Factors Affecting Reverse Osmosis Operation

 

Factors Affecting Reverse Osmosis Operation

Keeping water quality at optimal levels is essential to promoting process efficiency and user satisfaction while avoiding premature corrosion and equipment failures.

Clean water is used in various industries and processes for different end uses.  The end-use function defines the level of water treatment or purification level needed on a given feed source.  One of the most common water end uses is pure or ultra-pure water.

A reverse osmosis (RO) system, used in water purification, removes dissolved and suspended ions, gases, organics, and silica from feed water to generate high-purity water. By removing the salts, minerals, and other impurities, the RO process generates high-purity water that can be used in power generation, electronics manufacturing, and other applications such as:

• Pretreatment for deionizers, ultra-pure water, and Water for Injection (WFI)
• Production of zero-hardness water, used for boiler feedwater 
• Treatment of sanitary waste 
• Semiconductor manufacturing

How Does RO Work?

Osmosis (figure 1) is the natural tendency of a fluid — such as water  — to pass through a semipermeable membrane from a less contaminant-concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane. 

In RO (figure 2), pressure is exerted on the side with the concentrated solution to force the water molecules across the semi-permeable membrane to the permeate (pure) waterside. RO uses membrane separation which has been proven to be one of the most economical means to remove up to 100% of suspended solids and approximately 90% of dissolved solids, dissolved silica, alkalinity, and hardness from feed water.

The semi-permeable membrane inhibits the majority of dissolved impurities from passing through to the pure-water side. The amount of impurities carried over depends on the type and condition (age, cleanliness) of the membrane and the amount of pressure applied (energy) to the process. 

Not all the feedwater passes through the membrane. Some is diverted to flow over them to clean away the rejected impurities in a crossflow filtration mode. The RO system produces one purified water stream called permeate and a second stream referred to as concentrate, brine, or reject (figure 3).

Challenges in RO Measurement Points

Depending on the water end-use and the typical process flow, a facility determines which system is needed for their water quality requirements: a single-, double-, or triple-pass system.  Most industrial installations will see a 75% recovery using a double-pass stage system (figure 4).  

A double-pass RO system is typically installed upstream of the demineralizer. Its performance is pH-dependent with the second-pass section most dramatically affected. While these changes are not significant in the majority of applications, variations can become crucial to the success of high-purity water processing.

Manufacturers need to preserve the RO membranes because replacement costs are high. It’s important to keep the membranes clean, not put too much pressure on them, and make sure the upstream- and downstream conditions are suitable for membrane requirements.

Membranes lose efficiency over time as contaminants build up. By cleaning the membrane, most of the efficiency is restored. The greater the time between membrane cleaning, the more permanent the efficiency loss can be. Frequent cleaning is costly and needs to be balanced with the production targets and membrane replacement.

For optimal performance and good membrane health, a facility must measure and monitor points such as pressure, flow, temperature, pH, and conductivity. And in legacy systems, oxidation-reduction potential (ORP) must be measured and monitored. These measurement types have direct impacts on the system's efficiency and success.  

Pressure — The build-up of differential pressure across the membranes becomes a burden for the system because it causes the process to generate permeate water more slowly. The extra strain on the system requires the membranes to be cleaned or replaced more frequently. A differential pressure measurement can be used to determine when the membranes need to be cleaned or replaced before the membrane fails. A pressure transmitter can be used in these applications. Note that if the process is halted, problems can arise from vibration and pressure surges in the system

Flow — Flow rates can indicate if there is a problem with a pump or another piece of instrumentation upstream of the RO system. For example, flow combined with differential pressure assists in diagnosing membrane or filter clogging — low flow and low DP could mean a pump issue, while low flow plus high DP could indicate clogging. Proper monitoring of the feed water upstream of the RO system is critical for system success. Improper flow rates and membrane fouling result in reduced throughput capacity and shortened runs.

Temperature — Temperature has a significant impact on the amount of pressure that can be placed on the membranes. As the water temperature decreases, it becomes more viscous, and the RO permeate flow drops because more pressure is required to push the water through the membrane. Increased temperature lowers water viscosity, which lowers the pressure required to produce the same amount of permeate. That, in turn, creates higher total dissolved solids (TDS) in permeate because of the increased rates of diffusion. Therefore, monitoring and maintaining process temperature is key.

pH — The primary purpose of measuring pH is its quick response to the ingress process contamination during the production and distribution of pure water. While pH changes are not significant in the majority of applications, variations become crucial to the success of high-purity water processing. The low conductivity and limited buffering capacity of low ionic strength pure water causes pH electrodes to drift. The resulting measurements are non-reproducible and inaccurate. In addition to large drift, common problems are unacceptable flow sensitivity and poor temperature compensation.

Conductivity — Conductivity is a critical measurement in monitoring an RO system. The effect of minor feed-water constituents, such as alkalinity and ammonia, plays a role in achieving a high-purity permeate.  The overall efficiency of dissolved solids removal is usually determined using a pair of conductivity measurements, one at the inlet (cell 1) and one at the outlet (cell 2). This is referred to as % rejection and is calculated by the formula:

% rejection = [1-(cell2)/(cell 1)] x 100

For example, if the inlet water contained 200 ppm of dissolved solids and the outlet water contained 10 ppm, the efficiency would be a 95% rejection rate. A typical range for this type of application is an 80%-100% rejection rate.

To measure conductivity, an SC conductivity element (sensor) is shown (figure 4) at the start of the RO system and again at the end.  A second-pass system takes another conductivity reading at the outlet. Then the outlet measurement from the first pass is used as the reading of the inlet in the second pass.
A final conductivity measurement after the second stage is used to determine the quality of the outlet water. Ammonia also affects the production of high-purity water and may be present due to municipal chlorination of the feed water or from organic contamination. Ammonia will pass through the membrane system in either the molecular (NH₃) or ionic (NH₄+) form. 

Since ammonium hydroxide (NH₄OH) is less conductive than ammonium carbonate [(NH₄)₂CO₃], it is not uncommon to find off-line samples or storage tank water with conductivity higher than that of on-line readings. A shift in pH is due to the absorption of CO₂ from the air and the formation of carbonic acid in the water. Without the presence of ammonia, this type of contamination of high-purity water with CO₂ would generate higher conductivity and reduce the pH.

CO2 and its Downstream Impact on Conductivity

Because CO₂ can negatively impact conductivity, it is critical to understand carbon dioxide, its interaction with H₂O, and its effect on high-purity water treatment. Carbon dioxide is everywhere, dissolves into water, and changes form as it dissolves into the water.  
In water where the pH is below 5, essentially all of the CO₂ is in the dissolved CO₂ form (figure 5), called carbonic acid. As pH rises, CO₂ is lost and turns into bicarbonate ion.  The double-charged ions cause scaling issues within the process. Because the RO membrane does not reject CO₂, most downstream process issues are CO₂- related. 

Ensuring Good Conditions for the RO Process and for the Membranes

Manufacturers have found successful and efficient methods and devices to measure and monitor each point. These Yokogawa solutions overcome common issues in an RO process.

Pressure

To make sure the membranes do not clog and potentially require expensive replacement, manufacturers must change the filters as the pressure nears its set point. 

With ten-year stability, Yokogawa’s EJA110 differential pressure transmitters maintain a consistent zero level (zero-stability), which saves users calibration costs. In addition, they are able to perform well in extremes. Manufacturers reduce costs because Yokogawa’s digital, high-resolution devices can be used with a variety of common handheld devices that do not require special training. 

In addition to accurate pressure measurements, Yokogawa’s devices provide advanced diagnostics that can be used in process management systems such as PLCs. This ability provides total insight for users to understand and avoid issues that might lie ahead. 

For example, when a semiconductor facility that produces and uses deionized water decided to convert from switches to a non-Yokogawa transmitter, the process experienced pressure surges from the pumps. The events caused the transmitter’s zero to shift constantly, required the maintenance team to re-zero them every week, and resulted in additional maintenance headaches. The facility agreed to replace one of the units with Yokogawa’s EJA110 DPharp transmitter. With its superior overpressure protection (≤±0.1% @ 16MPa, 2300 psi) and vibration tolerance, the EJA110 operated without a zero shift for years, much longer than its initial six-month trial. The EJA110 produced consistently accurate output, resulting in better process efficiency and less maintenance downtime.

Flow 

Typically the upstream side of the RO, or the flow from the feed water, includes a magnetic flowmeter or pressure transmitter with the orifice to measure the flow rate of the incoming water. Also on the upstream side, additional pressure transmitters measure the pressure. 

On the downstream side (after the membrane), because the water has a very low conductivity, a magnetic flowmeter cannot be used. Instead, a vortex meter or pressure transmitter with an orifice plate must be used to measure flow. A pressure transmitter with an orifice plate is the most common and cost-effective choice. 
Yokogawa’s pressure transmitters with orifice plates lead to savings because no extra AC power is required to run to either Yokogawa’s ADMAG flow meter or the DP transmitter. In addition, Yokogawa devices offer displays that show operators the flow values so they can request that maintenance check conditions as needed. 

Temperature

Sometimes the temperature measurements for the RO process are far from the control system connection and require long runs of wires. In these cases, facilities may choose to use temperature transmitters rather than wiring the sensors directly. RTD (Resistance Temperature Detector) or thermocouple wiring can be expensive and can add complexity because the wiring needs to match the specific type of sensor. Long runs of sensor wires introduce errors because they change the electrical characteristics of the signal and they easily pick up noise from nearby equipment such as motors and drives.

Temperature transmitters from Yokogawa can be placed near sensors reducing both the wiring expense and errors due to distance and noise. The signal can be a 4 to 20mA analog signal, a digital signal, or a wireless signal. A digital signal can be communicated via protocols such as Hart® protocol. The Yokogawa transmitter sends a reliable, accurate signal to the desired system wirelessly or using a standard twisted pair.

pH/ORP and Conductivity

Achieving accurate and reliable readings using a traditional pH analyzer is challenging, however, with Yokogawa equipment, stable and accurate pure water pH measurements can be accomplished. 

With a traditional system, it is common to use a three-electrode system with independent electrodes for pH, reference, and temperature. All electrodes are placed together in a holder constructed of a stainless-steel body with a liquid-earth solution. In any pH sensor used to measure pure water, the reference electrode is the most important component besides a liquid-earth solution ground element. The purpose of the reference electrode is to provide a constant reference voltage as well as complete the pH measuring circuit. Because of the low conductive properties of high-purity water, a flowing reference is needed. The purpose of the flowing reference is to release just enough ions to carry the measurement signal and to complete the measurement circuit.

Yokogawa’s specialized Bellomatic pH sensor is the proven best solution for high-purity water applications. The Bellomatic design maintains a self-adjusting steady positive-pressure flow across the reference sensor and ensures immediate interior pressure equalization to the outside pressure making the sensor virtually insensitive to external pressure/flow variations. A slight overpressure caused by the bellow tension prevents fluid ingress and maintains a positive ion flow out of the sensor.

Building on the Bellomatic, the FU24 sensor incorporates the successful patented bellow system in an all-in-one body.

No refilling of the reference electrode is required. Users simply install the sensor and they are done. The long life of the sensor and its ability to retain stable readings up to 18 months bring the ownership costs very low. The FU24 benefits the facility through its ability to read both pH and ORP simultaneously with one sensor.   

The virtually maintenance-free sensor and Yokogawa’s SENCOM digital smart sensor technology means there is no need to worry about unplanned outages and lost production time. 

Conclusion/Key Advantages

The measurement improvements lead to cost reductions and efficiency advances, as follows:

• Pressure — By using the EJA to measure both pressure and differential pressure, facilities save costs by purchasing fewer transmitters. Overpressure protection and 10-year stability help reduce the need to calibrate and lower the total maintenance costs. 
• Flow — Yokogawa vortex flowmeters offer ½” to 16” meter sizes, ideal for RO skids that use smaller piping for compact shape. 
• Temperature — Temperature transmitters from Yokogawa can be placed near sensors reducing both the wiring expense and errors due to distance and noise.
• pH — Stable pH readings are achieved through immunity to noise and stray currents. In addition, the multi-channel process liquid analyzer provides the ability to measure pH, ORP, and conductivity in one analyzer, reducing panel footprint and installation costs. 
• Conductivity — Built-in USP23/24 functions make USP compliance pain free while dual display function allows for temperature-compensated measurement, valuable for process control. 

As facilities improve the monitoring of key RO performance indicators, they see improvements to their systems’ overall health. Performance increases while unnecessary and costly instrument maintenance falls.

For example, zero measurement drift results in reduced transmitter maintenance costs. Since this allows for better maintenance planning, throughput is improved. In addition, the more accurate membrane cleaning and maintenance enable reductions in energy consumption.