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Domestic wastewater treatment usingmicroaerophilic upflow sludge bed reactorSaibal Kumar Basu a & Takashi Mino ba Environmental Engineering Division, Department of Civil Engineering ,University of Manitoba , Winnipeg, Manitoba , R3T 2N2, Canadab Associate Professor, Environmental Engineering, Department of UrbanEngineering , The University Of Tokyo , 7–3–1 Hongo, Bunkyo, Tokyo, 113,JapanPublished online: 17 Dec 2008.

To cite this article: Saibal Kumar Basu & Takashi Mino (1993) Domestic wastewater treatmentusing microaerophilic upflow sludge bed reactor, Environmental Technology, 14:5, 413-422, DOI:10.1080/09593339309385309

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Environmental Technology, Vol. 14. pp 413-422© Publications Division Selper Ltd., 1993

DOMESTIC WASTEWATER TREATMENT USINGMICROAEROPHILIC UPFLOW SLUDGE BED

REACTOR

SAIBAL KUMAR BASU1* AND TAKASHI MINO 2

1Environmental Engineering Division, Department of Civil Engineering, University of Manitoba,Winnipeg R3T 2N2, Manitoba, Canada

2Associate Professor, Environmental Engineering, Department of Urban Engineering, TheUniversity Of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan

(Received 5 August 1992; Accepted 5 February 1993)

ABSTRACT

Microaerophilic Upflow Sludge Bed Reactor (MUSB) is a new wastewater treatment processoriginally developed as Multi-Stage Reversing Flow Bioreactor (MRB) in Japan. The processrelies on the interaction between sulphate reducing bacteria (SRB) and microaerophilic sulphideoxidizing bacteria (SOB), Beggiatoa which dominates in organic matter removal because of alimited oxygen supply in a sludge blanket type bioreactor. A five stage laboratory MUSB reactorwith a volume of 173 litres was operated for 120 days using settled domestic wastewater from thecampus of Asian Institute of Technology, Bangkok, Thailand. The objective of this study was todefine the system under various hydraulic retention time and organic loading conditions. Thestabilization of organic matter was found to be essentially via assimilation into biomass and tosome extent through aerobic metabolism by the use of dissolved oxygen added in the aerobicvessels. At an HRT of 4.5 hours and organic loading of 2 Kg COD m-3 d-1, maximum efficiencycorresponding to 93 % and 94 % of total and filtered COD removal were obtained. Mass balancestudies indicated a very low sludge production. By introducing plastic tube media in the 4th and 5thaeration vessels, low effluent solids and negligible sulphide concentration could be maintained.Presence of a series of sludge blankets ensured a very good effluent quality and avoided the needfor a settling unit. Sulphur reducing/oxidizing activity were also detected.

Keywords: Microaerophilic Upflow Sludge Bed Reactor (MUSB), Beggiatoa, sulphate reduction,sulphide oxidation, sludge production

INTRODUCTION a near anaerobic conditions favouring the growthof sulphate reducing bacteria (SRB) in the BV.

The Multi-Stage Reversing Flow Bioreactor The low oxygen microgradient (oxygen(MRB) developed in 1986 by the Public Works concentration less than 10 micromoles or 0.34Research Institute in Japan, utilizes the mg I"1) also creates an ideal environment forinterrelationship among micro-organisms sulphide oxidizing bacteria (SOB), Beggiatoarelated to sulphur metabolism in organic matter which are microaerophilic and utilize most of theremoval (1,2). The MRB consists of several oxygen due to their higher consumption ratestages of interconnected downflow aeration compared to organic substrate oxidizing bacteriavessels (AV) and upflow biological vessels (BV) (1). Little suspended solids remain in the AVin series (Fig. 1). Influent enters the first AV while the BV retain those suspended solids whosewhich supplies oxygen via air diffusers placed at settling velocities are higher than the upflowthe bottom and subsequently flows to the velocity of wastewater in the BV. Limited oxygendownstream BV. Dissolved Oxygen (DO) is conditions and interaction between SRB and SOBlimited to the saturation concentration and is results in the formation of self-granulatedrapidly consumed in the BV when organic sludge (1). Once organic substrate is removed insubstrate in the wastewater is high. This creates the BV's the downstream vessels start receiving

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Influent(DomesticWostewoter)

Rotation Speed1.5-2.5 rpm

14cm Internal Dia

(Typ) 14 cm (Typ)

Q.

= - — E f f l u e n t

Drain

(Typ)

AV

BV

BV,

Recycle Line

. Air Oiffuser

MMr* Paddles

Aeration Vessel

BWooicol Missel

Operotionol Detail*

Tblol Effective VWume • 173 Litres

Wo*tew«ter Flow Rates • 0 .64 -1.13 L/m

Figure 1. Laboratory scale MUSB unit.

low organic loads where oxygen is not fullyconsumed. This results in the inability of thedownstream BV's to maintain anaerobic bacteriathat thrive under low Oxygen Reduction Potential(ORP) and are therefore replaced by aerobicmicro-organisms. Successful accumulation ofaerobic sludge in the downstream BV's results invery good effluent quality (1).

Low sludge production, ability to operatewithout a secondary clarifier, overcoming longstart-up time associated with conventionalanaerobic processes and comparable effluentquality with aerobic processes are the majoradvantages associated with the MRB process. Onthe other hand, the success of the process in bothlab and pilot scale (1,3) has been mainlyrestricted to Japan and no. full scale operatingexperience is available. The essential principleof MRB is to provide the upflow mode formicroaerophilic granulation and therefore theprocess will be called Microaerophilic UpflowSludge Bed Reactor (MUSB). The papersummarizes the major findings of the researchon the potential of this process in treatment ofdomestic wastewater (4).

MATERIALS AND METHODS

Apparatus

A laboratory scale 5 stage MUSB reactor ofeffective volume 173 litres was constructed of a

14 cm ID acrylic tube. A schematic diagram ofthe lab scale MUSB system used is shown inFigure 1. The aeration (AV) and biologicalvessels (BV) had an internal diameter of 14 cmand were 1 meter and 2 meters high respectively.The BV's had tapered bottoms to impart higherupflow velocity to incoming wastewater while theoutlet pipes of AV's were extended into the BV'sfor better distribution of wastewater and to avoidshort-circuiting. The BV's 2 to 4 were providedwith mixing paddles rotating at the rate of 1.5 -2.5 rpm to avoid bridging of sludge and tofacilitate biomass flocculation in these vessels,100% recycling of wastewater was done with apump from top of BV4 to bottom of BV2.

Air was supplied via air diffusers at thebottom of all AV's and air flow rates werecontrolled to maintain a residual DOconcentration of 5-6 mg I'1 in the effluent ofAVs. Several sampling ports along the height ofvarious vessels facilitated withdrawal of reactorcontents for analysis. Due to very low strength ofAIT wastewater (COD = 100-150 mg I"1), a dilutesolution of acetic acid was added directly into thewastewater stream entering the system. Thecharacteristics of the influent is given in Table1.

Start-Up

The start-up of a system like MUSB is shortand less complicated as compared to

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Table 1. Influent wastewater characteristics.

Parameter

COD (Total)COD (Filt)TDCpHS3VSSNH3-NTKNNO3 & NO2 - NTotal - PSulphateSulphideOil & Grease

Unit

mgl"1

mgl'1

mgCl"1

—m g Hmgl"1

mg I"1

mgl'1

nig I"1

mg 1-1mgl-1

mgl"1

PPm

Range

159 - 66484 - 41953 -1385.9 - 7.620 -1958-1477-1710-21

Negligible0.8 - 2.735 -1822.5 - 3.570 -150

Average

450350120

6.655461216

—1.6

603.0

110

TDC = Total Dissolved Carbon; TSS.VSS = Suspended and Volatile Suspended Solids

conventional high rate anaerobic systems inpractice. Although domestic wastewater containsall the necessary micro-organisms, eachbiological vessels was inoculated with freshanaerobically digested sludge from a domesticwastewater treatment plant in Bangkok. Thesludge was screened through a sieve (12 openingsper inch), slightly diluted with tap water to aflowing consistency (TS = 39 g H, TVS = 22 g H)and was introduced to each BVs to a final volumeof 2.5 1.

Experimental Program

Batch experiments:The performance of MUSB is essentially

dependent on the activities of SRB and SOB inorganic matter removal. In order to study thesulphate reducing activity, sludge samples weretaken from the anaerobic zones of the BVs in a

one litre conical flask and purged with nitrogengas. Additional sulphate and carbon source in theform of Na2SO4 and acetic acid were added and theflask properly stoppered. The changes insulphate concentration were monitored at onehour intervals till a steady concentration wasreached. Activity was quantified in terms of mg

Continuous feed experiments:Table 2 summarizes the operational

conditions for the MUSB system in this study.The whole study was basically carried out inthree phases during which the system was studiedunder different hydraulic and organic loadings.The efficiency of the process was evaluated interms of Total and Filtered COD along with TSS.The parameters shown in Table 1 were regularlymonitored and analyzed in accordance withStandard Methods (5).

Table 2. Experimental conditions.

Operating Phase:Experimental Period (days)Flow rate (1 min"1)HRT (h)Organic Loading (Kg COD m'3 d"1)Upflow Velocity (m d"1)

BV1 & BV5BV2, BV3 & BV4

Recycle Ratio

I62

0.644.50

0.45-1.92

601201.00

II33

0.8253.50

2.5-2.9

771531.00

III25

1.152.50

3.7-3.96

1082151.00

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Estimation of sludge production by materialbalance

The amount of sludge produced in anywastewater treatment process is an importantaspect since it affects the design of sludgehandling and disposal systems consequentlydictating overall treatment costs. In order toestimate the biomass yield in a process likeMUSB, nitrogen and phosphorus balance (Fig. 2)was made three times during each phase underdifferent organic loadings. These two elementswere chosen since they are required as principalnutrients for proper functioning of anybiological treatment system and MUSB being anopen system did not permit COD or carbonbalance.

From the material balance expressed inFig. 2 we have:

( i i )P0-Pc=ACYpPx

No = Influent Total Nitrogen, mg I'1; Ne =Effluent Dissolved Nitrogen, mg I"1; AC =Organic matter removed, mg I'1; YJJ = YieldCoefficient from Nitrogen balance; Nx =Nitrogen content of cell tissue, mg mg"1; Nd =Nitrogen loss through denitrification, mg 1-1.

Similarly we have, Po = Influent TotalPhosphorus, mg I"1; Pe = Effluent DissolvedPhosphorus, mg I*1; Yp = Yield Coefficient fromPhosphorus balance; Px = Phosphorus content ofexcess sludge, mg mg*1.ACYNNx and ACYpPx

represents the amounts of nitrogen andphosphorus assimilated to cells. It was seenfrom experiments that the extent ofdenitrification in the system was negligible. Itwas thus assumed that the amount of Nitrogenremoved was utilized for cell building processand an average cell tissue composition ofC5H7Q2N in which nitrogen constituted 12.4 % byweight (Nx) was considered for calculation. TP =Total-P, mg I"1; DP = Dissolved-P, mg I'1 andMLSS (Mixed Liquor Suspended Solids), mg I"1

were calculated from samples taken from AVs.From equations (i) and (ii) the cell yield Y*j andYp were thus calculated as follows:

N 0 - N e - N d N 0 - N eN " AC.NX "0.124 AC

as Nd = 0 and Nx = 0.124

P « - P e

where(TP)-(DP)

MLSS

(iii)

(iv)

(v)

Nitrogen Produced byDenftrlficotlon 2 Nd

Influent TotalNitrogen

MUSB

Nitrogen Utilized inAssimilations

AC. Y. N,

Effluent TotalDissolved Nitrogen:

N.

MUSB

AC • Organic Matter Removed

p • Influent Total Phosphorus

|> • Effluent Olssolvtd Phosphorus

Px. Phosphorus Content of Excess Sludge

.Phosphorus UtilizedIn Assimilation • Biomass

Produced-ACY-P,

Figure 2. Nitrogen and phosphorus balance.

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RESULTS AND DISCUSSION

COD and SS removal

The viability of the MUSB system in treatingdomestic wastewater was evaluated on the basisof effluent total and filtrate COD along with SSduring 120 days of operation. These results areshown in Figures 3, 4 and 5 respectively. Afterinitial washout of the particles of lighter seedsludge, the effluent quality greatly improved inPhase I and the system apparently achieved asteady state in 10 days in terms of effluent CODconcentration. The best COD removalperformance was observed during 50-60 days ofoperation in which total and filtrate COD removalwere 93% and 94% respectively with SS in therange of 8-10 mg I*1 although no granulationoccurred. This high COD and SS removal is partlydue to the insertion of plastic media which will bediscussed in a separate section. These resultswere comparable to results of any conventionalaerobic processes.

On day 63 (start of Phase II), the HRT wasreduced from 4.5 to 3.5 hours causing somesludge washout from the system. Due to atechnical difficulty in reducing upflow velocityin BV5 to control the biomass loss, a recycle linewas started which cycled mixed liquor frommiddle to bottom of BV5. The modificationworked well except that some SS mainlycomposed of Beggiatoa persisted in the effluent.A steady state was achieved after 80 days withtotal and filtrate COD removal of 87% and 91%

respectively and a SS concentration of about 35mg I'1 in the effluent. It was evident that thesystem was performing quite well and the lowerpercentage removal of total COD should be due tothe presence of Beggiatoa in the effluent. On day96 the HRT was further reduced to 2.5 hours forPhase III which caused a deterioration in theeffluent quality due to some sludge washout.Analysis done over a week during steady state(110 - 120 days) gave a removal efficiency of 86%and 89% respectively for total and filtrate CODwith average effluent SS concentration of 33 mg 1"1 in the effluent. On the basis of above results wecan say that the system performed extremely well ~at a HRT of 4.5 hours although very high removalsof COD (89 %) could still be achieved even at anHRT of 2.5 hours with organic loadings in therange of 3-4KgCODm^d"1. With lower HRT of3.5 and 2.5 hours however some sludge washouttook place which is not desirable for this processas it operates without any secondarysedimentation tank. Due to major organicremoval in the upstream BV's, aerobicconditions in the last BV could be maintained.

Effect of Organic Loading

To study the effect of organic loading on theperformance of MUSB, the strength of influentwastewater was varied by controlling the aceticacid feed. Since the major organic constituent ofthe influent was in soluble form, the COD loadingwas calculated on basis of the soluble componentas most of the incoming SS was successfully

"0 10 20 30 40 50 60 70 80 90 100 110 120Days of Opera fion

Figure 3. COD (total) removal pattern.

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500

400

, I ,10 20 30 40 50 60 70 80 90 100 110 120

Days of Operation

Figure 4. COD (Filt) removal pattern.

Phase III

10 20 30 40 50 60 70 80 90 100 110 120Days of Operation

Figure 5. Suspended solids (SS) removal pattern.

entrapped in the sludge blanket of BV1. Duringthe 120 days of study the organic loading wasvaried from 0.45 - 3.96 Kg COD m"3 d"1 andcorresponding percentage removal were plottedin a graph shown in Figure 6. It can be seen in theabove figure that the maximum removal oforganics (94%) takes place at a loading of about 2Kg COD nr3 d'1 though high removal of 89% couldstill be achieved at loadings of about 3.9 Kg CODm"3 d"1. As already seen from Figure 4, theincrease in organic loading at the start of PhaseII did not affect the effluent quality significantly

and therefore accounts why such a higherpercentage removal could be effected.

Effect of Plastic Media in Aeration Vessel

After successful operation of MUSB for aperiod of 21 days (Phase 1) it was observed that theSS in the effluent increased and varied between17 - 29 mg I'1 and were mainly composed of SOB,Beggiatoa which also contributed to a higheffluent total COD of 47 - 83 mg I"1. Although somesulphide (1.5 - 2.1 mg I"1) were detected in the

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100

90

o

80

70

o COD (Filtrate)

o 9

HRT 4.5 h

J_1 2 3Organic Loading • Kg COD m3 d l

Figure 6. Effect of organic loading.

effluent this did not affect the COD measurementsince it was removed under highly acidicconditions prior to analysis. It was realized thatthe AV's provided a potential for trapping SOBwhich would also oxidize sulphide. To maintainSOB in the system, plastic tube media of length2.8 cm and 2.5 cm dia were introduced on day 40in AV4 and AV5. Figure 7 outlines the effect ofsuch a media on sulphide removal. No sulphideremain in the effluent and those produced in BV3and BV4 were effectively removed in AV4 &AV5. The quality of the effluent improvedsignificantly and the effect on effluent SS andTotal COD just before and after media insertionis shown in Figure 8.

Sludge Production in MUSB

In order to get an idea of sludge produced inthe MUSB process and compare it to the amount ofexcess sludge produced from conventionalaerobic and anaerobic processes, nitrogen andphosphorus balance as explained earlier wereapplied. From the study of nitrogen profileduring each phase it was seen that denitrificationwas negligible and hence it was concluded that thetotal nitrogen removed was equal to the amount ofnitrogen assimilated in the biomass. The resultsof such material balance are summarized inTable 3. The value of Yield Coefficient (Y)ranged from 0.106 - 0.157 and were hardly

3.5 -j

*fc 2 ,1CO

I-

0.5-

0

I Without Media EZ3 With Medial -o, l-b rtfert to firstaeration & biologicalvessels respectively

Jnf l-a l-b 2-o 2-b 3-o 3-b 4-a 4-b 5-a 5-bSampling Points

Figure 7. Effect of plastic media on sulphide removal in MUSB.

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100

40 49 55Days

Figure 8. Effect of plastic media on effluent quality.

62

affected by changes in hydraulic and organicloadings. From these set of values a typical valueof Y = 0.13 was established with a standarddeviation of 0.014. When compared to otherprocesses, Y value for MUSB can be seen to bemuch less than a conventional aerobic processlike activated sludge although it was greater thanthat of an anaerobic process like anaerobicdigestion (Table 4). During the 120 days ofoperation, no sludge wasting was carried out.However it is recommended to withdraw excess

sludge in practical operation of MUSB process.

Granulation in MUSB

Development of a granular sludge werereported from studies on MRB in Japan (1,3).After operating the present MUSB reactor withwastewater from AIT campus, no granulationwas observed. The sludge however exhibited ahighly dense flocculated characteristics withvery good settleability (Average SVI=16 ml g -1).

Table 3. Summary of calculated yield coefficient (Y).

Phase SI. No.

1I 2

3

1II 2

3

1III 2

3

Table 4. Comparison of Y values

Processes

Anaerobic DigestionConventional Activated SludgeMUSB (present study)

Y(Nitrogen Balance) Y(Phosphorus Balance)

0.1320.1230.130

0.1330.1310.127

0.1190.121

among different processes.

Range of Y values

0.04 • 0.100.40 - 0.80

0.106 - 0.157

0.149- 0.135

0.142

0.1430.1370.156

0.1570.1060.142

Typical Y value

0.0600.6000.133

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While the system was subjected to varioushydraulic and organic loading conditions tosimulate conditions leading to granulation, itwas realized that the nature of wastewater couldbe the most important factor behind thisphenomenon. Moreover, the presence of oil andgrease in sufficient amounts (70-150 ppm)cannot be ruled out also and further studies aresuggested to look into this aspects. While thepresence of pelletized microbiological granulesfacilitates the retention of high biomass even athigher hydraulic conditions and optimalperformance of the system, it was seen from thisresearch that granulation was not a necessity ashigh performance of the process could still beestablished with dense flocculated sludge. Therewas a tendency however of this type of sludge to bewashed out of the system at higher hydraulicloadings which could have been prevented with apelletized sludge.

Sulphur Metabolism

The success of a process like MUSB or MRBis highly dependent on the activity of micro-organisms related to sulphur metabolism. It hasbeen suggested (3) that the main mechanism ofpellet formation is one in which the filamentousbacteria, Beggiatoa wrap around the anaerobicsludge interfering with the penetration of O2 intoit. The symbiotic relationship of the SRB coreand the filamentous SOB, Beggiatoa covering,makes the pellet stable. In the present study no

granulation was observed although the results ofbatch study indicate sulphate reducing activity. Atypical result of a batch study showing the changein sulphate concentration with time for a sampleof sludge is shown in Figure 9. The extent ofsulphate reducing activity was found to be 2.73 mgSO4-8(gVSS)r1.hr1.

Regular observation of influent and effluentsulphate concentration show a little change(Figure 10) which indicate that the sulphideproduced as a result of sulphate reduction by theSRB is converted to sulphate by SOB, Beggiatoa.The dense flocculated sludge was seen to becovered with SOB, Beggiatoa which proliferated ~the system. The presence of Beggiatoa werevalidated by studies under a phase contrastmicroscope. Based on the activities of SRB andSOB, Beggiatoa it can be concluded that evenunder the condition that no granulation occurred,sulphur metabolism must be one of thesignificant mechanisms that took place in theMUSB system in the present study besidesproving the fact that the existence of SRB and SOB,Beggiatoa are not only the prerequisites forgranulation.

CONCLUSIONS

Based on the results of this study on MUSBprocesses the following pertinent conclusionscould be drawn:-1. The MUSB process is a feasible organic

carbon removal system for the treatment of

250

t?

rati

on

(e

Con

cent

Su

lfat

200

150

100

50

6 8 10Time in Hours

12 (4 16

Figure 9. Sulphate reducing activity.

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200

150

oC/l

B 100

OT 5 0

- o Influent

Phase III

10 20 30 40 50 60 70 80 90 100 110 120Days of Operation

Figure 10. Sulphate variation in MUSB.

domestic was tewa te r a l thoughmicrobiological granules did not develop.Good settleability of sludge (SVI) wasobserved.Organic removal efficiency is dependentupon the hydraulic and organic loading. At anHRT of 4.5 hours and an organic loading ofabout 2 Kg COD m'3 d"1 removal efficiency of94 % was achieved. A loading of 3.9 Kg CODm'3 d'1 could still be achieved with a 89 %removal.The effluent from MUSB process was as goodas any conventional aerobic process and thesystem could be effectively operated withoutany secondary sedimentation tank.

4. Less sludge is produced in a process likeMUSB with a typical yield coefficient (Y)value of 0.133 which is low compared toconventional aerobic processes.

ACKNOWLEDGEMENTS

This research was jointly supported by theRoyal Norwegian Government through NORADEscholarship provided to the first author and JapanInternational Co-operation Agency (JICA).Technical support from laboratory staff at theEnvironmental Engineering Division, AsianInstitute of Technology, Bangkok is alsogratefully acknowledged.

REFERENCES

1. Takahashi M. and Kyosai S., Development of multi-stage reversing flow bioreactor (MRB) forwastewater treatment. Water Sci. Technol., 20, 361-367 (1988).

2. Matsui T., Kyosai S. and Takahashi M., Application of biotechnology in municipal wastewatertreatment. Water Sci. Technol., 23, 1723-1732 (1991).

3. Takahashi M. and Kyosai S., Pilot plant study of microaerobic self granulated sludge process(multi-stage reversing flow bioreactor: mrb). Water Sci. Technol., 23, 973-980 (1991).

4. Basu S.K., Application of Multi-Stage Reversing Flow Bioreactor in Domestic WastewaterTreatment, Thesis No. EV - 91 - 13, Asian Institute of Technology, Bangkok, Thailand (1991).

5. APHA, AWWA and WPCF, Standard Methods for Examination of Water and Wastewater, 15th Ed.,American Public Health Associaiton, Washington D.C. (1981).

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