This paper aims to explain results of respirometry experiments using Activated Sludge Model No. 1. In cases of insufficient fit of ASM No. 1, further modifications to the model were carried out and the so-called “Enzymatic model” was developed. The best-fit method was used to determine the effect of the proposed modifications. Increased agreement was found between simulated data and respirometry results, particularly for repeated respirometric tests with acetate as the sole substrate. Additionally, the influence of different biomass pre-conditioning methods was examined. Results from repeated respirometric tests suggest that presence of residual products in an activated sludge sample before respiration testing may decrease the sample's activity and significantly affect results from respirometric tests. An innovative approach to recover original activity is suggested by washing activated sludge samples with tap water or “mineral medium”. As allylthiourea (ATU) was used in most experiments to suppress endogenous nitrification, its influence on kinetic parameters was reviewed. Respirometric tests confirmed that ATU addition has a significant effect on activity and respiration rate of activated sludge samples and could affect results of respirometric analyses.

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