Environmental Health & Safety

A. Introduction

These procedures describes methods of disinfecting newly constructed potable-water mains; mains that have been removed from service for planned repairs or for maintenance that exposes them to contamination; mains that have undergone emergency repairs because of physical failure; and mains that, under normal operation, continue to show the presence of coliform organisms. These disinfection procedures are based on the AWWA Standard for Disinfecting Water Mains (§C651-05 or the most current). The disinfecting agents discussed in this standard are chlorine solutions that may be derived from liquid chlorine (Cl2), calcium hypochlorite (Ca(OCl2), or sodium hypochlorite (NaOCl). Combinations of free chlorine residual and contact time are provided.

B. General Requirements

Installation of all plumbing fittings and valves necessary for full-diameter flushing and chlorination shall be done by the contractor.

Chlorination and final flush shall be done by a commercial disinfecting company which has demonstrated its ability, equipment, and knowledge of chlorination procedures in compliance with AWWA §C651-05 or the most current. Contractor shall submit the name of the proposed company for approval. Approvals shall be given by Environmental Health & Safety (EH&S) on each specific job. Notify EH&S, on the Attachment 1: Chlorination Notification Form, at least 5 working days prior to the date of chlorination. Provide EH&S the Attachment 2: Chlorination Report, indicating the date of chlorination, lines disinfected, method of chlorination, amount and type of disinfection used, concentration applied, chlorine residual attained in system, holding time, chlorine residual after retention period, date of flushing, and chlorine residual concentration after flushing.

Bacteriological analysis shall be done by a state certified testing laboratory for drinking water quality. Disinfecting company shall submit the name of the proposed laboratory as well as the proposed number and location of samples for approval. Approval shall be given by Environmental Health & Safety on each specific job. Notify EH&S, on the Attachment 3: Bacteriological Sampling Notification Form, at least 3 working days prior to the date of sampling. Provide EH&S the Bacteriological Report, indicating the date of sampling, sample location, presence or absence of coliform organism(total and fecal coliform), and heterotrophic plate count. A microbial laboratory analysis report is acceptable as Bacteriological Report. Upon satisfactory completion of all disinfection procedures approved by EH&S and receipt of acceptable bacteriological results, a written approval of the system will be provided to the responsible contractor (Attachment 4: Chlorination and Bacteriological Testing Verification and Approval).

C. Nonemergency Water Main Disinfection Procedures

The basic steps in nonemergency disinfection of water mains are the following;

Preventive Measures

During construction, the interior as well as all sealing surfaces of pipe, fittings, and other accessories should be kept as clean as possible. Inspect the interior of all pipes prior to installation. If dirt enters the pipe, it should be removed and the affected interior of the pipe swabbed with a 1‰ chlorine solution. All openings in pipelines should be closed with watertight plugs whenever the trench is unattended. Sealing, lubricating, or gasket materials used in pipe installation should be stored and handled in a manner that avoids contamination and be suitable for use with potable water.

Preliminary Flushing of Mains

Before being chlorinated, the main should be completely filled with water to eliminate air pockets and then flushed to purge the line of dirt and debris. This is typically conducted after the completion of the leakage and pressure tests. Adequate drainage must be provided during flushing. Drainage should take place away from the construction area.

Ineffective removal of dirt and debris from lines prior to disinfection often leads to failed bacterial tests, requiring repeated disinfection. Preliminary flushing should be accomplished at a rate of at least 2.5 ft/sec and no more than 5 ft/sec.

Fittings and valves should be thoroughly cleaned before applying chlorine to a main. Special attention should be given to mechanical joints, fittings, and valves that may contain spaces that are difficult to chlorinate once they become filled with water.

Table 1 shows the required flow rate to obtain a velocity of 2.5 ft/sec in commonly used sizes of pipe. Flushing can be enhanced by the used of soft pigs to remove dirt, debris, and air from the main prior to disinfection. The use of pigs can also conserve water and is particularly useful when there is insufficient water supply to attain a 2.5 ft/sec minimum flushing velocity.

Preliminary flushing, however, should not be conducted if tablets or granules of calcium hypochlorite have been placed in the pipe during construction. In this case, special care must be exercised in ensuring that the main does not become contaminated with dirt or other materials during construction.

Flushing of water mains shall not be allowed after ambient temperature has exceeded 85°F without the prior approval of the building engineer.

Disinfecting Agent and Method of Chlorination

The forms of chlorine that may be used in the disinfection operations are liquid chlorine (AWWA B301), sodium hypochlorite solution (AWWA B300), or calcium hypochlorite granules or tablets (AWWA B300). Contact with organic material or high temperatures must be avoided due to the danger of fire or explosion.

AWWA Standard §C651 describes three methods of chlorination for water mains: tablet, continuous and “slug.” The chlorine dose and minimum contact time for each AWWA method are summarized in Table 2. Recommendations for disinfection of small section of mains under emergency repair are also included in Table 2. Methods for measurement of free chlorine residual are summarized in Appendix A. Before any disinfection method is utilized, valves must be positioned so that the highly chlorinated water in the main being treated does not flow into water mains in active service.

Factors to consider when choosing a method of chlorination include length and diameter of the main, type of joints present, equipment and materials necessary for disinfection, skills, and training of personnel, safety concerns, and whether the main must be put into service on a rapid basis. The continuous and slug methods require the use of appropriate chlorine feed equipment and the determination of the necessary chlorine feed rate for the chlorine solution. In long, large-diameter mains, the slug method has the potential for reduction in water and chemicals as compared to the continuous method.

The tablet method is convenient to use for mains with diameters less than 24 inches and does not require special chlorine feed equipment. There are, however important limitations with this method: 1) The use of the tablet method precludes preliminary flushing. Flushing of the lines is often necessary to remove dirt and debris and assists in the removal of air from the lines. 2) Calcium hypochlorite granules or tablets may be dislodged from the lines upon filling and accumulate at points of restriction. 3) The tablet method should not be used in large-diameter mains where a worker must enter the main for inspection due to the potential of toxic fumes.

Refer to AWWA §C651 for detailed procedures and specifications of the chosen chlorination method.

Final Flushing of Mains

After the applicable minimum retention period, highly chlorinated water should be flushed from the main until chlorine residual measurements show that the concentration in the water leaving the main is no higher than that generally prevailing in the distribution system. Care must be exercised when flushing water with high free chlorine residuals. Chlorine is toxic to fish and other aquatic life. Disposal of highly chlorinated water to storm sewers should be avoided without neutralization of the chlorine residual where the sewer discharges directly to a creek, river, or lake.

Flushing of water mains shall not be allowed after the ambient temperature reaches 85°F without the prior approval of the building engineer.

Bacteriological Testing

Redisinfection

If the initial disinfection fails to produce satisfactory bacteriological results or if other water quality is affected, the new main may be reflushed and shall be resampled. If check samples also fail to produce acceptable results, the main shall be rechlorinated by the continuous-feed or slug method until satisfactory results are obtained — that being two consecutive sets of acceptable samples taken 24 hrs apart.

D. Emergency Water Main Disinfection Procedures

When repairs require that mains be opened and depressurized under emergency conditions such as a break or other physical failure of the pipeline, the necessity to restore water service as soon as possible prevents complete compliance with the routine main disinfection procedures of AWWA Standar §C651. Alternate disinfection procedures under such conditions were described in detail in an article by R. Scott Yoo, “Procedures for Emergency Disinfection of Mains,” Opflow, Vol 12, No. 1, (January 1986). The recommended procedures, based in part on this article are described in Appendix C.