Clean Water Advocacy - Newsroom - AMSA in the News
Bacterial Levels in Lake Michigan Increase After Release of Blended Sewage, Study Says
A person who swims near the discharge pipe of a sewage treatment plant where
partially treated wastewater has been released is 50 percent more likely to get
sick than if the effluent had undergone the full treatment process, according to
results of a study released by an environmental group.
The study by Joan Rose, a microbiologist at Michigan State University, looked at
the effects on water quality when effluent treated to meet Clean Water Act
standards is blended with wastewater that has only undergone partial treatment.
The Natural Resources Defense Council, which opposes a draft Environmental
Protection Agency policy to codify the practice known as "blending," made the
results of the study available March 25.
Blending Process
Blending is employed usually during rain storms when sewage treatment plants
route excess stormwater around the secondary, or biological, treatment process
before recombining it with treated effluent.
This is done to keep the biological treatment process from being washed out by
excessive flows. The blended wastewater is treated with chlorine before being
released and still meets the mandatory secondary treatment standards, but may
contain higher levels of bacteria.
The study looked at monitoring data provided by the Milwaukee Public Health
Department from the Jones Island Sewage Treatment plant, which discharges
effluents from the Metropolitan Milwaukee Sewerage District into Lake Michigan.
"Those data ... show definite spikes in levels of waterborne parasites
(specifically giardia) and E. coli bacteria from monitored effluent during
sewage treatment bypasses occurring in May and December 2003," Rose said in a
letter summarizing the study to Nancy Stoner, an NRDC attorney.
The blended effluent from which the data were derived had undergone chlorination
and dechlorination on two specific events, Rose said.
More Monitoring Data Needed
For the microbe giardia, which can cause gastrointestinal illness, Rose found an
average of 387 cysts per liter in blended effluent compared to 0.2 per liter
when no blending had occurred. The average number of cryptosporidium oocysts
found in effluent that had undergone blending was 0.86 per liter compared to the
0.29 per liter in effluent that had not been blended.
"Often, oocysts are not detected in effluents, so the question remains if they
are present, what are the levels during blending versus treatment," Rose said.
"Only more monitoring data will provide the information needed."
EPA released a draft policy in November 2003 to attempt to create consistency on
when the practice of blending should be allowed (213 DEN A-13, 11/4/03 ; 68 Fed.
Reg. 63,042).
The policy is needed, treatment operators said, because some EPA regions do not
allow blending even though there is no specific regulation banning it. Moreover,
if they are not allowed to blend, excess flows could wash out the biological
process resulting in the possible release of untreated sewage.
NRDC and other environmental groups said the practice should not be allowed
because of the high bacterial levels that can result. Exposure to these types of
bacteria can cause severe gastrointestinal illness.
Criteria for Bacteria
EPA does not have ambient water quality criteria for cryptosporidium or giardia,
so Rose's study only compares bacterial levels between discharges that have
blended effluents and those that have been fully treated.
She noted, however, that the August 2003 proposed long-term 2 enhanced surface
water treatment rule under the Safe Drinking Water Act would set treatment
requirements for any drinking water facility that draws in water containing more
than 0.075 oocysts per liter of cryptosporidium (154 DEN A-4, 8/11/03 ; 68 Fed.
Reg. 47,640).
E. coli bacterial concentrations the day before blended effluent was released
from the Jones Island facility were 11 per milliliter, compared to a range of
300 to 2,900 /mL during the blending incident, the study showed.
Water quality criteria set in 1986 for E. coli established daily concentrations
of 235 colony forming units per 100 milliliter or a monthly limit of 126/100 mL
for multiple samples for waters used for swimming, according to EPA documents.
These figures assume an "acceptable illness rate" of eight cases per 1,000
swimmers, EPA said. The maximum acceptable bacterial density is higher for a
single sample than multiple samples to avoid unnecessary beach closings based on
a single sample, EPA said.
Increased Chance of Illness
Regarding the high concentrations of giardia in Lake Michigan after a blending
event, Rose said a person has a 50 percent chance of getting sick from swimming
near a discharge pipe, also called an outfall.
"Based on this analysis, I would recommend EPA conduct an analysis of public
health risks associated with blending before proceeding to issue policy guidance
that would encourage the use of this approach to address peak wet weather
flows," she said.
EPA officials have said the bacteria criteria would be published soon. The draft
was made available in June 2002.
Representatives from the Association of Metropolitan Sewerage Agencies, which
represents large publicly owned treatment works, have been pushing for the
blending policy. Treatment plants have used blending since the 1970s, AMSA
officials have said, explaining that it is needed to keep the biological
treatment system from being inundated.
In comments on the draft blending policy, Thomas Grisa, public works director
for the city of Brookfield, a suburb of Milwaukee, said blending is necessary.
"The biological process is the heart and soul of the treatment process yet very
sensitive and prone to upset," he said. "It's not inconceivable that an attempt
to quickly add microbial biomass or take it back offline could result overall in
a poorer quality effluent due to an imbalance in the process."
Copies of the letter summarizing the results of the Michigan State study on wastewater discharges are available for a charge from BNA PLUS at 800-452-7773 toll-free or (202) 452-4323 in the Washington, D.C., metropolitan area.