How to Monitor Conductivity in Reverse Osmosis

14 Oct.,2022


Seawater Desalination RO Membranes

The conductivity of a material is a measurement of how easily electricity can move through it.Electrical charges are sent between two implanted probes by conductivity sensors. The resistance of the water may be measured by measuring the consequent current in the circuit since the probes are spaced at a specific distance apart. The conductivity is measured in Siemens/cm (S/cm), which is the conductance (or inverse resistance) along a certain path length. For defining common waters, milli Siemens/cm (mS/cm) or micro-Siemens/cm (S/cm) units are more useful.

The conductivity of RO water is used to evaluate how much salt a RO membrane rejects. In water, dissolved salts exist as ions, which help to make water more conductive. The amount of total dissolved solids (TDS) corresponds with conductivity, and the relationship is nearly linear over short distances. The correlations are built-in and applied automatically when using a TDS meter. Some meters additionally let you tailor the conversion factor to your unique needs and applications, such as waters with a lot of ions other than sodium and chloride. Conversions must be made on the collected data when using a conductivity meter to compute TDS.By measuring a recognized standard, conversion factors can be simply determined. For example, if 64 mg of NaCl generates a conductivity of 100 S/cm in one liter of water, the conversion factor between conductivity and TDS is 0.64, with TDS = Conductivity x 0.64.

Monitoring the operation of a reverse osmosis (RO) system is critical for avoiding problems that degrade water quality and shorten the membranes' usable life.Instrumentation is critical for providing the system operator with a way to continuously check the water quality and system performance. Without it, the operator is unaware of possible issues until they present themselves as a decline in water quality or membrane function.Manufacturers provide a wide range of alternatives for equipping the RO with the necessary monitoring equipment.

This article goes through some of the critical monitoring equipment that should be included in every RO system. This includes the following:


The standard temperature for RO membranes is 77 degrees Fahrenheit. Because cold water is denser than warm water, the flux (gallons per square foot per day) across the membrane is reduced. The flux increases at temperatures above 77 degrees Fahrenheit and decreases at temperatures below 77 degrees Fahrenheit.


In general, industrial RO systems are designed to recover 75% of the feedwater as permeate. The remaining 25% of the feedwater is discharged as concentrated brine into the drain (also known as reject or waste). The dissolved particles in the feedwater are concentrated four times in the brine stream under these operating conditions.


The electrical conductivity of water is measured in micro Siemens per cm or micromhos per cm at 25 degrees Celsius, and is expressed in micro Siemens per cm or micromhos per cm. The higher the specific conductance, the higher the dissolved solids concentration. This is a relatively basic test that is frequently used to determine the quality of water.The dissolved solids in the feedwater can be rejected by RO membranes to the tune of 99 percent or higher. The permeate is formed when the residual solids pass through the membrane.


The pressure decreases between the high-pressure feedwater stream and the brine (reject or wastewater) stream happens as water travels through the water channels in the RO module. The pressure differential grows if the water passageways get clogged with inorganic scale, organic debris, or suspended solids. This is particularly prevalent in RO operation, as deposition and fouling of the membrane happens over the module's lifetime.


The placement of critical instrumentation in a RO system gives a gauge of the system's reliability and performance. The data collected from monitoring and recording instrument readings is utilized to determine essential operating performance metrics like percent recovery, percent salt passage, pressure differential, normalized permeate flow, and net driving pressure. When this data is compared to the performance metrics, the operator has the information he or she needs to extend the time between membrane cleanings and/or replacements. This helps to save money while also extending the plant's useful life.