Chemical Oxygen Demand (COD) in Caustic Wastewater


Testing the wastewater at any manufacturing site is paramount in ensuring the water released into the waste stream meets the required standards. If the quality of effluent water does not meet predetermined standards, hefty fines can be issued by local, state, or federal regulatory agencies. In food manufacturing, large amounts of organic waste are produced. Inorganic waste is also released during the necessary cleaning operations for safe and hygienic manufacturing facilities. Large influxes of organic matter in wastewater treatment plants can overwhelm the plant and result in ineffective waste treatment or diversion of pollutants. When waste containing high levels of organic matter enters a natural stream, valuable dissolved oxygen is consumed during the breakdown of organic matter, adversely affecting the aquatic ecosystem. Due to these potential consequences, onsite wastewater treatment is often required in food processing and production to adequately remove organic matter.

Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are parameters used to determine the amount of organic matter in water. BOD measures the amount of oxygen consumed by microbes to breakdown organic matter. COD measures the amount of organic pollutants by the amount of oxygen consumed during the chemical oxidation of organic matter and inorganic chemicals such as ammonia and nitrite. BOD is a more accurate quantification of the readily available organic matter than COD; however, BOD requires a five-day incubation to obtain a measurement, making it difficult to make real-time adjustments.


A food processing facility contacted Hanna Instruments. They were interested in a colorimetric COD measurement, inquiring specifically about the HI83399 multiparameter photometer. After further investigation of the application requirements, it was determined that their caustic wastewater sample had a high level of turbidity. Because the cloudiness of the sample would interfere with the readings and accuracy of measurement, the photometer would not be a suitable option. However, measuring COD by titration instead of photometrically would allow the customer to obtain accurate results in the highly turbid sample. Hanna Instruments suggested the HI902C Automatic Titration System with the HI3131B ORP Electrode and HI839800 COD Test Tube Heater using the HI93754C-25 High Range COD Reagents.

HI902The sample is prepared for titration the same way as for a colorimetric test: a blank is prepared by adding 0.2 mL of deionized water to a reagent vial, and samples are prepared by adding 0.2 mL of sample to a reagent vial. Both the blank and samples are then heated in the HI839800 at 150°C for 2 hours. After cooling, the contents of the vials are carefully transferred and rinsed into 100 mL glass beakers for titration. COD is determined by titrating the excess potassium dichromate in the samples with a ferrous ammonium sulphate titrant. The endpoint is detected using an ORP electrode. First, the blank is titrated to determine the volume of titrant used to reach the endpoint. The blank volume is then entered into the method options prior to running the sample titration, where it can be automatically accounted for in the calculation. The samples are then titrated and results are reported in mg/L COD.

Prior to contacting Hanna Instruments, the customer was outsourcing all of the testing for their caustic wastewater samples. Laboratory outsourcing was costing the company a large amount of money and they were not able to analyse the COD content as frequently as they wished. The customer is now confident with the COD levels in their wastewater, and use the data obtained by the titrator to change their treatment accordingly. In addition, they are no longer worried about failing water quality tests when audited by the regulatory body. The customer was very happy with the ability of the titrator to graph, save, and download results, should the local regulatory body ever require this information. The customer appreciated the ability to automatically account for the blank volume in their titration calculation, and the precise detection of the endpoint provided by an electrode as opposed to a colour indicator. The customer also realised that the titrator could perform other tests required for their wastewater analysis, such as total acidity in the water. This HI902C was an all-in-one solution for their wastewater analysis.

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