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2.1.2 Reverse Osmosis Desalination (RO)

Technology Description

This is a technology that uses pressure to separate the salt from the water and is capable of reducing water salinity from 4,000 to 400 ppm. With this technology, the seawater pressure is increased above the osmotic pressure, which allows the water to pass through semipermeable membranes, but retains the solid salt particles (Figure 46). There are several types of membranes, including cylindrical or tubular, flat plastic layers, very thin flat membranes and smooth clay fiber (known as the hollow fine fiber), and those with spiral forms (known as the spiral wound). The most commonly-used membrane is the spiral wound membrane. The required pressure to desalinate saline water is 25-35 Bar, and about 65-70 Bar to desalinate seawater (the latter requires more energy). This technology is generally very suitable for desalinating brackish water. Some experts estimate that the efficiency of this technology can achieve 99% salt removal, while the membrane can remove up to 100% of the pollutants, bacteria, germs and harmful elements, and between 95-99% of the organic compounds.

Figure 46. Flow diagram of reverse osmosis process

The main components of reverse osmosis (RO) plants are as follows:

• The initial treatment units, which remove the large dissolved solids from the seawater, prior to the feedwater flowing through the membranes. This is done to protect the membranes and to reduce salt deposits that can diminish the efficiency;
• The high pressure pumps, which increase the feed water pressure on the membrane to the point that it exceeds the osmotic pressure, thereby providing enough energy to move the water in the opposite direction to the osmotic pressure;
• The membrane collection;
• The final treatment units, in which the acidity of the water is neutralized and chlorine added to disinfect it.

This technology is well suited for desalination of brackish water and industrial effluents. Thus, it is established in inland areas where brackish water is available to provide communities and industrial zones with desalinated water.

The efficiency of water production with this method depends on the salinity level of the feed water and the number of desalination stages (number of membranes). Most reverse osmosis membranes allow less than 1% of the salt content in a single stage, leading to production of water with a salinity between 300-400 ppm. With additional costs, the salinity can be further reduced with a second-stage membrane desalination. Enhancing the potability of the produced water is very important, especially since the salinity of the produced water consists primarily of sodium chloride salt.

Extent of Use

The reverse osmosis technology (RO) was introduced in the 1970’s. After the MSF technology, it is the most-used technology in all the Gulf countries. The production capacity of existing reverse osmosis desalination units is approximately 2.54 million m³/day, representing about 22% of the total production of desalinated water. The largest quantities of desalinated water are produced in Saudi Arabia, followed by Bahrain. Although this technology was generally suitable for desalination of brackish water, it also has been used to desalniate seawater in the 1980’s. This has reduced the gap in the efficiency between this method and the MSF technology in the ability to desalinate highly-saline seawater, such as found in the Gulf. With efficient use of this technology, it is possible to produce desalinated water with a salinity less than 500 ppm using only one membrane stage. On the international scale, this technology is used to desalinate saline groundwater.

The desalinated water produced with this technology is used mainly for potable water, either directly or by mixing it with saline groundwater to reduce the costs and increase the salt concentrations.

Operation and Maintenance

Reverse osmosis is considered the best alternative to distillation, due primarily to its low energy consumption, low deposition rate, smaller required space compared to other desalination facilities, and lower costs. It also is easy to assemble, operation and maintain reserse osmosis systems, since they consist of separate, stand-alone units. Reverse osmosis equipment, however, can be badly affected if the feed water receives inadequate initial treatment, requiring replacement of the membranes. Thus, it is necessary to conduct a routine daily monitoring of the operation of the initial treatment units, in addition to cleaning the membranes and replacing damaged ones. The operation and maintenance processes also require skilled staff, since the system requires highly-accurate operation, and treatment of the raw feed water.

Level of Involvement

Interest in this technology to provide desalinated seawater for dry areas has increased over the last decade, due in part to the possibility to adjust the production capacity to meet changing demands. Advances in manufacturing membranes with high efficiency also has boosted the popularity of this technology in the countries in the West Asia region, to the extent that it has become competitive to the use of distillation units (especially with its lower operating costs. The reverse osmosis desalination process also requires less time than distillation units. The reverse osmosis systems also are less expensive, since much of it is made of plastic.

Costs

The major costs of desalination plants, particularly reverse osmosis plants, are the investment costs, which are typically distributed over their approximately 30-year operational life time. The investment costs are estimated to be about 50-60% of the costs of the produced water.

The estimated cost for water produced with this technology is US$ 0.48/m³ in Saudia Arabia and US$ 0.56/m³ in Bahrain. The overall range in countries in the region is between US$ 0.7-2.5/m³. Based on 1989-1991 prices, the construction cost for a new new desalination plant, using the two main methods of RO or MSF, are estimated to be about US$ 1,687m³.day. The estimated operational cost is estimated to be about US$ 105/m³.day (Tables 10 and 11).

Studies demonstrates that the energy required to produce 1 cubic meter of freshwater from desalination of seawater with this technology is less than 5 kilowatts, compared to the other types of technologies, which can consume up to five times this quantity. The energy costs depend on the feed water salinity. The cost of desalinating seawater can reach up to 7 times that of desalinating groundwater. The consumption of chemicals is estimated to be about 20% of production costs, while it is less than 10% for the MSF technology.

Table 10. Comparison of 1981 cost estimates for seawater distillation plants
with capacities of 3,800 and 19,000 m³/day ( x US$ 1,000)

Table 11. Comparison of 1981 cost estimates for seawater distillation plants
with capacities of 3,800 and 19,000 m³/day ( x US$ 1,000)

Effectiveness of the Technology

This technology is considered very effective for desalinating saline groundwater. Significant improvements in the field of membrane manufacturing also have made it possible to use this technology to desalinate seawater. Thus, it is possible to produce good quality water with this technology, although still less than the quality of distillation water (less than 500 ppm salinity).

Suitability

This technology is suitable for providing water for small residential and rural communities lacking sufficient freshwater sources, mainly because it does not have a high operational cost, especially with regard to energy.

Advantages

The advantages of this technology are as follows:

• It is suitable for desalinating both seawater and saline groundwater;
• It has flexibility in regard to quantity and quality of water;
• It requires less energy than the he MED and VC technologies;
• It has flexibility in regard to selection of plant locations;
• It has flexibility in regard to starting and stopping;
• It exhibits easy operation;
• It has flexibility in increasing production capacity, based on water demands, primarily because new units can be added without affecting the overall plant production;
• It requires a short construction period;
• It has low initial capital costs.

Disadvantages

The disadvantages of this technology are as follows:

• The desalinated water quality ranges between 250-500 ppm salinity;
• It requires large capital investments, and has relatively high operational costs;
• Large desalination units have long construction periods.

Cultural Acceptance

Interest in this technology is receiving increasing attention, following advances in the manufacture of suitable membranes, which can retain salt with high efficiency even from very high salinity waters

Further Development of the Technology

Additional needed research on this technology includes:

• Developments of methods for prolonging the useful life of the membranes (service duration and replacement time), which currently range between 3-7 years;
• Development in the context of less optimal initial development conditions.

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