Removal Activated Sludge Systems Research Articles

Retrofitting a poorly designed system into a full performing nitrogen‐removal activated sludge process—a scientific challenge

AbstractBACKGROUNDThis study intended to detect the inherent deficiencies of prescription‐type empirical design procedures often adopted for nitrogen ‐removal activated sludge systems. Principles of process stoichiometry and kinetics were used to identify a series of hotspots that were likely to impair and disrupt system performance and efficiency. Available data, derived from an existing plant subject to a similar design approach, were utilized to numerically assess significant hotspots and to define a sustainable remedial action in compliance with applicable scientific principles.RESULTSCritical appraisal of the implemented design exposed a large spectrum of deficiencies affecting system efficiency and performance, including nitrification efficiency, denitrification potential, anoxic volume and sludge age, internal recycling and nitrate removal. The major role of the inorganic suspended solids as a design parameter was also ignored. The remedial strategy adopted for retrofitting corrected all the observed discrepancies and it devised a sustainable approach of flipping around the existing reactors so that they could be operated as a series of carousel/fast recycle reactors, each including anoxic and aerobic volume fractions to provide full nitrification.CONCLUSIONThe merit of the proposed remedial strategy was justified by offering two additional options of equally effective system operation with either reduced reactor volume or increased sewage flow. The study presented conclusive evidence to show why prescription‐type design should be avoided, providing numerical proof that this methodology is incompatible with stoichiometric and kinetic characteristics of sewage under local conditions. © 2024 Society of Chemical Industry (SCI).

Modeling nitrous oxide production by a denitrifying-enhanced biologically phosphorus removing (EBPR) activated sludge in the presence of different carbon sources and electron acceptors

In this study, the IWA Activated Sludge Model No. 2d (ASM2d) was expanded to identify the most important mechanisms leading to the anoxic nitrous oxide (N2O) production in the combined nitrogen (N) and phosphorus (P) removal activated sludge systems. The new model adopted a three-stage denitrification concept and was evaluated against the measured data from one/two-phase batch experiments carried out with activated sludge withdrawn from a local, large-scale biological nutrient removal wastewater treatment plant. The experiments were focused on investigating the effects of different external carbon sources (acetate, ethanol) and electron acceptors (nitrite, nitrate) on the mechanisms of N2O production in enhanced biological P removal by polyphosphate accumulating organisms (PAOs) and external carbon-based denitrification by ordinary heterotrophic organisms (OHOs). The experimental results explicitly showed that N2O production was predominantly governed by the presence of nitrite in the reactor regardless of the examined carbon source and the ratio COD/N in the reactor. The model was capable of accurately predicting (with R2 > 0.9) the behavior of not only N2O-N, but also NO3-N, NO2-N, soluble COD, and PO4-P. The simulation results revealed that only OHOs were responsible for N2O production, whereas the present denitrifying PAOs reduced only nitrate to nitrite.