Distribution System Disinfection
There are two important steps in the disinfection process for existing water distribution systems - flushing and treatment with chlorine. (For purposes of this article, the distribution system includes the pressure tank, the water heater, water treatment equipment, piping system and fixtures.) Both are essential to assure that contaminants that have been introduced into the distribution system are removed effectively.
Flushing the SystemFlushing of the distribution system helps remove loose materials that may be present. These materials can interfere with the chlorination process by using up available chlorine and preventing the chlorine from coming in contact with the bacteria that may be lodged in the materials. The materials may include:
- debris that has accumulated in the pipes, including loose bits of solder, metal shavings, dirt or loose scale
- flux residuals from the soldering of copper pipes
- solvents and glues from the solvent welds used with plastic pipes
- insects that may have entered the distribution system during construction
- sand or other material from the well that may have been pumped into the distribution system
Water moving through the distribution system can effectively help remove bacteria by the scouring and diluting action of the water. The distribution system flushing process should not take place until the water from the well is free of sand, silt or other turbidity. If a problem of this nature exists, it must be corrected before flushing of the distribution system. Flushing must take place before the chlorine treatment of the well or distribution system.
The flushing of the distribution system should take place in the following order:
1. Pressure tank. If the pressure tank has debris in it, it may be discharging debris into the distribution system. It would be counter productive to flush the pipes first, only to have debris reintroduced into them from the tank.
2. Hot water tank
3. Water treatment systems
4. Distribution system piping and fixtures
Pressure Tank FlushingConventional galvanized tanks normally have an inlet and outlet above the bottom of the tank. This design leaves a basin at the bottom where scale and other sediment accumulate. The older style tanks found in residential installations have no access openings into the tank that would allow for cleaning of the tank, so the only way to attempt cleaning of these is by flushing, which is, at best, marginal. The tank can be filled and drained repeatedly until discharge water no longer is turbid, but this will in no way assure removal of contaminants that may be in the debris that remains in the bottom of the tank.
Removal of an old galvanized tank and replacing it with a bladder/diaphragm type tank is the most practical, efficient way of attempting to correct a coliform bacteria problem that may be related to the tank.
Bladder/diaphragm tanks normally have only one pipe entering the tank. This opening serves as both the inlet and outlet for the tank and is located at the bottom center of the tank. These tanks are flushed out during each pump cycle, since the tank is nearly emptied of all water when the pump comes on. Most of the sediment or scale that may have entered the tank with the water is flushed out as the water is discharged from the tank.
The Water HeaterHot water tanks have a basin at the bottom of the water heater tank that allows for the collection of scale and other debris that may have been introduced into the tank from the well or minerals in the water. Biofilms also may be present in the tanks. These biofilms and debris must be removed to effectively disinfect a water heater tank. However, thorough cleaning of most domestic water heaters is not possible, because there are no access openings into the tank. The only way to attempt cleaning is by flushing, using the boiler drain (faucet) near the bottom of the tank. Flushing may remove much of the debris in the tank, but complete removal of all materials cannot be assured. The flushing of the water heater must take place prior to the addition of chlorine into the water supply system.
Suggested water heater flushing procedures:
1. If electric, turn off power to the water heater element or, if gas, set the control to the lowest temperature or “off” position.
2. Turn off the valve on the water inlet pipe delivering water to the hot water tank.
3. Open the faucet at the base of the water heater, allowing water in the water heater tank to drain. It may be necessary to open a hot water tap at one of the distribution system fixtures to relieve the vacuum that is created as water drains from the tank.
4. After the tank is empty, close the faucet.
5. Open the water inlet pipe valve to refill the tank.
6. Repeat steps 2 through 5 until the water being discharged from the boiler drain is free of any debris or turbidity.
7. Open the water inlet pipe valve, and turn the water heater control back to the operating position.
After the water heater tank has been cleaned to the extent possible by following the above flushing process, treat the tank with chlorinated water. This chlorinated water normally comes from the well, where the chlorine has been introduced into the system.
As a supplement to treatment with chlorine, the hot water tank, as well as the hot water piping system, may be disinfected by turning the heat control on the hot water heater to its highest setting. This normally will generate hot water temperatures at or above 140 degrees F - temperatures that will kill most bacteria. This hot water can be circulated through the hot water distribution piping to enhance the disinfection of that portion of the water supply. Extreme caution must be used to avoid scalding injuries by restricting use of the water supply when the hot water temperature has been increased.
Treatment EquipmentHousehold water treatment systems commonly encountered in a home include water softeners, reverse osmosis systems, iron removal systems, hydrogen sulfide treatment systems and cartridge filters. Organic and inorganic (calcium, magnesium, iron, etc) nutrients that are present in many water supplies will concentrate in these treatment systems. The presence of these nutrients, along with the large surface area provided in resin beds and on filter cartridges, facilitate bacterial growth, and often are the source of bacterial contamination of water supplies.
Some general water-treatment unit cleaning and disinfection procedures to consider:
1. Follow water treatment systems manufacturers' recommended cleaning and disinfection procedures.
2. Water softener resin bed disinfection procedures, in the absence of the unit manufacturer's recommendations for disinfection:
- Fully regenerate the resin bed to the sodium form before starting the disinfection process. This will help prevent metals from precipitating in the bed, causing fouling. Most units have a control setting for this manual regeneration.
- Place 4 ounces of standard household bleach into the brine well of the salt tank for every 1 cubic foot of resin in the treatment unit.
- Again put the unit into the regeneration mode. This regeneration pulls the bleach into the resin bed.
- It is advantageous to interrupt this second regeneration process and let the bleach sit in the tank for an hour. This will allow greater contact time for more complete disinfection. Make sure there is a residual chlorine level in the effluent before shutting down the regeneration.
- After the completion of the second regeneration, put the unit through a third regeneration. This will help remove any debris that may have broken loose during the disinfection process.
3. The brine tank associated with a water softener should be periodically cleaned. Water softener salt may contain sand and other impurities that accumulate as sludge in the bottom of the tank as the salt dissolves. The sludge that builds up should be removed periodically to prevent this from becoming a place to harbor bacteria that may then contaminate the distribution system piping when the water softener regenerates.
4. Canister-type cartridge filters should periodically be cleaned by putting the filter unit in the bypass position and removing the canister housing and filter. Clean and rinse the canister housing with chlorinated water, and install a new cartridge filter (properly dispose of the old one). Do not rinse the new cartridge filter with chlorinated water.
Piping and FixturesGeneral recommended procedures for flushing piping and fixtures:
1. Put water treatment equipment (such as water softeners or filters) in the bypass position.
2. Remove and clean faucet aerators or similar devices on the outlets.
3. Each outlet (hot and cold) in the distribution system needs to be individually flushed - one at a time. Outlets include, but are not limited to, sinks, outside taps, bathtubs, showerheads, toilets, laundry sinks, dishwashers, refrigerator icemakers and clothes-washing machines.
4. Start with the outlet closest to the source of water (pressure tank).
5. Turn on the outlet to full volume and flush it until the water being discharged is free of any debris or turbidity.
6. If the outlet is a mixing-type fixture (hot and cold water using the same outlet), flush each side separately.
7. Turn off that outlet and proceed to the next closest outlet. Repeat the flushing process.
8. Continue this process, one outlet at a time, until all outlets have been initially flushed.
9. By flushing the outlets one at a time, the flow volume and velocity are maximized to get the most effective flushing possible.
10. The longer the system is flushed, the better.
11. Reinstall the cleaned aerators or similar devices that were removed from the outlets prior to the flushing procedure.
Chlorine TreatmentThe objective of the treatment process is to assure that chlorinated water is flushed through every part of the distribution system, including all pipes, fixtures, outlets and water heaters. After the entire distribution system has been flushed as described above, it is ready for treatment with chlorine. Once the chlorine has been introduced into the well and pressure tank, the chlorine can be run into the distribution system. This chlorinated water should be tested to assure that at least 50 ppm chlorine concentration is present. Test at several points in the distribution system.
Introduce the chlorinated water from the well into the distribution system as follows:
1. Assure that all water treatment equipment is in the “by-pass” position.
2. Starting with the outlet closest to the pressure tank, turn it on until the strong smell of chlorine can be detected. Check with test strips. Outlets include, but are not limited to, sinks, outside taps, bathtubs, shower heads, toilets, laundry sinks, dishwashers, refrigerator ice makers and clothes-washing machines.
3. Turn off the outlet and proceed to the next closest outlet.
4. Continue this process until all fixtures and pipes have been filled with the chlorinated water.
5. Both hot and cold water piping and fixtures must be turned on (separately) until the chlorine smell can be detected.
6. It may take an extended flushing period until the chlorine smell can be detected in the first hot water outlet flushed. This is because it will take some time for the water heater to fill with the chlorinated water from the well.
7. After all outlets have been flushed and chlorinated water is in the entire distribution system, the chlorine should remain in the system at least overnight, and as long as possible.
8. The chlorine may be flushed from the system the next day. The chlorinated water should not be discharged into a septic tank/tile field system. It is suggested that a hose be attached to an outside tap, and the chlorinated water discharged to a roadside ditch, a sprinkler head, into the yard, etc. Do not discharge to a lake, stream or other body of water.
9. Continue to run the water until all traces of chlorine are gone from the system. Use a chlorine test kit to verify.
10. Allow the water to run an additional one-to-two hours to assure that all traces of chlorine have been removed from the system.
11. Put water treatment equipment back on line. The water treatment equipment must have been flushed and disinfected as described earlier.
12. Collect and submit a water sample for coliform bacteria analysis.
Difficult CasesIf chlorination and flushing of a distribution system is not successful in removing coliform contamination, consider flushing the system with a 10 percent vinegar solution (prepared from white, distilled vinegar) in order to remove any biofilm accumulated. There also are biodispersants on the market that are very effective at biofilm removal. In most cases, however, pH-controlled chlorination is more effective than biodispersants are in disinfecting distribution systems, especially where high heterotrophic plate counts are found.