Current through Register Vol. 41, No. 3, September 23, 2024
A. Disinfection can
be achieved through microorganism exposure to a sufficient level of Ozone (O3)
in solution for a proper contact period. Ozone is an unstable gas that is
produced when oxygen molecules are dissociated into atomic oxygen which
subsequently collides with other oxygen molecules.
B. Parameters. The following parameters are
important factors in the design of ozonation disinfection:
1. The applied ozone dosage is the mass of
ozone from the generator that is directed to a unit volume of the wastewater to
be disinfected.
2. The transferred
ozone dosage is the mass of applied ozone that is dissolved into the
wastewater. This dosage depends on the physical characteristics of the
contractor and the residual ozone concentration, which is affected by the
quality of the wastewater.
3. The
dew point is the measure of the relative moisture content of a gas,
specifically the temperature at which a gas under a precise pressure is
saturated with water.
4. Off-gas is
the excess ozone transferred from the contact basin to the surrounding
atmosphere.
5. Ozone destruction
involves the changing of ozone to a less reactive molecule. This occurs
naturally because of ozone's inherent instability. However, deactivation by
thermal or catalytic destruction units is usually necessary to reduce excess
ozone in the off-gas to acceptable levels for human health.
6. Dose/response curve is a mathematical
relationship between coliform destruction and transferred ozone dosage. A
threshold level of dosage may exist that indicates no response until the dosage
exceeds that threshold.
C. Design. This process can be considered for
disinfection of filtered secondary effluents. Documentation of process
effectiveness must be provided for ozone disinfection of secondary effluents
that are not filtered. The transferred ozone dosage shall exceed the threshold
level as necessary for adequate disinfection. The presence of reducing
compounds such as nitrates shall be addressed in the unit operation design.
1. The contact basin design shall ensure
uniform mixing of ozone with the wastewater as well as flow retention equal to
or exceeding the design contact period. Ozone addition shall be staged to
provide a uniform ozone concentration throughout the entire volume of the
contact basin. Multiple staged contactors that are positively isolated from
each other are recommended. The design shall provide continuous disinfection
while contact basins are dewatered for cleaning and shall include provisions
for foam control, including adequate collection space and a removal mechanism.
In addition, the design (flow path width to length ratio of 20 or more) shall
minimize short-circuiting and optimize the contact period through the provision
of baffles or other approved methods. A minimum contact period of 10 minutes
shall be provided at average daily flow.
2. Ozone recycling and destruction shall be
considered.
a. Moisture and foam removal
should be considered in the design of catalyst type destruction
units.
b. The use of activated
carbon for destruction is not recommended.
c. A pressure/vacuum relief valve is required
between the destruction unit and the contact basin to protect the contact basin
from excessive pressure or vacuum.
3. Generation and feeding equipment shall be
capable of providing disinfection, as specified by the issued certificate or
permit, under variable operating conditions such as peak flows and ozone
demand.
D. Ozone supply.
Ozone production shall be sufficient to disinfect to achieve effluent
disinfection requirements at the maximum daily wastewater flow. The applied
ozone dose shall produce the design transferred ozone dosage at the calculated
transfer efficiency. Pilot scale tests or development of a dose/response curve
from the current literature shall be provided to establish the design
transferred ozone dose.
1. The ozone generator
should produce the design ozone concentration while operating at 75% or less
maximum power to reduce stress on generator dielectrics and decrease
maintenance problems. Likewise, high voltages and frequencies should be
avoided.
2. The ozone generator
design shall provide for cooling. Watercooled systems are recommended. The
effectiveness of air cooled systems shall be verified.
3. The feed gas shall be oil-free,
particle-free and dry. Pure oxygen normally has these characteristics. If air
feed is used, the following shall be required:
a. The feed gas shall be filtered or
electrostatically precipitated so that it does not contain particles greater
than 0.4 microns in diameter.
b.
The feed gas moisture content shall not be greater than 0.011 grams per cubic
meter (dew point temperature of -60°C at standard pressure).
c. Desiccant type dryers shall have a design
cycle time of 12 hours or more under maximum moisture conditions.
d. Feed gas dryers shall have a source of
purge flow that is monitored and controlled.
4. Standby ozonation capability shall be
provided which will ensure adequate disinfection with any unit out of operation
for maintenance or repairs. An adequate inventory of parts subject to wear and
breakage shall be maintained at all times.
E. Features. Measurement equipment and alarms
shall be provided to ensure proper operation of all system units and continuous
disinfection to permit limits under expected operating conditions. Monitoring
should be provided for the parameters listed below:
1. Inlet temperature, pressure, flow rate,
and moisture concentration of generator feed gas.
2. Outlet temperature, pressure, flow rate,
and ozone concentration of generator discharge gas.
3. Frequency, voltage, wattage, and amperage
of generator power supply.
4. Inlet
flow, and inlet and outlet temperature of generator cooling water.
5. Ozone concentration in contact basin
off-gas.
6. Inlet temperature and
flow, and outlet ozone concentration of destructor gas.
Materials shall be suitable for use with ozone. Piping
systems should be as simple as possible, and specifically selected and
manufactured to be suitable for ozone service with a minimum number of joints.
Piping should be well supported and protected against temperature
extremes.
Requirements for housing shall be the same as for
chlorination. Floor space shall be sufficient to provide access for equipment
maintenance and to allow adequate equipment ventilation.
F. Safety. Safety requirements
shall be the same as for chlorination. Employee exposure to ozone in the
working environment is limited by VOSH requirements and such exposure should
not exceed the permissible exposure level in VOSH regulation. Monitoring and
purging shall be provided to prevent development of an explosive atmosphere in
the contact basins and other susceptible areas in accordance with federal and
state standards.
G. Monitoring.
Monitoring requirements shall be the same as for chlorination.
1. Off-gas ozone monitoring is recommended
for use in a control loop. Residual ozone monitoring is not recommended unless
its reliability can be documented.
2. Monitoring of the final effluent for a
suitable pathogenic bacterial indicator organism, such as fecal coliform, shall
be required for a period of at least one year to ensure disinfection
effectiveness.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
