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

BACKGROUND: This 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. RESULTS: Critical 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. CONCLUSION: The 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.