Extraction processes and COD
Extraction processes are necessary for separating all substances in a complex inorganic or biological structure if one wants to carry out a comprehensive quantitative analysis. Heavy metals in electroplating, for example, tend towards complexing and therefore require extraction. In environmental analysis and in water treatment plants, total nitrogen extractions are used for the recording of cumulative parameters such as total phosphate.
The chemical oxygen demand (COD) plays an important role as a standard parameter in environmental analysis and water treatment plants. Its level of concentration is used in evaluating the quality of water. It serves as a cumulative parameter for quantifying water pollution caused by organic substances and, along with the biochemical oxygen demand (BOD), gives an indication of the level of water pollution.
The chemical oxygen demand depicts the amount of oxidizable substances that are present in water under specific conditions. It indicates the amount of oxygen in mg/L that will be used for the oxidisation of all oxidizable substances present in the water if oxygen is the oxidising agent. The biological oxygen demand is the amount of oxygen (direct measurement) that the living organisms in the sample will use within 48 hours (BOD2) or 5 days (BOD5). The chemical oxygen demand is always higher than its biological counterpart. Together, the two readings give a reference point for the level of pollution.
In environmental analysis COD & BOD levels give additional information to chemical analytics with regard to the condition of a body of water. All higher animals in water depend on oxygen for survival. However, oxygen does not dissolve in water very well and the maximum amount that can be dissolved is also dependent on the temperature. During the day, plants in the water supply the oxygen, ensuring sufficient levels for the inhabitants. At night however, plants consume the oxygen too.
If oxygen consumption is also increased by decaying processes (BOD) and oxidisation, high temperatures in summer and lack of oxygen can lead to a high number of animal deaths.
Water Treatment Plants
In water treatment plants the material flows of organic carbon compounds can be depicted by COD (the COD volume balance). Oxygen demand in the aeration tanks can be estimated and the rate of the sludge stabilisation process can be determined.
Other values can be checked for their plausibility and the overall performance of the plant can be evaluated as well as its compliance with the prescribed limit values. COD is also needed for the reaction kinetics in the activated sludge process. If one knows the system one can calculate the amount of dry matter (dm) in the surplus sludge from the COD levels. If the COD/dm ratio is around 1.4 there is only organic matter in the surplus sludge, if it is around 1 there is a considerable amount of inorganic content.
In photometry the amount of potassium-dichromate (K2Cr2O7) that is expended during its application as an oxidizing agent is measured. In samples containing chloride, the chloride needs to be removed or masked in order to rule out any readings caused by its oxidisation to chlorine. In order to record all oxidizable substances in a sample, it is necessary to extract any particulate or bound substances. Besides the extraction process for COD there are other extraction processes for parameters which can be determined with photometry. An example is the total nitrogen value (Nges) – an alkaline extraction process is used here - and for total phosphate content an acid extraction process is necessary. And then there are other extraction methods which are used in the photometric analysis of heavy metals.
A standard process is implemented here following the DIN. A high concentration of sulphuric acid is added to the sample, and with a defined amount of potassium-dichromate and silver-sulphate as catalyst it is heated at 148°C for 2 hours.
After it has been shaken, cooled and allowed a period of time for settling, the amount of potassium-dichromate that was used up is measured and from that the equivalent demand for oxygen can be calculated. Plastic particles in the samples, from bottles and pipette tips for example, can result in artificially high results. Therefore the equipment needs to be cleaned thoroughly. In the case of very high COD readings, re-homogenization and the volume of the sample have a limiting effect.
Unfortunately, up to now, photometric determination of important environmental analytic parameters was only possible through the use of poisonous and environmentally harmful substances such as potassium-dichromate. What is more, the prescribed extraction processes can only be carried out by personnel in a well-equipped laboratory, which means that extensive and regular environmental monitoring is only possible under restricted conditions. An extraction and reagent-free process is already being used nowadays for the photometric online-measuring of COD in the outflow of water treatment plants. If this new process could be used extensively on surface waters by unpractised personnel in the near future, there would be an enormous improvement in our understanding of what is happening in the environment. You can read more about this in the final part of our series on the Photometry Compendium in issue 12 2015.