Hotel Service Unit 13 Q4

Feedwater pretreatment in reverse osmosis (RO) plants is critical to ensure the long-term performance and longevity of the membranes. It involves various processes designed to remove or reduce contaminants that can cause blockage, scaling, or fouling, ultimately safeguarding the efficiency and productivity of the system.  

1. What are the feed water requirements and process flow of industrial reverse osmosis system?

2. Water Pre-treatment for RO – Dutch Filtration

Here’s a breakdown of the key treatment stages for blockage prevention:

1. Screening and Filtration:

• Coarse Screening: Removes large debris like seaweed, plastics, and other floating objects that could damage pumps or clog subsequent filters.   1. Coarse screen – Quality and Long Service Life! – Hubert – Water Intake Systems hubert.nl
• Fine Screening: Further removes smaller particles and debris, typically using mesh screens with finer openings.
• Media Filtration: Multi-media filters, often containing layers of sand, anthracite, and garnet, remove suspended solids and reduce turbidity, ensuring a clearer feed water stream.   1. Multimedia Filtration | Multi-Media Water Filters www.waterprofessionals.com
• Cartridge Filtration: Additional cartridge filters may be used for further polishing and removal of fine particulate matter.   1. What is the Importance of Cartridge Filtration in RO Systems – Netsol Water www.netsolwater.com

2. Coagulation and Flocculation:

• Coagulation: Chemicals called coagulants (e.g., alum or ferric chloride) are added to destabilize and neutralize the surface charges of colloidal particles, causing them to clump together.   1. water treatment – coagulation-flocculation general comments – Degremont® www.suezwaterhandbook.com
• Flocculation: Gentle mixing encourages the formation of larger, heavier flocs (aggregates of particles) that can be easily removed in subsequent stages.   1. Lesson 5: Coagulation/Flocculation water.mecc.edu

3. Sedimentation or Dissolved Air Flotation (DAF):

• Sedimentation: In sedimentation tanks, the flocs settle to the bottom due to gravity and are collected as sludge for disposal.   1. Sedimentation as a water and wastewater treatment process – Issuu issuu.com
• DAF: Alternatively, dissolved air flotation (DAF) uses microbubbles to attach to the flocs, causing them to float to the surface for removal. This process is often faster and more efficient than sedimentation, particularly for removing algae and other light particles.

4. pH Adjustment and Antiscalant Dosing:

• pH Adjustment: The pH of the feedwater may be adjusted to an optimal range for RO membrane performance and to prevent scaling. This typically involves adding acid or alkali to the water.
• Antiscalant Dosing: Antiscalant chemicals are injected into the pre-treated water to prevent the formation of scale (mineral deposits) on the membrane surface. Scale can block membrane pores and reduce water flow and salt rejection.   1. RO Membrane Antiscalant Water Treatment Additives for Scale Prevention www.appliedmembranes.com2. Scale formation in reverse osmosis desalination: model development – ResearchGate www.researchgate.net

5. Disinfection (Optional):

• Depending on the feed water source and quality, disinfection may be included to kill or inactivate microorganisms like bacteria and viruses.
• This helps prevent biofouling of the RO membranes, which can lead to reduced performance and premature failure.
• Common disinfection methods include chlorination or ultraviolet (UV) light treatment.   1. Effects of chlorine disinfection on RO membrane biofouling at low feed water temperature for wastewater reclamation – IWA Publishing iwaponline.com

Why These Treatments are Important:

• Prevents Membrane Fouling: Fouling occurs when contaminants accumulate on the membrane surface, blocking pores and reducing water flow. Pre-treatment removes many of these contaminants, ensuring efficient operation and extending membrane life.   1. The Basics of Reverse Osmosis | Puretec Industrial Water puretecwater.com
• Prevents Membrane Scaling: Scaling occurs when dissolved minerals, like calcium carbonate, precipitate and form deposits on the membrane. pH adjustment and antiscalant dosing help prevent scaling.   1. Antiscalant dosing in Reverse Osmosis VS Softening – Nordic Filtration nordicfiltration.com
• Protects against Biofouling: Biofouling is the growth of microorganisms on the membrane surface, which can lead to decreased performance and membrane damage. Disinfection and other pre-treatment steps minimize biofouling risk.
• Improves Permeate Quality: By removing contaminants effectively, pre-treatment ensures the production of high-quality permeate (purified water) that meets the required standards.

By implementing these pre-treatment steps and monitoring the feed water quality regularly, RO plant operators can prevent membrane blockage, optimize system performance, and extend the lifespan of the membranes, leading to more efficient and cost-effective water purification.  

1. Optimizing Water Quality: Seawater RO Pretreatment Steps – Genesis Water Technologies

Several key parameters are measured to assess the purity of permeate (treated water) in RO plants:

1. Total Dissolved Solids (TDS):

• Measurement: TDS meters measure the electrical conductivity of the water, which is directly proportional to the concentration of dissolved salts and minerals.   1. Indicators: Conductivity | US EPA www.epa.gov
• Limit: The World Health Organization (WHO) recommends a maximum TDS limit of 500 mg/L (or ppm) for drinking water. However, RO permeate typically has much lower TDS, often in the range of 5-50 ppm.

2. Salt Rejection Percentage:

• Measurement: This is calculated by comparing the TDS of the feed water (incoming water) to the TDS of the permeate. The formula is: Salt Rejection (%) = [(Feed TDS - Permeate TDS) / Feed TDS] x 100
• Interpretation: Higher salt rejection percentages indicate better membrane performance and higher permeate purity. RO systems can achieve salt rejection rates of 95% to 99% or even higher.   1. The Basics of Reverse Osmosis | Puretec Industrial Water puretecwater.com

3. Other Parameters:

• pH: The pH of the permeate is measured to ensure it’s within the acceptable range for drinking water (6.5 – 8.5).
• Turbidity: Turbidity meters measure the cloudiness or haziness of the water, indicating suspended solids. Low turbidity values show high clarity and purity.   1. Turbidity measurement and monitoring in water quality analysis www.ysi.com2. River water quality: clarity and turbidity – Stats NZ www.stats.govt.nz
• Microbiological Testing: Samples may be sent to a laboratory for analysis to ensure the absence of harmful bacteria or other pathogens.
• Specific Contaminants: Depending on the application, the permeate may be tested for specific contaminants like heavy metals, pesticides, or other chemicals, with acceptable limits set by the WHO or local regulations.

Protecting the Purity of Water

Several measures are taken to protect the purity of permeate water:

• Pre-treatment: The feed water undergoes extensive pre-treatment (filtration, coagulation, etc.) to remove contaminants that could foul or damage the RO membranes and affect permeate quality.   1. Drinking Water Treatment: Reverse Osmosis – Nebraska Extension Publications extensionpublications.unl.edu
• Membrane Integrity: Regular monitoring and maintenance of the RO membranes ensure they are functioning correctly and rejecting contaminants effectively.   1. Seawater RO Membrane Maintenance: Best Practices for Optimal Performance – TK Water www.tk-water.com
• Post-treatment: Depending on the intended use, the permeate may undergo additional treatment, such as disinfection (UV or chlorination) to ensure microbiological safety or remineralization to adjust the mineral content for drinking purposes.
• Storage and Distribution: Proper storage and distribution systems are crucial to prevent recontamination of the purified water.
• Monitoring and Quality Control: Continuous online monitoring of key parameters like TDS, pH, and pressure, along with periodic laboratory testing, helps maintain the desired water quality and identify any potential issues early on.

Limits on Purity set by the World Health Organization

The WHO provides guidelines for drinking water quality, including maximum permissible limits for various contaminants. Some key limits relevant to RO permeate are:  

1. Drinking-water quality guidelines – World Health Organization (WHO)

• Total Dissolved Solids (TDS): 500 mg/L
• Chloride: 250 mg/L
• Sulfate: 250 mg/L
• Nitrate: 50 mg/L
• Arsenic: 0.01 mg/L
• Lead: 0.01 mg/L
• Total Coliforms: 0 (absent in 100 ml sample)
• E. coli: 0 (absent in 100 ml sample)

It’s important to note that these are guideline values, and specific regulations or standards may vary depending on local authorities and intended use of the water.

By carefully monitoring water quality and implementing appropriate treatment and protection measures, RO plants can consistently produce high-quality permeate that meets or exceeds the WHO guidelines and other relevant standards.

Even after passing through the reverse osmosis (RO) membranes, the permeate water may require further treatment before it can be considered safe and suitable for domestic use. This post-treatment is essential to address any remaining contaminants, adjust mineral content, and ensure microbiological safety.

Here’s an outline of the typical post-treatment processes for RO permeate intended for domestic use:

1. Disinfection:

• Purpose: To eliminate or inactivate any remaining microorganisms, such as bacteria and viruses, that might have passed through the RO membrane or entered the system through leaks or contamination.
• Methods:
  • UV Disinfection: Exposing the permeate to ultraviolet (UV) light at specific wavelengths (typically UV-C) disrupts the DNA of microorganisms, rendering them unable to reproduce and effectively killing them.
  • Chlorination: Adding a controlled amount of chlorine or chlorine dioxide to the permeate provides residual disinfection, protecting against recontamination in the storage and distribution system.

2. Remineralization:

• Purpose: RO removes not only contaminants but also essential minerals like calcium and magnesium. Remineralization adds back these minerals to improve the taste of the water and provide potential health benefits.
• Methods:
  • Calcite Filters: Passing the permeate through filters containing calcium carbonate (calcite) gradually dissolves minerals into the water.
  • Mineral Dosing: Adding specific mineral salts, such as calcium chloride or magnesium sulfate, to the water in controlled amounts.

3. pH Adjustment:

• Purpose: To ensure the final product water has a pH within the acceptable range for drinking water, typically between 6.5 and 8.5. RO permeate can sometimes be slightly acidic due to the removal of buffering minerals.
• Method: Adding a small amount of an alkaline solution, such as sodium hydroxide or soda ash, to raise the pH to the desired level.

4. Final Filtration (Optional):

• Purpose: To remove any remaining fine particles or turbidity that may have been introduced during post-treatment processes.
• Method: Typically, a final polishing filter, such as a carbon filter, is used to enhance the water’s clarity and taste.

5. Storage and Distribution:

• Storage: The treated water is stored in clean and sanitized tanks or reservoirs.
• Distribution: The water is then distributed through a network of pipes to various points of use, such as taps, showers, and other domestic applications.

Monitoring and Control:

• Throughout the post-treatment process, various parameters like chlorine residual, pH, and turbidity are continuously monitored to ensure the final product water meets the required quality standards.

By implementing these post-treatment steps and adhering to strict quality control measures, RO plants can produce safe and palatable drinking water that complies with all relevant health and safety regulations.

It’s important to emphasize that the specific post-treatment processes may vary depending on the initial quality of the feed water, local regulations, and the intended use of the water.