The most common odor complaint, "rotten egg smell", is derived from hydrogen sulfide gas dissolved in the water. Concentrations as little as 1 ppm can result in this odor. Most often "smelly water" will be noticed when a water heater has not been used for a long time allowing the accumulation of this hydrogen sulfide gas.
The smell is the result of four factors that must all be present for the odor to develop. These factors include:
- A high concentration of sulfate in the raw water
- Sulfate reducing bacteria, non-toxic to humans (sulfate is reduced to a sulfide state by the bacteria)
- Little or no dissolved oxygen in the water
- Hydrogen (a component of water which may be present due to water conditions reacting with the anode)
With these factors the hydrogen and sulfur combine to form the hydrogen sulfide gas that gives off the rotten egg odor. Active use of the water heater may reduce the problem. The following steps may also be helpful in clearing the water lines and flushing the hot water heater.
How to Flush the Hot Water Heater
Step 1: Turn all faucets inside and outside the home fully open. Let the water run for 10 minutes, then turn OFF the water. This will flush out all the pipes in the home. If the odor is still present follow the instructions in Step 2.
Step 2: How to flush out the hot water heater
- DO NOT TURN OFF incoming water supply.
- Connect a garden hose to the drain valve on the lower side of the water heater. Run the other end of the hose outside and open the valve to drain the hot water heater.
- After 10 minutes take a sample of the water coming out of the garden hose. Use a white cup to collect the sample.
- If there are NOT particles settling to the bottom of the cup, turn off the drain valve on the lower side of the water heater. Disconnect the hose from the valve and return the hose to normal use.
- If there ARE particles settling to the bottom of the cup, continue to let the water run. Check the water every 10 minutes until no particles are present in the cup. Then turn off the drain valve on the lower side of the water heater. Disconnect the hose from the valve and return the hose to normal use.
- Follow this procedure every 6 months to extend the life of your water heater.
- If the odor still persists, the hot water heater will need to be disinfected to kill the bacteria producing the odor. Follow the instructions in Step 3.
Step 3: How to disinfect the hot water heater
- Turn the temperature setting on the hot water heater to HIGH for two hours. This should kill the sulfate-reducing bacteria.
- Flush the water heater to remove the dead bacteria cells by completing Step 2 above. CAUTION: The water heater must have an operable pressure relief valve, otherwise this method of treatment may be dangerous. The temperature setting must be reduced following treatment to prevent scalding hot water and to avoid high energy costs.
- If the odor still persists, chlorination of the hot water system will need to be performed. Follow the instructions in Step 4.
Step 4: How to chlorinate the hot water system
- TURN OFF the water supply to the hot water heater.
- Turn the control to "Pilot" position on a gas hot water heater or turn off the circuit breaker on an electric hot water heater.
- Relieve the tank pressure by opening a convenient hot water faucet. Open for approximately 30 seconds (the water may stop flowing at this point).
- Disconnect the cold water inlet piping on top of the hot water heater.
- Drain approximately one gallon of water from the drain valve located on the lower side of the water heater.
- Add liquid household bleach to the water heater at the cold water inlet. Use a half gallon of bleach for a 40-gallon water heater.
- Reconnect the cold water inlet piping.
- Turn on the water supply to the water heater.
- Open each hot water faucet in the house, one at a time, until the chlorine bleach odor can be detected.
- Allow the chlorinated water to remain in the system for at least 8 hours. DO NOT USE ANY WATER (HOT OR COLD) DURING THIS TIME PERIOD.
- After 8 hours have elapsed, flush the water from each hot water faucet until no chlorine odor remains before returning hot water heater to normal use.
- Turn control back to "On" position with a gas water heater or turn on the circuit breaker for an electric water heater.
Please contact Soquel Creek Water District customer service at 831.475.8500 if you have any questions.