The role of the external mass transfer resistance in nitrite oxidizing bacteria repression in biofilm-based partial nitritation/anammox reactors
Journal article, 2020

A model-based study was developed to analyse the behaviour of Moving Bed Biofilm Reactor (MBBR) and Integrated Fixed-Film Activated Sludge (IFAS) reactor configurations for the removal of nitrogen in the main water line of municipal wastewater treatment plants via partial nitritation/anammox (PN/AMX). The basic principles and underlying mechanisms linking operating conditions to process performance were investigated, with particular focus on nitrite oxidizing bacteria (NOB) repression and resulting volumetric conversion rates. The external mass transfer resistance is a major factor differentiating granular sludge PN/AMX processes from MBBR or IFAS systems. The external mass transfer resistance was found to promote the metabolic coupling between anammox (AMX) and ammonia oxidizing bacteria (AOB), crucial for NOB repression in the biofilm. Operation at low bulk DO prevents NOB proliferation in the flocs of IFAS systems as AMX activity limits nitrite availability (the so-called AMX nitrite sink). Importantly, the effectiveness of the AMX nitrite sink strongly depends on the AMX sensitivity to oxygen. Also, over a broad range of operational conditions, the seeding of AOB from the biofilm played a crucial role in maintaining their activity in the flocs. From a practical perspective, while low DO promotes NOB repression, lower nitrogen loads have to be applied to maintain the same effluent quality. Thus, a trade-off between NOB repression and volumetric conversion capacity needs to be defined. To this end, IFAS allow for higher volumetric rates, but the window of operating conditions with effective NOB repression is smaller than that for MBBR. Ultimately, this study identified the principles controlling NOB in MBBR and IFAS systems and the key differences with granular reactors, allowing for the interpretation of (seemingly contradictory) published experimental results.

partial nitritation



boundary layer


nitrogen removal


Julio Pérez

Delft University of Technology

Universitat Autonoma de Barcelona (UAB)

Michele Laureni

Delft University of Technology

Aalborg University

Mark C.M. van Loosdrecht

Delft University of Technology

Frank Persson

Chalmers, Architecture and Civil Engineering, Water Environment Technology

David J. I. Gustavsson


Sweden Water Research

Water Research

0043-1354 (ISSN)

Vol. 186 116348

Nitrogen removal from wastewater with anammox for low resource usage and small carbon footprint

Formas, 2015-01-01 -- 2017-12-31.

Driving Forces

Sustainable development

Subject Categories

Water Engineering


Water Treatment


Basic sciences



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