RO Plant Systems - Data Collection & Monitoring

Advanced Reverse Osmosis Technology

RO Plant Systems - Data collection is critical for monitoring the performance of the membrane system. Without it, there will be no idea if the system is fouling, suffering from scale formation, or if the membranes are deteriorating.

When 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 the potential problem.

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

Silt Density Index (SDI)

The SDI is an on-site measurement of the suspended solids concentration in the feed water. It should be used to monitor the performance of the pre-treatment equipment.

SDI measurements should be made pre and post multimedia 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 replacing of the cartridge filters to give an SDI before the membranes of < 3.0.

For further details on the SDI procedure please refer to Silt Density Index Kit - manual or automatic.

RO System Pressure Drop

The difference between the inlet to the initial 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 .P 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 .P across each stage of the system. An increase in .P can then be isolated as lead end, tail end or both to indicate possible cause.

Salt Rejection

Since the RO systems are used to remove (or concentrate) dissolved salts,
measuring salt rejection is a direct way to monitor the performance. Salt rejection is 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 will then help determine if a high salt passage problem is universal (indicating membrane damage), isolated to a certain stage (possible fouling) or isolated to an individual pressure vessel (indicating O-ring problems). Probing of individual pressure vessels can be carried to isolate a salt rejection problem to an individual membrane element.

Normalized Permeate Flow

The permeate (product water) flow of the RO system is related to 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 membranes.

The formula used to calculate Normalized Permeate flow is :

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.

The membrane manufacturer provides 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. We can multiply by a ratio of the startup driving pressure to the current driving pressure to obtain the permeate flow rate if we were 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 ROCIsoft 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.

For further information or assistance please contact Reverse Osmosis Chemicals International on +44 (0) 161 877 2334 or e-mail info@rochemicals.com.