Virginia Law

Article 7. Effluent Polishing and Disinfection Processes

9VAC25-790-740. Disinfection.

Article 7
Effluent Polishing and Disinfection Processes

A. Disinfection processes are designed to inactivate actual or potential pathogenic microorganisms present in treated sewage effluents. Disinfection of treated sewage effluents shall be provided to prevent the occurrence of public health hazards in either receiving streams, land treatment sites, or reuse applications from wastewater effluents. Disinfection shall be accomplished in a manner that meets standards for indicator microorganisms but does not result in a violation of toxicity standards.

B. Policy. The need for disinfection of a sewage treatment works effluent is primarily based on standards for either the receiving waters and the land application site or public exposure to reuse as determined by the following requirements:

1. Discharges located within 15 miles upstream or one tidal cycle downstream of a water supply intake shall be disinfected at all times.

2. When sewage discharges are permitted to or within five miles upstream of shellfish waters, they shall be disinfected at all times.

3. Discharges located in all other waters shall be disinfected at all times unless it can be demonstrated, through the use of a Site Specific Beneficial Use-Attainability Analysis of the recreational and other beneficial seasonal uses of the receiving stream, that disinfection is not needed throughout the year, or on a seasonal basis, to protect those uses.

4. Discharges for land treatment or reuse purposes shall be disinfected as necessary to protect the public health and welfare. The public shall not be directly exposed to treated effluent.

C. Toxicity reduction. The need for reducing the effect of toxicity from wastewater effluents is based on the characteristics of the discharge and receiving stream and is established at the time of the permit or certificate issuance. Where the need is established, dechlorination or alternate disinfection methods shall be provided.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-800 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-740, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-750. Chlorination.

A. Disinfection can be accomplished through the controlled application of chlorine compounds to treated sewage to accomplish a sufficient dose, or contact exposure level, over a sufficient period of time, to achieve compliance with the indicator microorganism standard.

B. Chemical. Conventional types of chlorine compounds (chemical) include:

1. Chlorine gas is a greenish-yellow gas with a density greater than the density of air at room temperature and pressure. When compressed to pressures greater than its vapor pressure, chlorine gas condenses into a clear amber liquid.

2. Dry chlorine, liquid or gaseous, contains no more than 150 ppm of water (by weight). Unless otherwise indicated, the word "chlorine" wherever used in this section refers to dry chlorine.

3. A chlorine solution is a mixture of chlorine and water.

4. A hypochlorite solution is a mixture of either sodium or calcium hypochlorite and water.

5. A hypochlorite tablet is a solid formulation of a hypochlorite compound designed to dissolve in a liquid at a controlled rate.

C. Design. Chlorination feed equipment capacity shall be based on the degree of treatment, flow variations, and other uses in the treatment processes. For disinfection, the capacity shall be adequate to produce the residual required in the certificate or permit issued, in the effluent, after the required contact period. Conventional chlorination should be designed to provide a Total Residual Chlorine (TRC) level of up to 1.5 mg/l following a design contact period of 30 minutes or more. Chlorination should be considered for the control of odors and sludge bulking.

1. For normal domestic sewage the dosing capacities listed in Table 7 are recommended:

2. Standby chlorination capabilities shall be provided that will ensure adequate disinfection with any essential equipment of the unit operation out of service for maintenance or repairs. An adequate inventory of parts subject to wear and breakage shall be maintained at all times. An automatic changeover system shall be provided for either (i) treatment works with a design flow of 1.0 mgd or greater or (ii) discharges to critical waters, unless the treatment works are manned 24 hours per day. Where several cylinders are needed to feed sufficient chlorine, separate connections shall be provided for the duplicate gas supplies.

3. A sufficient supply of water shall be available for operating the chlorinators. Where a booster pump is required, duplicate pumping equipment shall be provided, except for discharges to critical waters, in which case duplicate pumps shall be installed. Where an onsite well is used for operating the chlorinators, an adequate back up shall be provided to ensure continuous disinfection. When connection is made from domestic water supplies, equipment for backflow prevention shall be installed. Pressure gauges shall be provided on chlorinator water supply lines.

4. Equipment for measuring the amount of chlorine used shall be provided. Where chlorine gas cylinders are used, scales shall be provided for weighing the cylinders. Scales should be manufactured with a material that is resistant to corrosion by chlorine. Adequate means for supporting the cylinders on the scales shall be provided. At large treatment works, multiple scales of the indicating and recording type are recommended. The recessing of scales is recommended to aid in changing of cylinders if hoists are not provided. Where manifolding of several cylinders will be required to feed sufficient chlorine, consideration shall be given to the installation of evaporators.

D. Dose control. The introduction of chlorine compounds (chemical) at a controlled feed rate is a critical area of disinfection system design.

1. Manual control is the simplest strategy for controlling the chemical feed rate. Generally the feed rate will be constant with minor adjustments made by the operator. This method is normally utilized at smaller treatment facilities.

2. Flow proportioning control in which the chemical feed rate is paced in proportion to the effluent flow rate by appropriate equipment is typically used at treatment works receiving more than 0.1 mgd influent flow.

3. Residual control may be used where the pacing of the chemical feed rate is based on residual analysis of a chemical compound or oxidation-reduction potential in the sample stream.

4. Compound loop control involves a system with interlocking controls that combines the regulation of chemical feed by flow proportioning with subsequent adjustment of the flow proportion dosage in reference to the chemical compound residual. This system is used at treatment works receiving more than 1.0 mgd of influent flow.

5. Solution-feed vacuum-type chlorinators are generally preferred for gas chlorination. Positive displacement type feeders are preferred for hypochlorite solution. Tablet chlorinators may be considered on a case-by-case basis for design flows up to 50,000 gpd.

6. The control system requirements for chlorine feed shall be in accordance with Table 8 as follows:

Notes:

⁽¹⁾Manual, or residual control, may be allowed for flows up to five mgd if equalization of flow prior to disinfection is provided, or allowed for unequalized flows up to one mgd when the discharge is not to critical waters. Flow proportioning control may be allowed for discharges up to five mgd to other than critical waters.

⁽²⁾Required for discharges to critical waters and when dechlorination is necessary to meet effluent requirements for maximum chlorine residuals (TRC) of 0.5 mg/l or less.

E. Dose application. The applied chlorine compound shall be uniformly mixed with the influent to the contact basin. The flow shall be retained within the contact basin for the time period necessary to achieve the design dose.

1. Provisions for mixing shall be made to ensure uniform mixing of the chlorine solution or chemical with the wastewater flow near the point of application prior to and without interfering with the design contact period. This may be accomplished by either the use of turbulent flow regime or a mechanical mixer. A mean velocity gradient (G) value of 500 to 1,000 per second (Sec^-1) is recommended. The engineer shall provide calculations to justify adequate mixing.

2. A minimum contact period of 30 minutes at average daily flow or 20 minutes at maximum daily flow shall be provided within basins or channels immediately following the application of chlorine. A minimum contact period of 60 minutes at average daily flow or 30 minutes at the maximum daily flow shall be required for treatment works that are not continuously manned and that discharge to shellfish waters as defined in the state Water Quality Standards (9VAC25-260). The contact period shall be based on whichever criterion is more stringent.

3. A chlorine contact tank is a basin specifically designed to retain chlorinated effluent for the design contact periods following the application of chlorine. Continuous disinfection shall be provided. The design shall provide continuous chlorination while the chlorine contact tanks are dewatered for cleaning. Multiple basins will be required when mechanical sludge collection equipment is utilized in the contact tanks. For all treatment works with a design flow of 40,000 gpd or greater, multiple tanks shall be provided unless other provisions are made to prevent discharge of nondisinfected effluent. The contact tanks shall be designed to provide plug flow type hydraulics, with baffling provided to achieve a flow path length to flow path width ratio of at least 20 to 1 and a basin depth to basin width ratio of approximately 1.0.

F. Features. Disinfection piping systems shall be well supported, adequately sloped to allow drainage, and protected from mechanical damage. Suitable allowance shall be provided for pipe expansion due to changes in temperature. It is recommended that joints in chlorine piping be flanged or welded.

1. Piping materials shall be suitable for use with chlorine gas or solution, in conformance with the latest standards of the Chlorine Institute.

2. Where adequate superheat is not provided by an evaporator, condensation should be prevented by reducing the pressure with a pressure reducing valve.

3. Where odor control is accomplished by prechlorination, solution piping shall be arranged such that the necessary chlorine application can be accomplished.

4. Any building that houses chlorine equipment or containers shall be designed and constructed to protect all elements of the chlorine system from fire hazards in accordance with applicable codes and regulations. If flammable materials are stored or processed in the same building with chlorination equipment other than that utilizing hypochlorite solutions, a fire wall shall be erected to separate the two areas. If gas chlorination equipment and chlorine cylinders are to be in a building used for other purposes, a gas-tight partition shall separate this room from any other portion of the building. Doors to this room shall open only to the outside of the building and shall be equipped with panic hardware. Such rooms shall be at ground level and should permit easy access to all equipment. The storage area should be separated from the feed area. At least two means of exit should be provided from each separate room or building in which chlorine, other than hypochlorite, is stored, handled, or used. All exit doors shall open outward or roll-upward. A clear-glass, gas-tight window should be installed in an exterior door or interior wall of the chlorinator room to permit the chlorinator to be viewed without entering the room.

5. Chlorinator rooms shall be provided with a means of heating so that a temperature of at least 15°C (60°F) can be maintained. The room shall also be protected from excess heat. Forced, mechanical ventilation that will provide one complete air change per minute shall be installed in all chlorine feed rooms and rooms where chlorine cylinders are stored. The entrance to the air exhaust duct from the room shall be near the floor and the point of discharge shall be so located as not to contaminate the air inlet to any building or inhabited areas. The air inlet shall be so located as to provide cross ventilation with air at such a temperature that will not adversely affect the chlorination equipment. The vent hose shall run without traps from the chlorinator and shall discharge to the outside atmosphere above grade.

6. The controls for the fans and lights shall be such that they can automatically operate when the door is opened if a remote disconnect or override switch is provided in an identifiable, safe, remote location and they can also be manually operated from the outside without opening the door.

G. Safety. Respiratory protection procedures and equipment in compliance with VOSH and other applicable standards (National Institute for Occupational Safety and Health (NIOSH)/Mine Safety and Health Administration (MSHA)) should be available where chlorine gas is handled, and should be stored at a convenient location, but not inside any room where chlorine is used or stored. For treatment works designed for one mgd or greater, it is recommended that at least two complete sets be provided.

1. Instructions for using the equipment shall be posted. The use of compressed air or oxygen, with at least a 30-minute capacity, as compatible with such units used by fire departments (responsible for the treatment works) is recommended in accordance with applicable local, state, and federal standards.

2. A bottle of approximately 50% ammonium hydroxide solution shall be available for detecting chlorine leaks. Where 150-pound cylinders, ton containers, or tank cars are used, a proper leak repair kit (as the type approved by the Chlorine Institute) shall be provided.

3. Consideration should be given to the provision of chlorine gas containment scrubber system with caustic soda solution reaction tanks for absorbing the contents of leaking ton containers where such containers are in use.

4. For treatment works designed for a one mgd or greater average influent flow, automatic gas detection and related alarm equipment should be provided in accordance with VOSH and other applicable requirements.

H. Monitoring. Facilities shall be included for collecting a sample following the contact period to determine the effectiveness of the disinfection method.

1. Equipment shall be provided for measuring chlorine residual in accordance with EPA approved methods.

2. For discharges to critical waters, equipment or services, or both, shall be provided for monitoring the level of indicator microorganisms for pathogenic organisms, in accordance with EPA approved methods, in order to verify the disinfection efficiency.

3. Requests to establish a chlorine (TRC) reduction program, for maintaining a TRC below 1.0 mg/l in the chlorine contact effluent, shall be evaluated based on submission of at least one year of adequate monitoring results comparing TRC values and corresponding indicator microorganism results.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-810 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-750, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-760. Bromochlorination.

A. Disinfection by bromochlorination is accomplished with bromine chloride (BrCl) in a manner similar to chlorine disinfection. Bromine chloride is an equilibrium mixture of bromine and chlorine in both the gas and liquid states. The chemical is highly soluble in water and hydrolyzes to hypobromous (HOBr) and hydrochloric (HCL) acids. Due to the rapid decay of bromine chloride in wastewater, generally there will not be any measurable bromine chloride residuals in the final sewage effluents. Pure bromine chloride is a heavy, fuming, dark, red liquid which is about 20% disassociated into molecular bromine and chlorine.

B. Design. This disinfection process can be considered for treated effluents with BOD₅ and suspended solids concentrations of 30 mg/l or less. Prior documentation should be furnished which shows that adequate disinfection of a specific sewage effluent can be obtained with this process.

C. Dose control. Bromochlorination feed equipment capacity shall be based on degree of treatment, flow variations, and other uses in the treatment processes. For disinfection, the capacity shall be adequate to produce the control point residual required in the permit or certificate issued. The dosing capacity of this process for normal domestic sewage should usually be 80% of that recommended in the chlorine dosage Table 7.

1. Bromochlorination equipment and spare parts are essentially the same as similar requirements for chlorination.

2. Gas feeder systems may be used for feed rates less than 500 pounds per day. Direct liquid bromine chloride feed systems should be used for feed rates greater than 500 pounds per day.

D. Features. Where adequate heat is not provided by the vaporizer to prevent condensation, the use of auxiliary heating and insulation shall be provided as necessary.

1. Materials for piping and appurtenances shall be suitable for handling gas, pure liquid or solutions of bromine chloride as appropriate.

2. The required housing shall be the same as for chlorination, as per VOSH requirements.

3. An evaporator shall be provided for all gas feed systems. The equipment should be designed to minimize the time out of service for maintenance. A backup system shall be provided to ensure adequate disinfection for all discharges when the vaporizer is out of service for maintenance. The vaporizer system should provide superheated gas to the inlet of the vacuum-operated bromine chloride feeder.

E. Safety. The requirement for safety shall be the same as for chlorination and should be in accordance with VOSH requirements. A physical barrier shall be provided for the separation of storage areas if bromine chloride and chlorine chemical supply containers and gas cylinders are located in the same room.

F. Monitoring. Facilities shall be included for collecting samples for bromine chloride residual determinations at the five minute contact time control point and for pathogenic bacterial indicator organism determinations following the total contact period. There should be no readily detectable bromine residual within the final effluent.

1. As bromochlorination equipment represents new technology and limited performance data is available for these systems, an initial period of increased sampling frequency and testing requirements for pathogenic bacterial indicators, such as fecal coliform, may be required. The required initial testing program should take place over a period of one year or more under reasonable operating conditions with a minimum sampling frequency of at least once per week.

2. Disinfection of secondary or better quality effluent should consistently maintain a fecal coliform level below 200 per 100 milliliters of sample volume, or the allowable level contained in the certificate or permit issued, whichever is more restrictive.

3. Indicator organism test results should be correlated with other measurements at the time of sampling, including flow rate, effluent suspended solids, bromine dose rate, and residual measurements.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-820 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-760, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-770. Ultraviolet light irradiation (UV).

A. Disinfection can be achieved through exposure of microorganisms to a sufficient level of UV at the germicidal wavelength for an adequate period of time.

B. Design parameters. The following parameters are important to UV disinfection design:

1. The absorbance coefficient is a measure of the UV absorbing characteristics of the irradiated fluid as measured by a single beam spectrophotometer at 253.7 nanometers, using both filtered and unfiltered fluid samples. The units of this parameter are absorption units per unit distance from the UV source.

2. The contact period is the period of time that a microorganism is exposed to a given intensity and is a function of the residence time distribution (RTD) of flow moving past an arrangement of UV lamps which can be determined from tracer tests.

3. The UV dose is a function of the product resulting from multiplying the average UV intensity, by the contact period (T) and is expressed as (microwatts)(seconds)/square centimeter (UW/SQ.CM/SEC).

4. The dose response is a measure of the inhibition of cell replication, and is indicated by the ratio of the monitored log counts of an indicator organism prior to and following exposure to a given UV dose.

5. The dispersion coefficient (E) is a measure of turbulent mixing (square centimeters per second) within the fluid passing through an arrangement of UV lamps. The value of E established by the RTD variance should be correlated with the contact time necessary to provide the required dose response.

6. The intensity is an expression of the rate (units of microwatts per square centimeter) at which energy is delivered from the source into the surrounding liquid. UV intensity will dissipate by dilution and will be absorbed by the medium as the distance from the source increases. The UV intensity provided for disinfection purposes should be approximated on the basis of the physical properties of the UV lamps, the physical arrangement of lamps within a flowing liquid stream, and the properties of the wastewater effluent (Kab).

7. Lamp assemblies are defined as the arrangement or grouping of UV lamps occupying the cross-section of a channel or reactor.

8. Photoreactivation is a process whereby certain organisms regain the ability to reproduce upon exposure to secondary light.

C. Design dose. This disinfection process shall only be considered as conventional when designed to treat effluent with BOD₅ and suspended solids concentrations of no more than 30 mg/l and that consistently maintains a filtered KAB(Base e) of no more than 0.4/centimeter. The minimum average design intensity and dosage provided by each lamp assembly shall be specified. Conventionally designed lamp assemblies shall not receive a maximum flow in excess of three mgd unless sufficient operating data is submitted to verify disinfection performance for similar wastewater flows in excess of three mgd.

1. Conventional UV process design shall provide a minimum average dose of 50,000 microwatt-seconds per square centimeter after the UV lamps have been in operation for 7,500 hours or more unless sufficient information is provided to demonstrate that the required level of disinfection can be obtained at a lower dose level.

2. UV designs based on dose-response models shall be verified by acceptable bioassay test results, and the expected influent level of indicator microorganisms shall be determined to verify the design.

3. Photoreactivation effects should be accounted for by the UV design.

D. Features. The current configurations acceptable for UV disinfection equipment include contact systems with submerged UV lamps enclosed in quartz tubes and noncontact systems with UV lamps situated adjacent to the flow surface or adjacent to teflon-lined tubular channels carrying treated effluent. Conventional UV disinfection system design shall include, as a minimum, two separate lamp assemblies with each assembly capable of providing the level of disinfection necessary to meet the disinfection standard at average daily flow. If no more than two lamp assemblies are provided for treatment works discharging to critical waters, then each assembly shall be capable of disinfecting the maximum daily flow. Upstream screens should be provided for unfiltered effluent to prevent breakage of quartz tubes by debris. In addition, these systems should be protected against "shock" hydraulic loads from pump station flows.

1. As quartz effectively passes the germicidal portion of light emitted by UV lamps, a quartz tube should be used to enclose UV lamps that are submerged in the treated effluent. The quartz tubes shall be watertight and not subject to breakage under normal usage. As teflon also passes the germicidal portion of light emitted by UV lamps, teflon lined channels may also be used to separate UV lamps from direct contact with treated effluent. Lamp alignment should provide for maximum contact periods and for reduced opportunity for blockage by debris around the submerged lamps. The downstream fluid head should maintain full flow within teflon lined channels. The strength needed to prevent channel deformation in relation to wall thickness should be established by the designer for these channels. The teflon tubes should normally be supported to prevent sagging during operation. Provisions should be made for air bleeding of this system by the operator when necessary.

2. Lamp spacing in channels or reactors should be sufficient to use the light in the solution rather than absorb it on adjacent lamps and walls. The lamp spacing should provide for the absorbance of the fluid disinfected. For good quality secondary effluent (absorbance (Base e) 0.3/cm or less) the spacing between lamps should be no more than eight cm with good mixing provided along intensity gradients. The arrangement and numbers of lamps included in each assembly shall be designed to facilitate proper maintenance. All electrical connections to submerged lamps shall be watertight and designed so as to remain dry during maintenance operations.

3. UV lamp specifications should include as minimum the following or demonstrated equivalent:

a. Availability (at least two manufacturers).

b. 90% or more emitted light output at 253.7 nanometers.

c. A minimum arc length that exceeds lamp length.

d. A rated output of 120 UW/SQ.CM. or more at 1.0 meter from the source.

e. A rated operating life in excess of 7500 hours during which time the UV output exceeds one-half of the rated output.

f. The lamps should not produce significant ozone or hydrogen peroxide.

g. Temperature control should provide for maintaining 105°F to 120°F surface temperature.

4. A single ballast should be utilized to provide power to no more than two UV lamps. Ballasts may be mounted side by side in a control box and shall be specified or labeled to indicate their corresponding UV lamps. A set of lights should indicate the on-off status of each lamp and should be visible without opening the control box. The ballasts generate a significant amount of heat, and forced-air ventilation or positive cooling of control boxes shall be provided. The set of ballasts serving each assembly of UV lamps shall be mounted in separate (physically separated) arrangements or control boxes. Control boxes shall be designed and installed in such a manner that replacement of individual ballasts will not result in discharge of undisinfected effluent.

5. The system of electrical connections shall be designed so as to minimize maintenance problems associated with breakage and moisture damage. The electrical system shall be designed so that routine maintenance can be achieved without loss of disinfection efficiency.

6. UV lamp assemblies shall be so located as to provide convenient access for lamp maintenance and removal. Provisions shall be made so that lamp assemblies may be observed and the channel surface physically inspected. Flow channels should be entirely accessible for cleaning to remove film deposits of material interfering with UV disinfection.

7. At least one UV intensity meter within each assembly of lamps shall be provided to indicate operating conditions. The intensity reading should be indicated on the control panel for each lamp assembly. For treatment works with a design average daily flow of one mgd or higher, flow metering shall be provided and appropriate spectrophotometric equipment shall be provided to measure the UV absorbance of the wastewater. An elapsed time meter shall be provided to indicate the total operating time of the UV lamps.

E. Dose control. For treatment works with a design average daily flow of one mgd or more, UV system design should include a control system to turn appropriate lamps on or off in order to conserve energy. The reliability of proposed automated control systems connected to flow sensors shall be demonstrated through submission of acceptable supporting information. Manual control should be based on diurnal flow variations.

1. A spare UV lamp (and quartz tube, if appropriate) shall be provided as a minimum at all UV installations. The number of additional spare lamps (and quartz tubes if appropriate) provided shall equal the nearest whole number equivalent to 10% of the number of lamps required to disinfect the maximum flow rate. Spare ballasts shall also be provided at all UV installations in numbers sufficient to operate the spare lamps.

2. UV equipment design shall provide for routine chemical cleaning with a proper acid/detergent cleanser. A chemical mix tank, circulation pump and upstream/downstream connections should be provided. A weak acid such as citric acid may be utilized for chemical cleaning of quartz tubes, but a stronger acid is recommended for more effective and more economical maintenance. Acid levels with flows returned to the treatment process should be monitored and controlled through pH measurements. A high pressure wash of the quartz tubes or teflon-lined channels should be utilized as a follow-up to chemical cleaning. The system design shall provide for direct scrubbing of surfaces and lamp removal for testing of UV output. As UV transmissibility of quartz and teflon will diminish with time, the design should provide for periodic measurements of these values. As continuous methods of cleaning UV lamp and channel surfaces have not been established as reliable means of maintenance, these methods, including mechanical wipers and ultrasonics, shall not be accepted as sole maintenance methods, i.e., they may be used together with conventional maintenance methods as previously described in this section.

F. Hydraulics. The distances across light intensity gradients for flow past UV lamps should be short compared to the length of the chambers in the direction of flow, and measures should be taken to assure mixing across these gradients, with minimal longitudinal mixing, as measured by the dispersion coefficient. UV system design should provide an estimated E value of no more than 100 square centimeters per second.

1. For lamp assemblies with a dispersion coefficient equal to or more than 50 square centimeters per second, the minimum contact period shall be 10 seconds, assuming that the flow path length is equivalent to the linear distance that the design dosage is provided. The contact period of the UV system flow pattern shall be of sufficient duration to provide the design dose response in relation to the established E value.

2. All UV systems shall be furnished with a means for dewatering as necessary for cleaning. The depth of irradiated flow shall be controlled as necessary to meet the disinfection standard at all flow rates.

G. Safety. UV lamps should not be viewed in the ambient air without proper eye protection as required by VOSH and other applicable regulations. A minimum of one pair of UV protective eye glasses shall be provided. The system design should prevent exposure of bare skin to UV lamp emissions for durations exceeding several minutes. Electrical interlocks should be provided to shut off high voltage systems in accordance with VOSH requirements and as requested by other local and state standards when such energized connections are exposed and could come into contact with operators.

H. Monitoring. Facilities shall be included for collecting a sample following the contact period prior to discharge, to determine the effectiveness of the disinfection method.

1. As most UV disinfection equipment represents new technology and limited performance data is available for these systems, an initial period of increased sampling frequency and testing requirements for pathogenic bacterial indicators, such as fecal coliform, may be required. The required initial testing program should take place over a period of one year or more under reasonable operating conditions with a minimum sampling frequency of at least once per week.

2. Disinfection of secondary effluent by UV irradiation should consistently maintain a fecal coliform level below 200 organisms per 100 milliliters of sample or the level established by the permit or certificate issued.

3. Indicator organism test results should be correlated with other measurements at the time of sampling, including flow rate, effluent suspended solids, UV absorbance coefficient, and lamp operating conditions such as total operating time, the number in operation, and voltage and intensity.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-830 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-770, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-780. Ozonation.

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.

Historical Notes

Former 12VAC5-581-840 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-780, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-790. Other disinfection methods.

A. Design standards for disinfection methods not specifically addressed in this chapter will be established upon evaluation of performance data.

B. Chlorine dioxide (ClO₂). Chlorine dioxide is characterized as a yellow-green to orange gas, its color changing toward red with increasing concentration. Upon cooling, it forms a red, highly unstable liquid which freezes at -59°C and boils at 11°C. Due to the sensitivity of ClO₂ gas to pressure and temperature, it shall be generated at the location where it will be used as a disinfectant. Chlorine dioxide is quite soluble in water, its solubility depending upon temperature and pressure. At temperatures less than 25°C and above 30 mm partial pressure, it is soluble to the extent of 10 grams per liter. Unlike chlorine, ClO₂ does not react with water; it is a true dissolved gas.

1. Chlorine dioxide gas is very toxic but, when dissolved, it is stable and safe to use in water solution. Since concentrated chlorine dioxide gas is unstable under pressure, chlorine dioxide shall be generated under controlled conditions.

2. The generation of chlorine dioxide involves the reaction between chlorine and sodium chlorite:

Cl₂ + 2 Na ClO₂——>2 NaCl + 2 ClO₂

Side reactions that also produce sodium chlorate (Na ClO₃) are also possible in dilute solutions, especially if the concentration of molecular chlorine, Cl₂, is low. Research has shown that high concentrations of sodium chlorite and molecular chlorine favor the formation of chlorine dioxide. Accordingly, chlorine dioxide generators should be designed and operated to provide these reaction conditions while minimizing the amount of chlorine gas that is mixed with the generated ClO₂.

3. As with chlorine, adequate disinfection with chlorine dioxide is achieved by maintaining a sufficient chlorine dioxide residual after a specific contact time in order to achieve the desired microbiological quality of the treated effluent. All the principles of good chlorination practice, proper pretreatment, rapid initial mixing, adequate residual, plug flow contacting, etc., are also applicable to disinfection with chlorine dioxide.

4. Thus, the required levels of residual ClO₂ shall be equivalent to the residual concentrations that would be required for chlorination of a specific effluent unless adequate information is submitted to the regulatory agencies verifying that acceptable disinfection can be achieved with a lower residual of ClO₂.

5. Design dosages of ClO₂ applied to treated effluent should be similar to the recommended levels for chlorination. The results of limited research to date indicate that for certain effluents, lower dosages of ClO₂, in comparison to Cl₂, may accomplish adequate disinfection. However, all proposals specifying design dosages of ClO₂ below the levels approved for chlorination, must provide supporting information based on field measurements or laboratory studies acceptable to the regulatory agencies.

6. The introduction of ClO₂ shall be in a manner to maximize mixing with the influent flow to the contact basin while minimizing vaporization. The same basic principles as for chlorine are to be adhered to in chlorine dioxide physical contacting with the wastewater. However, chlorine dioxide use should be optimized by appropriate selection of application points within the process scheme.

7. Contact periods approved for chlorination shall be directly applicable to chlorine dioxide contacting unless adequate supporting information is submitted verifying that the use of a particular design contact period can result in the acceptable level of disinfection.

8. Chlorine dioxide disinfection requires maintenance of a residual throughout the contact period. Conventional amperometric titration systems should be used to monitor chlorine dioxide residuals and, with some modifications, should be used to control the residual and generation of chlorine dioxide. Operator exposure to ClO₂ shall be minimized. Adequate ventilation shall be provided in areas where ClO₂ is generated and where concentrated mixtures of ClO₂ are sampled and tested. As ClO₂ to ambient air mixtures containing 10% or more ClO₂ are potentially explosive and highly corrosive, provisions shall be made to prevent this occurrence.

C. Electrolytic oxidants. Electrolytic processes produce a mixed group of oxidants consisting of ozone, hydrogen peroxide and chlorine constituents. This process is typically monitored and controlled by the chlorine residual level in the wastewater effluent. All electrolytic oxidant processes should be evaluated under the provisions for conventional disinfection of wastewater in accordance with this chapter. The department will evaluate the development of these methods of disinfection and the approval of this process will be handled on a case-by-case basis in accordance with the provisions of this chapter.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-850 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-790, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-800. Dechlorination.

A. Dechlorination is a process which effectively reduces free and combined chlorine residuals. Sulfur compounds applied to chlorinated effluents have been established as effective dechlorination agents as follows:

1. Sulfur dioxide (SO₂) is a nonflammable, colorless gas with a suffocating, pungent odor and a density greater than that of air. It rapidly dissolves in water to form a weak solution of sulfurous acid (H₂SO₃) which dissociates to produce sulfite ions (SO₃)^-2, which are the active dechlorinating agents.

2. Sulfite salts used for dechlorination include sodium sulfite (NaHSO₃), sodium disulfite (NaHSO3), and sodium metabisulfite (Na₂S₂O₅). Sodium metabisulfite is the most commonly used. Sulfite salts are available in dry form and are more safely handled than sulfur dioxide. On dissolution in water they produce the same active sulfite (SO₃)^-2 ion.

B. Usage. Both sulfur dioxide gas and sulfite compounds may be considered for use for dechlorination purposes. However, the use of sulfur dioxide gas or sodium metabisulfite in accordance with this chapter will be considered as conventional technology for dechlorination of flows equal to one mgd or more.

1. Sulfur dioxide shall be fed as a gas similar to chlorine gas. Since sulfur dioxide is more prone to reliquification, consideration should be given to heating the sulfur dioxide header. Sulfonator capacity shall be adequate to dechlorinate the maximum chlorine residual anticipated on at least a one-to-one basis at maximum daily flow rates to meet the effluent requirements contained in the issued permit or certificate. Requirements for equipment type, standby capability, spare parts, water supply, measurement equipment, control equipment, and evaporators are the same as for chlorination although the materials of construction may differ.

2. Sulfite salts may be fed in dry form with dry chemical feeders or they can be made up as a solution and fed with a diaphragm pump. With either method, proper feed controls shall be provided. Equipment capacity shall be adequate to dechlorinate the maximum chlorine residual anticipated on the basis of 1-1/2 parts or more sulfite salt to one part chlorine.

C. Features. Gas and dry feed equipment requirements shall be similar to those used for chlorination.

1. The dose mixing shall occur following the design chlorine contact period. Normally, this will require the use of a separate basin designed to thoroughly mix the dechlorinating agent with the contact tank effluent within a period of approximately one minute.

2. As the dechlorination reaction is essentially instantaneous, no further contact time is needed other than that required for mixing.

3. Piping materials shall be suitable for use with the sulfur chemical utilized.

4. Housing for feed equipment required shall be the same as for chlorination. However, sulfur dioxide feed equipment and storage containers shall be physically separated by sufficient distance, or by partition barriers, from the chlorination equipment and storage containers in order to prevent cross contamination of feed lines and to satisfy fire codes. Sulfite salts should be stored in unopened shipping containers until ready for use.

D. Safety. Handling requirements shall be the same as for chlorination, except for sulfite salts, which are nonhazardous.

E. Monitoring. Monitoring provisions shall be the same as for chlorination, except that facilities shall also be provided for securing a sample after dechlorination.

F. Other methods. Other means of dechlorination will be evaluated based on submission of adequate performance data.

1. Granular activated carbon may be used for dechlorination of high quality effluents. The dechlorination reaction is dependent on the chemical state of the chlorine, chlorine concentration, flow rate, physical characteristics of the carbon, and wastewater characteristics. Design considerations are similar to those utilized for other wastewater processing unit operations.

2. For small facilities with a design flow less than one mgd, dechlorination may be accomplished through the use of a holding pond such as effluent polishing pond or a constructed wetlands.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-860 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-800, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-810. Polishing ponds.

A. On-line effluent polishing ponds (OLEPP) can be provided to receive discharges at locations where use of the receiving water requires a degree of performance reliability exceeding that provided by the design, operation and maintenance of the sewage collection system and treatment works. The design and construction of OLEPP's may be similar to that of stabilization ponds.

B. Useage. All sewage treatment works designed to produce a secondary effluent quality of 24 mg/l or more of BOD or suspended solids that discharge to shellfish waters such that shellfish harvesting restrictions may be imposed, shall be provided with an OLEPP, or sufficient off-line emergency storage, unless an exemption is granted by the director subsequent to a public hearing held to discuss the impacts of the discharge. An OLEPP should be required for all sewage treatment works (i) for which the design either does not achieve Class I reliability requirements, or is considered nonconventional in accordance with this chapter; (ii) that discharge to critical waters; or (iii) that are located where water quality conditions dictate the need for maximum protection of public health and welfare.

1. These effluent polishing ponds may be required for any Class I reliability discharge from treatment works that are not daily attended by operational personnel for a minimum period of 16 hours.

2. Those sewage treatment works for which sufficient information is provided to the department verifying that adequate performance reliability will exist in the form of continuously available operational staff and supplemental systems and resources, so that water quality and resources will not be damaged in a manner that produces socio-economic losses, may be granted an exception to the requirements for an OLEPP or emergency storage.

3. An OLEPP can be utilized in instances where an additional removal of BOD₅and suspended solids up to a maximum of 3.0% is desired from the effluent of a properly operated and properly loaded secondary treatment facility.

4. An OLEPP can be utilized to control residual chlorine through natural processes such as oxidation and UV light irradiation. The chlorine dosage applied to the pond influent shall be monitored and controlled.

5. A closure plan shall be provided in accordance with this chapter and standards contained in this chapter, prior to issuance of an operating permit.

6. Effluent from an effluent polishing pond shall be disinfected in accordance with this chapter, unless adequate disinfection can be provided for the pond influent, so that effluent disinfection is not deemed necessary.

7. Adequate disinfection of a three-day capacity effluent-polishing pond influent may require special consideration such as:

a. A minimum flow path length-to-width ratio within contact tanks of 40:1.

b. Expansion of detention volume to 60 minutes residence time.

c. Use of mixing devices for chlorine dosing to replace or supplement standard diffusers.

C. Design. The actual liquid depth of facultative polishing ponds shall not be less than five feet or more than 10 feet. The detention time shall not be less than one day nor more than three days, based on average daily flow.

1. In most cases, it should be necessary to provide postaeration facilities following facultative polishing ponds to meet effluent dissolved oxygen requirements, due to the depletion of oxygen in facultative ponds. If postaeration facilities are not provided, calculations shall be submitted to show that the required effluent dissolved oxygen concentrations can be maintained on a continuous basis. Postaeration shall occur during or following disinfection.

2. The influent line shall discharge below the liquid level of the pond near the edge of the pond embankment. The influent line shall enter the pond at a point opposite the effluent structure to prevent short-circuiting and to provide maximum detention time.

3. The effluent structure can be a single draw-off type with a draw-off point 12 to 18 inches below the normal liquid level or a multiple draw-off structure.

D. Aeration. The selection of aeration equipment shall be consistent with the depth of the lagoon.

1. The aeration equipment shall be sized to provide uniform dissolved oxygen concentration throughout the pond. Surface aerators should provide a minimum horsepower capacity of 0.01 hp per 1,000 gallons or provide equipment for which existing performance data has shown it to be sufficient to maintain solids in suspension and capable of dispersing the required level of oxygen uniformly. Diffused aeration systems must be adequately located and sized to provide uniform oxygen dispersion and maintain solids in suspension.

2. The number of surface aerators required shall be determined by the circle of influence of the aerator. The circle of influence shall encompass the entire pond and is defined as the area in which the return velocity is greater than 0.15 feet per second as certified by performance data. Without supporting data, the following may be used as a guide.

E. Features. For aerated OLEPP's the influent sewer shall discharge near one of the mechanical surface aerators. The outlet should be arranged to withdraw effluent from a point at or near the surface. In-pond baffling may be considered to improve hydraulics.

1. A sedimentation zone that has at least 1-1/2 hours of design detention or settling period and a surface loading not to exceed 700 gallons per square foot per day shall be provided. Provisions for sludge removal from the OLEPP, as necessary, shall be addressed in the final design.

2. Either concrete bottom, walls, or embankment walls, or soils-cement stabilization of bottom, walls and embankments should be evaluated in the final design. Earthen embankment walls one foot above and one foot below the normal water level shall be riprapped or stabilized with other suitable material to prevent erosion from wave action.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-870 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-810, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-820. Postaeration.

A. Postaeration design may involve mechanical aeration, diffused air injection, or cascade type aeration. Other methods may be utilized and will be evaluated on a case-by-case basis by the department.

B. Mechanical aeration. Multiple aeration basins for continuous operability should be provided at all treatment works with a design flow of 40,000 gallons per day or more, unless other means of maintaining an adequate level of dissolved oxygen (D.O.) in the effluent are available.

1. The aeration equipment transfer efficiency shall be determined utilizing the manufacturer's certified rating for the particular equipment being considered. The transfer efficiency shall be adjusted to reflect anticipated field conditions of temperature, atmospheric pressure, initial D.O., and composition of the wastewater being oxygenated. When the detention time within the aeration basin exceeds 30 minutes, consideration shall be given to oxygen requirements resulting from biological activity in the postaeration basin. For aeration basins equipped with a single mechanical aeration unit, a minimum of one mechanical aeration unit shall be maintained in storage at the treatment works site for immediate installation.

2. Aeration basins shall be designed to minimize short circuiting of flow and the occurrence of dead spaces. Vortexing shall be prevented.

C. Diffused aeration. Multiple aeration basins shall be provided for continuous operability of treatment works having a design flow capacity of 40,000 gallons per day or greater, except where diffusers may be removed from the basin for maintenance.

1. Diffused aeration basins shall be designed to eliminate short-circuiting and the occurrence of dead spaces. For maximum efficiencies, sufficient detention time shall be provided to allow the air bubbles to rise to the surface of the wastewater prior to discharge from the basin.

2. When the detention time in the aeration basin exceeds 30 minutes, consideration shall be given to the oxygen requirements resulting from biological activity in the aeration unit.

3. Diffused air aeration systems shall be designed utilizing Fick's Law (the rate of molecular diffusion of a dissolved gas in a liquid) in the determination of oxygen requirements. Supporting experimental data shall be included with the submission of any proposal for the use of diffusers which are considered nonconventional. Such proposals will be evaluated on a case-by-case basis by the department.

4. Blower design shall be such that with any single unit out of operation, the oxygen requirements will be provided for maintaining effluent D.O. A minimum of one standby blower shall be stored at treatment works where single aeration basins are utilized.

D. Cascade type. Effluent aeration may be achieved through a turbulent liquid-air interface established by passing the effluent downstream over either a series of constructed steps, or a rough surface that produces a similar opportunity for transfer of dissolved oxygen to the effluent.

1. The following equation shall be used in the design of cascade type aerators:

2. The equation for determining the number of steps is dependent upon equidistant steps; and, if unequal steps are used, transfer efficiencies must be determined for each separate step.

3. The effluent discharge to a cascade type aerator shall be over a sharp weir to provide for a thin sheet of wastewater. Consideration shall be given to prevention of freezing.

4. The final step of the cascade type aerator shall be above normal stream flow elevation and the cascade aerator shall be protected from erosion damage due to storm water drainage or flood/wave action.

5. When pumping is necessary prior to discharge over the cascade aerator, multiple, variable speed pumps must be provided except when preceded by flow equalization.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-880 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-820, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.