Estimation of BODst, respiration rate and kinetics of activated sludge

Tight control of activated sludge processes is hampered by the lack of reliable information about important process characteristics. Some of those important characteristics are sludge kinetics, short-term biochemical oxygen demand (BODst) and respiration rate in the aeration tank. Spanjers et al. (1994) developed a technique to estimate those characteristics from data collected from a continuously fed respirometer with a specially developed operation strategy. Essential in their estimation technique is the instable backward integration of a forwardly stable differential equation. In this paper, an improved on-line implementable estimation methodology is presented. The estimation problem is forulated as a non-linear optimization problem, avoiding backward integration and estimating the respirometer flow. This novel methodology has been applied to the data set of Spanjers et al. (1994), and the resulting estimates turn out to be comparable. But, contrary to expectation, the estimates still suffer from unacceptable inaccuracy due to large parameter correlation. However, a slight modification in the measurement strategy is proposed which will eliminate the problem of parameter correlation to a large extent and so enable more accurate estimation. The new measurement strategy is only advantageous when combined with the novel estimation methodology presented in this paper.