Why ORP is not for me! (in warm water systems)

What is ORP?

ORP is the acronym for Oxidation-Reduction Potential, also known as the 'redox potential'. Most things in our environment have a redox potential. Currently electrical measurement of ORP is being used to control disinfection in built water systems. This may work really well in some applications - but in my opinion - it is not a good idea in others. So why is it 'not for me'?

Think about rockets?! Rocket fuel is oxygen and hydrogen. The two elements combine to provide energy and water. Oxygen is (of course) the oxidising agent and 'burns' hydrogen to produce water. Bushfires use oxygen to burn carbon (trees). So oxygen has a high redox potential. High redox potentials tend to 'burn' things.

The reduction potential is the opposite. You've most likely heard of foods that are 'high in antioxidants'. These compounds 'quench' oxidising agents. For example Turmeric is high in antioxidants (curcumin, see below). They have a high reduction potential.

The redox potential is an electrical measure of how much a solution wants to ‘give’ electrons to it’s environment. Oxidising agents have a positive potential, reducing agents a negative potential. Simply, ORP measures how much the water is likely to ‘burn’ organic material – like bacteria. What does that mean in practice?

Well, water with a high redox potential (negative ORP) wants to give electrons to anything with a lower potential (oxidise them). Water with chlorine in it has a higher redox potential than iron. So the electrons will flow to the iron and it will be oxidised (rust). Adding ‘antioxidants’ (tannins) to water will give positive redox potentials and reduce corrosion.

Pure water has a redox potential of 0.00 - it is neutral. Of course pure water is not a common occurrence; so all water we use has a ‘background’ redox potential (ORP) depending on what it contains. This baseline may be positive or negative. This is important and we’ll touch on this again later.

Chlorine and ORP

A given concentration of chlorine as a disinfectant dissolved in pure water has a constant ORP. That is, in pure water the same concentration of dissolved chlorine always has the same ORP value. This means that the disinfection ability of the chlorine is a measurable value.

By measuring the amount of free electrons to do the job of disinfection an ORP controller can deliver a constant disinfection level to a stable system. The key word there is ‘stable’, changes to the water supply, or more to the point what is in it (chemistry), can cause problems.

It is important to know that an ORP probe immediately measures the water passing through the sensor. The sensor them immediately directs the chemical to be dosed. Naturally, the location of the sensor and the probe is critical. The further the probe is from the dosing point the greater the 'lag time' between dosage and the controller.

Water Supply and ORP

Water almost always has a redox potential of it's own, either positive or negative. So an ORP probe needs to be calibrated against this 'background' ORP. If the water quality changes then the background changes and the probe loses calibration. So regular maintenance and calibration of probes is important to ensure the right level of chlorine dosing.

This value also changes with water temperature. It also changes with pH. A very variable supply will need frequent calibration. Storm events, disruption to supplies (works on mains supplies) or changes to the supply by the water provider should all trigger a re-calibration.

In general, organic material (tannins) and slimes (biofilms) tend to reduce the ORP of the water. Incidentally, ORP can be used to measure ‘antioxidants’. For instance green tea has a negative ORP that means it will ‘soak up’ oxidants. Foods with negative ORP like turmeric and olives are a healthy dietary option because they ‘mop up’ the harmful ‘oxidisers’ in our food that can damage our cells.

Recirculating Water Systems

ORP control systems are particularly suitable for recirculating systems like cooling towers and swimming pools. The re-circulation time for these systems is fairly constant and relatively short.Recirculating water becomes quite chemically stable if the system is operating efficiently. For instance, cooling water systems often have quite complex dosing and monitoring systems that maintain water chemistry within set limits. This is partly achieved by controlled water losses (blow down).

The problems of a moving baseline and re-calibration are for the most part overcome by recirculating the water and chemical management. Installing probes at the return to the dosing point will take into account dilution effects from the ‘top-up’ from the mains supply. This can ensure that the target chemical dose is flowing through the whole system, and will prevent overdosing.

‘Once through’ Systems

Once through systems where water does not recirculate but flows straight to the outlet are another matter. The chemistry and temperature of the supply will be variable. As water flows through a building it’s chemistry and temperature may change. This makes reliable ORP control problematic simply because the baseline is likely to change at very short notice.

It also means that the probe needs to be a long way from the dosing point at an outlet that runs continually to be effective. Otherwise the ORP controller may not dose enough chemical to account for the decay of the chemical as it moves through the water system.

ORP is unlikely to give good control in ‘once through’ distribution systems and will tend to under-dose or over-dose, or both. It is rare for providers of these controllers to use them for once through systems – either hot or cold - because of this uncertainty.

Warm Water Systems

Although Warm Water systems are usually re-circulating loops they differ from other systems. In Cooling towers and swimming pools water chemistry is controlled by set points that include a known and continuous bleed off of water. This is used to control the conductivity, hardness and background ORP of the water.

In warm water system the only bleed off is by direct usage. So there are peaks and troughs in usage. Morning activities like showering, cooking, etc place a high demand on water usage. Lunch time may also show peaks in water use. Little water is used after late evening. The dynamic nature of water usage means the re-circulation time through the system is very variable. If the dosing point is at the start of the loop and the probe at the end of the loop the effectiveness of ORP treatment is at the mercy of the circulation rate and hydraulic design.

Naturally, this is made worse by poor hydraulic design, stagnation, sub-ordinate loops, long runs etc. All the common deficiencies in large buildings conspire against effective ORP control!

Yet again, even though this is a re-circulating system ORP will tend to under-dose or over-dose, or both. This is certainly what we have experienced in Hospital and aged care premises.

This is why I don't believe they are a good fit for a warm water system.

So what's the alternative?

The Australian enHealth guidelines endorse a ‘systemic treatment’ for the disinfection of the entire system. This means that a treatment should address the hot, warm and cold supplies as part of a water safety and risk management plan. Unless a second disinfection treatment deals with the cold water system any ‘once through systems’ ORP cannot reliably provide a systemic disinfection treatment. If you are considering a single disinfection method for your system its worth considering a solution that delivers to all of the water supply.

This approach may get you around the 'headaches' of disinfection control in warm water systems. It will also, as part of a Water Risk Management Plan, make your facility a safer place for all who use it.

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