ORP controllers can monitor and control the amount of disinfectant dosing into a system. They measure the Oxidation-Reduction Potential of the water and this measures and controls the concentrations of some disinfectants. So let’s explore what ORP is and how and where it can work for you.
ORP is the acronym for Oxidation-Reduction Potential. It is also known as the redox potential. Most things in our environment have a redox potential. The ultimate oxidising effect is fire. Fire has a high redox potential and oxidises wood. Oxygen converts wood to carbon dioxide and water. Water stops fires because it has a low redox potential that neutralises the high redox potential of the burning wood.
ORP is also a property of water that is variable depending on the water quality. The redox potential is a measure of how much a solution wants to ‘give’ electrons to it’s environment. Simply, it measures how much the water is likely to ‘burn’ organic material – like bacteria. What does that mean?
What does Redox potential mean?
Well, water with a high redox potential (ORP) wants to give electrons to anything with a lower potential. 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). Pure water has a redox potential of zero – it is neutral. But dissolving chemicals with different redox potentials in the water will change this behaviour. So adding a chlorine solution to water will make the water more likely to oxidise anything it contacts. This gives a positive redox potential. Adding ‘antioxidants’ (alkali) to water will give negative redox potentials. 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 water has a constant ORP. That is, in pure water dissolved chlorine always has the same ORP value. This means that the disinfection ability of the chlorine is a measurable value. An ORP probe can measure this value and then adjust the dose of chlorine added to the water to keep it constant. These probes are quite complex instruments and the most accurate ones are too delicate and expensive for everyday use. They are usually used to calibrate ‘working’ probes. Working probes in water treatment systems are less accurate but more robust. This is the basis of ORP controllers for chlorine dosing system. Incidentally the same system can also be used for other oxidising disinfectants like monochloramine and chlorine dioxide. 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, can cause problems.
Water Supply and ORP
ORP probes measure the voltage between the water and a calibrated reference probe. This voltage measures the amount of chemical in the water. By setting a voltage range and linking the probe to a pump the chemical dose is kept at a reasonably constant level. This value also changes with water temperature.
Of course, the supply water will have dissolved chemicals in it that will give it a ‘baseline’ voltage. The probe is calibrated to zero against the background voltage so that it only measures the chemical. Routinely probes need to be re-calibrated against a standard. How often this occurs really depends on the water supply. 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 and slimes (biofilms) tend to reduce the ORP of the water. Incidentally, ORP can be used to measure ‘antioxidants’. For instance green tea (picture left) has a negative ORP that means it will ‘soak up’ oxidants. Foods with negative ORP like green tea and olives (right) are a healthy dietary option because they ‘mop up’ the harmful ‘oxidisers’ in our food that can damage our cells.
Recirculating Water Systems
This ORP control system is particularly suitable for recirculating systems. 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. In looped warm and hot water systems there is little variation in the chemistry or temperature of the supply water circulating through the system. Of course, changes occur when large volumes of fresh water enter the system. For this reason the probes need re-calibrating regularly.
The problems of a moving baseline and re-calibration are for the most part overcome by recirculating the water. 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. (Diagram: SA Health Guidelines for the Control of Legionella)
‘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. Otherwise the ORP controller may not dose enough chemical to account for the decay of the chemical as it moves through the water system. A direct result of this is a tendency to ‘overdose’ at the dosing point if the controller is a long way off, leading to corrosion issues. It is rare for providers of these controllers to use them for once through systems – either hot or cold because of this uncertainty.
ORP and Building Water Systems
ORP systems can effectively control disinfection levels in recirculating water loops. Of course this can only happen if the system design and operation is up to speed. ORP is unlikely to give good control in ‘once through’ distribution systems and will tend to under-dose or over-dose, or both! This means that cold water supplies to buildings are probably out of the scope of these controllers.
The 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.
Oxidation-Reduction Potential (ORP) for Water Disinfection Monitoring, Control, and
Documentation.(2004) Suslow, T.V. UC Agriculture and Natural Resources.