Virginia Law

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.