Chlorine Monitor, no reagents required!

More water companies are now using chloramination as the final stage in the treatment of potable water. Control of the addition of ammonia is vital to the success of this process. Analytical Technology has a developed a reagentless monochloramine monitor to allow simple reliable measurement of monochloramines.

Chlorine Monitor, no reagents required!

Summary of terminology below:

Free Chlorine
This is the concentration of hypochlorous acid and hypochlorite ions present in water. Addition of hypochlorite solution gives a solution of HOCl and OCl-, (the proportions are pH dependant which leads to the need for buffering of most chlorine monitors).

NaOCl + H2O HOCl + Na+ + OH-

Free chlorine HOCl + OCl-

When ammonia is added to water containing free chlorine chloramines are formed; if the dosing is correct mono chloramine is formed

HOCl + NH4 + OH- NH2Cl + H2O

Total Chlorine
Total chlorine is the sum of free and combined chlorine plus any chlorine bound to organic compounds.

Measurement Techniques
Combined chlorine measurement can be carried out in a number of ways. Only one method is direct.

Amperometric monitors with reagent addition
By adding potassium iodide solution to the sample water, iodine is liberated in direct proportion to the total amount of chlorine in the sample. If a measurement of the same sample without potassium iodide addition is made a measure of free chlorine is obtained. By subtracting one measurement from the other, combined chlorine can be calculated.

Amperometric monitors will need addition of buffer to measure free chlorine and buffer plus KI to measure total. The direct costs (chemicals) and indirect costs (labour costs to change reagents, maintenance of pump systems etc) of reagents are very high. They can reach 400-500 per monitor per year. There is also a detrimental environmental impact caused by the disposal of acetate and phosphate buffers. In many cases the samples have to be disposed to a tank and then removed from site, further increasing the real costs of using such systems.

Amperometric monitors do not measure chloramines directly.

Colorimetric monitors
Colorimetric monitors use the DPD method to measure either free or total chlorine. To obtain combined chlorine measurements, two measurements must be made with the combined chlorine calculated by subtraction of the free chlorine value from the total value.

Reagent addition is always needed to make a colorimetric measurement. The true running costs and associated problems are similar to those of an amperometric system.
Colorimetric monitors do not measure chloramines directly.

Polarographic membraned sensors
Polarographic membraned sensors isolate the measuring electrodes from the sample behind a membrane. The species to measured, (monochloramine), diffuses through the membrane into the measuring cell. An electrochemical reaction takes place and a direct measure of monochloramine is made.

The sensor does not need any reagents. Polarographic membraned sensor monitors measure chloramines directly.

Environmental impact
Of the available methods polarographic membraned sensors have the least environmental impact. They do not use any reagents.

Running costs
Of the available methods a polarographic membraned sensor technology has by far the smallest running costs.

To obtain combined chlorine measurements from any other system involves the use of two monitors or a combination monitor with two measuring cells. In either case the running costs of the system will be around 800-1000 per annum.

In comparison the costs of running a direct measuring sensor are limited to the costs of changing membrane and electrolyte around twice per year. In the case of an ATi A15/63 combined chlorine monitor this cost would be around 7 per year.

Effect of di and tri chloramines species

If ammonia dosing is incorrectly carried out, di and tri chloramines species can form. These species are very undesirable. They create taste and odour problems. It is important that their presence is detected.
Amperometric and colorimetric monitors
Because these systems both measure total chlorine they will not differentiate between mono, di and tri chloramine. A reading of 0.5ppm total chlorine could result from a sample with 0.5ppm monochloramine or from a sample with e.g. 0.3ppm monochloramine, 0.15ppm dichloramine and 0.05ppm trichloramine.

Polarographic membraned sensors
The polarographic membraned sensor will have an increased sensitivity to dichloramine species. The presence of volatile trichloramine species will cause the sensor to become unstable. This is a good indication that dosing of ammonia is not being controlled correctly.
What happens in fault conditions?

If the objective of the process is to produce water with monochloramine as the residual disinfectant, a couple of things are worth considering with respect to what happens under certain fault conditions.

Ammonia fails
1. A total chlorine monitor will see no change
2. A monochloramine monitor will go to zero

Ammonia under-dosed
1. A total chlorine monitor will see no change, mono, di and tri will be measured as total chlorine
2. A monochloramine monitor will see the change.

The ammonia under-dosed is probably the most important as it will lead to formation of di and tri chloramines

Recommended method for best control of chloramination

Post ammoniation
For control of ammoniation and to monitor for formation of di and trichloramines the use of a polarographic combined chlorine monitor along with polarographic membraned free chlorine probe will give a truer picture of what is happening than any other method.

When the process is under control with a nominal e.g. 0.5ppm monochloramine concentration the ATi Q45H/62 free chlorine monitor will read 0.005ppm (100:1 free to combined sensitivity ratio) and the Q45H63 will read 0.5ppm.

If ammonia dosing fails, the free monitor will read 0.5ppm and the combined monitor will read zero.
If di and tri chloramines are formed the free chlorine monitor will start to respond at a level dependant on the ratio of species present. The sensitivity to di chlorine is around 0.5:1 and to tri is around 100:1.

Pre ammoniation
To obtain the maximum information about the process and to obtain maximum control the above monitors should be used in combination with pre-ammoniation free chlorine monitor. This allows feed forward control of ammonia based on the free chlorine measurement with feedback control and or alarm monitoring based on post ammoniation combined and free monitoring.

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