Use of Reverse Osmosis membrane technology in the desalination of sea water

Last Updated 6 years ago by Kenya Engineer

The U.S geological Survey reported that 96.5 % of earth’s water is located in seas and oceans. Approximately 0.8% of earth’s water is considered to be fresh water (Kim, Amy, Karanfil, 2015). The scarcity of fresh water and the need for additional fresh water is already critical in many islands and coastal regions. Sea water is saline and requires desalination to make it fit for human consumption. There has been notable growth in the installation of sea water desalination facilities in the past few years as a means to produce additional water supply in countries with water shortage.

Reverse osmosis, commonly referred to as RO, is a process where you demineralize or deionize water by pushing it under pressure through a semi permeable reverse osmosis membrane. In RO salt water is forced against membranes under high pressure where fresh water passes through. Other pressure driven membrane filtration systems include: microfiltration, ultrafiltration, and nanofiltration. The application of these systems depend on pore size and charge of solute.

Reverse Osmosis has been applied to a variety of salty water resources using tailored pretreatment and membrane system design. Desalination by RO requires the use of a permeable membrane which allows water to pass through it at much higher rate than dissolved salts can, therefore leaving behind the salts. The water flowing through the membrane is encouraged to flow through the membrane by the pressure differential created between the pressurized incoming sea water and the product water (El-Sadek, 2010). The remaining feedwater continues through the pressurized side of the reactor as brine. No heating or phase change takes place. The major energy requirement is for the initial pressurization of the feed water.

RO membranes is capable of removing up to 99%+ of the dissolved salts (ions), particles, colloids, organic compounds, bacteria and pathogens from the feed water. However, an RO system should not be relied upon to remove 100% of bacteria and viruses.  Any contaminant that has a molecular weight greater than 200 is likely to be rejected by a properly running RO system. Likewise, a greater ionic charge value of the contaminant makes it harder for the contaminant to pass through the RO membrane. Previous RO system were unable to remove gases such as CO₂ efficiently because the gas is not adequately ionized while in solution and it has a low molecular weight. This resulted in the permeate water having a slightly lower than normal pH level depending on CO₂ levels in the feed water as the CO₂ is converted to carbonic acid.

This study explores a double pass RO system where the permeate from the first RO becomes the feed water to the second RO thus producing a much higher quality permeate because it has gone through two RO systems. Adding caustic after the first pass, increases the pH of the first pass permeate water through converting Na₂CO₃ to bicarbonate (HCO₃^-1) and carbonate (CO₃^-2) for better rejection by the RO membranes in the second pass. This can’t be done with a single pass RO because injecting caustic and forming carbonate (CO3^-2) in the presence of cations such as calcium will cause scaling of the RO membranes.

The performance of RO membrane is usually measurement of water flux and salt rejection for the membranes, which indicates the suitability of the membrane for the application. To ensure good performance, membrane type, flow control, feed water quality, temperature and pressure are factors that enable maximizing output of water.

THEORETICAL CONCEPTS

Osmosis is a naturally occurring phenomenon where a weaker saline solution tends towards a strong saline solution, which means RO is essentially the process of osmosis in reverse. While osmosis occurs without requiring energy, reversing the process requires energy. A RO membrane is a semi-permeable membrane that allows the passage of water molecules but not majority of the dissolved salts, organics, bacteria and pathogens. (Puretec, n.d) However, the water molecules need to be pushed through the RO membrane by applying a pressure greater than the naturally occurring osmotic pressure. This allows desalination of water in the process, allowing pure water through while holding back a majority of contaminants. The figure below shows a diagram of RO process.

Proper pretreatment of feedwater using both mechanical and chemical treatments is critical for an RO system to prevent fouling, scaling and costly premature RO membrane failure as well as frequent cleaning requirements. Pretreatment should include a multimedia filter, softener and activated carbon filter. The backwashable filter and softener vessels are fibre glass reinforced plastic (FRP). A stainless steel needle valve is used to control concentrate flow, and permeate flow varies. (Tate J., 2008). Cleaning RO membranes is not only about using the appropriate chemicals but also factors such as flows, water temperature, and quality, as well as properly designed and sized cleaning skids. RO pretreatment is optimized based on the feed water characteristics and source. Suspended solids are removed by filtration.

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 membrane. This leaves the dissolved salts behind in the reject stream. The pressure required to overcome the osmotic pressure depends on the concentration of the feed water.

The feed water is pumped into the RO system resulting in the desalinated water (permeate /product water) and the concentrate (retentate (Puretec)/brine/reject stream). The reject stream can either be drained or fed back into the feed water to be recycled in the first stage thus increasing the system recovery. 

MODEL DEVELOPMENT AND RO SYSTEM PERFORMANCE

The schematic diagram of the RO desalination system with Turbine is shown in Fig 2 and the modelling equations follow after. The main components of the RO system are a pump unit which supplies high pressure feed water, at pressure Pf and flow rate, Q;f to a RO membrane, and an energy recovery turbine. The turbine generates energy from the rejected brine stream and directly powers the pump. The model was designed to predict the system performance and support the optimisation of the permeate quality and flow rate.

Since the flow rates and the concentration of permeates and rejects were seen to remain constant, only the feed concentrations varied with time.

The two most meaningful methods of measuring suspended solids are turbidity and Silt Density Index (SDI). Turbidity is most commonly measured in Nephelometric Turbidity Units (NTU) and is increased as the water’s ability to scatter light (transparency) decreases. SDI is a calculation of fouling potential according to test standard ASTM D-4189. (Qadir, 2011)

Turbidity should be less than 1.0 NTU for optimal performance. Acceptable SDI levels at the RO inlet are less than 5.0 (15 min test), but SDI should be less than 3.0 for optimal performance. (Tate, J., 2008).

ECONOMIC ANALYSIS

Among the major determining factors for estimating the cost of water is the cost of available energy. The principal cost factors considered include capital investment, maintenance cost and the cost of supplying saline water to the desalination system. The labour cost can vary greatly, and is subject to the local economy. Cost balance equations for the required components in the system are presented in Table 1. The expenditure connected with setting up and operation of a desalination plant, include the initial concept, design, obtaining of permits, finance, construction and the commissioning and acceptance testing for normal operation is defined here as the capital cost.

The total water cost (TWC) is estimated by adding the capital cost to the operating cost for the length of the contract and dividing the total of the annual capital costs and the annual Operating and Maintenance costs by the average annual potable water production volume. As is typical, the TWC excludes distribution costs, especially where alternative delivery contracts are concerned. (M. Sarai Atab, et al., 2016)

DISCUSSION

The efficiency of RO system is high due to the fact that it consumes absolutely no energy except for the initial pressurization of the feed water. This is further enhanced by high amounts of water that are recovered during the process.

If the recovery % is too high for the RO design, it can lead to larger problems due to scaling and fouling. The % Recovery for an RO system is established with the help of design software taking into consideration numerous factors such as feed water characteristics and RO pretreatment before the RO system. A high % recovery leads to more concentrated salts and contaminants in the concentrate stream. This can lead to higher potential for scaling on the surface of the RO membrane.

Reverse Osmosis can remove between 95 and 99 percent of TDS. RO can also remove fluoride, chlorine, and other impurities. This technique of desalinating salty water improves the overall taste of water, appearance, and odor. Families and Industries who have installed the system often benefit from better quality water which in turn saves a lot money involved in buying water.

Here are some disadvantages of Reverse Osmosis: 

• Inadequate maintenance can cause clogging, scaling, and fouling of the RO system
• The system takes time to desalinate the feed water
• The RO membranes ought to be regularly replaced while sterilization and cleaning should be done annually.

CONCLUSION

In summary, several technologies for concentrate treatment are emerging and some may offer the potential of enhanced water recovery and reduced concentrate. However, no one technology is appropriate for all instances. Reverse Osmosis is an effective and proven technology to produce water that is suitable for many industrial applications that require demineralized water especially in coastal regions (Greenlee, et al., 2009). Further post treatment after the RO system such as mixed bed deionization can increase the quality of the RO permeate and make it suitable for the most demanding applications. Proper pretreatment and monitoring of an RO system is crucial to preventing costly repairs and unscheduled maintenance. With the correct system design, maintenance program, and experienced service support, the RO system should provide many years of high purity water.

Table 1: Cost balance equations