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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/276454794 Determination of the external mass transfercoefficient and influence of mixing intensity inmoving bed biofilm reactors for...  Article   in  Water Research · May 2015 DOI: 10.1016/j.watres.2015.05.010 CITATION 1 READS 212 6 authors , including: Some of the authors of this publication are also working on these related projects: Biofilm Reactor Modeling   View projectnothing special   View projectBruno NogueiraFederal University of Rio de Janeiro 1   PUBLICATION   1   CITATION   SEE PROFILE Mark Van LoosdrechtDelft University of Technology 1,005   PUBLICATIONS   38,952   CITATIONS   SEE PROFILE Argimiro R. SecchiFederal University of Rio de Janeiro 178   PUBLICATIONS   661   CITATIONS   SEE PROFILE Marcia DezottiFederal University of Rio de Janeiro 108   PUBLICATIONS   2,312   CITATIONS   SEE PROFILE All content following this page was uploaded by Marcia Dezotti on 09 June 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  Determination of the external mass transfercoefficient and influence of mixing intensity inmoving bed biofilm reactors for wastewatertreatment  Bruno L. Nogueira  a,b , Julio P  erez  b , Mark C.M. van Loosdrecht  b , Argimiro R. Secchi  a, *  , M  arcia Dezotti  a , Evaristo C. Biscaia Jr. a a Programa de Engenharia Quı´mica, COPPE  e  Universidade Federal do Rio de Janeiro  e  Centro de Tecnologia,Bl. G-115, Ilha do Fund ~ ao, CEP: 21941-972 Rio de Janeiro, RJ, Brazil b Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands a r t i c l e i n f o Article history: Received 5 February 2015Received in revised form25 April 2015Accepted 5 May 2015Available online 10 May 2015 Keywords: MBBRExternal mass transferMixing intensity a b s t r a c t In moving bed biofilm reactors (MBBR), the removal of pollutants from wastewater is due tothe substrate consumption by bacteria attached on suspended carriers. As a biofilm pro-cess, the substrates are transported from the bulk phase to the biofilm passing through amass transfer resistance layer. This study proposes a methodology to determine theexternal mass transfer coefficient and identify the influence of the mixing intensity on theconversion process in-situ in MBBR systems. The method allows the determination of theexternal mass transfer coefficient in the reactor, which is a major advantage whencompared to the previous methods that require mimicking hydrodynamics of the reactorin a flow chamber or in a separate vessel. The proposed methodology was evaluated in anaerobic lab-scale system operating with COD removal and nitrification. The impact of themixing intensity on the conversion rates for ammonium and COD was tested individually.When comparing the effect of mixing intensity on the removal rates of COD and ammo-nium, a higher apparent external mass transfer resistance was found for ammonium. Forthe used aeration intensities, the external mass transfer coefficient for ammoniumoxidation was ranging from 0.68 to 13.50 m d  1 and for COD removal 2.9 to 22.4 m d  1 . Thelower coefficient range for ammonium oxidation is likely related to the location of nitrifiersdeeper in the biofilm. The measurement of external mass transfer rates in MBBR will helpin better design and evaluation of MBBR system-based technologies. ©  2015 Elsevier Ltd. All rights reserved. 1. Introduction Moving bed biofilm reactors (MBBR) have been used for thebiological treatment of industrial and municipal effluents.They are applied also for the upgrade and retrofit of existing treatment plants (Ferrai et al., 2010). In these systems, thegrowth of microorganisms occurs on carriers which freelymove inside the reactor. This movement can be achieved byaeration or mechanical stirring in aerobic or anaerobic/anoxic *  Corresponding author . Tel.:  þ 55 21 3938 8307; fax:  þ 55 21 3938 8300.E-mail address: arge@peq.coppe.ufrj.br (A.R. Secchi).  Available online at www.sciencedirect.com ScienceDirect  journal homepage: www.elsevier.com/locate/watres water research 80 (2015) 90 e 98 http://dx.doi.org/10.1016/j.watres.2015.05.0100043-1354/ ©  2015 Elsevier Ltd. All rights reserved.  processes, respectively (Rusten et al., 2006). The carriers areretained inside the reactor by a sieve arrangement at thereactor outlet and, therefore, the microorganisms are keptinside the reactor favouring the retention of slow growing bacteria as nitrifiers (Wang et al., 2005; Rusten et al., 2006;Bassin et al., 2011).In biofilm processes, the substrates are transported fromthe bulk phase to the biofilm, where they diffuse through andare consumed by bacteria. The compounds diffusion in andout of the biofilm plays an important role (Rusten et al., 2006),being the performance of the reactor controlled by both con-sumption rate and substrate transport (Rasmussen andLewandowski, 1998). Before reaching the biofilm, the pollut-antspassthroughamasstransferresistancelayer.Withinthebiofilm, the substrates are transported by diffusion due theconcentration gradient generated by the consumption of thepollutants.The external mass transfer resistance is usually describedas a stagnant film between the bulk phase and the biofilmsurface where all external mass transfer processes areincluded (Beyenal and Tanyolac¸, 1998). One important factorwhich affects the external mass transfer to the biofilm is themixing intensity within the reactor (Kugaprasatham et al.,1992; Chen et al., 2006). High mixing intensities increase theexternal mass transfer coefficient causing an increase in themass transfer rate and an improvement on the pollutantremoval performance (Wanner et al., 2006).Several works (Zhang and Bishop, 1994; Stoodley et al.,1997; Rasmussen and Lewandowski, 1998; W € asche et al.,2002; Boessmann et al., 2004) utilized microelectrode mea-surementsforthedeterminationoftheexternalmasstransfercoefficient in different biofilm processes. The use of thismethodologyprovidestheoxygenprofilealongandnearbythebiofilm, which makes it possible to determine the masstransfer coefficients (Rasmussen and Lewandowski, 1998). MicroelectrodemeasurementsonMBBRcarriersrequirefixing the mobile carriers in a flow cell, which influences theexternal mass transfer boundary and make the measure-ments non representative.Despite the crescent use of MBBR, there are few studiesconcerning external mass transfer, and all of them in a nitri-fying system. Hem et al. (1994) found a near first order nitri-fication kinetic for oxygen when it was the limiting substrate.Since the oxygen concentrations studied were above valuesfor substrate half saturation constant, this could be explainedby a strong influence of the external mass transfer in thesesystems. Gapes and Keller (2009) studied the influence of twodifferent carrier types (Kaldnes K1, Natrix C10/10) under twodifferent growth conditions using a titrimetric and off-gasanalysis (TOGA) sensor. Differences in external mass trans-fer coefficient values were observed for biofilms grown underdifferentammoniumloadingrates.Athigherloadingratesthemore heterogeneous biofilm surface resulted in higher masstransfer coefficients. Ma  sic et al. (2010) using BiofilmChip Pcarriermeasuredtheoxygenprofilebymicroelectrodes.Thesemeasurements showed the strong drop in oxygen concen-tration in the boundary layer.In this study, a method to determine the external masstransfer coefficient in-situ in MBBR systems is presented. Themethod employs the same reactor configuration in which thebiofilms are formed at normal operation, i.e., the measure-ments are performed under the same mixing conditions.Moreover, this methodology is also capable to evaluate theeffect of mixing intensity on the external mass transferresistance and consequently on the conversion of the sub-strates by MBBR systems. The studied system was operatedperforming simultaneous nitrification and COD removal inorder to allow comparing the effect of external mass transferresistance on both processes. 2. Material and methods 2.1. Reactor The experiments were performed at a lab-scale moving bedbiofilm reactor operated in continuous mode to remove COD Nomenclature AbbreviationsCOD  Chemical Oxygen Demand DO  Dissolved Oxygen EMT  External Mass Transfer MBBR  Moving Bed Biofilm Reactor vvm  Volume of Gas per Volumeof Liquid per Minute SymbolsA  Total superficial area (L 2 ) D  Coefficient of diffusion (L 2 T  1 )  J  Flux (M L  2 T  1 ) k  External mass transfer coefficient (L T  1 ) L  Thickness of the biofilm (L) q  Substrate specific conversion rate (M M  1 T  1 ) r  Volumetric removal rate (M L  3 T  1 ) R  Volumetricremovalratewithoutexternalmasstransfer resistance (M L  3 T  1 ) S  Mass concentration (M L  3 ) V   Volume of the liquid phase (L 3 ) X  Biomass density (M L  3 ) z  Spatial variable in the biofilm (L) g  Stoichiometric factor d  Boundary layer thickness (L) s 2 Variance of the volumetric removal rate(M 2 L  6 T  2 ) Superscriptsbiof   Biofilm phase BL  Boundary layer bulk  Bulk exp  Experimental interf   Interface water  Water SubscriptsHet  Heterotrophic bacteria Max  Maximum Nit  Nitrifier Bacteria water research 80 (2015) 90 e 98  91  and ammonium nitrogen. The carriers K1 of Anoxkaldnes ® wereusedto performthegrowthofthebiofilm.Thesecarriersare made of polyethylene (density of 0.95 g cm  3 ) and have auseful surface area of 500 m 2 m  3 (or 0.00049 m 2 carrier  1 )(GapesandKeller,2009).Thevolumeofthereactorwas2 Landwas operating under a hydraulic retention time of 2 h withabout 815 carriers (around 40% of filling ratio).Before the beginning of the experiments for determining the influence of external mass transfer, the reactor wasalready being operated for some months using the syntheticmedium detailed in Table 1. A short hydraulic retention timewas used to minimize the contribution of the suspendedbiomass inside the reactor for the performance of the reactor.ThepHwaskeptaround7.5usingphosphorusbuffersolution,the temperature inside the reactor was kept at 20   C using aheat exchanger, and the dissolved oxygen (DO) was keptaround 5 mg L  1 using a air-flow rate controller maintained in1Lmin  1 .Theoff-lineanalysesofN  NH þ 4 ,N  NO  2 ,N  NO  3 and COD were made three times per week spectrophotomet-rically using a commercial cuvette test kit (Hach Lange, Ger-many) to verify the efficiency of the reactor. 2.2. External mass transfer experiments The experiments to determine the external mass transfer in-fluence on the performance of the reactor were performeddirectlyinsidethereactor,andthemajoradvantageisthatthetests are made in the same hydrodynamic conditions andreactor geometry that in the normal operation of the reactor.In these experiments the mixing intensity inside the reactorwas increased until no effect at the removal rate of thepollutant was observed, as can be visualized by the scheme inFig. 1. Since no mechanical stirrer was used in this aerobicsystem, the mixing intensity was evaluated by the gas flowrate(literperminute)pervolumeofliquidinthereactor(liter).Gas flow rate divided by the reactor section (116.5 cm 2 ) couldbe also used to express mixing intensity as gas superficialvelocity. The gas flow rate was composed by air and nitrogengas flow rates, combined to a specified total value for obtain-ing a desired and constant DO concentration for all the ex-periments inside the reactor.In orderto evaluate the influence of external mass transferon COD removal disregarding the effects on the ammoniumoxidation, and vice-versa, independent experiments wereperformed in batch mode. For this, before starting each one of the experiments, the supernatant was removed and newmedium was carefully added inside the reactor. Thecomposition of the media utilized for the tests were similar tothat used for the normal operation of the reactor (Table 2). Inthe tests for COD removal, only the necessary amount of ammonium for assimilatory activity was added at the me-dium.Inthetestsforammoniumoxidation,noorganicmatterwas added and NaHCO 3  was added in excess. The pH andtemperature were kept at the same level than those used atthe normal operation of the reactor, i.e., 7.5 and 20   C,respectively.Each experiment lasted one hour, with sampling time of 15 min. Off-line measurements were made for ammoniumnitrogen and COD. The ammonium concentration in the su-pernatant was determined spectrophotometrically using acommercial cuvette test kit (Hach Lange, Germany). The ace-tate concentration in the supernatant was measured using high-performanceliquidchromatographyandthenconvertedin COD concentration. During the experiments, the DO wasalwaysthelimitingsubstratefortheremovalofthepollutantsand its concentration was measured online and kept at aspecified value.The experimental removal rate ( r exp ) for each experimentwas obtained from the slope of the linear decrease obtainedfrom the values of ammonium nitrogen or acetate concen-trationsasfunctionoftime.Oxygenwasthelimitingsubstrateand the fluxes were, therefore, defined as a function of theoxygen concentration. The calculation of the experimentalflux of oxygen to the biofilm (  J exp O 2 ) was computed by Eq. (1),where  g O 2 = substrate  is the stoichiometric factor between the ox-ygen and substrate uptake,  V  bulk  is the volume of liquid inside Table 1  e Composition of the influent. Substance Value Unit Acetic acid (C 2 H 4 O 2 ) 80.80 mg L  1 NH 4 Cl 66.60 mg L  1 NaHCO 3  64.80 mg L  1 NaH 2 PO 4 .7H 2 O 305.37 mg L  1 KH 2 PO 4  9.05 mg L  1 MgSO 4 .7H 2 O 24.30 mg L  1 CaCl 2 .2H 2 O 12.15 mg L  1 NaOH 22.28 mg L  1 Trace metal solution (Vishniac and Santer, 1957) 0.08 mL L  1 Fig. 1  e  Schematic variation of the substrate removal rateas a function of mixing intensity within the reactor.Table 2 e  Initial Concentrations of acetic acid andammonium chloride on the external mass transferexperiments. Substance Value Unit COD experiments Acetic acid (C 2 H 4 O 2 ) 220 mg L  1 NH 4 Cl 47 mg L  1 Ammonium experiments Acetic acid (C 2 H 4 O 2 ) 0 mg L  1 NH 4 Cl 95.5 mg L  1 water research 80 (2015) 90 e 98 92
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