Encyclopedia of Environmental Science and Engineering, Volume I and II

DISINFECTION 225

that pathogens likely to be associated with such equipment

and with eating and drinking vessels will be killed. Davis

(1968) has employed the name “detergent-sterilant” through-

out his review, although, as pointed out earlier, his definition

of a sterilant differs from that above. Throughout the present

chapter, “sanitizer,” without quotes, will be employed rather

than these other terms.

KINETICS

It is tempting to imagine that the application of an appropriate

disinfection procedure will result in immediate elimination of

all microorganisms from the site of interest. This temptation

is often fostered by various advertising interests in pursuance

of their sales campaigns. However, a cursory inspection of

the literature soon dispels this cosy, over-simplified view.

Disinfection has been shown repeatedly to be not only a

gradual or even prolonged, process, but also a complex one.

Almost invariably, investigation into the course of disin-

fection processes have involved the study of purified cultures

of microorganisms (usually bacteria) under specified condi-

tions. This has led to certain criticisms that such systems are

too far removed from reality to be of practical significance.

While it is true that considerable caution must be exercised

in applying the results of these studies to practical situations,

the experimental systems are still far from simple, and have

yielded much useful information.

Survivor Curves

The most common method of monitoring the progress of

a disinfection process is by means of viable counting tech-

niques. These suffer from certain inherent limitations. In

particular, the absolute values obtained are dependent on the

specific technique and the experimental conditions associated

with it; and in addition, cells which have been exposed to the

process, may respond quite differently from those examined

prior to exposure. In order to obviate this difficulty, alter-

native methods of assessing “vital activity” have been sug-

gested, usually biochemical in nature. Unfortunately, while

the greater simplicity of these methods allows more precise

measurements to be made, the killing of microorganisms

usually involves a whole series of complex reactions, which

makes correlation of the results rather difficult. Despite their

faults, viable counting methods do reflect the complexity of

the killing process.

The usual scheme of events is to expose the chosen cul-

ture of microorganisms to the disinfection process of inter-

est, under controlled experimental conditions. Estimates of

the viable population density of the system are made by per-

forming viable counts on representative samples removed

from the system: see, for example, Prince et al. (1975). For

convenience, these estimates are usually plotted graphically

against time of exposure or occasionally dosage of the dis-

infection agent employed. While the estimated numbers of

organisms may be plotted directly, they are usually converted

to a proportional basis such as “surviving fraction” or “per-

centage survivors,” since this facilitates visual comparison

of the results.

The simplest graph so obtained is the arithmetic plot

which invariably exhibits a curve of similar general form to

Figure 1. The main point of interest about this curve is that

it indicates that the rate of disinfection varies inversely with

the number of surviving organisms. This is interpreted as

an indication that the individual cells of the culture exhibit

differing sensitives to the process, i.e., there is a distribution

of resistances. Unfortunately, curves of this type are difficult

to analyze or to compare visually, and so the survivors are

often plotted in a logarithmic fashion. This results in a whole

“family” of possible results, as shown in Figure 2.

Figure 2(a) shows the simplest result, the familiar

straight line which is often prized for its ease of charac-

terization. It also possesses the sometimes dubious advan-

tage of ease of extrapolation; a property which should be

utilized only with extreme caution. This graph indicates

that the rate of disinfection is inversely proportional to

the logarithm of the number of surviving organisms. The

similarity between this situation and the kinetics of a first-

order chemical reaction has caused this type of response

to be described as unimolecular or monomolecular. It is

important to stress, however, that the description applies

to the graphical response of the system; for it would be

extremely naive to assume from this that the mode of death

of the cells is attributable to a first-order chemical reac-

tion. The straight line may be described mathematically

by the equations:—

k t

N

N

(^10)
log
⎛
⎝
⎜
⎞
⎠
⎟
where
k
rate constant or slope of line
t
time elapsed
N 0
number of viable cells initially
N
number of viable cells at time t
time
Surviving Fraction
10
FIGURE 1
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