OXYGEN TRANSFER AND INDUSTRIAL WASTEWATER TREATMENTEFFICIENCY OF A VERTICALLY MOVING BIOFILM SYSTEM

MICHAEL RODGERS, XIN-MIN ZHAN∗ and ANGELA CASEYDepartment of Civil Engineering, National University of Ireland, Galway, Galway, Ireland(∗ author for correspondence, e-mail: Xinmin.zhan@nuigalway.ie, Fax: (+353) 91 750507)

(Received 3 February 2003; accepted 10 July 2003)

Abstract. A vertically moving biofilm system (VMBS) was developed to treat wastewater. In thissystem, the biofilm grows on a biofilm module consisting of plastic media that is vertically andrepeatedly moved up into the air and down into the water. The objectives of this study were toinvestigate the oxygen transfer efficiency and industrial wastewater treatment performance of theVMBS. The oxygen transfer coefficient (KLa) depended on the movement frequency (n) of thebiofilm module and was proportional to n1.67. KLa values measured were within the range of 0.0001to 0.0027 s−1. The VMBS exhibited good carbonaceous removal when treating industrial wastewaterproduced in a factory manufacturing synthetic fibres. Removal efficiency of filtered chemical oxygendemand (COD) and biological oxygen demand (BOD5) was up to 93.2 and 97.9%, respectively.The volumetric removal rates of filtered COD and BOD5 reached 1320 g COD m−3 day−1 and700 g BOD5 m−3 day−1. The areal organic removal rates, based on the surface area of the biofilmsubstrata, were 16 g BOD5 m−2 day−1 and 39 g COD m−2 day−1. No clogging occurred during theexperiment. The mean areal biofilm mass increased with increasing the mean areal BOD5 removalrate. The new biofilm process has such advantages as high carbonaceous oxidation, energy saving,simple construction and easy operation for industrial wastewater treatment.

Keywords: biofilm, carbonaceous oxidation, clogging of biofilm, oxygen transfer, vertically movingbiofim system, wastewater treatment

1. Introduction

Suspended-growth activated sludge processes have been used successfully andwidely to treat municipal wastewater during the past hundred years. However, theyare not always suitable for treating wastewaters from small populations, remotefarms and small industries due to their big land requirement, high running costsand operational complexity.

In comparison with suspended-growth activated sludge processes, biofilm sys-tems have a low land requirement and flexible operation. Biofilm systems includetrickling filters, biological aerated filters (BAFs), rotating biological contactors(RBCs), fluidized bed reactors (FBRs) and moving bed reactors (MBRs). Advant-ages cited for biofilm systems over suspended activated sludge processes includethe following (Ødegaard et al., 1994; Loukidou and Zouboulis, 2001): the treat-ment plant may be more compact due to the availability of biofilm media with highspecific surface area; the operational performance is far less dependent on final

Water, Air, and Soil Pollution 151: 165–178, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

166 M. RODGERS ET AL.

Figure 1. Laboratory biological treatment unit (r1: the industrial wastewater reservoir; r2: thesynthetic wastewater reservoir; P1, P2: peristaltic pumps).

sludge separation in a clarifier; it is not necessary to return sludge to the biologicalreactor; co-existence of aerobic and anoxic metabolic activity within the biofilm;and lower sensitivity to adverse environmental conditions.

A new vertically moving biofilm system was developed and has shown volu-metric filtered chemical oxygen demand (COD) removal rates up to 7.2 kg CODm−3 day−1 (Rodgers, 1999; Rodgers and Burke, 2001). This process is based on theuse of a plastic media module, moving vertically up into the air and down into thewater, while the activated biomass grows as a biofilm on the surface of the module.In comparison with other frequently used biofilm systems, including BAFs, FBRsand MBRs, the main advantages of this vertical moving biofilm system (VMBS)are: no backwashing is required to prevent clogging as occurs in BAFs; no need tocontrol moving-bed expansion due to biofilm growth; and no need for compressedair as is used in BAFs, FBRs and MBRs.

The specific objectives of the present study were to investigate the oxygen trans-fer occurring in the VMBS and its performance in treating industrial wastewater.

2. Methods and Materials

2.1. LABORATORY VMBS UNIT

The laboratory VMBS unit, which is shown in Figure 1, comprised a four-chamberwastewater tank, four biofilm modules and a pneumatic cylinder. The pneumatic

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cylinder moved the modules vertically up into the air and down into the water andthe movement was controlled by pneumatic limit switches and timers.

The entire experimental system was located in a controlled temperature roomat an average temperature of 11 ◦C. The wastewater tank was manufactured in thelaboratory from 10 mm thick polypropylene sheets. It consisted of four identicalseparate chambers, each with a bulk fluid volume of 0.04 m3 and dimensions of400 mm long, 200 mm wide and 500 mm high. Each chamber had a drainage port,and three weirs were located at alternate sides of the panels separating the adjoiningchambers to encourage serpentine flow.

BIOdek (Munters, U.K.), a commercially available media with a specific sur-face area of 150 m2 m−3 and porosity greater than 95%, was used as the biofilmsubstratum. This media consists of corrugated polyethylene sheets seam weldedat the corrugation troughs. The corrugation angle is 30◦ to the horizontal. Thedimensions of each biofilm module were 300 mm long, 300 mm high and 100 mmwide, providing a surface area of 1.35 m2. The modules were held in position by asupport frame manufactured from stainless steel.

For the industrial wastewater treatment, the biofilm modules were moved bythe pneumatic cylinder into and out of the bulk fluid in the chambers in a cycleconsisting of 3 sec static in the bulk fluid, 1 sec coming out of the fluid, 3 sec staticin the air and 1 sec going into the fluid. Consequently, the modules had an overallmotion frequency of 7.5 cycles per minute, which is similar to the motion of aRBC.

2.2. AERATION TEST

The oxygen transfer coefficient (KLa) of a clean biofilm module was measured inaccordance with Zeevalkink et al. (1979). Forty litres of tap water were fed intoone chamber and the water was deoxygenated with sodium sulfite, and cobalt (II)chloride was used as a catalyst. Once the vertical movement of the biofilm modulewas started, dissolved oxygen (DO) concentrations in the water were measureduntil the water was saturated. Values of KLa were calculated from the rate withwhich the dissolved oxygen concentration of the water in the reactor increased.KLa was measured at module movement frequency (n) of 2 to 12 cycles per min.The tap water was replenished after every measurement.

2.3. INDUSTRIAL WASTEWATER TREATMENT

The industrial wastewater used in the study was produced in a factory manufac-turing synthetic fibres. Its pH was adjusted at the factory before collection. Itcontained ethylene glycol and terephthalic acid, which were raw materials used inthe industrial process, and concentrations of pollutants in the wastewater fluctuatedwith the industrial process. Overall, this wastewater exhibited slow biodegradabil-ity since the BOD5: COD ratio was on average about 0.18. Furthermore, comparedwith the mean filtered COD concentrations, which were up to 22730 mg L−1,

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TABLE I

Composition of the synthetic sewage

Constituent Concentration

(mg L−1)

Glucose 266

Yeast 40

Dried milk 160

NH4Cl 80

Na2HPO4·12H2O 133

KHCO3 66.5

NaHCO3 173

MgSO4·7H2O 66.5

FeSO4·7H2O 2.66

MnSO4·H2O 2.66

CaCl2·6H2O 4

Urea 40

Bentonite 53.2

nitrogen and phosphorous concentrations were low with the mean concentrationof ammonium-N 1 mg L−1 and the concentration of phosphate-P 0.

Twenty five litres of sludge, collected from a local extended aeration wastewatertreatment plant treating domestic sewage from a nearby town and animal wastefrom a livestock mart, were divided among the four chambers to provide seed mi-croorganisms for the development of the biofilm. Before the industrial wastewatertreatment was commenced, biofilms were grown on the plastic media by indi-vidually feeding the four chambers with a synthetic sewage for one month. Thecomposition of the synthetic sewage is given in Table I.

When the industrial wastewater treatment was commenced, the four separateoutlet ports were sealed and the weirs between adjourning chambers were opened.The direction of wastewater flow was from Chamber 1 to Chamber 4. Since theindustrial wastewater was slowly biodegradable and was deficient in nutrients,synthetic sewage, which had been used to develop and maintain the biofilm, waspumped into Chamber 1 along with the industrial wastewater so as to supply thenecessary nitrogen and phosphorus for microorganisms. Figure 2 gives the percent-age of the industrial wastewater in the influent in terms of filtered COD and BOD5.The industrial wastewater contribution to the influent was on average 85% filteredCOD and 64% filtered BOD5. The experiment was conducted in two stages withdifferent hydraulic retention times (HRT): the first stage lasted 45 days and its HRTwas 1.1 days; the second stage lasted 64 days and its HRT was 0.75 days.

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Figure 2. Percentage of influent filtered COD (�) and BOD5 (�) contributed by the industrialwastewater.

2.4. METHODS OF ANALYSIS

Filtered chemical oxygen demand (COD) and filtered five-day biological oxygendemand (BOD5) were measured in accordance with the standard APHA meth-ods (APHA, 1998). Filtered samples were obtained by filtering the wastewaterthrough a Whatman GF/C glass microfiber filter paper (pore size 1.2 µm). Dis-solved oxygen (DO) was measured in situ with an electrochemical membraneelectrode (WTW CellOx 325, Wissenschaftlich-Technische Werkstätten GmbH,Germany) and a digital DO meter (WTW Oxi, 330). The electrode was calibratedin accordance with the manufacturer’s procedures. Biofilm mass on the moduleswas measured by weighing the modules on a laboratory scale. The modules weretaken from the chambers and allowed to drip for a period of 5 min before they wereweighed.

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Figure 3. KLa measured versus the module movement frequency (n) using tap water.

3. Results and Discussion

3.1. OXYGEN TRANSFER IN THE VMBS UNIT

The vertical movement of the biofilm modules supplied oxygen to the microor-ganisms present in the biofilm and in the bulk fluid, along with mixing the bulkfluid to make efficient contact between the wastewater and the biofilm. The aer-ation mechanism of the vertically moving biofilm modules is similar to oxygentransfer occurring in RBCs. It includes: oxygen absorption at the liquid film overthe biofilm’s surface when the modules are in the air; direct oxygen absorption bythe microorganisms during the air exposure; and direct oxygen transfer happeningat the air-water interface caused by the turbulence created by the movement of thebiofilm modules.

Measured KLa values at various movement frequencies (n) are given in Fig-ure 3. The results clearly indicate that KLa notably increased with increasing n

and KLa = 4×10−5 n1.67 (The regression coefficient, R2, is 0.996). KLa values

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Figure 4. Profiles of bulk fluid DO in the four chambers for HRT = 1.1 days (�) and HRT = 0.75days (�) when treating industrial wastewater.

are within the range of 0.0001 to 0.0027 s−1, close to KLa of RBCs measured atthe corresponding rotating speeds obtained by Sant’Anna (1980), Wuidar (1994)and Boumansour and Vasel (1998) cited by Boumansour and Vasel (1998).

In the industrial wastewater treatment experiment, average DO concentrationsin the bulk fluid (given in Figure 4) were within the range of 3.0–9.4 mg L−1

and increased along the flow direction, from Chambers 1 to 4. This followed thedecrease of the volumetric organic loading rates along the flow direction, becausemore oxygen was consumed when more organic matter was degraded.

3.2. CARBONACEOUS OXIDATION

Figures 5 and 6 illustrate the overall removal of filtered COD and BOD5, respect-ively. In the steady state operation phase for the two stages, removal efficiency of

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Figure 5. Filtered influent COD (�), effluent COD (�) and COD removal efficiency (�) againsttime.

filtered COD and BOD5 was up to 93.2 and 97.9%, respectively. Filtered COD inthe effluent was never more than 180 mg L−1 though filtered COD in the influentfluctuated greatly around an average value of 2800 mg L−1 and correspondingvolumetric COD loading rates varied within the range of 500 to 1320 g COD m−3

day−1. Filtered BOD5 in the effluent was 7 ± 6 mg L−1 in the steady state operationof HRT = 1.1 days. Shortening HRT from 1.1 days to 0.75 days produced a shock tothe system. When the HRT was shortened to 0.75 days, the effluent filtered BOD5

first increased to about 100 mg L−1 and then decreased gradually. The filteredBOD5 in the effluent in the steady state operation at HRT of 0.75 days was 6 ± 3 mgL−1. From BOD5 values in the effluent, it may be concluded that the remainingfiltered COD in the effluent was mostly non-biodegradable organic matter.

One of the reasons that the studied VMBS system exhibited steady removal oforganic matter is the four-chamber biological treatment train. The four chambersplayed different roles in degrading the organic matter (Table II). During the 108-

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Figure 6. Filtered influent BOD5 (�), effluent BOD5 (�) and BOD5 removal efficiency (�) againsttime.

TABLE II

Carbonaceous oxidation occurring in the four chambers

Chamber 1 Chamber 2 Chamber 3 Chamber 4

Filtered COD removala (%) 68.3 (3.5)b 85.6 (5.3) 90.3 (7.5) 93.2 (14.2)

Filtered BOD5 removala (%) 80.8 (10.2) 93.1 (7.4) 96.2 (5.3) 97.9 (3.5)

Areal filtered COD removal 38.8 10.4 2.7 1.7

rate (g COD m−2 day−1)c (12.4) (7.1) (1.5) (1.5)

Areal filtered BOD5 removal 16.5 2.4 0.7 0.4

rate (g BOD5 m−2 day−1) (8.4) (1.1) (0.7) (0.6)

a Filtered COD and BOD5 removal efficiency was calculated from the influent filtered COD andBOD5 entering Chamber 1.b Mean values are shown in this table and data in the brackets are the standard deviations.c Areal filtered COD and BOD5 removal rates were calculated for each chamber.

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Figure 7. Profile of BOD5 concentrations through the series of four chambers. �, HRT = 1.1 days;�, HRT = 0.75 days.

day operation, the average cumulative removal in Chambers 1, 2, 3 and 4 of filteredBOD5 was 80.8, 93.1, 96.2 and 97.9% and of filtered COD was 68.3, 85.6, 90.3and 93.2%, respectively. There was little difference in the removal of filtered BOD5

and COD from Chamber 2 to Chamber 4. Chamber 1 played a significant role inthe wastewater treatment. Areal removal rates of filtered BOD5 and COD in thischamber, based on the surface area of the biofilm substrata, were up to 16.5 gBOD5 m−2 day−1 and 38.3 g COD m−2 day−1. Figure 7 shows that filtered BOD5

concentrations decreased in an almost exponential manner as the liquid movedfrom chamber to chamber. This implies that the fractional removal from chamberto chamber is almost constant, which is the characteristic of a first-order reaction.This behaviour makes it easy to predict the effect of an additional chamber on theremoval performance.

3.3. GROWTH OF THE BIOFILM

A substantial change was observed in the mass of the biofilm during the first 20days of the study (Figure 8). The biofilm on the modules in Chambers 1 and 2 had a

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Figure 8. Biofilm growth in the four chambers. �, Chamber 1; �, Chamber 2; �, Chamber 3;�, Chamber 4.

grey/cream colour and the biofilm in Chamber 1 produced an unpleasant smell. Thebiofilms in Chambers 3 and 4 were dark brown. The mass of the biofilm in Cham-ber 1 was generally greatest and the biofilms in Chambers 3 and 4 showed leastgain in mass. When HRT decreased from 1.1 days to 0.75 days, the biofilm massin Chambers 1 and 2 rose sharply and the maximum biofilm mass was up to 4500and 3000 g, respectively. Sloughing of the biofilm mass occurred in Chamber 1 onthe 84th day and in Chamber 2 on the 105th day.

The growth of biofilm on the plastic modules is the net result of various pro-cesses such as adsorption, desorption, attachment, microbial growth and detach-ment (Peyton and Characklis, 1992). The relatively stable biofilm mass in Cham-bers 3 and 4 probably resulted from the balance between microbial growth and

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Figure 9. Dependence of biofilm mass on BOD5 removal for HRT = 1.1 days (�) and HRT = 0.75days (�).

the detachment. Microbial growth was caused by the utilization of the organicsubstrates. However, the detachment of biofilm is still one of the least studiedand least understood processes amongst the mechanisms controlling immobilizedcell bioreactor performance (Gikas and Livingston, 1999). Different processes areresponsible for the detachment of biomass from biofilms and four categories ofdetachment process can be distinguished: (1) abrasion; (2) erosion; (3) sloughing;and (4) predator grazing (Morgenroth and Wilderer, 2000; Gikas and Livingston,1999). Erosion is the continuous removal of the surface of the biofilm by liquidshear stress. Abrasion is caused by collision of biofilm solid particles. Sloughingis a discrete process, and often occurs in relatively old and rather thick biofilmsand is the detachment of relatively large flocs with characteristic size comparableto or greater than the thickness of the biofilm itself. Predator grazing refers to theconsumption of bacteria from the external surface of the biofilm by protozoa. In the

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VMBS, the vertical movement of the biofilm module with the speed up to 0.4 m s−1

produced the hydraulic shear forces. Hence, erosion was likely to be the dominantdetachment process in Chambers 3 and 4. Cheng et al. (1997) observed that in athree phase draft-tube fluidized bed using granular activated carbon (GAC), whilethe mean liquid velocity increased from 0.12 to 0.16 m s−1, the biomass attachedonto the GAC decreased from 30.4 to 15.6 mg VSS g−1 GAC. Similarly, Trulearand Characklis (1982) found that biofilm detachment rate occurring on an annularreactor increased with rotational speed.

Figure 9 gives the dependence of the mean areal biofilm mass in the four cham-bers, which was based on the surface area of the biofilm substrata, on the meanareal BOD5 removal rate. The mean areal biofilm mass increased with increasingthe mean areal BOD5 removal rate. This is also seen in Figure 8. When the meanareal BOD5 removal rate was below 3 g BOD5 m−2 day−1, the mean areal biofilmmass increased linearly with the mean areal BOD5 removal rate. However, whenthe mean areal BOD5 removal rate was higher than 3 g BOD5 m−2 day−1, theincrease in the biofilm mass slowed down.

4. Conclusion

From the studies on the oxygen transfer and industrial wastewater treatment usinga VMBS system at 11 ◦C, the results shown below were obtained:

(1) The oxygen transfer coefficient (KLa) of the vertically moving biofilm modulewas dependent on the movement frequency of the biofilm module and gavevalues from 0.0001 to 0.0028 s−1, which are similar to those of RBCs.

(2) This process efficiently removed slowly biodegradable organic pollutants.Filtered COD and BOD5 removal efficiency was up to 93.2% and 97.9%,respectively.

(3) The volumetric removal rates of filtered BOD5 and COD for the whole systemwere up to 700 g BOD5 m−3 day−1 and 1320 g COD m−3 day−1, respectively.For an individual biofilm media module, the areal filtered BOD5 and CODremoval rate was 16 g BOD5 m−2 day−1 and 53 g COD m−2 day−1.

(4) The biofilm mass growth on the biofilm module was dependent on the organicloading rate and the areal BOD5 removal rate. The mean areal biofilm massincreased with the mean areal BOD5 removal rate. Clogging, which occurs inother biofilm processes, was not found in the operation of the system for 108days.

This study has the potential to supply a simple and efficient system for industrialwastewater treatment.

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