P08401: Water Desalination and Purification Device
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Reverse Osmosis

Reverse Osmosis:

Process:

The natural process of osmosis is reversed in order to gain more fresh water. Natural osmosis occurs when salt water and fresh water is introduced on opposing sides of a semi permeable membrane. Osmotic pressure is responsible for these phenomena. Reverse osmosis occurs when pressure is applied to the salt water side of the membrane and fresh water is effectively drawn from the salt water. Much of the water is rejected by the membrane and must be recycled or discarded after the water is too concentrated with salt and other contaminants. The amount of contaminants discarded by the membrane is dependant on the amount of pressure applied and the amount of fouling on the membrane. This relationship is linear. Membrane fouling can be a more significant problem when using a spiral wound or hollow-fiber membrane type. Membrane fouling can significantly affect the R.O. process and to minimize fouling pretreatment of the water to eliminate any large particles should be done. The general consensus among membrane manufacturers is to clean the RO element when there is a loss of 10-15% of normalized permeate flow rate, an increase of 10-15% of the pressure differential, or a decrease of salt rejection of 1-2%. At pressures greater than 250 psi, the total dissolved solids (TDS) rejection rate is around 90%. There are various types of membranes available with many choices in surface-to-volume ratios and flux capacities (gal/day/sq^2). The most popular choices for membrane are plate and frame, spiral wound, and hollow-fine fiber. Typically the process is used for medium brackish (2-3000 ppm TDS) or greater. Reverse osmosis, does effectively make drinking water with out the need for further treatment as it is a very good barrier to viruses and bacteria. However, should any defect occur in the separation membrane, there would be no insurance that the water is drinkable, therefore typically traditional disinfecting methods are employed to the product.

Energy Considerations:

Reverse Osmosis is a reasonably energy efficient process as it does not involve a change in phase. However, as stated earlier, the effectiveness of RO depends on the amount of pressure applied. For systems treating water with very low TDS, the pressure required is very low. Systems such as these do not require booster pumps and the natural line pressure of the feed water is typically sufficient. For highly brackish water a booster pump is required to build the feed water pressure and sufficiently remove contaminant s and bacteria. The energy required to power a pump is only energy that need be considered. Despite the mechanical energy requirements, RO does not require any thermal energy making it very competitive with other industrial methods of desalination.

Cost Considerations: Among desalination processes, RO is one of the most cost effective. There are many home systems available to treat municipal water supplies. The cost of RO increases when more brackish water is treated. Membrane fouling is increased, thus requiring more frequent maintaince.

Existing Products: There is a variety of existing RO systems for domestic and light commercial use. Typically systems designed to treat sea water have larger dimensions and require a reserve tank to hold water before reaching the booster pump. There are some smaller systems used on marine craft as well. In either case, sea water RO treatment products seem to have a significantly higher cost, making any benchmarking or reverse engineering impossible given our budget. Systems used for treatment of fresh water however are readily available and relatively inexpensive ($2-600).

How does RO meet our project goals?: The main goal of our design project is to make clean drinkable water at a reasonable cost to people with an abundance of sea water but little fresh water. RO is a reasonable answer to this goal because it can process the water with a low amount of energy and consequently a low cost as opposed to traditional methods. In addition, the process produces a reasonable amount of drinking water per day. This goal can be tarnished by our other goal of safety. The process involves very high pressures and in order to ensure safety an extremely robust design must be considered. When considering such a system material cost is increased over a system being used to treat municipal water supplies. I believe that the initial cost for an RO system that treats seat water would be high; however maintained costs would be comparable to that of a fresh water system.

Sources:

Handbook of Industrial Membrane Technology

Water Encyclopedia, Volumes 1-5

Handbok of Waste and Wastewater Treatment