By Nick Camin and Omar Betancourt
Users of reverse osmosis (RO) membrane water systems are burdened with numerous challenges such as optimizing energy consumption, system cleaning and maintenance. Other issues include keeping the RO system operating at peak efficiency, and extending the system life cycle.
To help users meet these challenges, suppliers are required to provide instruments with widely varying ranges in chemically resistant materials, and with compact footprints and smart technologies - whether a fast-response chlorine measurement is required or a simple pressure reading.
For a completely functional and optimized RO membrane system, pressure, flow, level, pH, conductivity, turbidity, temperature, total organic carbon, particle count, total dissolved solids and chlorine sensors are often incorporated into the instrumentation package. These sensors provide information critical to the automation systems which protect membranes from damage and keep them running at top efficiency.
Continuous access to sensor data and diagnostic information in a timely fashion and at a competitive cost has gone from being a wish to a requirement in modern designs. In this article, we'll look at the instrumentation challenges in RO membrane treatment systems, and at some of the newest solutions.
Keeping Membranes Operating
RO membrane systems are under continuous attack in several ways. Damage to the membrane can be caused by residual disinfectants, high levels of salts and particles of inappropriate or excessive size. Problems start with the feed water.
Feed water is often treated with chemicals prior to release to an RO system. These chemicals can include coagulants and chlorine which help remove suspended solids, undesired ions like Fe+ or Mn+, or organic compounds. The effectiveness of pretreatment must be monitored to avoid membrane plugging and damage.
In many applications, parameters such as raw water dissolved oxygen (DO), total dissolved solids (TDS), temperature and many others are constantly changing. Variations can be caused by any number of factors present in the feed water, and these changes can affect the quality of the finished product as well as potentially damage or foul membranes.
Here are some of the potential problems in a RO system, and where sensors are used:
- Differential pressure transmitters are used to determine whether membranes are fouled or plugged.
- Particle counters can check membrane effectiveness.
- Temperature changes alter the permeability of the membranes, requiring temperature sensors at various locations.
- Increases in TDS require measures to counter higher osmotic pressure requirements.
- Excessive chlorine content can damage or destroy membranes with direct impact on product water quality.
- Differential pressure and TDS/conductivity measurements across the membrane and outlet can provide an indication as to the condition of the system and the final quality of the permeate.
Pressure, flow and differential pressure transmitters are required, and should be selected to minimize pressure loss. These instruments, plus conductivity measurements, help monitor membrane performance and efficiency.
Conductivity is most critical because it indicates that the RO system is fulfilling its purpose. Temperature and pH measurement can help to monitor permeate (finished water) quality. ORP and/or chlorine sensors ensure chlorination and dechlorination processes are completed. Permeate flow measurement, along with concentrate (waste) flow and feed flow, allow calculation of system efficiency.
A typical membrane measurement system can have dozens of these instruments.
Automation System Challenges
A RO control system monitors and controls operating pressure, permeate flux, salt rejection and recovery rate. All of these parameters are highly interrelated, so they must be understood and correlated to ensure the system is operating within the required ranges.
The automation system monitors several parameters which indicate the performance of the RO system, and which can impact on the membrane integrity.
Percent recovery compares the feed and permeate flowrates (typically 50 - 75%) and indicates the status of the membrane flux of the array.
Salt rejection is linked to water quality produced by the RO system and is calculated with feed, concentrate and permeate conductivity. The amount of salt rejection will indicate if the membrane surface is in good condition or if a CIP (Clean in Place) operation has to be run.
The monitoring of turbidity, temperature and feed pressure is important because if the raw water quality meets the design parameters of the membrane manufacturer, a long life cycle of the most expensive consumable of the RO system, the membrane array, is ensured.
The automation system monitors the feed pressure to the boosting pump; if too high, the pump can be damaged. In the case of high temperature (95 to 113 °F depending on the membrane manufacturer), the system has to be able to interrupt the feed flow rate into the membrane array to avoid membrane damage.
In many cases, the RO system comes with its own controller, as supplied by the RO manufacturer, but the entire water treatment plant may use a distributed control system (DCS), a PC-based controller, or a Programmable Automation Controller (PAC).
The RO system controller is designed to run the RO system, but it may not have the necessary software to perform diagnostics or asset management-such as determining when to do a CIP, or when an instrument needs calibration.
Therefore, when sensors are selected for a RO system, they must be able to communicate not only with the local controller (usually by 4-20 mA or pulse outputs)-but also with a DCS, a PC-based controller or a PAC (typically over a digital fieldbus protocol).
Sensors and instruments are available to provide all the information needed by the local and main automation systems. Table 1 describes the typical sensors used in a reverse osmosis membrane system.
However, sensors for water systems often have unique challenges. While most standard flow, level, pressure, temperature and other sensors used in process industries will suffice, additional attention is frequently needed when selecting an instrument for water treatment applications. For example, sometimes the TDS content of the permeate concentration is below the minimum conductivity for a magmeter.
An ultrasonic flowmeter is often the most cost-effective option for measuring flow anywhere in an RO system, while Coriolis flowmeters should be used only when it's necessary to also measure mass flow.
For flow measurements, it's important to select a flowmeter with a minimum pressure drop because the operating pressure on the permeate side could be as low as 30 psi, making flow measurements difficult. This tends to favor magnetic, ultrasonic and Coriolis meters, and rules out other types of flowmeters.
Several analyzers are available from instrument makers that perform multiple measurements. The Endress+Hauser cm442, for example, accepts any two sensor inputs and measures turbidity/suspended solids, nitrate, SAC (UV254), ISE, dissolved oxygen, pH/ORP, chlorine, concentration and conductivity. Analyzers used for these applications must be carefully selected to work with the unique requirements of RO systems.
While fresh water is corrosive to an extent, brine-especially in desalination plants-is an even more powerful electrolyte which often requires more exotic metals in the construction of wetted sensors. To deal with brine, appropriate metals and seals are required.
Water desalination projects are increasing all over the world. The water desalination plant in Fujairah in the United Arab Emirates, for example, is one of the world's largest in terms of capacity. It supplies one million people with drinking water at a rate of 450,000 cubic meters per day, and produces 500MW of power. This is the first time a RO unit has been combined with a power plant. The facility is also equipped with a thermal desalination unit that employs distillation.
Endress+Hauser has supplied instruments for Fujairah and Taweelah desalination projects in the United Arab Emirates; the Tlemcen Seawater Desalination plant in Algeria; and the Pasir Ris Variable Salinity Water Desalination Plant in Singapore. Some of these require a large number of instruments. The Taweelah project, for example, required more than 300 sensors, some of which are specifically designed to work with brine.
Conventional water treatment plants also have instrumentation problems. These plants are often built in environmental conditions where dust, humidity and condensation are present, and extreme hot and cold temperatures can be reached. Although they are installed in a somewhat hostile environment, these sensors are often expected to provide lab-quality reliability and accuracy.
Also, because of the need for more information than just flow, pressure and temperature, the sensors should have the capability to provide information for trending analysis. Some instruments can store events, such as high temperature or low-high pH, giving a user the ability to detect events in the operation that could affect membrane performance and life cycle.
All the instruments mentioned so far have been available for decades. What makes modern instruments so different than those used 10 or 20 years ago are the intelligence, diagnostics and communications that are now built in.
Instruments and analyzers with digital communication protocols such as Profibus, HART and Foundation Fieldbus provide near-instantaneous transmission of primary and secondary parameters plus sensor diagnostic data, all included in a low-cost yet highly robust package.
For sensor to instrument/analyzer connections, some manufacturers employ proprietary digital communications technologies to improve performance and reliability. Endress+Hauser developed Memosens technology for this purpose, and licensed it to several other instrument vendors, so the technology is becoming widespread.
Memosens and other smart sensors convert analog signals to digital in the sensor heads prior to communication with a transmitter or analyzer. This eliminates troublesome mV level signals and ground loops, and allows standard cables to be used.
Memosens sensors are immune to EMF interference, can be lab-calibrated prior to field installation, and can digitally store and transmit diagnostic data from the sensor to the instrument or analyzer. The technology is available for pH, conductivity, turbidity, free chlorine, nitrate and a variety of other sensors.
Smart sensors, instruments and analyzers work together to not only transmit and store process data which can be used for predictive maintenance programs, but to also store current calibration data such as the slope and zero point of pH electrodes. This gives users the ability to insert lab-calibrated sensors into the process without additional field calibrations, eliminating the additional time and equipment required for separate field calibrations.
In a RO system, sensors are the primary components which must be inspected and calibrated on a regular basis. Using smart sensor technology, technicians can accomplish this in a temperature-controlled lab environment. Being able to take out one sensor and install a calibrated replacement-without having to shut down the system for a field calibration-saves time and labor costs.
Thanks to advances in sensors, instruments and analyzers, comparatively complex measurements in RO systems such as pH, TDS and chlorine have caught up to their flow, level and pressure counterparts. Smart sensors, instruments and analyzers provide diagnostic parameters and a new level of information for better predictive maintenance, longer life, and easier asset management.
About the Authors: Omar Betancourt is a Chemical Engineer graduated of Instituto Politecnico Nacional in Mexico City, with over 11 years experience within the water treatment market. He holds a Master degree of Business Administration degree with emphasis on Marketing from EGADE Business School. He has been collaborating with Endress+Hauser since November 2008 as Water Industry Manager in Sales Center Mexico. Nick Camin has worked in the instrumentation field for over 13 years specializing in environmental industry applications for over 10 of them. As an employee of Endress+Hauser, headquartered in Greenwood, IN, he has held the positions of application engineer, project manager, municipal business manager, regional sales manager and currently occupies the position of marketing manager for the environmental industry.