RO Trouble Shooting

Replacing Reverse Osmosis Membrane Elements

Over a period of time, it is necessary to replace the reverse osmosis membrane element(s) of a system. This can be ascertained by

Following the steps below as and remedial actions.

To replace the reverse osmosis membrane, the system should be stopped and drained.
Before installing of membrane, serial number(s) and the position of the new membrane in the pressure tube. This is useful when comparing the membrane manufacturer’s test data against the actual performance of the membrane.
Remove both end caps of the RO pressure vessels and remove the membranes in the direction of flow. This will prevent the brine seals from obstructing the smooth removal of reverse osmosis membrane element from the pressure vessels.
The replacement elements to be inserted from the feed side of the pressure vessel. Then push the other elements one by one in the pressure vessel (s).
The brine seal and the inter-connector O – rings should be lubricated with glycerin for easy movement of membrane elements in the pressure vessel.
Brine should be put on one end of the reverse osmosis element.
Any gaps, at the time of replacing of membranes should, be limited with shims.
The end caps can be secured firmly and the reverse osmosis system started.
The system should be filled with low-pressure water prior to starting the high-pressure pump.
Any or all new elements should be rinsed thoroughly with water flowing to drain to remove residual preservative chemicals and also increase the hydrophilicity of dry membranes.
The reverse osmosis plants stabilize normally within 24 hours of operations. The system operating data should be collected after the plant performance stabilizes.

Replacing O – rings

Movement of reverse osmosis membrane elements within their pressure vessel may cause abrasion and failure of the O-Rings that seal the inter-connector to the element permeate tube

A sudden increase in permeate conductivity, without any noticeable increase in permeate flow rate can be because of a broken or missing O-Ring. Probing will determine the particular pressure vessels/membrane elements where O – ring is damaged causing increase in conductivity.
To replace the O-Ring, the reverse osmosis system should be shut down and allowed to drain. The end-cap to be removed. The damaged O – Ring is visible easily visible. The damaged O – ring to be replaced and system to be put back into service.

Probing

Probing is a technique used to identify problems affecting reverse osmosis systems. Once a particular pressure vessel is identified an increase in conductivity / salt rejection problem probing can further help pin point the membrane giving the problem.

Probing is done by inserting flexible tube through the vessel permeate connections and a part of the permeate collected from a particular membrane in the tube. This water is then tested for conductivity with a hand held TDS meter.
Probing should be performed / done while the reverse osmosis system is in service. The tube is pushed to the last membrane in the vessel through the permeate port. Then the tube is slowly pulled back and samples collected and tested for TDS. It is then gradually pulled back as diverted water samples are tested. The end of the vessel should be sampled and then every 12 inches (30 – cm) through the length of the pressure vessel. Sample readings at the new point should be noted after 20 seconds allowing sufficient time to for the new sample to completely displace the sample from the old point

Washer (Shim)

Shims are nothing but small rings of metal / plastic that has the same diameter as the end connector. These are used on the end connector between the lead, last element and the pressure vessel end cap(s). One should insert enough shims to eliminate any movement of membranes in the pressure vessel during operation. The shim(s), when added, should make it slightly difficult to install the end cap.

Reverse Osmosis Membrane Cleaners

The use of high quality membrane cleaners is an important part of a reverse osmosis system maintenance program.

It is always advantages to clean membranes at an early stage of fouling, as irreversible damage can occur when cleaning excessively fouled membranes.

Although cleaning techniques should not harm the membrane, nevertheless, the operator should always refer to the manufacturer’s instructions (Chemical) before starting any cleaning program.

Though the membrane foulant maybe often unknown, however, effective cleaning usually requires some knowledge of the type of foulant and the various cleaning options available.

Sometimes the causes of fouling can be identified from plant operation. For example, a pressure build up at the membrane inlet may indicate suspended solids, such as iron or colloidal particles in the feed water; in this case there may be an apparent increase in the silt density index (SDI). An increase in pressure at the reject discharge usually indicates scaling. The reject water analysis can be examined for clues as to its possible nature.

The rate of membrane fouling is often depending on the efficiency of the pretreatment system and the performance of the antiscalants being used.

When to clean a membrane Cleaning of reverse osmosis membranes should take place when one or more of the main operating parameters namely flux, conductivity or pressure differential, changes by 10 – 15 %. It is important to clean the membrane at an early stage of fouling. If performance deteriorates by 30 – 40%, or more, it may be impossible to recover the plant. In a highly fouled membrane the flow of the cleaning solution may fail to penetrate the fouling medium.

Reverse Osmosis Biocides

Non-oxidizing, non-ionic biocides can be used either on line or as part of a cleaning program to control biofouling in reverse osmosis (RO) membranes. When used as an on-line treatment, the biocide should be dosed prior to the RO system to control bio-growth in the membranes.

Plant Startup

Following the correct start – up procedure is an important operational part of reverse osmosis system. Prior to system start-up it is important to verify that all pretreatment systems are working according to their specifications. It may also be necessary to take water samples for analysis to confirm this.

Water quality

In the case of polyamide (thin film composite) membranes the free chlorine levels must be 0.0 ppm. The silt density index (SDI) should be according to plant system design guidelines, which are typically < 5.0.

If case there is a change in ions, temperature, pH then it is strongly recommended that a fresh water analysis maybe conducted and a new projection analysis maybe run on the new parameters.

Start-up procedures

On start-up, the inlet valve should be kept open prior to the starting of the high-pressure pump, completely fill the system with low pressure water (<100 psi / < 7 Bars). The “soft start” will prevent hydraulic shock(s).

Pre-treatment chemical addition should begin at this stage in the right quantity.

After taking the above steps the high-pressure pump to be started and the system slowly brought in – line till the recommended design permeate flow is achieved.

When starting – up the reverse osmosis system after a period of shutdown, you should flush the permeate to drain for a minimum of 30 minutes to ensure the removal of any residual preservatives.

Plant Shut-Down

Shutting-down a reverse osmosis plant system is an important operation and one that must be completed correctly. If this operation is not done correctly, then water quality and performance issues may be affected once the system is restarted.

1. Permeate flush

Prior to shut-down rinse the reverse osmosis system for 15 – 20 minutes with permeate water or till the input water conductivity is equal to te output water conductivity. This will help remove high concentration of salts, colloids and bacteria from the membrane surface. The flushing flow rate should be done as recommended by the membrane manufacturer. However, the normal flushing flow rate is 30 – 40 GPM (7 – 9.5 m3 /hr) per pressure vessel.

2. Biological growth

For system to be shut down for 24 hours or more, biological growth in the reverse osmosis system has to be controlled. To do this one will require the introduction of a suitable chemical i.e disinfectant or biocide to kill bacteria and prevent re-growth. Remember to fully rinse the system of any biocides or disinfectants prior to restarting of the reverse osmosis system.