Whether you're thinking of a megalopolis or a suburban city, all cities have, at some point in their lives, the need to incorporate a water treatment plant into their operations. As many people know, these plants allow cities to provide quality drinking water to the residents and businesses within its territory.
The production of drinking water is a challenge in itself, as the contaminants found in the water supply can be very diverse in both type and quantity. Fortunately, there are a wide range of water treatment technologies available to meet all needs. If you are interested in this subject, and since we will not go into the subject of drinking water in detail, here is an article on the production of drinking water in remote locations.
Producing Drinking water in Remote Area
In short, one step of water treatment that is often found in water treatment plants is disinfection. This step reduces the risk of water contamination by bacteria, viruses, protozoa and other types of organic contaminants. Again, there are several types of disinfection technologies. Among these technologies, we can think of ozonation, thermal disinfection or chemical disinfection.
On the subject today, one of the chemical disinfection techniques is chlorination. It consists of adding chlorine to the water in order to deactivate contaminants. Well, for some time now, many treatment plants have been moving away from the use of chlorine to the addition of chloramine.
In this article, we will try to answer, "what is chloramine?", "what are its advantages over chlorine?", "why are cities switching to chloramine?", "how and why chloramine can damage reverse osmosis membranes" and "what can be done to remedy the problems caused by chloramine?"
What is Chloramine?
As its name suggests, chloramine is a derivative of chlorine. Also known as monochloramine and with the formula NH2CL, chloramine is an oxidizing agent most often used in dilute solution. NH2CL is used in dilute solution since in concentrated form it is highly unstable and even more unstable as a pure liquid. For example, NH2Cl decomposes violently at temperatures above -40°C.
Chloramines are made by adding ammonia to water containing free chlorine. The free chlorine can be in the form of HOCl or OCl, depending on the pH of the solution. Ideally, for the reaction to be ideal, the water should be alkaline and have a pH of 8.4. The importance of the pH of the solution is explained by the fact that its value will determine the type of chloramine that will be formed. There are three different types of inorganic chloramines that can be formed from this mixture. Trichloramine is usually formed when the pH is below 3. Dichloramine will form when the pH is between 3 and 7. Lastly, monochloramine, the desired result, is formed when the pH is above 7.
More technically, the synthesis is performed in dilute solution, as the reaction occurs when HClO is attacked by the nucleophile NH3. This synthesis will allow generating monochloramine in solution for use in water treatment. As information, the preparation of the pure compound is done thanks to the contact of flluoramine with calcium chloride: NH2F + CaCl2 -> NH2Cl + CaCIF.
The Uses of Chloramine in Water Treatment
More and more common in municipal water treatment plants, chloramine is used in the disinfection stage and is installed to take the place of chlorine.
Why are Cities Switching to Chloramine?
Chloramine has two main advantages over chlorine that may motivate some cities to opt for the use of this oxidizing agent. In general, the primary reason why some cities opt for chloramines is that chloramines are more stable and will provide a longer-lasting treatment than chlorine. For example, cities such as Los Angeles are switching to chloramines because water is scarce and must be purchased externally, resulting in the water spending more time in the water treatment system. Thus, the chlorine is less durable and deteriorates in the water and does not offer any oxidizing power once it reaches its destination.
Secondly, water treated with chloramines has a better smell and taste than water treated with chlorine. In other words, since the concentration of free chlorine is lower, the smell and taste of chlorine are much more subtle.
The Disadvantage of Monochloramine
Like anything else, the use of monochloramine in water treatment does not have only advantages. Although very subtle, this type of treatment gives a greenish colour to the water. More importantly, chloramine can increase the by-product production of chlorine disinfection and its by-products are generally more genotoxic.
As such, the consumption or use of chloramine-treated water does not pose a health hazard to humans. Nevertheless, for some people who are more fragile, such as young children, people undergoing dialysis or undergoing chemotherapy, certain harmful impacts on health have been identified. Then, people with fish or amphibians must also be careful because the ammonia present in the water is very dangerous for their health.
From a water treatment equipment optimization point of view, since chloramine remains active for a longer period of time in the water and does not dissipate by itself, reverse osmosis membranes can be damaged by the oxidizing power of chloramines.
Chloramines and Reverse Osmosis Membranes
Reverse osmosis membranes commonly used in municipal water treatment are not manufactured to resist chlorine. When the chlorine concentration is too high, the membranes can last between 200 and 1000 hours. Once the chlorine comes into contact with the membranes, the oxidizing power of the chlorine damages the materials used to make the semi-permeable liners. After some time of contact, the treatment efficiency of the membranes is greatly affected and contaminants can be overlooked.
There are several ways to remedy this problem, starting with the use of special membranes with increased resistance against oxidizing products such as chlorine and its derivatives. Although these membranes are available, they are much more expensive than the osmotic membranes generally used for drinking water production.
The problems caused to osmotic membranes are very often misunderstood by water treatment plants since the problems seem to come from nowhere. This is because when a water treatment plant uses chlorine as an oxidizer, the sensors will monitor the amount of free chlorine present in the water. The tests used to calculate free chlorine in water do not work the same way as the one used to identify total chlorine in water. Since chloramine leaves amounts of total chlorine rather than free chlorine, the tests performed before the transition become useless. So, even if the tests seem to indicate a low chlorine concentration, it can be much higher and have adverse impacts on the treatment system and water quality.
What can be done to Remedy the Problems Caused by Chloramine?
As mentioned above, the use of special membranes is an option to avoid problems. However, to make your equipment more profitable and optimize costs, other solutions should be prioritized.
Besides this solution, the addition of a dechlorination step can be advantageous for you. There are several dechlorination alternatives such as activated carbon adsorption, the addition of sodium metabisulfite or hydrogen peroxide. Since dechlorination techniques are varied and complex, we refer you to this article to learn more about the strengths and weaknesses of these techniques as well as the costs associated with each of them.
In summary, the use of chloramine as an alternative to chlorine has many advantages in the treatment of water in municipal water treatment plants. However, it is important to consider all aspects including the health of citizens and the costs associated with the use of this oxidizing product. Although more and more cities are opting for the use of chloramines, they often adopt an end-of-pipe dechlorination system to ensure the absence of chlorine ions in the water in order to avoid complications related to reverse osmosis membranes.
If you have any other questions or comments, please feel free to leave a comment below. In the meantime, here are some other articles that might interest you.