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Keywords: Membrane bioreactor (MBR); Sequencing batch reactor (SBR); Membrane fouling; Sludge retention time (SRT); Specific oxygen uptake rate

NR)

system can be significantly influenced by SRT. In fact, many

MBR researchers have operated their systems with longer

reported that properties of mixed liquor, such as viscosity,

amount and composition of microbial product and cell

Process Biochemistry 40 (200surface properties were changed at longer SRT [6,7]. These

properties can also influence membrane fouling. Since

membrane fouling is one of the most important problems of* Corresponding author. Tel.: +82 2 880 4621; fax: +82 2 873 2285.

E-mail address: tmtak@snu.ac.kr (T.-M. Tak).

0032-9592/$ see front matter # 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.procbio.2004.09.0171. Introduction

It is well known that sludge retention time (SRT) is the

one of the important factors, which can change the state of

biomass in an activated sludge system [1,2] and the con-

centration of mixed liquor suspended solid (MLSS) in the

bioreactor increased with SRT [3]. A membrane bioreactor

(MBR) system can maintain higher MLSS compared to a

conventional activated sludge system through membrane

separation technology, which can accomplish perfect solid/

liquid separation [4]. Therefore, it is not difficult to expect

that biomass properties and membrane fouling in a MBR

SRT compared to conventional biological treatment since

they believed that a higher biomass concentration, which

was derived by longer SRT, gave rise to higher treatment

efficiency. Some MBR plants were operated with an infinite

SRT in order to maintain large amounts of biomass.

However, it is not difficult to expect that treatment efficiency

would not be linearly proportioned to biomass concentration

because the specific bioactivity can be reduced at substrate

deficient states.

On the other hand, higher MLSS concentrations can

accelerate membrane fouling via rapid deposition of sludge

particles on the membrane surface [5]. Furthermore, it was(SOUR); Specific nitrification rate (SNR); Specific denitrification rate (SDAbstract

Sludge retention time (SRT) can produce significant effects on biomass properties in a membrane bioreactor (MBR) system. In this study,

the membrane separation process was coupled to a sequencing batch reactor (SBR), which is one of the biological nutrient removal (BNR)

processes, and the influence of SRT on membrane fouling and biological activity was investigated. Membrane fouling increased with SRT

since sludge particles were more severely deposited on the membrane surface at longer SRT. Regardless of SRT change, COD removal

efficiency was high and stable (over 92%) throughout the experiment. Nitrogen removal efficiency also attained a high treatment level.

However, it was not proportioned to SRT increase and rather decreased at the longest SRT. Phosphorus removal decreased at prolonged SRT

since excess sludge was reduced. Biological activity such as specific oxygen uptake rate (SOUR), specific nitrification rate (SNR), and specific

denitrification rate (SDNR) did not increase with SRT but decreased at prolonged SRT.

# 2004 Elsevier Ltd. All rights reserved.Influence of sludge retentio

and bioactiviti

bioreact

Sung-Soo Han a,b, Tae-Hyun Bae a,a School of Biological Resources and Materials

Shinlim-dong, Kwanak-kub School of Civil and Environmental En

Atlanta, Georg

Received 7 June 2004; received in revisedtime on membrane fouling

in membrane

system

ung-Gug Jang a, Tae-Moon Tak a,*

neering, Seoul National University, San 56-1,

ul 151-744, South Korea

ring, Georgia Institute of Technology,

30332, USA

3 July 2004; accepted 25 September 2004

www.elsevier.com/locate/procbio

5) 23932400

MBR processes, the influence of SRT on membrane fouling

needs to be investigated.

It was reported that adjustment of SRT value is essential

for better biological nutrient removal processes (BNR),

which have attracted great attention for economical nitrogen

and phosphorus removal [8]. Studies on BNR-conducting

MBR systems have been conducted by several researchers in

recent years and there is growing interest on those processes

[911]. Thus, the influence of longer SRT in MBR system on

BNR performance needs to be clarified.

Considering the aspects mentioned above, SRTs have

significant effects on biological activity and membrane per-

formance in BNR-conducting MBR systems. In this study, a

membrane unit was coupled to a sequencing batch reactor

(SBR), which is one of the BNR processes, and the system was

operated with synthetic wastewater. The main purpose of this

study was to investigate the effect of SRT on specific biolo-

gical activity including BNR performance and membrane

fouling in a membrane coupled sequencing batch reactor.

2. Materials and methods

2.1. Experimental set-up

membrane modules were directly submerged in the reactors.

The air diffusers were installed under the membrane module

to optimize the contact between the air bubbles and the

membrane surface. Thus, solid accumulation on the

membrane surface could be prevented by a sheering stress

generated by the uplifting flow of bubbling air. The pressure

gauges were installed in order to monitor the variation of t

he trans-membrane pressure (TMP) between the membranes

and suction pumps. To maintain a constant level in the

reactor, a level sensor was used. Submerged circulation

pumps were used for sludge mixing during the non-aeration

period.

The bioreactor is a rectangular tank of 200 mm 250 mm 350 mm. The membrane module used here was ahollow fibre membrane module made of polyethylene

(Mitsubishi Rayon Engineering Co., Japan) with a pore size

0.4 mm and a filtration area of 0.3 m2 and the effluent wassucked out by metering-pump (FMI QD PUMP, USA).

Operating conditions are summarized in Table 1. The four

bioreactors were operated at the same hydraulic residence

time (HRT) and air flow rate. Temperature and pH are

adjusted in order to eliminate their influences. Since the

the SRT of the reactors were 30, 50, 70 and 100 days

MLSS concentrations were maintained at approximately

(a) 7000 mg/l, (b) 10,000 mg/l, (c) 14,000 mg/l and (d)

S.-S. Han et al. / Process Biochemistry 40 (2005) 239324002394The experimental set-up is shown in Fig. 1. Four-parallel

reactors were operated simultaneously and controlled by

computer connected to a programmable logic controller

(PLC: LG Industrial System, Korea). U-shape hollow fibreFig. 1. Schematic diagram o18,000 mg/l, respectively. Food to microorganism ratios

(F/M) of all reactors were about 0.060.15 kg COD/kg

MLSS-day. These values are lower range than those of

conventional systems.f experimental system.

The SBR operation sequence is described in Fig. 2.

Reactor conditions, i.e. anaerobic, anoxic, and anaerobic

was cultivated with synthetic wastewater over 100 days for

acclimation of microorganisms.

2.3. Membrane performance assessment

First of all, membrane separation characteristics were

investigated. The air scouring effect by air blowing and

critical flux, which can be viewed as the flux at which solid

deposition starts to occur [13], were measured.

Since it was reported that cake formation on membrane

surfaces plays a main role in MBR systems, sludge

deposition on the membranes was calculated quantitatively

using the resistance model [14].

R DPthJ

1

S.-S. Han et al. / Process Biochemistry 40 (2005) 23932400 2395

Table 1

Operating conditions of Membrane coupled SBR system

Operating conditions

Working volume (L) 12

Feeding volume (L) 4

Hydraulic retention time (h) 12

Four sludge retention time (days) 30 50 70 100

MLSS concentration (mg/L) 7000 10,000 14,000 18,000

F/M (kg COD/kg MLSS d) 0.15 0.10 0.07 0.05

Aeration intensity (L/min) 15 20 20 25

Dissolved Oxygen (mg/L) 23

Temperature 25 2 8CpH 7.08.0condition, were changed during the process time by power

on/off of air blower controlled automatically by PLC.

After the initial 60 min of feeding and anaerobic phase for

phosphorus release, 40 min of aeration was applied to the

reactors for organic decomposition and nitrification. The

reactors then were not aerated but mixed constantly by

submerged circulation pumps for 70 min. During that

period, denitrification could occur after oxygen was con-

sumed. Before the suction period, 30 min of aeration was

applied for removal of residual COD and phosphorus luxury

uptake. During the suction period, the reactor was aerated

and 8 min/2 min intermittent suction was applied in order to

control membrane fouling.

2.2. Synthetic wastewater and microorganism

A synthetic wastewater used for this study contained

glucose, (NH4)2SO4, and KH2PO4 as the sources of carbon,

nitrogen, and phosphorus, respectively. COD, nitrogen and

phosphorus concentrations of influent were set at 500, 100

and 50 mg/l, respectively. Seeding sludge was supplied from

a nearby sewage treatment plant. After seeding, the sludgeFig. 2. Operation mode of membwhere R: filtration resistance (m ), J: permeation flux

(m3/m2 s), DPt: TMP (Pa), h: viscosity of permeate (Pa s).Sub-critical flux operation is essential for long time use of

membranes without washing or substitution. It has been

known that irreversible membrane fouling is developed

rapidly beyond the critical flux [15]. This study measured

critical flux by monitoring the flux with filtration time at

constant TMP using a metering pump.

2.4. Analytical method

Standard methods [16] were adopted for the measure-

ment of water quality. Measurement of DO concentration

was carried out with OX22 (AQUA LYTIC, Germany).

COD, total phosphorus (TP) and ortho-phosphate concen-

tration are determined using a spectrophotometric method

with Fotometer AL282 (AQUA LYTIC, Germany) and

reagent kits. Total Kjeldahl nitrogen (TKN) was measured

with a Kjeltec Auto 2300 analyzer (Tecator, Sweden).

Oxidized nitrogen, including the nitrite and nitrate concen-

trations, were determined using ion chromatography

(Shimadzu, Japan).rane coupled SBR system.

activity, specific nitrification rate (SNR; mg NO3N/

chemg MLSS h) and specific denitrification rate (SDNR; mg

NO3N/g MLSS h) batch experiments were conducted [12].

SNR was measured by the following method. Six hundred

millilitres of MLSS solutions was prepared in a flask and

200 ml synthetic wastewater added. The mixed sample was

aerated and samples for NO3N analysis obtained at timed

intervals using directly submerged mini membrane modules

made by the authors. SNR was calculated by monitoring the

increased rate of NO3N concentration versus times at

aerobic condition. SDNR was measured through the

same procedure of SNR. However synthetic wastewater

for SDNR measurement was prepared using sodium nitrate

as nitrogen source instead of ammonium sulphate. SDNR

was calculated by monitoring the rate of decrease of NO3N.

3. Results and discussion

3.1. Membrane performance according to the

filtration condition

A typical MBR process using submerged membranes is

implemented by causing a shearing stress through uplifting

air bubbles. In order to measure the air scouring effect, flux

declines according to air blow intensity were monitored.

Variation of total filtration resistance according to SRT

changes are shown in Fig. 3. This represents the effect of

air flow intensity on membrane fouling in the activated

sludge MBR process. Resistance changes of non-aeration

and aeration condition show that sludge deposition was

significantly prevented by air flow. The scouring effect was

increased with air flow intensity. However, it was not

linearly proportioned to air flow and there was critical air

flow intensity at which scouring effect hardly improved.

This tendency was reported in our previous studies [11,17].2.5. Biological activity measurement

Since oxygen is required for microorganisms to decom-

pose organic compounds in a biological treatment process,

biological activity for COD removal can be characterized by

specific oxygen uptake rates (SOUR). SOURs were

measured using a Winkler bottle method. Hundred milli-

litres synthetic wastewater added to 300 ml sludge sample in

the bottle and the mixed samples were aerated until the DO

reached constant level. Aeration was stopped and DO con-

centration was constantly measured using an oxygen sensor.

During the experiment, samples were constantly stirred

using a magnetic stirrer. The decreases in DO-concentra-

tions were transferred into a utilization rate, assuming

linearity in the decreasing slope. SOUR was calculated by

the following equation:

SOUR (mg O2/g MLSS h) = O2 consumption rate (mg

O2/l min) MLSS1 (l/g) (60 min/h)For the measurement of nitrificationdenitrification

S.-S. Han et al. / Process Bio2396The existence of critical air flow intensity indicates limitflow velocity due to the resistance of fluid. Air scouring

effect was hardly increased above the 15 l/min of air blow

intensity at SRT 30 days and this trend was also observed at

the other SRTs. As shown in Fig. 3, critical air flow intensity

gradually increased with the increase of SRT due to higher

MLSS concentration and viscosity.

It should be noted that increase of air flow intensity for

fouling control can result in an over-supply of DO and poor

denitrification which is reported as the limiting step for

nitrogen removal in a membrane coupled BNR system [10].

Furthermore, since power consumption increases with air

blow intensity, the operation cost of MBR process increases.

Thus, over aeration for fouling control may cause a

deterioration effect on MBR process. Since oxygen supply

to a reactor is sufficient to maintain aerobic condition, each

reactor was operated at a critical flow intensity in this study

(Table 1).

The concept of critical flux was shown experimentally by

Defrance and Jaffrin [15]. The critical flux is defined as the

flux below which flux decline with time does not occur.

Fig. 4 shows the result of critical flux measurement. At SRT

30 days, flux decline did not occur until the TMP increased

up to 23 K Pa, however, 47 l/m2 h flux began to decrease

slightly at 27 K Pa. Then, flux decreased more rapidly and

TMP was not controlled consistently at 37 K Pa. Therefore,

it can be concluded that the critical flux of SRT 30 days was

about 47 l/m2 h. The observed critical flux decreased

according to the increase of SRT, 43 l/m2 h, 42 l/m2 h and

36 l/m2 h were observed at SRT 50, 70 and 100 days,

respectively. Membrane performance studies clearly showed

that membranes are fouled more severely and the control

of membrane fouling with air scour is more difficult at

prolonged SRT.

3.2. Treated water quality

The average treatment efficiencies for about 40 days

operation are shown in Fig. 5. As shown in Fig. 5(a), the

COD removal rate slightly increased with SRT due to the

higher concentration of biomass which can decompose

organic compounds. Although COD removal in the

bioreactor slightly decreased with shortened SRT, the total

removal efficiency of organic compound could be kept over

92% regardless of SRT. This high and stable COD removal

could be achieved by the maintenance of higher MLSS

concentration compared to conventional system and

membrane separation of macromolecular COD components

which might be generated from microbial metabolism. It

was reported that membrane separation plays an important

role in maintaining high and stable COD removal [18].

The removal efficiency for total nitrogen (TN) also

showed a satisfactory level, less than an effluent nitrogen

concentration of 10 mg/l. This is because of the longer

sludge retention times of the nitrifying and the denitrifying

bacteria. However, the treatment efficiency was not linearly

istry 40 (2005) 23932400proportioned to SRT. It appeared that nitrogen removal of

ochemS.-S. Han et al. / Process BiSRT 100 days was less than that of SRT 50 and 70 days. This

may be due to the lower growth rate of microorganism and

specific biomass activity.

Since the nitrogen in the reactor could be consumed by

new microbial synthesis, TN removal could be lowered at

extremely prolonged SRT because of lower growing rate.

Specific microbial activity can be influenced by the amount

of oxygen and substrate available. Organic decomposition

and nitrification, which mainly occurred in aerobic condi-

tions, required sufficient oxygen as electron acceptor.

Although even the DO concentration of the bulk solution

is at a sufficient level to maintain aerobic condition,

inefficient oxygen transfer can be derived by increased fluid

resistance due to the high MLSS concentration and viscosity.

Since the substrate deficient state, which was generated by

low food to microorganism ratio, gave rise to competition of

biomass, specific bioactivity could be reduced. The effect of

SRT on nitrificationdenitrification will be discussed with

experimental evidence in the following section. Since

inefficient oxygen transfer and substrate deficient state gave

rise to lowered specific biological activity, it can be

concluded that maintaining appropriate SRT is required

for efficient nitrogen removal.

As shown in Fig. 5(c), influent total phosphorus (TP)

concentration was very high. Therefore, TP was not well

removed due to the limitation of biological process. The

Fig. 3. Variation of total filtration resistance according to SRT change (TMP: 15 kP

days, d: SRT = 100 days).istry 40 (2005) 23932400 2397limitation would be attributed to the fact that removal of

phosphorus ultimately depends on the amount of excess

sludge wasting. Thus, maintaining relatively shorter SRT,

which means a large amount of excess sludge discharge, can

have an advantage for phosphorus removal compared to

maintaining longer SRT.

3.3. In situ nitrification and denitrification analysis

Oxidized nitrogen concentrations of sludge mixed liquor

were monitored during the aerobic and anoxic phases.

Nitrification and denitrificaton profiles of membrane

coupled SBR systems are shown in Fig. 6. Nitrification

and denitrification performance was improved with increase

of SRT until SRT 70 days since the amount of nitrifier and

denitrifier increased. Thus, nitrogen removal increased from

87 to 96% as shown in Fig. 5(b). However, nitrogen removal

efficiency was decreased to 89% at SRT 100 days. Fig. 6

shows that nitrification rate of SRT 100 days was not

different from those of SRT 50 and 70 days but denitrificaton

profile was worse than that of shorter SRT. Since the MLSS

concentration increased to about 18,000 mg/l, mass transfer

of electron acceptor and carbon source might be hindered by

great amount of solid matter and increased fluid viscosity.

Accumulation of inert matter, which has lower bioavail-

ability, also caused poor denitrification profile.

a, temperature: 25 2 8C; a: SRT = 30 days, b: SRT = 50 days, c: SRT = 70

S.-S. Han et al. / Process Biochemistry 40 (2005) 239324002398

Fig. 4. Variation of critical flux according to SRT change (a: SRT = 30 days, b: SRT = 50 days, c: SRT = 70 days, d: SRT = 100 days).

Fig. 5. COD, TN (total nitrogen is the sum of TKN and NOxN) and TP removal behavior according to SRT change (a: COD, b: TNandc:TP).

(4) Nitrogen removal was increased with SRT until SRT 70

days. But the removal efficiency decreased at SRT 100

Tec

und

S.-S. Han et al. / Process Biochem3.4. Specific sludge activities measurement according to

SRT change

Changes of sludge activities with different SRT are

illustrated in Table 2. With respect to organic decomposi-

tion, SOUR was not influenced by increase of SRT until

SRT 70 days. However, the biomass of SRT 100 days had a

lower oxygen utilization rate compared to the biomass

of shorter SRT. This might be explained by an impeded

transfer rate of both substrate and oxygen according to

an increase of the sludge viscosity at long SRT and

accumulation of inert matter according to the endogenous

respiration [18].

The nitrifying activity of sludge according to the different

SRT was also investigated. As shown in Table 2, although

there is no big difference between SRT 50 and 70 days, SNR

slightly decreased with increase of SRT. The decrease of the

specific nitrification rate at longer SRT might be due to the

lower oxygen transfer and deficient substrate. Although total

nitrification rate slightly increased, specific nitrification rate

decreased by competition of biomass derived from low F/M

Fig. 6. Nitrification and denitrification profiles of the membrane coupled

SBR system during aerobic and anoxic phase.ratio. Specific denitrification rate increased with SRT, but

decreased significantly at extremely prolonged SRT. This

might also be due to the competition of denitrifier for

substrate.

As mentioned above, prolonged SRT give rise to

deterioration effects on specific biomass activities. Further-

more, since high viscosity and large amounts of inert matter

can cause severe membrane fouling, extremely prolonged

SRT give adverse effects on MBR performance.

Ref

[3]

Table 2

Sludge activities measurement according to SRT change

SRT (days) SOUR (mg O2/g MLSS h) SNR (mg/l h) SDNR (mg/l h)

30 3.41 1.29 1.34

50 3.45 1.22 1.38

70 3.50 1.23 1.43

100 2.71 1.09 1.12tion tank. Water Sci Technol 1989;21:43.

[5] Takeshi S, Yasuhiko I. Effects of activated sludge properties on water

flux of ultrafiltration membrane used for human excrement treatment.

Water Sci Technol 1991;23:16018.

[6] Shin HS, Kang ST. Characteristics and fates of soluble microbial

products in ceramic membrane bioreactor at various sludge retention[4] Y

streatment sludge production and modeling approach. Water Sci Tech-

nol 1991;23:158390.

Yamamoto K, Win KM. Tannery wastewater treatment using a

sequencing membrane batch reactor. Water Sci Technol 1991;23:1639.

amamoto K, Hissa M, Mahmood T, Matsuo T. Direct solidliquid

eparation using hollow fiber membrane in an activated sludge aera-[2] Cbiological reactor system for treatment of oily wastewaters. Water

Environ Res 1994;66(2):1339.

haize S, Huyard A. Membrane bioreactor on domestic wastewater[1] Kerences

noblock MD, Sutton PM, Mishra PN, Gupta K, Janson A. Membranehnology, Republic of Korea for their financial support

er Grant M1-0214-00-0303-03-B15-00-043-10.The authors are grateful to the Ministry of Science &Acknexplained by impeded transfer rate of both substrate and

oxygen and accumulation of inert biomass due to

endogenous respiration. The findings of this study show

that prolonged SRT gives rise to deterioration effects on

the process of BNR conducting MBR system in aspects

of membrane fouling and bioactivities. Thus, appro-

priate SRT need to be maintained for efficient operation

of MBR process.

owledgementdays because of low biomass growth and poor denitri-

fication.

(5) Phosphorus removal also showed poor performance at

prolonged SRT since amount of excess sludge reduced.

(6) Specific biological activities such as SOUR, SNR,

SDNR were decreased at prolonged SRT. This might be4. Conclusions

Major findings from this study are summarized as

follows:

(1) The membrane fouling rate increased with SRT,

presumably due to large amount of foulants and high

fluid viscosity. Thus, higher air flow intensity was

needed for fouling control at prolonged SRT.

(2) Critical flux decreased with increase of SRT, indicating

that membrane fouling started to occur even at low flux

condition.

(3) COD removal in the bioreactor slightly decreased with

shortened SRT but total removal efficiency could be

maintained at over 92% regardless of SRT.

istry 40 (2005) 23932400 2399times. Water Res 2003;37:1217.

[7] Chang IS, Lee CH. Membrane filtration characteristics in membrane-

coupled activated sludge systemthe effect of physiological states of

activated sludge on membrane fouling. Desalination 1998;120:22133.

[8] Randall CW, Barnard JL, Stensel HD. Design and retrofit of waste-

water treatment plants for biological nutrient removal. Lancaster,

Pennsylvania: Technomic Publishing, 1992.

[9] Yoon SH, Kim HS, Park JK, Sung JY. Influence of important opera-

tional parameters on performance of a membrane biological reactor,

Membrane Technology in Environmental Management, Tokyo, 1999.

[10] Ueda T, Hata K, Kikuoka Y. Treatment of domestic sewage from rural

settlements by a membrane bioreactor. Water Sci Technol 1996;34:

18996.

[11] Bae TH, Han SS, Tak TM. Membrane sequencing batch reactor

system for treatment of dairy industry wastewater. Process Biochem

2003;39:22131.

[12] Yeom IT, Na YM, Ahn KY. Treatment of household wastewater using

an intermittently aerated membrane bioreactor. Desalination 1999;124:

193204.

[13] Field RW, Wu D, Howell JA, Gupta BB. Critical flux concept for

microfiltration fouling. J Membr Sci 1995;100:25972.

[14] Choi JG, Bae TH, Kim JH, Tae TM, Randall AA. The behavior of

membrane fouling initiation on the crossflow membrane bioreactor

system. J Membr Sci 2002;203:10313.

[15] Defrance L, Jaffrin MY. Comparison between filtration at fixed

transmembrane pressure and fixed permeate flux: application to a

membrane bioreactor used for wastewater treatment. J Membr Sci

1999;152:20310.

[16] American Public Health Association, Standard Methods for the

Examination of Water and Wastewater, 20th ed., Washington DC,

1998.

[17] Hong SP, Bae TH, Tak TM, Hong S, Randall AA. Fouling control

in activated sludge submerged hollow fiber membrane bioreactors.

Desalination 2002;143:21928.

[18] Huang X, Gui P, Qian Y. Effect of sludge retention time on microbial

behaviour in a submerged membrane bioreactor. Process Biochem

2001;36:10016.

S.-S. Han et al. / Process Biochemistry 40 (2005) 239324002400

Influence of sludge retention time on membrane fouling and bioactivities in membrane bioreactor systemIntroductionMaterials and methodsExperimental set-upSynthetic wastewater and microorganismMembrane performance assessmentAnalytical methodBiological activity measurement

Results and discussionMembrane performance according to the filtration conditionTreated water qualityIn situ nitrification and denitrification analysisSpecific sludge activities measurement according to SRT change

ConclusionsAcknowledgementReferences