Contact time disinfectants

The effectiveness of disinfection increases with the amount of contact time available. 

In order to ensure the destruction of pathogens, the process of chlorination must achieve certain control of at least one factor and, preferably two, to compensate for fluctuations that occur. For this reason, some authorities on the subject stress the fact that the type and concentration of the chlorine residual must be controlled to ensure adequate disinfection. Only this way, they claim, can chlorination adequately take into account variations in temperature, pH, chlorine demand and types of organisms in the water. While possible to increase minimum contact times, it is difficult to do so. Five to ten minutes is normally all the time available with the type of pressure systems normally used for small water supplies. Many experts feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, superchlorination-dechlorination does the best job. Briefly, what is this technique and how does it operate ... 

It has also been demonstrated that the germicidal effectiveness of free and combined chlorine is markedly diminished with decreasing water temperature. In any situation in which the effects of lowered temperature and high pH value are combined, reduced efficiency of free chlorine and chloramines is marked. These factors directly affect the exposure time needed to achieve satisfactory disinfection. Under the most ideal conditions, the contact time needed with free available chlorine may only be on the order of a few minutes combined available chlorine under the same conditions might require hours. 

Whitehead, K. and McCue, K. A. (2010). Virucidal efficacy of disinfectant actives against feline calicivirus, a surrogate for norovirus, in a short contact time. Am. J. Infect. Control 38, 26-30. 

Disinfectants come from various chemical classes, including oxidants, halogens or halogen-releasing agents, alcohols, aldehydes, organic acids, phenols, cationic surfactants (detergents) and formerly also heavy metals. The basic mechanisms of action involve de-naturation of proteins, inhibition of enzymes, or a dehydration. Effects are dependent on concentration and contact time. 

Mucosal disinfection Germ counts can be reduced by PVP iodine or chlor-hexidine (contact time 2 min), although not as effectively as on skin. 

The membrane filtration technique is used once, prior to the introduction of a new disinfectant within the production department. The surface testing technique is used prior to any changes in the recommended procedure for evaluating its effectiveness on surfaces to be treated and demonstrating activity against contamination for various contact times. 

Water Treatment. The source of water for this experiment was a pilot plant located on the Seine River upstream from Paris, France (Figure 1). The pilot plant uses an upflow solids contact clarifier (Pulsator, Degremont, Rueil Malmaison, France) followed by rapid sand filtration (RSF). The filtered water is then distributed over four treatment lines to evaluate the efficiency of various ozone-GAC combinations (ozonation rates of 1 or 5 ppm O3 and 10-30 min of contact time). The GAC used in this study was Calgon F-400 (Calgon Corp.). Disinfection by chlorine or chlorine dioxide completed the process. In this chapter, line 3 treatment was not considered a complete treatment for the water supply. This line was studied to evaluate the efficiency of a high ozonation rate. 

GAC-filtered effluent containing 10-12 mg/L of NH3N is routinely disinfected with an average dose of 7 mg/L of chlorine to achieve a typical residual of 3-4 mg/L of chloramine following a 2-h contact time. Effluent turbidity is routinely <2 turbidity units and color is <10 color units GAC is normally regenerated at 6-week intervals to maintain this effluent quality. 

The kinetics of the microbiological kill reaction is more favorable for bromine. Thus bromine tends to kill microbes more quickly, and for any disinfection time period it will achieve a higher magnitude of kill. This can be important for cooling systems with short disinfection contact times, e.g., once-through systems. 

The effectivity of disinfectants are affected by the following factors time of contact between disinfectant and the microorganism and the intensity of the disinfectant, age of the microorganism, nature of the suspending liquid, and temperature. Each of these factors are discussed next. 

Having obtained m and k, the time t can be solved using Equation (17.2) from a knowledge of the value of /. This time is called the contact time for disinfection, and the intensity / is called the lethal dose. From Equation (17.2) any reasonable amount of dose is lethal when administered in a sufficient amount of contact time as calculated from the equation. We call Equation (17.2) the Universal Law of Disinfection. 

Example 17.1 It is desired to design a bromide chloride contact tank to be used to disinfect a secondary-treated sewage discharge. To determine the contact time, an experiment was conducted producing the following results ... 

The effectiveness of a disinfectant also depends upon the age of the microorganism. For example, young bacteria can easily be killed, while old bacteria are resistant. As the bacterium ages, a polysaccharide sheath is developed around the cell wall this contributes to the resistance against disinfectants. For example, when using 2.0 mg/L of applied chlorine dosage, for bacterial cultures of about 10 days old, it takes 30 min of contact time to produce the same reduction as for young cultures of about one day old dosed with one minute of contact time. In the extreme case are the bacterial spores they are, indeed, very resistant and many of the chemical disinfectants normally nsed have little or no effect on them. 

We have learned from previous chapters that equilibrium and reaction constants are affected by temperature. The length of time that a disinfection process proceeds is a function of the constants of the underlying reaction between the microorganism and the disinfectant thus, it must also be a function of temperature. The variation of the contact time to effect a given percentage kill with respect to temperature can therefore be modeled by means of the Van t Hoff equation. This equation was derived for the equilibrium constants in Chapter 11, which is reproduced next ... 

Contact time and chlorine dosage. The two most important parameters used in the design of chlorine contact tanks is the contact time and dosage of chlorine. These parameters have already been discussed the equation is given by the universal law of disinfection. Equation (17.2). Figure 17.12 shows a contact tank used to disinfect treated sewage. 

Example 17.15 A chlorine disinfection study was conducted to determine the constants of Equation (17.2). For a log 2 removal efficiency, the value of m is found to be 0.35 and the value of k is found to be 100. Calculate the contact time if the regulatory agency requires a chlorine dose of 20 mg/L. 

Example 17.16 A total flow of 1000 mVd is to be disinfected. What should be the cross-sectional area of the serpentine channel in order to maintain selfcleaning velocity What would be the total combined length of the channel if the contact time has been calculated to be 35 min ... 

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