Total Oxygen Demand (TOD) – An Alternative Parameter For Real-Time Monitoring Of Wastewater Organics
By Dr. Wolfgang Genthe and Mike Pliner
Today, due to the multitude of organics, the organic contamination of wastewater cannot be defined with passable analytical effort. Therefore, composite parameters become more important for the description of organic contamination. Parameters have been defined to enable a fast detection of organic loads.
The biological or biochemical oxygen demand (BOD5) can be considered the “mother” of the so-called composite parameters, as it dates to the 19th century. Afterward, further parameters based on oxygen demand have been defined, such as chemical oxygen demand (COD). In the U.S., both industries and municipalities are requested to comply and report at least one of these two parameters in order to prevent financial surcharges. However, both parameters require time-consuming laboratory methods that are not suitable for fast and real-time monitoring which presently is realized by automated online analyzers.
On the contrary, there is the parameter total oxygen demand (TOD), which has existed for more than 40 years, was standardized by the ASTM D6238-98 (re-approved in 2011), and is also very suitable for real-time measurements. Nevertheless, because manufacturers encountered problems on the technical implementation of the recommended thermal oxidation, this parameter almost fell into oblivion.
Common Parameters And Their Limitations
Either BOD5 or COD are used to indicate the demand of oxygen that is needed for the degradation of substances within a water sample. The BOD5 is internationally standardized and defines five days for the decomposition of loads by bacteria. It is only suited for laboratory use, due to its duration. However, the most consequential restriction, compared to a general approach, is the addition of a nitrification inhibitor, with which the biochemical oxygen demand of nitrogen compounds can be suppressed. There is often talk of carbonaceous BOD (cBOD) among experts.
The COD parameter depends on the method that is used to determine it. Different methods produce different results — which are not wrong, but refer to the particular method only. The current reference method is the dichromate method. It determines the COD indirectly by adding a chemical oxidant to the sample and by measuring its consumption. The value gives an idea of the amount of oxygen an aerobic treatment will need to decompose wastewater loads. Despite the dangerous chemicals used, in some cases the oxidation may not be complete leading to significantly lower results. The method is not suitable for online measurements, because the reagents are dangerous and the time needed to oxidize the sample is about two hours. Additionally, it is well disputed among experts that the COD methods based on the dichromate oxidation principle are not appropriate today, especially because of environmental and labor security concerns.
In reference to the carbonaceous BOD, total organic carbon (TOC) is also measured in order to gain information on organic loads of water. It is widely used as its standardized determination, and methods may be easily automated. Additionally, correlations between TOC and COD are possible. However, the correlation factors range from two to six depending on the source of pollution and matrix of the sample. Hence, the TOC in correlation to COD may only be used in applications that do not have frequently changing water compositions.
Although the reporting parameters BOD5 and COD are requested and used to regulate organic discharge from industrial sites and municipal wastewater treatment plants, their fast online monitoring is hard to achieve or not recommended. Operators are therefore faced with the challenge to find an adequate parameter that is possible to determine within minutes and similar to the parameters required.
TOD is based on the same idea as COD, which is that all organic compounds shall be oxidized completely in order to determine the oxygen demand required. However, using thermal oxidation at 1,200 degrees C guarantees complete oxidation of all organic compounds and no chloride disturbances where the dichromate method is detectable. Table 1 shows a comparison of the different methods for the determination of oxygen demand in wastewater. Non-catalytic, high-temperature methods not only provide accurate results, they are also safer for the operator and environment.
Table 1. Comparison Of Parameters And Methods
For the determination of TOD, the sample is thermally oxidized at high temperature in a reactor of high-purity alumina and the oxygen consumption of this reaction is measured directly in the gas phase. Until this point, the sample is fed into a combustion furnace, similar to TOC analysis. The furnace has to be continuously pervaded by an oxygen-containing carrier gas in a “closed” system. During the sample injection, a gas exchange with the environment has to be prevented to avoid measurement failures. After cleaning the measuring gas, the oxygen content is measured directly with a zirconiumoxide-based detector, and the reduction of the oxygen content is directly an extent for the oxygen consumption.
Within the furnace, the water of the injected sample will evaporate immediately and all organic compounds contained will be oxidized completely at the temperature of 1,200 degrees C. The used reactor is filled with inert ceramic material, which is not affected by ingredients of the sample water. No catalyst is necessary at the temperature used for the oxidizing process, and thus the risk of poisoning of the catalyst that may cause a malfunction of the oxidation process is avoided. The process takes only about one to two minutes, which allows the measurement frequency to be three to five minutes, depending on the application.
This is a clean and fast method of analysis to define oxygen demand of a water sample. Demanding developments of new, improved, and environmentally friendlier methods of COD measurements, as done in previous professional journals, is not necessary since this method has existed for decades already and is available on the market.
Figure 1 shows recovery rates of some exemplary chemical compounds of measurements of the total oxygen demand with an online TOD analyzer in comparison to the chemical oxygen demand with the dichromate method (COD) and the calculated theoretical oxygen demand (TODth).
Figure 1. Recovery Rates Of Synthetic Samples
Contrary to the COD value, there are no different retrieval rates for the diverse organic compounds with TOD measurement. In general, the theoretical value is retrieved about 100 percent. Therefore, the TOD result differs from the COD result, since the contamination is determined completely. In addition to the cleanliness and speed of the TOD as the desired parameter, it also determines all organic compounds in water, completely and without any restrictions. Furthermore, TOD is not affected by chloride concentration the way COD is.
Special process controls on the TOC analyzer can prevent any blockages of the high-temperature reactor, even with the highest salt concentrations. In practice, individual applications have salinities of up to 50 g/L NaCl (sodium chloride). A large proportion of the salt falls as powder into a collecting device or is discharged together with the condensate. In contrast to the TOC, good correlations are possible between TOD and COD.
However, at very low TOD concentrations the results may be affected by a few factors. Some inorganic substances may also release oxygen molecules during the combustion, which may reduce the value of the organic oxygen demand. Such interferences are usually rare and minimal. In addition, to minimize these disorders described in the Annex to ASTM Standard, no sulfuric acid is used and the effect from nitrate is possible to be compensated arithmetically on the basis of the prevailing measurement values of the inorganic compounds.
The TOD analyzer construction corresponds to a TOC analyzer. The analytical processes are similar and the results are available within a few minutes. If required both parameters TOC and TOD may be realized in one unit.
An analysis system including the sample preparation, which is optimized to the requirements of online composite parameter determination, removes only very large particles and those that do not contribute to the composite parameter, such as sand. At the beginning of the standardization of composite parameter methods, it had already been emphasized that the samples shall be homogenized if applicable with a mechanical diminishing device. As a result, filtration removes a significant amount of sample before analysis when online analysis is used. However, particulates may be a part of the organic matter to be monitored and must be considered for the online analysis as well.
Today the usage of composite parameters in environmental monitoring and process control is of increasing importance. Regarding the environment and labor safety, special attention should be given to the TOD as a clean and fast substitute for COD in online analysis. The latest technical approaches result in innovative, fast, and reliable online TOD measurements that are easily correlated to COD and bring the online analysis to the next step.
Dr. Wolfgang Genthe (TU Berlin, MIT) is an expert in the fields of engineering and natural science referring to water and wastewater analysis, with more than 20 years of experience.
Mike Pliner is expert of water analysis equipment for industrial applications with over 40 years of practical experience.