Post-treatment with disinfection

1. Introduction. In this chapter, a little more extensive vision in what is known as disinfection in the post-treatment phase using chlorine as disinfectant agent together with other devices widely used such as ultraviolet light and ozone will be shown. In another chapter, a quick overview of the same theme was shown but in spite of this, or rather as a result, there were several colleagues who reported that this information lacked sufficient quality to be published in this website. So, therefore in order to do this, it has been decided to intensify the information published on January the 24th of 2015 to offer the reader an improved vision of post-treatment with disinfection process while in fact that these methods in practice are less used in comparison to the chlorination.

2. Chlorination.

Disinfection can be produced by chlorine in its most varied forms such as Chlorine gas, NaOCl (sodium hypochlorite) and ClO2 (chlorine dioxide).

Chlorine keeps being the most economic chemical reagent, and with better control and higher security, it can be applied to water for disinfection which, depending on its characteristics, requires a greater or lesser period of contact and a higher or lower dose. Generally, a relatively clear pure water, with a pH almost neutral, practically without organic matter and strong contamination, requires about five to ten minutes of contact time with doses lower than 1 mgL. of chlorine. In each case, should be determined the minimum dose required to set a small free waste that ensures a free water, at any time, of living pathogens.

Reactions in aqueous medium.

Reactions using chlorine as a disinfectant agent, are most important those that occur in aqueous solutions and are those that most affect our study. As a result, two chemical reactions occur:

2.1 Hydrolysis reaction.

It consists of the combination with water which responds to the expression:


The dissolution of large amounts of chlorine is possible due to the magnitude of the hydrolysis coefficient whose value is enormous compared to the rest of the chemical reactions.

The disinfection process by chlorine responds to a curve known as the curve of the Breakpoint in which the formation of chloramine compounds, apart from others, occurs.

During the initial phase, all the chlorine added is used to be combined with organic matter so that, as a result, the residual chlorine level is zero.

Arriving at the stage BB', the level of chlorine residuals increases, but all this chlorine is combined in the form of chloramines, which are products that have a low disinfectant power hence producing an unpleasant smell. These compounds are the cause of the so-called pool smell.

During BC phase the added chlorine is used to destroy chloramines by which the measured residual chlorine decreases up to a minimum in C called Breakpoint. From this point on, all the added chlorine is used to increase the residual chlorine that would be as free chlorine and with more sanitizing power rather than combined chlorine forming chloramines.

Therefore, this Breakpoint must be overcome so that the combined chlorine be the minimum possible.

2.2. Ionization reaction.

It explains the breakage of hypochlorous acid (HClO) in its respective ions. The present amount of hypochlorous acid and hypochlorite ion in water is called free available chlorine.

HOCl = H+ClO-

3.-Disinfection with ozone.

Ozone is considered the most powerful compound to carry out water disinfection. Therefore water is inserted into an ozone generator package plant which really produces the total disinfection because of the elimination of bacterial agents with an average of 0.5 ppm dosing during short periods of contact time (approximately 5 minutes).

Ozone is enriched oxygen product and it is unstable and decomposes into oxygen gas and free radical oxygen, as a disinfectant reagent in killing the bacteria through the breakdown of cell membranes. This process, known as cells destruction by lysine, produces the dispersion of the cell cytoplasm in water: non-saturated lipids are the major compounds of the cytoplasmic membrane the bacteria have, causing the death of pathogenic microorganisms.

The figure shows the arrangement of the different elements that make up the ozone plant layout to do so.

4 Disinfection with UV.

Disinfection by ultraviolet rays consists of the absorption of radiation by water requiring to be disinfected in an effective manner.

The modification of the main dwelling bacteria and virus mechanism, hence transforming their DNA occurs when the rays are absorbed by water. As a consequence, the formation of difficult to control products also appear.


About Author:Alejandro Núñezis an Industrial Engineer specialized in Water Treatment. His career has been linked to both Sea Water and Sewage Water Treatmentsince 1997 until now. You can read his other interesting articleshere.