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Application Summary
ChemScan® Application Summaries
Rev. 11/97

ChemScan® Application Summary

Water Chloramination Process Control

Statement of the Problem

Chloramination of water is performed to produce a more stable disinfectant compared to free chlorine. Chloramine has been shown to reduce the formation of THM and other disinfection by-products by as much as 80%, while reducing certain taste and odor problems. In some parts of the US, especially the midwest and south, as many as one third to one half of the utilities serving over 50,000 customers currently practice chloramination.

Chloramination Process Strategies

There are three typical strategies for creation of chloramine in potable water. All involve the addition of ammonia and free chlorine in the process:

  1. Concurrent addition allows a very short direct contact time for free chlorine and water before reacting with ammonia. This method optimizes use of free chlorine as a primary disinfectant while reducing the formation of THM in the water.
  2. Preammoniation, where ammonia is fed upstream from the chlorine addition point, provides the lowest possible THM formation and better taste and odor control, but somewhat less effective disinfection than either concurrent addition or post ammoniation.
  3. Post ammoniation provides a substantial free chlorine contact period prior to the addition of ammonia and the formation of chloramine. This achieves maximum disinfection but may also maximize THM formation.
The optimum chlorine dose for monochloramine formation is based on a Cl2 to NH3-N ratio of 4.5 to 1). Many utilities practicing chloramination routinely add chlorine and ammonia at a ratio of between 3 to 1 and 5 to 1. A 5 to 1 weight ratio or less will suppress the decomposition of monochloramine into dichloramine and trichloramine in the water. The objective is to maintain the process at the peak of the breakpoint curve, to the left of the breakpoint, which represents the maximum concentration of monochloramine (NH2Cl) prior to any dichloramine formation (See Figure 1).

Traditional Monitoring Methods

Optimization of chloramine treatment variables requires accurate feed rate control to ensure a consistent CL2 to NH3-N ratio and a desired total chlorine residual in water leaving the treatment plant typically in the range of 2.0 to 4.0 mg/l as Cl2. Control systems used by many utilities are inadequate for accomplishing this objective1. Manual feed control and flow pacing strategies may not be adequate for control in the presence of a variable chlorine demand. Downstream chlorine residual monitoring may not adequately define the state of the process, since the same total chlorine residual can be achieved at any of three different locations on the breakpoint curve, each representing a different chlorine species resulting from very different chlorine feed rates.

ChemScan® Analytical Method

Monochloramine (NH2Cl) has a strong light absorbance signature in the ultraviolet wavelength range, the intensity of which is proportionate to monochloramine concentration. A direct (primary) analysis of monochloramine can therefore be provided by ChemScan. This concentration can be reported in terms of the equivalent NH2Cl or the equivalent ammonia or chlorine fraction, as shown on Table 1. An alternate method of chloramine analysis injects a pH buffer to force monochloramine into the di- or tri-chloramine state, and compares the difference in absorbance spectra before and after the pH change.

In addition, ChemScan® Process Analyzers can detect free ammonia. When chlorine and ammonia are combined at ratios of less than 5 to 1, an excess of free ammonia will be present. Nitrification problems can be avoided and ammonia or chlorine feed rates can be optimized based on the analysis of free ammonia in the chloraminated water. Free ammonia may also be a desirable parameter for control of a preammoniation process, especially if there is a substantial distance between the point of ammonia addition and the point of chlorine addition.

Although ChemScan® can detect free chlorine, when chlorine and ammonia are combined in ratios of 5 to 1 or less all of the chlorine should be in combined form. However, measurement of free chlorine in a post ammoniation process, just prior to the point of ammonia addition, can help optimize feed rates which would otherwise be unstable based on variable chlorine demand in the water.

Apparatus

A single ChemScan® Analyzer can detect monochloramine, ammonia and/or free chlorine from multiple sample points in the process or a dedicated ChemScan® Analyzer can detect one or more of these parameters at specific sample points, dictated mainly by sample line distance and time intervals between analysis cycles.

Table 1
  NH3 NH3-N NH2Cl Cl C12
Mol Wt. 17 14 51 35 71
 
Eq ppm1.000.823.002.064.18
5.004.1215.0010.292.88
10.008.2430.0020.5941.76
ppm species B = ppm species A *MW Species B/MW Species A (MW = molecular weight)


1 Kirmeyer, G.J., et al, "Optimizing Chloramine Treatment", 1993, AWWA Research Foundation, Denver, CO.

Theoretical Breakpoint Chlorination


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