Treatment and Recycling of Wastewater by Submerged Hollow Fiber Membrane

doi:10.4236/nr.2011.22009

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Natural Resources, 2011, 2, 71-74

doi:10.4236/nr.2011.22009 Published Online June 2011 (http://www.scirp.org/journal/nr)

Treatment and Recycling of Wastewater by

Submerged Hollow Fiber Membrane

Dewen He, Huangnian Zhou, Lei Liu, Dingmin Liang, Lu Du

School of Metallurgical Science & Engineerin, Central South University, Changsha, China.

E-mail: [email protected]

Received October 10th, 2010; revised April 11th, 2011; accepted April 20th, 2011.

ABSTRACT

In this study, the effects of experimental conditions including the MBR equipped novel device and different operating

modes on permeate flux were studied. The results show that the MBR equipped novel device can reduce the resistance

and enhance the flux, decreasing the total resistance (t = 9.649) to 5.962 and increasing the permeate flux to 15-20

l/m2 hr. The permeate flux of intermittent operating mode is more than that of continuous operation and the value of the

permeate flux is between 15 l/m2 hr and 20 l/m2 hr. The MBR equipped novel device which adopting intermittent oper-

ating mode is most effective in this study and the value of permeate flux is between 20 l/m2 hr and 25 l/m2 hr.

Keywords: Membrane Bioreactor, Separation, Fouling, Hollow Fiber Membrane

1. Introduction

Membrane bioreactor (MBR) is an effective technology

for wastewater treatment and recycling, in which gravity

settling of the activated sludge system is replaced by a

membrane separation process such as micro-filtration

(MF) or ultra-filtration (UF) [1-2]. The resulting high-

quality and disinfected effluent implies that MBR proc-

esses can be especially suitable for reuse and recycling of

wastewater [3]. A major factor against the application of

membrane micro-filtration or ultra-filtration with the

activated sludge process is the flux decline due to mem-

brane fouling [4-5].

Membrane fouling is often referred to as any form of

flux decline [6]. This includes reversible effects (such as

cake formation), as well as irreversible effects (such as

adsorption). Concentration polarization, though it does

cause a decline in membrane flux, is a phenomenon

which occurs in solution, and will therefore not be con-

sidered a form of fouling. However, when speaking of

reversible flux decline, or reversible hydraulic resistance,

concentration polarization will be categorized with cake

formation, since all of their effects can be reversed by the

hydraulic back-flushing of the membrane. Irreversible

flux decline, which is always a form of fouling, is caused

primarily by the adsorption of natural organic matter

(NOM) onto the surface or inside the pores of the mem-

brane [7-8]. Though this fouling can be reversed by

chemical treatment, it is not considered “reversible”

since it cannot be reduced by standard physical proc-

esses.

During a long term operation, membranes are periodi-

cally back-washed by pumping a fraction of permeate

back through the membranes [9-12]. Backwashing is

used for removing the cake layer (reversible fouling).

Concentration polarization is reduced by non-suction as

well as reverse flow. Thus backwashing can remove re-

versible hydraulic resistance due to cake formation and

concentration polarization. Backwashing entails reversal

of the permeate flow and power consumption of back-

washing pump.

In this paper, a novel device was introduced during the

filtration. This device could be used to counteract fouling

during filtration (especially for cake formation) without

reverse flow. The behavior of MBR under the combina-

tion of this device and intermittent permeation was then

observed.

2. Materials and Methods

2.1. Activated Sludge MBR Operation

The configuration of the activated sludge MBR system

used in this study is schematically described in Figure 1.

This system consisted of hollow fiber (HF) flat-plat

membrane module and bioreactor, similar to typical

MBR [13]. The bioreactor has a working volume of 30 l.

72 Treatment and Recycling of Wastewater by Submerged Hollow Fiber Membrane

(1. feed tank, 2. electromagnetic valve, 3. level controller, 4. level probe 5.

vacuum gauge, 6. pump, 7. fiber membrane, 8. pump, 9. flow meter, 10.

aerator, 11. bioreactor solenoid e valve)

Figure 1. Configurations of activated sludge MBR system.

Table 1. Characteristics of the wastewater.

COD (mg·L−1) 20~80

NH3-N (mg·L−1) 4~18

TP (mg·L−1) 0.3~2.0

The hydraulic retention time of the bioreactor was set to

6 h for the majority of experiments.

The wastewater used in the study was pumped from a

local drain ditch. Characteristics of the wastewater are

shown in Table 1.

2.2. Membranes and Module Configuration

The micro-filtration membranes were polypropylene

hollow fibers with a pore size of 0.1-0.2 μm. An effective

filtration area was 0.25 m2.

In order to investigate the reversible resistance re-

moval, the membranes has been continuously operated

for two months prior to this study. Irreversible resistance

(adsorption fouling) during this study reached steady and

kept constant. To ensure that the condition and perform-

ance of the membrane module was almost the same in all

experiments, backwashing was performed after every

experiment to removing the reversible resistance.

2.3. Membrane Performance Assessment

The rate and extent of membrane fouling were quantified

by measuring permeate flux at constant suction pressure,

which is defined as permeate vol/unit membrane surface

area and filtration time. The variation of membrane pro-

ductivity was evaluated by plotting permeate flux against

filtration time.

To characterize membrane fouling, the water flux was

initially measured with tap water. Also, water flux was

measured at each step of the cleaning procedure, that is,

after taking out and putting the membrane module in tap

water and then after flushing and backwashing the mem-

brane module. The fluxes and filtration resistances were

measured and calculated collectively.

3. Results and Discussion

3.1. Continuous Operation

The permeate flux decline with time during the mem-

brane filtration of activated sludge was first studied with

a polluted membrane. The permeate flux decreased

sharply after the start of filtration, and reached a pseudo-

steady state after 2-3 h of operation.

The initial sharp decline in permeate flux observed

was mainly due to concentration polarization and cake

layer formation on the membrane surface. Further de-

crease in permeate flux over time was caused by the in-

creased thickness of the cake layer due to additional fou-

lant transport to the membrane surface, cake compaction

and concentration polarization.

3.2. Continuous Operation with the Novel

Device

In order to maintain a steady permeate flux during opera-

tion, the cake layer deposited on the membrane surface

needs to be removed by a shear, which can be created by

various means.

A novel device was employed to control concentration

polarization and cake layer formation on the membrane

surface. The fibers through this device made to-and-fro

motion along the direction vertical to two pipes, and the

continuous deformation of the fibers themselves created

a shear to the cake layer on the membrane surface, and

the mutual rubbing between the fibers due to the con-

tinuous deformation of the fibers also created a shear to

the cake layer on the hollow fiber membrane. In addition,

the continuous movement of the fibers in water enhanced

the stirring of water on the membrane surface, and de-

creased concentration polarization.

Compared to continuous operation in Figure 2, con-

tinuous operation with the device reduced the rate and

extent of membrane fouling [14-15].

The resistance-in-series model was applied to evaluate

the characteristics of membrane fouling. According to

this model, the permeation flux (J) takes the following

form.





 (1)

tmprfi

RR RR R



  (2)

where p



is the transmembrane pressure;



is the

dynamic viscosity of the permeate; t is the total re-

sistance; is the intrinsic membrane resistance;

Treatment and Recycling of Wastewater by Submerged Hollow Fiber Membrane 73

is the polarization layer resistance caused by the concen-

tration gradient; rf is the reversible fouling resistance

formed by a strongly deposited cake layer; irf is the

irreversible fouling resistance due to some irreversible

adsorption. Here, the

term experimentally defined

as the portion of the total resistance dislodged only by

water dropping of the membrane module and then im-

merging in tap water.

These equations and the flux data at the end of con-

tinuous operation with the novel device have been used

to calculate the values of each resistance term given in

Table 2.

Figure 2 shows that rf (the percent of total resis-

tance) was very small during continuous operation with

the novel device and cake layer were almost removed

due to this device.

3.3. Non-Continuous Operation

In general, the membrane coupled with bioreactors is

continuously operated for the effective utilization of

given membranes. Continuous membrane filtration,

however, tends to increase fouling, thus decreasing per-

meate flux through the membrane over time. In this study,

non-continuous suction operation was introduced as a

fouling alleviating alternative. Figure 3 compares the

continuous and non-continuous suction operation modes.

Change in permeate flux was plotted in terms of time (28

Figure 2. Flux variation with respect to time / 26kPa suction

pressure.

Table 2. A series of resistances for the membrane at the end of

continuous operation and continuous operation with the novel

device.

with this device without this device

Resistance Va l u e

(1012 m−1)

Percentage*

(%)

Va l u e

(1012 m−1)

Percentage

(%)

mirf

 4.123 69.2 4.123 42.7

1.707 28.6 3.687 38.2

0.132 2.2 1.839 19.1

5.962 100.0 9.649 100.0

minutes’ suction of 2 minutes’ suspension). The results

clearly showed that membrane performance was im-

proved with intermittent suction. This finding can be

explained by the enhanced foulant back transport under

pressure relaxation. From Figure 3, as soon as trans-

membrane (suction) pressure was reduced to zero at

non-suction period, the foulants not irreversibly attached

to the membrane surface, diffused away from the mem-

brane surface because of the concentration gradient. As a

result, foulant accumulation near the membrane surface

was lessened and the rate of membrane fouling was re-

duced.

Flux recovery by pressure relaxation has been re-

ported by Hong et al. [10], who studied the ul-

tra-filtration of activated sludge. Like their study, the

permeate flux was only partially recovered, indicating

that reversible hydraulic resistance due to cake forma-

tion and concentration polarization existed.

3.4. Non-Continuous Operation with the Device

With operation of the device, the cake layer was almost

removed. After two minutes’ pressure relaxation, con-

centration polarization was removed. As shown in Fig-

ure 4, almost 100% flux recovery was observed under

the intermittent suction with the device. In consideration

of long-term operation of fibers, the fibers kept loose and

cannot tension during to-and-fro motion. Thus a portion

of fibers close to catchments pipes hardly made continu-

ous deformation and no movement of fibers. The cake

layer on the membrane surface of this portion of fibers

was not eliminated.

4. Conclusions

The MBR equipped novel device can improve the per-

meate flux. The value of the MBR equipped novel device

is between 15 l/m2 hr and 20 l/m2 hr. It is more than the

Figure 3. Flux variation with respect to time with a device /

26kPa suction pressure.

Treatment and Recycling of Wastewater by Submerged Hollow Fiber Membrane

[5] X. C. Wang and J. Wang, “Kinetic Study of Membrane

Fouling under Cross-Flow Ultrafiltration Opteration,”

Environmental Chemistry, Vol. 21, No. 6, 2002, pp.

552-558.

[6] G. E. Wetterau, M. M. Clark and C. Anselme, “A dynamic

model for predicting fouling effects during the ultrafiltra-

tion of a groundwater,” Journal of Membrane Science,

Vol. 109, No. 2, 1996, pp. 185-204.

doi:10.1016/0376-7388(95)00200-6

[7] M. Cheryan, “Ultrafiltration Handbook,” Technomic,

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[8] S. Kunikane, Y. Magara, M. Itoh and O. Tanaka, “A

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membrane science, Vol. 102, 1995, pp. 149-154.

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Figure 4. Flux variation with respect to time / 26kPa suction

pressure; non-continuous suction operating mode with the

device was not eliminated.

[9] T. Jiang, M. D. Kennedy, G. J. Walter, van der Meer, P. A.

Vanrolleghem and J. C. Schippers, “The Role of Blocking

and Cake Filtration in MBR Fouling,” Desalination, Vol.

157, 2003, pp. 335-343.

doi:10.1016/S0011-9164(03)00414-4

value of MBR. (10-15 l/m2 hr)

The permeate flux of intermittent operating mode is

more than that of continuous operation and the value of

the permeate flux is between 15 l/m2 hr and 20 l/m2 hr. [10] E. H. Bouhabila, R. B. Aïm and B. Hervé, “Fouling Char-

acterization in Membrane Bioreactors,” Separation and

Purification Technology, Vol. 22-23, 2001, pp. 123-132.

doi:10.1016/S1383-5866(00)00156-8

The MBR equipped novel device which adopting in-

termittent operating mode is most effective in this study

and the value of permeate flux is between 20 l/m2 hr and

25 l/m2 hr. [11] C. Albasi, Y. Bessiere, S. Desclaux and J. C. Remigy,

“Filtration of Biological Sludge by Immersed Hol-

low-Fiber Membranes: Influence of Initial Permeability

Choice of Operating Conditions,” Desalination, Vol. 146,

2002, pp. 427-431. doi:10.1016/S0011-9164(02)00527-1

5. Acknowledgements

The authors acknowledge the financial support of Na-

tional 863 Project (2007AA06Z374).

[12] C. Albasi, Y. Bessiere, S. Desclaux and J. C. Remigy,

“Filtration of Biological Sludge by Immersed Hol-

low-Fiber Membranes: Influence of Initial Permeability

Choice of Operating Conditions,” Desalination, Vol. 146,

2002, pp. 427-431. doi:10.1016/S0011-9164(02)00527-1

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