Disinfecting Public Water Systems

The purpose of this document is to provide a step-by-step outline for emergency disinfection of a public water system (PWS). Public water systems would typically have dug or bedrock wells and relatively small atmospheric and hydropneumatic tanks.
 

ACTION PRIOR TO DISINFECTION

When water supplies have positive bacterial tests that are not due to sample error, the following initial actions need to be taken before disinfection.

Inspect Facilities

Carefully inspect the facilities (i.e. wells, tanks) to identify the pathway(s) that allowed the bacteria to enter the water system. See the DES fact sheets concerning recommended "Dug Well Design" WD-WSEB-1-4 and "Bedrock Well Design" WD-WSEB-1-2.

Take Additional Bacterial Samples

Take additional bacterial samples from the well(s), storage tank(s), and various points in the distribution piping. These samples could possibly aid in determining the location where the bacteria first entered the system. Remember that once the disinfection begins, all past bacterial evidence down stream of the point of disinfection will be killed. Failure to locate the point where the bacteria entered the water system potentially allows the contamination to occur again.

Flushing the System

The third step before adding a disinfectant is to flush the system. Chlorine is not able to kill bacteria entrapped within mud, rust, or other solids. In order to achieve total bacterial kill, all components of the system must be flushed to a clean condition. If you have already cleansed tanks or piping, it may not be necessary to repeat the process if you are sure that the interior conditions are clean.

Flushing Wells For dug wells, open the cover, look for poor construction, scrub the walls with a long handled brush and wash down. Flush the well clean by long duration pumping to waste. Also try to pump out accumulated sediments from the bottom of the well using a construction mud sucker type pump. In bedrock wells, the cascading water, caused by the drawdown, will flush the inside of the well. Flush the well by long duration pumping to waste.

Flushing Tanks If the storage tank has never been cleaned, and if access to the inside of the tank is available, the tank could be drained, the floor and walls scrubbed with a brush, and the inside washed down. If there are appreciable solids, try to collect them by shovel or scraper. Pump out the remaining muddy water. Do not let this muddy water drain into streams or ponds. If access to the inside of the tank is unavailable, try hosing down the inside through the top air vent. This may require unscrewing the vent pipe or removing the water level detection mechanism. If plumbing unions exist on the incoming water line(s), a hose could be inserted along the bottom of the tank to loosen accumulated solids.

Flushing Pipes The velocity of flow, rather than the flow duration, is the most critical factor in flushing water lines. The velocity should exceed 3 feet per second (fps). To achieve this flow rate, the size of the flushing point may need to be increased or multiple faucets opened simultaneously. The blowoff size should be approximately as large as the pipe being flushed. Shown below are representative flow rates to achieve 3 fps. See fact sheet WD-WSEB-7-5 for details of distribution system blow-offs.

 

Diameter 2" 4" 6"

3 fps   30  Gallons per minute 120        " 350        "

ANALYSIS OF SYSTEM - WHERE TO ADD THE DISINFECTANT

The disinfection must be carried out either upstream, or at the point, where the bacteria entered the system. If there is not sufficient evidence to determine the entry point of the bacteria, you need to disinfect the entire system. If you can localize the origin of the bacteria entry, you only need to disinfect that component of the system and any remaining components down stream. (If you find a construction defect, the defect must be repaired immediately in addition to the disinfection.)

WHAT CHEMICAL TO USE TO DISINFECT

Chlorine is the standard chemical used to disinfect all components of a water system. Chlorine comes in two common forms: 5 1/4 percent sodium hypochlorite, a liquid; and 70⁺ percent HTH calcium hypochlorite, a solid. The liquid can be purchased at supermarkets and is commonly known as chlorine bleach. Read the label to ensure purchase of only sodium hypochlorite. The solid HTH can be purchased from swimming pool supply dealers in either tablets or powder.

DETERMINING THE VOLUME OF WATER NEEDING DISINFECTION

To determine the amount of chlorine to use, it is necessary to determine the volume of water in the
facility(ies) to be cleansed. The volume of water in a circular shape can be determined by using the formula:

Shown below are tables which provide the interior volume of water for various size cylinders.
 

* = to achieve minimal filtration, the soil backfill around a well should be at least 5 feet above the water table.

VOLUME IN STORAGE TANKS
(Approximate Capacity of a Water Storage Tank in Gallons)
Diameter (ft)

In atmospheric tanks, assume the top 10 percent of the tank is air space. In pressure tanks, assume the top 30 percent of tank volume is air space.

CONCENTRATION OF CHLORINE TO USE

The chlorine concentration used to disinfect water facilities can be varied based on the bacterial contaminantion level expected. If the system is believed to be reasonably clean (no pipe breaks, no pipe replacement), then 5 parts per million (ppm) is a good disinfecting concentration. If stronger contamination is expected, then a 50 ppm concentration should be used. In either case, flushing is a critical step before beginning the disinfection.

A 5 ppm chlorine concentration can be achieved by adding one gallon of 5 3 percent chlorine bleach to 10,000 gallons of water (i.e. 1 quart per 2,500 gallons). For an additional reference, see AWWA Specification C651-86 concerning disinfecting water works facilities.

If the entire system will be disinfected, consideration should be given to the additional water in the tanks and piping.

DISINFECTING THE SOURCE

Before beginning the disinfection process, provide advance notice to customers that the system will be chlorinated. If dissolved iron or manganese is present in the water, expect staining complaints.

Existing Dug Wells

Remove the cover. Pour in the proper volume of chlorine. Mix by running a hose stream back from a sill faucet or other pump house, into the well. Circulate the chlorine water solution through the storage tank, and distribution system. Measure the arrival of the chlorine by taste or odor at user taps or by test kit. Let stand preferably overnight. Then flush the heavy chlorine to waste.

Existing Bedrock Wells

With bedrock wells there may be difficulty in dispersing the chlorine throughout the well depth.

- When liquid chlorine is used, add water to the top of the well to force the liquid chlorine deeper into the drill hole. Typically a garden hose would be used to bring water from the pump station or home to the well. The water would be added to the top of well until the chlorine odor is noted in the running water. This method is often used for a shallower well, less than 100 feet.

- Solid chlorine tablets can also be used. The advantage of chlorine tablets is that they sink to the bottom of the well fully dispersing the chlorine through the overall depth. The tablets should be reduced in size by placing in a heavy bag and breaking with a hammer. Pour the dry chlorine pieces into the well. Let the chlorine set over night, and then proceed as indicated for shallow wells. This method is often used for wells greater than 100 feet deep.

As a typical rule of thumb, the chlorine should be in contact with the system components overnight. The more contact time, the more assured the bacterial kill. Chlorine is normally added to a water system in the early evening. This allows the chlorine ample contact time for all facilities before being flushed out the next morning.

When a water system can not be shutdown over night, the chlorine residual will need to be re-strengthened as new water comes from the wells. The goal is to maintain a continuous level of chlorine through out the suggested 12 hour contact period. It is not necessary for the same chlorine atom to be in contact with a particular point in the system so long as the general chlorine concentration remains at an effectively high level. In a case where the system cannot be shutdown, the maximum tolerable chlorine level by customers is approximately 5 mg/L. DES suggests using : of the calculated chlorine amount immediately and 3 of the calculated amount added at 3 hour intervals including : 3 hour, 6 hour and 9 hour points in order to replenish the chlorine which is being pumped out. This will results in the overall addition of 150 percent of the calculated chlorine over the 12 hours. Other ratios and intervals are also acceptable.

FLUSHING OUT THE CHLORINE

Early the next morning, the excess chlorine should be flushed to waste via a blow-off and/or hydrant. At present there are no EPA limits on the maximum amount of chlorine allowable in drinking water. DES suggests flushing to waste until the free chlorine level is less than 1.0 mg/l free chlorine.

When flushing, it is preferable to run the flushed water into a sump in the ground. Do not directly dispose of the chlorinated water into a stream or pond as it will kill aquatic life.
 

DISINFECTING OTHER COMPONENTS OF THE SYSTEM

Shown below are instructions for disinfecting other components of the system.

Adding Chlorine To Water Storage Tanks

Chlorine can be added to a non pressure tank through the air vent at the top of the tank. The equipment needed would include a funnel and flexible rubber or plastic small diameter tube. The flexible tube would be snaked through the vent; the funnel would be attached; and the concentration liquid chlorine poured into the funnel. Run the well pump(s) to help mix the chlorine and stored water.

For pressure tanks, it is normally easiest to add the chlorine to the well or vented storage tank prior to the pressure tank and then run the appropriate pumps to force the chlorinated water solution through the system. To feed chlorine directly into the pressure tank a chemical feed tap must be installed on the feed line and a chemical feed pump must be purchased. Ensure that the chemical feed pump has a pressure capability comparable with that of the operating water system. The amount of chlorine added would recognize all volumes down stream of the injection point.

Chlorine can be added to the distribution system by attaching a chemical feed pump. The chemical feed pump would discharge into the water main through a tap similar to that used for a house service connection. The chlorine chemical feed pump would be run while water is flushed out through a hydrant or blowoff at the end of the distribution system. This configuration is similar to that used for pressure testing new water mains. Note that the service line must be filled with the chlorine solution before the chlorine will reach the main distribution pipe.

In some cases bacterial contamination can be attributed to a one time event rather then to an ongoing deficiency in a facility. These one time events often include:

newly installed wells,

recent repair work performed on the well pumps or pump station piping,

cleaning water storage tanks,

recent repair of water main breaks or extension of the main.

FOLLOW-UP ACTION

Bacterial sample(s) should be taken after the chlorine has been flushed from the system to ensure that the disinfection has been successful. Repeat the bacteria sampling the next month (or possibly even weekly) to insure that recontamination does not occur. Bacterial samples can not be taken until all chlorine has been flushed away.

FOR MORE INFORMATION

We would appreciate your suggestions concerning this fact sheet. In particular we are interested in identifying other solutions (or problems) that you encountered when carrying out disinfection. For an overall listing of water supply related fact sheets, please request fact sheet WD-WSEB-15-2. For more information please call DES's Water Supply Engineering Bureau at (603) 271-3139. Drinking water fact sheets are available through the DES web site at: http://www.des.state.nh.us/ws.htm then select: publications, fact sheets, water division, water supply.