Ultrafiltration of activated sludge: Flocculation and membrane fouling

Desalination 281 (2011) 142–150

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Ultrafiltration of activated sludge: Flocculation and membrane fouling María Matos a, José M. Benito b, Ángel Cambiella c, José Coca a, Carmen Pazos a,⁎ a b c

Department of Chemical and Environmental Engineering, University of Oviedo, Julián Clavería 8, 33006 Oviedo, Spain Department of Chemical Engineering, University of Burgos, Plaza Misael Bañuelos s/n, 09001 Burgos, Spain Wehrle Medioambiente S.L., Belice 1-3°C, 33212 Gijón, Spain

a r t i c l e

i n f o

Article history: Received 27 April 2011 Received in revised form 22 July 2011 Accepted 24 July 2011 Available online 8 September 2011 Keywords: Activated sludge Ultrafiltration Organic membranes Fouling Flocculation

a b s t r a c t The effect of flocculants on the ultrafiltration (UF) of activated sludge from a wastewater treatment plant, using organic membranes, was investigated. Flocculation trials were performed with commercial flocculants, ZETAG 7197 (an aliphatic polyamine) and Nalco MPE30 and MPE50 (cationic polymers). Optimal dosage was determined as a function of the mean floc size. Experiments were performed in a dead-end UF unit using polyethersulfone (PES) and regenerated cellulose (YM) flat membranes (10, 30, and 100 kDa) and in a crossflow UF-pilot plant unit, with a polyethersulfone (PES) membrane (100 kDa). Best results were obtained for the MPE50 flocculant. Membrane fouling was reduced at the optimal dosage of flocculant, while permeate collected was up to 20% of the feed volume. The total amount of permeate at the optimal dosage of MPE50 flocculant was 16–69% higher than without additives, depending on the MWCO (molecular weight cut-off), with an initial permeate flux increase of up to 80%. In the UF-pilot plant unit, a slight increase of permeate flux was observed at the optimal dosage of the flocculant, and more than 98% of chemical oxygen demand (COD) reduction was achieved. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Membrane bioreactors (MBRs) are widely used for industrial and urban wastewater treatment because of their high operation efficiencies. The membrane process coupled with the activated sludge treatment allows degradation of the organic material and removal of the suspended solids. Higher biomass concentrations can be treated with high-quality permeate [1,2]. Membrane fouling is the main limitation, increasing operating costs associated with membrane cleaning and reduced membrane life [2–5]. Aeration rate, mechanical mixing, and feed flow through the membrane have been studied to improve the performance of MBRs. These variables affect the size distribution of the activated sludge flocs and consequently the performance of the membrane [5,6]. Membrane parameters such as pore size, crossflow velocity and transmembrane pressure have also been investigated [4,7–9]. The influence on biofouling of activated sludge parameters, such as suspended solids concentration, particle size distribution, viscosity, hydrophobicity, extracellular polymeric substances (EPS), soluble or colloidal material, etc., has been reported [4,10–13]. The identity and diversity of bacterial populations on MBR operation also affect fouling, though to a lesser extent. Several approaches to reduce biofouling in MBRs have been attempted. Modified biopolymers with a net cationic charge are

⁎ Corresponding author. Tel.: + 34 985103509; fax: + 34 985103434. E-mail address: [email protected] (C. Pazos). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.07.058

membrane performance enhancers (MPE) that react with biomass, decreasing fouling [14–16]. Flocculation of the activated sludge leads to organic colloids aggregation, increasing membrane filterability and fouling control, due to the decrease of soluble foulant concentration, like SMP (soluble microbial products). Thus, by increasing the porosity of the cake layer on the membrane, an increase in filterability and cleaning reduction may be achieved. The effect of other enhancers such as powdered activated carbon, polyaluminium chloride, chitosan, starch, alumina, ferric salts, etc., on MBRs performance, as a function of COD, TOC, SMP, and N and P removal, transmembrane pressure (TMP) and oxygen uptake rate, has been reported [14–26]. The purpose of this study was to determine the effect of flocculants on membrane fouling for the UF of activated sludge from an industrial wastewater treatment plant. Laboratory tests were performed in a dead-end UF unit with flat membranes and crossflow tests in a UF-pilot plant unit with tubular organic membranes. 2. Materials and methods 2.1. Activated sludge and flocculants Activated sludge samples with a COD of 12,000 mg/L from a local wastewater treatment facility in northern Spain were used as the raw influent in this study. They were taken before a set of experiments was carried out. Their properties (pH, conductivity and mixed liquor suspended solid (MLSS) concentration) are shown in Table 1. Three liquid commercial flocculants were used: two cationic polymers (MPE30 and MPE50, PermaCare® MPE), supplied by Nalco

M. Matos et al. / Desalination 281 (2011) 142–150

143

concentration, ±0.2 mg/L; conductivity, ±3 μS/cm; pH, ±0.1 units; and zeta potential, ±0.2 mV.

Table 1 Properties of activated sludge samples. Sample

pH

Conductivity (μS/cm)

MLSS (g/L)

1 2 3 4 5

7.1 6.2 6.3 7.2 6.4

826 931 732 1204 854

18.0 20.9 16.2 11.3 17.0

Table 2 Characteristics of flat-sheet UF membranes. Membrane

Material

MWCO (kDa)

Jw (L/m2 h)

PES10 PES30 PES100 YM10 YM30 YM100

Polyethersulfone Polyethersulfone Polyethersulfone Regenerated cellulose Regenerated cellulose Regenerated cellulose

10 30 100 10 30 100

70 266 810 33 259 768

2.2. Membranes Six Millipore flat-sheet UF polyethersulfone (PES) and regenerated cellulose (YM) membranes (MWCO 10, 30, and 100 kDa, effective membrane area of 28.7 cm 2), were used and their characteristics are summarized in Table 2. Crossflow UF experiments were performed with a HyMem 37.03 I10 tubular polyethersulfone (PES) membrane module (length 600 mm, total area 0.02 m 2, MWCO 100 kDa), supplied by Berghof Filtrations- und Anlagentechnik GmbH & Co. KG (Eningen, Germany). The membrane is suitable for the filtration of liquids containing up to 30 g/L of non-dissolved material. These membranes have an average life that depends heavily on the operating conditions. An average life of 4 to 5 years may be estimated, although there are several plants that have been working with the same type of membranes for about 10 to 12 years. 2.3. UF experiments

Europe B.V. (Leiden, The Netherlands), and a highly cationic flocculant ZETAG 7197, an aliphatic polyamine, supplied by Ciba Especialidades Químicas S.L. (Barcelona, Spain). Flocculants are generally supplied as liquids, thus their shelf-life is highly dependent on storage conditions, and they should not be exposed to direct sunlight. Their shelf-life is one year at the most. For industrial applications flocculants are diluted in water to facilitate pumping. After dilution, they should be used within the next two days. 100 mL samples of activated sludge were mixed with increasing concentrations (0–2500 mg/L) of flocculant. Optimal dosage was determined by measuring floc size distribution by laser light scattering, using a Mastersizer S long bench equipment (Malvern instruments Ltd., UK). Conductivity, pH, zeta potential and stability of the activated sludge samples with flocculant were also studied. The zeta potential was determined with a Zetasizer NanoZS unit (Malvern Instruments Ltd., UK) with triplicate measurements for each sample at 25 °C. Stability was determined by a Turbiscan LAb Expert equipment (Formulaction Co., L'Union, France). Activated sludge samples, with and without flocculant addition, were placed in cylindrical glass cells and the transmitted (180° from the incident light) and backscattered light (45° from the incident light) monitored as a function of time and cell height for 3 days at 30 °C. The effect of flocculant addition on UF performance at optimal dosage was compared with the UF of activated sludge with no flocculant addition. Three to five replicates of the analytical measurements were conducted for each sample and the average value was taken. The confidence limits of the techniques used were: COD, ±4 mg/L; MLSS

2.3.1. Dead-end UF Batch UF experiments were performed in an Amicon 8200 stirred cell at room temperature, with 100 mL feed samples, a transmembrane pressure (TMP) of 100 kPa and 300 rpm of stirring speed. Permeate was collected for 30 min with cumulative mass monitored by an electronic balance (A&D Instruments Ltd., model FX 2000, Tokyo, Japan), as illustrated in Fig. 1. The permeate pH was measured with a Crison 25 pH-meter. COD analyses were determined by the reactor digestion method [27] using a Hach DR2010 UV spectrophotometer. 2.3.2. Membrane cleaning A membrane cleaning protocol was established using PES30 membranes previously fouled under constant conditions. A 100 mL sample of activated sludge was filtered and 70 g of permeate were collected at room temperature, at a TMP of 200 kPa and at 300 rpm. The fouled membrane was then rinsed with distilled water at room temperature, distilled water at 50 °C, and ultrasounds at room temperature for 15 min. A cleaning step followed with three different detergents: neutral (P3-Ultrasil-53), basic (P3-Ultrasil-110) and acid (P3-Ultrasil-75), supplied by Ecolab (Barcelona). Each cleaning step was carried out at 50 °C and several detergent concentrations: 1.5% w/w neutral detergent, 0.5% w/w basic detergent, and an acid detergent solution at pH = 1. Cleaning was conducted until 100 g of permeate was collected. Finally, membranes were rinsed twice with distilled water at 50 °C after each cleaning step. Water flux was measured at room temperature, at a TMP of 100 kPa and at 300 rpm to compare the effectiveness of each step.

Fig. 1. Dead-end ultrafiltration experimental setup.

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M. Matos et al. / Desalination 281 (2011) 142–150

Fig. 2. Crossflow ultrafiltration experimental setup.

a) 50 PES 10 kDa YM 10 kDa

J (L/m2h)

40 30 20 10 0 0

500

1000

1500

2000

Time (s)

b) 50 PES 30 kDa YM 30 kDa

40

J (L/m2h)

2.3.3. Crossflow ultrafiltration A flow diagram of the crossflow UF-pilot plant is shown in Fig. 2. The feed solution was circulated by a Grundfos CHN 4–60 centrifugal pump from a 30 L internally-cooled feed tank to the membrane module. The circulation flow rate was measured with a Comaquinsa R-005 flowmeter (FT) and the permeate flux by collecting a volume of fluid in a given time. Crossflow velocities and transmembrane pressures were monitored by flow control valves (V1, V2 and V3). Operational variables, measured by pressure (PT1, PT2) and temperature (TC) transducers, were kept constant during the experiments. Oxygen concentration was monitored by a Crison OXI 49 oxygen meter. A check valve (V4) controlled the compressed air flow. Crossflow UF experiments were performed in total recycle mode at room temperature, TMP of 200 kPa, feed flow rate of 800 L/h (corresponding to a crossflow velocity of 2.83 m/s) and an oxygen concentration in the range of 7–8 mg/L. The membrane was cleaned following a two-step standard protocol after each UF run. First, it was rinsed with water and then cleaned with 1.5% w/w neutral detergent P3-Ultrasil 53 (Ecolab, Barcelona, Spain) in distilled water for 60 min at 35–40 °C, TMP = 100 kPa and a crossflow velocity of 5 m/s. Then, the membrane was rinsed again with freshly deionized water and the permeate flux measured at 20 °C and different TMPs to evaluate the flux recovery. Data points for ultrafiltration experiments in this work are the average of at least three replications, with high reproducibility in the results within deviations of ±5%.

30 20 10 0

3. Results and discussion

0

500

1000

1500

2000

Time (s)

Batch UF experiments were performed with the same feed sample under the same experimental conditions with two types of membranes. Fresh activated sludge samples were used to avoid changes in its properties over time [18]. Fig. 3 shows the permeate flux (J) for PES and YM flat-sheet UF membranes. Permeate fluxes for YM membranes are similar, around 30 L/m 2 h, for all the MWCO's, while they increase with MWCO for PES membranes. Therefore, membrane filterability is higher for 10 kDa YM: it is similar for both PES and YM 30 kDa and is still higher for the 100 kDa PES. PES membranes were selected for subsequent experiments to study the influence of MWCO on membrane filterability.

c) 50 PES 100 kDa YM 100 kDa

40

J (L/m2h)

3.1. Dead-end ultrafiltration experiments

30 20 10 0

3.1.1. Effect of flocculant on activated sludge properties Preliminary flocculation tests were performed with activated sludge samples at several flocculant concentrations. The optimal concentration was determined by measuring the floc size distribution

0

500

1000

1500

2000

Time (s) Fig. 3. Permeate flux vs. time for activated sludge (sample 4) filtered with PES and YM flat-sheet membranes according to their cut-off.

M. Matos et al. / Desalination 281 (2011) 142–150 Table 3 Effect of flocculant addition on pH and conductivity of activated sludge (sample 3). Flocculant concentration (mg/L)

MPE30

MPE50

ZETAG 7197

pH

Conductivity (μS/cm)

pH

Conductivity (μS/cm)

pH

Conductivity (μS/cm)

0 500 1000 1500 2000 2500

6.3 6.4 6.4 6.2 6.2 6.1

732 775 794 911 1057 1016

6.3 6.1 6.1 6.2 6.2 6.2

732 979 1200 1457 1672 1899

6.3 7.1 6.9 6.8 6.2 6.1

732 1016 1269 1430 1514 1702

and the following parameters: pH, conductivity, zeta potential and stability. Conductivity increased upon flocculant addition, especially for MPE50 and ZETAG 7197. However, no pH changes were noticed, as shown in Table 3 for sample 3 of the activated sludge. Zeta potentials as a function of flocculant concentration (samples 3 and 4 of activated sludge) are shown in Fig. 4. A similar trend was observed for each flocculant. The zeta potential provides information on solids agglomeration. Microbial flocs of activated sludge usually exhibit a negative surface charge, and therefore a negative zeta potential. A small absolute value suggests a weak repulsion between flocs. Negative or close to zero zeta potentials were obtained for the optimal concentration of flocculant. This fact justifies the settling of the solid phase of activated sludge, after a certain time, and the appearance of a clear supernatant liquid phase. The surface charge of microbial flocs is neutralized when a cationic polymer is added to the activated sludge. Neutral flocs attract other flocs producing larger flocs by a charge neutralization mechanism. Furthermore, an excess of flocculant above the optimal dosage causes deflocculation as the surface charge of the flocs becomes positive [16,18]. This

Zeta potential (mV)

a) 15 MPE30 MPE50 ZETAG 7197

10 5 0 -5 -10 -15 0

500

1000 1500 2000 2500 3000

Flocculant concentration (mg/L)

behavior is apparent for ZETAG 7197, where deflocculation is observed and the zeta potential increased with flocculant concentration, as shown in Fig. 4. This is typical of a charge neutralization mechanism. A similar behavior was observed for MPE50, the zeta potential also increasing with concentration while deflocculation was not as pronounced. Thus, MPE50 suggests two destabilization mechanisms: bridge formation and charge neutralization. It is difficult to ascertain the actual destabilization mechanism as the flocculant composition is unknown. For MPE30, once neutralization of flocs surface charge occurs, the zeta potential does not change, even when flocculant concentration increases. This behavior might be explained by a bridging mechanism. Light backscattering profiles of activated sludge (3 days at 30 °C) without additives and adding the optimal dosage of flocculant (for sample 3) are shown in Fig. 5. No large variations with concentration were observed for each flocculant, likely because a quick destabilization process with a considerable increase in floc size occurring before taking the stability measurement in the Turbiscan apparatus. The lowest value, which corresponds to a larger floc size, occurs when MPE50 is added. However, the floc size increase with time, which corresponds to the slight backscattering reduction, is small in all cases. 3.1.2. Effect of flocculant on permeate flux The UF process depends on the sludge quality/properties, which vary greatly with sample storage [18]. Flocculation tests were performed with fresh activated sludge samples, and immediately before each UF experiment. Fig. 6 shows floc size distributions (sample 4) before and after the addition of flocculants. Similar distributions were obtained for the other samples. For ZETAG 7197, a clear optimum concentration was observed for all samples. Deflocculation occurred with an excess of flocculant yielding lower permeate fluxes and SMP removal [14]. The optimal dosage of MPE30 and MPE50 was determined for the largest floc size. In other works optimal concentrations were determined as a function of SMP or COD removal of supernatant or for the largest filtrate volume through a 0.45 mm filter in a fixed time [18,20,23]. It has been reported that membrane filterability is related to microbial floc size [5–21] that can be reduced by the MBR pump shear. A comparison of floc size with the optimal flocculant dosage for sample 4 is shown in Fig. 6d. MPE50 yields the largest floc size for most of the samples. Table 4 summarizes the volume mean diameter or D[4,3] of samples 1–5 and the ratio mg flocculant/mg MLSS calculated for each optimal dosage. It is observed that MPE50 leads to a maximum floc size in most of the samples, 6–12 times larger than the original volume mean diameter of the raw activated sludge. Although ZETAG produces large flocs at lower concentrations, it leads to a poor UF behavior because of deflocculation. Koseoglu et al. [18] have reported that the performance of cationic polymers does not vary for a small range of concentration, while other additives decrease

MPE30 MPE50 ZETAG 7197

5 0 -5 -10

15 No flocculant

% Backscattering

Zeta potential (mV)

b) 15 10

145

1500 mg/L MPE30 1500 mg/L MPE50

14

500 mg/L ZETAG 7197

13 12 11 10

-15 0

500

1000 1500 2000 2500 3000

Flocculant concentration (mg/L) Fig. 4. Zeta potential as a function of flocculant concentration for activated sludge samples 3 (a) and 4 (b).

0

1

2

3

Time (days) Fig. 5. Backscattered light data measured at 10 mm cell height vs. time for activated sludge (sample 3) as a function of the optimal dosage of flocculant.

146

M. Matos et al. / Desalination 281 (2011) 142–150

a) 16

b) 0 mg/L 500 mg/L 1000 mg/L 1500 mg/L 2000 mg/L 2500 mg/L

Volume (%)

12 10

16 0 mg/L 500 mg/L 1000 mg/L 1500 mg/L 2000 mg/L 2500 mg/L

14 12

Volume (%)

14

8 6

10 8 6

4

4

2

2

0

0 1

10

100

1000

1

10

Diameter (µm)

c) 16

d) 0 mg/L 500 mg/L 1000 mg/L 1500 mg/L 2000 mg/L 2500 mg/L

Volume (%)

12 10

1000

16 No flocculant 1500 mg/L MPE30 1500 mg/L MPE50 500 mg/L ZETAG 7197

14 12

Volume (%)

14

100

Diameter (µm)

8 6

10 8 6

4

4

2

2

0

0 1

10

100

1000

1

10

Diameter (µm)

100

1000

Diameter (µm)

Fig. 6. Floc size distributions of activated sludge (sample 4) before and after the addition of flocculants: (a) MPE30, (b) ZETAG 7197, (c) MPE50, (d) optimal dosage.

filterability. Furthermore, Lee et al.[14] have observed that the mean floc size can decrease for a certain dosage of a cationic polymer due to deflocculation. Figs. 7–9 illustrate the permeate flux as a function of time for UF of activated sludge, with and without flocculant addition, for each MWCO. An initial increase of permeate flux was observed for all flocculants. Best results were obtained with MPE50 probably due to the larger flocs formed. Some authors have confirmed that coagulant addition minimizes MBR biofouling because particle aggregation reduces the cake resistance [13,25]. For PES10 membranes, an 80% initial permeate flux increase was observed when flocculant was added. Moreover, a sharp decrease in permeate flux over a short period of time occurs, as shown in Fig. 7, when the sludge concentration in the retentate is over 40 g/L. It must be pointed out that experiments were carried out with no permeate recirculation. For the 100 mL samples, a large amount of permeate was eluted in a small period of time, leading to high retentate concentrations and a relatively thick cake layer. The total amount of permeate collected was up to 20% of the feed volume for all flocculants. Results for the PES30 membranes are shown in Fig. 8. An increase of 60% in the initial permeate flux with MPE50 was observed. However, a

rapid flux decline took place and the total amount of permeate collected was similar to that obtained without flocculant addition. An improvement of 12% was observed with MPE30 and ZETAG 7197. For experiments with PES100 membranes (Fig. 9), a filterability enhancement over 17% with MPE50 was observed. An improvement of 10% was obtained with MPE30 and ZETAG 7197. A smaller increase of the initial permeate flux was observed compared to the other membranes. Flocculant addition had a slight influence on permeate COD, being lower than 250 mg/L. Only permeate conductivity was affected by the addition of several flocculants. The largest values were obtained for MPE50 and ZETAG 7197, as shown in Table 5. The effect of MWCO on filterability with flocculant addition was also studied using PES membranes and activated sludge (sample 3) without additives and with addition of optimal concentration of MPE50. Results are shown in Fig. 10.

200 No flocculant 1500 mg/L MPE30 1500 mg/L MPE50 500 mg/L ZETAG 7197

Table 4 Volume mean diameter (D[4,3]) and optimal dosage of each flocculant per mg MLSS for samples 1–5.

Sample Sample Sample Sample Sample

1 2 3 4 5

Activated sludge

ZETAG 7197

MPE30

D[4,3] (μm)

D[4,3] (μm)

mg/mg MLSS

D[4,3] (μm)

mg/mg MLSS

D[4,3] (μm)

mg/mg MLSS

35.5 24.0 25.6 21.2 22.0

228.1 163.8 124.3 57.1 176.3

0.028 0.072 0.031 0.044 0.059

99.2 54.2 33.1 61.6 49.3

0.056 0.072 0.062 0.133 0.088

204.8 156.6 185.5 235.6 275.5

0.056 0.072 0.093 0.133 0.059

J (L/m2h)

150

100

MPE50

50

0 0

500

1000

1500

2000

Time (s) Fig. 7. Permeate flux vs. time for UF of activated sludge (sample 2) with and without flocculant addition using PES10 flat membranes.

M. Matos et al. / Desalination 281 (2011) 142–150

200

J (L/m2h)

Table 5 Properties of permeates obtained after UF of activated sludge (sample 1) with and without flocculant addition using PES100 flat membranes.

No flocculant 1000 mg/L MPE30 1500 mg/L MPE50 500 mg/L ZETAG 7197

150

147

Feed

Permeate pH

Conductivity (μS/cm)

COD (mg/L)

7.9 8.2 7.2 7.3

833 846 1333 1070

188 225 203 181

100 Activated Activated Activated Activated

50

sludge sludge + 1000 mg/L MPE30 sludge + 1000 mg/L MPE50 sludge + 500 mg/L ZETAG 7197

0 500

1000

1500

2000

Time (s) Fig. 8. Permeate flux vs. time for UF of activated sludge (sample 3) with and without flocculant addition using PES30 flat membranes.

The filterability for the three membranes increased. Several studies have been reported on the efficiency of MPE50 at full-scale processes [14,15,20,21,26]. For the 10 kDa PES membrane, the flux increase is steady, while for the other two membranes fluxes are lower at the end than those obtained without flocculant addition. The total amount of permeate collected for UF with MPE50 flocculant is 69% higher than that obtained without additives for 30 kDa membranes. The flux increase for 10 and 100 kDa membranes was 16% and 33%, respectively. 3.1.3. Fouling resistances The flux decline is mostly caused by fouling of the membrane due to deposition of biosolids inside the pores and the formation of a cake layer. The total filtration resistance, Rt (m − 1), may be described according to Darcy's law:

The addition of flocculants to the activated sludge samples caused a rapid cake layer build-up, and concentration polarization was only relevant during the first 15–30 s of the UF process. Therefore, the concentration polarization effect is included into the cake layer resistance (Rc).

a) 100 No flocculant 1500 mg/L MPE50

80

J (L/m2h)

0

60 40 20 0 0

500

1000

1500

2000

Time (s) J=

TMP μRt

ð1Þ

b) 100 No flocculant 1500 mg/L MPE50

and Rt according to the resistance-in-series model can be expressed as:

where TMP is the transmembrane pressure (N/m 2), μ is the viscosity of the permeate (Ns/m 2), J is the permeate flux (m 3/m 2s), and Rm, Rp, and Rc (m − 1) are the intrinsic membrane, pore fouling and cake layer (reversible fouling) resistances, respectively [9]. Pore narrowing by adsorption of dissolved matter and/or pore plugging is considered irreversible fouling.

60 40 20 0 0

500

1000

1500

2000

Time (s)

c) 200

300

150

1500 mg/L MPE50

160 140

J (L/m2h)

200

No flocculant

180

No flocculant 1000 mg/L MPE30 1000 mg/L MPE50 500 mg/L ZETAG 7197

250

J (L/m2h)

J (L/m2h)

ð2Þ

Rt = Rm + Rc + Rp

80

120 100 80 60

100

40 50

20 0 0

0 0

500

1000

1500

2000

500

1000

1500

2000

Time (s)

Time (s) Fig. 9. Permeate flux vs. time for UF of activated sludge (sample 1) with and without flocculant addition using PES100 flat membranes.

Fig. 10. Permeate flux vs. time for UF of activated sludge (sample 3) using 10 kDa (a), 30 kDa (b) and 100 kDa (c) PES flat membranes, with and without addition of the optimal dosage of MPE50 flocculant.

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M. Matos et al. / Desalination 281 (2011) 142–150

Rf x 10-12 (m-1)

40

Table 6 Cake resistances (αCw) for UF of activated sludge with and without flocculant addition using PES100 flat membranes.

No flocculant, PES 10 kDa 1500 mg/L MPE50, PES 10 kDa No flocculant, PES 30 kDa 1500 mg/L MPE50, PES 30 kDa No flocculant, PES 100 kDa 1500 mg/L MPE50, PES 100 kDa

30

UF test Activated Activated Activated Activated

20

αCw × 10− 15 (m− 2)

0.999 0.995 0.992 0.999

3.44 2.33 2.93 2.63

10

Assuming that Rm and αCw remain constant, integration of Eq. (4) gives:

0 0

500

1000

1500

2000

Time (s) Fig. 11. Fouling resistance vs. time for UF of activated sludge (sample 3) using 10 kDa, 30 kDa and 100 kDa PES flat membranes, with and without addition of the optimal dosage of MPE50 flocculant.

Rt can be readily determined from sludge filtration experiments and Rm from pure water filtration through the membrane. Rc and Rp are difficult to measure because a complete decoupling cannot be assumed, but a fouling resistance, Rf, can be defined as the sum of both (Rf = Rc + Rp) and determined as: Rf = Rt − Rm. A plot of Rf vs. time is shown in Fig. 11. It is observed that Rf increases in all cases with time because of pore fouling and cake layer formation. An initial decrease of Rf is noticed for the three membranes with the addition of MPE50. However, for the 30 kDa and 100 kDa membranes Rf increases abruptly after 1000– 1300 s of operation because of fast cake layer build-up due to the high initial permeate fluxes. This behavior is not observed for PES10 membranes likely because of its larger pore resistance. If it is assumed that with flocculant addition and for the larger membrane pore size Rc NN Rp, and Rt can be expressed as in conventional constant pressure filtration as: R t = Rm +

αCw V A

ð3Þ

where α is the cake specific resistance per unit mass (m/kg), Cw is the rejected particle concentration near the membrane (kg/m 3), Vf is the volume of permeate (m3) and A is the total membrane area (m2). Eq. (3) shows that Rc increases proportionally with permeate volume. The filtration flow rate, Qf (m 3/s), can be expressed as: Qf =

sludge sludge + 1500 mg/L MPE30 sludge + 1500 mg/L MPE50 sludge + 1500 mg/L ZETAG 7197

r2

dV f A TMP  =  dt μ Rm + αCAw Vf

40000

ð4Þ

PES 100 kDa, no flocculant PES 100 kDa, MPE30 PES 30 kDa, no flocculant PES 30 kDa, MPE30

35000

t Rm μαCw + = Vf Vf A TMP 2A2 TMP

ð5Þ

A plot of t/Vf vs. Vf for batch dead-end UF for the 30 kDa and 100 kDa PES membranes is shown in Fig. 12. Fitting of the data to Eq. (5) determines αCw. Results for the PES100 membrane follow a linear behavior as predicted by the model, with a high correlation, while deviations are observed for the PES30 membrane. Rp is higher than Rc during the initial filtration time for PES30 (and also for PES10) membrane. Only at the end of filtration time Rc becomes more important and a behavior change is noticed. Huang et al.[17] determined that the porosity of the formed biofilm increased when membrane fouling reducers were added. Table 6 shows that the cake resistance parameter, αCw, decreases with the addition of flocculant, being 33% for MPE50, 24% for MPE30, and 15% for ZETAG 7197. 3.1.4. Membrane cleaning A membrane cleaning protocol was established to evaluate membrane fouling under fixed conditions. The reference sample had a MLSS concentration of 8 g/L. The three membranes were rinsed following the three methods described previously. Fig. 13 shows the scanning electron micrograph (SEM) of the cake layer formed using a MEB JEOL-6100 over the PES100 membrane after UF. Two typical bacteria morphologies may be noticed: rod-shaped for bacilli and spherical for cocci. Water fluxes are shown in Table 7a, before and after each rinsing step. Best results were obtained for rinsing with distilled water at 50 °C. Results for the next step, cleaning with detergent solutions, are shown in Table 7b. The neutral detergent was the most effective and higher values than the initial water flux were achieved. This might be explained

PES 100 kDa, MPE50 PES 100 kDa, ZETAG 7197 PES 30 kDa, MPE50 PES 30 kDa, ZETAG 7197

t/Vf (s/L)

30000 25000 20000 15000 10000 5000 0 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Vf (L) Fig. 12. Variation of t/Vf vs. permeate volume (Vf) for UF of activated sludge using PES30 and PES100 flat membranes, with and without addition of the optimal dosage of flocculant, in order to test the cake filtration model shown in Eq. (5).

Fig. 13. SEM micrograph of PES100 flat membrane surface after activated sludge UF.

M. Matos et al. / Desalination 281 (2011) 142–150

149

Table 7 Water flux before (J0) and after UF experiment (J1), the rinsing step 1 (J2) and the cleaning step 2 (J3) with detergent. a. Rinsing step 1 Membrane

J0 (L/m2 h)

J1 (L/m2 h)

J1/J0 (%)

Step 1

J2 (L/m2 h)

J2/J0 (%)

1 2 3

209 242 258

64.9 74.4 66.2

31.0 30.9 25.7

Water Hot water Ultrasounds

67.2 86.3 74.0

32.0 35.9 28.7

b. Cleaning step 2 after hot water rinsing step Membrane

J0 (L/m2 h)

J1 (L/m2 h)

J2 (L/m2 h)

J2/J0 (%)

Step 2

J3 (L/m2 h)

J3/J0 (%)

2 4 5

242 230 257

74.4 67.9 67.3

86.3 68.5 75.8

35.9 29.8 29.4

Neutral Acid Basic

276 38.1 114

132 16.5 44.2

because rinsing at 50 °C may cause a temporary pore expansion. Thus, the cleaning protocol was a rinsing step with distilled water at 50 °C, followed by cleaning with neutral detergent at a concentration of 1.5% w/w. A solution of 1500 mg/L (optimum dosage) of MPE50 was filtered through a membrane fouled with the same activated sludge sample to evaluate the effect of MPE50 on the cleaning protocol. Water flux was similar to that obtained with samples filtered without flocculant addition. It appears that the addition of MPE50 flocculant had no influence on membrane cleaning for the experiments carried out in this work.

3.2. Crossflow ultrafiltration experiments Crossflow UF experiments were conducted with 100 kDa PES tubular membranes at pilot plant scale, based on the aforementioned results, as this membrane gave the highest permeate fluxes. Flocculant was added at the optimal dosage determined from preliminary flocculation tests. MPE50 produces the largest floc size, followed by ZETAG 7197 and MPE30. An increase in permeate flux (Fig. 14) was observed for ZETAG 7197 and MPE50. The total amount of permeate collected was around 5 and 10% of the initial feed, respectively. These are small amounts compared to those obtained with flat membranes with MPE50. A permeate flux reduction was noticed with MPE30 because a deflocculation process might have happened due to the pump shear in the system [5]. Permeate CODs were lower than 200 mg/L, with COD reductions higher than 98% for all experiments, as shown in Table 8.

200

No flocculant 1500 mg/L MPE30 1000 mg/L MPE50 1000 mg/L ZETAG 7197

J (L/m2h)

150

4. Conclusions The addition of several flocculants on the performance of UF membranes for the filtration of an industrial activated sludge was studied. The main conclusions on the effect of flocculants on membrane fouling are the following: 1. Cationic flocculants neutralized the surface charge of activated sludge producing larger flocs. The zeta potential changed slightly, being negative but very close to zero, for the optimal dosage of flocculant. Sludge pH remained almost constant and conductivity increased. Furthermore, an excess of flocculant can lead to a restabilization process and floc sizes decreased to their original values. 2. MPE50 flocculant produced larger flocs with a more uniform size distribution, and a higher increase in permeate flux than with the other flocculants was observed. 3. The total amount of permeate collected for UF of activated sludge with the optimal dosage of MPE50 flocculant was 16–69% higher than that obtained without additives, depending on MWCO, with initial permeate flux increase up to 80%. 4. The fouling resistance (Rf) increased with time because of pore fouling and cake layer formation. In the case of 100 kDa PES membrane the fouling resistance was mainly due to the cake layer formed and its cake resistance parameter was decreased with flocculant addition. Best results were obtained when MPE50 was added. 5. Initial water flux was completely restored after application of a membrane cleaning protocol consisting of a rinsing step with distilled water at 50 °C, followed by cleaning with a neutral detergent solution (1.5% w/w in water). Flocculant addition had no effect on membrane cleaning process. 6. Pilot plant scale crossflow UF experiments were conducted using 100 kDa PES tubular membranes. A slight improvement on permeate flux was observed only when MPE50 and ZETAG 7197 flocculants were added at their optimal dosages, being higher for MPE50. More than 98% of COD reduction was achieved in the permeate, with concentrations lower than 200 mg/L. 7. The MWCO has an important influence on membrane filterability, and it should be studied in more detail considering other properties, such as SMP and COD removal, besides the mean floc size.

100

Table 8 Properties of permeates obtained after crossflow UF of activated sludge (sample 5) with and without flocculant addition using 100 kDa PES tubular membranes.

50

Feed

Permeate

0 0

1000

2000

3000

pH

Conductivity (μS/cm)

COD (mg/L)

7.2 7.1 8.5 8.6

1279 1286 2370 1481

148 192 185 163

4000

Time (s) Fig. 14. Permeate flux vs. time for UF of activated sludge (sample 5) with and without flocculant addition using 100 kDa PES tubular membranes.

Activated Activated Activated Activated

sludge sludge + 1500 mg/L MPE30 sludge + 1000 mg/L MPE50 sludge + 1000 mg/L ZETAG 7197

150

M. Matos et al. / Desalination 281 (2011) 142–150

References [1] S. Rosenberger, U. Krüger, R. Witzig, W. Manz, U. Szewzyk, M. Kraume, Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water, Water Res. 36 (2002) 413–420. [2] T. Stephenson, S. Judd, B. Jefferson, K. Brindle (Eds.), Membrane Bioreactors for Wastewater Treatment, IWA Publishing, London, 2000. [3] I.-S. Chang, C.-H. Lee, Membrane filtration characteristics in membrane-coupled activate sludge system — the effect of physiological states of activate sludge on membrane fouling, Desalination 120 (1998) 221–233. [4] H. Choi, K. Zhang, D.D. Dionysiou, D.B. Oerther, G.A. Sorial, Effect of activated sludge properties and membrane operation conditions on fouling characteristics in membrane bioreactors, Chemosphere 63 (2006) 1699–1708. [5] J.-S. Kim, C.-H. Lee, I.-S. Chang, Effect of pump shear on the performance of a crossflow membrane bioreactor, Water Res. 35 (2001) 2137–2144. [6] S.P. Hong, T.H. Bae, T.M. Tak, S. Hong, A. Randall, Fouling control in activated sludge submerged hollow fiber membrane bioreactors, Desalination 143 (2002) 219–228. [7] R. Bai, H.F. Leow, Microfiltration of activated sludge wastewater — the effect of system operation parameters, Sep. Purif. Technol. 29 (2002) 189–198. [8] H. Choi, K. Zhang, D.D. Dionysiou, D.B. Oerther, G.A. Sorial, Effect of permeate flux and tangential flow on membrane fouling for wastewater treatment, Sep. Purif. Technol. 45 (2005) 68–78. [9] H.H.P. Fang, X. Shi, Pore fouling of microfiltration membranes by activated sludge, J. Membr. Sci. 264 (2005) 161–166. [10] D.J. Barker, D.C. Stuckey, A review of soluble microbial products (SMP) in wastewater treatment systems, Water Res. 33 (1999) 3063–3082. [11] H. Nagaoka, S. Ueda, A. Miya, Influence of bacterial extracellular polymers on the membrane separation activated sludge process, Water Sci. Technol. 34 (9) (1996) 165–172. [12] S. Rosenberger, H. Evenblij, S. te Poele, T. Wintgens, C. Laabs, The importance of liquid phase analyses to understand fouling in membrane assisted activated sludge processes — six case studies of different European research groups, J. Membr. Sci. 263 (2005) 113–126. [13] Z. Ahmed, J. Cho, B.-R. Lim, K.-G. Song, K.-H. Ahn, Effects of sludge retention time on membrane fouling and microbial community structure in a membrane bioreactor, J. Membr. Sci. 287 (2007) 211–218.

[14] S.-H. Yoon, J.H. Collins, D. Musale, S. Sundarajaran, S.-P. Tsai, G.A. Hallsby, J.F. Kong, J. Koppes, P. Cachia, Effects of flux enhancing polymer on the characteristics of sludge in membrane bioreactor process, Water Sci. Technol. 51 (2005) 151–157. [15] T. Wozniak, MBR design and operation using MPE-technology (Membrane Performance Enhancer), Desalination 250 (2010) 723–728. [16] W.-N. Lee, I.-S. Chang, B.-K. Hwang, P.-K. Park, C.-H. Lee, X. Huang, Changes in biofilm architecture with addition of membrane fouling reducer in a membrane bioreactor, Process. Biochem. 42 (2007) 655–661. [17] B.-K. Hwang, W.-N. Lee, P.-K. Park, C.-H. Lee, I.-S. Chang, Effect of membrane fouling reducer on cake structure and membrane permeability in membrane bioreactor, J. Membr. Sci. 288 (2007) 149–156. [18] H. Koseoglu, N.O. Yigit, V. Iversen, A. Drews, M. Kitis, B. Lesjean, M. Kraume, Effects of several different flux enhancing chemicals on filterability and fouling reduction of membrane bioreactor (MBR) mixed liquors, J. Membr. Sci. 320 (2008) 57–64. [19] J. Wu, F. Chen, X. Huang, W. Geng, X. Wen, Using inorganic coagulants to control membrane fouling in a submerged membrane bioreactor, Desalination 197 (2006) 124–136. [20] S.-H. Yoon, J.H. Collins, A novel flux enhancing method for membrane bioreactor (MBR) process using polymer, Desalination 191 (2006) 52–61. [21] A. Drews, Membrane fouling in membrane bioreactors — Characterisation, contradictions, cause and cures, J. Membr. Sci. 363 (2010) 1–28. [22] V. Iversen, R. Mehrez, R.Y. Horng, C.H. Chen, F. Meng, A. Drews, B. Lesjean, M. Ernst, M. Kraume, Fouling mitigation through flocculants and adsorbents addition in membrane bioreactors: comparing lab and pilot studies, J. Membr. Sci. 345 (2009) 21–30. [23] V. Iversen, H. Koseoglu, N.O. Yigit, A. Drews, M. Kitis, B. Lesjean, M. Kraume, Impacts of membrane flux enhancers on activated sludge respiration and nutrient removal in MBRs, Water Res. 43 (2009) 822–830. [24] K.-G. Song, Y. Kim, K.-H. Ahn, Effect of coagulant addition on membrane fouling and nutrient removal in a submerged membrane bioreactor, Desalination 221 (2008) 467–474. [25] S. Omer, S. Yaxi, H. Ailing, G. Ping, Effect of PAC addition on MBR process for drinking water treatment, Sep. Purif. Technol. 58 (2008) 320–327. [26] W.S. Guo, S. Vigneswaran, H.H. Ngo, J. Kandasamy, S. Yoon, The role of a membrane performance enhancer in a membrane bioreactor: a comparison with other submerged membrane hybrid systems, Desalination 231 (2008) 305–313. [27] L.H. Keith, Compilation of EPA's Sampling and Analysis Methods, CRC Press, London, 1996.