Hotel Service Unit 14 Q1

Onboard an ocean-going vessel, there are primarily two methods for producing fresh water:

1. Distillation (Evaporation):

• Principle: This method utilizes the difference in boiling points between water and dissolved salts. Seawater is heated, causing the water to evaporate while leaving behind the salts and other impurities. The water vapor is then condensed back into liquid freshwater.   1. 2.2 Desalination by distillation www.oas.org2. Distillation and Freshwater Generation | Oceans Technology Higher Education oceanstechnology.co.uk
• Types of Evaporators:
  • Flash Evaporators: Utilize multiple stages of flash chambers operating at progressively lower pressures to evaporate and condense water.   1. Evaporator (marine) – Wikipedia en.wikipedia.org
  • Vacuum Evaporators: Operate under vacuum conditions to lower the boiling point of seawater, reducing the energy required for evaporation.   1. Vacuum evaporation – Wikipedia en.wikipedia.org
  • Vapor-Compression Evaporators: Use a compressor to increase the pressure and temperature of the water vapor, enhancing condensation efficiency.   1. Vapor-compression evaporation – Wikipedia en.wikipedia.org

2. Reverse Osmosis (RO):

• Principle: This method uses a semi-permeable membrane and high pressure to force water molecules through the membrane, leaving behind dissolved salts and other contaminants.   1. Preventing Scale Formation in Reverse Osmosis Systems – Kurita America www.kuritaamerica.com
• Process:
  • Seawater undergoes pre-treatment to remove larger particles and contaminants.   1. Seawater Pre-treatment Solutions | IDE Tech ide-tech.com
  • High-pressure pumps force the pre-treated water through the RO membrane.   1. Reverse Osmosis (RO) Process Water Treatment – Accepta accepta.com
  • Purified water (permeate) is collected on one side, while the concentrated brine is discharged.
  • The permeate may undergo further post-treatment for disinfection and remineralization.   1. How to produce fresh water at sea pt.1 – Vessel Automation vesselautomation.com

Comparison of Methods:

Feature	Distillation	Reverse Osmosis
Principle	Evaporation and condensation	Membrane filtration
Energy Consumption	Higher	Lower
Water Quality	Very high purity	High purity, but may require remineralization
Maintenance	Moderate	Can be higher due to membrane fouling
Space Requirements	Can be bulky, especially for large capacities	More compact and modular
Cost	Higher initial investment	Lower initial investment, but higher membrane replacement costs
Suitability	Suitable for various ship types	More common on larger vessels or those with high water demand

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Additional Methods (Less Common):

• Rainwater Harvesting: Collecting rainwater on deck can supplement freshwater supply, but it’s dependent on weather conditions and requires proper storage and treatment.
• Atmospheric Water Generators: These extract moisture from the air to produce fresh water, but their output is typically low and dependent on humidity levels.   1. Atmospheric water generator – Wikipedia en.wikipedia.org

Choice of Method:

The selection of the most appropriate freshwater production method on board a vessel depends on various factors, including:

• Ship type and size: Larger vessels with higher water demand may favor RO, while smaller ones may opt for distillation.
• Operational requirements: The need for high-purity water for specific purposes (e.g., boilers, laboratories) may influence the choice.
• Energy availability: The availability and cost of energy onboard can impact the decision, as distillation is more energy-intensive than RO.   1. How does the energy intensity of thermal desalination compare with reverse osmosis? | 4 Answers from Research papers – SciSpace typeset.io
• Maintenance capabilities: The crew’s expertise and resources for maintenance should be considered, as RO systems require specialized membrane cleaning and replacement.
• Environmental regulations: Some regions may have restrictions on brine discharge from RO plants, influencing the choice of technology.

Conclusion:

Both distillation and reverse osmosis are viable methods for producing fresh water on board ocean-going vessels. The optimal choice depends on a careful evaluation of various factors, including operational needs, energy considerations, maintenance requirements, and environmental regulations.  

1. Fresh Water Generator Explained – saVRee

There are several methods commonly used to kill or inactivate bacteria in water, especially potable water onboard vessels:  

1. How to Remove Bacteria from Drinking Water

1. Chlorination:

• Process: Involves adding chlorine or chlorine compounds (like sodium hypochlorite or calcium hypochlorite) to the water. Chlorine acts as a powerful oxidizing agent, disrupting the cell membranes and internal structures of bacteria, leading to their death.   1. Water Disinfection with Chlorine and Chloramine | Public Water Systems | Drinking Water | Healthy Water | CDC www.cdc.gov2. Chlorine and its importance in the inactivation of bacteria, can it inactivate viruses? www.scielo.org.mx
• Advantages: Effective against a wide range of bacteria, provides residual disinfection (continues to protect the water in the distribution system), relatively inexpensive.
• Disadvantages: Can produce disinfection byproducts (DBPs) that may have health concerns, can affect the taste and odor of water, requires careful handling and dosage control.   1. Disinfection and Disinfection Byproducts – MN Dept. of Health www.health.state.mn.us

2. Ultraviolet (UV) Disinfection:

• Process: Exposes the water to UV light, specifically UV-C radiation, which damages the DNA of bacteria, preventing their replication and effectively killing them.   1. DNA Damage Kills Bacterial Spores and Cells Exposed to 222-Nanometer UV Radiation journals.asm.org
• Advantages: No chemical addition, no taste or odor impact, effective against most bacteria and viruses, relatively low maintenance.   1. UVC Water disinfection | effictive without chemicals – Sterilsystems sterilsystems.com
• Disadvantages: No residual disinfection (protection only while the water is exposed to UV), less effective against some protozoa and spores, requires a power source.   1. Ultraviolet disinfection of drinking water – UV light – WA Health www.health.wa.gov.au

3. Ozonation:

• Process: Involves injecting ozone gas into the water. Ozone is a powerful oxidant that destroys bacteria and other microorganisms.   1. Effects of ozone treatment on cell growth and ultrastructural changes in bacteria – J-Stage www.jstage.jst.go.jp
• Advantages: Highly effective disinfectant, removes taste and odor, leaves no residual disinfectant, can oxidize other contaminants like iron and manganese.   1. Drinking Water Disinfection | Ozonation | Ozone Water Treatment – Water Professionals www.waterprofessionals.com
• Disadvantages: More complex and expensive than chlorination or UV, requires on-site ozone generation, no residual disinfection.

4. Boiling:

• Process: Heating water to its boiling point (100°C or 212°F) for a specific duration (usually 1 minute) kills most bacteria and viruses.
• Advantages: Simple and effective in emergency situations or when other treatment options are not available.
• Disadvantages: Not practical for large-scale water treatment, can affect taste, requires energy for heating.

5. Distillation:

• Process: Involves boiling water and condensing the steam to produce pure water, leaving behind contaminants, including bacteria.
• Advantages: Highly effective in removing most contaminants, including bacteria, viruses, and dissolved solids.
• Disadvantages: Energy-intensive and expensive, not suitable for large-scale water production.

Additional Methods:

• Filtration: While filtration alone may not completely kill bacteria, it can remove larger particles and some bacteria, reducing their overall numbers.
• Reverse Osmosis: This process effectively removes most contaminants, including bacteria, but it’s primarily used for desalination and may require remineralization for drinking water.
• Iodine: Iodine tablets or solutions can be used for emergency water disinfection, but they have a distinct taste and may not be suitable for long-term use.

The choice of method depends on factors like:

• Water source and quality
• Intended use of the water (drinking, sanitation, etc.)
• System capacity and cost considerations
• Environmental regulations

Often, a combination of methods, such as filtration followed by UV disinfection or chlorination, is used to achieve comprehensive water treatment and ensure its safety for consumption or other purposes.  

1. How To Remove Bacteria From Water – Pure Aqua, Inc.

It’s important to regularly monitor and maintain water treatment systems, including testing for bacterial contamination, to ensure the water remains safe and free of harmful pathogens.

Hydrated lime, also known as calcium hydroxide (Ca(OH)₂), is commonly added to water produced onboard ocean-going vessels primarily for two key reasons:

1. pH Adjustment and Alkalinity Control:

• Raising pH: The water produced by desalination processes like reverse osmosis tends to be slightly acidic due to the removal of buffering minerals. Adding hydrated lime increases the pH of the water, making it less corrosive and more suitable for drinking and other domestic purposes.
• Increasing Alkalinity: Hydrated lime also increases the alkalinity of the water, providing a buffering capacity that helps resist changes in pH. This further reduces the corrosive potential of the water and protects the ship’s piping and equipment.

2. Hardness Control and Scale Prevention:

• Reducing Hardness: Hydrated lime reacts with calcium and magnesium ions in the water, forming insoluble precipitates that can be removed through filtration or sedimentation. This process reduces the water’s hardness, preventing scale formation in pipes, boilers, and other equipment.
• Scale Prevention: Scale buildup can reduce heat transfer efficiency, clog pipes, and damage equipment, leading to operational issues and increased maintenance costs. Hydrated lime helps prevent these problems by reducing the concentration of scale-forming minerals in the water.

Additional Benefits:

• Improved Taste and Odor: By adjusting the pH and reducing hardness, hydrated lime can improve the taste and odor of the water, making it more palatable for drinking and other uses.
• Disinfection: While not a primary disinfectant, hydrated lime can contribute to microbial control by increasing the pH, making the environment less favorable for some microorganisms.

Important Considerations:

• Dosage Control: Careful control of hydrated lime dosage is essential to achieve the desired pH and alkalinity levels without overdosing, which can lead to problems like excessive scale formation or clouding of the water.
• Mixing and Settling: Adequate mixing and settling time are required to ensure the hydrated lime reacts with the water and forms precipitates that can be removed effectively.
• Filtration: Post-treatment filtration is necessary to remove any remaining suspended solids or precipitates formed during the lime treatment process.

In summary, hydrated lime is added to water produced on board ocean-going vessels to adjust pH, increase alkalinity, reduce hardness, and prevent scale formation. These benefits help protect the ship’s systems, improve water quality, and ensure its suitability for various domestic purposes.

Before bringing a freshwater tank back into service on a vessel, it’s crucial to follow a specific procedure to ensure the water is safe and potable for consumption. Here’s an outline of the recommended steps:

1. Inspection and Cleaning:

• Empty the Tank: Completely drain the tank, ensuring all residual water is removed.
• Internal Inspection: Thoroughly inspect the tank’s interior for any signs of corrosion, rust, sediment buildup, or other contaminants. If necessary, enter the tank (following confined space entry procedures) for a closer inspection.
• Cleaning: Clean the tank thoroughly using appropriate cleaning agents and disinfectants. High-pressure washing may be required to remove stubborn deposits. Ensure that all cleaning agents are thoroughly rinsed out.
• External Inspection: Inspect the tank’s exterior, piping, valves, and connections for any signs of leaks, corrosion, or damage. Repair any defects before proceeding.

2. Disinfection:

• Chlorine Solution: Prepare a chlorine solution of appropriate concentration (typically 50-100 ppm) using calcium hypochlorite or sodium hypochlorite. Follow manufacturer instructions and safety precautions when handling chlorine.
• Fill and Circulate: Fill the tank with the chlorine solution, ensuring it reaches all corners and surfaces. Circulate the solution throughout the tank and associated piping for a specified contact time (usually 30 minutes to several hours) to ensure thorough disinfection.
• Drain and Rinse: Drain the chlorine solution completely and rinse the tank thoroughly with clean water until the chlorine residual is within acceptable limits for drinking water (typically 0.2-0.5 ppm).

3. Water Quality Testing:

• Sampling: Collect water samples from various points in the tank and the distribution system.
• Laboratory Analysis: Send the samples to a certified laboratory for testing to ensure compliance with drinking water standards, including parameters like coliform bacteria, pH, turbidity, and residual chlorine.
• Onboard Testing: Conduct onboard tests for basic parameters like chlorine residual and pH using test kits or portable meters.

4. Refilling and Monitoring:

• Refill with Fresh Water: Once the tank is clean, disinfected, and tested, refill it with fresh water from a safe and approved source.
• Monitor Chlorine Residual: Regularly monitor the chlorine residual in the tank and distribution system to ensure ongoing disinfection.
• Periodic Testing: Conduct periodic water quality testing to ensure the water remains safe and potable throughout the voyage.

Additional Precautions:

• Tank Ventilation: Ensure adequate ventilation of the tank during cleaning and disinfection to prevent the buildup of hazardous fumes.
• Personal Protective Equipment (PPE): Use appropriate PPE, including gloves, eye protection, and respiratory protection if necessary, when handling cleaning agents or disinfectants.
• Record Keeping: Document all cleaning, disinfection, and testing procedures in the ship’s logbook or maintenance records.
• Crew Awareness: Educate crew members on the importance of proper water hygiene and the procedures for maintaining the potable water system.

By following these procedures and precautions, ship operators can ensure the safety and quality of the freshwater supply onboard, safeguarding the health and well-being of the crew and passengers.