Environmental Engineering Reference
In-Depth Information
instead be added after filtration. Another current approach is using alterna-
tive disinfection methods. We briefly discuss a few alternative disinfection
methods in the following section.
Note:
Because of OSHA's Process Safety Management (PSM) Standard
and USEPA's Risk Management Program (RMP), many facilities cur-
rently using elemental chlorine have used or are actively pursuing the
use of alternative disinfection methods. We further reemphasize that the
problem of THMs is also helping spur interest in alternatives to chlorina-
tion as the preferred method of disinfection.
Alternative Disinfection Methods
Currently, several alternative disinfection methods are available for use in
treating water; however, in this text (for water treatment) we focus on two
alternatives: ozonation and ultraviolet (UV) radiation. These commonly used
alternatives (especially in small water treatment systems) are also increas-
ingly being substituted for existing chlorination systems at larger plants
because of regulatory pressure.
Note:
Before discussing the ozonation and ultraviolet disinfection alter-
natives, we must point out that neither one of these two alternative disin-
fectants is an easy solution to problems created by chlorination. It is true
that each has the advantages of not creating THMs and not being cov-
ered by the requirements under the PSM standard and RMP, but each has
uncertainties and known disadvantages that have restricted their more
widespread use. In addition, ozonation and ultraviolet irradiation cannot
be used as disinfectants by themselves. Both require secondary disinfec-
tant (usually chlorine) to maintain a residual in the distribution system.
Ozonation
Ozone
(O
3
), a gas at ordinary temperature and pressures, is a very power-
ful disinfectant that breaks up molecules in water; it is even more effective
against some viruses and cysts than chlorine. It has the added advantage of
leaving no taste or odor and is unaffected by pH or the ammonia content
of the water. When ozone reacts with reduced inorganic compounds and
with organic material, an oxygen atom instead of a chloride atom is added to
the organics, the end result being an environmentally acceptable compound.
But, because ozone is unstable and cannot be stored, it must be produced on-
site. Ozonation usually costs more than chlorination.
