Understand Water Parameters BOD, COD, TSS, etc in a Simple Way

bod vs cod

Chemical Oxygen Demand (COD) and Biological Oxygen demand (BOD) are the two main characteristic properties that indicate the strength of sewage. Both the parameters test the oxygen-demanding strength of the wastewaters. If there are
few or very little bacteria in the sample, it will show that the BOD is low.

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Simple enough, because most NPDES stormwater & process water permits require that folks test for these two parameters. Usually we see these parameters limited by an effluent limitation. More sensitive to complex organic compounds that are resistant to biological degradation. Generally flocculation reaction requires water pH around 8.5, if the wastewater is with too small or too large pH value, then an Automatic pH Regulator with Online pH Meter will be needed to monitor water pH and dose Acid/Alkali to adjust water pH. Before I start, I want to kindly inform you that, don’t forget to take a PDF of this whole article at the end.

What is DO (dissolved oxygen)?

Sewage was
dumped in a river and it took five days for it to reach the ocean, hence the
five-day incubation requirement in the BOD method. The carbon
dioxide diffuses into a colored pH indicator solution where it is converted
to carbonic acid. The resulting color change is directly proportional to the
concentration of carbon present in the sample. (3) CONDUCTIVITY- UV oxidation / Conductivity based TOC methods oxidize
the TOC that is present to CO2 using UV radiation typically in the presence
of a titanium oxide catalyst. EOS services center on energy conservation, water conservation, water and air quality, solid waste and other environmental and sustainability issues.

bod vs cod

Chemical oxygen demand is typically performed where a strong oxidising chemical is used. For example, organic matter is oxidised into carbon dioxide and water in acidic conditions. The quantity of organic matter or the oxygen demand is calculated by determining how much oxidising chemical was consumed during the test. Biochemical Oxygen Demand (BOD) reduction is used as a gauge of the effectiveness of wastewater treatment plants. BOD of wastewater effluents is used to indicate the short-term impact on the oxygen levels of the receiving water.

One type of new methodology for TOC analyses is using closed-loop
photocatalytic oxidation. The new TOC methodology includes a titanium
dioxide slurry and a 400 nm light source for the oxidation process. The
closed-loop TOC system incorporates a closed-loop design to eliminate the
need for carrier gases and uses what is called dynamic endpoint detection,
where all of carbon in the sample is oxidized to completion. The reaction is
measured using a non-dispersive infrared (NDIR) detector. This new
technology can directly measure TOC from a single sample eliminating the
loss of purgable organics and results in comparable recoveries of various
organic compounds when compared to the combustion TOC method.

Why are we talking about COD & BOD?

Generally, COD is preferred to BOD for process control measurements because
results are more reproducible and are available in just two hours rather
than five days. By the time you have the results from a five day test, the
wastewater treatment plant conditions are no longer the same, so real time monitor and control
can not be relied upon by the use of BOD. In summary, both BOD and COD play crucial roles in assessing water quality and pollution levels. While BOD focuses on measuring the oxygen required by microorganisms to degrade organic matter, COD determines oxygen demand through chemical oxidation of organic and inorganic substances. BOD tests are more suitable for biological treatment evaluation, while COD tests are effective in assessing industrial pollution.

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Understanding the differences between BOD and COD is crucial for effective environmental management, wastewater treatment, and pollution control strategies. By employing these parameters, researchers, environmentalists, and policymakers can make informed decisions to protect and preserve our water resources. Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. The term also refers to a chemical procedure for determining this amount. This is not a precise quantitative test, although it is widely used as an indication of the organic quality of water.

There’s a fair amount of natural, organic matter which makes its way to waterbodies via stormwater runoff. Same as chemical sources, biological sources place a burden on the ecosystem by needing oxygen to breakdown. Requires incubation of water samples for a specific duration (typically five days) at a constant temperature. This analytical method is sensitive and accurate to down to levels of ± 0.1 mg/L dissolved oxygen.


COD value is always greater than the BOD value of a particular water body. BOD measures the oxygen demand for the decomposition of organic material by the microbes in the wastewater. COD measures the oxygen demand for the decomposition of both organic and inorganic materials in wastewater. COD, or Chemical Oxygen Demand, is a measure of the amount of oxygen required to chemically oxidize organic and inorganic substances in water. Unlike BOD, COD determines both the biodegradable and non-biodegradable organic content present in the water sample. The natural and organic waste and foliage from the wastewater treatment plants fall in urban runoff, and the whole system acts as the source food for the bacteria borne in water.

NPDES permits usually specify limits related to oxygen demand based on either the Chemical Oxygen Demand (COD) or Biochemical Oxygen Demand (BOD) testing methods. BOD measures the amount of oxygen required by the aerobic organisms to decompose organic matter and COD measures the oxygen required to decompose organic and inorganic constituents present in the wastewater by chemical reaction. In environmental and wastewater management, two critical parameters used to assess water quality and pollution levels are Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). These measures provide valuable insights into the organic pollution present in water bodies and help determine the effectiveness of water treatment processes. This article aims to explore and compare BOD and COD, highlighting their definitions, measurement methods, significance, and differences in applications. COD measures how much DO is consumed by the chemical oxidation of organic matter under controlled conditions.

Characteristics of BOD and COD

These Bacterial matters decompose the organic wastes using the dissolved oxygen present in the water. Therefore, Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are the main properties that show the characteristics and power of sewage. Using these BOD & COD, the oxygen demand in wastewater is tested during the Sewage Treatment Plant treatment. BOD, or Biochemical Oxygen Demand, is a measure of the amount of oxygen required by microorganisms to degrade organic matter in water. It is an essential parameter used to evaluate the organic pollution level in natural waters and wastewater. Total organic compound concentration in water may be estimated using chemical oxygen demand (COD).

bod vs cod

Calibration of the redox electrode of this membrane electrode still requires the use of the Henry’s law table or the Winkler test for dissolved oxygen. This analytical method is sensitive and accurate down to levels bod vs cod of ± 0.1 mg/L dissolved oxygen. Concentration (most often reported in wastewater samples as mg/L or ppm) tells how much of a substance (e.g., mg of BOD) is present in a known volume of wastewater (e.g., 1 Liter).

COD is normally higher than BOD because more organic compounds can be chemically oxidised than biologically oxidised. This includes chemicals toxic to biological life, which can make COD tests very useful when testing industrial sewage as they will not be captured by BOD testing. A COD test measures all organic carbon with the exception of certain
aromatics (benzene, toluene, phenol, etc.) which are not completely oxidized
in the reaction. COD is a chemically chelated/thermal oxidation reaction,
and therefore, other reduced substances such as sulfides, sulfites, and
ferrous iron will also be oxidized and reported as COD. Some municipalities want to measure the amount
of chemicals in the incoming stream in order to asses surcharges as a way of
measuring how much additional treatment their plant will have to do in order
to get the incoming water clean.

  • The dilution water blank is used to confirm the quality of the dilution water that is used to dilute the other samples.
  • CHEMetrics also offers a line of mercury-free vials (K-7351S, K-7361S, and K-7371S) that are more convenient for disposal.
  • This temperature may be significantly different from the temperature of the natural environment of the water being tested.
  • TOC test procedures are relatively simple and straight-forward, but are specific to the type of carbon-analyzing instrument utilized in the laboratory.
  • Furthermore, BOD is a time-consuming test, while COD can finish within a few hours.

TOC doesn’t differentiate between that portion of organic carbon,
which can be metabolized (assimilated). • This includes chemicals toxic to biological life, which can make COD tests very useful when testing industrial sewage as they will not be captured by BOD testing. • COD is normally higher than BOD because more organic compounds can be chemically oxidized than biologically oxidized. • Aquatic organisms depend on oxygen in water or dissolved oxygen (DO) for their respiratory needs. The amount of DO in a body of water depends on the temperature of the water, the amount of sediment, the amount of oxygen removed from the system and the amount of oxygen returned to the water. However, COD is the only method for measuring the amount of industrial waste in the water, which cannot be measured in the DBO.

As an added bonus, the BOD of polluted water is often greater than that of clean water. Sewage from homes, oil spills, and agricultural and animal waste may all lead to elevated BOD levels. The term “BOD” refers to the “biological oxygen demand” that is present in water.

The overall goal of the EOS is to decrease the cost of doing business, increase environmental sustainability and competitiveness of the state’s industries, lower costs for government agencies and facilities, and enhance economic development. Along with the potassium dichromate in 50% sulfuric acid solution, pre-prepared COD vials also contain silver sulfate as a catalyst and mercuric sulfate to eliminate chloride interference. Thus, COD vials are considered hazardous waste and must be handled and disposed of in an approved manner.

So it doesn’t need microbes to decompose the organic matter in the sample through aerobic respiration. Moreover, COD is usually high in contrast to BOD since it measures a full breakdown of pollutants. In contrast, BOD is a measurement of a biochemical breakdown, which is not necessarily as thorough as a chemical breakdown. The sample is kept in a sealed container fitted with a pressure sensor.

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