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OZONE AS A DISINFECTANT IN WATER PURIFICATION 1
By Joseph W. Ellms 2
Ozone was first observed by the Dutch chemist Van Marum in
1785, while operating a static electrical machine. In 1840, Schoen-
bein while investigating the properties of the gases produced in elec-
trolysing water and electrifying air, noted the odor and oxidizing
properties of the gas and gave it the name of "ozone," because of its
peculiar odor. It was not until the results of Soret's work in 1865
were known, that scientists agreed that ozone was tri-atomic oxygen,
although it had been generally held that it was some form of oxygen.
Ozone is an unstable gas requiring a large amount of energy for
its formation (34,000 calories). It is but slightly soluble in water,
and undergoes decomposition when heated. When strongly ozonized
oxygen is liquefied and the product subjected to fractional distilla-
tion, a mixture of ozone and oxygen results, of which about 85 per
cent is ozone. The factors governing the "ozone-oxygen system"
are none too well understood. The state of equilibrium which exists
in a mixture of the two gases is dependent upon several factors,
such as temperature, pressure, and electrical conditions that are
extremely complex.
Ozone is an extremely energetic oxidizing agent. It attacks
many inorganic oxidizable substances readily, and is particularly
destructive of organic matter even at low temperatures. Its value
as a bactericidal agent is probably due to this property, and is the
reason for its use in the disinfection of drinking water.
For practical purposes, the production of ozone can best be effected
by certain forms of electrical discharges through oxygen gas or
through air, which, of course, consists in part of oxygen. It may be
well to describe the nature of these discharges so that the techni-
cal difficulties of the practical production of ozone will be better
appreciated.
1 Read before the Central States Section. Discussions of this paper are
requested, and should be sent to the Editor.
» Consulting Sanitary Engineer, Cleveland, Ohio.
60
OZONE AS A DISINFECTANT 61
The phenomena of an electrical discharge through a gas are com-
plicated. Ordinarily gases are non-conductors, but may become
conductors under certain physical and electrical conditions brought
about by the flow of the current through them. When two elec-
trodes are separated by a gas and connected to some source of
high tension electricity, and between them the potential difference
is being gradually increased, there is first produced an invisible
electrical discharge which gradually becomes visible by a glow upon
one of the electrodes. As the potential difference increases the
corona effect is produced, which changes to the true brush discharge.
It is this latter form of discharge that is regarded as the most effec-
tive in the production of ozone. By still further increasing the po-
tential difference between the electrodes there is formed in succes-
sion the spark discharge, the flame and finally the well known electric*
arc.
The brush discharge is of a dark blue violet color and is accom-
panied by a peculiar hissing sound and the "electric wind," which
latter, according to J. J. Thomson, is due to a current of electrified
ions that set the air in the vicinity of the discharge in motion.
Brush discharges are readily formed on electrodes having sharp
points or roughened edges. Alternating or direct current may be
used, but usually the alternating current is preferable since it may
be obtained more readily under high tension. A better production
of ozone appears to be obtained with alternating currents of high
frequency. A 500 cycle alternating current is more commonly
used in ozone installations than one with lower frequencies.
It has been found, as a result of much experimentation, that the
use of a dielectric between the electrodes increases the yield of
ozone. In other words, imposing even greater resistance than
that offered by the air itself, effects a larger production of ozone
by the discharge. Dielectrics are made from various substances,
such as glass, mica, fused quartz and baekelite. While many other
substances will act as dielectrics, the above named include the
practical materials available in ozone work.
It will probable be appreciated from the little that has been cited
of the complex character of the phenomena of ozone production that
the pressure, temperature and humidity of the air being electrified,
the form, size, spacing and material of the electrodes, the kind of
current employed, the frequency in the case of an alternating cur-
rent, the voltage, amperage and other secondary influences of an
62 JOSEPH W. ELLMS
electrical nature in general, and the kind and arrangement of the
dielectrics employed, are factors that must all be given careful
consideration in an efficient ozone apparatus. A brief descrip-
tion of one or two of the ozonizers that have been developed and
have been used may be of interest.
There are two general types of ozone apparatus. The Siemens-
Halske ozonizers are of the vacuum tube type, in which air is drawn
through an annular space across which high-tension electrical dis-
charges occur. An inside metallic cylinder acts as one electrode.
It is covered by a slightly larger tube of glass covered with tin foil.
The electrodes are water cooled. The General Electric and the
Gerard apparatus are of the tube type also. The Small-Linder,
Abraham-Marmier and Vosmaer apparatus use plate forms of
electrodes and dielectrics. In some cases their electrodes are hollow
and water cooled, but in others this cooling is not attempted. Volt-
ages varying from 2000 or 3000 to 50,000 or 60,000 have been used
in various ozonizers, but from 10,000 to 20,000 volts are more com-
monly employed. The yield of ozone increases with an increase in
wattage for any given area of electrodes, that is, with the density
of the current per unit of area.
The application of the ozonized air to water, where the ozone is to
act as a disinfecting agent, has been given considerable attention by
investigators, but much more study of the problem is needed. In
some cases the flowing water is used to suck the ozonized air through
the ozone generator and into the water to be treated. This method
is not susceptible of very close control. Another method consists in
pumping the ozonized air into the bottom of towers down through
which the water descends. A modification of this latter method
consists in placing the air compressor back of the ozonizer, thereby
avoiding handling the corrosive gas in the compressor. In this
case the ozonizer must be in a container that will withstand the air
pressure required to overcome the hydrostatic head of the water
columns and the friction head resulting from the flow of the air
through pipes and towers. A third but expensive method has been
used in which the water has been sprayed into the atmosphere of
ozonized air. Baffled towers where the counter-current system is
used have not proven very successful. A great deal is yet to be
learned regarding the proper method of distributing the ozonized
air at the bottom of the column of water, so that a maximum absorp-
tion of ozone may be effected.
OZONE AS A DISINFECTANT 63
Since the expenditure of so much electrical energy in ozonizing
the air creates more or less heat, and in consequence subsequent de-
composition of the ozone formed, cooling and drying the air by
refrigeration have been usually resorted to.
In small plants, passing the air over chemicals that would absorb
the moisture, has also been successfully utilized. The yield of ozone
apparatus naturally varies greatly, depending upon the manner in
which the air is handled both before, during and after the passage
of the gas through the ozonizer. Russell Spaulding in a report to
the New York State Department of Health in 1913, very concisely
states the desirable features of a good ozonizing apparatus. He
estimates the theoretical yield of ozone for an expenditure of 1
kilowatt of electrical energy to be 1386 grams. Since the actual
yields of apparatus will vary all the way from 10 to 60 grams per
kilowatt, it is evident that their efficiencies are very low. Mr.
Spaulding summarizes his conclusions as follows:
To summarize, then, the main desiderata in generating ozone are :
1. A supply of alternating electric current at low cost.
2. An efficient transformer to obtain high tension.
3. Ozone electrodes that do not generate heat to
a. Disrupt the dielectrics.
b. Cause reversion of ozone to oxygen.
c. Require external means for cooling.
4. Ozone electrodes that will approach the theoretical efficiency much
more closely than the various systems now in use.
To this the author would add, that unless the ozonized air is
effectively applied to the water to be disinfected, that is, unless prac-
tically 100 per cent absorption of the ozone by the water is effected,
the over-all efficiency of the entire apparatus may still be far from
satisfactory. This phase of the subject still warrants considerable
investigation.
The ability of ozone to reduce the bacterial content of a water
has been too frequently demonstrated to doubt its inherent dis-
infecting properties. Nevertheless, it has its limitations in this
direction like other disinfectants. In waters containing too large
an amount of organic matter some of the bacteria may escape being
killed. Pathogenic organisms probably are more readily killed
than the ordinary water forms, because of the unfavorable conditions
imposed upon them in the water, as well as from the destructive
effect of the disinfectant. Spore-forming bacteria may also escape
64 JOSEPH W. ELLMS
destruction. Ozone, because of its strong oxidizing powers, has
the merit of being able to oxidize and remove tastes and odors due
to organic matter in suspension or solution, and to reduce the color
due to vegetable stain. If ferrous iron exists in a water, it is able to
oxidize the iron to the ferric condition, in which form it is practi-
cally insoluble, and hence may be removed by sedimentation and
filtration.
In conclusion, it may be well to point out that ozone used as a
disinfecting agent is no more a cure-all for a polluted water supply
than are other disinfecting agents that are at present more widely
employed. As a supplement to filtration processes ozone can be
used with good effect, and thereby render the water safer for drink-
ing purposes. That the process needs investigation and scientific
development in order to make it economical and efficient, cannot be
denied by its most ardent advocates. When such a development is
brought about, its use in the purification of water will become more
general and its points of real merit better appreciated.