Desalination 174 (2005) 221–230 0011-9164/05/$– See front matter © 2005 Elsevier B.V. All rights reserved *Corresponding author. Sub-critical flux fouling in membrane bioreactors — a review of recent literature Alfieri Pollice a *, Adam Brookes b , Bruce Jefferson b , Simon Judd b a CNR – Istituto di Ricerca Sulle Acque, Viale F. De Blasio 5, 70123 Bari, Italy Tel. +39 (080) 5820531; Fax +39 (080) 5313365; email: pollice@area.ba.cnr.it b School of Water Sciences, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK Received 11 June 2004; accepted 24 September 2004 Abstract Membrane fouling is one of the main factors affecting the diffusion of membrane bioreactors to wastewater treatments. Fouling is generally held to persist above a so-called “critical flux”, below which a steady-state membrane permeability is assumed to be attainable. However, a number of recent studies have demonstrated that low-level fouling can take place at very low fluxes which are substantially below the critical value. This short survey is focused on reported evidences of membrane fouling under sub-critical conditions and examines possible links between operational paramenters, mixed liquor characteristics and fouling propensity. The reported experimental evidences suggest no unequivocal connection between parameters such as permeability decline (dK/dt) and fouling rate (dP/dt) and sustainable long term sub-critical operation. In this respect, test size and plant configuration also play a relevant role. Step-flux experiments may provide information on fouling propensity under sub-critical flux, but the fouling rates determined with this method are generally not applicable to long term operation. Mixed liquor quality determinants such as EPS and SMP and their relative protein/carbohydrate ratios are likely to contribute to sub-critical fouling in MBRs. Possible relationships between these parameters and plant operation (sludge age, biomass concentration, hydraulic retention time, etc.) are briefly discussed. Keywords: Fouling; Membrane bioreactors; Sub-critical flux; Permeability 1. Introduction Critical flux (J c ) was originally defined for microfiltration as a flux below which a decline of permeability with time does not occur and above which fouling is observed [1,2]. However, sub- critical flux fouling in membrane filtration has recently been reported and membrane bioreactors (MBRs) operated at sub-critical fluxes appear to be affected by a similar phenomenon over the long term. In some studies, a noticable change in the rate of transmembrane pressure (TMP) increase was observed after some critical time period,