Amonia NH3, is mixed with chlorinated water to form chloramines. Ammonia is normally added to drinking water as aqua ammonia, NH4OH, or as anhydrous ammonia, a liquified gas. It is also sometimes obtained in solid form as either ammonium sulfate or ammonium chloride but this approach is generally used only for smaller systems. Ammonia is in equilibrium with the ammonium ion and this ionized species is the predominant form in most drinking water systems:
NH3 + H2O <--> NH4+ + OH-
The relationship between free ammonia, NH3, and ammonium ion, NH4, is highly pH dependent. For water systems with a pH between 7 and 8.5, the predominant form is ammonium ion; as the pH increases, a higher percentage of ammonia gas is formed until at pH 11 over 95% will be present as NH3.
Chloramines exist in water as three compounds:
NH2Cl monochloramine
NHCl2 dichloramine, and
NCl3 trichloramine/nitrogen trichloride
The two primary forms of interest are mono- and dichloramine. Under normal operating conditions, trichloramine will not be present. The preferred residual is monochloramine as even trace amounts of dichloramine often give objectionable tastes and odors to the water.
Monochloramine forms as chlorine reacts with the ammonia. The figure above illustrates the relationship between chlorine dose, chlorine residual, and the form of chloramines for a hypothetical water containing 0.5 mg/L NH3-N. Normally sufficient ammonia is added to the disinfected water to produce a chlorine to ammonia ratio between 3:1 and 5:1. This ratio is expressed as mg/L Cl2: mg/L NH3-N and is based on the chlorine residual, not the chlorine dose.
The chlorine demand of the water is defined as the difference between the chlorine dose and the chlorine residual. In the example shown in the figure above, a chlorine dose of about 2.7 mg/L is necessary to produce a desired residual of 2.2 mg/L because for this set of conditions the chlorine demand is about 0.5 mg/L. The chlorine demand varies with contact time, temperature, pH, and chlorine dose. Thus, to properly operate with chloramines, the operator will need knowledge regarding the chlorine demand for the water being treated for the present condition. For a particular solution, the chloramine residual will reach its maximum concentration at approximatley a 5:1 chlorine:ammonia ratio. Monochloramine is formed at a chlorine:ammonia ration of 5:1 or less, and increasing the chlorine:ammonia ratio greater than 5:1 will cause favorable conditions for the undesirable formation of dichloramine. Note that if the chlorine:ammonia ratio is increased beyond 5:1, the total chlorine residual will actually decrease even though the chlorine dose is increasing. This is an important point for operators who are attempting to maintain a chloramine residual within a specified range. If an operator notices that the chloramine residual is less than desired, the normal first reaction is to turn the feed rate on the chlorinator to increase the chlorine dose. However, this response may lead to a further drop in the total chlorine residual (see figure below), and the operator should first calculate the current chlorine:ammonia ratio to ensure which portion of the chlorine curve is applicable before adjusting any chemical feed systems.
Source: A Guide for the Implementation and Use of Chloramines by Harms and Owen. © 2004 AwwaRF. Reproduced with permission. This material is presented solely for informational purposes. More details are available at http://www.awwarf.org/research/TopicsAndProjects/projectSnapshot.aspx?pn=2847.
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