Desalination 199 (2006) 490–492

Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy.

Influence of loading rates on production and characteristicsof retentate from a biofilm membrane bioreactor (BF-MBR)

Igor Ivanovic, TorOve Leiknes*, Hallvard ØdegaardNTNU – Norwegian University of Science and Technology, Department of Hydraulic and Environmental

Engineering, S.P. Andersensvei 5, N-7491 Trondheim, Norwayemail: torove.leiknes@ntnu.no

Received 22 October 2005; accepted 4 March 2006

1. Introduction

A hybrid biofilm membrane reactor basedon the moving-bed-biofilm reactor (MBBR)combined with solids separation in animmersed membrane reactor (IMR) has beeninvestigated for the treatment of municipalwastewater. This process enables the design ofcompact treatment plants with high biodegrada-tion efficiencies that produce high quality efflu-ents. Retentate characteristics play an importantrole in the efficiency of the solids separationand subsequently the overall performance ofthe membrane unit. The aim of this study was toinvestigate and compare characteristics of theretentate in the membrane reactor as a functionof organic loading rates in the bioreactor of thisBF-MBR process.

2. Experimental setup and methods

A schematic of the process configuration andpilot plant studied is shown in Fig. 1. Municipalwastewater is mechanically pretreated (primarysedimentation) before being fed to the pilot plant

where biodegradation takes place in the MBBRfollowed by solids separation in the IMR. Fourbiofilm reactors placed in series were used toallow for operation with varying organic loads,each with a volume of 60 L. The membranereactor had a volume of 27 L and contained 1 m2

of membrane area. The pilot plant was operatedunder two loading rates; a low-loading rate wherenitrification occurs (HRT ~ 4 h, average organicloads COD = 12.1 g O2/m

2d, FCOD = 6.3 g O2/m2d) and a high-loading rate where only BODremoval occurs (HRT ~ 1 h, average organicload COD = 47.9 g O2/m

2d, FCOD = 27.3 g O2/m2d). The membrane unit was operated with aconstant flux of 50 L/m2 h with a 96% recovery ina 10 min cyclic mode consisting of 9.5 min pro-duction with a 0.5 min backwash sequence. Dewa-tering and filtering characteristics were measuredby capillary suction time (CST) and time to filter(TTF) analysis and further related to suspendedsolids concentrations (SS) and organic matter(COD/FCOD), and particle size distributions(PSD) according to standard methods. The mem-brane performance was measured by recording thetransmembrane pressure (TMP) development overthe operating time.

*Corresponding author.

doi:10.1016/j.desal.2006.03.1960011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

I. Ivanovic et al. / Desalination 199 (2006) 490–492 491

3. Results and discussion

Results for the two operating conditionstested show that average CST and TTF-25values were higher for high-rate operating con-ditions compared to the low-rate case (Fig. 2)indicating poorer dewatering and filtering char-acteristics of the retentate. Higher solubleorganic matter (FCOD) concentrations werealso found in the high-rate case. PSD analysisalso showed higher relative number % of sub-micron particles (Fig. 3). The results are inagreement with reported values for sludgecharacteristics giving poor settling and filter-ing characteristics [1].

Microscopic observations showed inherentlydifferent floc structures in the low and high-rateretentate. The high-rate retentate showed a highernumber of filamentous bacteria, smaller flocsand less compact particles compared to the low-rate retentate. The microscope observations are inagreement with measurements of CST and lead tothe conclusion that floc structure and number offilaments present a significant effect in dewateringcharacteristics. Around 10% less sludge produc-tion, expressed as SS, was also measured duringthe low-rate.

Membrane performance also varied withoperating conditions where higher fouling rates

Fig. 1. Schematic of process configuration and pilot plant layout.

Fig. 2. Average measured values in retentate during thelow and high-rate operating cycles.

Fig. 3. Particle size distribution (PSD) of the retentateas a function of operating condition.

492 I. Ivanovic et al. / Desalination 199 (2006) 490–492

were observed during the high-rate conditions.A fouling parameter, represented as average dailyTMP decline (DTMP, Fig. 2), was 2.5 timeslower during the low-rate conditions compared tohigh-rate, giving approximately twice as longoperation before chemical cleaning. The result isin agreement with other studies in which the maincontribution to membrane fouling was attributedto the soluble organic fraction and concentrationsof submicron particles [2,3].

4. Conclusion

Operating the BF-MBR process with loworganic loading rates appears to produce a reten-tate with more favorable characteristics withrespect to dewatering, filterability and sludge pro-duction compared to high loading rates. Lowermembrane fouling rates are also observed, i.e. lessfrequent cleaning required. From the results in thisstudy the differences in dewatering and filteringcharacteristics and fouling rates may be attributedto higher concentrations of soluble organic matter,higher relative number % of submicron particles,

undesirable floc structure with higher number offilaments found in the high-rate retentate com-pared to low-rate operation. The retentate charac-teristics for the low-rate condition also are morefavorable for further sludge treatment, i.e. easierand less expensive to process.

References

[1] R.P. Merlo, R.S. Trussell, S.W. Hermanowicz andD. Jenkins, Physical, chemical and biologicalproperties of submerged membrane bioreactorand conventional activated sludges, WEFTEC®,2004, 1–18.

[2] T.H. Bae and T.M. Tak, Interpretation of foulingcharacteristics of ultrafiltration membranes duringthe filtration of membrane bioreactor mixedliquor, J. Membr. Sci., 264 (2005) 151–160.

[3] R.M. Åhl, T. Leiknes and H. Ødegaard, Trackingparticle size distributions in a moving bed biofilmmembrane reactor for treatment of municipalwastewater, in: Proceedings, IWA InternationalConference on Particle Separation 2005, 1–3 June2005, Seoul, Korea. Accepted in Water Scienceand Technology.