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Current digestion models cannot properly simulate processes that are operated under high loadings or high temperatures. A modification to existing models has been implemented by adding important missing degradation pathways, to accommodate these high loaded and high temperature anaerobic systems without artificially recalibrating the model parameters. This degradation pathway relies on the use of an alternate acetate oxidizing mechanism that is more tolerant to ammonia than the aceticlastic pathway. Inhibition values have been estimated and a logistic function has been used to apply ammonia inhibition. The model also relates metabolic activity to unionised species such as undissociated acetic acid as substrate (though not obligatory for all organisms) and unionized ammonia as inhibitor. The model also incorporates all important chemical species and activity coefficients in the equilibrium chemistry module (such as the phosphate buffer), resulting in more accurate pH predictions which is crucial for proper modeling of CO 2 and NH 3 stripping. This model can now be used to simulate processes that are operated under conditions where free ammonia inhibition can be an important factor for process efficiency and substrate conversions.

In current process models activated sludge consists of biodegradable and unbiodegradable organic fractions. Recent evidence suggests that this approach may not be accurate because some of this "unbiodegradable" material may indeed be degradable. To improve sludge production predictions, it is important to know to what extent the "unbiodegradable" organic fraction is degradable. Assuming that volatile suspended solids (VSS) is a measure of the sum of biodegradable and unbiodegradable organic solids and the integral of the oxygen uptake rate (OUR) is representative of the biodegradable organics, the combination of these measurements can be used to predict the change of unbiodegradable organic solids within an aerobic digestion batch experiment. This procedure was used to estimate degradation rates of "unbiodegradable" VSS between 0.006 to 0.029 d(-1). The advantage of the proposed method is that the degradation rate can be determined directly based on measurements and relies on a limited number of assumptions.

The purpose of this paper is to demonstrate how dynamic modelling and simulation can effectively be used to assess the operation of an on-site respirometer. A dynamic model of the respirometer was developed using the General Purpose Simulator (GPS-X). The approach is appropriate for operational design of the respirometer in allowing the user to experiment with the instrument, through simulation, before it is actually operated. The model was able to replicate the operational modes of the respirometer. The dynamic model was found to be particularly useful in assessing difficulties associated with multiple respiration rate calculations, the effect of temperature on the respiration rate, and the detection of the endogenous respiration rate.