Monitoring to ensure effective water sanitation

The monitoring and disciplined record keeping of sanitiser levels and other parameters including pH and oxidation and reduction potential (ORP) are critical in ensuring effective microbial inactivation potential in treated water.

Monitoring sanitised water is not straightforward and the use of chlorine test strips alone may not give a true indication of the disinfection potential of the chlorinated water. Technical assistance must be sought, particularly when dealing with halogens other than chlorine and with oxidising compounds such as chlorine dioxide. This advice should preferably come from a competent technical advisor.

Most methods for testing for effective water sanitation look at the level of the particular sanitiser in the water as an indicator (e.g. chlorine at 1 to 2 ppm at the drinker level) and if this is achieved then it is assumed that the water is effectively sanitised. This is true for reasonable quality water, but for poor quality water the effectiveness of the sanitiser may be compromised despite its level appearing to be correct. ORP does not measure the chemical—instead, it measures the capacity of the sanitised water to kill microorganisms.

Determination of the ORP has become the procedure of choice for monitoring, and can be performed with incorporated systems or a hand-held apparatus. The quality of the testing unit should be evaluated prior to purchase. ORP, measured in millivolts (mV), operates much like a digital thermometer or pH probe and ORP sensors allow easy monitoring and tracking of critical disinfectant levels in water systems. ORP for water system monitoring provides the operator with a rapid and single-value assessment of the disinfection potential of water. Research has shown that at an ORP value of 650 to 700 mV, spoilage bacteria and bacteria such as E. coli and salmonellae are killed within a few seconds. Other microorganisms such as protozoa and viruses are inactivated over longer contact times, generally measured in minutes.

The ORP is a valuable tool where water quality is poor. For example, where water pH is high, measurable chlorine levels may be high but the level of active sanitising agent, hypochlorous acid, may be below effective levels, resulting in an ORP measurement significantly below 650. The routine measurement of ORP in mV is not a linear relationship at typical use rates. In chlorine sanitation systems, increasing pH will lower the ORP and decreasing the pH will increase ORP, reflecting the increased availability of hypochlorous acid. In 1972, the World Health Organisation adopted an ORP standard for drinking water disinfection of 650 mV. At this level the sanitiser in the water is active enough to destroy harmful organisms almost instantaneously.

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