Bentzen, Thomas Ruby

Christensen, Morten Lykkegaard

Jørgensen, Mads Koustrup

Membrane bioreactors (MBR) are an attractive alternative solution for municipal and industrial wastewater treatment. The MBR, which is a combination of a bioreactor for sludge degradation and a membrane for separation, has the advantages of a low footprint, ability to handle high sludge concentrations and a clear effluent with no bacteria present in the permeate [1]. However, the process performance is limited by membrane fouling, which results in a lower productivity and higher energy demand and hence places demands for limitation of fouling and/or cleaning of the membranes. One way to do this is to enhance the shear on the membrane surface by either air scouring or by generation of a crossflow [1], e.g. by membrane rotation. However, this is associated with high energy consumption, and the gain in permeability should compensate for the additional energy requirements in order to be competitive to conventionally activated sludge systems. Therefore, it is essential to understand the impact of enhanced shear on filtration performance.The influence of shear and sludge concentration on the limiting flux is investigated using the limiting flux as a measure of the degree of fouling. The utilized MBR system uses rotating ceramic membrane discs for creation of shear, which can be changed by controlling the membrane rotation speed of the membrane. Furthermore, the influence of shear on fouling is studied at different radii from the center of rotation, by dividing membranes into different concentric rings. An empirical model is proposed to describe limiting flux as a function of shear stress and concentration. The shear stress is calculated from the local shear rate on the membrane disc, expressed as a function of rotation speed and radial distance [2]. The activated sludge is considered to have non-Newtonian shear-thinning properties in the calculation of shear stresses from the shear rates [3]. The modeled limiting flux fits the experimental limiting flux well at varying sludge concentration, rotation speed and radial distance on the disc. The limiting flux increases with rotation speed and radial distance and decreases with increasing sludge concentration. From the proposed model, the filtration performance can be estimated as a function of shear (rotation speed) and concentration in order to improve filtration performance and productivity.References1. A. Drews, J. Membr. Sci., 363, 1–28 (2010).2. R. Bouzerar, L. Ding, M. Jaffrin, J. Membr. Sci., 170, 127-141 (2000).3. S. Rosenberger, K. Kubin, M. Kraume, Eng. Life Sci., 2, 269-275 (2002).<br/

    Aalborg UniversitetFouling in a MBR system with rotating membrane discsJørgensen, Mads Koustrup; Bentzen, Thomas Ruby; Christensen, Morten LykkegaardPublication date:2014Document VersionEarly version, also known as pre-printLink to publication from Aalborg UniversityCitation for published version (APA):Jørgensen, M. K., Bentzen, T. R., & Christensen, M. L. (2014). Fouling in a MBR system with rotating membranediscs: Influence of concentration and shear. Poster session presented at ICOM 2014, Suzhou, China.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: April 25, 2017  • The limiting flux is used to describe the back transport, depending on operating conditions such as concentration and shear.  • Fouling is assumed to be due to reversible cake formation • A mass balance for particle transport to the membrane is solved to give the limiting flux [1]. • For larger particles (< 1 µm), e.g. sludge flocs and colloids, back transport is a result of shear induced diffusion Fouling in a MBR system with Rotating membrane discs – influence of concentration and shear Mads Koustrup Jørgensen1*, Thomas Ruby Bentzen2, and Morten Lykkegaard Christensen1.     1 Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, Sohngaardsholmvej 57, 9000 Aalborg, Denmark  2 Department of Civil Engineering, Aalborg University, Sohngaardsholmvej 57, 9000 Aalborg, Denmark (*mkj@bio.aau.dk) The following empirical expression for the limiting flux is proposed:   Shear stress (τ) is related to shear rate (γ̇) with the following expression for non-Newtonian fluids [2]:   With m and n representing the concentration dependent flow consistency and behavior index. The local shear rate is determined as a function of rotation speed (Ω) and kinematic viscosity (υ) [3]:    The shear dependency of the dynamic viscosity (μ) is determined as      • Four membranes were blinded to have concentric rings at different radii open for filtration and used to filter 10 g/L MBR sludge at rotation speeds varying from 100-280 rpm.      • The influence of concentration was studied filtering 4.7, 10 and 12.7 g/L SS with varying rotation speeds and TMP.  • For each operational condition, TMP-step filtrations in the range of 0.3 – 2.5 bar were performed to determine the limiting flux [1]. Experimental The functionality of a membrane bioreactor (MBR) is limited by fouling, which places demands for fouling reduction and membrane cleaning. Membrane fouling can be reduced by rotating membrane discs which enables stable operation at high sludge concentrations. As membrane rotation is associated with high energy consumption the gain in permeability by higher membrane rotation should compensate for the additional energy requirements. Therefore, it is essential to understand the impact of enhanced shear on filtration performance.     Study limiting flux distribution at varying rotation speed and sludge concentration.  Develop a mathematical model to describe limiting flux as function of membrane disc rotation speed and sludge concentration.   Describe and simulate flux as a function of rotation speed, sludge concentration and the radial distance from the center of rotation.  Introduction    Conclusion  • The developed model describes the limiting flux (back transport) as function of disc rotation speed and sludge concentration.  • The limiting flux increases with higher radius from the center of rotation and higher membrane rotation speed due to enhanced shear • The limiting flux decreases at high concentrations;  • From the proposed model, the filtration performance can be estimated as a function of shear (rotation speed) and concentration and used to improve filtration performance and productivity. ACKNOWLEDGEMENTS This study is funded by the EcoDesign MBR strategic research center. Malene Thostrup Pedersen is acknowledged for experimental assistance. REFERENCES 1. M.K. Jørgensen, T.V. Bugge, M.L. Christensen, K. Keiding, J. Membr. Sci., 409-410, 335-345, (2012). 2. S. Rosenberger, K. Kubin, M. Kraume, Eng. Life Sci., 2, 269-275 (2002). 3. R. Bouzerar, L. Ding, M. Jaffrin, J. Membr. Sci., 170, 127-141 (2000). Aim  Theoretical Background  Model • Due to higher shear, the limiting flux increases at higher rotation speed and higher radial distance from center of rotation.  • The limiting flux decreases with higher sludge concentration.  • The model fits experimental limiting fluxes   • The proposed model can describe the limiting flux from concentration and rotation speed locally on the membrane discs.  Results 

Fouling in a MBR system with rotating membrane discs:Influence of concentration and shear

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Fouling in a MBR system with rotating membrane discs:Influence of concentration and shear

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