Water disinfection

Methods for disinfection of water.

The aim of water disinfection is to eliminate pathogenic organisms in the water. Some harmful microorganisms in water storage and distribution systems can be viruses, bacteria and protozoa. The reduction of these can be achieved by filtration or by inactivation. The filtration process consists of a physical barrier (usually a membrane) that does not allow the passage of microorganisms into the effluent; while in inactivation, the microorganisms are transformed (their reproductive mechanism is altered) so that they can no longer cause disease. To ensure the microbiological integrity of the water in the distribution network, it is necessary to add a residual of the disinfecting agent.

The disinfection agents commonly used in the treatment of drinking water are (1) free chlorine, (2) combined chlorine (chloramines), (3) chlorine dioxide, (4) ozone and (5) ultraviolet light. In the following table we can see of the mentioned disinfectants.

Table 1. Comparison and characteristics of various disinfectants.

Disinfectants

Parameter

Free chlorine

Combined Chlorine

Chlorine Dioxide

Ozone

Ultraviolet light

Effectiveness in disinfecting bacteriaExcellentGoodExcellentExcellentGood
Virus Disinfection EffectivenessExcellentOn the edgeExcellentExcellentOn the edge
Effectiveness in disinfection of protozoaDeficientDeficientGoodGoodExcellent
Effectiveness in Endospore disinfectionDeficientDeficientOn the edgeExcellentOn the edge
Maximum Residual Concentration4 mg / l4 mg / l0.8 mg / l
Formation of regulated chemical by-productsForms 4 species of THMa and 5 species of HAAbForms traces of THM and HAAChloriteBromateNone
Formation of chemical by-products that can be regulated in the futureMiscellaneousCyanogen halidesChlorateBiodegradable organic carbonUnknown
Typical application rate, mg / l1 – 62 – 60.2 – 1.51 – 520 – 100 mJ / cm2
Chemical sourceDelivered to the plant as liquefied gas under pressure in tank cars or as liquid chlorine.

Generated on site from salt electrolysis

Calcium hypochlorite powder is used for small applications.

Same chemical sources as chlorine.

Ammonia is delivered to the plant as ammonium hydroxide (liquid), ammonia (liquefied gas) or ammonium sulfate (solid).

Ammonia and chlorine are mixed in the disinfection process.

ClO2 is manufactured on site with a chlorine and chlorite generator.Manufactured on site using an ozone generator (consists of passing pure oxygen or dry air through an electric field).Use of UV (radiation) lamps.
a THM = trihalomethanes
bHAA = haloacetic acids

MWH’s water treatment: principles and design. John C. Crittenden et al.

The design of a disinfection system consists of:

1. Selection of the appropriate disinfectant and dosage.

The dose depends on the type of disinfectant used, the selection of which will depend on the conditions of the water as well as the microorganisms to be eliminated. When chemical disinfectants are added to the water, they are consumed by the rapid oxidation of the reduced species in the water; this phenomenon is known as initial demand. By covering the initial demand and continuing with the addition of the disinfectant a residual dose is generated, the maximum concentration of which depends on local regulations. Its function is to protect the tanks and pipes of water distribution systems from pathogenic organisms.
An important parameter when choosing a disinfectant is the product of the disinfectant concentration (C) by the contact time (t. The Ct has been a useful measure to compare the performance of different disinfectants in inactivating a microorganism. Figure 1 shows the Ct required by each disinfection method to inactivate 99% of different microorganisms. In the case of ultraviolet light, the concentration factor is changed to the intensity factor (I), obtaining an intensity value for time (It).

Disinfection requirements for an inactivation of 99%. John C. Crittenden et al.

Figure 1. Disinfection requirements for 99% inactivation. John C. Crittenden et al.

2. Dosing system design

2.1) For liquid disinfectants

, these can be injected directly into the pipe (example of dosing device, Figure 2), dosed into a mixing device such as a static mixer, or simply poured into a high turbulence point.
Dosing system for liquid disinfectants.

Figure 2. Dosing system for liquid disinfectants.

2.2) For gaseous disinfectants

, like ozone, is usually injected by bubbling to the bottom of a column dimensioned to obtain the necessary contact time between the gas and the water. Another more common design is dosing by means of a Venturi (example of ozone dosing by Venturi, Figure 3). Ozone dosing with Venturi in a barrel filling line.

Figure 3. Ozone dosing with Venturi in a barrel filling line.

2.3) In the case of ultraviolet light

, the radiation dosage is carried out inside a reactor designed by the manufacturer (example of reactor, Figure 4), to maximize the interaction between water and light.

Example of UV reactor

Figure 4. UV reactor example.

 

Last updated 14/02/2020.

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