What are the terms bod and cod

BOD, COD, TOC and TSB - sum parameters in environmental analysis

About 40 million organic compounds are known in the environment [1], which cannot be determined individually - at least not with an analytically justifiable effort and within a very short time. This is why so-called sum parameters are used, which summarize the action and substance parameters of one or more substance groups. The most important wastewater parameters are the biochemical or biological oxygen demand (BOD, English BOD), the chemical oxygen demand (COD, English COD), the total organic carbon (TOC) and the total oxygen demand (TSB, English TOD). The TOC reflects the organic contamination based on a direct carbon determination. The other parameters are based on the oxygen required to break down or oxidize the sample ingredients.

Creation of the sum parameters

The biochemical oxygen demand can be considered the mother of the sum parameters. This was first defined in the 19th century as a consequence of the sewer system [2]. The sewer system of the cities brought decisive hygienic advantages, since sewage and rubbish were now discharged "underground". The wastewater that got into the water bodies had a high need for oxygen, which quickly reduced the oxygen content in the water bodies concerned to zero. Low-oxygen waters and fish deaths were the result. Since then, one has dealt intensively with the BSB. Further parameters were defined in the following years.

BOD - Biochemical Oxygen Demand (BOD)

The BOD indicates the amount of oxygen that is required for the biological breakdown of organic compounds in wastewater by bacteria. As a rule, the BOD is determined using the specially standardized laboratory parameter BOD5 (5 for 5 days analysis time) [3, 4]. To determine the BOD5 nitrification inhibitors are added to the sample, which suppress the breakdown of nitrogen compounds. The determination results from the degradation of carbon compounds (carbon-BOD, English carbonaceous BOD). Due to this restriction, an essential process of wastewater treatment is not considered: nitrification. Since a sewage treatment plant can only be reliably controlled and monitored by determining the total BOD, the BOD is suitable5 insufficient for this purpose. Since the BOD measurement is actually a respiration measurement, it is preferred due to its speed in online operation. Under known conditions, the respiration measurement [mg / (l * min)] can be converted into BOD [mg / l].

Due to the 5-day determination, the BOD is5 or carbon BOD for assessing the current degradability of the wastewater by bacteria. The parameter is only used to provide delayed information about the pollution of the wastewater. Alternatively, BOD or respiration analyzers can be used, which allow the determination of the biological consumption within 5-60 minutes. In this way, assessments can be made promptly about the biodegradability of the wastewater and its behavior in the plant.

Short-term consumption measurements using O dominate the market2-Electrode. The low oxygen solubility of the wastewater results in very small consumption, which are extrapolated to the end result by means of factors. Essentially, only easily biodegradable substances are recorded in this way.

Alternatively, LAR AG offers an online respirometer that works like a miniature sewage treatment plant: the BioMonitor. The BOD measuring device breaks down nitrogen and carbon compounds in a wastewater cascade so that the meaningful total BOD is determined. Thanks to the multi-level cascade structure, even ingredients that are difficult to break down are reliably recorded. The oxygen required for degradation is marked with an O2-Sensor measured. The use of the plant's own activated sludge is particularly advantageous, which simulates the conditions in the sewage treatment plant very well.

COD - Chemical Oxygen Demand (COD)

The COD parameter indicates the amount of oxygen that is used to oxidize all of the organic substances contained in the water. For chemical oxidation, an oxidizing agent is added to the sample and its consumption is determined. Nowadays, the standardized laboratory procedure, the dichromate method [5, 6], which acidifies the water sample with sulfuric acid and adds silver sulfate, dominates. In order to avoid falsified measured values ​​in the case of samples containing chloride, the chloride must first be masked by mercury sulphate. Due to the use of hazardous chemicals and a duration of around 2 hours, the process is not suitable for online use.

TSB - Total Oxygen Demand (TOD)

The TSB value has the same basic idea as the COD: It indicates the amount of oxygen that is required for the complete oxidation of all ingredients. In Germany the TSB was forgotten, in contrast to it it was standardized in the USA as a reference parameter for the assessment of organic substances in wastewater [7].

For thermal oxidation, the sample is fed to an incinerator, in which the water evaporates explosively and the components of the sample become CO2 oxidize. The TSB is then determined with a special oxygen detector. The TSB value can be influenced by inorganic compounds in the sample. As a rule, these disturbances are rare and can be compensated mathematically. The parameter is well suited for the correlation with COD and BOD, since inorganic carbons do not affect the correlation and the non-carbon BOD (nitrogen compounds) is also considered in the BOD correlation. Due to the similarities in determining the TOC, the TSB is well suited for online monitoring.

TOC - Total Organic Carbon

The TOC content reflects the organic pollution in the water. To determine the TOC, the inorganic carbon (TIC, Total Inorganic Carbon), i.e. H. Carbon dioxide and its ions dissolved in the water can be excluded or subtracted from the sample, as this is contained in the total carbon (TC, English Total Carbon) (see Fig. 1).

The TOC parameter is determined using thermal or wet chemical oxidation, in which CO2 arises, which then i. d. Usually measured by an NDIR detector. The TOC measurement is ideally suited for online use, as it delivers fast and meaningful results depending on the process management of an analyzer. The TOC has a special position in well-known regulations.

Correlations between COD and TOC

Common COD methods are based on the dichromate oxidation principle, so that they often do not meet the usual environmental and occupational health and safety requirements. Furthermore, the method is difficult to implement for online operation. This leads to an increasing tendency to replace the COD with the TOC. The background for the legitimate concern is based e.g. Partly due to ignorance of the TSB method.

Correlations between TOC and COD are defined in the German Waste Water Ordinance (AbwV) and can be determined for each individual substance. However, depending on changing sample matrices, correlation factors show weaknesses, which means that online operation of TOC / COD correlations is only recommended if the substance mixtures in the water remain the same.

The main reasons given for the COD replacement by the TOC were the dangerousness of the method and the poor automatability. According to the current state of knowledge, it must be checked whether the TSB can be used as a COD replacement method. The equipment required for a TSB device is roughly the same as that of a TOC device, with no analytical problems arising from the inorganic carbon. The analysis processes are correspondingly similar, so that the respective result is available in a few minutes. In addition, the parameters TSB and TOC can be implemented in one device.

Importance of sample preparation

The sample preparation is a decisive criterion for the accuracy and reliability of a measurement result. In water and especially in wastewater, particles can occur that contain organic carbon and should be included in the measurement of the TOC [8]. Other particles that do not contribute to the sum parameters, such as B. sand should not be taken into account during processing.

With the FlowSampler (Fig. 2), LAR AG supplies an optimal sampling system that sucks the sample through a stainless steel tube into the measuring device against the direction of flow. Large and heavy solids are drawn past the tube by the velocity of the flow. However, all waste water components relevant for the measurement are recorded so that the sample taken corresponds to over 98% of a professionally drawn "scoop sample".

QuickTOCultra with a robot-guided injection module - an analyzer plays a role in the laboratory

Application-specific measurement technology

The continuous monitoring of different types of water poses enormous challenges for online measurement technology. In general, the more different the application, the more different the requirements for the analysis system. Commercially available products can only be slightly adapted with modules or are equipped with several ovens. The second reactor is used for safety as soon as the first one fails or becomes clogged with salts. Other analyzers are equipped with homogenizers which, due to their low rotational speed, perform the task of a sieve. Instead of making the sample homogeneous, this step filters out particles, although these may contain components that are important for the TOC value. In contrast to this, LAR AG supplies specialized analyzers with the Quick series, which are designed for the respective requirements of the different applications. The use of a second reactor is superfluous, since with high salt concentrations even salt concentrations of up to 300 g / l NaCl can be treated without any problems with the high salt option. In general, the LAR analyzers differ in the injection system, which is selected depending on the sample structure.

QuickTOCpurity is designed with loop injection for low measuring ranges

For samples with a high concentration of particles with up to 50,000 mg / l TOC, a robot-guided injection module is used, in which hoses and components in contact with the media have been minimized so that the raw wastewater samples can be fed to the measuring device without filtration. For low-particle samples with low measuring ranges, a loop injection is used which, among other things, injects the sample. seals against contamination from the ambient air. However, the oxidation process is the same in all high-temperature devices in the Quick series - the LAR ultra-high-temperature process at 1,200 ° C. It has been proven that all carbon compounds are oxidized at this temperature. The use of catalysts is therefore superfluous.

Years of dialogue with customers resulted in an extremely versatile family of devices that optimally meet the requirements of the respective application - the LAR AG Quick series.

[1] Wikipedia: Organic Chemistry, de.wikipedia.org/wiki/Organische_Chemie, accessed on July 25, 2013.

[2] Royal Commission on Sewage Disposal, 1898.

[3] DIN EN 1899-1, Edition 1998-05: Determination of the biochemical oxygen demand in n days according to the dilution principle (dilution BODn). Water quality - Determination of the biochemical oxygen demand after n days (BODn) - Part 1: Dilution and inoculation method after adding allyl thiourea.

[4] DIN EN 1899-2, edition 1998-05: Water quality - Determination of the biochemical oxygen demand after n days (BODn) - Part 2: Method for undiluted samples.

[5] DIN 38409-41, edition 1980-12: German standard methods for the examination of water, waste water and sludge; Summarized effect and substance parameters (group H); Determination of the chemical oxygen demand (COD) in the range above 15 mg / l (H 41).

[6] ISO 3199, edition 1975-02: Sodium chlorate for industrial purposes; Determination of the chlorate content; titrimetric dichromate method.

[7] ASTM D 6238, 1998 edition: Standard Test Method for Total Oxygen Demand in Water.

[8] DIN EN 1484, Edition 1997-08: Water analysis - Instructions for determining total organic carbon (TOC) and dissolved organic carbon (DOC).