Getting the most out of your RO system

Summary

Product identification

This document describes various reverse osmosis (RO) membrane types and considerations for their application, focusing on fouling, pretreatment, cleaning, and troubleshooting. Key membrane types discussed include:

• Low Fouling Composite (LFC) polyamide membrane: A new generation membrane designed with a neutral surface charge and increased hydrophilicity to minimize adsorption of organic and colloidal foulants. It offers similar rejection and feed pressure requirements to durable polyamide (PA) membranes and a neutral surface charge like Cellulose Acetate (CA) membranes.
• Composite Polyamide (PA) membranes: Generally have a negative surface charge.
• Cellulose Acetate (CA) membranes: Characterized by a neutral surface charge and good resistance to biocidal chlorine.

The following table compares key characteristics of these membrane types:

Operating limits

Feed Water Quality

• Turbidity:
  • Maximum limit: 1 NTU
  • Recommended average for long-term reliable operation: ≤ 0.5 NTU
• Silt Density Index (SDI):
  • Maximum limit: 4 units
  • Recommended average for long-term reliable operation: ≤ 2.5 SDI units
  • For colloidal/suspended fouling, pretreatment aims for turbidity < 1.0 NTU and a 15-minute SDI < 4.0.
• Langelier Saturation Index (LSI): Used as an indicator for calcium carbonate (CaCO₃) saturation. Negative values suggest water is aggressive and may dissolve CaCO₃; positive values indicate possibility of CaCO₃ precipitation.
• pH Range:
  • LFC and PA membranes: 3-10
  • CA membranes: 4-6
• Temperature Limit:
  • LFC and PA membranes: 113 °F (45 °C)
  • CA membranes: 104 °F (40 °C)
• Chlorine Tolerance:
  • LFC and PA membranes: 1000 ppm-hr (feed should be dechlorinated to < 0.1 ppm)
  • CA membranes: 26,280 ppm-hr
• Chloramines Tolerance (LFC and PA membranes): 150,000 to 300,000 ppm-hours. (300,000 ppm-hours correlates to 11.4 ppm for an operating period of 3 years).
• Organic Fouling Alert Levels (for considering LFC over PA membrane):
  • Total Organic Carbon (TOC as C): 3 ppm
  • Biological Oxygen Demand (BOD as O₂): 6 ppm
  • Chemical Oxygen Demand (COD as O₂): 8 ppm

Cleaning Operations

• Cleaning Trigger: Clean when any of these conditions occur:
  • Normalized pressure drop increases by 15%.
  • Normalized permeate flow decreases by 15%.
  • Normalized percent salt passage increases by 15%.
• Cleaning Temperature: Warm water, 90-100 °F (32-37 °C), is significantly more effective.
• Cleaning Pressure Drop: Do not exceed 10 psi pressure drop per element during cleaning.

Feed water

The feed water may contain various concentrations of suspended solids (inorganic particles, colloids, biological debris like microorganisms and algae) and dissolved matter (highly soluble salts like chlorides, sparingly soluble salts like carbonates, sulfates, and silica).

Pretreatment Process Steps

Depending on raw water quality, pretreatment may include:

• Removal of large particles using a coarse strainer (e.g., mesh strainers for well water, traveling screens for surface water).
• Water disinfection with chlorine (common for surface water; some well water applications for oxidizing iron and manganese).
• Clarification with or without flocculation (using flocculants like iron or aluminum salts).
• Clarification and hardness reduction using lime treatment (increases pH, forms CaCO₃ and magnesium hydroxide particles).
• Media filtration (e.g., sand filter).
• Reduction of alkalinity by pH adjustment.
• Addition of scale inhibitor.
• Reduction of free chlorine using sodium bisulfite or activated carbon filters.
• Water sterilization using UV radiation.
• Final removal of suspended particles using cartridge filters.
  • Common nominal rating: 5-15 microns.
  • Some systems use cartridges as low as 1 micron.
• New pretreatment equipment: Backwashable capillary microfiltration (MF) and ultrafiltration (UF) membrane modules.

Pretreatment for specific sources:

• Well water: Usually limited to screening of sand, addition of scale inhibitor, and cartridge filtration due to low suspended particles. Chlorination is usually not required unless for iron/manganese oxidation.
• Surface water: Typically requires disinfection to control biological activity and removal of suspended particles by media filtration. Flocculants and organic polymers can increase filtration efficiency. Dissolved organics can be removed by activated carbon filters. Acidification and scale inhibitor addition may be required.

Mechanical / hydraulic

RO System Design

• Flux Rate (conservative design for troubleshooting avoidance):
  • Surface waters: 8-14 GFD (gallons per square foot of membrane area per day).
  • Well sources: 14-18 GFD.
• Cross Flow Velocity: Maximize cross-flow velocity in elements to reduce concentration of salts and foulants at the membrane surface.
• Cartridge Filter Rating: 5-15 microns (nominal), some as low as 1 micron.

Cleaning System Specifications

• Solution Volume: The cleaning tank should hold a minimum of 340 gallons (1270 liters) for a system with six 8" vessels (six elements per vessel) and 40 feet of 4 inch pipe (3.82" ID).
• Materials/Components:
  • Tank: Fiberglass reinforced plastic (FRP) or polypropylene.
  • Piping: PVC schedule 80 or Nylon reinforced flex hose.
  • Victaulics: Stainless Steel.
  • Valves: Stainless Steel.
  • Pump: Centrifugal type, Stainless Steel or Non-metallic composite polyesters.
  • Cartridge filters: 5 micron rating string wound modules.
• Flow Rates for Cleaning (per vessel):

Disclaimer

This summary is based on extracted text and may miss figures, footnotes, or the full context of the original document. For contractual data or complete information, please refer to the specific OEM PDF revision used on your project.

Official datasheet (PDF)

PDF datasheet