Living with RO

Reverse osmosis (RO) technology is used in a wide variety of healthcare applications, as a proven and cost effective method of producing high purity water for duties ranging from renal dialysis and decontamination, to boiler feed.

Reverse osmosis systems are used to remove inorganic ions and larger, non-ionic contaminants from water streams. Each system operates by passing pressurised feedwater through a specialised semi-permeable membrane. This is capable of rejecting up to 98% of the mineral salts, silica and organic compounds and over 99% of bacteria that may be present. It produces a stream of purified water known as the permeate. A waste or concentrate stream containing the rejected impurities is also produced that, in many instances, can be recycled for use elsewhere within the hospital estate.

A carefully designed reverse osmosis system will operate at an efficiency of up to 75% recovery, dependant on the feedwater quality. This is, however, dependent on the use of effective pre-treatment systems; indeed, it should be noted that the efficiency and reliability of even the best reverse osmosis plant will rapidly decline if there is insufficient provision for pre-treating the feed stream, especially in areas where feedwater has high levels of organic contamination or hardness.

To optimise the performance and longevity of the reverse osmosis system it is necessary to take a holistic approach to the construction, operation and maintenance of the entire treatment plant and to consider both pre-treatment and reverse osmosis as a combined system or process.

In essence, the pre-treatment stage is designed to maximise the operating life of the reverse osmosis membranes by removing contaminants that might otherwise foul or degrade them.

The type of pre-treatment required depends entirely on the quality of the feedwater. Pre-treatment systems normally include a water softener, to help prevent scale deposition on each 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 ultra-filter can also be inserted at the front end of the pre-treatment system.

Regardless of the configuration, it is important to ensure that each pre-treatment process is maintained in optimum condition. For example, the carbon filter needs to reduce the concentration of chlorine to a level of 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 reverse osmosis system.

Similarly, the system needs to be managed correctly to minimise running costs and extend its operating life. This involves a consistent approach to system operation and routine and preventative maintenance.

Reverse osmosis membranes may have an almost continuous flow of water through them and, even with the best pre-treatment 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 system and by pre-programmed automatic flushing, 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 reverse osmosis system to require cleaning once a quarter, subject to the quality and variability of the feed water stream.

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 that is 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 1ºC 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 pre-treatment 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 our 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, while parts such as electrodes should be cleaned of scale and contaminants and calibrated in order to ensure the accuracy of measurement.

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 SUEZ 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, maximising uptime and productivity.

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