(a) General policy. Facilities for disinfection shall be provided to protect the public health and as an aid to plant operation.
(b) Chlorination facilities.
  (1) Chlorination equipment. Chlorination equipment shall be selected and installed which is capable of applying desired amounts of chlorine continuously to the effluent. Chlorination equipment may also be installed to control odors and generally assist treatment. To accomplish these objectives, points of chlorine application may be established at the head of the plant for prechlorination, in the effluent chlorine contact chamber, or other suitable locations.
    (A) Capacity. Chlorination equipment shall have a capacity greater than the highest expected dosage to be applied. Chlorination systems shall be capable of operating under all design hydraulic conditions. Duplicate equipment with automatic switchover should be considered for standby service, so that continuous chlorination can be provided.
    (B) Controls. Means for automatic proportioning of the chlorine amount to be applied in accordance with the rate of effluent being treated is encouraged for all plants and may be required if a maximum chlorine residual is required in the applicable discharge permit. Manual control will be permitted where the rate of effluent flow is relatively constant and for prechlorination applications. Consideration shall also be given to controlling chlorine feed by use of demand.
    (C) Measurements. A scale for determining the amount of chlorine used daily, as well as the amount of chlorine remaining in the container, shall be provided.
    (D) Safety equipment. Self-contained breathing apparatus shall be available for use by plant personnel. The equipment should be located at a safe distance from the chlorine facilities to insure accessibility. Self-contained breathing apparatus shall be located outside the entrance to the chlorine facility.
    (E) Housing. Housing of chlorination equipment and cylinders of chlorine shall be in separate rooms above ground level, with the door opening to the outside, as a measure of safety. Doors should be equipped with panic hardware. The chlorination room should be separated from other rooms by gas-tight partitions and should be equipped with a clear glass, gas-tight window which permits the chlorinator to be viewed without entering the room. Forced mechanical ventilation shall be included in chlorination rooms which will provide a complete air change a minimum of every three minutes. The exhaust equipment should be automatically activated by external light switches and gas detectors that are provided with contact closures or relays. No other equipment shall be installed or stored in the chlorinator room. Vents from chlorinators, vaporizers, and pressure reducing values should be piped to the outdoors at a point not frequented by personnel, nor near a fresh air intake. Detectors and alarms should be located in each area containing chlorine gas under pressure. If gas withdrawal chlorine storage cylinders are subjected to direct sun, pressure reducing devices must be provided at the cylinders. Fire protection devices and fireproof construction is required for all chlorine storage areas. Electrical controls in chlorine facilities must be replaceable or protected against corrosion. Separate, trapless floor drains or a drain to an ample dilution point shall be provided from the chlorine storage room and from liquid feed chlorinator rooms.
    (F) Emergency chlorination. Emergency power should be provided for chlorination facilities.
    (G) Other. Chlorine rooms shall maintain a minimum temperature of 65 degrees Fahrenheit. Chlorinate solution should be prepared using treated effluent. If potable water is used, the potable water supply system must be protected by an adequate backflow prevention device. When a booster pump is required, duplicate equipment should be provided.
  (2) Pellets. The use of pellet systems will be considered for approval on a case-by-case basis.
  (3) Chlorine contact chamber design criteria.
    (A) Initial mixing. Rapid initial mixing of the chlorine solution and wastewater is essential for effective disinfection. Effective initial mixing can be accomplished by applying the chlorine solution in a highly turbulent flow regime created by in-line diffusers, submerged hydraulic structures, mechanical mixers, or jet mixers. The mean velocity gradient in the area of turbulent flow, or G value, shall exceed 500 sec.-1 with residence times of three to 15 seconds. Calculations supporting the design G value shall be presented in the engineering report. Mixing devices for which the mean velocity gradient is difficult to verify shall be justified by pilot or full-scale performance data.
    (B) Contact time. Contact chambers shall be designed to provide a minimum average hydraulic residence time (chamber volume divided by flow) of 20 minutes at the design peak hydraulic flow.
    (C) Contact chamber configuration. Pipe contact chambers shall be sized so that a scour velocity of at least one foot per second will be obtained at the existing maximum daily dry weather flow rate. If adequate initial mixing is not provided, contact chambers shall have a flow pathway length-to-width ratio of at least 40 and a maximum depth-to-width ratio of no greater than 1.0. This length-to-width ratio may be accomplished by baffling.
    (D) Sludge and scum removal. Contact chambers shall either be provided with a means to remove sludge and scum, such as a small hydraulic dredge and skimmers, without taking the contact tank out of service, or shall be configured so that one-half of the contact chamber can be drained for cleaning without interrupting flow through the other half.
(c) Other means of disinfection.
  (1) Chemical disinfection is not normally required when the total residence time in the wastewater treatment system (based on design flow) is at least 21 days.
  (2) Ultraviolet light (UV) disinfection.
    (A) General. Ultraviolet disinfection systems are considered applicable to treated wastewaters with daily average five-day biochemical oxygen demand (BOD5 ) and total suspended solids (TSS) concentrations consistently less than 20 milligrams per liter (mg/liter).
    (B) Definitions.
      (i) Ultraviolet module--A grouping of UV germicidal lamps of a specified arc length in a quartz or teflon sleeve, sealed and supported in a single stainless steel or some other noncorrosive frame.
      (ii) Ultraviolet bank--A grouping of UV modules which span the entire width and depth (of flow) of the reactor.
    (C) Sizing, configuration, and required dosage. Ultraviolet disinfection units will be designed in accordance with methodologies presented in the United States Environmental Protection Agency Design Manual, Municipal Disinfection, EPA/625/1-86/021. Turbulent flow is necessary due to non-uniform intensity fields in an ultraviolet reactor. The proposed design shall have a Reynolds' number of greater than 6,000 at average design flows. Disinfection systems shall consist of a minimum of two ultraviolet banks in series and shall be capable of providing disinfection to permitted fecal coliform levels at the design daily average flow with the largest bank out of service.
    (D) System details. The ultraviolet unit shall be configured so that there is adequate space for the removal and maintenance of lamps. One person should be able to replace lamps without the aid of mechanical lifting devices, special tools, or equipment. Drains shall be provided to completely drain the ultraviolet reactor unless the equipment can be easily removed from the effluent channel, but lamps shall be replaceable without draining the unit. The materials used to construct the reactor shall be resistant to ultraviolet light. Ballasts and other electrical components shall be consistent with the ultraviolet lamp manufacturer's recommendations. Temporary screens shall be installed to protect the lamps and other fragile components from construction debris.
    (E) Controls. Each individual ultraviolet lamp shall be provided with a remote operation indicator. Lamp failure alarms shall also be provided for a predetermined number of lamp failures. Techniques that result in nonirradiated flow pathways are prohibited. Each ultraviolet bank shall be equipped with at least one ultraviolet intensity meter or some means to monitor changes in ultraviolet dosage; however, intensity meters shall not be relied upon to automatically control system operation. A flow control device, such as an automatic level control, shall be provided to ensure that the lamps are submerged in the effluent at all times regardless of flow rate. The automatic level control shall be arranged so that it will allow suspended solids, which may settle, to be washed out of the area of UV disinfection. Proper heating and ventilation are critical to ultraviolet system operation. Cabinets containing ballasts and or transformers shall be provided with positive filtered air ventilation and automatic shutdown alarms at high temperatures. Provisions shall also be made to maintain the ultraviolet lamps at or near their optimum operating temperature and to filter ventilating air so as to limit ultraviolet Cont'd...