Living with RO systems
27 Oct 2005
A carefully designed RO membrane plant will operate at an efficiency of up to 75% recovery, dependant on the feedwater quality. This is, however, dependent on the use of effective pretreatment systems. Indeed, it should be noted that the efficiency and reliability of even the best RO plant will rapidly decline if there is insufficient provision for pretreating the feed stream, especially in areas where feedwater has high levels of organic contamination or hardness.
To optimise the performance and longevity of the RO plant it is necessary to take a holistic approach to the construction, operation and maintenance of the entire treatment plant and to consider both pretreatment and reverse osmosis as a combined system or process.
In essence, the pretreatment stage is designed to maximise the operating life of the RO membranes by removing contaminants that might otherwise foul or degrade the membranes. The type of pretreatment required depends entirely on the quality of the feedwater. Pretreatment systems normally include a water softener, to help prevent scale deposition on the membrane, followed by a carbon filter to de-chlorinate the feedwater. A separate cartridge filter can also be used as a media trap to catch any fines carried over from upstream processes, while a polishing filter will remove residual colloidal or suspended solids. If the raw water supply contains a high level of organic matter, then an organic scavenger or ultrafilter can also be inserted at the front end of the pretreatment system.
Regardless of the configuration, it is important to ensure that each pretreatment process is maintained in optimum condition. For example, the carbon filter needs to reduce the concentration of chlorine to less than 0.1mg/l (1ppm), while the scavengers should reduce the silt density index (SDI) to a figure of 3 or less. In addition, softeners should output water at a ‘commercial zero’ hardness, which is less than 4ppm measured as calcium carbonate hardness.
Maintaining these levels requires regular monitoring and, when appropriate, system cleaning and disinfection, in order to maximise the performance of the downstream RO plant.
Similarly, the RO system needs to be managed correctly to minimise running costs and extend its operating life. This involves a consistent approach to system operation and to both routine and preventative maintenance.
RO membranes may have an almost continuous flow of water through them and, even with the best pretreatment technology or maintenance, will gradually blind through the accumulation of scale, organic contamination or biofilm forming agents. Although the rate of build up can be reduced by continuous operation of the plant and by preprogrammed automatic flushing of the system to dislodge contaminants before they can adhere to the membrane surface, the membranes will eventually need to be cleaned in order to remove residual contamination.
The frequency of cleaning will vary from application to application, but a normal expectation is for an RO system to require cleaning once a quarter, subject to the quality and variability of the feed.
It is important to monitor a number of process factors to ensure that membrane cleaning and disinfection is carried out at the correct intervals. The key factors include the flow rate of the permeate, concentrate and feedwater, the feedwater temperature, pump pressures and the percentage of salt rejection across each membrane. In addition, the quality of the feedwater and permeate should be monitored, especially in processes where contamination cannot be allowed to rise above critical levels.
One further point worth considering is that fluctuations in the operating efficiency of RO systems can be caused by changes elsewhere in the process. For example, something as simple as a 1degC change in the temperature of the feed stream can affect the operating efficiency of the membrane by up to 3%. In addition, blinding or failure of pretreatment filters will cause the quality and possibly the flow of the permeate stream to fall.
Regular quarterly cleaning of RO membranes is relatively straightforward and is typically carried out using specialised cleaning solutions such as Purite’s range of Roclean chemicals. Acid based cleaners are used to remove scale, and alkaline based solutions to remove organic matter. If required, special chemicals can also be used for disinfecting, but should only be added once all scale and organic matter has been removed. Solutions are simply circulated through the RO system and then flushed to drain.
Similarly, mechanical units such as pumps, valves and pipework should be inspected at regular intervals, with seals, diaphragms or other components that are prone to wear being replaced before they begin to fail. Parts such as electrodes should also be cleaned of scale and contaminants and recalibrated.
Regular maintenance will ultimately reduce operating costs by preventing unexpected downtime or component failures. Costs can be reduced still further by recovering the concentrate stream for use in low-level duties such as plant wash-down, vehicle cleaning or toilet flushing.
For many process companies, it has become increasingly cost-effective to outsource the routine maintenance of capital plant and equipment and, as a result, most leading RO manufacturers such as Purite now offer comprehensive planned or preventative maintenance programmes.
Ultimately, however, the performance and successful operation of any RO system depends on the quality of the equipment and installation, and on the ability of the end-user to monitor a number of simple, but nevertheless key, process parameters on a regular basis. This, combined with a sensible programme of maintenance, will ensure a long and reliable operating life.
Mark Bosley is Purite’s technical manager.
