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Oxidation-reduction potential (ORP) or redox monitors the condition of the reverse osmosis (RO) plant. The ORP value indicates if the membrane will be susceptible to attack by chlorine or other oxidizers, which can cause significant damage, and shorten the lifespan of the RO plant, thus increasing maintenance costs. ORP is also used to detect the amount of sodium bisulfite, which is commonly used to reduce the level of chlorine in the solution/water.
Osmosis is the process where water molecules pass through a semipermeable membrane from an area with low dissolved solids to an area with high dissolved solids until near equilibrium is established.
Reverse osmosis, shortened to RO, is a membrane process of purification where the majority of total dissolved solids (TDS) are removed in water by reversing the osmosis process. An RO plant purifies or desalinates contaminated water by allowing it to pass through a membrane.
In RO plants ORP plays a significant role in monitoring the condition of the RO membrane, and therefore, it is essential to understand how to affect the water quality and maintenance during operations. In this article, we will look at what oxidation-reduction potential (ORP) is and understand the role that ORP has in a RO plant.
Oxidation-reduction potential, shortened to ORP, sometimes called redox, is a quantification that shows how oxidizing or reducing a liquid is, measured in the unit millivolts (mV). The extent to which a fluid exhibits reducing or oxidizing properties depends on the strength and abundance of different oxidizing and reducing agents present in the fluid. As an example, a higher level of oxidizing agents indicated by ORP is not good. Whereas a higher level of antioxidants indicated by the ORP reading, is beneficial.
Oxidation and reduction are related chemical processes that refer to the exchange of electrons. In a reaction, oxidation refers to when a chemical loses electrons, and reduction refers to when a chemical acquires electrons, so reduction is the opposite of oxidation. Both reduction and oxidation can occur in the same reaction, this is why reactions involving oxidation and reduction are generally called redox reactions; the “red” stands for reduction, and the “ox” for oxidation.
ORP is often expressed as an electrical potential of voltage. A reducing environment is indicated by a negative reading, and so reduction is the net gain of electrons by an atom, molecule, or ion. On the other hand, an oxidizing environment is indicated by a positive reading, as oxidation is the loss of electrons by an atom molecule or ion. Typically, the lost electrons are replaced by oxygen. The most common unit unit for expressing ORP is millivolts (mV). Most ORP meters can read values, ranging from -1000 mV to +1000 mV.
An increase in temperature speeds up the dissociation of molecules in the liquid which increases the ORP value and redox reaction rate. Therefore, temperature is measured during calibration with a temperature sensor. The value of an ORP calibration solution at different temperatures is predictable and can be compensated for, as part of the calibration process to increase accuracy.
pH is a measurement of the hydrogen ion concentration in an aqueous solution. ORP is affected by any positive or negative charged particle, therefore, pH will directly influence the ORP value in an RO plant.
While pH and ORP change together, the rate of change may not correlate exactly, and so the redox reaction must be taken into account.
To understand their relationship, let’s take a look at chlorine (Cl), as it is commonly used in RO plants as part of the disinfectant stage.
When the pH has a value of 1.9, chlorine is CL2 (dichlorine) in water (H2O). When the pH increases, the chlorine reacts with H2O to form hypochlorous acid (HOCl) and the ORP value decreases.
Here is the formula:
CL2 + H2O = HOCl + HCl
When the pH level is 7.3 pH or greater, HOCl reacts to form hypochlorite (ClO –), and the ORP value continues to decrease:
HOC = OCl- + H+
The stages from a strong oxidizer like chlorine to a mild oxidiser such as hypochlorous acid, to a weak oxidizer like hypochlorite directly impacts how capable chlorine is as a common disinfectant in water. So, this example shows us that ORP decreases with increasing pH, and as the pH decreases the redox potential increases.
Reverse osmosis (RO) plants are a manufacturing plant where RO occurs. As mentioned RO is a common process to purify or desalinate contaminated water by allowing the water to pass through a membrane.
The water that is produced in a RO plant can then be utilized for a variety of uses such as desalination wastewater treatments, reclamation of dissolved minerals, and understanding the concentration of contaminants in water.
RO plants require a variety of pre-treatment processes including dechlorination, softening, and antiscalant treatment. After completing the pre-treatment, high levels of pressure are applied to the water through a semipermeable membrane. Here, the pure water can pass through the membrane while 100% of suspended solids and approx. 90% of dissolved silica, dissolved solids, hardness, and alkalinity are removed.
It is recommended that an alarm be set in the RO plant to take immediate action when the ORP reaches 250 mV on the feed/concentrate side. When the ORP value reaches 200 mV, you should set a high alarm to shut down the RO plant, to protect the RO membranes at the feed/concentrate side.
In RO plants, ORP is a key parameter to control chemical doses (namely chlorine), assess the water quality, and monitor the plant’s membrane.
An effective tool for measuring water quality in RO plants is ORP. By measuring ORP, you can assess the oxidative potential of the water and detect changes from the desired conditions.
As mentioned, a high ORP value suggests the presence of oxidizing agents, which can damage the membrane and compromise the quality of water. On the other hand, a low ORP value indicates reduced conditions, which may result in biological fouling in the RO plant or corrosion of equipment.
ORP measurements serve as a guideline for dosing processes in RO plants. Chlorine is one of the most common chemicals used in RO plants during the disinfection stages of water treatment. Measuring ORP levels therefore optimize chlorine dosages to ensure that the disinfection process is effective, without overusing chemicals. The overuse of chemicals can cause damage to the RO membrane or cause residual disinfectants in the water.
ORP can be used to establish the activity of an oxidizer. ORP membranes are susceptible to attack by oxidizers, such as hydrogen peroxide, ozone, bromine, and chlorine. RO membranes are vulnerable to attack by oxidizers, such as chlorine, bromine, ozone, and hydrogen peroxide The undertaking of the oxidizer provides more information than the chemical residual because it determines the ability and speed of oxidation. When the ORP reading is high, it indicates a need for pre-treatment. When the ORP value is low, it can indicate biological activity which could cause fouling of the RO membranes.
ORP can also be used to determine an overfeed of sodium bisulfite, which is used to reduce chlorine. If the ORP reading is under 210 mV, you have a reduction reaction. This overfeeding has additional costs and can cause environmental discharge problems. Therefore, it is best to check the rejected water where the ORP concentration is at its highest. Measurement of the ORP will even show small quantities of oxidizers or reducers in the water.
Chlorine will attack thin, film, and composite membranes, resulting in the degradation of the membrane, and a corresponding loss of performance, as indicated by an increase in salt passage. If the membrane is exposed to any level of chlorine, it will damage over time. Therefore, manufacturers highly advise the complete removal of any leftover chlorine particles ahead of the reverse osmosis plant system. This is accomplished by either carbon filtration or the chemical injection of sodium bisulfite.
ORP measurements make it easier to process optimization in RO plants as they provide real-time insights into water chemistry. By corresponding ORP measurements with other important parameters like pH, temperature, and conductivity, you can fine-tune operating conditions to maximize water production efficiency while ensuring the plant’s membrane condition and water quality.
Improper calibration of an ORP probe can lead to inaccurate ORP measurements. To prevent the issue, you should ensure you properly calibrate the ORP probe before use, including using the proper calibration solution, ensuring it is fresh, and also verifying the probe’s stability.
Other chemicals in the solution or water can cause interference with ORP readings. For example, if the solution or water contains high levels of organic compounds or reducing agents, it can mask the true oxidative potential. To troubleshoot the issue, you should use appropriate ORP probes and conduct additional water analysis that can identify interfering chemicals and substances.
ORP probes can experience electrode contamination, which results in erratic ORP measurements and reduced ORP probe performance. To solve the issue, you should regularly maintain the ORP probe and clean it properly, following the manufacturer’s instructions.
As mentioned, temperature can affect ORP readings in RO plants as ORP values are temperature-dependent. When measuring ORP, temperature should be compensated using temperature sensors or correction factors during probe calibration.
Improperly placing the ORP probe within the RO plant can lead to inaccurate measurements. For representative readings, the ORP probe should be placed in locations of the RO plant where water quality is most important, such as downstream of chemical dosing points or at the membrane feed and permeate outlets.
The easiest way to measure ORP in an RO plant is to use an ORP probe which is connected to a controller/handheld meter. As mentioned earlier, this will give you the oxidizing potential in mV.
When using a handheld ORP meter, it is key to note that ORP readings and values can drift, taking up to 30 minutes to stabilize, so patience is key. The ORP readings may also not exactly match the controller readings. Therefore, ROP readings should only be used to confirm the desired halogen residuals within the RO plant.
When using an ORP meter, it is important to carry out routine probe maintenance. Like pH probes, ORP sensors must be submerged in a solution at all times, so when you are not using the ORP probe, immerse it in the correct pH/ORP probe storage solution – alternatively, you can use a pH buffer. We recommend a Probe Tip Soaker Bottle to keep your probe properly submerged; Should you need a spare, or have damaged the original soaker bottle.
This is essential as if you were to store the ORP probe in demineralized water, it can cause the electrolyte in the probe to leach out, degrading the performance over time. We also recommend monthly cleaning with a very soft brush, or cloth to remove any deposits on the ORP probe. By maintaining an ORP probe, you can expect it to have a longer lifespan of 12-18 months before you need to replace the probe.
At Atlas Scientific, we have highly accurate ORP probes and all the right solutions for maintaining your probe. Contact the team today to learn more.
Oxidation-reduction potential (ORP) plays an important role in reverse osmosis (RO) plants as it indicates the susceptibility to oxidizers such as chlorine. Measuring ORP in RO plants can also detect biological activity, preventing damage to the RO plant membrane, and therefore optimizing RO treatment processes.
If you have any questions regarding ORP and RO plants or you would like to learn more about the ORP probes and meters we have to offer, do not hesitate to contact the world-class team at Atlas Scientific.
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