Environmental Engineering Reference
In-Depth Information
In addition to the desirable characteristics of a disinfectant listed above, the
disinfectant chosen must be able to kill off or deactivate pathogenic micro-
organisms by one of several possible methods, including: (1) damaging the
cell wall, (2) altering the ability to pass food and waste through the cell mem-
brane, (3) altering the cell protoplasm, (4) inhibiting the cells' conversion of
food to energy, or (5) inhibiting reproduction.
Chlorination
For the past several decades, chlorine dispensed as a solid (calcium hypo-
chlorite), liquid (sodium hypochlorite), or gas (elemental chlorine, Cl
2
) has
been the disinfectant of choice, particularly in the United States. Chlorine
(sometimes referred to as the workhorse of disinfection) has proven its worth
both because of its effectiveness and because it is relatively inexpensive; it
also provides a chlorine residual in the water distribution system, ensuring
that the water remains disease free.
Gaseous chlorine
(Cl
2
), 2.5 times as heavy as air, is a greenish-yellow toxic
gas. One volume of liquid chlorine confined in a container under pressure
yields about 450 volumes of gas. Large water treatment works usually use
chlorine gas, supplied in liquid form, in high-strength, high-pressure steel
cylinders. The liquid immediately vaporizes in the form of gas when released
from these pressurized containers. Chlorine gas is lethal at concentrations as
low as 0.1% air by volume. In nonlethal concentrations, it irritates the eyes,
nasal membranes, and respiratory tract.
Sodium hypochlorite
is most commonly used in smaller systems, because it is
simpler to use and has less extensive safety requirements than gaseous chlo-
rine; in the form used, it is less toxic. Recently, many larger water facilities
that have used chlorine for disinfection are beginning to substitute sodium
hypochlorite for chlorine because of regulatory pressure.
Note:
The Occupational Safety and Health Administration's Process
Safety Management Standard (29 CFR 1910.119) and USEPA's Risk
Management Program (Clean Air Act, Section 112(r)(7)) have come to be
known in the industry as the “chlorine killers,” because of their effect
on industrial processes. The USEPA is attempting to steer industry away
from the use of chlorine. Although the Agency cannot absolutely outlaw
this substance from use, it is following the path of simply regulating it to
death. In an effort to avoid having to comply with strict (in some cases,
unworkable) regulations, many water treatment and wastewater facili-
ties in the United States are substituting some other chemical product
that is not regulated (at least for the moment) such as sodium hypochlo-
rite. Sodium hypochlorite provides 5 to 15% available chlorine (common
laundry bleach is a 5% solution of sodium hypochlorite). Usually diluted
with water before application as a disinfectant, it is very corrosive and
should be handled and stored with care and kept away from equipment
