Activated sludge monitoring with combined respirometric-titrimetric measurements

A short review of different respirometric methods is presented, and advantages and disadvantages of different principles are discussed. In this study a combined respirometric-titrimetric set-up was applied to monitor the degradation processes during batch experiments with activated sludge. The respirometer consists of an open aerated vessel and a closed non-aerated respiration chamber. It is operated with two oxygen probes resulting in two sources of information on the oxygen uptake rate; both collected at a high frequency. The respirometer is combined with a titrimetric unit that keeps the pH of the activated sludge sample at a constant value through the addition of acid and/or base. The cumulative amount of added acid and base serves as a complementary information source on the degradation processes. Interpretation of respirometric data resulting from validation experiments (additions of acetate and urea as ammonium source) showed that the set-up provided reliable data. Data interpretation was approached in two ways: (1) via a basic calculation procedure, in which the oxygen uptake rates were obtained by an oxygen mass balance over the respiration chamber, and (2) via a model-based procedure in which substrate transport was included for a more accurate data interpretation. Simulation examples showed that the presence of substrate transport in the model may be crucial for a correct data interpretation, since experimental conditions (e.g. low flow rate) and/or the biodegradation kinetic parameters (e.g. high Ks) may otherwise lead to data interpretation errors. Earlier studies already pointed out that titrimetric data can be related to nitrification, and this was also confirmed in this study. However, 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 analysis was however only applied for the nitrification process. For an experiment with ammonium, it was illustrated that the estimation of biodegradation 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.