Manual and Automated Measurement of the Chemical Oxygen Demand (COD) of Wastewaters

Abstract

The problem of water pollution in the United Kingdom and the usefulness of oxygen demand tests to measure the organic pollution is outlined. Because oxyeen demand tests are arbitary the relative merits and shortcomings of the commonly used tests are discussed, as are some new techniques for measuring organic pollution.The correlation between the results of the different tests is examined and is shown to be very limited. An historical review of tests based on dichromate oxidation shows that many different procedures have been proposed, but that an almost universally accepted procedure now seems to have evolved. Automation of the procedure is shown to be possible but the difficulties in doing so are stressed, in particular the problem of the time parameter. A twofold requirement for an automated COD. system is outlined. Experiments with a commercial continuous flow system showed it to be unsuitable for the both requirements. Work with a hybrid part dioscrete, part continuous flow system is described. Some useful informations was gained but the system proved unreliable for monitoring purposes. A completely discrete system was finally developed. A laboratory version for discrete samples and a version for monitoring sewage works final effluent have both been extensively used. The development and performance of both systems is described. The analytical precision and comparative agreement using different analytical parameters is reported. It is suggested that the COD. test is becoming increasingly important and is widely applicable across the whole spectrum of water pollution technology. The future of automated COD. procedures is seen as one of wider acceptance and further development. The manual procedure could possibly benefit from further­ investigation, and some preliminary work and suggestions for further nvestigations are included. IMPORTANT NO'I'E. The term COD. at one time referred to any oxidation demand test using any chemical oxidising agent. More recently the term has come to mean oxygen demand tests where potassium dichromate is employed as the oxidising agent.This is certainly the case in America and acceptance of the term in this sense is becoming widespread. To avoid any ambiguity, the term COD. in this thesis refers exclusively to oxygen demand tests using potassium dichromate as the oxidising agent.