Measuring Disinfection: What You Need to Know
When it comes to potable and pool water, effective disinfection is obviously extremely important. If you get it wrong, the effects can be felt by an entire community! In fact, water disinfection is a concept that dates back to 2,000 BC (granted, our technology has improved to this day). Two options for assessing disinfection power include monitoring of free chlorine, and ORP (oxidation reduction potential). The two measures are different, but how do they differ and what do the numbers actually tell us?
Free chlorine and ORP targets are commonly set in water treatment plants and public pools to make sure that an appropriate level of disinfection is maintained.
So, what is free chlorine?
Free chlorine (sometimes also referred to as residual chlorine or free available chlorine) is the portion of chlorine that remains available for disinfection at any given time. You do need a minimum concentration of free chlorine in potable and pool system,s as it confirms that disinfection occurred, and ensures residual chlorine is available to prevent recontamination. Chlorine can react with organic and inorganic compounds, including bacteria, metals, and ammonia. Chlorine which reacts with ammonia (to form chloramines) can still disinfect water, but are less efficient.
Free chlorine is comprised of hypochlorite ions (OCl–)and hypochlorous acid (HOCl).
Disinfection occurs best with hypochlorous acid, since it does not have the negative charge that pathogens have (and similarly-charged particles do not react easily — which is why chloramines take much longer for disinfection).
The balance of hypochlorite ions and hypochlorous acid depends on the pH of the water. A pH below 7.4 is ideal for disinfection (where hypochlorous acid is more abundant), and pH in the low-7 range is typically seen at treatment plants.
What is ORP?
ORP stands for oxidation-reduction (redox) potential. ORP is measured in millivolts, and is commonly monitored for swimming pools and potable water supplies.
Since chlorine disinfection is an oxidation process, ORP can tell us whether the conditions within the water are favourable to disinfection. A positive ORP value indicates an oxidative environment, which is desired for water treatment, whereas a negative ORP value suggests that there is a greater potential for reduction (insufficient disinfection).
Measuring Free Chlorine
Free chlorine is typically measured using the DPD method — a quick and simple method that makes use of a colorimeter, which measures the colour of a dye produced when reagents react with chlorine.
What does a free chlorine reading tell us?
A presence of free chlorine tells us that breakpoint chlorination has been achieved.
Breakpoint chlorination is a very important aspect of water treatment, as achieving breakpoint means that the chlorine demand has been fully satisfied. Any additional chlorine added to the water remains as free chlorine. A reasonable residual chlorine concentration (typically 0.2 to 0.5 mg/L for potable water) also tells us that the water will remain disinfected as it travels through pipework.
What can free chlorine not tell us?
If a free chlorine residual test returns a result of zero, it is difficult to determine how much additional chlorine must be added to achieve breakpoint chlorination. Free chlorine also does not tell us how much hypochlorous acid we have, although we can estimate that if we also know the pH of the sample (which in turn allows us to judge the disinfection power of the chlorine).
Ideally, free chlorine measurement is followed up with a total chlorine measurement, in order to determine how much combined chlorine (chloramines) is present in the sample. Not only are chloramines far weaker disinfectants than free chlorine compounds, they are also the primary cause of taste and odour problems, especially in swimming pools.
ORP is typically measured with an ORP probe that contains an ORP electrode and a reference electrode, similar to how pH is measured.
What can an ORP measurement tell us?
The primary benefit of ORP measurement is that it can be an indicator of disinfection rates, with higher ORP values correlating with faster kill times of micro-organisms.
What can ORP measurement not tell us?
ORP is not a direct substitute for free chlorine measurement. Free chlorine can consist of hypochlorite ions, which hold a negative charge (like many micro-organisms but unlike hypochlorous acid), which can actually decrease ORP readings – despite still having a degree of disinfection power. This issue will occur at higher pH values, as shown in the figure below.
ORP could be high despite low concentrations of hypochlorous acid, if the water is already “clean”. This means that a high ORP reading cannot guarantee that a disinfection buffer will be available for water flowing into reticulation. Similarly, ORP could be low at high concentrations of hypochlorous acid, if the water is highly contaminated. This is more of an issue for sewage treatment plants.
Free Chlorine and ORP are commonly measured in the water industry, and the results of these measurements can reveal useful information about the disinfection power of disinfectants in a sample of water. There are limitations to relying on one measurement over the other, and ideally both should be used to gain a full understanding of the disinfection power in water.
Sometimes additional information is needed to qualify the disinfection power in water, such as the pH of the water, total chlorine, or ammonia concentrations.