Activated sludge monitoring with combined respirometric-titrimetric measurements

A short review of different respirometric methods is presented, and advanta ges and disadvantages of different principles are discus sed. In this study a combined respirometric-titrimetric set-up was applied to monitor the deg radation processes during batch experiments with activated sludge. The resp irometer consists of an open aerated vessel and a closed non-aerated respir ation chamber. It is operated with two oxygen probes resulting in two sourc es of information on the oxygen uptake rate; both collected at a high frequ ency. The respirometer is combined with a titrimetric unit that keeps the p H of the activated sludge sample at a constant value through the addition o f acid and/or base. The cumulative amount of added acid and base serves as a complementary information source on the degradation processes. Interpreta tion of respirometric data resulting from validation experiments (additions of acetate and urea as ammonium source) showed that the set-up provided re liable data. Data interpretation was approached in two ways: (1) via a basi c calculation procedure, in which the oxygen uptake rates were obtained by an oxygen mass balance over the respiration chamber, and (2) via a model-ba sed procedure in which substrate transport was included for a more accurate data interpretation. Simulation examples showed that the presence of subst rate transport in the model may be crucial for a correct data interpretatio n, since experimental conditions (e.g, low flow rate) and/or the biodegrada tion kinetic parameters (e.g. high Ks) may otherwise lead to data interpret ation errors. Earlier studies already pointed out that titrimetric data can be related to nitrification, and this was also confirmed in this study. Ho wever, in addition, it was shown here for experiments with acetate that the amount of acid dosed was clearly related to the amount of acetate degraded . This indicates that the titrimetric data can be used to study the carbon source degradation. For the titrimetric data in this study, a model-based a nalysis was however only applied for the nitrification process. For an expe riment with ammonium, it was illustrated that the estimation of biodegradat ion kinetics on a combined respirometric-titrimetric data set significantly improves confidence intervals of the parameters compared to the parameter estimation based on respirometric or titrimetric data separately. (C) 2001 Elsevier Science Ltd. All rights reserved.