Transient response of aerobic and anoxic activated sludge activities to sudden substrate concentration changes

Abstract

The state-of-the-art understanding of activated sludge processes as summarized in activated sludge models (ASMs) predicts an instantaneous increase in the biomass activity (which is measured, e.g., by the corresponding respiration rate OUR, NUR, etc.) under sudden substrate concentration changes. Experimental data (e.g., short-term batch respiration experiments under aerobic or anoxic conditions) collected for the calibration of the dynamic models (ASMs) often exhibit a transient phenomenon while attaining maximum activity, which cannot be explained by the current understanding of the activated sludge process. That transient phenomenon exhibits itself immediately upon addition of a substrate source to an endogenously respiring activated sludge sample and it usually takes a few minutes until the activated sludge reaches its maximum possible rate under given environmental conditions. This discrepancy between the state-of-the-art model and the experimental data is addressed in detail in this investigation. It is shown that the discrepancy is not caused by an error in the experimental set-up/data but it is rather due to model inadequacy. Among the hypotheses proposed, it appears that this transient response of the activated sludge most likely results from the sequence of intracellular reactions involved in substrate degradation by the activated sludge. Results from studies performed elsewhere with pure cultures (S. cerevisae and E. coli) support the hypothesis. The transient phenomenon can be described by a dynamic metabolic network model or by a simple first-order model, as adopted in this study. The transient phenomenon occurring in short-term batch respiration experiments is shown to interfere severely with parameter estimation if not modeled properly (2.8%, 11.5%, and 16.8% relative errors [average of three experiments] on Y(H), micro(maxH), and K(S), respectively). Proper modeling of this transient phenomenon whose time constant is on the order of minutes (1 to 3 min) is expected to contribute fundamentally to a better understanding and modeling of Orbal, carousel, and SBR-type treatment plants with fast-alternating process conditions, although such studies are beyond the scope of this report.

A new approach for modelling simultaneous storage and growth processes for activated sludge systems under aerobic conditions

“…) is used to model the transient response observed in OUR data obtained from batch experiments (Guisasola et al, 2003;Vanrolleghem et al, 2004).…”

Characterization and Quantitation of a Novel β-Lactamase Gene Found in a Wastewater Treatment Facility and the Surrounding Coastal Ecosystem

“…3A). The high metabolic activity of microorganisms contained in activated sludge (14,56) combined with the potential presence of antibiotics may provide a selective environment for the enrichment of the bla M-1 gene, resulting in the highest GCN within the treatment process. The bla M-1 GCN ml Ϫ1 sample was significantly reduced (P Ͻ 0.0001) in the posttreatment principal effluent compared to that in other sites within the facility.…”

A structured model for the simulation of bioreactors under transient conditions

“…32 Its fate within the cell is highly dependent on the biological phenomena considered in the metabolic model. Although attempting to model storage and growth processes simultaneously for activated sludge systems, Vanrolleghem et al 17 have proposed modeling the dynamics of growth and storage using a first order time delay function (1 À e Àt/s) . Sin et al 19 have stipulated that the ratio between the substrate uptake rate and the substrate flux diverted to storage is constant.…”

“…14,46 • The history of the culture or the initial conditions before the perturbation. 17,22,46 • The sign of the perturbation: O'Neil and Lyberatos 47 have identified different time constants for step-up and stepdown experiments performed on continuous cultures of Saccharomyces cerevisiae.…”

“…Reasons for this failure are numerous and have already been cited many times. 4,[11][12][13][14][15][16][17] Basically, unstructured models are issued from conservation (mass balance) equations over the entire reactor. They result from a double averaging (in time and space) of a wide variety of intracellular reactions and lead to a description of the mean behavior of the population in terms of an apparent kinetic rate.…”