Science News -
April 6, 2005
E. coli thrive outside wastewater treatment plants
After noticing increased numbers of E. coli downstream from a wastewater treatment plant on a New Hampshire estuary,
researchers became concerned that fecal bacteria were surviving the disinfection process. The result is a lab study
published on the ES&T research ASAP website (es048643s) this week. The research suggests that if bacteria are only
injured by exposure to low levels of chlorine disinfectant, they can go undetected by current wastewater testing
procedures and then recover after the water is discharged into a river or estuary. Whether that is happening in
treatment facilities is still not known.
New Hampshire Estuaries Project
Researchers find that bacteria thrive in water discharged from wastewater treatment plants into estuaries, but this effect
may be offset in some areas where the water is saline.
The study does raise concerns that bacteria might survive disinfection at a water treatment plant and then become a
health hazard downstream, says Alan Roberson, director of regulatory affairs with the American Water Works
Association: “There’s not a high possibility, but they are right, it’s definitely an issue.” Roberson points out that
wastewater plants typically use much higher concentrations of chlorine and longer exposure times than were used in this
study, and those tougher conditions are likely to kill any bacteria.
With current procedures, wastewater is disinfected according to a formula based on chlorine concentration and contact
time, then dechlorinated to prevent harm to the environment. The researchers simulated this process by exposing E. coli
to chlorine bleach at a concentration of 0.5 parts per million for 5 minutes and then dechlorinating the water. These
injured bacteria were then injected into 2-liter vessels containing water collected from 9 sites in an estuary on the New
Hampshire seacoast; these locations varied greatly in water quality and salinity.
“This is important because we used actual environmental water samples,” says Stephen Jones, a research associate
professor at the University of New Hampshire and study coauthor.
Initially, the injured E. coli could not be cultured, a typical means of detection used by treatment plants, but after only 10
hours the bacteria began to recover and grow on culture plates. Bacteria showed the greatest recovery in water
samples with high levels of organic pollutants. For example, at one site 360 meters downstream of a water treatment
facility, bacterial recovery increased 50-fold after 74 hours.
“Whether that was regrowth or resuscitation of injured bacteria, we don’t know,” says study coauthor Carl Bolster, a
hydrologist with the federal Agriculture Research Service. Increases in detectable E. coli could signal that water
discharged from these plants encourages growth of bacteria that are already in the environment. “It’s a good question
that we couldn’t answer with this study design,” says Jones.
A paradigm within microbiology holds that E. coli only grow in intestines, but Jones and other experts say that evidence
exists that the bacteria might also be expanding their numbers in nutrient-rich environments, such as sewage waste. —
PAUL D. THACKER