Important Of Reverse Osmosis Systems For Industrial, Commercial And Residential Purposes

Reverse Osmosis has been around for many decades

. As the name applies the process involves reversing of the osmosis pressure. The liquid is forced with high pressure using a pump through the membranes housed in pressure vessels. The dissolved solids are separated from the feed and disposed as reject and the low TDS (Total Dissolved Solids) product is collected and forwarded to the end using point.

The feed water may have a low TDS of a few hundred or a few thousand ppm (Parts Per Million) or a high TDS of 35,000 to 45,000 ppm generally the sea water. The two major types of systems are generally called Brackish Water for lower TDS and Sea Water for the high TDS water. The higher the TDS, the higher pressure is required to reduce the TDS to the desired level.

The membranes are installed in ASME code rated pressure vessels generally anywhere from one membrane to 7 membranes are housed in one pressure vessel. Depending on the flow rate, a number of pressure vessels are all connected to each other via feed inlet, product outlet and reject outlet manifolds.

The RO system can be built very simple or more sophisticated. Flow, feed pressure and effluent water quality are the main factors used in designing of a RO system. Other factors such as temperature variation will have some impact in the design. Instruments such as flow indicators, pressure indicators, temperature indicators, conductivity indicators and ph indicators are used for monitoring, recording and controlling of a RO system.

The three most common design terminologies you may hear referred to for a RO system are pass, array and recovery. These three arrangements along with influent and effluent water quality and pressure are design bases of a RO system. Other factors such as temperature and type of the membrane used are secondary factors. Pass is referring to a combined number of pressure vessels feeding another combined number of pressure vessels. This is called a two pass system. The most common are single pass, two pass and sometimes a three pass system. Array is number of pressure vessels in each pass, so a 20 to 10 array is referring to 20 pressure vessels in first pass and 10 in second pass. The recovery is the percentage of the feed water produced as product. It could range as high as 75 % to 80 % or as low as 50 % for high TDS sea water. The feed water quality, product water quality required type of membranes and economics normally dictate the number of passes, the array and number of membranes in each pressure vessel. The foot print availability can also dictate the length of the pressure vessels and how they are stacked on top of each other. This is normally a concern for larger flow rates.

There are near half dozen or more membrane manufacturers around the world each manufacturing one or more different type of membranes, i.e. spiral wound, hallow fiber, thin film, etc. The most common sizes of the membranes used in municipal, industrial and commercial projects are 4 inch and 8 inch diameter with 40 inch length. Smaller diameter than 4 inch and larger diameter than 8 inch and 60 inch long membranes are also available. However generally the municipal and industrial plants with higher flow rates use 4 or 8 inch diameter, while the smaller diameter below 4 inch are used for residential and very small flow rates. On the average membranes are capable of removing 95 percent plus of the TDS from the feed water.

As the history of the RO systems and RO membranes goes, there have been many improvements to the membranes. Companies like Dow/Filmtec, Hydranautics,Toray, Koch and others have thrived to improve the flow capacity of the membranes and lowering the required pressure to reduce the capital and operating cost of the RO systems. As we stated earlier, the RO system can be very simple or more advanced. It is very common to use VFDs (Variable Frequency Drives) with the high pressure pump motors to allow the RO system more flexibility in operation and the use of ERS (Energy Recovery System) to take advantage of the reject flow pressure to reduce the power consumption and cost.

The required pressure for a brackish water system can be anywhere between 100 to 300 PSI, while the sea water system will require pressure between 800 to 1000 PSI. Pressure vessel manufactures such as Code Line, Protec and others manufacture different diameter, length and pressure rating of the pressure vessels made of FRP material and code stamped per ASME section X.

Other types of membranes are also available for ultrafiltration, nano filtration and also specialty manufactured membranes for waste water treatment.

Based on customers desire and the application, sophisticated control systems with PLC control panels, HMIs, Ethernet connection with SCADA capabilities are incorporated in a RO system.

As a final and very important note, the pretreatment for an RO system is essential to increase the life of the RO system and the membranes. Media filters and cartridge filters are the most common pretreatments used in front of the RO system to remove the suspended solids ahead of the RO membranes. Also the chemical feed systems such as coagulant, antiscalant, chlorination, dechlorination and acid for pH adjustment are used as required. Membrane cleaning is also required and the frequency depends on effectiveness of pretreatment, feed water quality and care of the plant operators.

For further information, please visit our website: www.pureaqua.com

by: James Armstrong

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