ELSEVIER Desalination 170 (2004) 83-90

DESAHNATION

www.elsevier.com/locate/desal

Influence of filtration conditions on the performance of nanofiltration and reverse osmosis membranes in dairy

wastewater treatment

Mustafa Turan Department of Environmental Engineering, Civil Engineering Faculty, Istanbul Technical University,

34469 Ayazaga, Istanbul, Turkey Fax +90 (212) 285-6587; email: mturan@ins.itu.edu.tr

Received 9 October 2003; accepted 10 February 2004

Abstract

Filtration performance and fouling of nanofiltration (NF) and reverse osmosis (RO) membranes in the treatment of dairy industry wastewater were investigated. Two series of experiments were performed. The first one involved a NF membrane (TFC-S) for treating the chemical-biological treatment plant effluents. The second one used a RO membrane (TFC-HR) for treating the original effluents from the dairy industry. The permeate flux was higher at higher transmembrane pressures and higher feed flowrates. The curves ofperrneate flux exhibited a slower increase while the feed flowrate decreased and the pressure increased. Membrane fouling resulted in permeate flux decline with increasing the feed COD concentration. Furthermore, the flux decline due to the COD increase was found higher at higher pressures for both NF and RO membranes.

Keywords: Flux; Feed flowrate; COD; Nanofiltration; Reverse osmosis; Dairy industry

1. Introduction

Dairy industry effluents usually include highly dissolved organic matter with varying character- istics. Membrane technology has some advan- tages including a high degree of reliability in removing dissolved, colloidal and particulate matter; selectivity in size of pollutants to be removed and the possibility of very compact treatment plants. Reverse osmosis (RO) tech-

nology has been successfully applied for the treatment of municipal leachates and contami- nated reservoir waters [1,2]. On the other hand, nanofiltration (NF) membrane treatment is becoming a viable alternative to the conventional treatment because it can operate at lower pressures, lower energy consumption and higher permeate recoveries than RO [3]. NF membranes remove all particulates present in feedwater such

0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved

doi: 10.1016/j.desal.2004.02.094

84 M. Turan / Desalination 170 (2004) 83-90

as dissolved inorganic solids hydrated divalent ions, other higher complexes and low TDS and COD [4,5].

The performance of crossflow filtration sys- tems is limited by membrane fouling and by the concentration polarization layer [6]. Fouling is a major problem in the efficient operation of mem- brane treatment plants. It forms as a result of the attachment, accumulation, or adsorption of foul- ants onto the membrane surface and/or within the membrane pores, which causes a decline in permeate flux over time [7]. Meanwhile, RO membrane fouling can also occur as precipitate fouling [8] or fouling by biological growth [9,10]. Winfield [ 11,12] investigated the fouling of RO membranes by secondary wastewater effluents and reported that dissolved organic materials play a much more significant role in membrane foul- ing than large suspended particles. On the other hand, several works using NF and RO mem- branes for dairy effluents have been made in recent years [13-15]. The goal of the present study is to investigate the effect of hydrodynamic conditions (i.e., feed flowrate, transmembrane pressure) and fouling effects of pollution para- meters - - especially chemical oxygen demand (COD) - - on the filtration performance of NF and RO membranes during the treatment of dairy industry effluents.

2. Materials and methods

2. I. Experimental procedure

The investigated dairy industry in the Istanbul province of Turkey has a two-stage (chemical and biological) treatment plant consisting of coagula- tion, flocculation, clarification, flotation and aeration units. A typical composition of the influent and effluent of this treatment plant is shown in Table 1. Experimental studies have been performed on a pilot-scale membrane filtra- tion system having a thin-film composite type of spiral-wound crossflow module (2 m 2 area) with

Table 1 Influent and effluent characteristics of a chemical- biological treatment plant of the dairy industry

Constituents Influent Effluent

COD, mg/1 2,000-10,000 55-500 BOD 5, mg/1 1,300-1,500 45 TSS, mg/1 800-1,000 85 Total P, mg/l - - 4.1 Total N, mg/l 40--65 20 Oil-grease, mg/l - - 35 Total hardness, mg 105-300 1,200

CaCO3/1 Conductivity, #S/cm 1,800-2,000 1,750 Turbidity, NTU 120-300 1.5

the NF (TFC-S) for the chemical-biological treat- ment plant effluents and the RO (TFC-HR) for the dairy industry original effluents, respectively (Fig. 1). Membrane material is a polyamide composite. TFC-S and TFC-HR membranes are manufactured by Koch-Fluid Systems.

The feedwater was stored in a 60-1 tank and pH was adjusted to 7 using sulphuric acid and passed through a fine filter at 2 bar pressure and then pumped to the membrane. The concentrate and the permeate were recirculated back to the feed tank, maintained at a temperature of about 28+1 °C. After filtration the equipment was rinsed with tap water, and the membrane was cleaned using 2% sodium meta bisulfite (Na2S205). The cleaning agent was circulated through the membrane module at 2 bar pressure and 500 1/h for 30 min while the temperature was 28±1°C. The pilot system was operated at an average water recovery (RC) of 16% and 30% for the RO and NF membranes, respectively. The treatment plant effluents from the dairy industry were applied to the NF membrane (TFC-S). Its trans- membrane pressures were 5, 7.5, 9, 10 and 12 bar using flowrates of 350, 400, 500,600 and 700 I/h. In the second series of the experiments, the dairy industry original effluents were treated by using

M. Turan / Desalination 170 (2004) 83-90 85

Permeate Flow

[ [ FIowR°'°ntate [ I

Feed Tank

Cooling System

Pressure gauge

Pressure gauge

~ 9 Fl°wmeter

Membrane

Fig. 1. Flow diagram of the membrane pilot plant.

the RO membrane (TFC-HR) at 6, 10, 14 and 18 bar pressures and flowrates of 350, 450 and 550 1/h. Each run was performed during 3-4 h, and flux decline was about 5-10%.

2.2. Methods of analysis

Samples to determine the permeate water flux and solute rejection were taken at various time intervals. Turbidity, conductivity, chemical oxy- gen demand (COD), total suspended solids (TSS) and total hardness were measured in the feed and permeate side of each membrane, pH was measured using an Orion SA type 720 pH meter. Temperature and conductivity were measured using the AGB-10001 Laboratory Data Logging System. COD and SS were determined according to Standard Methods [ 16].

In a membrane unit, feedwater was pres- surized and introduced to the feedwater side at a flowrate Qfand permeates through the membrane into the permeate side at a flowrate Qp. The following equations give the transport of the permeable component of water as the permeate flux (J) and the permeate recovery, respectively:

J = A (Ap-A ) (1)

RC = Q/Q: (2)

where A is the water permeation coefficient, Ap is the applied pressure difference across the mem- brane, and A~z is the osmotic pressure gradient. Eq. (1) expresses that the permeate flux increases with an increase in the transmembrane pressure (Ap). Due to the concentration polarization layer, the solute concentration next to the membrane wall (C,,) is higher than the bulk solute concen- tration (Cb), and this phenomenon is defined as

J = k In [(C,, - Cp) / (Cb-Cp)] (3)

where Cpis the permeate concentration, k = d/D is the mass transfer coefficient, d is the concen- tration polarization layer thickness and D is the solute diffusion coefficient. In RO membranes, the term Cp can be neglected due to their high solute rejection [6].

On the other hand, fouling is related to growth in the foulant layer; the deposition behaviour results in a thick foulant layer, the thickness of which is highly dependent on the hydrodynamic conditions [ 17,18]. Consequently, cake formation on membranes causes flux decline (A J). Accord- ing to the cake filtration model, the permeate flux

86 M. Turan / Desalination

is expressed by the following relationship in a form similar to Darcy's Law:

J= AP/(R,. + Rf) (4)

where Rm is clean membrane resistance and Rfis the fouling (cake) layer resistance. Similarly, the clean membrane (initial) flux can be given as:

Jo = AP/Rm (5)

For this reason, the flux decline can be described by substracting Eq. (4) from Eq. (5) as follows:

AJ = Jo - J = A p (Ru/R,.) / (R,. + Rf) (6)

Eq. (6) shows that increasing cake layer resis- tance (Rf) results in an increase of membane fouling, and hence a decrease of permeate flux during the permeation process.

3. Results and discussion

Two series of experiments were performed. The first one was carried out using a NF mem- brane for treating the chemical-biological treat- ment plant effluents. In the second one, the original effluents of dairy industry were treated using a RO membrane. The effluents of treatment plant show high levels of electrical conductivity and hardness [5]. The influent and effluent COD concentrations in this treatment plant varied in the range of 2,000-10,000 mg/1 and 55-500 mg/1, respectively; thus, COD removal efficiency was found to be approximately 97% (Table 1). Since the chemical-biological treatment process caused an increase in the total hardness of original dairy effluent, the total hardness in the treatment plant effluent rose to 1200 mg CaCO3/1. The NF membrane was effectively used for such an effluent because this membrane is classified as a softening membrane with up to 98% hardness removal [ 19]. During the NF membrane experi-

170 (2004) 8~-90

Table 2 Operating data of the TFC-S (NF) and TFC-HR (RO) spiral-wound membranes

Membrane Feed COD, Permeate COD type mg/1 COD, mg/1 rejection, %

TFC-S 450 9 98 TFC-S 40 0.8 98 TFC-HR 10,000 28 99.7 TFC-HR 5,000 19 99.6

ments, the feed COD concentrations of 40 and 450 mg/1 were used. On the other hand, the original effluents from the dairy industry were treated with the RO membrane and their COD concentrations were 5,000 and 10,000 mg/1. As a result, the performance of RO and NF membranes was excellent with COD removals of 99.7% and 98%, respectively (Table 2).

3.1. Evaluation o f the nanofiltration membrane process

The NF membrane was operated in a range of transmembrane pressures between 5 and 12 bar and feed flowrates between 500 and 700 1/h. In Figs. 2a and 2b, the effect of feed flowrates on the permeate flux is shown as a function of transmembrane pressures. Permeate flux linearly increased when the pressure was increased up to 10 bar at different flowrates. At 10 bar of pres- sure, permeate fuxes were measured as 22 1/m2h at the flowrate of 500 1/h and 33 1/m2h at the flowrate of 700 I/h with a corresponding COD concentration of 450 mg/1 in the feed water (Fig. 2a). At the same pressure, permeate fluxes for NF also varied between 32 and 42 1/m2h for a feed COD of 40 mg/1 (Fig. 2b). The flux in- creased by 32-50% while the flowrates increased by approximately 40% during the filtration pro- cess. After 10 bar, the permeate fluxes also showed a slow increase. Thus, the decrease in the filtration can be attributed to the increase in the

M. Turan / Desalination 170 (2004) 83-90 87

(a) 45 ~ 55

4020 T ~ 50 45

E 35 E 40

= 25 ~ ~" 30 2s

--0-- 500 I/h E 20 -4- 500 I/h E 15 --=- 600 I/h ~. --=- 600 I/h ~. 15 ~. 700 I/h

10 700 I/h 10 5

4 8 10 12 14 4 6 8 1'0 12 Pressure, bar Pressure, bar

Fig. 2. Permeate flux as a function oftransmembrane pressure for three different flowrates in the NF membrane. Feed COD concentration: (a) = 450 mg/1; (b) = 40 mg/1.

50

45 TFC-S

gao 25

20 E 15 o COD=40 mcj/I

10 ..-~ C ~ rng/I 5

4 6 8 1'0 12 1~4 Pressure, bar

Fig. 3. Permeate flux as a function of transmembrane pressure for two different COD concentrations in the NF membrane (feed flowrate = 600 I/h).

transmembrane pressure lead to the concentration polarization layer and, as a consequence, the per- meate flux decreased [6,20].

The permeate flux is higher at higher pressures and higher flowrates, but this positive effect decreased as the feed COD concentration in- creased because of increasing pollution level of the dairy wastewater (Figs. 3 and 6) [6,20,21]. Major pollutants of dairy wastewater include TSS, COD, total hardness and other organic con-

taminants. Deposition of the suspended particles or biological growth onto the membrane surface and/or membrane pores results in a fouling layer and causes flux decline (rid). The thickness of this layer varies as highly dependent on the hydrodynamic conditions of the membrane sur- face [10,22]. At a flowrate of 600 1/h and a pressure of 10 bar, the flux decline of the NF membrane was about 45% (from 39 to 27 l/m2h) because of the increase of the feed COD concen- tration from 40 to 450 mg/1. This difference between the permeate flux curves due to in- creasing COD represents the net contribution of pollutants to membrane fouling. Thus, the in- crease of the COD concentration caused a decline in the permeate flux in the range of 33-52% upon increasing the transmembrane pressure from 5 to 12 bar (Fig. 3).

3.2. Evaluation o f the reverse osmosis membrane process

In the second series of experiments, a RO membrane was used for the treatment of high- strength dairy industry wastewater. Transmem- brane pressures varied between 6 and 18 bar with feed flowrates within the range of 350 and

88 M. Turan / Desalination 170 (2004) 83-90

(a) 30

25 E v

ff 20

~ 15

~ 10

(b) 40

35

s : 30 E

25

20

15

lO

,t 550 I/h

i

lb 15 2b Pressure, bar

- - , i . - - 5 5 0 I /h

4 8 1 2 1 6 2 0

Pressure, bar

Fig. 4. Permeate flux as a function of transmembrane pressure for three different flowrates in the RO membrane. Feed COD concentration: (a) 10,000 mg/1; (b) 5,000 mg/1.

550 l/h. It was found that the permeate flux in the RO membrane rapidly increased with increasing transmembrane pressure up to about 14 bar and then a slow increase appeared, though the curves tended to be stable with decreasing the feed flowrate (Figs. 4a and 4b). At a pressure of 14 bar, permeate fluxes of 20 and 25 1/m2h were measured with flowrates of 350 and 550 l/h, respectively, at a feed COD of 10,000 rag/1 (Fig. 4a). At the same pressure and the feed COD of 5,000 mg/1, permeate fluxes of the RO also changed from 24 to 30 1/m2h (Fig. 4b). Conse- quently, the permeate flux increased by 26-28% while the feed flowrate increased by about 50%.

E

Q fl_

40

35

30

25

20

15

10

5

Pressure, bar

Fig. 5. Permeate flux as a function of transmembrane pressure for two different COD concentrations in the RO membrane. Feed flowrate = 550 1/h.

50 .-. 45 -* oaz(~-)=4orr~

--*-- CCO(RO)=~,000 rnoj1 x- 35 --e-oco(RO):lOOOOrr~ ./" m

= 30

25

n 15

10 100 200 300 4 ~ 500 600 700 800

Feed flovxate, I/h

Fig. 6. Permeate flux variation as a function of feed flowrate in the NF and RO membranes. Transmembrane pressure = 10 bar.

On the other hand, Fig. 5 illustrates the effect of feed COD concentration on the rate of fouling of the RO membrane at feed flowrate of 550 1/h. At a pressure of 14 bar, the permeate flux of the RO membrane showed 19% decrease (from 30 to 25 1/m2h) while the feed COD concentration increased from 5,000 to 10,000 mg/l, respect- ively. Thus, the decline in the RO flux because of increasing the feed COD varied between 15-31% with increasing the transmembrane pressure from 6 to 18 bar. According to these results, at a

M. Turan / Desalination 170 (2004) 83-90 89

4,5

40

" " 3 0

¢=

20

EL 10

5

, 5r:~o ~ (Ro) - - ~ 6oo~ (NF}

\

I I

100 10000

Fig. 7. Permeate flux variation as a function of COD concentration in the NF and RO membranes Trans- membrane pressure = 10 bar.

range of 33-52% while the feed COD and the transmembrane pressure increased from 40 to 450 mg/1 and 5 to 12 bar, respectively. The decrease in the filtration can be attributed to the increase both in the fouling layer and the concentration polarization layer. Similarly, a 27% increase in the permeate flux through the RO membrane was achieved with increas- ing the feed flowrate by about 50%. Besides, RO flux decline also varied between 15-31% while COD values ranged from 5,000 to 10,000 mg/1 and pressure ranged from 6 to 18 bar. Consequently, the flux decline in the RO membrane because of increasing the feed COD was lower than that in the NF membrane.

constant pressure, permeate fluxes as a function of feed flowrate remained practically stable for both the NF and RO membranes (Fig. 6). The flux decline in the RO membrane due to COD increase was lower than that in the NF membrane (Figs. 6 and 7).

4. Conclusions

The effect of hydrodynamic conditions and fouling on the filtration performance of the NF and RO membranes in dairy wastewater treatment was investigated. The following conclusions can be drawn from this work: • The RO and NF membranes showed excellent

performance by removing at 99.7% and 98% of the COD, respectively, from a feed COD concentrations of 40 and 450 mg/1 for NF feedwater and 5,000 and 10,000 mg/1 for RO feedwater.

• The permeate flux in the NF membrane increased by about 45% while the flowrates increased by 40%. On the other hand, the flux showed a slower increase at higher feed COD concentrations and NF flux declined in the

5. Symbols

A - - Water permeation coefficient, m/s bar

Cb - - Bulk solute concentration, mol/1 Cm - - Solute concentration next to the

membrane wall, mol/1 Cp - - Solute concentration in permeate,

mol/1 D - - Solute diffusion coefficient d - - Concentration polarization layer

thickness, m J - - Permeate flux, m 3 / m 2 s

J0 - - Clean membrane (initial) flux, m3/m z s

zk/ - - Flux decline, m a / m 2 s

k=d/D - - Mass transfer coefficient Ap - - Transmembrane pressure, bar Qf - - Feed flowrate, 1/h Qp - - Permeate flowrate, 1/h R m - - Clean membrane resistance, s bar/m R/ - - Fouling (cake) layer resistance,

s bar/m Ar~ - - Osmotic pressure gradient, bar

90 M. Turan / Desalination 170 (2004) 83-90

Acknowledgments

This work was partially supported by a grant from the ITU Research and Development Center (Project No: 1947).

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