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Measuring Water Sanitation: Don't Be Fooled by Parts Per Million

01 May 2014

NSW Department of Primary Industries

If you are measuring chlorine levels in poultry drinkers, you are only seeing part of the picture on the effectiveness of your water sanitation procedure, according to Byron Stein in 'The Drumstick' from the New South Wales Department of Primary Industries.

If you are measuring the level of a particular sanitiser in the water as an indicator of how ‘clean’ the water is  - for example, chlorine at 1 to 2 parts per million (PPM) at the drinker level - you might be way off track, and you may wrongly assume your birds are protected.

Why might measuring chlorine in PPM at the drinker level be a problem?

When measuring chlorine in ppm at the drinker level, you are measuring total free chlorine in the water. This measure is made up of two forms of chlorine. But not all chlorine was made equal, with one form (hypochlorous acid) of chlorine being an excellent water sanitiser, and the other form (hypochlorite ion) being relatively hopeless.

The proportion of these two forms of chlorine depends on the pH of the water and the water quality. This essentially means that if the water is clean, and the pH is low (under 7), then measuring ppm of chlorine may be OK.

However, if the pH is high (more than 7), the ppm level might be the same but the chlorine will be in a form (hypochlorite ion) that is not nearly as good at sanitising the water.

Another way of thinking about this is if you imagine chlorine wearing two different kinds of shoes. What shoes the chlorine wears will depend on what the water pH is. Under pH7 (in more acidic conditions), chlorine wears runners and can race around killing pathogens quite effectively. However above pH7, chlorine wears gumboots, and is pretty slow and sluggish and is not as effective at killing pathogens. However, when you measure for chlorine in ppm, you have no way of knowing which shoes chlorine happens to be wearing at the time.

So, PPM could be misleading and very much depends on how clean the water is and its pH.

What is the best way to measure how ‘sanitised’ my water is?

Measuring the oxygen reduction potential (ORP) of the water is the most effective measure of water sanitation. Rather than reading the level of sanitiser, which may be in a form that may or may not be working, ORP is a measure of how effective the sanitiser is in killing 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 700mV, 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.

"Measuring the oxygen reduction potential (ORP) of the water is the most effective measure of water sanitation."

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 650mV.

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 650mV. At this level, the sanitiser in the water is active enough to destroy harmful organisms almost instantaneously.

Monitoring sanitised water is not straightforward. Technical assistance should be sought to ensure that your water testing method is accurate and takes into account your sanitation method and local water supply chemistry and quality. This advice should preferably come from a competent technical advisor.

This article is based on the Australian National Water Biosecurity Manual for Poultry Production (2009).

May 2014

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