P2

Question 1

What are the two types of desalination seawater intakes?

Answer 1

• Surface (open) intakes located above the seafloor.
• Subsurface intakes located beneath the seafloor or sandy beach.

Question 2

What are the major types of surface water intakes?

Answer 2

• Velocity Caps: minimize the velocity at the intake to prevent impingement.
• Travelling Water Screens: Traveling screens used with velocity cap minimize entrainment and protect downstream processes.
• Vertical and Cylindrical Wedgewire Screens: Wedgewire screens use narrow slot size and low velocities to protect organisms.
• Aquatic Filter Barriers: These have worked well in lakes with minimal current forces on the fabric barrier. Bio-growth may be an issue in seawater.

Question 3

What are the two issues associated with surface water intakes?

Answer 3

Impingement: fish get stuck to intake screen due to high intake velocity.

Entrainment: organisms that are smaller than the screen are drawn into the intake.

Question 4

What are the major types of subsurface intakes?

Answer 4

• Vertical Wells (Beach Wells)
• Horizontal Collector Wells (Ranney Collector)
• Slant Wells (Directionally Drilled Wells)
• Engineered Infiltration Gallery

Question 5

What are the advantages of subsurface intake?

Answer 5

• Potential for natural filtration pre-treatment
• No impingement and entrainment issues
• Minimizes growth of marine life on the inside of the intake pipeline
• Favoured by regulatory agencies

Question 6

Which feedwater intake option is predominantly used by thermal seawater desalination and large seawater RO facilities?

Answer 6

Thermal seawater desalination and large seawater RO facilities predominantly use open-water intakes. (surface water)

Question 7

What other flashing system can be used for desalination other than SSF?
What are the advantages and disadvantages for the proposed system?

Answer 7

• MSF : Multistage flash

Advantages:
- a lower number of pumps and pipes and thus lower maintenance and repair costs
- a simple and reliable operation
- overall a higher level of availability

Disadvantages:
- high costs for the pre-treatment
- a low level of flexibility in operation
- reduced thermal efficiency by rejection of hot brine (40 °C)

Question 8

What are the main types of evaporators used?

Answer 8

1- Submerged Evaporator
2- Falling Film Evaporator
a- Horizontal Tube
b- Vertical Tube
3- Plate Evaporator

Question 9

Which type of evaporator is most widely used in industry?

Explain answer.

Answer 9

The horizontal tube falling film evaporator is the most widely used one due to:
o Efficient water distribution and tube wetting.
o High heat-transfer rates.
o Absence of dry patches.
o Low scale formation and tube damage.
o Efficient disengagement of vapours and non-condensable gases.
o Proper venting of the non-condensable gases.
o Simple monitoring of scaling and fouling.

Question 10

What are the assumptions for an evaporator system?

Answer 10

• Steam condenses at given pressure and gives up latent heat.
• Distillate discharge temperature is the same as the evaporator discharge temperature.
• There is no salt in the distillate.
• Feed boils at the same temperature as pure water at the given pressure.

Question 11

What is the typical yield for a simple evaporator?

Answer 11

40 - 60 %

Question 12

When might MVC be used over RO?

Answer 12

If thermal desalination is required i.e. very low salt content of the distillate is required.

Question 13

Explain the difference between RO and FO.

Answer 13

Reverse Osmosis is the application of pressure to drive water out of a concentrated system, i.e. the reverse of osmosis.

Forward osmosis (FO) is a process that utilizes osmotic pressure to drive water from a contaminated or saline water feed solution across a semi-permeable membrane that retains the dissolved solutes.

Question 14

When would a 2 stage RO staging unit be used?

Answer 14

Seawater RO systems, which operate on a very high salinity feed water, at high recovery ratio and/or at high feed water temperature.

Brackish RO applications which require very low salinity permeate such as supply of makeup water for pressure boilers or production of rinse water for microelectronics applications.

Question 15

What are the main factors that affect the cost of desalination plants?

Answer 15

• Quality of Feedwater
• Plant Capacity
• Site Characteristics
• Regulatory Requirements
• Qualified manpower
• Energy cost
• Plant life and amortization

Question 16

How can the cost of desalinated water be reduced to make the desalinated water more affordable?

Answer 16

1) The economy of scale of the desalination systems’ manufacturing enterprises.
2) The continuous development of better desalination equipment.
3) Improvement of desalination system manufacturing procedures and techniques.
4) Finding better solutions to existing operation problems, stemming from accumulating (and hopefully shared) experience.
5) Competition between desalination system suppliers
6) Accumulating experience with building, operation, and management of desalination plants will reduce the level of technological and managerial risk associated with desalination projects and might bring down the interest rates and lower the capital costs.