The Nuts and Bolts of Chemical Oxygen Demand
High Tech Focus
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
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.
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.
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.
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.
For more information, phone Hach at 800-227-4224 .