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CASCADIAN®
Professional Water
Treatment Products |
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O3
Water Systems, Inc.
17700 147th St SE
Suite F
Monroe, WA 98272
USA
Phone:
360-794-9511
Fax: 360-794-0856
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Nuts and Bolts of Chemical Oxygen Demand
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| Biochemical Oxygen Demand (BOD) is a
definitive indicator of required treatment in wastewater, and estimating
BOD is an important part of wastewater treatment process control. The U.S.
Environmental Protection Agency (USEPA) requires wastewater treatment
plants to bring BOD within limits before discharging treated wastewater,
and accurate test results must be recorded for regulatory reporting. Thus,
measuring BOD in treated water is an important part of the monitoring
process. Many wastewater treatment facilities use a faster Chemical Oxygen
Demand (COD) test to estimate BOD levels.
Oxygen demand is an important parameter for determining the amount of
organic pollution in water. Testing oxygen demand has its widest
application in measuring waste loadings of treatment plants and in
evaluating treatment efficiency. Biochemical Oxygen Demand (BOD) is a
definitive indicator of required treatment in wastewater, and estimating
BOD is an important part of wastewater treatment process control. High
influent BOD requires extensive treatment to provide the oxygen necessary
to break down the water's organic contents.
Left untreated or partially treated, discharged water contains effluent
organics that compete with downstream organisms for oxygen. This oxygen
demand can kill or inhibit life downstream of the discharge area.
The U.S. Environmental Protection Agency (USEPA) requires wastewater
treatment plants to bring BOD within limits before discharging treated
wastewater, and accurate test results must be recorded for regulatory
reporting. Failure to do so is a violation of federal law that can lead to
extensive fines. Thus, measuring BOD in treated water is an important part
of the monitoring process. In addition, plants that treat wastewater from
commercial operations measure the oxygen demand of waste as it comes to
the facility to determine how much the commercial customer must pay in
fees to have its waste treated.
By monitoring wastewater entering the treatment facility, technicians
are able to respond to changes in oxygen demand and adjust the treatment
process accordingly. The challenge of this process lies in the fact that
the BOD of incoming wastewater can vary substantially over days or hours,
and accurate BOD measurement takes five days to complete.
BOD testing uses microorganisms that consume oxygen while feeding on
organic compounds in a wastewater sample over a five-day period. While
this test is a good model of the aerobic waste treatment process, in some
cases the microorganisms can become poisoned by toxic substances in the
untreated wastewater. The five-day test also does not provide the
real-time information necessary to make process control decisions.
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| For this reason, many wastewater treatment
facilities use a faster Chemical Oxygen Demand (COD) test to estimate BOD
levels.
The EPA-approved COD test is useful for performing rapid, frequent
monitoring of treatment plant efficiency, and results allow quick response
to changing conditions in the waste stream. The evaluation is complete in
just two hours, and the reproducible results correlate with BOD and often
can be used for NPDES reporting. Toxic materials in the sample do not
affect the oxidant, so the test provides a good indicator of organic
pollution in industrial wastewater containing heavy metals and cyanides.
COD testing assesses all chemically oxidizable substances and can be
directly related to the true oxygen demand imposed by the effluent if
released into the environment. Because each organic compound differs in
the amount of oxygen necessary for complete oxidization, the COD test
reflects the effect of an effluent on the receiving stream more directly
than measurement of carbon content. For more perspective on a waste
stream's true organic load, Total Organic Carbon testing can be performed
as a complementary analysis method to COD.
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| The COD/TOC ratio can be an important tool in
the evaluation of wastewater treatment. While the TOC test directly
assesses the carbon atoms present in organic compounds, operators of
processes dependent upon on biochemical oxidation (e.g. wastewater
treatment) find COD provides a measure of the oxygen-demanding substances.
The ratio of COD value to TOC value at a given point in the process may
provide information on the type of organic wastewater constituents
present.
A high COD/TOC ratio may indicate organic compounds that are easily
oxidized (alcohols, for example). A shift in the COD/TOC ratio in the
influent means a change in the type of organic compounds entering the
system, which can impact the effectiveness of the process. In the
wastewater treatment, the amount of oxygen required (reflected by the COD
value) may change while the carbon concentration (reflected by the TOC
value) does not.
By-products of the traditional dichromate COD test, including mercury,
chromium and silver, can be costly and troublesome to dispose of. A
patented COD test available from HACH uses manganese III oxidant instead
of the heavy metal reagents used in the traditional test. This methodology
allows the fast, accurate estimation of BOD, and the end products do not
contain toxic metals. Mercury-free dichromate COD reagents are also
available and help minimize disposal costs.
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| Users should be aware that, with mercury-free
alternatives, some organic compounds are not oxidized completely, and
chloride ions cause interference with the oxidation process, necessitating
their removal for accurate results. The HACH Manganese III method includes
a chloride pretreatment method for samples with interfering chloride
levels.
COD testing, via the dichromate or mercury-free method, is an efficient
alternative or complementary method to the BOD test for wastewater
monitoring. Its relatively quick turnaround time, simplicity and
correlation to BOD make it a valuable tool for those analyzing wastewater
for pretreatment planning or process monitoring.
Source: Water & Wastes Digest
May 2003 Vol: 43
Num: 5
Copyright © 2004 Scranton Gillette Communications |
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