistance” and “anti-scaling” are key challenges

Water Characteristics High salinity (TDS ~35,000 mg/L), high hardness, high corrosivity.

Core Challenges Extreme scaling tendency, microbial proliferation, demanding equipment corrosion resistance.

→ Recommended Process Flow

Raw water first passes through intake screens or grates to remove large suspended solids. It then enters the coagulation stage, where chemical dosing destabilizes and aggregates colloidal impurities. Coagulated water proceeds to media filters for deep retention of suspended particles. Effluent undergoes precision security filtration to remove micron-sized particles. Subsequently, RO chemicals are added based on water quality characteristics to protect RO membranes and enhance system performance; finally, pretreated water enters the core RO reverse osmosis system for desalination and purification.

Process Analysis Coarse Screen Interception: Coarse and fine screens intercept large particulate matter and marine organisms. Enhanced Coagulation: Coagulants are added to enhance the removal of colloidal and minute particles by the media filter.

Security Filtration: Serving as the final barrier, 5μm precision filter cartridges intercept any potential particulate leakage.

Specialized Chemical Dosage: Addition of scale inhibitors and reducing agents tailored for seawater characteristics prevents scaling and membrane oxidation.

✓ Critical Control Points Residual Chlorine Control: Add reducing agents to ensure zero residual chlorine in RO feedwater.

Scale Inhibitor Selection: Use inhibitors tailored for seawater's high calcium and sulfate characteristics.

Filter Differential Pressure: Closely monitor and promptly backwash to address potential biological fouling from seawater.

04. ♻️ Recirculating Cooling Water Effluent

“Ultimate Softening” and “Multi-Technology Integration”

Water Characteristics: A “concentrated version” of complex water bodies, featuring high TDS, hardness, silica, and organic content.

Core Challenges: Scaling risks, interference between organic matter and chemical agents (scale inhibitors).

→ Recommended Process Flow

Raw water first enters the chemical softening and clarification system, where chemicals are added to completely remove calcium/magnesium hardness and partial alkalinity. The effluent passes through a media filter to remove residual suspended solids before entering a weak acid cation exchanger for further deep removal of hardness related to alkalinity. Subsequently, water flows through a decarbonation tower to strip dissolved carbon dioxide. Before entering the RO system, water undergoes precision filtration via a security filter while simultaneously dosing scale inhibitor to provide final-stage protection for RO membranes. Finally, pretreated water enters the core RO reverse osmosis system for desalination.

Process Analysis

Chemical Softening: Addition of lime (Ca(OH)₂) and soda ash (Na₂CO₃) to chemically reduce hardness and silica content.

Weak Acid Exchange: The weak acid cation exchanger serves as a deep softening unit, further removing residual hardness.

Decarbonation Treatment: The decarbonation tower removes excess CO₂ generated during softening, reducing the load on the RO system.

Scale Inhibitor Dosage: Adds high-concentration-ratio scale inhibitors to mitigate scaling risks.

✓ Critical Control Points

Softening pH Control: pH regulation in chemical softening tanks is pivotal for removal efficiency and effectiveness.

Hardness and Alkalinity: Monitors effluent from the weak acid system to ensure compliance with water quality standards.

TOC/COD: Closely monitors organic content to assess fouling risks.

05. Other Typical Raw Water Pretreatment Considerations

🏭 Municipal Sewage/Reclaimed Water

Core Challenges: High organic content (COD/BOD), elevated nitrogen/phosphorus, high microbial load, residual pharmaceuticals.

Process Approach: Core treatment involves biological processes (e.g., MBR) + advanced oxidation (e.g., ozone, Fenton) + ultrafiltration (UF).

Focus on preventing organic fouling and biological contamination.

🏭 Industrial wastewater (electronics/textile dyeing/pharmaceutical)

Core challenges: Complex composition containing toxic/hazardous substances (e.g., heavy metals, high salinity, refractory organic compounds).

Process approach: Targeted pretreatment (e.g., chemical precipitation for heavy metals, advanced oxidation for bond breaking/ring cleavage, homogenization/conditioning), typically followed by ultrafiltration (UF) as a reliable barrier before RO.

II. Process Selection Decision Guide: Four-Step Method

Selecting pretreatment processes for actual projects requires systematic decision-making. How can one rapidly determine pretreatment for a specific project? Follow these four steps:

01 Precise Analysis

Obtain a comprehensive water quality analysis report. This forms the foundation for all decisions—avoid relying solely on experience.

02 Identify Primary Contaminants

Determine core pollutants and match them to corresponding process approaches: Hardness/Iron/Manganese → Groundwater process; Turbidity/Colloids/Organics → Surface water process; High Salinity/High Corrosion → Seawater process; High Pollutant Concentration → Recirculating wastewater process.

03 Select Core Process Units

Tailor solutions to specific challenges: Iron/Manganese Removal: Oxidation + Filtration; Hardness/Silica Removal: Softener or Chemical Softening + Ion Exchange; Turbidity/Colloidal Removal: Coagulation/Clarification + Multi-Media Filtration; Organic Matter/Residual Chlorine Removal: Activated Carbon Adsorption.

04 Establish the Ultimate Barrier

Standard Choice: Security Filter (Low Cost, Limited Protection Capability)

Preferred Choice: Ultrafiltration (UF) — Capable of consistently producing high-quality permeate with SDI<3, currently the most reliable process for complex water conditions.

💡 Summary

There is no one-size-fits-all pretreatment solution. Only deep insight into source water and precise combination of process units matter.

Remember: Pretreatment isn't a cost—it's the most enduring protection for your RO system.