U.S. Geological Survey


Sewage Effluent modelling for water generally assumed that if the treatment worked the bacteria didn't show up at the
point of testing.
1979 study, Injury induced in Escherichia coli cells by chlorination was studied from a physiological
2005 study, Chlorination is one of the most widely used forms of disinfection for wastewater effluent,
however, studies have shown that in many case chlorination results in the injury, rather than death, of fecal-borne
microorganisms. Injured cells, although incapable of growing on standard growth media may retain their virulence.

After 25 years, the problem is being studied to death, with the same answer -- treatment doesn't kill or eliminate the
bacteria. The next best study is to find out how many bacteria show up if the treatment doesn't even injure the bacteria
when the effluent is diluted in water. The
next study is to find out how many bacteria show up on food crops irrigated
with sewage effluent not affected by dilution.

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Simulation of chlorination breakdown at the existing and future outfall locations
One of the design considerations for the future outfall site was that it should be more than one tidal excursion offshore
so that in the event of a chlorination failure, untreated effluent would not be swept into Boston area beaches on the
incoming tide. Signell et al. (1992) showed that even at spring tide, the future outfall location is well offshore of the
region from which Boston Harbor draws water on flood tide. To quantify the expected benefit of the offshore locations,
however, chlorination failures were simulated in ECOM-si and the bacteria concentrations were monitored at beaches
throughout Massachusetts Bay. Chlorination failure was modeled as a breakdown that releases concentrations of
5,000,000 cells/100 ml for a period of 1 day. A e-folding decay timescale of 12 hours was used to simulate die-off of the

Since currents are variable, chlorination failures were simulated every 10 days over a one year period from October
1989 to October 1990, and the observed concentration levels were monitored at 15 different beaches and harbors
along the coast (Table 5.1).

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Two runs were conducted, one for the existing outfalls at Nut and Deer Islands, and one for the future outfall near the
Boston Buoy. For each location, the number of events where the concentration (counts/100 ml) exceeded various
values were calculated, and are shown in Tables 5.2 and 5.3. Events were only counted if they occurred at least 5 days
after the previous event.

The future outfall location obviously has a large beneficial effect, as the total number of events with concentrations
greater than 5 counts/100 ml dropping from 136 to 6 and the number of events greater than 10 counts/100 ml dropping
from 130 to 0. The existing outfall caused 12 events greater than 1000 counts/100 ml, whereas the highest
concentration for the future outfall was less than 10 counts/100 ml.


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