Membrane pre-treatment for RO
Reverse osmosis (RO) is used in a variety of applications, ranging from use in effluent treatment either to concentrate the dissolved material to aid its recovery (or to recover water), to producing both process and drinking water from seawater or brackish water (desalination).
It continues to be essential in producing useable water supplies in areas throughout the world, either where fresh water is not available or where water demand is greater than can be supplied by the local supply. RO has many applications in areas where the water infrastructure is not well established.
In an RO system suspended particles are first removed from pre-treated water with a particle filter. A high-pressure pump then pumps the raw water to the RO membrane where the flow splits into two: the permeate, which is the purified water, and the concentrate which is now highly concentrated raw water.
The concentrate flows
into the drainage system. Depending on the initial condition of the raw water, up to 80% of the raw water can be recovered as permeate. The RO process can remove ionic species, macro-molecules, suspended solids, dissolved solids, organics, bacteria and viruses. Up to 99% of dissolved salts can be removed from the permeate.
The key element of an RO system is the membrane itself and efficient operation of the reverse osmosis system is dependent a good maintenance regime. Chemical pre-treatment of the feed water can be of critical importance in keeping the membrane in good condition.
RO is a popular and effective alternative to ion exchange methods of water purification. It has lower chemical consumption. In general, less service and maintenance is required and the unit can produce pure water continuously; there is no down time for ion exchange column regeneration.
However for efficient operation of RO systems, it is essential that the membrane is kept in the best possible condition. In particular, everything should be done to prevent clogging or fouling of the membrane because as the membrane clogs up, increased water pressure is required to maintain a given flow through the RO system.
If the membranes become clogged, then the plant needs to be shut down for cleaning, which can be both expensive and time-consuming. In fact, cleaning should be considered whenever the normalised water output rate drops by 15% from its initial flow rate (the flow rate established during the first 24 to 48 hours of operation) or when salt content in the product water rises noticeably.
Membranes are generally manufactured from organic polymers and can become blocked either by fouling or scaling. Fouling of membranes results from suspended or emulsified materials such as silica, oil, clay, iron, sulphur and organic compounds such as humic and fulvic acids and proteins that may be present in the feed water to the RO system.
Bacteria can also stick to the membrane, creating a biofilm. Chemical scaling occurs on RO membranes when the concentration of scale-forming species as
calcium carbonate, calcium sulphate, barium sulphate, strontium sulphate, and reactive silica exceeds saturation. Saturation can occur since as permeate is removed through the membrane, all impurities are left behind near the membrane surface.
The layer of water next to the membrane surface gets more and more concentrated in the dissolved and suspended materials until saturation is reached. Scale on a membrane also provides nucleation sites that increase the rate of formation of additional scale. Chemical dosing systems are being increasingly used for pre-treatment of the membranes in RO systems.
The choice of dosing pumps, controllers and chemicals is very dependent on the RO application, the nature of the feed water and the local climatic conditions. Not surprisingly, these units are often installed in inhospitable parts of the world to produce clean water where temperatures may be very high or low.
One solution to prevent scale formation is to install softening systems but for large installations this can be very expensive. An alternative is to dose proprietary antiscalants which bind calcium and magnesium and prevent them from precipitating. Acid dosing can also be used to stop calcium precipitation. Removal of scale is also important for in-situ cleaning of the membrane. Acid and alkali dosing is used for cleaning. The introduction of 98% sulphuric acid takes the pH down to two, removing any scale, then caustic soda is dosed for cleaning to remove oil and grease.
There are two ways in which chemical dosing can be used to help prevent fouling. The first approach is to dose something that will deactivate the foulant. The second is to dose a polyelectrolyte flocculant, which allows the small particles that would cause the fouling to coagulate together to form much larger particles that can then be removed by filtration or sedimentation techniques.
Chlorination (generally through the dosing of sodium hypochlorite) is the primary technique to minimise microbiological foulants because it is very effective against a wide variety of micro-organisms and can be easily deactivated using sodium metabisulphite. Other biocides may also be dosed.
In addition, sodium metabisulphate also removes dissolved oxygen from the water making it less conducive for the growth of bacteria, reducing the risk of biofilm formation. Care must be taken, however, since some membrane materials can be damaged by chlorination.
Post-membrane treatment may include the dosing of hypochlorite for disinfection. If the water is to be used for drinking, dosing with sodium chloride or calcium carbonate may be necessary for remineralisation to improve the taste. In addition, pH adjustment may be required.
Specialised dosing skids such as the one shown in above, designed and manufactured by Prominent Fluid controls are frequently supplied for these metering needs. Of particular importance are the safety features that need to be built into the packages because many of the chemicals used (for example 98% sulphuric acid) are extremely corrosive.
The skids themselves are made from polypropylene and the chemical containers are enclosed in polypropylene cabinets fitted with transparent polycarbonate doors. The pumps themselves should be carefully specified for the chemicals to be dosed. For example, stainless steel liquid ends should be used for pumping 98% sulphuric acid and pumps for liquids that might outgas should be equipped with self-bleeding liquid ends to prevent pressure build-up in the pump.
In severe climatic conditions, pumps and controllers that can withstand extremes of temperatures should be used. For many applications, proportional chemical dosing is used, where the flow of chemicals is proportional to the flow of water. In some cases, shock dosing may be necessary.
A host of safety parameters are built into the skids. These are leak detection devices, pressure relief valves, pulsation dampeners, pressure gauges and flow switches. They can be interlinked to the pump to prevent further dosing in the event of a failure and to send a warning signal to the control panel.
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