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Mass balances for carbon (as COD), nitrogen, and phosphorus are excellent tools for the evaluation of WWTPs in order to estimate fluxes of these substances, to compare operational data of different WWTPs with one another and, finally, to draw conclusions of general validity. In the case of activated sludge systems, mass balances can also be very helpful for model calibration, in particular for the ‘Activated Sludge Model No. 1’ (ASM1), because this model is consequently based on balances for COD and nitrogen. Hence, the results of mass balances can be utilised directly for the estimation of model parameters. Accordingly, it is recommended to evaluate the fluxes of COD and nitrogen, before the model calibration is started. By means of mass balances only the state of equilibrium can be described. Therefore, as regards model calibration, only parameters responsible for long-term behaviour can be estimated, i.e. the heterotrophic yield YH, the fraction of biomass yielding particulate organics fP, the heterotrophic decay rate bH and the concentration of particulate inert organics XI in the influent. A correct estimate of these parameters is a prerequisite for the validity of the simulation results, not only in regard to excess sludge production and SRT, but also with respect to the oxygen consumption for carbon removal and, therefore, to denitrification capacity.

Since more or less severe nitrification inhibition was observed in several pilot and full-scale activated sludge plants, a means to quantify the extent of inhibition under operational conditions has been sought. Thus a method for the estimation of the actual maximum autotrophic growth rate μ^A,T was developed. This parameter helps to detect low nitrification capacity, before the effluent ammonia concentration increases. Two case studies show that nitrifying activated sludge plants may become acclimatized to inhibitory compounds but have to be protected from peak loads of both nitrogen and inhibitory compounds.

In lab-scale experiments at the 2-stage activated sludge pilot plant of Vienna's central WWTP it is shown that the wastewater soluble COD concentration, which is inert to a sludge with SRT < 1 d (SI(A)) is about double compared to the S(I) concentration in sludge with SRT > 10 d (SI(B)). Unexpectedly the ratio of SI(A)/SI(B) is independent of the sludge age between SRTs of 0.4 and 1.0 days. The difference between the two S(I) fractions is soluble COD that is readily biodegradable by the sludge with SRT > 10 d. However, it is degraded at a lower maximum growth rate. These results comply with earlier results gained with different methods and at different WWTPs. It is hypothesised that very low sludge ages result in a selection of fast growing bacteria, which can utilise only part of the S(S) in the raw wastewater. The other part of S(S) therefore remains in the wastewater and can thus be utilised for enhanced denitrification in the second stage. It is still unknown beyond which sludge age the soluble inert COD SI(A) starts to decrease, finally reaching the value SI(B) for low loaded systems (SRT > 5 days). From this point on S(I) and S(S) are assumed only to depend on the wastewater composition and not on the sludge age. The assumption of the Activated Sludge Model No.1 that the biodegradable fractions can be modelled as a single substrate and by a single removal kinetic (one Monod term) appears not to be applicable for low sludge ages. Some suggestions for mathematical modelling, design and operation of 2-stage activated sludge systems are given.

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