Section 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:

TABLE 7. MINIMUM DESIGN CHLORINE DOSAGES.
EFFLUENT BOD/SS CONCENTRATION	DOSAGE (Based on Maximum Daily Flow)
30/78 mg/l	20 mg/l
45/45 mg/l	15 mg/l
30/30 mg/l	8 mg/l
20/20 mg/l	6 mg/l
10/10 mg/l	4 mg/l
<10/10 mg/l	<4 mg/l
Odor/Sludge Bulking Control	>20 mg/l

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:

TABLE 8. CHLORINE DOSAGE CONTROL SYSTEMS.
Design Flow MGD	Type of Control System Recommended
<0.04	Manual Control
0.04 to 5.0	Flow Proportioning(1)
1.0 to 5.0	Compound Loop(2)
5.0 or greater	Compound Loop

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.

Historical Notes

Former 12VAC5-581-810 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-750, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.