Automation and comparison of different respirometric methods to estimate BOD
The usual way of measuring BOD is the dilution method. The consumption of oxygen is measured in the dark at 20 °C during 5 days. Manometric methods were proposed already some decades ago. Their principle is that every molecule of oxygen consumed la transformed into CO2 which is absorbed in a solution of KOH, creating a pressure variation in the bottle. From this pressure variation the BOD can be calculated. By this way experiments were made easier but the duration of the measure remained unchanged. Other respirometric methods were also proposed, especially Short Term Biological Oxygen Demand (STBOD) to evaluate BOD rapidly. Usually such determinations were done in closed respirometers with acclimated sludge under endogeneous respiration condition. Recently, it was suggested to proceed in an open respirometer aerated continuously during the test. We set up this type of respirometer with acquisition and automatic processing of the data, using a microcomputer. In this way it was easy to test different procedures. Among these procedures we tested : - the method of integration of curve in sack-from, - the method of peak level, - the method of comparison to standards, - the method of internal standards. The principles of these methods are given in more detail in the text. We examined the influence of the type of substrate, of the type of biomass used and of the sludge retention time on the results. The substrates tested were : - glucose, - domestic waste water with and without detergent, - diluted eggs, - diluted beer, - effluents from the paper industry. Three different types of biomass and these sludge retention times (fresh, 5 days and 20 days) were used. Automatisation of the test and automatic processing of the data were carried out in basic language with an « Apple II » microcomputer. The test lasted between 37' and 3 hours 10', depending on the conditions (substrate, concentration, type of sludge,...) Analysis of the results were computerized by the Analysis of Variance (ANO-VA) method and results are lasted in 3 tables. From this study we can conclude that STOD methods have some advantages : - they are much faster than the classical method, - they can be easily automatized, - they are cheap, - oxygen is really consumed by biological processes. For simple substrates these methods can lead to good results but for complex substrates reproductibility and precision are rather poor. Among the methods tested the only acceptable one seems to be the integration of the curve in sack-form. Lack of reproductibility appears to be intrinsic to the method. Some possible improvements are presented, such as washing the activated sludge between two experiments, but are not very practical. Despite these limitations the method can be useful to control the variability of influent in a sewage treatment plant. In such a case, the biomass is acclimated to the substrate and variations of influent BOD can be measured.