DESALINATION FAQ

Q. What is "Desalination" ?

 

A. Desalination means removing salt from saline (salty) water. Sea water is the basic source of water for our planet. But we can not use it for drinking or other domestic purposes because it is too salty. Desalination renders sea water usable for human inhabitation.

 

Q. What is the difference between  water "Desalination"  and water "Purification" ?

 

A. Scientifically, there is no difference. It is a matter of terminology. Generally speaking, when the salinity level is low and a number of other impurities are also present, water needs "purification".  The common salinity limit is 5,000 mg/L.  When the salinity is as and removing salt to make the water usable is the primary objective, water needs "desalination". 

 

Q. How much salt is in sea water ?

 

A. This varies slightly from ocean to ocean, but the range is 32,000 mg/L - 45,000 mg/L. For inland seas like the Red Sea, the salinity (salt content) is about 45,000 mg/L. For the contiguous oceans like the Pacific Ocean, Atlantic Ocean, Indian Ocean, the salinity is about 32,500 mg/L. Near the river delta, it could be as low as 25,000 mg/L

 

Q. How much salinity is good for my health ?

 

A. Too much salinity in water is harmful for human health. Salinity up to 500 mg/L is normal, but less the better. Up to 1,000 mg/L salinity is considered the limit for long-term human consumption. Anything over 1,000 mg/L has long-term harmful health effects , for example, hypertension (high blood pressure) which can cause heart failure or stroke.

 

Q. What methods can be used to desalinate sea water ?

 

A. The traditional method is to distill water. In other words, heat the sea water to vaporize it, remove the vapor to a different container and condense the water vapor by cooling it.  Rainfall is an example of the natural distillation process. The next alternative is to freeze the sea water and then thaw the ice thus formed.  Icebergs are examples of natural freeze-thaw process.  Reverse osmosis is a man-made process which can also desalinate sea water. In reverse osmosis,  sea water is forced through a semi-permeable membrane that allows only water molecules to pass through, and which retains all other impurities in water, including the ions of salt.

 

Q. Why reverse osmosis for sea water desalination ?

 

A. Reverse osmosis is the most energy-efficient process in comparison with the other two desalination processes, namely, distillation and freeze-thawing. It takes certain minimum amount of mechanical energy, also known as "free energy" to remove one gallon of water from saline solution.  For example, the  power requirement of a Blue Spring reverse osmosis desalination unit with a capacity to produce 1 gallon per minute of fresh water from sea water is  0.5 -1.4  horse power, depending on the model chosen. By comparison, for distillation process, this power requirement is 30- 186 horse power!

 

If you have a lot of energy to waste, use distillation. Otherwise Blue Spring reverse osmosis desalinators are the best bet for you.  They are designed for minimum use of energy. Only in exceptional cases in which heat energy may be available free from another source, will distillation process prove to be economical. One such example is low-pressure exhaust steam from steam turbines used in power generation industry.

 

Freeze-thawing is not a very economical or practical process because it requires refrigeration and mechanical handling of icy sludge.

 

Reverse osmosis desalinators are much smaller in size than their distillation counterparts.  Reverse osmosis plants are also easier to startup, and easy to take care of. They cost less to purchase and to operate. They are best suited for majority of applications.

 

Q. What about electrodialysis ?

 

A.  Electrodialysis is not a true water purification process. It removes only ionized salts but leaves other impurities like organic material, fluorides, iodides, poisonous chemicals, in the product water. This water will not be very appealing  to drink, neither will it be healthy. Reverse osmosis is a true water purification process. That means it extracts water from sea water, leaving all impurities behind in the reject stream. Water desalinated by reverse osmosis is tasty and healthy.

 

Q. Does desalinator remove harmful bacteria from sea water ?

 

A. Definitely. Reverse osmosis membranes are capable of removing 99.9+ % of bacteria. This level of removal is sufficient for most sea waters which contain some bacteria and other micro-organisms. The feed water intake pipe should not be located close to some source of contamination like sewer discharge point. Otherwise the reverse osmosis membranes will be overloaded with bacteria.

 

Q. Should I pass the desalinated water through a "marble"  filter ?

 

A. It is not necessary. Marble is a mineral form of almost pure calcium carbonate. The marble filter does three things. First, it corrects the pH of the freshly desalinated water. The freshly desalinated water may have pH in the range of  6.5 -7.5 pH units. If the pH is lower than 7.0, then the water may be slightly corrosive to iron piping used for fresh water.  But the corrosion problem can be avoided by using plastic piping and plastic storage tanks and in any case, it is not a serious problem. Second, the marble filter adds some calcium bi-carbonate to the desalinated fresh water. This also prevents corrosion in iron piping by coating them with calcium. The calcium also narrows down the piping, due to internal scaling. Third, it  imparts a "taste" to water. Natural water contains 20 - 80 parts-per-million of calcium which imparts it characteristic taste. The desalinated sea water has only a few parts-per-million of calcium. If you want the water to have "calcium" taste just like the natural water that you are used to, you may consider the "marble" filter post-treatment.  But the marble filter  does not offer any health benefits. We get our calcium intake from fruit and vegetables. Calcium received from these sources is more digestible than calcium received from water. Rain water does not contain calcium either, but it is still healthy water.

 

Q. Do I need UV lights to purify fresh water produced by desalinator ?

 

A. In nearly all cases, U.V. light or any further treatment of water in unnecessary. The UV light only kills bacteria which rarely exists in the reverse osmosis processed water. It is recommended that the desalinator membranes be cleaned periodically to prevent internal growth of bacteria.  For prolonged storage of drinking water, chlorination is necessary regardless of UV light disinfection of the freshly purified water. UV light has no preservative action.

 

Q. How do I select between electric powered, diesel powered and solar powered desalinator ?

 

A. This depends on what source of power you have available.

 

When economic source of electric power is  readily available, select the regular, electric powered desalinator. It is economical to purchase, easy to install and it will be economical to operate provided that the electricity is reasonably priced.

 

When electric power is not readily available, or is very expensive, you have a choice between diesel powered desalinator and a solar powered desalinator.  Diesel powered desalinator is recommended when space is limited, Sunshine is not plenty or if large quantities of fresh water needs to be produced. The solar powered system requires large array of solar electric panel and the solar panels take a lot of outdoor space.  There is no difference in the indoor space requirements. The solar electric panels are also quite expensive to purchase, compared with other energy engines like electric motor or diesel engine, for the same power output.

 

Solar powered desalinator is recommended when neither electricity nor diesel fuel is readily available, or these power sources are too expensive. Solar powered desalinators are particularly suitable for emergency water systems and for remotely isolated locations. You can always depend on the Sun, when you are stuck on an island !  One nice thing about the solar electric power is that it is free. The solar electric panels last about 25 - 30 years. Solar powered desalination systems are high-priced to begin with, but if the price is amortized over 25 -30 years, then the payoff is attractive. The solar electric panels do occupy a lot of space. For example, Blue Spring System SW-2MS solar powered desalinator which has a capacity to produce 2 m3 of fresh water in 8 hours of Sunshine period requires  33 m2 of outdoor surface to install. This surface can be on roof-top. Apartment dwellers living in high-rise coastal buildings can forget about solar powered desalination.

 

Q. What is involved in installation of desalinators ?

 

A. Every installation is different. Please consult with our technical support department, for installation requirements.

 

In general, you need a sea water intake pump, at the source of the sea water. This can be an open-intake with a surface-mounted pump, or it can be a beach-well.  The sea water pump must be installed very close to the source, within 15 meters or so. In the marine situation, the intake pump is not required if the desalinator is installed at the bottom of the vessel. This this case, there may be sufficient water head available to feed the desalinator. Intake pumps are available from Blue Spring for electric, diesel or solar power applications.

 

You also need to have a housing for installation of the main desalinator and some associated equipment.  The desalinator location can be far away from the source of water, as long as an intake pump is used. The desalinator and any support equipment need to be protected from the elements inside a housing which should preferably have a concrete slab, but a heavy-duty steel floor will do. Mobile installations inside a trailer or a truck have been done successfully. The housing should have adequate ventilation, to exhaust the heat dissipated by the desalinator.

 

You need a storage tank to collect the fresh water produced by the desalinator. The size depends on the usage pattern, but generally 1 - 7 days of fresh water supply is recommended. The tank can be erected locally with concrete or steel, or it can be supplied by Blue Spring. Field-erectable sectional steel tanks with corrosion-resistant epoxy coating are available from Blue Spring. For smaller installation, plastic tanks are the most convenient and economical.

 

You need to layout two pipelines: an intake pipe from the intake pump to the desalinator and the reject pipe from the desalinator to the ocean or a discharge point. The desalinators recover 25% - 60% of water. The remaining water is concentrated in salt and it must be discharged. The discharge point should be widely separated from the intake point, to avoid cross-mixing of the intake and the discharge water, or else the capacity of the desalinator will be reduced, and the quality of the fresh water will be affected.

 

Q. What to do with the reject stream ?

 

A. In case of brackish water well, the reject water may be collected in a shallow pond and allowed to evaporate. Or, it may be injected back into the ground at a point widely separated from the intake point. In case of sea water, the reject water may be discharged into the ocean, also at a point widely separated from the intake point. Some governments put regulations on ocean discharge, but there is no scientific basis for concern for environmental damage to the ocean due to reverse osmosis desalinator discharge because the reverse osmosis desalination process does not add any extraneous chemical impurities to the sea water. It merely extracts water from it. The natural rain does exactly the same thing.

 

Q. What kind of maintenance is involved in operating a desalinator ?

 

A. The main maintenance is to change the fine-grade prefilters, to clean the reverse osmosis membranes, to backwash the media filter - if used.  Once every few years, you will have to replace the reverse osmosis membranes.  With our module exchange program, it is easy to do.  There may be some mechanical maintenance  associated with the pumps,  the electric motors or  diesel engines.  Solar electric panels do not need maintenance. If you use storage batteries, then they may need addition of water from time to time and may need replacement after a few years.

 

Q. What is the operating cost for desalinators.

 

A. The range of operating costs for brackish water desalinators is US$ 0.22 - 0.65 per m3 of purified water.  The range of operating costs for electric powered desalinators is  US$ 0.60 - 1.80 per m3 of fresh water.  The range of operating costs for diesel powered desalinators is US$ 0.43 - 1.20 per m3 of fresh water. The operating cost for solar powered desalinators is about  US$ 0.55 per m3 of fresh water for the SW-1MS through SW-3MS models which have small capacities. Naturally, larger, commercial models have lower operating costs than smaller, portable models.

 

These operating costs are all-inclusive, and not just energy costs. They include energy, replacement membranes, filters, cleaning chemicals, operating and maintenance labor etc. Actual costs will vary from country to country, depending on local cost factors.

 

Q. Where can I get more information about my desalination project ?

 

A. Click here to get started. You can also contact our technical support department.

 

 

 

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