Relationships between water quality parameters in rivers and lakes: BOD5, COD, NBOPs, and TOC Environmental Monitoring and Assessment

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The TOC test can take several minutes to several hours to complete, and information obtained from a TOC analysis is less useful than information obtained from the BOD5 or the COD analysis. The TOC test does not differentiate between compounds with the same number of carbon atoms in different stages of oxidation and will thus produce different oxygen demand results. Because BOD5 and COD tests directly measure the amount of oxygen required to stabilize a waste sample, results reflect the original oxidation state of the chemical pollutants. The analysis of BOD5 methods shows there is unification in the systems, which allows them to be performed in situ or, in the best of cases, with an online identification of the results. Most of the studies reviewed focus on the use of microorganisms or biological systems, which can ascertain BOD in less than 1 day.

The test measures only the approximate amount of oxygen that will be required (absorbed or consumed) by a wastewater when it is exposed to air or oxygen for an extended period of time. Toxic substances in the wastewater inhibit or even prevent bacterial growth and, therefore, oxidation of the organic matter. When this happens, the test result is lower than the actual amount of organic matter present would suggest. The amount of oxygen that is required for the chemical oxidation of the organic and inorganic chemicals present in the wastewater by utilising oxidising agents like Potassium permanganate, Potassium dichromate etc. is called as chemical oxygen demand (COD).

  1. COD is the oxygen demand that is consumed by both inorganic and organic matter present in the wastewater sample.
  2. Total organic carbon is a good parameter to measure and actually a more accurate indication of some of the pollutants that cause the most problems than a BOD test.
  3. Before it began its long-term validation study, the Lewiston plant had never conducted COD analysis before.
  4. This analytical method is sensitive and accurate down to levels of ± 0.1 mg/L dissolved oxygen.

Chemical Oxygen Demand (COD) is one of the fastest tests available to monitor water quality during treatment [9, 20]. For example, in one of these analyzers, TOC concentration is not directly measured; the Analyzer measures total carbon (TC) and total inorganic carbon (TIC) and subtracts TIC from TC to obtain TOC. The acid reacts with bicarbonate and carbonate ions present in the sample to release carbon dioxide (CO2). The CO2 released from bicarbonate and carbonate ions represents the TIC in the sample. The sample is then subjected to ultra-violet (UV) radiation, which reacts with the oxidant and breaks down all remaining carbon bonds in the sample to release CO2.

In conclusion, it is preferable to highlight the importance of TOC and the possibility of using this technique as a complementary tool to verify the results obtained by other techniques. COD is always higher than BOD because chemical oxidation is easier than biological oxidation. PH is a scale used to specify how acidic or basic a water-based solution is. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. Before it began its long-term validation study, the Lewiston plant had never conducted COD analysis before.

There are only 11 data points so we need to be careful in drawing too many conclusions. Still, the mean of these 11 values is 2.3 with a sample standard deviation of 0.39. The BOD5 test is limited in some applications such as industrial wastewaters, which often contain heavy metal ions, cyanides, and other substances toxic to microorganisms. When microorganisms become poisoned by toxic substances, they are unable to oxidize waste, in which case the BOD5 test becomes an ineffective measure of organic pollution. 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.

TOC Test Procedures

Additionally, two more techniques are considered; measurement by fluorescence (a previously studied method, adding more recent applications), and miniaturized biological systems. Initially, the COD technique was improved in processes such as scaling [11]. Besides, the cost could be lowered, and the importance of adding silver salts (or not) was discussed [11, 19]. Moreover, the characteristics of COD allow us to divide it into different stages, all of which can be modified. That is the case of the elimination of interferences, other oxidation or digestion methods, the elimination of mercury, or the measurement of the response. Adding, new novel alternatives have emerged, e.g., the use of thermal biosensors [27, 52, 59].

How can my wastewater have a BOD of 1,500 mg/L when clean water at 68 °F can only contain 9.1 mg/L of DO (Table ?

For measurement of carbonaceous BOD (cBOD), a nitrification inhibitor is added after the dilution water has been added to the sample. The inhibitor hinders the oxidation of ammonia nitrogen, which supplies the nitrogenous BOD (nBOD). When performing the BOD5 test, it is conventional practice to measure only cBOD because nitrogenous demand does not reflect the oxygen demand from organic matter. This is because nBOD is generated by the breakdown of proteins, whereas cBOD is produced by the breakdown of organic molecules. To measure oxygen demand, biochemical oxygen demand (BOD) relies on bacteria to oxidize readily available organic matter during a five-day incubation period.

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. On the other hand, only modifications in the detection systems are discussed in the analysis of TOC. These systems are expensive, but they have high sensitivity, so it was unnecessary to carry out further modifications. (2)WET OXIDATION- Persulfate oxidation / NDIR MethodThere are two types of this method, persulfate oxidation supported with UV (Ultraviolet) irradiation activation and heated persulfate oxidation. This temperature may be significantly different from the temperature of the natural environment of the water being tested.


The University of Georgia Faculty of Engineering Outreach Service (EOS), working cooperatively with Bio & Ag Extension Engineering, provides engineering services and outreach to commercial, industrial and institutional clients throughout the state of Georgia. EOS services center on energy conservation, water conservation, water and air quality, solid waste and other environmental and sustainability issues. 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.

The photogenerated electrons react with organic matter in a redox-type reaction. Also, an electrode is irradiated with UV light to generate electrons that will be transferred to a working electrode that produces an analytical signal, which may quantify COD values. For some authors, the disadvantages relation between bod and cod of this detection are the small specific surface areas and limited use of UV light to generate wide bands [63, 72, 82, 83]. BOD and COD are water’s biochemical and chemical oxygen demands to break down the organic components, whereas TOC is total organic carbon found in an organic compound.

Wastewater generated by commercial, industrial and institutional facilities is typically referred to as “high-strength” compared to typical household wastewater. Table 1 shows the typical concentrations (mg/L) of organics found in untreated domestic wastewater. This table can be used to understand how non-sanitary process wastewater compares to typical domestic wastewater.

In contrast, COD methods use chemical oxidants to oxidize organic matter. BOD simulates the actual treatment plant process by measuring the organic material that can be oxidized with the oxygen in the sample when catalyzed by bacterial enzymes. Although COD is comparable to BOD, COD actually measures chemically oxidizable matter.

BOD is defined as the amount of oxygen demanded by themicro-organisms in the sewage for the decomposition of bio-degradable matterunder aerobic condition. This is the most commonly used parameter to determinethe strength of municipal or organic quality of the water. At the same time, population and production increases have increased water use, creating a corresponding rise in wastewater quantity. This increased water use and process wastewater generation requires more efficient removal of by-products and pollutants that allows for effluent discharge within established environmental regulatory limits. Since the implementation of the Clean Water Act and subsequent creation of the U.S.

Chemical Oxygen Demand (COD) v/s Biological Oxygen Demand

The good news is these two units are equal and thus are interchangeable! Some wastewater parameters (e.g., heavy metals) are often reported in smaller units such as micrograms per liter (µm/L) or parts per billion (ppb). For its part, COT has been the only technique that has been able to be perfected over time, however, there was no change in the initial technique, only changes emerged in the detection method. The possibility of using more and more sophisticated equipment for this technique and not for the others can be explained in the acquisition of the equipment by the laboratories.

COD is usually a measurement of chemicals and the test is simple and easy to perform with the right equipment (COD analyser and digestor) and can be done in 2 hours. BOD usually takes 5 days and TOC used to require large sophisticated pieces of equipment that could measure the sample in minutes, but was cost prohibitive. The BOD is used in measuring waste loadings to treatment plants and in evaluating the BOD-removal efficiency of such treatment systems. Based on the strength of BOD and COD present, the domestic wastewater can be classified as weak, medium, strong and very strong as shown in table-2 below. 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.

Biochemical oxygen demand (BOD) is the amount of oxygen microorganisms require to break down organic materials. In contrast, chemical oxygen demand (COD) is the amount of oxygen required to break down the organic material via oxidation. It requires samples injection by syringe into a high temperature furnace with a platinum or cobalt catalyst. The regulator came up with these test methods as a way to determine the amount of pollution in a water stream to try to control and limit the amount of chemicals that can pollute the sewerage, lakes and rivers if left in a final effluent or discharge stream. 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. 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.

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