Encyclopedia of Environmental Science and Engineering, Volume I and II

228 DISINFECTION

experimental conditions. In addition, they usually involve

preliminary assumptions concerning the nature of the

disinfection process. Consequently, attempts to deduce

mechanisms of killing from these models tend to be rather

disappointing. They are, however, useful in terms of con-

cise descriptions of the results of the process; and as fur-

ther biochemical information on modes of killing becomes

available, their usefulness and reliability should increase

further.

DISINFECTANT TESTING

The testing of disinfectants is a topic with a long history of

controversy. Differing opinions on the merits and relevance

of various methods are legion, and have been the cause of

many heated exchanges between those holding them. The

wide variety of methods may most conveniently be consid-

ered under the headings of screening, standardization and

“in-use” tests, although there is a degree of overlap between

the three categories. The interested reader should consult the

papers by Forsyth (1975), Miner et al. (1975) and Reybrouck

(1982) for further information.

Screening Tests

These are usually of the simplest type, since they are specula-

tive by nature, and often involve the testing of large numbers

of disinfection agents or formulations; both time and cost

usually dictate this simplicity. The actual methods employed

vary according to the physical characteristics of the disinfec-

tant and the type of use envisaged. Ideally, any test should be

as realistic as possible, although for screening purposes this

will be subject to the requirements of simplicity discussed

above.

Disinfectants which are soluble or miscible in water are

often incorporated in microbiological culture media which

are then inoculated with suitable microorganisms. After

incubation under appropriate conditions, the inoculated

media are examined for growth of the organisms, absence

of growth indicating that the inoculum has been inhibited.

Where the continuous presence of the disinfectant is inap-

propriate, then the method is modified to include a suitable

means of removing or inactivating the disinfectant after a

predetermined exposure time.

Other disinfectants may be tested in a similar manner,

by arranging for inocula of suitable organisms to be sub-

jected to standardized exposure to them. After exposure, the

organisms are inoculated into suitable culture media, and

incubated as before. Since the end-point in these tests is the

death of all the organisms involved, this type of test is often

referred to as an Extinction Test.

The phrase “suitable organisms” used above, represents

one of the key factors in the testing of disinfectants. A disin-

fectant is expected to kill all undesirable organisms, which

usually refers to organisms injurious to health (see previous

section). Test organisms may be chosen, therefore, either

for their resistance to disinfection or, like Salmonella typhi,

for their medical significance. Although the designation

“undesirable organisms” covers a wider range of life-forms,

most disinfectant testing has employed various species of bac-

teria for reasons of convenience. In retrospect, this does not

appear to have been a significant disadvantage, since activity

against bacteria usually coincides with that against the other

life-forms. However, although it is recognized that bacterial

spores are much more resistant to disinfection than the veg-

etative forms, the latter have, with few exceptions, invariably

been used in testing. This is partly due to convenience, but it

should be noted that there are many disinfectants marketed

which under normal conditions of use are incapable of killing

bacterial spores. This has been countered, in certain quarters,

by re-defining a disinfectant as a chemical agent capable of

killing bacteria but not necessarily bacterial spores (see pre-

vious section).

Standardization Tests

This is the area which has given rise to the greatest amount

of contention. In most cases, this has been due to misin-

terpretation and misapplication of the results of testing

in situations where they have little, if any, relevance. The

standardization of disinfectants usually requires the use

of microbiological techniques rather than chemical assay.

Even when chemical characterization of the active agent(s)

involved is possible, the activity of a disinfectant will usu-

ally be significantly influenced by factors concerned with

the formulation and method of use of the product. However,

although the disinfectant may be standardized in terms of its

intended biological effect, this is carried out under specific,

controlled conditions. Depending on the similarity of their

circumstances, the results from standardization tests may, or

may not, be capable of extrapolation to practical situations.

This is where the controversy tends to arise.

The most popular methods of testing for standardiza-

tion purposes have been extinction tests. These are basically

similar to the general method discussed in the previous

section, except that the procedure and materials used are

rigidly standardized in order to achieve best reproducibility

of results. In addition, the most widely used tests employ

the pure chemical, phenol, as a control of the resistance

of the test organisms used. Since results are expressed by

comparison of the activities of phenol and the disinfectant

being tested, these tests are referred to as Phenol Coefficient

methods. The most popular and official versions in use are

the Rideal–Walker Method (as modified by B.S. 541), the

Chick–Martin Method (as modified by B.S. 808), and the

Association of Official Agricultural Chemists (AOAC)

Phenol Coefficient Method (1970). Details of these tests

may be found in the appropriate publications.

While the use of phenol as a control on the resistance

of the test organisms is extremely valuable, it has led to

widespread assumptions that the ratio of activities of disin-

fectant and phenol indicated by the phenol coefficient will

hold true in all circumstances. This, of course, is far from

true. The AOAC (1970) attempted to improve this situation

by introducing a Use-Dilution Method. This test is based on

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