Characterization of foulants in conventional and simultaneous nitrification and denitrification membrane bioreactors

Separation and Purification Technology 69 (2009) 153–160

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Characterization of foulants in conventional and simultaneous nitrification and denitrification membrane bioreactors Sara Arabi, George Nakhla ∗ Department of Chemical and Biochemical Engineering, University of Western Ontario, 1151 Richmond Street, London, Ont., Canada N6A 5B9

a r t i c l e

i n f o

Article history: Received 20 April 2009 Received in revised form 10 July 2009 Accepted 13 July 2009 Keywords: Membrane bioreactor Membrane fouling Simultaneous nitrification and denitrification Extracellular polymeric substances Soluble microbial products

a b s t r a c t Membrane fouling was compared in two submerged membrane bioreactors (MBRs): conventional and simultaneous nitrification and denitrification (SND) MBRs operated at a membrane flux of 14 L/m2 h and an aeration rate of 4 L/min. The mixing rate was adjusted to maintain a DO level of 1–1.2 mg/L to achieve SND. Steady state membrane permeability and fouling rates as well as the mixed liquor filterability and characteristics such as protein, carbohydrate, and humic acids in extracellular polymeric substances (EPS) and soluble microbial products (SMP) were determined. Higher membrane fouling rates were observed in the SND MBR despite the larger floc size. The increased concentrations of SMP and EPS for the SND reactor resulted in higher fouling rate as well as higher modified fouling index (MFI) both for suspended solids and soluble components. Higher EPS concentration and relative hydrophobicity in the SND MBR increased the attachment on the membrane surface. Also, higher rejection of carbohydrate SMP and the increased concentration of this fraction retained in the membrane pores indicated that carbohydrate SMP was the major foulant indicator. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Despite the widespread use of membrane bioreactors (MBR) in wastewater treatment, the application of MBRs to biological nutrient removal (BNR) is only recently emerging [1,2]. Generally BNR systems operate at different conditions of dissolved oxygen (DO), nutrients, hydraulic, solids retention times (HRT, SRT), and mixed liquor suspended solids (MLSS) concentrations, all of which strongly impact extracellular polymeric substances (EPS) and soluble microbial products (SMP) which have been recognized as significant membrane foulants. However, evidence regarding the relative importance of EPS and SMP concentrations and composition in membrane fouling is controversial [3]. Although higher EPS concentrations have been reported by many studies to be responsible for the increase in membrane fouling rates [4,5], Yun et al. [6] showed that lower EPS concentrations increased hydraulic resistance due to a more even distribution of EPS over the membrane surface compared to higher EPS concentrations. Additionally, Jang et al. [7] reported higher fouling rates attributed to lower EPS and consequently floc deterioration in denitrification assays. Although increased hydrophobicity was reported to enhance bioflocculation [8,9] resulting in larger more permeable flocs and reduced fouling [10], Geng and Hall [11] found no correlation between sludge

∗ Corresponding author. Tel.: +1 519 661 2111x85470; fax: +1 519 850 2129. E-mail addresses: [email protected] (S. Arabi), [email protected] (G. Nakhla). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.07.013

relative hydrophobicity and fouling propensity in BNR systems while Le-Clech et al. [12] reported higher fouling with increasing hydrophobic microbial flocs. The protein to carbohydrate ratio within the EPS was found to impact hydrophobicity and flocculating ability of the sludge more than total EPS concentration [13] and many studies reported on membrane fouling concluded that EPS composition is more important than the total quantity of EPS from the fouling perspective [5,14]. Many recent studies have attributed higher fouling rates to SMP concentration in the MBRs [15,16]. Furthermore, several attempts have shown that carbohydrate SMP contribute to fouling more than protein SMP [1,17]. The impact of anoxic conditions on EPS/SMP production and hence membrane fouling has been controversial. The filterability of sludges from anoxic/oxic systems has been found to be worse than aerobic systems [1,5]. As a general trend, higher dissolved oxygen tends to lead to better filterability and lower fouling rate [12,18]. Although there is a consensus on the detrimental impact of low DO level on membrane fouling, there is a conflict on the fouling mechanism. Although physico-chemical properties of cake layer, i.e. floc size and porosity, both of which are enhanced by high DO, have a greater impact on mitigating membrane fouling by reducing cake resistance [5,19], other studies suggested that the relatively lower SMPs under high DO concentrations are responsible for the lower fouling [20]. Nonetheless, evidence from another study [5] suggests that soluble COD concentrations, an indicator of SMP concentration, decreased from 37 mg/L at DO of 3.4 mg/L to 27 mg/L at DO of 0.9 mg/L, respectively and therefore cannot rationalize the

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increase of fouling rate at low DO levels. Ng et al. [21] investigated membrane fouling at various SRTs in pre-anoxic MBRs and reported severe membrane fouling for the 3-day and 5-day SRT compared to 10 and 20 days due to higher SMP, EPS, and carbohydrates concentrations in the mixed liquor. Furthermore, Wang et al. [22] reported that the outer shell of the aerobic granules contained poorly soluble, not readily biodegradable, and rather hydrophobic EPS which tend to enhance bioflocculation and reduce cake resistance, whereas the anoxic inner core retained readily soluble and biodegradable EPS. Simultaneous nitrification and denitrification (SND), based on the development of an anoxic core within the biological floc is different from nitrification and denitrification, and is only recently that it has been employed in MBRs such as in intermittently aerated MBRs [23,24] and sequencing batch MBRs [25,26]. The focus of the aforementioned studies has been COD and nitrogen removal efficiencies. To the author’s best knowledge, there has been very limited research on the extent of membrane fouling in SND MBRs, not to mention characterization of foulants. This study aims primarily at the characterization of membrane foulants in two MBRs: SND and conventional MBRs. The continuous-flow lab-scale MBRs were operated for more than 100 days at an SRT of 25 days. The concentrations, relative hydrophobicities of EPS and SMP as protein and carbohydrate, and humic acids components were measured to characterize the membrane foulants. Also, the filtration resistances expressed as modified fouling index (MFI) of the soluble and suspended solid fractions of the mixed liquor were determined to assess membrane fouling in SND MBRs. 2. Materials and methods 2.1. Membrane bioreactor Laboratory scale 6.6 L Plexiglass MBRs (Fig. 1), each employing a submersible membrane module ZeeWeed-1 (GE Water and Process Technologies, Oakville, ON, Canada) with a total surface area of 0.09 m2 were used in this study. The membranes are made of strong polymers with hydrophilic coating and nominal pore size of 0.047 ␮m. Continuous aeration was provided underneath the membranes to supply air and prevent membrane fouling. In both MBRs, an aeration rate of 4 L/min was applied. The DO in the conventional MBR was enhanced by increasing the mixing intensity. Intermittent permeate suction was applied, with 9 min of suction followed by 30 s of relaxation. Alum at 4 g/d (110 mg/L) was added to the system continuously to chemically remove phosphorous. The reactors were seeded with returned activated sludge from

Table 1 Operating conditions of the MBR. Parameters

Values

MBRs

Conventional

SND

Flux (L/m2 h) Temperature (◦ C) Aeration rate (L/min) Dissolved oxygen (mg/L) pH TSS (g/L) VSS (g/L) Velocity gradient (G-value) (s−1 )a (for mixing) Velocity gradient (G-value) (s−1 )b (for aeration) Combined velocity gradient (G-value) (s−1 )c

14 ± 0.6 (55) 20 ± 3 (20) 4 3–4 7 ± 0.3 (13) 10.6 ± 1.1 (10) 6.8 ± 0.7 (10) 628 157 785

14 ± 0.8 (55) 20 ± 3 (20) 4 1–1.2 6.8 ± 0.5 (15) 9.8 ± 1.2 (10) 6.2 ± 0.7(10) 314 157 498

±: Standard deviation ( ): number of measurements. a

G=



P/ · V where P is the power in W, V is the volume in m3 , and  is the



viscosity in Pa s [42]. b

G=

QHw /V where Q is the air flow rate in m3 /s, H is the depth of water

column in m, V is the reactor volume in m3 ,  is the viscosity of water in Pa s, and w is the specific weight of water in N/m3 [38]. c The combined G-value is the sum of the G-value for mixing and the G-value for aeration.

the Adelaide treatment plant in London, Ont., Canada. Membranes were removed for chemical cleaning once the transmembrane pressure (TMP) reached 69 kPa by soaking in 200 mg/L sodium hypochlorite solution for a minimum of 5 h. In order to prevent overflow, a level sensor was used to maintain a constant liquid level in the reactor by controlling the operation of the feed pump. The MBRs were operated under a constant flux mode. The target flow rate through the membranes was 36 L/day corresponding to a hydraulic retention time (HRT) of 4.5 h. However, due to deterioration in filterability over time, flow through the membrane varied slightly within 10–15% of the targeted value. A target solids retention time (SRT) of 25 days was maintained through direct removal of sludge from the bioreactor (1/25 of the bioreactor volume) on a daily basis. The effect of simultaneous nitrification and denitrification (SND) was examined on the membrane permeability. The experimental conditions are described in Table 1.

2.2. Composition of synthetic wastewater The synthetic wastewater (total influent COD of 325 mg/L) was made by adding 125 mg/L casein, 84.4 mg/L starch, 96.9 mg/L sodium acetate, 12.0 mg/L glycerol, 93.0 mg/L (NH4 )2 SO4 , 69.6 mg/L MgSO4 ·7H2 O, 11.0 mg/L FeCl3 , 22.5 mg/L CaCl2 ·2H2 O, 0.08 mg/L CuSO4 ·4H2 O, 0.15 mg/L NaMoO4 ·2H2 O, 0.13 mg/L MnSO4 ·H2 O, 0.23 mg/L ZnCl2 , 0.42 mg/L CoCl2 ·6H2 O, 5.9 mg/L K2 HPO4 , 23.6 mg/L KH2 PO4 , 216 mg/L Na2 CO3 , and 169 mg/L NaHCO3 . Sulfuric acid was used to maintain a pH of 7.

2.3. Analytical methods

Fig. 1. Schematic diagram of MBR experimental setup.

The influent and permeate quality as well as the performance of the system with respect to filterability of sludges, floc size distribution, permeability, and TMP were routinely monitored. The MLSS, mixed liquor volatile suspended solids (MLVSS), COD, NH3 , total nitrogen (TN), NO3 , NO2 , and total phosphorous (TP) were measured using Standard Methods [27]. The pH was measured using Orion pH meter model 410A and a pH probe (VWR model SympHony). Particle size distribution was determined by Malvern Mastersizer 2000 (Malvern Instruments Ltd., Worcestershire, United Kingdom).

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trate were determined as representing the SMP. The difference between the measurements of treated mixed liquor (representative of EPS + SMP) and the untreated mixed liquor (representative of SMP) was the EPS concentration. Carbohydrates were determined according to Dubois et al. [30] and samples absorbances were measured at 490 nm in duplicates. Glucose was used as a standard for calibration from 0 to 100 mg/L. Proteins and humic acids were determined according to the method published by Frolund et al. [29] which is a modification of the Lowry method [31]. Calibration for proteins was done using bovine serum albumin (BSA) from 0 to 150 mg/L. For humic acid measurements, a humic acid salt (humic acid, sodium salt, Sigma–Aldrich) was used for calibration from 0 to 150 mg/L. Samples were measured in duplicates. 2.6. Relative hydrophobicity

Fig. 2. Schematic diagram of stirred batch cell system.

2.4. Batch filtration test Batch filtration tests were conducted to measure the modified fouling index (MFI) [28]. A stirred batch cell (8400, Amicon, USA) shown in Fig. 2 was used to measure the permeate volume with an ultrafiltration (UF) membrane (nominal molecular weight cut-off 300 kDa, regenerated cellulose, PALL Life Sciences, Ont., Canada), under constant pressure. Two samples were applied to fractionate the membrane foulants into the soluble and suspended solids (SS) components. First, the mixed liquor (ML) of the MBR sludge containing the soluble and SS components was filtered through UF membranes in the batch filtration set-up. Second, the supernatant of the mixed liquor centrifuged at 12,000 × g for 15 min and filtered through a 0.45 ␮m filter paper (same procedure as SMP) was called the soluble component. Then, the soluble component was filtered through UF membranes in the batch filtration set-up. Last, the SS component was calculated by subtraction of the soluble component from the ML. The MFI was measured to compare fouling characteristics according to the following procedure originally proposed by Schippers and Verdouw [28] wherein the SS and soluble fraction, resistance was determined by filtering a 300 mL sample under constant pressure of 10 psi (69 kPa) and measuring the flow rates as a function of time. A plot of t/V versus V (t in s and V in L) was then constructed to determine the MFI. The slope (tan ˛) of the straight part of the curve is then calculated. MFI is found from the following equation (Eq. (1)) and corrected for the pressure and temperature of 210 kPa and 20 ◦ C MFI =

20 P . . tan ˛  210

A hydrocarbon–hexane extraction was used to measure the hydrophobicity in the sludge, EPS, and SMP as protein and carbohydrate. The procedure is as follows: a 50 mL sample was agitated for 10 min, with 50 mL n-hexane, in a separating funnel. After 10 min, when the phases were separated completely, of the 50-mL aqueous phase, only 40 mL of the aqueous solution were transferred to glassware prior to protein and carbohydrate analysis. The relative hydrophobicity is expressed as the ratio of the aqueous phase concentration after emulsification (Se ) to that of the initial sample concentration (Si )



Relative hydrophobicity (%) = 100 × 1 −

Se Si



2.7. Membrane performance The permeate flux (J) was calculated from the permeate flow (Q) and the membrane area (A) using Eq. (2). This was determined through the one day time period (t) and the corresponding permeate volume (V). J=

1 V Q = · [L/m2 h] A A t

Temperature corrected permeability to 20 ◦ C (LP20 from Eq. (3) [32] as follows: Lp20

◦C

=

J −0.0239(T −20) e [L/m2 h bar] p

(2) ◦C

) was found

(3)

where p is the transmembrane pressure (averaged over a day, using 3 data points). Fouling rate is calculated as the slope of permeability versus time graph.

(1)

where 20 = viscosity at 20 ◦ C,  = viscosity at the water temperature, P = pressure applied in kPa. 2.5. EPS and SMP analysis The EPS and SMP concentrations were measured as carbohydrate, protein, and humic acids using a cation exchange resin (CER) (DOWEX R Marathon C, Na+ form, Sigma–Aldrich, USA) extraction method [29]. The mixed liquor sample was cooled to 4 ◦ C to minimize microbial activity. The exchange resin (75 g of CER/g VSS) was added to a 200 mL sample and mixed at 600 rpm for 2 h at 4◦ C. The mixture was then centrifuged for 15 min at 12,000 × g to remove the MLSS. The centrifuged supernatant of the sample, after CER addition, represented the sum of EPS and SMP concentrations. Untreated mixed liquor was centrifuged for 15 min at 12,000 × g, followed by filtration through a 0.45 ␮m filter paper and the sum of protein, carbohydrate, and humic acid concentrations of the fil-

2.8. Fouled membrane resistance parameters The resistance-in-series model [33] was applied to determine the membrane filtration resistances. Based on this model, the permeate flux (J) on the applied transmembrane pressure (TMP) can be described by Darcy’s law (Eq. (4)) J=

P 1 dV P = = · A dt (Rt ) (Rm + Rc + Rf )

(4)

where Jp is the permeate flux (L/m2 h), V is the total volume of the permeate (L), A is the membrane area (m2 ), P is the transmembrane pressure (kPa),  is the permeate viscosity (Pa s), Rt is the total membrane filtration resistance (m−1 ) which is the sum of membrane resistance (Rm ) in m−1 , the filtration resistance due to membrane fouling (Rf ) in m−1 , and the cake layer resistance (Rc ) in m−1 . Rf in this study was calculated to be the sum of pore blocking resistance (Rp ) and irreversible fouling resistance (Rir ) in m−1 . It should be noted that the term irreversible in this study refers to the

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membrane resistance that cannot be removed by chemical cleaning. The values for each resistance were estimated through a set of batch experiments as described below. The fouling resistances are determined from the following equations: Rm

TMP =  · JDI

(5)

Rt =

TMP − Rm ·J

(6)

Rp =

TMP − Rm  · Jc

(7)

Rir =

TMP − Rm  · Jbw

(8)

Rf = Rp + Rir Rc = Rt − (Rf + Rm )

(9) (10)

Membrane filtration was carried out using a batch stirred cell (8400, Amicon, USA) as shown in Fig. 2. A UF membrane with nominal molecular weight cut-off 300 kDa (regenerated cellulose, PALL Life Sciences, Ont., Canada) was used under constant pressure of 50 psi and stirring speed of 600 rpm. The permeate flux was determined by monitoring the volume of permeate with time. Prior to each experiment, the pure deionized water (DI) flux (JDI ) was measured to find Rm from Eq. (5). The stirred cell was then emptied and filled with 300 mL of activated sludge. The activated sludge flux (J) is measured based on the time required for filtration of 300 mL of sludge sample and Rt was measured using Eq. (6). After the filtration test with the sludge sample, the membrane was washed in a cross-flow manner with DI water to remove the cake layer and the clean water flux was measured to find Jc and Rp from Eq. (7). Next, a backwash, simulated by reverse filtration, with 100 ppm NaOCl was used. The clean water flux (Jbw ) was used to estimate the Rir value from Eq. (8). Finally, Rc was found by subtraction of the total membrane resistance from the sum of Rf and Rm from Eq. (10). 3. Results and discussion 3.1. MBR performance The operational conditions for the two MBRs, as summarized in Table 1, were identical except for the mixing rate which was adjusted to maintain a DO level of around 1 mg/L to achieve SND. The experiments were carried out for more than three turnovers of SRT. It should be mentioned that the steady state phase in this study refers to the operational period from day 50 to day 106. Throughout the experimental period, effluent COD was 5 mg/L higher in the SND MBR compared to the conventional reactor and the COD removal efficiencies were 98% and 96% for the conventional and SND MBR, respectively. The effluent ammonia concentration increased from 0.1 mg/L in the conventional MBR to 0.5 mg/L in the SND MBR. Nitrification efficiencies were 99% and 97% for the conventional and SND MBRs, respectively. The amount of nitrogen that is utilized for biomass was based on the observed yield and the measured N content of VSS of 7.5%. Nitrogen removed through assimilation and SND are calculated from Eqs. (11) and (12), respectively. As shown in Table 2, the amount of nitrogen used for biomass assimilation was 4.5 and 3.7 in the conventional and SND MBRs, respectively, attesting to the 15% reduction in biomass yield achieved by the SND MBR. Comparison of effluent NO3 concentrations shows that the effluent NO3 was around 5.5 mg/L lower in the SND reactor compared to the conventional MBR. It is obvious that 100% higher N removal by denitrification was observed in the SND reactor compared to the conventional MBR (Table 2) Nassimilation = 0.075 × Yobs (Sin − Sout )

(11)

Table 2 Steady state MBR operational conditions and performance. Parameter

Conventional

SND

COD in (mg/L) COD out (mg/L) Influent NH4 –N (mg/L) Effluent NH4 –N (mg/L) Influent NO3 (mg/L) Effluent NO3 (mg/L) Influent NO2 (mg/L) Effluent NO2 (mg/L) Influent TN (mg/L) Effluent TN (mg/L) Influent TP (mg/L) Effluent TP (mg/L) Yield (g VSS/g COD) N removed by assimilation (mg/L) N removed by denitrification (mg/L) Particle size (␮m)a Fouling rate (L/m2 h bar d)

320 ± 30 (11) 5.6 ± 1.9 (11) 24 ± 3 (23) 0.1 ± 0.07 (23) 0.6 ± 0.3 (11) 19 ± 1.7 (23) 0.1 (11) ≤0.002 32 ± 1.9 (15) 23 ± 1.5 (15) 6 ± 1.3 (15) 0.7 ± 0.6 (15) 0.19 4.5 5.3 55 ± 6 (15) 7.8 ± 0.8 (9)

318 ± 15 (11) 10.5 ± 2.3 (11) 22 ± 5 (23) 0.5 ± 0.2 (23) 0.5 ± 0.3 (11) 13.5 ± 2.7 (23) 0.1 (11) 0.25 ± 0.1 (11) 33 ± 1.7 (15) 18 ± 1.9 (15) 5.5 ± 2 (15) 0.4 ± 0.2 (13) 0.16 3.7 10.5 88 ± 5 (15) 14.8 ± 3 (11)

Washwater characteristics TSS (gSS/m2 ) Carbohydrate (g/m2 ) Protein (g/m2 ) Humic acid (g/m2 )

1.26 ± 0.2 (8) 0.02 ± 0.006 (6) 0.04 ± 0.005 (6) 0.025 ± 0.003 (6)

3.12 ± 0.5 (8) 0.02 ± 0.007 (6) 0.069 ± 0.006 (6) 0.047 ± 0.007 (6)

± Standard deviation ( ): number of measurements. a Volume weighted mean.

Ndenitrification = (TNin − TNout ) − 0.075 × Yobs (Sin − S-+ )

(12)

where 0.075 = mass of nitrogen per biomass (mg N/mg/VSS), Yobs = observed yield (mg-VSS/mg COD), Sin and Sout = influent and effluent COD concentrations (mg/L), TNin and TNout = influent and effluent total nitrogen concentrations (mg/L). The observed yield and P removal were also influenced by the introduction of anoxic conditions in the SND MBR. The observed yields were 0.19 and 0.16 g VSS/g COD for the conventional and SND MBR, respectively, which is consistent with the lower observed yields for the BNR systems. The effluent phosphorous concentrations were 0.7 and 0.4 mg/L for the conventional and SND MBR, respectively, corresponding to 88% and 92% P removal efficiencies. Although high P removal efficiencies are due to chemical addition, the lower effluent P concentration in the SND reactor might be due to enhanced biological phosphorus removal in the SND reactor under anoxic conditions. Fig. 3 shows the variations of permeability with time for MBRs throughout the operation. The cyclical variations of the permeability are explained by the regular membrane washing. The membranes were washed when the TMP reached 0.69 bar. The fouling rates presented in Table 2 are calculated from the slope of permeability versus time graphs within each cycle. It is evident that the steady state fouling rate in the SND MBR was almost double the conventional MBR. The frequency of membrane cleaning for the conventional MBR decreased from once every 4–5 days in the transient period to once every 6 days in the steady state

Fig. 3. Time-variation of permeability during MBR operations.

S. Arabi, G. Nakhla / Separation and Purification Technology 69 (2009) 153–160 Table 3 The estimated values for the membrane resistances. Resistances (m−1 )

Conventional

Rm Rt Rc Rf Rp Ric

(3.8 ± 0.03) × 10 (7%) (5.2 ± 0.1) × 1012 (1.12 ± 0.02) × 1012 (21%) (3.7 ± 0.2) × 1012 (71%) (3.1 ± 0.15) × 1012 (0.6 ± 0.1) × 1012

a b

a

SND 11

b

(3.8 ± 0.03) × 1011 (5%) (7 ± 0.25) × 1012 (1.82 ± 0.06) × 1012 (26%) (4.8 ± 0.25) × 1012 (68%) (3.9 ± 0.05) × 1012 (0.9 ± 0.3) × 1012

Average of six data points. Percentages in parenthesis shows the % of total resistance.

period. For the SND MBR, the cleaning frequency was once every 3–4 days throughout the operation. The changes in the EPS and SMP were investigated to evaluate the higher fouling rates under SND conditions. The alum dosage used in this study of 110 mg/L (11 g/kg MLSS) can be compared with a recent research work [34]. They investigated the filterability of sludges from a pilot-plant MBR, operated at an HRT of 6 h, SRT of 15 days and MLSS concentration of 9.1 g/L and pretreated with chemical coagulants. Their results showed that the alum dosage may have potentially reduced the time to filter (TTF) by a maximum of 25%, and an average of 17%, and did not impact particle size distribution. Therefore, since both reactors were operated at the same alum dose, the alum concentration used does not have a significant impact on the differences in permeability reported herein between the conventional and SND MBR. 3.2. Membrane filtration resistance Table 3 shows the relative contribution of various resistances under the conventional and SND conditions. Statistical analysis using MINITAB (Version 14, Minitab, State College, PA) was performed on all the data presented in this research to detect the differences in means (averages) between the parameters in the two MBRs. Prior to the t-test analysis, normality has been investigated using the Anderson–Darling test to ensure that the data follows normal distribution. All the data presented in this paper were found to be normally distributed at the 95% confidence level, thus validating the use of the t-test. The observed differences between the resistances have been confirmed to be statistically significant at the 95% confidence level using paired t-test. The total membrane resistance was 35% higher in the SND MBR compared to the conventional MBR. Close to 70% of total resistance was attributed to fouling resistance (Rf ) for both MBRs. The cake resistance (Rc ) represented 21% and 26% of the total resistance for the conventional and SND MBRs, respectively. The pore blocking and irreversible fouling resistances accounted for around 80% and 17% of the fouling resistance (Rf ) for both MBRs but the corresponding values increased by 25% and 50%, respectively from the conventional to the SND MBR. The cake resistance was almost 1.5 times higher in the SND MBR compared to the conventional MBR which was confirmed with a higher cake density on the membrane surface. It is concluded that the membrane fouling under SND conditions was significantly higher than the conventional MBR as reflected by 38% increase in the cake resistance (Rc ) and 23% increase in the fouling resistance (Rf ). It may be argued that the higher fouling rate observed in the SND MBR is due to the relatively lower shear as reflected by combined G values of 498 s−1 versus 785 s−1 ; however, cross-flow velocity was found to predominantly impact reversible fouling due to formation of a cake layer and only slightly impacts the irreversible fouling which is mainly due to pore blocking [12,35]. Shear is mainly used to control particulate deposition on the membrane surface in MBRs and there exists a critical shear rate, in the form of air scour, above which no flux enhancement occurs [36] due to irreversible fouling of the membrane surface or pore blocking. In the present

157

study, fouling was found to be mainly due to pore blocking resistance and irreversible fouling as their combined values represented more than 70% of the total resistance in MBRs while the cake resistance was only 21–26% of the total resistance. Thus, the lower shear rate in the SND MBR may have impacted the cake layer resistance, which was found to be around 60% higher in the SND MBR than the conventional MBR. However, the impact of shear on overall membrane performance can be ignored as the total membrane hydraulic resistance was mainly governed by pore blocking and irreversible fouling. This was further confirmed with the results of modified fouling index (MFI) analysis for the soluble and suspended solids fraction, as the soluble MFI accounted for more than 55% of the total mixed liquor resistance in both MBRs. It should be noted from Table 3, that the cake resistance in the SND reactor increased by 62%, from 1.12 × 1012 m−1 in the conventional to 1.82 × 1012 m−1 in the SND MBR, very comparable to the 57% increase in the combined G-value from 498 s−1 in the conventional to 785 s−1 in the SND MBR. 3.3. Mixed liquor hydraulic resistance determined by batch filtration tests The hydraulic resistance attributed to the individual components of the mixed liquor (ML) was determined in batch ultrafiltration experiments. To validate the relevance of MFI tests to MBR fouling rates, an attempt was made to find the correlation between total MFI and steady state fouling rate in the reactors. As shown in Fig. 4a and b, positive linear correlation was found for both the SND and conventional MBR, with R2 of 84% and 64%, respectively. The results of Fig. 4 prove the correlation between the MFI test and the reactors fouling rate despite the different filtration conditions of constant flux mode in the MBR versus constant pressure mode in the MFI test, and dead-end (batch) versus the hollow fiber outside-in Zenon (MBR). The MFI values for the soluble (sol), suspended solids (SS), and mixed liquor (ML) are presented in Fig. 5 for the two MBRs. Statistical analysis using paired t-tests indicated that the differences between averages of various MFI, i.e. SS and soluble between the two MBRs were significant at the 95% confidence level.

Fig. 4. Correlation between fouling rate and MFI for (a) SND MBR and (b) conventional MBR.

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Fig. 5. Fouling characteristics for the MBRs.

The SS MFI accounted for almost 40% of the ML resistance while the soluble MFI accounted for more than 55% of the ML resistance for both MBRs emphasizing that the soluble fraction of mixed liquor is the predominant foulant, consistent with finding of other studies [37,38]. The total ML resistance was 37% higher in the SND MBR compared to the conventional MBR. A large difference was observed in Sol MFI from 4.9 s/L2 in the conventional to 7 s/L2 in the SND MBR. This can explain the higher fouling rates in the SND MBR. The resistance due to suspended solids (SS MFI) was 31% higher in the SND MBR compared to the conventional MBR. This can be explained by higher EPS levels observed in the SND reactor as well as increased hydrophobic EPS concentration that resulted in higher EPS attachment to the membrane surface.

although literature often relates higher floc size to lower membrane fouling [7,10], the results of this study showed that larger anoxic flocs do not improve membrane filtration performance. Considering the comparison of aerobic and anoxic cores of biological flocs [22], where the outer surface of anoxic flocs remained readily soluble and more biodegradable compared to aerobic granules which comprised poorly soluble and not readily biodegradable, the increased filtration resistance can be attributed to the higher solubility and biodegradability. Therefore, despite the larger floc size for the SND MBR, membrane fouling rate tended to increase, which shows that although floc sizes were larger and according to many reported studies on aerobic MBRs should have been more permeable, for anoxic flocs, the increased size translated to lower permeability. Analysis of the TSS and foulants extracted from the membrane surface (washwaters) indicates that the biomass surface density was higher under SND conditions (3.12 gSS/m2 ) compared to the conventional MBR (1.26 gSS/m2 ). The average amount of carbohydrate extracted from both reactors were almost equal at 0.02 g/m2 but the concentrations of humic acids and proteins were 0.025 and 0.04 g/m2 for the conventional and 0.047 and 0.069 g/m2 for the SND MBR. Thus, the extracted proteins and humic acids from the membrane in the SND MBR were 72% and 88% higher than the conventional MBR, respectively. Since protein and humic acid EPS are hydrophobic [39], it is apparent that the higher protein and humic acid EPS concentration has caused higher attachment of EPS to the membrane surface under SND conditions, thus resulting in higher cake resistance. The results of EPS analysis showed that the concentration of EPS were 20% higher in SND MBR. This, in conjunction with the higher hydrophobicities of EPS under SND conditions, translated to 60% higher cake density and 47% higher fouling rate for the SND MBR.

3.4. EPS 3.5. SMP EPS is known to comprise carbohydrates, proteins, nucleic acids, lipids and humic substances but carbohydrates and proteins were found to be the major constituents of the EPS [8]. Table 4 illustrates protein, carbohydrate, and humic acid concentrations for EPS in both MBRs as mg/g-VSS and mg/m2 surface area of the membrane. It should be noted that the EPS concentration available per surface area of the membrane (expressed as mg/m2 ) was obtained using the EPS mass in the mixed liquor divided by the surface area of the membrane. Differences in EPS concentrations between the two MBRs were found to be statistically significant at the 95% confidence level using the paired t-test. Total bound EPS concentration was 20% higher in the SND compared to the conventional MBR. Among the three EPS fractions, the highest EPS increase was observed in the protein EPS as shown by an increase of 36% from the conventional to SND MBR. Carbohydrate and humic acid EPS were 14% and 29%, higher in the SND MBR compared to conventional MBR, respectively. The higher EPS concentration resulted in the formation of larger flocs as confirmed by the results of floc size distribution which shows that the floc size in the SND MBR was 37% larger than the conventional MBR (Table 2). It should be mentioned that Table 4 EPS quantity and composition in the MBRs. EPS concentration

Unit

Carbohydrate

mg/g-VSS mg/m2

SND 5.5 ± 0.6 (8) 415 ± 45(8)

Conventional 4.8 ± 0.4 (8) 330 ± 27(8)

Protein

mg/g-VSS mg/m2

8.6 ± 0.6 (8) 650 ± 45(8)

6.3 ± 0.5 (8) 434 ± 34(8)

Humic acid

mg/g-VSS mg/m2

4.8 ± 0.5(8) 362 ± 37(8)

3.7 ± 0.3 (8) 254 ± 20(8)

Total

mg/g-VSS mg/m2

19 ± 1.2 (8) 1435 ± 90(8)

15 ± 0.5 (8) 1033 ± 34(8)

Table 5a presents the SMP composition and concentration over the experimental period in terms of carbohydrate, proteins, and humic acids in mg/L in both MBRs. The observed differences between the SMP concentrations have been confirmed to be statistically significant at the 95% confidence level using paired t-test. Total SMP concentrations of 69 mg/L in the SND MBR were 50% higher than the conventional MBR while both humic acid and carbohydrate SMP were 40%, and proteins 70% higher, resulting in greater fouling rates. Although higher shear was observed to increase the production of SMPs as a result of physical hydrolysis of EPS [40], the observed higher SMP concentrations in the SND MBR compared to the conventional MBR, despite the lower shear rate, emphasizes that the fouling propensity of the SND sludge as a result of anoxic conditions was a more important factor impacting membrane fouling than the shear rate. Examination of the permeate concentrations for SMPs in Table 5a showed that the percent rejection of carbohydrate, protein, and humic acids were 71 ± 5, 38 ± 1.8, and 30 ± 1.6 respectively for the conventional MBR while the corresponding values for the SND MBR were 77 ± 5, 44 ± 1, and 45 ± 1.3, respectively. The higher carbohydrate rejection relative to proteins and humic acids were consistent with another study [20], which reported protein and carbohydrate rejections in the ranges of 20–65% and 75–95%, respectively. Therefore, higher SMP rejections in the SND reactor relative to the conventional MBR reflected the higher retention of SMPs in the SND MBR and increased membrane fouling as confirmed by higher sol MFI values. A mass balance of the SMPs removed by the membrane through the biofilm on the outside and the pores inside the membrane is presented in Table 5b. It was found that higher concentrations of SMP were retained inside the pores in case of SND reactor relative to the conventional MBR, which in conjunction with the higher Rp

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159

Table 5a SMP concentration and composition in the reactors and permeates and SMP rejections for both MBRs. SMP conc. (mg/L)

SND MBR

Carbohydrate Protein Humic acid Total

32 22 14 69

Conventional MBR

Reactor ± ± ± ±

Permeate

2.8 3.4 3.2 7

7.1 12 7.8 26

± ± ± ±

Rejection (%)

1.4 1.5 1.1 1

77 44 45 63

± ± ± ±

Reactor

5 1 1.3 5

23 13 10 46

± ± ± ±

3.2 3.3 2.7 5.4

Permeate 6.5 7.7 6.4 20

± ± ± ±

1.7 1.5 2 1.2

Rejection (%) 71 38 30 55

± ± ± ±

5 1.8 1.6 2

Table 5b Mass balance of the SMPs removed by the membrane, on the biofilm layer and inside the pores for the MBRs. SMP conc. (mg/L)

Carbohydrate Protein Humic acid

SND MBR

Conventional MBR

Removed

Biofilm

Pores

Removed

Biofilm

Pores

25 ± 2 (6) 10.2 ± 2 (6) 6.2 ± 2.3 (6)

8.5 ± 1.3 (6) 5.5 ± 1.2 (6) 4.3 ± 0.9 (6)

16.3 ± 2 (6) 4.7 ± 2 (5) 2.1 ± 1.9 (6)

16.5 ± 3 (6) 5.4 ± 2.5 (6) 5 ± 2 (5)

10.5 ± 1 (6) 6.7 ± 1 (6) 5 ± 0.8 (6)

6 ± 3.7 (6) – –

resistance (Table 3) proves that the pore blocking was higher in the SND MBR. Among the three fraction of SMP, for both MBRs, pore retention of carbohydrate SMP was found to be the highest, indicating that carbohydrate fraction was the major contributor to pore blocking resistance. Statistical analysis was performed to identify the key foulant among EPS and SMP components (SPSS for Windows, Version 14). The foulants studied were the three components of EPS (mg/g VSS): protein EPS, carbohydrate EPS, humic acid EPS concentrations and three fractions of SMP (mg/L): protein SMP, carbohydrate SMP, and humic acid SMP concentrations. Multiple regression analysis was chosen to evaluate which of the six independent variables (EPS and SMP components) exerted the greatest effect on the dependent variable (membrane fouling). Modified or standardized regression coefficient, called the Beta coefficients were used to interpret the regression model since they have a common unit of measurement and thus allow direct comparison of the impact of each independent variable [41]. The results of the analysis showed that carbohydrate EPS and carbohydrate SMP had the largest impact on the fouling rate in the SND MBR. For the conventional MBR, the effect of carbohydrate EPS and protein SMP on membrane fouling rate was more noticeable than the effect of the other components.

protein and humic acids in EPS had a strong positive influence on the hydrophobicity of microbial flocs, while carbohydrates had no remarkable influence. Therefore, increased hydrophobicity of protein EPS and humic acid EPS is more likely to enhance attachment of flocs on the membrane surface than hydrophobic carbohydrate EPS. While increased particle size as a result of bioflocculation tends to decrease membrane fouling as a result of reduced hydraulic resistance, attachment onto membrane exacerbates membrane fouling by increasing the cake resistance. The analysis of wash waters indicated that biomass, protein, and humic acids surface densities were all higher under SND conditions. This further confirms the greater attachment of EPS to the membrane surface under SND conditions due to hydrophobic interactions between EPS and membrane surface. Examination of the data presented in Table 6 for SMPs, indicates that the relative hydrophobicity of carbohydrate SMP remained almost constant in both MBRs while those of protein SMP and humic acids SMP were 57% and 50% higher in the SND MBR. Higher protein hydrophobicity enhances the protein aggregation which can serve as an attachment site for the bulk protein and increase membrane fouling [37]. Therefore, it is evident that the higher hydrophobicity of SMP as proteins and humic acids in the SND MBR contributed to higher fouling rate as evidenced by the increase in the soluble MFI (Fig. 4).

3.6. Relative hydrophobicity of SMP and EPS 4. Summary and conclusions Table 6 presents the relative hydrophobicities of EPS and SMP for both MBRs. Statistical analysis using paired t-tests indicated that the differences between average RH values in SND and conventional MBR were significant at the 95% confidence level except for the carbohydrate SMP. As shown in Table 6, the RH of EPS increased from the conventional MBR to the SND, with the highest increase for the carbohydrate EPS (62%). Interestingly, the higher hydrophobicity of carbohydrate EPS did not translate to higher membrane attachment but the higher hydrophobicities of the protein and humic acid EPS did (Section 3.4) and increased membrane fouling rate as shown by higher cake resistance in Section 3.5. Wilen et al. [39] stated that the Table 6 Relative hydrophobicities of EPS and SMP for the MBRs. Relative hydrophobicity (RH) (%)

Conventional MBR

Protein EPS Carbohydrate EPS Humic acid EPS Protein SMP Carbohydrate SMP Humic acid SMP

21 8 15 14 10 12

± ± ± ± ± ±

9 (8) 5 (8) 6 (8) 4 (8) 2 (8) 4 (8)

SND MBR 31 13 20 22 12 18

± ± ± ± ± ±

7 (8) 4 (8) 8 (8) 6 (8) 3 (8) 5 (8)

This study examined the membrane fouling in two MBRs: SND and conventional MBRs. Based on this study, the following conclusions are drawn: (1) Membrane fouling under SND conditions was significantly higher than the conventional MBR as reflected by 38% increase in the cake resistance (Rc ) and 23% increase in the fouling resistance (Rf ). (2) The sol MFI accounted for more than 55% of the ML resistance for both MBRs, confirming that the soluble fraction of the mixed liquor was the predominant cause of decrease in permeability and increase in membrane fouling. (3) The results of EPS analysis showed that the concentration of EPS were 20% higher in SND MBR. This, in conjunction with the higher hydrophobicities of EPS under SND conditions, is translated to 60% higher cake density and 47% higher fouling rate for the SND MBR. (4) Larger floc sizes were observed for the SND MBR compared to the conventional reactor. Despite numerous reports that shows larger floc sizes decrease the cake resistance and improve per-

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