RO Part IV: System operation and monitoring

This is the fourth part of a five-part series on the design, operation, and maintenance of reverse osmosis systems for powerplants compiled by Wes Byrne, U.S. Water’s consultant on membrane technologies. Parts I, II, III, and V are identified below; the final segment will appear in an upcoming issue of CCJ ONsite.

Large RO systems will include several membrane pressure vessels. These vessels will be staged so that the concentrated salt stream from one set of parallel-plumbed vessels will be plumbed into a smaller number of membrane vessels, then possibly plumbed to another stage with an even smaller number of vessels.

Staging is based on maintaining flow velocities sufficient to keep suspended particles moving and to assist dissolved salts in diffusing back into the bulk stream from the membrane surface.

RO systems usually are operated by adjusting the membrane feed pressure as needed to achieve the desired RO permeate flow rate. This may be done with a variable frequency drive (VFD) to control the high-pressure pump motor’s rotational speed, or by using a throttle valve located directly downstream of the pump. With VFD control, the adjustment may be automatic. The RO system will also have a concentrate stream throttle system to achieve the desired concentrate flow rate. This may be a fixed system that uses an orifice plate, or more commonly a manual or automatic valve.

Along with the permeate and concentrate flow meters, pressure sensors are installed in the system piping to monitor the pressure entering the membrane elements, the concentrate pressure exiting the membrane, and possibly the pressures within the plumbing manifolds that connect the membrane vessel stages. A permeate pressure sensor may be needed, especially if there is significant or variable permeate backpressure on the system. 

The electrical conductance of the water streams is used to monitor how well the RO is removing dissolved salts. The RO permeate water conductivity is monitored along with the makeup water conductivity.

A percent salt rejection for the system is calculated by subtracting the permeate conductivity from the feed stream conductivity and then dividing this value by the feed conductivity. The salt rejection percentage is probably the most commonly monitored performance variable.

Additional instruments may be needed if there is variability in the feedwater characteristics, or if a chemical is being added. If the water acidity changes naturally or through chemical addition, then the water pH should be monitored continuously.

The pH can have a dramatic impact on the RO salt rejection. If chlorine is being removed upstream, an online chlorine monitor or possibly an oxidation-reduction potential (ORP) monitor may be used to warn against its presence.

Key variables. Monitoring the percent salt rejection is important, but is limited in its ability to show the state of the RO membrane and is generally not a good gauge of membrane fouling or scale formation.

The relative ability for water to permeate the RO membrane can be tracked using a variable called the normalized permeate flow rate, which is the RO permeate flow rate standardized for the effects of operating pressures, dissolved salt content, and water temperature. The feed-to-concentrate pressure drop tracks the resistance to water passage through the flow channels of the various membrane elements. This value may be calculated for the entire RO vessel array, or if inter-stage pressures are available, it can be calculated for the individual vessel stages.

If flow rates are not kept constant when operating the RO, it will be necessary to standardize the pressure drop for the effect of changing flow rates in calculating normalized pressure drop values. This then allows direct comparison of these values over time regardless of whether any flow rates have changed.

Small suspended particles or salt particles that coat the RO membrane surface will cause the RO normalized permeate flow rate to decline. Larger particles that get caught within the membrane flow channels and subsequently block the flow will cause the normalized pressure drop to increase.

If something in the water is chemically reacting with the RO membrane, the effect will likely be apparent in the normalized permeate flow rate, and possibly in the salt rejection. For example, if chlorine is allowed to come into contact with the RO membrane, the extent of membrane oxidation will be apparent as an increase in the normalized permeate flow rate soon followed by a decline in RO salt rejection.

A thorough understanding of the state of the RO system can thus be gained by routinely calculating and graphing the salt rejection and the two normalized performance variables. But their values may be misleading if any of the instrument readings are inaccurate. It is absolutely critical that monitoring instruments be routinely calibrated and repaired/replaced if in error.

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