Monitoring and Controlling Water Quality in Membrane Systems

From wastewater treatment facilities to food and beverage manufacturers, obtaining the right water chemistry balance is crucial to ensuring the success of these processes. Many industrial processes will fail if the water is too acidic or too alkaline. When water becomes contaminated, you can filter and purify it with several water treatment technologies, which include reverse osmosis, ultrafiltration, and nanofiltration.

Reverse osmosis is a highly effective water purification method that involves using a semipermeable membrane to remove contaminants and other substances from the water molecules. Pressure is applied to the water to ensure it flows through the membrane.

Ultrafiltration (UF) is a similar membrane treatment process that uses hydrostatic pressure to push water through a semipermeable membrane. As for nanofiltration, this pressure-driven liquid separation technology sits between ultrafiltration and reverse osmosis.

When removing contaminants from a solution, you must regularly monitor the water quality to ensure contaminants are being properly eliminated. Eventually, membranes need to be cleaned from mineral buildup. By monitoring the water quality in your membrane systems, you can accurately identify when the membrane should be flushed or replaced. This article explains the role of sensors and chemical dosing in maintaining membrane performance.

Understanding Membrane Filtration Systems

There are several types of membrane filtration systems that you can use to filter water and improve its quality.

Reverse Osmosis (RO) Systems

Reverse osmosis is a type of water treatment process that’s designed to get rid of most of the contaminants that are present in feed water. Water is pushed into a semipermeable membrane with pressure. The contaminants are then caught by the membrane, after which the water flows out of the RO system.

During the reverse osmosis process, the water goes through three or more stages of treatment. Each stage reduces the water’s contaminant levels. During the last stage, the solution flows through the RO membrane to deliver fresh water. The contaminants and water that are left behind are referred to as brine or waste.

The semipermeable membrane that’s contained within the RO system consists of small pores that capture most contaminants but allow water to be pushed through. If water containing a lot of salt goes through the reverse osmosis process, the salt will be left behind. Any chlorine or sediment in the water is removed with a prefilter. The primary filters in a reverse osmosis system are:

• Carbon filter: Chlorine and volatile organic compounds (VOCs) are removed with the carbon filter to improve the water’s odor and taste
• Sediment filter: Particles like dust, rust, and dirt are removed
• Semipermeable membrane: As much as 98% of total dissolved solids are captured by the membrane

The types of contaminants that can be removed with an RO system include the following:

• Arsenic
• Fluoride
• Salt
• Chlorine
• VOCs
• Sediment
• Heavy metals
• Pesticides and herbicides
• Microplastics

The only downside to reverse osmosis membrane systems is that they don’t remove some viruses and bacteria. On the other hand, nearly all other contaminants can be removed with reverse osmosis filters.

Ultrafiltration (UF) Systems

Ultrafiltration is another type of membrane filtration that’s similar to reverse osmosis. It uses hydrostatic pressure to push water into a semipermeable membrane. Unlike RO systems, ultrafiltration filters can get rid of viruses, bacteria, and endotoxins. They can also remove most suspended solids from the water. You’ll be left with water that contains low contaminant levels and high purity.

This type of filtration can eliminate molecules that range from 1,000 daltons in molecular weight to upwards of 500,000 daltons. While ultrafiltration is effective and should be used to get rid of viruses and bacteria, it works best when combined with an RO system. Ultrafiltration removes contaminants with a pore size of 0.02 microns. When comparing ultrafiltration vs reverse osmosis, RO systems have a pore size of 0.0001 microns, which makes them more effective.

Contaminants that have a molecular weight of around 100,000 daltons can range from 0.05-0.08 microns in diameter. Ultrafiltration membranes can remove these microns with ease. However, some dissolved solids will get through the membrane. If you need to remove turbidity from water, ultrafiltration is the way to go. The main advantages associated with ultrafiltration include the following:

• It’s easy to automate the process
• Ultrafiltration is environmentally friendly
• You don’t need to use chemicals
• The results of ultrafiltration are consistent

There are, however, a couple of downsides associated with using this system to filter contaminants out of water, the primary of which is that it has high energy costs. A lot of energy is used to create the pressure needed to constantly move the wastewater through membranes. Keep in mind that several innovations have been made in recent years that help to reduce the energy consumption of ultrafiltration.

If you want to create potable water with this solution, you’ll likely need to recirculate it, which costs even more energy. Water needs to be routinely recirculated to avoid losing some of it during processing.

Nanofiltration (NF) Systems

Nanofiltration technology is based on the same liquid-separation technology as reverse osmosis and ultrafiltration systems. Reverse osmosis technology is powerful enough to remove nearly all types of dissolved solutes. While nanofiltration is effective at removing calcium and other multivalent ions, it doesn’t properly filter chloride.

This technology is often used in wastewater treatment facilities as one component in a larger treatment system. Many plants use a combination of nanofiltration and reverse osmosis. The membrane in this system can remove contaminants that are as small as one nanometer. While nanofiltration isn’t as effective as reverse osmosis at removing finer particles, it provides a higher contaminant reduction than ultrafiltration. The many types of contaminants that you can remove with nanofiltration are:

• Herbicides
• Antibiotics
• Sugars
• Nitrates
• Metal ions
• Insecticides and pesticides
• Dissolved organics

It will get rid of anywhere from 20%-98% of dissolved salts. Magnesium sulfate has a rejection rate that ranges from 90%-98%. These systems are primarily used to:

• Reduce the concentration of total dissolved solids
• Remove total organic carbon and color from surface water
• Soften well water
• Separate inorganic and organic matter in wastewater applications

Importance of Monitoring Water Quality

No matter the application, it’s highly recommended that you closely monitor the quality of the water that you put through a membrane system.

Inlet Water Quality

Whether you use reverse osmosis or filtration, there are several parameters you need to monitor within the water you send through the system. Make sure you measure the water’s pH, turbidity, and conductivity. If the water quality is poor, it will effectively degrade the membrane’s performance over time. You can usually replenish the membrane by flushing it. However, it may eventually need to be replaced altogether.

When measuring water quality, pH is among the most important indicators. You can use it to measure the physical, biological, and chemical changes in a body of water. If you place a pH sensor in the water, it will send back a reading that ranges from 0-14. A reading of 7.0 is neutral. Anything below the neutral number is acidic, while readings above 7.0 are alkaline. You may need to treat your water if it gets too acidic or too alkaline.

When water becomes more acidic, there’s a good chance that it will contain high concentrations of dissolved solids, metals, and other organic matter. If the water is too alkaline, it likely consists of high levels of calcium and magnesium. In industrial processes, alkaline water can cause scale buildup and similar issues that worsen efficiency.

When the pH is lower, metals like mercury, arsenic, aluminum, and lead are more soluble, which means that they can be absorbed directly into the tissues of various organisms. In bodies of water, metals are toxic to aquatic life. When you’re using a membrane system to treat water in an industrial facility, measuring the pH allows you to determine how contaminated the inlet water is before you treat it. It can also help you identify the types of contaminants that are currently present in the water.

As for turbidity, this measurement can be used to identify the concentration of suspended particles, which include everything from silt to organic matter. An increase in turbidity usually means that the water is highly contaminated. In a body of water, turbidity is the haziness of the fluid, which is caused by a large concentration of particles that are effectively muddying the liquid. High turbidity means that water clarity and quality are low. In most cases, water with a low pH reading will also have a high turbidity measurement.

As for conductivity, it’s the measurement of the ability that water has to pass an electrical current. If the water contains a high amount of inorganic dissolved solids, you’ll receive a high conductivity reading. Substances like sulfate, magnesium, aluminum, calcium, and chloride can significantly impact conductivity measurements. On the other hand, organic compounds like alcohol, oil, and phenol don’t properly conduct electrical current, which means that they’ll produce low conductivity readings.

Your conductivity measurements will also be affected by the temperature of the water. When the water is warm, the conductivity will be high. Conductivity is often measured at 25 degrees Celsius, which is the same as 77 degrees Fahrenheit. Inlet water with high conductivity will usually have a low pH and high turbidity.

Outlet Water Quality

When water flows out of a membrane system, it’s considered outlet water. There are many quality indicators you should be on the lookout for in treated water, which include pH, conductivity, and turbidity. Once the water has been treated, you can measure different indicators and compare them to the initial results you received from the inlet water. Whether you intend to reuse the water or dispose of it, you need to identify how contaminated it is.

The Environmental Protection Agency (EPA) sets strict standards and regulations that all industrial facilities must adhere to. These regulations apply to more than 90 different contaminants that can be found in drinking water. If you send wastewater effluent out of your industrial facility, contaminant levels must be kept below specific thresholds. If the EPA finds that you are bypassing these regulations, they can assess large fines.

To maintain compliance with EPA’s national standards and the rules put forward by the Safe Drinking Water Act (SDWA), make sure you continuously monitor the outlet water in your membrane systems. You can use conductivity, pH, and turbidity sensors to perform continuous monitoring.

Chemical Dosing and Membrane Protection

While flushing and cleaning a membrane can get rid of the built-up contaminants, there’s more that must be done to enhance membrane longevity. Your best option is to perform chemical dosing.

Purpose of Chemical Dosing

Chemical dosing is a solution that treats sewage effluent in treatment plants and other industrial facilities. This process involves injecting reagents into the wastewater to improve pH levels, allow dissolved solids to settle, and remove odors. It can boost membrane longevity by increasing the quality of the water before it’s treated.

While there are numerous chemicals that can be used for this process, the main ones include antiscalants and biocides. An antiscalant is a type of pre-treatment that’s sent to the RO membrane. It ensures that scale deposits don’t grow on the membrane.

When particles build up in the RO system, the membrane’s pores can become plugged, which makes it more difficult for the water to get through. Make sure to combine antiscalants with biocides, which are designed to eliminate microorganisms within the water. Chlorine is among the most effective biocides.

Monitoring and Controlling Chemical Dosing

You need to use the right doses of chemicals when you’re trying to reduce scale and treat the water that you send through your membrane systems. For example, the ideal antiscalant dosage can be anywhere from 0.5-4.0 mg/L. By placing sensors in dosing systems, you can continuously monitor the chemical concentration to ensure the dose isn’t too high or too low. If you’re able to maintain accurate dosing, you’ll benefit from:

• Regulatory compliance
• Cost efficiency
• Excellent water quality

Role of pH Sensors in Water Treatment

The success of a water treatment is best measured with a pH sensor. The readings you obtain will tell you if the contaminants have been effectively eradicated during treatment.

Function and Importance of pH Sensors

You can use pH sensors to measure the performance of the membrane system you’re using. This implies that the water is highly contaminated. If you send it through a high-efficiency reverse osmosis system, the water should get much closer to a 7.0 reading. If you take another measurement of the outlet water and discover that the pH is around 5.5-6.0, the membrane may be damaged and in need of replacement. Perform pH monitoring at different stages of water treatment.

Direct Connection to PLCs and Remote Monitoring

Industrial buildings are increasingly relying on programmable logic controllers (PLCs) and other automated technology to monitor the quality of water. You can connect your pH sensors with PLCs to receive real-time readings at a remote location. Once this integration occurs, you’ll no longer need to have operators manually take pH measurements during and after treatment. Remote monitoring gives you more control of your water treatment efforts. Real-time readings allow you to act instantly when you notice a change in the water’s pH.

ORP and Conductivity Sensors in Water Treatment

While there are many different tools you can use to measure the quality of water, ORP and conductivity sensors can be useful for water treatment applications.

ORP Sensors

Oxidation reduction potential (ORP) is a measurement that informs you of a system’s ability to reduce a specific substance in the water. If the ORP is higher than 0 mV, the body of water will have oxidizing characteristics. An ORP that’s lower than 0 mV indicates that the water has a reducing environment.

You can use ORP sensors to estimate how much oxidizing biocide residuals are in the water. For example, let’s say that you place chlorine in contaminated water to reduce the concentration of microbiological organisms. If the chlorine has been effective and most of the contaminants have been destroyed, some oxidizing biocide residuals should be left behind. A high ORP reading means that the water is healthy. If you don’t detect oxidizing biocide residuals, there’s a good chance that the water is still contaminated.

Whether you’re treating wastewater or drinking water, ORP sensors can help you monitor water quality by telling you if your treatments have produced the intended results. ORP sensors consist of probes in a handheld meter. Once the probes obtain a reading, the meter will display the oxidation potential in millivolts. Keep in mind that these readings can drift. It might take upwards of 30 minutes for you to get a solid ORP reading.

To ensure accuracy, you’ll need to maintain the sensor’s probe regularly. ORP sensors must be kept wet constantly. When you aren’t using your ORP sensors, store them in a neutral pH buffer. It’s also a good idea to gently clean the probe every month to prevent scale buildup.

Conductivity Sensors

As touched upon previously, conductivity is an essential water quality measurement. It tells you if the water can pass an electrical current. Conductivity increases when the water’s salinity gets higher. This measurement is useful because water typically has a constant conductivity range. Once you identify this range, you can use it as a baseline when treating water.

If the readings change significantly, it may indicate that a major pollutant has entered the water. Conductivity sensors can provide continuous monitoring within your membrane systems. You can place them in the inlet and outlet water to make sure you’re properly filtering out contaminants.

High-Efficiency RO Systems

Among the most effective membrane solutions is a high efficiency reverse osmosis system. These systems are equipped with the most advanced filtering technology to deliver purified water.

Features of High-Efficiency RO Systems

High-efficiency reverse osmosis systems are able to provide you with ultrapure water. The purification process that this system uses is so comprehensive that it can handle highly contaminated water. It also consumes significantly less water than standard reverse osmosis systems. If you have wastewater or feedwater that contains high contaminant concentrations, using high-efficiency RO technology can help you save money.

During processes that cause heavy scaling, these systems allow you to treat more water at a lower overall cost. The service life of the membrane is also much longer than the one in a standard reverse osmosis system. The main benefits of using this membrane system in your facility include the following:

• Suitable for nearly all wastewater application
• Low fouling risk
• High water recovery
• Tolerates high concentrations of organics and silica
• Comprehensive pre-treatment isn’t necessary
• Can be retrofitted into traditional reverse osmosis systems without issue

There are also a few drawbacks to these systems, such as:

• High upfront costs
• Considerable chemical consumption
• Multiple process units are required

Conclusion

Maintaining water quality is essential in many industrial processes. When you use membrane systems to treat water, you must monitor and control its quality. Once you identify the concentration of contaminants in the inlet water, you can measure the quality of the outlet water and compare the results. From pH to conductivity, these measurements will help you determine if your treatments are effective and if the membranes need to be flushed.

Use chemical dosing alongside your sensors to maintain system performance. Chemical dosing is necessary to reduce contaminants in the inlet water and remove scale deposits from the membrane. Adopting advanced monitoring technologies will help you achieve better water treatment outcomes.