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Disinfection workshop summary
Weighing the risks and benefits of wet-weather disinfection
Presented at The Rosamond Gifford Zoo, Syracuse, NY
October 15, 2002
3. Total chlorine residual toxicity program
Mr. Apicella described a project performed in New York
City where acute and chronic toxicity testing was
performed at fourteen wastewater treatment plants.
Unchlorinated, chlorinated and dechlorinated effluents
were studied. Exposure duration effects relevant to
CSO discharges were evaluated.
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Organism response to dechlorinated effluent is similar to unchlorinated effluent
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48-hr LC50 TRC was 0.3 to 0.6 mg/L for most sensitive species (mysid shrimp)
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6-hr LC50 TRC was 0.9 mg/L (typical exposure to CSO)
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TRC decay in receiving water results in a substantial reduction factor in addition to dilution.
| NYSDEC water quality standards for total residual chlorine |
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Water use / class
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Aquatic protection
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Freshwater (µg/L)
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Marine water (µg/L)
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Fish propagation (A, B, C, SA, SB, SC, I)
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Chronic
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5.0
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7.5
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Fish survival (D, SD)
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Acute
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19.0
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13.0
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Acute toxicity at fourteen NYC water pollution control plants, unchlorinated effluent
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Acute toxicity at fourteen NYC water pollution control plants, chlorinated effluent
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Mysid 48-hr LC50 vs TRC, all fourteen WPCPs 1989-1993
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Effect of exposure duration mysid shrimp toxicity
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| NYSDEC's interim guidance for application of chlorine standard |
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Plant MA7CD10 dilution ranges
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SPDES effluent limit (µg/L)
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80:1
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2000
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> 30:1 <= 80:1
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500
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<= 30:1
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Dechlorination
Alt. disinfection
No disinfection
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Rockland, Maine facility
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Mr. Freedman presented bacteria reduction and effluent
chlorine residuals from two CSO treatment facilities
in Maine employing high-rate chlorination-dechlorination.
Mr. Zabinski presented his experience as Deputy Commissioner with vortex units followed by high-rate chlorination for the City of Yonkers, NY.
chlorination.
and as a result, constructed a second vortex facility at another location.
On behalf of Mr. Boner, Mr. Moffa presented the
full-scale wet-weather flow abatement technologies
that have been tested in Columbus, GA, for flows up to
24 MGD. Two main treatment processes were constructed.
and was simpler and less costly to operate. The filtration, UV process is currently offline and not in operation.
Q: What does "chlorine decay" mean and what causes it? What is the half-life?
A: Free chlorine reacts with organic material
including bacteria and viruses and in the process of
inhibiting the bacteria and viruses chlorine is
consumed. Free chlorine also decays more rapidly in
the presence of sunlight. The decay of TRC has been
demonstrated in receiving waters and the laboratory. A
first-order decay coefficient for TRC is generally
estimated to be 2/day, which is equivalent to a
half-life of approximately 8 hr.
Q: Bacteria reductions from the Rockland experience appear highly variable? What are the bacteria reductions associated with disinfection of wet-weather flows?
A: Fecal coliform densities in undisinfected CSO
can range from hundreds of millions to hundreds of
thousands units per 100 mL. Typical goals after
disinfection are less than 1,000 units or less than
200 units per 100 mL. Bacteria reductions are
presented in percent removal or log reductions. When
presented in percent removals, the removals are
generally greater than 99.99%. With respect to the
Rockland data, disinfected densities ranged from 1 to
848 units per 100 mL. When considering that the
undisinfected densities may have ranged up to
millions of units, a range from 1 to 849 units in the
effluent is not considered highly variable.
Q: How successful are CSO disinfection facilities?
A: Not knowing of all the facilities that may be
disinfecting, it was estimated at the workshop to be
in excess of 90%. Notwithstanding unusual flow
circumstances and operational disruptions, if a
disinfection facility is designed and operated
correctly, it should be 100% effective.
Q: What were the phosphorous reductions in the Columbus GA experience?
A: The figure presented stated 60% phosphorus removal, which represents an annual removal by the vortex. An additional 10% was removed for the filter/UV train for a total of 70%.
Clarification: In Onondaga County's situation, most of
the CSO phosphorus and ammonia will be treated at
Metro because approximately 90% of the annual
wet-weather flow will be stored and pumped back to
Metro for treatment after the event.
Q: What is the volume stored in the interceptor system as compared to the original County alternative and full storage?
A: The existing Main Interceptor Sewer (MIS)
collects the dry and wet-weather flow along the
Onondaga Creek corridor. The total volume of the MIS
itself is calculated as 3.9 MG. The MIS capacity is
partly taken up by daily dry-weather flow
(approximately one-third). During wet-weather flow the
MIS fills completely to maximize flow to Metro and
minimize CSO discharging to Onondaga Creek. For
comparison purposes, the volume associated with the
County's preferred alternative is 8.5 MG, which
includes pipeline volume as well as the volume
associated with the RTF. Similarly, the storage
alternative is estimated at a total volume of 17 MG.
Clarification: Most importantly for Onondaga County is
the fact that 90% of the total average annual CSO flow
will be stored and conveyed for treatment at Metro
leaving only 10% of the total left to be treated at
the regional sites.
Q: What are the benefits of vortex compared to storage?
A: Storage does not provide immediate treatment of
the larger storms as does vortex, but stores the
entire storm volume. This volume is kept in the tank
until the storm subsides and Metro can receive this
volume. During this time floatables float and solids
settle. Both floatables and solids need to be flushed
from the tank after the storm. This requires specific
flushing equipment.
Very large storms will exceed the storage volume and
result in discharge from the storage tank. Any
treatment within the tank is contingent upon time
within the tank, which is dependent upon the rate of
flow through the tank. Given equal treatment goals,
storage tanks can treat floatables and solids at flow
rates only one-quarter that of vortex beyond which
turbulence prevents treatment. Conversely vortex
continues to provide treatment up to 20 times that of
storage.
In summary, vortex tanks:
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Susan Miller, Project Deputy Director
Onondaga County Dept of Water Environment Protection