When running reverse osmosis (RO), operational efficiency is paramount. The comparison between the rate of water production and applied energy determines the cost of the water produced. Taking the system offline to conduct routine cleaning and maintenance can increase production rates and efficiency or extend membrane lifetime. However, spending extra effort to monitor membrane integrity and separations performance isn’t desirable. Fortunately, RO treatment plants can monitor salt rejection by measuring conductivity of RO water.

How to Measure Conductivity of RO Water

Conductivity measures how easily electricity can pass through a material. In electrical terms, the water poses resistance to the passage of electricity and conductivity is the inverse of resistivity. Conductivity sensors send an electrical charge between two embedded probes. Since the probes are a set distance apart, the resistance of the water can be calculated by measuring the resulting current in the circuit. Conductivity is reported in units of Siemens/cm (S/cm) which represents the conductance, or inverse resistance, over a given path length. Units of milliSiemens/cm (mS/cm) or microSiemens/cm (µS/cm) are more useful for describing common waters. Pure distilled and deionized water has a conductivity of 0.05 µS/cm, which corresponds to a resistivity of 18 megohm-cm (MΩ). Seawater has a conductivity of 50 mS/cm, and drinking water has a conductivity of 200 to 800 µS/cm. The permeate of an RO unit varies based on the feed concentration and operating pressure. Typically, the conductivity of RO water should fall between the value for deionized water and the value for drinking water (0.05 µS/cm-200 µS/cm).

Membrane Integrity

RO treatment systems are designed on the assumption that the RO unit provides a certain level of separation. If a failure occurs in the system, the entire treatment train is compromised. Membranes can be damaged by insufficient pre-treatment, such as a failure to remove large and gritty particles or insufficient dechlorination, which results in chlorine damage to polyamide membranes. Integrity problems can arise from a single treatment stage, which indicates problems with fouling or pre-treatment, or they can arise within a single module, which points to a mechanical failure such as an O-ring break. Therefore, profiling the system by collecting conductivity values from many points within the system is the best way to catch and diagnose membrane integrity issues.

Conductivity vs. TDS

Measuring the conductivity of RO water helps determine how much salt is rejected by an RO membrane. Dissolved salts are present as ions in water, which help to make water more conductive. Conductivity correlates with the total dissolved solids (TDS) content, and the correlation is approximately linear over short ranges. When using a TDS meter, the correlations are built-in and automatically applied. Some meters also allow customization of the conversion factor to specific needs and applications, such as waters containing significant quantities of ions besides sodium and chloride. When using a conductivity meter to determine TDS, conversions must be performed on the collected data. Conversion factors can be easily determined by measuring a known standard. For example, if 64 mg of NaCl in one liter of water produces a conductivity of 100 µS/cm, then the conversion factor between conductivity and TDS is 0.64, where TDS = Conductivity х 0.64.

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