Automatisation et comparaison de différentes méthodes respirométriques d'estimation rapide de la DBO Automation and comparison of different respirometric methods to estimate BOD

Résumé

II est bien connu que la DBO peut étre mesurée par des méthodes respiromédriques. Les manipulations sont ainsi rendues plus aisées mais la durée de la mesure reste inchangée. La demande biologique en oxygène à court terme (DBOACT) a été proposée pour évaluer rapidement la DBO. Habituellement, ces mesures étaient réalisées dans un respiromètre fermé avec une boue acclimatée et en état de respiration endogène. Récemment on a suggéré de travailler avec un respiromètre ouvert, aéré continuellement durant le test. Nous avons réalisé un tel appareil avec une acquisition et un traitement automatique des données, en utilisant un micro-ordinateur. De cette façon, il est facile de tester différentes méthodes (intégration de la courbe en sac, hauteur de pic, comparaison à des étalons, usage d'étalons internes) pour évaluer la DBO. Nous avons également examiné l'influence du type de substrat, de l'âge et des caractéristiques des boues. Notre conclusion est que aucune des méthodes testées ne fournit une détermination précise de la DBO₅ pour des substrats complexes. La méthode reste utile pour contrôler la variabilité de l'influent dans une station d'épuration, lorsque la biomasse est acclimatée au substrat et qu'on ne désire pas une mesure très précise.

Mots-clés:

• DBO,
• méthodes respirométriques,
• DBO à court terme

Abstract

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.

Keywords:

• BOD,
• respirometric methods,
• STBOD