Disinfection Is Key to Flood Recovery

In the summer of 1993, flooding in Iowa deluged the Des Moines Water Works and disrupted water service to 250,000 residents. More recently, in April 1997 an unprecedented 500-year flood hit the Grand Forks, North Dakota, area (including East Grand Forks, Minnesota). In both instances, local water departments had to act promptly to disinfect contaminated water supplies and restore delivery of safe drinking water to their communities. Floods can contaminate water supplies with sewage, garbage, debris, farm run-off, mud and silt, which leads to an increased risk of waterborne diseases, including diarrheal illnesses. Pathogens of potential concern include E. coli, Shigella, Leptospira, Giardia, Hepatitis A virus and Norwalk virus. Treatment of the water supply with increased chlorination following floods is the key to eliminating harmful contaminants.

The Des Moines Water Works flood response plan included locating and isolating washed-out pipelines and closing valves to broken water lines. High levels of chlorine throughout the entire distribution system were maintained for nearly a week. In addition, the entire system was flushed to destroy infectious pathogens in the water system.

L.D. McMullen, General Manager of the Des Moines Water Works, said that adding high levels of chlorine disinfects the water and alerts people that the water is being treated. Water samples from different distribution points were tested for sufficient levels of chlorine and to make sure all bacteria was destroyed. When lab tests showed absence of contaminants, chlorine levels were gradually returned to normal. The Des Moines water treatment plant's recovery efforts were successful: it began pumping water in seven days and began providing potable water for drinking, bathing, sanitation and cooking in less than 30 days.

The Grand Forks, N.D., water and sewer department used increased levels of chlorine in the distribution system and superchlorination in the clear wells when last year's flood damaged water mains and pump stations. Following the flood, the Grand Forks water and sewer department filled the distribution system with highly chlorinated water -- 10 to 15 ppm (parts per million) -- then flushed the entire system. All points were tested until a sufficient chlorine residual was achieved.

Hazel Fetters-Sletten, superintendent of the Grand Forks water department, said that they superchlorinated the clear wells according to American Water Works Association standards. Super-chlorinating continued until all coliform tests for fecal contamination were negative. The facility maintained high chlorine levels for 18 days prior to resuming the plant's pre-flood disinfection practices with chloramination.

Similar steps were taken to restore service in East Grand Forks, Minn. Virtual around-the-clock efforts to maintain water service were abandoned three days after all the pumps in the water treatment plant were found under water. Although the water mains were still intact, the plant was unable to hold water pressure due to the destruction of plumbing in severely damaged homes.

The water plant resumed operations a week later and began the process of flushing with heavily chlorinated water and continual testing for bacterial contamination. During the following two-week period -- until final samples tested clean -- a "boil water" advisory was in effect to ensure the potability of the available water supply. Despite considerable damage, plant workers and volunteers were able keep disruption of the water supply to a minimum.

The American Water Works Association (AWWA) standards, Disinfection of Water Treatment Plants (C653-95), are available for $10 for members and $15 for non-members by calling 800-926-7337. In addition, a manual with guidelines and practices for water utility emergency planning also is available from AWWA for $30 for members and $45 for non-members.