Technically speaking

Black & Veatch's Frank Rogalla looks at chloramination in the water industry

Several utilities use chloramines for disinfection, some systems have been in use for more than 50 years. Thus, chloramination is not a new technology but it is one that presently is capturing the attention of the water industry. Present and future needs to reduce byproducts of chlorination means chloramination is frequently considered or selected as a replacement for disinfection with free chlorine. However, chloramines have recently received some bad press and WTW operators and the public are concerned over health effects and related issues, ranging from high lead levels at the tap, to the creation of hazardous disinfection byproducts. Thus, a fresh look at using chloramines is both timely and important.

Chloramines are formed when free chlorine is combined in water with ammonia. The ammonia can be naturally present in the untreated water source or it can intentionally be added during treatment. Generally, insufficient ammonia is present in the raw water to form desired levels of chloramines and ammonia is therefore added at the treatment plant in solid, liquid or gaseous form. Ammonia in the gas form, anhydrous ammonia, is fed with equipment similar to that used to feed chlorine gas. Liquid ammonia and aqua ammonia is fed using metering pumps. Solid ammonia, such as ammonium sulfate is fed with dry feed equipment, with mixing in a solution tank prior to being metered into the treatment process. All chemical feed systems are normally designed for water systems by consultants with prior experience in this area. The disinfection capabilities of chloramines are not as efficient against most microbial organisms as free chlorine and for this reason chloramines are normally used as secondary disinfectants within the distribution system rather than for primary disinfection. However, the gap between disinfection with chloramines and free chlorine is not as wide as most professionals are lead to believe because many drinking water systems operate at a pH of 7.5 or higher.

In this pH range the majority of free chlorine has dissociated to hypochlorite ion (OCl-) and this form of free chlorine has disinfection characteristics much closer to monochloramine than the more effective form of free chlorine, hypochlorous acid. A key driving force for using or switching to chloramines in today's regulatory environment is the need to reduce chlorinated disinfection byproducts such as trihalomethanes (THMs) and haloacetic acids (HAAs). Both of these byproducts have regulatory limits. As each water company evaluates its current situation and the available options should change be necessary, chloramines enter into the evaluation process because they are normally one of the more economical alternatives.

Chemistry of Chloramines

Figure 1 shows a breakpoint chlorination curve similar to curves found in most water treatment texts. It can be used to illustrate some important aspects of chloramination. When ammonia and chlorine are added together monochloramine (NH2Cl) is formed at chlorine:ammonia ratios less than 5:1. Chlorine:ammonia ratios are calculated on a weight basis, for example, 5mg/l of chlorine with 1mg/l of ammonia-nitrogen would be a 5:1 chlorine:ammonia ratio. Monochloramine is the desired form of chloramines for use by drinking water systems because higher ratios favour the formation of dichloramines, which are often the source of obnoxious taste and odours that will generate customer complaints. Note beyond the 5:1 ratio the measured chlorine residual will go down when additional chlorine is added. This relationship between dose and residual is fundamental, but frequently overlooked by operators when the chlorine residual monitor starts to show a drop in residual. The normal tendency for an operator when the chlorine residual drops below the desired level in the plant discharge is to increase the chlorine dose. If the plant is then operating beyond the 5:1 level increasing the chlorine dose will result in an even lower chlorine residual - it will not resolve the low residual problem.

Challenges

As mentioned previously, training operators of treatment works to recognise the fundamental principles of chloramine chemistry is important. In addition, educating the public, controlling nitrification, blending of disinfectants and limiting the formation of other harmful DBPs are other challenges facing a utility using chloramines. Of these challenges, controlling nitrification and blending chlorinated water with chloraminated water are the two issues most frequently discussed by WTW operators.

Educating or informing the public regarding the use of chloramines is an important but often overlooked aspect of using chloramines. Some homeowners will recognise their household cleaning products carry warnings against mixing ammonia with chlorine, exactly what their water utility intends to do. Those who receive phonecalls from the public must be prepared to answer questions such as this. Also, it is important to notify pet shops and other users regarding the need to remove chloramines prior to using water for fish tanks. Although chloraminated water is not known to be harmful for pets to drink or for watering plants, these questions will be asked and users do have the option of removing chloramines prior to use with chemicals available at local shops. It will also be prudent to notify dialysis centres of any change in disinfection practise, although the standard procedure during dialysis is to remove any chlorine or chloramine prior to use. Any disinfection process that uses an oxidant will form byproducts as a result of the disinfection reaction. This statement is true for all oxidants including chlorine, chloramines, ozone and chlorine dioxide. Byproducts of chloramination that may be of concern are N-nitrosodimethylamine (NDMA) and cyanogen chloride. Presently neither of these compounds is regulated in drinking water, partially due to a lack of reliable information regarding their health risks and at what concentrations they may be cause for concern. Data to date indicates their presence in chloraminated water is at low concentrations. However, just knowing NDMA or cyanogen chloride could be present in drinking water has launched an initiative in California to require the state to discontinue the use of chloramines.

Controlling Nitrification

Nitrification is the biological oxidation process by which ammonia is converted to nitrate. It is a two-step process with nitrite as the intermediate contaminant. Two primary groups of organisms responsible for this biological oxidation are loosely categorised as ammonia oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB). Neither AOB or NOB are pathogenic but they can be responsible for a loss of disinfectant residual, a drop in pH or alkalinity and/or an increase in HPC concentrations.

A key step in controlling nitrification is to know your distribution system and what changes may be occurring. It is essential to monitor within the system periodically for ammonia, nitrite, nitrate, chloramine residual, pH, alkalinity and HPC concentrations. It is important to establish a baseline for these constituents so if nitrification does begin to occur, it is detected at an early stage. Nitrification can usually be kept under control by ensuring an adequate chloramine residual (2-2.5mg/l) throughout the system, reducing water age as much as possible by avoiding excessive storage time and using a chlorine:ammonia-nitrogen ratio of about 4.5:1.

Systems that operate for part of the year with cold water can usually avoid nitrification using these procedures. Systems that have warm water within their distribution system for the entire year will experience more difficulty avoiding nitrification episodes. Introducing free chlorine into the system periodic- ally has been another technique used for controlling nitrification. This method of control often produces taste and odour complaints during the change to the new disinfectant but it does generally restore disinfectant residuals to an acceptable level. The future use of this technique is not assured pending interpretation of the final Stage 2 DBPR, which may require sampling for DBPs during the period that chloraminated systems switch to free chlorine.

Blending of Disinfectants

Many systems have multiple water sources with different disinfectants, for instance, chlorinated groundwater may be pumped directly into a distribution system that also receives chloraminated water from a surface water treatment facility. As evident from Figure 1, if water disinfected with free chlorine is blended with chloraminated water, the additional chlorine in the blended water will change the chlorine to ammonia:nitrogen ratio. If the additional chlorine places this ratio beyond the 5:1 location on the breakpoint chlorination curve, the promotion of dichloramines with subsequent tastes and odours may result.

Some systems avoid this blending problem by isolating portions of the distribution system so waters with different disinfectant residuals are not blended in the same pressure zone. Elsewhere, attempts are made to limit the blending so the chlorine:ammonia-nitrogen ratio never exceeds 5:1. One large utility in the US has developed a sophisticated computer programme to track blending to ensure all portions of the system receive water with only monochloramine.

Although not a new technology, there is renewed interest in using chloramines for secondary disinfection to limit concentrations of DBPs. Regulatory limits have become more stringent for DBPs in recent years and they will continue to receive critical attention from future regulations. It is important for water companies using chloramines to have a basic understanding of the chemistry involved as well as an awareness of the challenges for successful implementation. Important activities to successfully implement chloramines include monitoring of water quality changes in the system, operating with suitable residuals, maintaining proper chlorine:ammonia ratios and controlling nitrification


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