Marine Desalination and Water Treatment
Salt water is desalinated to produce water suitable for human consumption. Reverse Osmosis occurs when, for example, you put salt water on one side of a semi-permeable membrane and pressure moves the water molecules through the filtering membrane as the larger molecules-including sale molecules- stay trapped behind. For salty sea or ocean water, a considerable amount of pressure is required to move the water through a filter, where each pore is just a fraction of the size of a human hair (source: American Chemical Society). This means a series of pumps are usually in play, all exerting pressure on the water (source: WorldPumps.com).
There can be a public perception that desalinated water doesn’t taste good and isn’t good for you. But desalinated water – straight from the tap – is safe to drink. A study in Saudi Arabia, for example, found no significant differences between desalinated water served on tap and bottled water.
Reverse osmosis Desalination Plants (also called water makers) are the marine industry standard for on-board freshwater production. From single-pass potable water plants to sophisticated multi-pass technical water machines the systems enable the desalination of seawater.
How does reverse osmosis work?
The term “watermaker” or “fresh watermaker” is not really accurate. Desalination of Seawater Reverse Osmosis Systems do not “make” water, it actually just extracts the fresh water out of existing (seawater).
The membrane used in the equipment is what makes this possible. All watermakers work on very similar basic principals, however they vary greatly with the quality of components, control and safety systems.
Desalination and water treatment by Reverse Osmosis occurs (RO) works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The RO membrane has a tight pore structure (less than 0.0001 micron or 500,000 times less than the diameter of a human hair- see above) that effectively removes up to 99% of all contaminants and impurities such as total dissolved solids.
Desalination and water treatment is reliant on use of a Filter, the most popular being a sediment cartridge style filter. For the most part these filters are disposable items once they have “blocked up” – not allowing enough flow thru the cartridge. Most filters are given ratings by their manufacturers that describe their effectiveness at removing particles down to a specified size. The “Water Quality Association” (WQA) states that the filter shall filter out at least 85% of the particles of the size it is rated for; therefore a filter that is rated at 5 micron will select 84% of those particles that a re 5 micron or larger from the water that passes thru it. If you need extra filtration you may place in line and before the 5-micron filter a 10/20-micron rates filter, which will reduce the load off the primary filter.
Cost of Desalination
Desalination and water treatment costs can vary depending on the needs of the vessel and size of the machine. You can purchase a locally manufactured 90 litre 1-3 phase Desalination Kit for $10,820 or a 200 litre 1-3 phase Desalination Kit for $14,750. Initial purchase price generally reflects the quality of the unit, however when seriously looking at purchasing a Desalinator the potential owner not only needs to look at the purchase price, but costs over the lifetime of the unit.
Consumables and spares can vary greatly and be expensive over a period of time. Check whether there is a service branch for the product purchased in their locality. Not a local mechanic who is willing to “give it a go” but a trained technician who has intimate knowledge of the machine and access to spare parts. With regards to spare parts if you do purchase a machine not locally produced you will be looking at a minimum of 10-15 days wait for shipping to Australia.
The buyer would be wise to check too that the system that he is purchasing is upgradable. Desalination plants in Australia marketed as Kits enable the installer to customise the unit or water purification system to meet specific requirements. A standard framed unit cannot always be installed in the space available.
An inbuilt Desalination plant is typically automated and you don’t need to move hoses around when in use. They can be used while underway, even in heavy seas and are self-contained.
View locally manufactured and service supported Citor Desalination Systems here.
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