TECH TIP - A Closer Look at Chloramine and Nitrification Monitoring

A Closer Look at Chloramine and Nitrification Monitoring

 

Introduction

Nitrification is one of the 19 Operational Performance Improvement Variables included in the Partnership for Safe Water distribution system optimization program’s self-assessment process.  This component of the self-assessment applies only to utilities that use chloramine as a distribution system residual disinfectant.  Utilities that use only free chlorine as a distribution system disinfectant are not required to complete self-assessment questions that are specific to chloramine use and nitrification. 

 

Approximately 50% of the Partnership’s distribution system program subscribers that submitted disinfectant residual data for the 2012-2013 reporting period reported using chloramine as a distribution system disinfectant.  This number compares with 30% of systems found to be using chloramine disinfection, as indicated by responses to the AWWA Disinfection Committee’s Disinfection Survey conducted in 2007.  This difference in percentage may be due to utility size.  Chloramine use tends to be more prevalent in larger systems, which also were more prevalent in reporting data to the Partnership during this time period.

 

Regardless of size, chloramine is used for distribution system disinfection for a variety of reasons.  Most commonly, it is used because, unlike free chlorine, it does not readily react with the natural organic matter present in water to form disinfection by-products (DBPs), such as trihalomethanes and haloacetic acids.  Chloramine disinfection may also be used due to its stability or if a utility may have the potential to blend water with neighboring systems that also use chloramine disinfection.  Chloramine may be used successfully in all of these applications – the most vigilant and successful systems will carefully monitor chloramine, and is associated parameters, to prevent water quality issues that have the potential to occur both at the time of chloramine formation and in the distribution system.

 

Chloramine Formation

Chloramines forms when free chlorine and ammonia react together.  The chemical equations below indicate, on a general level, what happens when these reactions occur.  As with any chemical reaction, water quality conditions, such as pH and temperature, can impact the effectiveness of these chemical reactions.

 

HOCl + NH₃ → NH₂Cl (monochloramine) + H₂O

 

HOCl + NH₂Cl → NHCl₂ (dichloramine) + H₂O

 

HOC l + NHCl₂ → NCl₃ (trichloramine) + H₂O

 

The continued reaction of free chlorine with trichloramine and other organic nitrogen compounds present in the water results in breakpoint chlorination.  At the breakpoint, all of the nitrogen compounds in the water have been oxidized, and any additional chlorine added to the water remains present as free chlorine.  While treatment applications exist for which breakpoint chlorination is the desired solution, systems that are utilizing chloramine disinfection for DBP control purposes, target the optimized formation of monochloramine.  The figure below indicates the species present as chlorine is added to a water containing ammonia. 

 

 

(Hach Company)

 

Monochloramine is formed at a specific ratio of chlorine to ammonia-N.  This ratio typically ranges from 4.5-5.0:1, although the specific ratio used at a particular utility is system specific and may fall outside of this range.  Monochloramine formation is considered optimal when approaching the first “hump” on the curve.  If the ratio of chlorine to ammonia used is too high, it can cause the formation of di- and trichloramines, which can contribute to taste and odor issues in the finished water.  If the ratio of chlorine to ammonia is too low, the formation of monochloramine may be incomplete, leaving a higher than optimal concentration of free ammonia in the finished water.  Excess free ammonia can contribute to distribution system nitrification.  As described in the following sections, chemical analysis can be used to monitor and control the formation of monochloramine, as a means to help prevent nitrification from the start.

 

Distribution System Nitrification

Nitrification is a microbially mediated process through which ammonia is oxidized to form nitrite and nitrate.  Nitrosomonas is responsible for oxidizing ammonia to nitrite, while Nitrobacter converts nitrite to nitrate.  The reactions associated with nitrification are displayed below:

 

Nitrosomonas – conversion of ammonia to nitrite

NH₃ + 3/2O₂ → NO₂^- + H₂O + H⁺

NH₄⁺ + 3/2O₂ → NO₂^- + H₂O + 2H⁺

 

Nitrobacter – conversion of nitrite to nitrate

NO₂^- + 1/2O₂ → NO₃^-

 

Nitrification has the potential to occur in the distribution system under various conditions, and is most commonly associated with elevated temperatures (above 15^oC), high water age, lower pH, high free ammonia concentrations, high chlorine demand (ie total organic carbon), and low monochloramine residuals.  Some distribution systems are impacted minimally, or not at all, by nitrification, while the process can present significant operational challenges to others.

 

Nitrification is a challenge due to the potential impact that it can have on distribution system water quality.  Nitrification can result in a significant drop in disinfectant residual, which has the potential to result in increased bacteria growth, higher heterotrophic plate count (HPC) and coliform levels, and increased biofilm growth.  As indicated above, the reactions associated with nitrification also consume oxygen and produce hydrogen ion, reducing pH and alkalinity and potentially impacting the corrosion properties of the water.  Nitrification also increases the concentration of nitrite and nitrate in the water.

 

Distribution system nitrification can be a challenging problem to address, once it is established in the system.  Therefore, optimized systems vigilantly monitor the parameters associated with nitrification and create action plans prompting them to take steps to prevent its occurrence.

Chloramination and Nitrification Monitoring

There is not one single parameter that can be used for chloramination and nitrification monitoring that will provide comprehensive information about the water’s chemistry.  Whether in the plant or in the distribution system, testing a combination of parameters is required to fully understand and control the process.  Areas where chloramination and nitrification parameters are tested are recommended to include the treatment plant where monochloramine is being formed, areas in the distribution system that may be at higher risk for nitrification, and any area in the system where disinfectant residual may be boosted.  A distribution system hydraulic model may assist in identifying and locating these sampling sites.  Examples of key sampling sites for nitrification monitoring include:

·         Water treatment plant (point of monochloramine formation for process control)

·         Distribution system storage tanks

·         Pressure zone boundaries

·         Areas with the potential for high water age and/or low usage

·         Dead ends

 

Parameters recommended for monitoring chloramine formation and nitrification include the following:

·         Chlorine residual

o   Monitoring for free and total chlorine is recommended in many cases.  However operators should be aware that accurately measuring free chlorine in the presence of total chlorine using the colorimetric DPD chemistry can be challenging.  Even when running a test using free chlorine reagent, total chlorine can react to oxidize DPD, potentially resulting in a false positive result.  Consult the reagent manufacturer for more information about performing tests in this application.

·         Monochloramine

·         Free ammonia

·         Total ammonia

·         Nitrite

·         pH

·         HPC

As the figure above indicates, a single monochloramine or total chlorine measurement does not provide sufficient information about  water chemistry to identify where the water is relative to the chloramination curve.  Without additional information, the same total chlorine results could indicate incomplete monochloramine formation (section I of the graph), dichloramine and trichloramine formation (section II of the graph), or even breakpoint chlorination (section III of the graph).  The combination of multiple parameters listed above, for example monochloramine, total chlorine, and free/total ammonia, allows utilities to better understand where they reside on the chloramination curve, their level of nitrification risk, and steps that must be taken to maintain process or system control.  Field, bench, and continuously monitoring instrumentation is available for the analysis of these parameters, and utilities are encouraged to select the analysis platform that best fits their needs. 

 

There are many steps that can be taken to prevent and control nitrification.  For example, nitrification prevention can begin at the plant by maintaining a chlorine to ammonia ratio that is optimal for monochloramine formation.  Steps can be taken to reduce water age in the distribution system, such as adding loops to the system.  Tank stratification can be prevented by the addition of mixing.  Once established, nitrification may require steps such as flushing to restore water quality.  A comprehensive description of nitrification prevention and control is beyond the scope of this article.  Refer to the resources listed below for additional information.

 

In any case, utilities that chloraminate should consider developing action plans and procedures to address the potential for nitrification and take steps to proactively address any issues to occur.  Many systems develop tiered action plans, in which different actions are to be taken depending on the relative concentrations of different parameters in the system.   As with any procedure, staff should be trained in its implementation to help ensure that is carried out correctly.

 

Good understanding of nitrification, its causes, and its impact on distribution system water quality and operations give utilities the tools they need to work towards achieving optimization.

 

References

This article presents a brief overview of chloramination and nitrification.  Many comprehensive resources exist to provide more detailed information about these topics including:

 

Water Chlorination Principles and Practices – AWWA Manual M20

Nitrification Prevention and Control in Drinking Water – AWWA Manual M56