Treatment of Textile Wastewater by a Coupling of Activated Sludge Process with Membrane Separation

This research was conducted to investigate the capability of a coupling of activated sludge and microfiltration processes with backflushing technique to reduce organic carbon and color in textile wastewater. In this study, the optimum condition of membrane operation was obtained at 0.88 m/s cross-flow velocity (CFV) and 0.4 bar transmembrane pressure (TMP). On the other hand, the optimum condition of backflushing technique was obtained at 1.6 bar pressure applied for 1 second at 1.5-minute interval. With this optimum condition, the flux was relatively stable at 5.04 L/m2.h for all SRT. At steady state, the effluent COD decreased with increasing SRT. The COD removal was more than 82 % and color removal was more than 95 %. The microorganisms involved in the system were found to be slowgrowing microorganisms. Therefore, the coupling of activated sludge and membrane separation processes successfully proceeded to treat textile wastewater. Complete solids removal as well as a significant degree of organic and color removal was achieved. Sludge production was also low amounting to less than 0.2 g dried cells/ g COD removed. Thus, the treatment of unsettled textile wastewater to a tertiary effluent quality in a single unit process was made possible

Treatment of Textile Wastewater by a Coupling of Activated Sludge Process

with Membrane Separation

Tjandra Setiadi, Suwardiyono and I Gede Wenten

Department of Chemical Engineering, Institut Teknologi Bandung (ITB)

Jl Ganesa 10, Bandung 40132, Indonesia Fax: 62-22-250 1438; Phone: 62-22-250 0989; Email: tjandra@che.itb.ac.id

ABSTRACT

This research was conducted to investigate the capability of a coupling of activated sludge and microfiltration processes with backflushing technique to reduce organic carbon and color in textile wastewater In this study, the optimum condition of membrane operation was obtained at 0.88 m/s cross-flow velocity (CFV) and 0.4 bar transmembrane pressure (TMP) On the other hand, the optimum condition of backflushing technique was obtained

at 1.6 bar pressure applied for 1 second at 1.5-minute interval With this optimum condition, the flux was relatively stable at 5.04 L/m2.h for all SRT At steady state, the effluent COD decreased with increasing SRT The COD removal was more than 82 % and color removal was more than 95 % The microorganisms involved in the system were found to be slow-growing microorganisms Therefore, the coupling of activated sludge and membrane separation processes successfully proceeded to treat textile wastewater Complete solids removal as well as a significant degree of organic and color removal was achieved Sludge production was also low amounting to less than 0.2 g dried cells/ g COD removed Thus, the treatment of unsettled textile wastewater to a tertiary effluent quality in a single unit process was made possible

Keywords: activated sludge, membrane separation, textile wastewater

INTRODUCTION

Textile industries progressed rapidly in Indonesia during the last ten years The development of textile industry is manifested in the rise of textile product exports In 1989, the exports amounted to US $ 2.02 billion and increased up to US $ 7.0 billion in 2002 The industries survived during the Asian economic-crisis became one of the most important sources of foreign exchange in Indonesia

Textile industries however, have caused serious environmental problems because of the wastewater produced Most textile industries produce wastewater with relatively high BOD, COD, suspended solids and color The wastewater may also contain heavy metals depending on the type of coloring substances used In general, the objective of textile industry wastewater treatment to reduce the level of organic pollutants, heavy metal, suspended solids and color before discharge into the river

Coloring substances are used for dyeing and printing processes The wastewater from these two processes is the most polluted liquid waste in a textile industry Biological, chemical, physical or the combination of the three treatment technologies can be used to treat textile industry liquid waste

Chemical coagulation is frequently practiced in Indonesian textile industries and sometimes it is the only method of treating wastewater The main disadvantage of this method is that it produces sludge that is needed to be disposed According to the current Indonesian Government Regulation (No 18, 1999), the textile sludge from chemical treatment is classified as a hazardous waste, so it should be treated in a proper way This means that the sludge disposal causes a substantial increase in wastewater treatment cost

The use of synthetic compounds in textile processes has increased Consequently, the amount of more complex compounds (characterized as having slow biodegradability in aerobic processes) in the textile effluent has increased On the other hand, conventional activated sludge processes (CASP) have been widely used for treating textile wastewater

To meet the stricter effluent standard, a more effective and efficient system to remove organic pollutants and color has to be established

The CASP has several disadvantages including high sludge production and large area requirement In addition, the performance of CASP is strongly affected by several parameters such as feed concentration sludge retention time (SRT), hydraulic retention time (HRT), biomass concentration in aeration tank (MLSS, Mixed Liquor Suspended Solid), organic loading, food to microorganism ratio (F/M ratio), sludge wastage rate, and sludge settling characteristic in sedimentation tank An increase in biomass concentration will increase degradation rate and reduce the area needed Nevertheless, high MLSS in aeration tank would cause settling problem because the settling qualities are poor at high sludge concentrations In addition, a disadvantage of secondary sedimentation tank is that its separation ability depends on the operating condition in aeration tanks Therefore, the performance enhancement of CASP by increasing MLSS can only be achieved through methods that do not use sedimentation technique An alternative method to replace sedimentation is the membrane separation technology

However, a widespread application of membrane technology has been hindered due to the relatively low flux and high energy needed for the membrane filtration The high-energy requirement for membrane separation is due to high cross-flow velocity and pressure needed to maintain flux stability [Dijk and Roncken, 1997] In order to reduce flux decline, highly sufficient cross-flow velocity over the membrane surface has to be maintained This

is subject to high operation cost due to recirculation An effort to minimize energy consumption has been done by several researchers Yamamoto [1989] incorporated hollow fiber membrane directly into the suspended solid aeration tank It enabled direct solid-liquid separation without requiring a recirculation pump However, it is still uncertain whether the process can be stable in a long time operation or not, as the hollow fiber membrane has inherent drawback of severe clogging at the neck of the module

Another technique is the rapid backflushing operation It offers an alternative to maintain flux stability at a low cross-flow velocity and pressure [Wenten, 1995] The backflush was dialed with a high frequency and extremely short time duration (backshock) and combined with the use of reverse asymmetric membrane structures This backshock mode allows filtration with very stable fluxes at extremely low cross-flow velocities and transmembrane pressure, which would extremely reduce energy requirement This technology integrated with an activated sludge process, has not been studied in detail for treatment of various industrial wastewater Therefore, this research was conducted to investigate the capability

of a coupling of activated sludge and microfiltration processes with backflushing technique

to reduce the organic carbon and color in textile wastewater

The main objective of this study was to find the optimum condition of membrane operation and backflushing technique Moreover, the influence of solid retention time (SRT) to the process performance, effluent quality, and kinetic parameters were also investigated

Fig 1 Schematic Diagram of the Coupling of Activated Sludge Process and Membrane Separation

MATERIALS AND METHODS

Wastewater preparation The wastewater was obtained from a textile plant carrying out

denim processing operations where only indigo dye was used The COD, BOD, color and

pH ranges were 795 – 1148 mg/L, 100 – 700 mg/L, 90 – 300 mg/L and 8 – 10.9, respectively A necessary amount of nitrogen and phosphorous were added to the wastewater to achieve COD : N : P ratio of 100 : 5 : 1 The pH of wastewater was also neutralized

Experimental Setup A schematic diagram of the activated sludge process employed in

this study is shown in Figure 1 The main unit consisted of an aeration tank and a microfiltration membrane as separation apparatus The aeration tank was made from Plexiglas with a working volume of 8 liters Air was supplied to the aeration tank through diffusers at a flow rate of 10 L/min Microporous membrane hollow fiber made of polypropylene with 0.2 µm pore size was used with a filtration area of 0.0226 m2 The backflush unit consisted of a three-way valve and timer and the backflusing technique was applied giving pressurized air from permeate side of the system A timer was used to set the time and interval of backflushing

Acclimatization and Reactor Operation The sludge from activated sludge process was

acclimatized by the textile wastewater in an aerobic batch tank without the membrane filtration unit Necessary amount of nitrogen and phosphorous were also added When the acclimatization period was complete, coupling of the activated sludge process with membrane separation was done This experiment was carried out in two sections , namely, preliminary step and main step The preliminary step was the optimization of flux stability

by manipulating operating conditions The main experiment was the treatment of textile wastewater through coupling of activated sludge process with membrane separation It was conducted with different solid retention times (SRT) The SRT was varied between 24 and 48 days and the hydraulic retention time (HRT) was maintained at 9 hours

INFLUENT

AIR

SLUDGE WASTE

COMPRESSED AIR PERMEATE

RESULTS AND DISCUSSION Optimization of membrane operating conditions

The purpose of this step was mainly to obtain the optimum conditions of process membrane at low cross-flow velocity (CFV) and transmembrane pressure (TMP) The membrane was operated at various CFV and TMP The liquor used was from the textile acclimatized activated sludge with the MLSS of around 8000 – 10000 mg/L The effects of TMP and CFV to the permeate flux are shown in Figures 2 and 3

0

15

30

45

60

75

TIME, MINUTES

2 h

TMP 0.2 bar TMP 0.4 bar TMP 0.6 bar TMP 0.8 bar

Fig 2 Effect of TMP on the membrane flux,

MLSS of 9300 mg/L, CVF (v) of 0.79 m/s,

without backflush

6 9 12 15 18

TIME, minutes

2 h

v 0.88 m/s

v 0.66 m/s

v 0.45 m/s

Fig 3 Effect of CFV (v) on the membrane flux,

MLSS of 9300 mg/L, TMP of 0.4 bar, without backflush

Figure 2 shows that at TMP of 0.4, 0.6 and 0.8, the flux was relatively unaffected and remained stable at about 22.04 L/m2h (LMH) This region was known as the mass controlled region and below 0.4 bar was known as the pressure controlled region [Cheryan, 1986] From this figure, it can be concluded that the optimum TMP of the membrane was 0.4 bar

To show the effect of CFV, the experiments were conducted at TMP of 0.4 bar and MLSS was kept constant at 9300 mg/L The CFV was varied but due to the limitation of the pump capabilities, the maximum CFV achieved was 0.88 m/s at TMP of 0.4 bar Figure 3 shows that higher CFV resulted to increased flux because of the higher shear stress of flow to sweep solutes adhering to the membrane surface Based on the above figures, TMP of 0.4 bar and CFV of 0.88 m/s were set to be used for the next experiments

Budiyono (1997) showed that the membrane operation without backflush would be difficult

to maintain a high flux Thus, in the following experiments, the backflushing condition was determined first before proceeding to the main experiments In this experiment, the backflush pressure, the interval between backflushes and the backflush duration applied were set Figure 4 and 5 show the results of the experiments

Since the backflush duration did not significantly affect the flux [Wenten, 1995], it was set

to 1 second Thus, only backflush pressure and the interval between backflushes were varied in this experiement Figure 4 shows the effect of time interval between backflushes

on flux The flux seemed to be stable when the backflushing interval was 1.5 minutes The increase in backflush interval caused reduction in the flux However, when the interval was lowered to 1 minute the flux also decreased Thus, a backflush interval of 1.5 minutes was chosen to determine the backflush pressure to be applied

Figure 5 shows that when the pressure was increased from 1.2 to 1.6 bar, the flux also increased from 23.3 to around 24.7 L/m2.h (LMH) However, the flux remained stable at 24.7 LMH, when the pressure was increased to 2 bar Results showed that the pressure

of 1.6 bar was enough to expel the particles blocking the membrane surfaces It was also confirmed that the fouling in this membrane was dominated by pore blocking [Mulder, 1991] From the experiments it can be concluded that the optimum condition of membrane operation was at 0.88 m/s cross-flow velocity (CFV) and 0.4 bar transmembrane pressure (TMP) The optimum condition of for backflushing technique was obtained at 1.6 bar pressure applied for 1 second at 1.5-minute interval

18

20

22

24

26

Time, minutes

2 h

1 minute 1.5 minutes 2 minutes 2.5 minutes

Fig 4 Effect of backflush interval on the flux,

MLSS of 8700 mg/l, TMP of 0.4 bar,

CFV of 0.88 m/s, backflushing pressure

of 1.5 bar and backflushing interval of

1 s.

22 23 24 25 26

0 10 20 30 40 50 60 70 80 90

Time, minutes

2 h

Fig 5 Effect of backflush pressure on the flux,

MLSS of 8700 mg/l, TMP of 0.4 bar, CFV of 0.88 m/s, backflushing pressure

of 1.5 bar and backflushing interval of

1 s.

Main experiments

In the main experiments, the textile wastewater having characteristics as mentioned in the wastewater preparation, was treated by the coupling of an activated sludge process and a membrane separation The backflush condition applied was that determined in the previous experiments The SRT was varied, to 24, 32, 40 and 48 days The permeate flux was observed daily and the data are shown in Figure 6

Figure 6 shows that the flux for all SRT declined as time passed and reached a relatively steady flux afterwards At SRT of 24 days, the flux decreased from 25 LMH to about 5 LMH within 12 days of operation On the 19th day, although the membrane was cleaned, the flux returned to about 5 LMH For other SRT operations, similar permeate trend was observed (steady flux of about 5 LMH) At SRT of 48 days, the flux decreased to 3 LMH after 15 days of operation due to some problems in the air compressor At that period, the backflush was not operated However, although there was no backflush, the flux was still maintained at about 3 LMH

During the experiments, MLSS in the aeration unit and COD in the influent and effluent were analyzed daily The MLSS data was presented by Suwardiyono (2000) In the steady state period, the average MLSS were 2900, 3500, 3800 and 4100 mg/L for SRT of 24, 32,

40 and 48 days, respectively The percentage of COD removal is shown in Figure 7 The figure shows that the COD removal increased with the time of operation until the steady state condition was achieved The average COD removal at steady state were 80.8 %, 87.6 %, 92.1 % and 96.3 % for SRT of 24, 32, 40 and 48 days, respectively However, fluctuations prior to steady state were observed more apparently in lower rather than higher SRT

0

5

10

15

20

25

0 2 4 6 8 10 12 14 16 18 20 22 24 26

TIME, DAYS

2 h)

SRT 24 Days SRT 32 days SRT 40 days SRT 48 days

Fig 6 Permeate Flux at various SRT in the

main experiments

0 20 40 60 80 100

TIME, Days

SRT 24 days SRT 32 days SRT 40 days SRT 48 days

Fig 7 Percentage of COD removal at various

SRT in the main experiments

Table 1 shows the ratio of BOD and COD of different samples at various SRT The BOD/COD of influent sample has an average of 0.66 This shows that the denim-processing wastewater can be classified as rather easily biodegradable waste by aerobic microorganisms It is evident in Table 1 that the increased SRT reduced the BOD/COD value thereby increasing the nonbiodegradable components in wastewater An increase in SRT on the other hand, increased the MLSS and reduced the F/M (food/mass) ratio This would then increase the possibility of a microbe cell in a lysis condition that might also increase the nonbiodegradable residue of the cell

Table 1 The ratio of BOD and COD at various SRT SRT,

days Sample COD, mg/L BOD mg/L 5 BOD/COD

Influent 1034 694 0.6712 Bioreactor 326 122 0.3742

24

Permeate 198 83 0.4192 Influent 1015 658 0.6483 Bioreactor 242 81 0.3347

32

Permeate 126 52 0.4189 Influent 997 649 0.6509 Bioreactor 168 55 0.3274

40

Permeate 79 32.78 0.4149 Influent 1014 666 0.6571 Bioreactor 92 24.44 0.2656

48

Permeate 38 16.33 0.4298

The COD removal data were presented by Suwardiyono (2000) Figure 8 gives an example of influent and effluent of the wastewater at SRT of 24 and 48 days Visually, the color removed almost completely with an average value of removal about 95 % from the original concentration of dyes of 90 to 300 mg/L

a b c d

Fig 8 Disparity of treated (effluent) and untreated (influent) wastewater

(a: effluent of 24-day SRT; c: effluent of 48-day SRT; b and d: untreated wastewater)

Kinetic Parameter

The kinetic parameters were determined by regression technique as has been described

in the literatures [Grady and Lim, 1980] The kinetic parameters determined were specific maximum growth (µm, day-1), cell yield (Y, mg cell/mg COD), half saturation constant (KS, mg/L), specific decay rate (b, day-1) and specific death rate (γ, day-1) Other symbols used

in the graphs were Vi as cell viability, and θc as SRT Figure 9 shows the graphs used to determine the parameters from the experimental data Figure 9.a gives the plot of (S0 -S)/(X.HRT) versus 1/SRT with the slope and intercept being 1/Y and b/Y, respectively Figure 9.b gives the plot of 1/Vi versus θc/(1 + b θc) with the slope and intercept being γ and b/Y, respectively Figure 9.c gives the plot of S/(1/θc + b + γ) with the slope and intercept being 1/µm and KS/µm, respectively

From the figures, the following kinetic parameter values were obtained: Y = 0.191; b = 0.151; γ = 3.073; µm = 3.273 and KS = 0.45 These values show that the specific growth rate (µm) and half saturation constant (KS) were relatively low and indicate slow-growing microorganisms [Grady and Lim, 1980]

y = 5,2453x + 0,7922

R 2 = 0,9628

0,85

0,9

0,95

1

1,05

1/SRT

R 2 = 0,9256

15 16 17 18 19 20

θc/(1+b.θc)

y = 0,3055x + 0,1375

R2 = 1

0 10 20 30 40 50 60 70

0 40 80 120 160 200

S

/θ

Fig 9 Graphs used in determining kinetic parameters

Conclusions

The following conclusions were inferred based on the results of the study:

1 The optimum condition of membrane operation was obtained at 0.88 m/s cross-flow velocity (CFV) and 0.4 bar transmembrane pressure (TMP) The optimum condition of backflushing technique was obtained at 1.6 bar pressure applied for 1 second at 1.5-minute interval With this optimum condition, the flux was relatively stable at 5.04 L/m2.h for all SRT

2 At steady state, the effluent COD decreased with increasing SRT The COD removal was more than 82 % and color removal was more than 95 %

3 The microorganisms involved in the system were found to be slow-growing microorganisms

Acknowledgement

This project was funded by the Indonesian Government through RUT VI, 1998-2000 with contract no, 40/SP/RUT/1998

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