Data Collection & Monitoring - Data collection is critical for monitoring the performance of any reverse osmosis (RO) membrane system. Without it, it is very difficult to determine if the RO system is fouling, suffering from scale formation; or if the RO membranes are deteriorating.

When such operating data is recorded, it should be compared to previously established alert and alarm levels. These levels should be associated with well-defined response procedures corresponding to each potential problem.

The alert and alarm levels are usually set for a 15% change from normalized start up data.

Silt Density Index (SDI)

The Silt Density Index or SDI is an on-site measurement of the suspended solids concentration in the RO feed water. The SDI measurements should be used to monitor the performance of the pre-treatment equipment.

SDI measurements should be made both pre and post multi-media filters and post cartridge filters. An SDI < 5.0 for the RO feedwater should be maintained at all times. Pre-treatment should be controlled efficiently using the designed flow rates and differential pressure limits for back-washing of the multi-media filters and replacement of the cartridge filters to give an SDI before the membranes of < 3.0.

For further details on Silt Density Index monitoring peocedures please refer to >> Silt Density Index.

Reverse osmosis system pressure drop

The pressure difference between the inlet to the initial RO membrane elements and the concentrate stream pressure coming off the tail end elements is what pushes the water across the membrane surface of all the elements. This is called the pressure drop or the hydraulic differential pressure (.P).

As long as the flows are constant, the hydraulic differential pressure will not change unless something physically blocks the passage of flow between the membrane envelopes of the elements (fouling). Therefore it is important to monitor the hydraulic differential pressure across each stage of the system. An increase in the hydraulic differential pressure can then be isolated as lead end, tail end or both to indicate the possible cause of any problems.

Salt rejection

Reverse osmosis systems are used to remove (or concentrate) dissolved salts; measuring salt rejection is therefore a direct way to monitor the performance of such systems.

Salt rejection describes the percentage of the feed water TDS that has been removed in the permeate water. The simple way to monitor the salt rejection is to measure permeate water conductivity.

The permeate water conductivity should be measured for each pressure vessel on a daily basis. This helps to determine if a high salt passage problem is universal (indicating membrane damage); if it is isolated to a certain stage (possible fouling) or if it is isolated to an individual pressure vessel (indicating O-ring problems). Probing of individual pressure vessels can also be carried out to isolate a salt rejection problem to an individual membrane element.

Normalized permeate flow

The permeate (product water) flow of the reverse osmosis system is related to both the water temperature and the net driving pressure. Permeate flow should therefore be standardized for the effects of these variables to allow better monitoring of how well water is permeating through the RO membranes.

The formula used to calculate Normalized Permeate flow is as folloes:

Qnorm = Qi * (NDPstart / NDPi) * (TCstart/TCi)

Qnorm = Normalized permeate flow

Qi = Permeate flow at point i

NDPstart = Net Driving Pressure at startup or reference condition

NDPi = Net Driving Pressure at point i.

TCstart = Temperature Correction Factor at startup or reference condition

TCi = Temperature Correction Factor at point i.

Individual membrane manufacturers provide the temperature correction factors (at a constant net pressure) to allow normalization for temperature effects.

The net driving pressure is the applied pressure minus the permeate back-pressure minus the osmotic pressure. This driving pressure is proportional to the permeate flow rate. Multiply by a ratio of the startup driving pressure to the current driving pressure to obtain the permeate flow rate if the system is at startup pressure conditions.

The calculated permeate flow rate can then be multiplied by the membrane temperature correction factor to give the normalized permeate flow.

To save time and give accurate measurements, either the membrane manufacturers or our ROCsoft software should be used to normalize all permeate flow readings.

A decline indicates that fouling or scale formation is reducing permeate flow through the membranes. An increase indicates that fouling/scaling has been removed or that membrane deterioration is occurring.

It is recommended that normalized permeate flow is monitored for each stage. This will help identify and isolate problems more accurately.

Technical support and advice

Reverse Osmosis Chemicals International work closely with a diverse range of global organisations, intelligently combining advanced treatment technologies with practical solutions to resolve complex issues. If you have a project you would like to discuss, or you require technical support and assistance; or if you simply have a question about our reverse osmosis technology solutions please contact one of our specialist advisors using our Technical Support page or call us on +44 (0) 330 223 31 31.