Encyclopedia of Environmental Science and Engineering, Volume I and II

DISINFECTION 233

The types of chlorine compounds which are frequently

used are (1) Hypochlorites. These are cheap and convenient

to use, and have a wide antibacterial spectrum (Davis, 1963).

They possess potent sporicidal activity (Truman, 1971; Kelsey

et al. , 1974; Waites, 1982) which may be potentiated by alco-

hols (Coates and Death, 1978; Death and Coates, 1979).

The hypochlorites are moderately effective against animal

viruses.

The antibacterial activity of the hypochlorites decreases

with increasing pH (Charlton and Levine, 1937; Weber,

1950; Ito et al. , 1967; Hays et al. , 1967), e.g., whereas 99%

of spores of B. cereus are killed after 2.5 min at pH 6 by a

solution containing 25 ppm available chlorine, nearly 8 hours

are required for a comparable kill at a pH of c. 13 (Rudolph

and Levine, 1941). At constant pH, the time to kill bacteria

depends on the concentrations of available chlorine. The spo-

ricidal activity of sodium hypochlorite may be potentiated by

various compounds, e.g., by the addition of ammonia (Weber

and Levine, 1944) or 1.5–4% sodium hydroxide (Cousins

and Allan, 1967), notwithstanding the earlier comment

about pH. In the presence of bromide, hypochlorite has an

enhanced effect in bleaching cellulosic fibres as compared

with hypochlorite alone, possibly because of a continuous

generation of hypobromite when hypochlorite is in excess.

A potentiation of the bactericidal effect of hypochlorite has

been achieved by the addition of small amounts of bromide

(Farkas–Himsley, 1964).

The antimicrobial activity of hypochlorites is considerably

reduced by organic matter. However, the hypochlorites are

used in the disinfection of water, dairy equipment and eating

utensils. (2) Chloramine-T (sodium p -toluene sulphonchlo-

ramide). Dakin et al. (1916) considered that chloramine-T had

a powerful germicidal action. It is bactericidal and sporicidal,

although the rate of kill is slower than with the hypochlorites.

Its activity is considerably higher at acid than at alkaline pH

(Weber, 1950), and a drop of 10°C in the reaction tempera-

ture results in a 3–4 fold increase in the time necessary to kill

microorganisms (Weber and Levine, 1944). Chloramine-T is

employed as a wound “disinfectant,” and as a general surgi-

cal disinfectant. It is nonirritant and nontoxic, in contrast to

Dichloramine-T (toluene- p -sulphon-dichloramide) which

although a powerful disinfectant is not used because of its

toxicity and instability.

The mode of action of chlorine compounds is unknown,

although several proposals have been made, e.g., the informa-

tion of chloramines as a result of combination of chlorine with

bacterial protoplasm, halogenation or oxidation reactions of

chlorine with bacterial cells, changes in cellular permeability

and an effect on enzyme systems. It has also been found,

however (Bernarde et al. , 1967) that chlorine dioxide causes

a marked and immediate cessation of protein synthesis in

growing cells.

Iodine and Iodophors

Iodine in aqueous or alcoholic solution is considered by

most authors (Gershenfeld and Witlin, 1950; Gershenfeld,

1956; Report, 1965; Sykes, 1970) to be a reliable and

effective germicide which is lethal to vegetative bacteria,

bacterial spores and acid-fast bacilli. Spaulding et al. (1977),

however, consider that alcoholic iodine (0.5% iodine in 70%

alcohol) possesses good activity against non- sporing bacte-

rial and M. tuberculoses but none against bacterial spores,

whereas Rubbo and Gardner (1965) state that bacterial

spores are moderately resistant to iodine. Viruses are consid-

ered by Rubbo and Gardner (1965) to be moderately sensi-

tive to iodine.

Iodine has a high fungicidal or fungistatic activity against

yeasts and various moulds, but its antimicrobial properties

are to a great extent inhibited in the presence of organic

matter, since iodine is a highly reactive element.

Iodine is sparingly soluble in cold water, but more solu-

ble in hot water. Stronger solutions can be made in potassium

iodide solution or in aqueous alcohol. Iodine is more effective

as a germicide at acid than at alkaline pH, but is less affected

by pH than are chlorine compounds. The concentration of

iodine to disinfect does not vary greatly with different types

of microorganisms. Various types of iodine solution are used

for the first-aid treatment of small wounds and abrasions,

and in pre-operative skin “disinfection.” Iodine has also been

employed for the sterilization of surgical catgut (although

this method is now little used) and is nowadays used for the

disinfection of drinking and swimming pool water, the disin-

fection of instruments and of clinical thermometers, and the

sanitization of eating and drinking utensils.

Unfortunately, iodine solutions stain fabrics and tissues

and tend to be toxic. However, certain non-ionic surface-

active agents can solubilize iodine to form compounds, the

iodophors (Blatt and Maloney, 1961; Davis, 1962, 1963,

1968) which retain the germicidal activity of iodine, but

not its undesirable properties; these iodophors are literally

“iodine-carriers.” They are active against bacterial spores,

including pathogenic anaerobic spores (Lawrence et al. ,

1957; Gershenfeld, 1962). It is the concentration of free

iodine in an iodophor which is responsible for its microbial

action; this has been well demonstrated by Allawala and

Riegelman (1953) who made a log-log plot of killing time

against amount of free iodine, and found that the 99% killing

time of B. cereus spores was a function of the concentration

of free-iodine in the presence or absence of added surface-

active agent. The bactericidal properties of the iodophors are

increased at low pH values, but their stability is unaffected

(cf. hypochlorites). They may thus be employed with acids,

e.g., phosphoric acid, to enhance their microbial action and

also to assist in preventing the formation of film or milkstone

(see later) in the dairy industry. The iodophors are consid-

ered (Davis, 1968) to be powerful detergents, although they

do not dissociate protein as readily as do alkalis. The formu-

lation of acidic solutions of iodophors is particularly useful

when calcium or magnesium scale is encountered, but they

can be corrosive, especially with galvanized iron.

Surface-Active Agents

Surface-active agents have 2 regions in their molecular

structure, one a hydrocarbon, water-repellent (hydrophobic)

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