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Impact of cation concentrations on fouling in membrane bioreactors
Journal of Membrane Science 343 (2009) 110–118
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Impact of cation concentrations on fouling in 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 24 March 2009 Received in revised form 8 July 2009 Accepted 10 July 2009 Available online 18 July 2009 Keywords: MBR Fouling Mg/Ca ratio M/D ratio EPS SMP
a b s t r a c t In this study, the interaction of calcium, magnesium, and sodium as well as impact of monovalent to divalent (M/D) cation ratio and magnesium to calcium (Mg/Ca) ratio in the feed wastewater on membrane fouling in submerged membrane bioreactor (MBR) was investigated. The protein and carbohydrate content of soluble microbial products (SMP) and extracellular polymeric substances (EPS) as well as their relative hydrophobicities was examined. The mixed liquor and its components (soluble and suspended solids) were analyzed for their filtration resistance, as reflected by the modified fouling index (MFI). Based on the findings of this study, the optimum conditions with respect to fouling rate were calcium and sodium concentrations of 36 and 140 mg/L, respectively, M/D of 1:1 and Mg/Ca of 5:1, with all parameters on an equivalent basis. High sodium concentration at high M/D ratio was found to decrease the floc size and increase the fouling rate. At the low M/D ratio of 1:1, introduction of magnesium was beneficial in reducing the fouling rate by increasing the EPS concentration and floc size and decreasing the SMP concentration and relative hydrophobicity in the supernatant. The fouling rate was found to be statistically correlated with the concentrations of Ca, Mg, and Na, with both Ca and Na adversely impacting fouling and Mg alleviating fouling propensity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Application of membrane bioreactors (MBRs) as an efficient technology in wastewater treatment has increased in recent years. However, membrane fouling and its consequences in terms of increased operating and maintenance costs represent a significant limitation to the widespread application of MBRs. Bound extracellular polymeric substances (EPS) and soluble microbial products (SMP) greatly influence filterability in MBRs [1]. The characteristics of the biological suspension flocs such as floc size, relative hydrophobicity (RH), and surface charge have been reported to have an impact on membrane fouling [1] with hydrophobicity increasing with proteins and humic acids while hydrophilicity is enhanced by carbohydrates [2,3]. Increased hydrophobicity was reported to enhance bioflocculation [4,5] resulting in larger more permeable flocs and reduced fouling [6]. Enhanced bioflocculation due to cation bridging is known to result in the formation of larger flocs [7] which are more permeable and consequently reduce cake fouling resistance [6]. The divalent cation bridging (DCB) theory, postulates that divalent cations, such as calcium and magnesium, play a major role in bioflocculation and bridging negatively charged functional groups within the EPS, thus helping bioflocculation [7,8]. Wilen et al. [9] found
∗ Corresponding author. E-mail address: [email protected] (G. Nakhla). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.07.016
that more hydrophobic fraction of EPS is bound to cations; therefore hydrophobicity of EPS varies with the change in divalent cation concentrations. Many studies confirmed the relatively high fouling role played by SMP compared to those of the suspended solids (biofloc and bound EPS) [10,11]. The soluble fraction of the mixed liquor or the SMPs is assumed to be responsible for the pore blocking of the membrane [10,12] which is not removed by physical cleaning and thus significantly reduces the membrane permeability [1]. Cations are also known to impact dissolved organic matter (DOM) in wastewaters [13–15]. An increase of ionic strength was found to decrease the DOM rejection of the membrane from 75% to 50% when Na+ ions (1.75 mmol/L) were added and to about 15% where Ca2+ (0.6 mmol/L) were added [13]. The decrease in DOM rejection with the addition of cations may be attributed to a decrease in the size of the DOM molecules due to the reduction of repulsion between the DOM molecules [13,16]. As a consequence, the DOM molecules may pass more easily through the membrane. Tests conducted at wastewater treatment plants revealed that the complex interactions between cations and sludge influenced the settling and dewatering properties in a manner that depended on the ratios and concentrations of monovalent to divalent cations in the activated sludge feed and solution [17–19]. Critical magnesium to calcium ratio and the monovalent to divalent (M/D) cation ratios of 1:1 and 2:1 (mequiv./L) were found to be important parameters in floc strength, settling and dewatering characteristics of activated sludge by Higgins and Novac [18], with floc deterioration
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occurring below the former and above the latter ratio. Addition of sodium caused deterioration in floc properties due to replacement of divalent cations with the monovalent cations from binding sites within the floc. The aforementioned authors suggested an optimum cation balance for settling and dewatering of activated sludge measured by sludge volume index (SVI), specific resistance to filtration (SRF), and capillary suction time (CST). Although the published studies [18,20,21] have used filtration tests for the optimization of activated sludge dewaterability, fouling propensity in MBRs is distinctively different from dewatering due to a number of factors, including different solids concentrations, membrane pore openings, operating pressures, and relative contribution of soluble microbial products to both cake resistance and pore plugging. This paper aims primarily at delineating the optimum cation concentrations for membrane filtration performance of activated sludge in MBRs, as well as characterization of membrane foulants. The MBR designation used henceforth represents the M/D, and the Mg/Ca milli-equivalent ratios as the first and second subscripts respectively.
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primarily focuses on characterization of membrane foulants and delineation of concentrations and RH of EPS and SMP in order to rationalize the change in fouling rates. A target solids retention time (SRT) of 15 days was maintained through direct removal of sludge from the bioreactor (1/15 of the bioreactor volume) on daily basis. The impact of cation concentrations in the feed wastewater was examined on the membrane permeability. The experimental conditions and cation concentrations are described in Tables 1 and 2, respectively. Two M/D ratios of 3:1 and 1:1 were examined. At each M/D ratio, two Mg/Ca ratios of 1:5 and 5:1 were evaluated at constant calcium concentrations. A fifth MBR was also tested at the optimum sludge dewaterability M/D and Mg/Ca ratios of 2:1 and 1:1, respectively identified by Higgins and Novac [18]. The M/D ratios of 3:1 and 1:1 were selected to reflect typical ratios in municipal wastewaters, consistent with the 1.58 and 2.39 observed by Higgins and Novac [18] in two municipal wastewaters. Similarly, the Mg/Ca ratios reflect variations typical of both municipal and industrial wastewater [18]. 2.2. Composition of synthetic wastewater
2. Materials and methods 2.1. Membrane bioreactor Five 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. Alum was added to the system continuously to chemically remove phosphorous. The reactors were seeded with returned activated sludge from the Adelaide treatment plant in London, ON, 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. MBRs were operated under constant flux mode. The target flow rate through the membrane 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. It must be noted that the Zenon membranes used in this study (ZeeWeed 1) are short and have fewer number of hollow fibers compared to real-scale membranes, and hence the extrapolation of the results of this work to full-scale MBRs must be undertaken with caution. This research
The synthetic wastewater (total influent COD of 325 mg/L, ammonia of 25 mgN/L, total phosphorous of 6 mgP/L) was made by adding the following chemicals to distilled water: 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 , 11.0 mg/L, FeCl3 , 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. MgSO4 ·7H2 O, CaCl2 ·2H2 O, and NaCl were added to maintain the Mg/Ca and M/D ratios accordingly. 2.3. Analytical methods The influent and permeate quality as well as the performance of the systems with respect to filterability of sludges, floc size distribution, permeability, and TMP were routinely monitored. The mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), COD, NH3 , total nitrogen (TN), NO3 , NO2 , and total phosphorous (TP) were measured using Standard Methods [22]. 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). Calcium, magnesium, and sodium were analyzed in the influent, effluent, mixed liquor and soluble mixed liquor by Inductively Coupled Plasma (ICP) (Vista-Pro, VARIAN) method No. 3120 of Standard Methods [22]. Total metal concentrations (in the mixed liquor) were measured after digesting the sludge samples according to method 3030D of Standard Methods [22]. To measure the soluble metal concentrations, activated sludge samples were filtered through 0.45 m filter paper prior to analysis. Deionized water was used for preparing all calibration standards and for dilution. A single element ICP standard solution (1000 mg/L) from Ultra Scientific (N. Kingstown, USA) was used to prepare a calibration curve for sodium. The mixed ICP standard No. 4 (100 mg/L) was used to prepare the calibration curves for magnesium and calcium. All samples were acidified with 2% nitric acid prior to analysis. 2.4. Batch filtration tests
Fig. 1. Schematic diagram of MBR experimental setup.
Batch filtration tests were conducted to measure the modified fouling index (MFI) [23]. A stirred batch cell (8400, Amicon, USA) was used to measure the permeate volume with an ultrafiltration (UF) membrane (nominal molecular weight cut-off 300 kDa, polyethersufone, 41.8 cm2 , Amicon, USA), under constant pressure.
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Table 1 Operating conditions of the five MBRs. Parameters
Values
Mg/Ca ratio M/D ratio HRT (h) SRT (days) Membrane surface area (m2 ) Flux (LMH)a Reactor volume (L) Operation time (day) Aeration (L/min)
1:5 3:1 4.5 15 0.09 14 ± 0.6 (55) 6.6 60 4
a
1:5 1:1 4.5 15 0.09 13.9 ± 1.1 (38) 6.6 55 4
5:1 1:1 4.5 15 0.09 14 ± 0.2 (63) 6.6 60 4
5:1 3:1 4.5 15 0.09 13.7 ± 0.8 (38) 6.6 55 4
1:1 2:1 4.5 15 0.09 14 ± 0.5 (55) 6.6 60 4
LMH = L/m2 h.
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. 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. Second, the soluble component was filtered through UF membranes. 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 [23] 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 seconds and V in liters) 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.
tration through a 0.45 m filter paper and the sum of proteins, and carbohydrate concentrations of the filtrate were determined as representing the SMP. The difference between these measurements was the EPS concentration. Carbohydrates were determined according to Dubois et al. [25] 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 were determined using Micro-Bicinchoninic Acid (BCA) protein assay (Pierce, Rockford, USA). Calibration for proteins was done using bovine serum albumin (BSA) from 0 to 150 mg/L. Samples were measured in duplicates. 2.6. Relative hydrophobicity
where 20 = viscosity at 20 ◦ C; = viscosity at the water temperature; P = pressure applied in kPa.
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 ):
2.5. EPS and SMP analysis
Relative hydrophobicity (%) = 100 × 1 −
MFI =
20 P tan ˛ 210
(1)
The EPS and SMP concentrations were measured as carbohydrates and proteins using a cation exchange resin (CER) (DOWEX R Marathon C, Na+ form, Sigma–Aldrich, USA) extraction method [24]. 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 fil-
Se Si
2.7. Membrane performance The permeate flux (Jp ) was calculated from the permeate flow (Qp ) and the membrane area (Am ) using Eq. (2). This was determined through the 1 day time period (t) and the corresponding permeate volume (Vp ): Jp =
Qp 1 Vp [LMH] = Am Am t
(2)
Table 2 Steady-state MBRs operational conditions and performance. Parameter
Values
Mg/Ca (mequiv./L) M/D (mequiv./L) Notation Na+ (mg/L) Ca2+ (mg/L) Mg2+ (mg/L) Total divalent (mequiv./L) Floc size (m) Fouling rate (LMH/bar day) TSS (g/L) VSS (g/L) COD removal (%) NH4 removal (%)
1:5 3:1 MBR3,0.2 140 38 5 2.3 37 ± 9 (10) 4.0 ± 0.6 (8) 10 ± 1.3 (10) 7.4 ± 1.2 (10) 97 97
1:5 1:1 MBR1,0.2 170 150 18 9.5 45 ± 5 (7) 8.6 ± 1.7 (7) 8 ± 0.6 (6) 5 ± 0.5 (6) 98 99
5:1 3:1 MBR3,5 645 36 96 9.8 34 ± 5 (7) 12.5 ± 2.4 (10) 7.8 ± 0.5 (6) 4.8 ± 0.3 (6) 96 99
5:1 1:1 MBR1,5 140 36 96 9.8 58 ± 8 (10) 1.9 ± 0.3 (5) 9 ± 1.4 (12) 6.5 ± 1.1 (8) 97 99
1:1 2:1 MBR2,1 140 36 21 3.5 43 ± 7 (10) 3.7 ± 0.8 (5) 10.2 ± 1 (10) 6.7 ± 0.7(10) 98 98
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[18] as confirmed by around 40% lower floc size in MBR3,5 compared to MBR1,5 . It is known that smaller flocs in activated sludge increase cake resistance and membrane fouling rate [6]. The performance of MBR3,5 (M/D ratio of 3:1 and Mg/Ca ratio of 5:1) with fouling rate of 12.5 ± 2.4 LMH/bar day could be compared with the results of another research study by Kim and Jang [27] using submerged MBR employing a hydrophilic membrane. In the abovementioned study, a MBR was operated at Mg/Ca ratio of 5:1 and M/D ratio of 33, exhibiting a fouling rate of 13.4 LMH/bar day. It is concluded that at the high Mg/Ca ratio of 5:1, increasing the M/D ratio has detrimental impact on membrane permeability for the MBRs.
Fig. 2. Metal concentrations in the influent and effluent for the MBR3,5 .
Temperature corrected permeability to 20 ◦ C (LP20 from Eq. (3) [26] as follows: LP20
◦C
=
Jp e−0.0239(T −20) pTM
[LMH/bar]
◦C
) was found
(3)
where pTM is the transmembrane pressure (averaged over a day, using three data points). Fouling rate is calculated as the slope of permeability versus time graph. 3. Results and discussion
3.1.3. Impact of Mg/Ca at high M/D ratio As apparent from Table 2, for the MBRs at the high M/D ratio of 3:1, the fouling rate was almost three times higher at Mg/Ca ratio of 5:1 than the 1:5. The floc sizes for the two aforementioned sludges were found to be almost equal (Table 2). Analysis of cation concentrations presented in Table 2 shows that both sodium and magnesium were higher for MBR3,5 compared to MBR1,5 with 4.5 times higher sodium and almost 19 times magnesium concentration. 3.1.4. Impact of Mg/Ca at low M/D ratio At the lower M/D ratio of 1:1, the MBR at lower Mg/Ca ratio (1:5) showed a 4.5 times higher fouling rate than the MBR at Mg/Ca of 5:1. The sodium concentration was equal for both MBRs; however,
3.1. MBR performance The steady-state performance data of the three MBRs is summarized in Table 2. Steady-sate data was collected after five turnovers of the mean SRT. As apparent from Table 2, throughout the experimental period COD and ammonia removal efficiencies were almost equal in the MBRs. It must be noted that the concentration of inorganic suspended solids was on average around 2.9 ± 0.4 g/L for the five MBRs which shows that the inorganic content of the mixed liquor in the MBRs was stable and no inorganic solids were accumulated in the MBRs to contribute to inorganic fouling (except for the calcium in MBR1,0.2 as shown in Section 3.5). Fig. 2 shows the typical temporal variation of influent and effluent calcium, magnesium, and sodium concentrations in the MBRs for MBR3,5 as an example. As apparent from Fig. 2, the influent and effluent concentrations of calcium and magnesium were almost constant in the MBR proving that the inorganic content of the MBR was constant and no inorganic precipitation occurred. 3.1.1. Impact of M/D at low Mg/Ca ratio Fig. 3(a)–(c) shows the temporal variation of permeability for the five MBRs throughout the operation. Fouling rate data is presented in Table 2. At the lower Mg/Ca ratio (1:5), the MBR at M/D ratio of 1:1 showed twice higher fouling rate than the MBR at M/D ratio of 3:1 despite 20% lower MLSS concentration. Comparing the floc sizes for MBR1,0.2 and MBR3,0.2 shows that the floc size was 21% higher in the MBR1,0.2 compared to MBR3,0.2 . This might be due to higher calcium concentration in MBR1,0.2 which enhances bioflocculation and increases the floc size. Analysis of the supernatant and hydrophobicity of EPS was performed to find the reason. 3.1.2. Impact of M/D at high Mg/Ca ratio The fouling rates data (presented in Table 2) show that at the higher Mg/Ca ratio (5:1), the fouling rate in the MBR at an M/D ratio of 3:1, was 6.6 times higher than at an M/D ratio of 1:1. The concentration of both calcium and magnesium was equal in MBR3,5 and MBR1,5 except for 4.5 times higher sodium concentration for MBR3,5 . Higher fouling rate for MBR3,5 is attributed to higher sodium concentration in the feed which deteriorates floc properties
Fig. 3. Time variation of permeability for: (a) MBR3,0.2 and MBR 1,0.2 ; (b) MBR1,5 and MBR3,5 ; (c) MBR2,1 .
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calcium concentration was four times higher for MBR1,0.2 compared to MBR1,5 while magnesium concentration was five times higher in MBR1,5 compared to MBR1,0.2 . It must be noted that the total divalent cation concentrations were equal (around 9.8 mequiv./L) in both reactors. Thus, the higher magnesium concentration in the feed appears to be beneficial for fouling mitigation through enhanced bioflocculation of EPS, as reflected by a 28% increase in floc size (Table 2). The optimum conditions for the cations in the feed found in this study is Mg/Ca ratio of 5:1 and M/D of 1:1 (MBR1,5 ), as reflected by the observed minimum fouling rate of 1.9 LMH/bar day. The MBR at Mg/Ca ratio of 1:1 and M/D of 2:1, chosen according to the critical ratios proposed by Higgins and Novac [18] for settling and dewatering of activated sludge, showed a fouling rate of 3.7 ± 0.8 LMH/bar day, very close to 4 LMH/bar day observed in MBR3,1 at an M/D ratio of 3:1 and Mg/Ca ratio of 1:5 but almost double the minimum fouling rate. The analysis of cations for the abovementioned MBRs (MBR3,0.2 and MBR2,1 ) showed that both MBRs have the same calcium and sodium concentrations (Table 2) while the magnesium concentration was almost four times higher in the MBR2,1 . Comparing MBR2,1 with the MBR1,5 (lowest fouling rate achieved) also shows that both MBRs had equal sodium and calcium concentration while the MBR at optimum conditions (MBR1,5 ) had 4.5 times higher influent magnesium concentration than MBR2,1 . The floc sizes presented in Table 2 also show that the activated sludge floc size increases from 43 m in MBR2,1 to 58 m in MBR1,5 , thus emphasizing the beneficial role played by magnesium in reducing fouling rate by bridging the EPS and increasing the floc size. 3.1.5. Impact of total divalent cations Although according to the divalent cation theory, bioflocculation is positively impacted by the total divalent cations [7], the findings of this study are not completely consistent with the DCB theory. Comparing the fouling rates in MBR1,5 and MBR1,0.2 both with comparable total divalent cation concentrations of 9.8 mequiv./L (MBR1,5 ) and 9.5 mequiv./L for (MBR1,0.2 ), shows that not only was the fouling rate 4.5 times higher in MBR1,0.2 than MBR1,5 , but also the floc size was 30% smaller. MBR1,5 had 5.3 times the magnesium concentration of MBR1,0.2 , but only 24% of the calcium (Table 2) clearly emphasizing the superiority of magnesium over calcium in fouling mitigation. Additionally, MBR1,5 (lowest fouling rate) and MBR3,5 (highest fouling rate) both with the same amount of total divalent cation concentration of 9.8 mequiv./L exhibited very different behavior, with the fouling rate at the high sodium concentration (MBR3,5 ) 6.6 times higher and floc size 40% smaller than MBR1,5 . Furthermore, comparing MBR3,0.2 (2.3 mequiv./L total divalent cation concentration) and MBR1,0.2 (9.5 mequiv./L total divalent cation concentration) in terms of fouling indicates that the fouling rate at the low divalent concentration was about half of that at the higher concentration despite a 20% reduction in floc size. While the aforementioned three comparisons clearly refute the principles of the DCB, a comparison of the fouling rates and floc sizes in MBR3,0.2 and MBR2,1 emphasizes that increasing the total divalent cation concentrations from 2.3 mequiv./L in MBR3,0.2 to 3.5 mequiv./L in MBR2,1 in fact reduced the fouling rate by 7.5% and simultaneously increased floc size by 15%. While the abovementioned comparisons emphatically demonstrate the beneficial role of magnesium in mitigating fouling and bioflocculation, it unambiguously corroborates that the total divalent cation concentration is not the sole factor impacting bioflocculation.
Table 3 Fouling characteristics for the MBRs. MFI (×103 s/L2 )
Sol
MBR3,0.2 MBR1,0.2 MBR2,1 MBR1,5 MBR3,5
4 4.5 3 1.5 5.7
SS ± ± ± ± ±
1.2 (6) 1.5 (6) 0.8 (6) 0.5 (6) 2.2 (6)
3 4.7 2.2 1 7.4
± ± ± ± ±
suspended solids (SS) MFI for the MBRs. Fig. 4 proves the correlation between the total MFI (suspended plus soluble) values and the reactors fouling rate despite the different filtration conditions of constant flux mode in the MBR versus constant pressure mode in the batch tests for MFI, and dead-end (in MFI test) versus the hollow fiber outside–in Zenon (MBR). Statistical analysis using MINITAB (Version 14, Minitab, State College, PA) was performed on all the data to assess the significance of the differences in means (averages) between the parameters in the MBRs. Prior to the t-test analysis, normal data distribution has been investigated using the Anderson–Darling test. 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 suspended and soluble MFI values have been confirmed to be statistically significant at the 95% confidence level using paired t-test. As apparent from Table 3, the highest and lowest MFI values were attributed to MBR3,5 and MBR1,5 , respectively. Soluble MFI was higher for all MBRs except MBR1,0.2 which had almost equal SS and Sol MFI and MBR3,5 where SS MFI was higher due to deterioration of floc properties as a result of the high Na concentration in the feed. Both SS and Sol MFI trends were consistent with the fouling rate trend from the highest to the lowest as follows: MBR3,5 > MBR1,0.2 > MBR3,0.2 > MBR2,1 > MBR1,5 . The changes in the EPS and SMP and their relative hydrophobicities were investigated to rationalize the MFI results. 3.3. EPS Table 4 shows the concentration and relative hydrophobicity of EPS in the MBRs. The highest and lowest EPS concentrations correspond to MBR1,5 and MBR3,5 , respectively, which experienced the lowest and highest fouling rates, respectively. Similarly, MBR1,5 and MBR3,5 showed the largest and smallest floc sizes (Table 2). Carbohydrate EPS concentrations in ascending order are as follows: MBR3,5 < MBR1,0.2 < MBR3,0.2 < MBR2,1 < MBR1,5 , which is exactly the same as the descending order with respect to both the SS MFI (Table 3) and the fouling rates (Table 2), clearly suggesting that carbohydrate EPS concentrations may be inversely related to cake resistance. Total EPS concentrations arranged in ascending order were similar, though not identical, to the carbohydrate concentrations, i.e., MBR3,5 < MBR3,0.2 < MBR1,0.2 < MBR2,1 < MBR1,5 .
3.2. Mixed liquor hydraulic resistance The hydraulic resistance attributed to the individual components of the mixed liquor (ML) was determined in batch ultrafiltration experiments. Table 3 shows the soluble (Sol) and
1.1 (6) 2 (6) 0.9 (6) 0.3 (6) 3.1 (6)
Fig. 4. Correlation between fouling rate and total MFI for the MBRs.
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Table 4 EPS concentration and composition for the MBRs in mg/g VSS. Parameter
Values
Mg/Ca M/D
1:5 3:1
1:5 1:1
5:1 3:1
5:1 1:1
1:1 2:1
EPSp EPSc EPSt RH (%)
6.4 ± 1.6 (8) 4.7 ± 1.2 (8) 11.6 ± 2.3 (8) 17 ± 7 (7)
7.5 ± 0.8 (5) 4.5 ± 0.5 (5) 11.9 ± 0.5 (5) 20 ± 8 (5)
4.5 ± 0.6 (5) 3.7 ± 0.6 (5) 8.3 ± 1.1 (5) 15 ± 7 (5)
9.4 ± 1 (8) 7.3 ± 0.8 (8) 16.4 ± 1.3 (8) 9 ± 6 (7)
7.2 ± 0.9 (8) 6.4 ± 2.2 (8) 13.6 ± 2.1 (8) 13 ± 5 (7)
Table 5 SMP concentration and composition for the MBRs in mg/L. Parameter
Values
Mg/Ca M/D
1:5 3:1
1:5 1:1
5:1 3:1
5:1 1:1
1:1 2:1
SMPp SMPc SMPt RH (%)
36 ± 4 (6) 57 ± 12 (6) 93 ± 12 (6) 19 ± 4 (7)
30 ± 3 (5) 40 ± 5 (5) 70 ± 6 (5) 22 ± 9 (5)
45 ± 5 (5) 65 ± 6 (5) 111 ± 11 (5) 28 ± 8 (5)
20 ± 3 (6) 39 ± 7 (6) 60 ± 9 (6) 10 ± 3 (7)
32 ± 7 (6) 50 ± 6.5 (6) 82 ± 6 (6) 12 ± 7 (7)
It was also found that the EPS protein concentration increased as the floc size increased in the order as follows: MBR3,5 < MBR3,0.2 < MBR2,1 < MBR1,0.2 < MBR1,5 . This shows that there is a correlation between the protein content of the EPS and the floc size. In general, EPS concentration tend to be lower at higher M/D ratio of 3:1 compared to M/D ratio of 1:1 which is due to presence of higher monovalent cations in the reactor and their replacement with divalent cations that leads to decrease in the floc size and strength. Examination of the relative hydrophobicity data presented in Table 4 shows that the lowest RH of EPS was observed in sludge from MBR1,5 with the lowest fouling rate and highest EPS concentration. Generally higher relative hydrophobicities were found for the MBRs at lower Mg/Ca ratios which indicate that in the absence of magnesium, more hydrophobic EPS is released or in other words, magnesium preferentially binds and bridges with the hydrophobic EPS than hydrophilic EPS.
3.4. SMP Table 5 presents the concentrations and relative hydrophobicities of SMPs in the MBRs. Statistical analysis using paired t-tests indicated that the differences between averages of SMPs for the MBRs were significant at the 95% confidence level except for the carbohydrate SMP in MBR1,0.2 and MBR1,5 . SMP concentrations as both protein and carbohydrate were higher for the MBRs at M/D ratio of 3:1 for both Mg/Ca ratio of 5:1 and 1:5, than at M/D ratio of 1:1. The MBR at optimum conditions (MBR1,5 ) showed the lowest SMP concentration and RH. The higher RH of SMP for MBR1,0.2 and MBR3,5 resulted in a higher soluble MFI as shown in Table 5. Comparing the MBRs at the lower M/D ratio of 1:1, carbohydrate SMP concentrations remained constant at 40 mg/L for the Mg/Ca ratio of 5:1 and 1:5 while protein SMP decreased by 33% from Mg/Ca ratio of 1:5 to 5:1. In general, SMP concentrations were higher for the MBRs at higher M/D ratio which is consistent with the findings of other studies showing that introduction of sodium caused a release of soluble proteins [17]. This shows that, at the low M/D ratio of 1:1, higher magnesium concentration (Mg/Ca of 5:1) was beneficial in reducing protein SMP compared to the MBR at higher calcium concentration (Mg/Ca of 1:5). Conversely, at the high M/D ratio of 3:1, increasing magnesium increased both carbohydrate and protein SMPs, resulting in a 40% increase in soluble MFI (Table 3).
3.5. Correlation of fouling rates with individual cation concentrations Statistical analysis was performed to study the influence of each individual cation concentration (Na, Ca, and Mg) on membrane fouling (SPSS for Windows, Version 14). Multiple regression analysis was chosen to evaluate which of the three independent variables (Na, Ca, and Mg concentration) exerted the greatest effect on the dependent variable (membrane fouling rate). The data used in the multiple regression analysis includes the five MBRs operated in the present study and two MBRs, previously reported by the authors [28], operating at Mg, and Na concentrations of 5 and 140 mg/L and two Ca concentrations of 36 and 250 mg/L, respectively. Modified or standardized regression coefficient, called the Beta coefficients were used to interpret the regression model. They have a common unit of measurement and so allow direct comparison of the impact of each independent variable [29]. They show the sensitivity of the dependent variable to each of the independent variables. A P value of 0.05, i.e., a 95% confidence level, was used as the cut-off for statistical significance. The fouling rate variations can be explained by following equation: Fouling rate = 0.517 + 0.237 Ca + 1.129 Na − 0.248 Mg (R2 = 0.96)
(4)
Ca and Na concentration had statistically positive impact on membrane fouling rate, i.e., an increase in dependent variable increased the fouling rate. The effect of Na on the membrane fouling rate was noticeably more pronounced than the effect of the other two parameters, i.e., Ca and Mg; Ca and Mg concentration affected the permeability with almost the same significance. Mg concentration showed a negative correlation with fouling rate while Ca showed a positive correlation with fouling rate. The Pearson’s product momentum correlation coefficients (rp ) were used for linear correlations between two parameters (Table 6). Table 6 Pearson’s (rp ) coefficient for linear correlations between fouling rate, Mg/Ca ratio, M/D ratio and the product of the two.
Fouling rate Mg/Ca M/D (Mg/Ca) × (M/D)
Fouling rate
Mg/Ca
M/D
(Mg/Ca) × (M/D)
1 0.339733 0.229331 0.77768
1 −0.02194 0.803966
1 0.329962
1
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The Pearson’s (rp ) coefficient is always between −1 and +1 where −1 means a perfect negative correlation and +1 a perfect positive correlation while 0 means absence of relationship. The data analysis showed that there was a relatively significant correlation between the fouling rate and the product of Mg/Ca and M/D ratios with no significant correlation found with the two independent parameters. This indicates that the two parameters must be simultaneously taken into consideration for the optimization of cation concentrations for the improvement of filtration performance in MBRs. 3.6. Impact of individual cations MBR3,0.2 , MBR2,1 and MBR1,5 are compared to study the impact of magnesium on membrane fouling since they have the same influent calcium and sodium concentrations. Magnesium concentration increased from 5 mg/L in MBR3,0.2 to 21 mg/L in MBR2,1 to 96 mg/L in MBR1,5 . Fig. 5(a)–(d) compares the three MBRs with respect to fouling rate, floc size, EPS, and SMP concentrations, respectively.
Fig. 6. Comparison of MBR characteristics with respect to sodium concentration.
Fig. 5. Comparison of MBR characteristics with respect to magnesium concentration.
As apparent from Fig. 5(a), fouling rate tends to decrease with increasing Mg concentration which is consistent with the results of statistical analysis discussed in Section 3.5. Fouling rate was almost constant at around 4 LMH/bar day for MBR3,0.2 and MBR2,1 but decreased to 1.9 LMH/bar day for MBR1,5 . The floc size and EPS concentration increased as the magnesium concentration increased which proves that magnesium was beneficial in reducing membrane fouling by increasing the floc size and hence lowering the cake filtration resistance. Fig. 5(d) presents that the SMP concentrations decreased by 11% upon increasing influent Mg concentration from 5 to 21 mg/L and by a further 26% from 21 mg/L to 96 mg/L. Therefore, increase of influent magnesium concentrations resulted in lower SMP concentrations in the reactors and decreased fouling rates. MBR1,5 (Na = 140 mg/L) and MBR3,5 (Na = 645 mg/L), both with the same calcium and magnesium, are also compared to exam-
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ine the impact of sodium concentration on membrane fouling. Fig. 6(a)–(d) compares the two MBRs with respect to fouling rate, floc size, EPS, and SMP concentrations, respectively. As apparent from Fig. 6(a), fouling rate increased by increasing sodium concentration from 140 mg/L in MBR1,5 to 645 mg/L in MBR3,5 which is consistent with the results of statistical analysis in Section 3.5. Fouling rate was around 6.6 times higher for the MBR3,5 at the Na concentration of 645 mg/L compared to MBR1,5 at Na concentration of 140 mg/L. Fig. 6(b) shows that the floc size was 41% lower at the higher sodium concentration which rationalize the higher suspended solids MFI for the MBR3,5 . EPS concentration decreased by about 50% while the SMP concentration almost doubled with the increase in influent sodium concentration from 140 to 645 mg/L (shown in Fig. 6(c) and (d)). This shows that the EPS bound to the flocs were released to the solution to form soluble EPS (SMP) as a result of the exchange of sodium with divalent cations in the floc structure which increased membrane fouling rate. The examination of the MFI data (Table 3) emphasizes the deterioration of both soluble and suspended solids filterability with the rise in sodium. Filtration performance can be compared with respect to calcium concentration in MBR1,0.2 and MBR2,1 where sodium and magnesium concentrations are close and calcium concentration is four times higher in MBR1,0.2 . Preliminary analysis of data showed that the fouling rate increased by increasing Ca concentration from 36 mg/L in MBR2,1 to 150 mg/L in MBR1,0.2 which is consistent with the results of statistical analysis (Section 3.5). However, cation analysis using MINTEQ program (MINTQA2, version 3.11) showed that calcium carbonate precipitated in the MBR1,0.2 and therefore the impact of calcium on bioflocculation cannot be assessed comparing the two MBRs. 4. Summary and conclusions The findings of this study illustrate the significant role of cations in membrane fouling expressed as Mg/Ca and M/D ratio. (1) Within the concentration ranges used in this research, Ca concentration of 36–150 mg/L, Mg of 5–96 mg/L, and Na of 140–645 mg/L, the optimum condition found was M/D of 1:1 and Mg/Ca of 5:1 in MBR1,5 , with Ca, Mg, and Na concentrations of 36, 96, and 140 mg/L, respectively. At the lower M/D ratio of 1:1, higher Mg/Ca was beneficial in reducing membrane fouling while at the higher M/D ratio of 3:1, a lower Mg/Ca showed better performance than the higher Mg/Ca ratio. (2) High sodium concentration of 645 mg/L at M/D of 3:1 resulted in a decrease in the floc size and increased the fouling rate. (3) The optimum ratios of M/D and Mg/Ca of 2:1 and 1:1 observed in the literature for sludge settling and dewatering did not result in the lowest membrane fouling rates or membrane filtrations resistance. The MBR (MBR1,5 ) at M/D and Mg/Ca ratios of 1:1 and 1:5, respectively not only exhibited the least fouling rate but also the largest floc size, highest EPS, the lowest SMP concentration and RH. The higher RH of SMP for MBR1,0.2 and MBR3,5 contributed to higher soluble MFI. (4) At the low M/D ratio of 1:1 and a total divalent cation concentration of about 9.5 mequiv., higher Mg concentration of 96 mg/L was beneficial in reducing SMP compared to the MBR at higher calcium concentration of 150 mg/L (Mg/Ca of 1:5) while at an M/D ratio of 3:1, higher magnesium increased both carbohydrate and protein SMPs, thus increasing both soluble and suspended solids MFI. (5) At constant influent calcium and sodium concentrations of 36 and 170 mg/L, respectively, increased magnesium concentration in the range of 5–96 mg/L simultaneously reduced fouling rate, increased floc size and EPS, and decreased SMPs.
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(6) At a constant influent calcium and magnesium concentrations of 36 and 96 mg/L, increasing sodium from 140 to 645 mg/L increased fouling rate by around six times, decreased both the floc size and EPS by 41% and 50%, respectively, while increasing SMP by 100%. (7) Statistical analysis of the impact of each individual cation (Na, Mg, and Ca) on membrane fouling rates revealed that Ca and Na increased membrane fouling rate while Mg concentration had a negative correlation with fouling rate. The Pearson’s product momentum correlation coefficient (rp ) showed that Mg/Ca and M/D ratios must be simultaneously taken into consideration for optimization of cation concentrations for improvement of filtration performance in MBRs. Acknowledgment This work was fully funded by Natural Sciences and Engineering Research Council of Canada (NSERC). References [1] P. Le Clech, V. Chen, T.A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284 (2006) 17–53. [2] I.S. Chang, C.H. Lee, Membrane filtration characteristics in membrane-coupled activated sludge system—the effect of physiochemical states of activated sludge on membrane fouling, Desalination 120 (1998) 221–233. [3] B.Q. Liao, D.M. Bagley, H.E. Kraemer, G.G. Leppard, S.N. Liss, A review of biofouling and its control in membrane separation bioreactors, Water Environ. Res. 76 (2004) 425–436. [4] Y. Liu, H.H.P. Fang, Influence of intracellular polymeric substances (EPS) on flocculation, settling, and dewatering of activated sludge, Crit. Rev. Environ. Sci. Technol. 33 (2003) 237–273. [5] V. Urbain, J.C. Block, J. Manem, Bioflocculation in activated sludge: an analytical approach, Water Res. 27 (1993) 829–838. [6] C. Wisniewski, A. Grasmick, Floc size distribution in a membrane bioreactor and consequences for membrane fouling, Colloid Surf. A: Physiochem. Eng. Asp. 138 (1998) 403–411. [7] D.C. Sobeck, M.J. Higgins, Examination of three theories for mechanisms of cation-induced bioflocculation, Water Res. 36 (2002) 527–538. [8] F.D. Sanin, P.A. Vesilind, Bioflocculation of activated sludge: the role of calcium ion and extracellular polymers, Environ. Technol. 21 (2000) 1405–1412. [9] B.M. Wilen, B. Jin, P. Lant, The influence of key chemical constituents in activated sludge on surface and flocculating properties, Water Res. 37 (2003) 2127–2139. [10] T. Itonaga, K. Kimura, Y. Watanabe, Influence of suspension viscosity and colloidal particles on permeability of membrane used in membrane bioreactor (MBR), Water Sci. Technol. 50 (2004) 301–309. [11] N. Cicek, M.T. Suidan, P. Ginestet, J.-M. Audic, Impact of soluble organic compounds on permeate flux in and aerobic membrane bioreactor, Environ. Technol. 24 (2003) 249–256. [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] F. Saravia, C. Zwiener, F.H. Frimmel, Interactions between membrane surface, dissolved organic substances and ions in submerged membrane filtration, Desalination 192 (2006) 280–287. [14] M.B. Müller, D. Schmitt, F.H. Frimmel, Fractionation of natural organic matter by size exclusion chromatography—properties and stability of fractions, Environ. Sci. Technol. 34 (2000) 4867–4872. [15] C. Jucker, M.M. Clark, Adsorption of aquatic humic substances on hydrophobic ultrafiltration membranes, J. Membr. Sci. 97 (1994) 37–52. [16] R.S. Swift, Molecular weight, size, shape, and charge characteristics of humic substances: some basic considerations, in Humic Substances II—in search of structure, Wiley VCH, 1989, pp. 449–465. [17] S.N. Murthy, J.T. Novak, R.D. De Haas, Monitoring cations to predict and improve activated sludge settling and dewatering properties of industrial wastewaters, Water Sci. Technol. 38 (1998) 119–126. [18] M.J. Higgins, J.T. Novak, Dewatering and settling of activated sludge: the case for using cation analysis, Water Environ. Res. 69 (1997) 225–232. [19] M.J. Higgins, L.A. Tom, D.C. Sobeck, Case study I: application of the divalent cation bridging theory to improve biofloc properties and industrial activated sludge system performance-direct addition of divalent cations, Water Environ. Res. 76 (2004) 344–352. [20] J.H. Bruus, P.H. Nielsen, K. Keiding, On the stability of activated sludge flocs with implications to dewatering, Water Res. 26 (1992) 1597–1604. [21] B. Jin, B.M. Wilen, P. Lant, Impacts of morphological, physical and chemical properties of sludge flocs on dewaterability of activated sludge, Chem. Eng. J. 98 (2004) 115–126.
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[22] APHA, Standard Methods for the Examination of Water and Wastewater, 20th edition, American Public Health Association/American Water Works, 2000. [23] J.C. Schippers, J. Verdouw, The modified fouling index, a method of determining the fouling characteristics of water, Desalination 32 (1980) 137–148. [24] B. FrØlund, R. Palmgren, K. Keiding, P.H. Nielsen, Extraction of extracellular polymers from activated sludge using a cation exchange resin, Water Res. 30 (1996) 1749–1758. [25] M. Dubois, K.A. Gilles, J.K. Hamilton, P.A. Rebers, F. Smith, Colorimetric method for determination of sugars and related substances, Anal. Chem. 28 (1956) 350–356.
[26] R.S. Trussell, R.P. Merlo, S.W. Hermanowicz, D. Jenkins, The effect of organic loading on process performance and membrane fouling in a submerged membrane bioreactor treating municipal wastewater, Water Res. 40 (2006) 2675–2683. [27] I.S. Kim, N. Jang, The effect of calcium on the membrane biofouling in the membrane bioreactor (MBR), Water Res. 40 (2006) 2756–2764. [28] S. Arabi, G. Nakhla, Impact of calcium on the membrane fouling in membrane bioreactors, J. Membr. Sci. 314 (2008) 134–142. [29] J.F. Hair, R.E. Anderson, R.L. Tatham, W.C. Black, Multivariate Data Analysis, Prentice-Hall, Englewood Cliffs, NJ, 1998.
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Impact of cation concentrations on fouling in 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 24 March 2009 Received in revised form 8 July 2009 Accepted 10 July 2009 Available online 18 July 2009 Keywords: MBR Fouling Mg/Ca ratio M/D ratio EPS SMP
a b s t r a c t In this study, the interaction of calcium, magnesium, and sodium as well as impact of monovalent to divalent (M/D) cation ratio and magnesium to calcium (Mg/Ca) ratio in the feed wastewater on membrane fouling in submerged membrane bioreactor (MBR) was investigated. The protein and carbohydrate content of soluble microbial products (SMP) and extracellular polymeric substances (EPS) as well as their relative hydrophobicities was examined. The mixed liquor and its components (soluble and suspended solids) were analyzed for their filtration resistance, as reflected by the modified fouling index (MFI). Based on the findings of this study, the optimum conditions with respect to fouling rate were calcium and sodium concentrations of 36 and 140 mg/L, respectively, M/D of 1:1 and Mg/Ca of 5:1, with all parameters on an equivalent basis. High sodium concentration at high M/D ratio was found to decrease the floc size and increase the fouling rate. At the low M/D ratio of 1:1, introduction of magnesium was beneficial in reducing the fouling rate by increasing the EPS concentration and floc size and decreasing the SMP concentration and relative hydrophobicity in the supernatant. The fouling rate was found to be statistically correlated with the concentrations of Ca, Mg, and Na, with both Ca and Na adversely impacting fouling and Mg alleviating fouling propensity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Application of membrane bioreactors (MBRs) as an efficient technology in wastewater treatment has increased in recent years. However, membrane fouling and its consequences in terms of increased operating and maintenance costs represent a significant limitation to the widespread application of MBRs. Bound extracellular polymeric substances (EPS) and soluble microbial products (SMP) greatly influence filterability in MBRs [1]. The characteristics of the biological suspension flocs such as floc size, relative hydrophobicity (RH), and surface charge have been reported to have an impact on membrane fouling [1] with hydrophobicity increasing with proteins and humic acids while hydrophilicity is enhanced by carbohydrates [2,3]. Increased hydrophobicity was reported to enhance bioflocculation [4,5] resulting in larger more permeable flocs and reduced fouling [6]. Enhanced bioflocculation due to cation bridging is known to result in the formation of larger flocs [7] which are more permeable and consequently reduce cake fouling resistance [6]. The divalent cation bridging (DCB) theory, postulates that divalent cations, such as calcium and magnesium, play a major role in bioflocculation and bridging negatively charged functional groups within the EPS, thus helping bioflocculation [7,8]. Wilen et al. [9] found
∗ Corresponding author. E-mail address: [email protected] (G. Nakhla). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.07.016
that more hydrophobic fraction of EPS is bound to cations; therefore hydrophobicity of EPS varies with the change in divalent cation concentrations. Many studies confirmed the relatively high fouling role played by SMP compared to those of the suspended solids (biofloc and bound EPS) [10,11]. The soluble fraction of the mixed liquor or the SMPs is assumed to be responsible for the pore blocking of the membrane [10,12] which is not removed by physical cleaning and thus significantly reduces the membrane permeability [1]. Cations are also known to impact dissolved organic matter (DOM) in wastewaters [13–15]. An increase of ionic strength was found to decrease the DOM rejection of the membrane from 75% to 50% when Na+ ions (1.75 mmol/L) were added and to about 15% where Ca2+ (0.6 mmol/L) were added [13]. The decrease in DOM rejection with the addition of cations may be attributed to a decrease in the size of the DOM molecules due to the reduction of repulsion between the DOM molecules [13,16]. As a consequence, the DOM molecules may pass more easily through the membrane. Tests conducted at wastewater treatment plants revealed that the complex interactions between cations and sludge influenced the settling and dewatering properties in a manner that depended on the ratios and concentrations of monovalent to divalent cations in the activated sludge feed and solution [17–19]. Critical magnesium to calcium ratio and the monovalent to divalent (M/D) cation ratios of 1:1 and 2:1 (mequiv./L) were found to be important parameters in floc strength, settling and dewatering characteristics of activated sludge by Higgins and Novac [18], with floc deterioration
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occurring below the former and above the latter ratio. Addition of sodium caused deterioration in floc properties due to replacement of divalent cations with the monovalent cations from binding sites within the floc. The aforementioned authors suggested an optimum cation balance for settling and dewatering of activated sludge measured by sludge volume index (SVI), specific resistance to filtration (SRF), and capillary suction time (CST). Although the published studies [18,20,21] have used filtration tests for the optimization of activated sludge dewaterability, fouling propensity in MBRs is distinctively different from dewatering due to a number of factors, including different solids concentrations, membrane pore openings, operating pressures, and relative contribution of soluble microbial products to both cake resistance and pore plugging. This paper aims primarily at delineating the optimum cation concentrations for membrane filtration performance of activated sludge in MBRs, as well as characterization of membrane foulants. The MBR designation used henceforth represents the M/D, and the Mg/Ca milli-equivalent ratios as the first and second subscripts respectively.
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primarily focuses on characterization of membrane foulants and delineation of concentrations and RH of EPS and SMP in order to rationalize the change in fouling rates. A target solids retention time (SRT) of 15 days was maintained through direct removal of sludge from the bioreactor (1/15 of the bioreactor volume) on daily basis. The impact of cation concentrations in the feed wastewater was examined on the membrane permeability. The experimental conditions and cation concentrations are described in Tables 1 and 2, respectively. Two M/D ratios of 3:1 and 1:1 were examined. At each M/D ratio, two Mg/Ca ratios of 1:5 and 5:1 were evaluated at constant calcium concentrations. A fifth MBR was also tested at the optimum sludge dewaterability M/D and Mg/Ca ratios of 2:1 and 1:1, respectively identified by Higgins and Novac [18]. The M/D ratios of 3:1 and 1:1 were selected to reflect typical ratios in municipal wastewaters, consistent with the 1.58 and 2.39 observed by Higgins and Novac [18] in two municipal wastewaters. Similarly, the Mg/Ca ratios reflect variations typical of both municipal and industrial wastewater [18]. 2.2. Composition of synthetic wastewater
2. Materials and methods 2.1. Membrane bioreactor Five 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. Alum was added to the system continuously to chemically remove phosphorous. The reactors were seeded with returned activated sludge from the Adelaide treatment plant in London, ON, 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. MBRs were operated under constant flux mode. The target flow rate through the membrane 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. It must be noted that the Zenon membranes used in this study (ZeeWeed 1) are short and have fewer number of hollow fibers compared to real-scale membranes, and hence the extrapolation of the results of this work to full-scale MBRs must be undertaken with caution. This research
The synthetic wastewater (total influent COD of 325 mg/L, ammonia of 25 mgN/L, total phosphorous of 6 mgP/L) was made by adding the following chemicals to distilled water: 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 , 11.0 mg/L, FeCl3 , 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. MgSO4 ·7H2 O, CaCl2 ·2H2 O, and NaCl were added to maintain the Mg/Ca and M/D ratios accordingly. 2.3. Analytical methods The influent and permeate quality as well as the performance of the systems with respect to filterability of sludges, floc size distribution, permeability, and TMP were routinely monitored. The mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), COD, NH3 , total nitrogen (TN), NO3 , NO2 , and total phosphorous (TP) were measured using Standard Methods [22]. 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). Calcium, magnesium, and sodium were analyzed in the influent, effluent, mixed liquor and soluble mixed liquor by Inductively Coupled Plasma (ICP) (Vista-Pro, VARIAN) method No. 3120 of Standard Methods [22]. Total metal concentrations (in the mixed liquor) were measured after digesting the sludge samples according to method 3030D of Standard Methods [22]. To measure the soluble metal concentrations, activated sludge samples were filtered through 0.45 m filter paper prior to analysis. Deionized water was used for preparing all calibration standards and for dilution. A single element ICP standard solution (1000 mg/L) from Ultra Scientific (N. Kingstown, USA) was used to prepare a calibration curve for sodium. The mixed ICP standard No. 4 (100 mg/L) was used to prepare the calibration curves for magnesium and calcium. All samples were acidified with 2% nitric acid prior to analysis. 2.4. Batch filtration tests
Fig. 1. Schematic diagram of MBR experimental setup.
Batch filtration tests were conducted to measure the modified fouling index (MFI) [23]. A stirred batch cell (8400, Amicon, USA) was used to measure the permeate volume with an ultrafiltration (UF) membrane (nominal molecular weight cut-off 300 kDa, polyethersufone, 41.8 cm2 , Amicon, USA), under constant pressure.
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Table 1 Operating conditions of the five MBRs. Parameters
Values
Mg/Ca ratio M/D ratio HRT (h) SRT (days) Membrane surface area (m2 ) Flux (LMH)a Reactor volume (L) Operation time (day) Aeration (L/min)
1:5 3:1 4.5 15 0.09 14 ± 0.6 (55) 6.6 60 4
a
1:5 1:1 4.5 15 0.09 13.9 ± 1.1 (38) 6.6 55 4
5:1 1:1 4.5 15 0.09 14 ± 0.2 (63) 6.6 60 4
5:1 3:1 4.5 15 0.09 13.7 ± 0.8 (38) 6.6 55 4
1:1 2:1 4.5 15 0.09 14 ± 0.5 (55) 6.6 60 4
LMH = L/m2 h.
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. 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. Second, the soluble component was filtered through UF membranes. 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 [23] 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 seconds and V in liters) 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.
tration through a 0.45 m filter paper and the sum of proteins, and carbohydrate concentrations of the filtrate were determined as representing the SMP. The difference between these measurements was the EPS concentration. Carbohydrates were determined according to Dubois et al. [25] 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 were determined using Micro-Bicinchoninic Acid (BCA) protein assay (Pierce, Rockford, USA). Calibration for proteins was done using bovine serum albumin (BSA) from 0 to 150 mg/L. Samples were measured in duplicates. 2.6. Relative hydrophobicity
where 20 = viscosity at 20 ◦ C; = viscosity at the water temperature; P = pressure applied in kPa.
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 ):
2.5. EPS and SMP analysis
Relative hydrophobicity (%) = 100 × 1 −
MFI =
20 P tan ˛ 210
(1)
The EPS and SMP concentrations were measured as carbohydrates and proteins using a cation exchange resin (CER) (DOWEX R Marathon C, Na+ form, Sigma–Aldrich, USA) extraction method [24]. 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 fil-
Se Si
2.7. Membrane performance The permeate flux (Jp ) was calculated from the permeate flow (Qp ) and the membrane area (Am ) using Eq. (2). This was determined through the 1 day time period (t) and the corresponding permeate volume (Vp ): Jp =
Qp 1 Vp [LMH] = Am Am t
(2)
Table 2 Steady-state MBRs operational conditions and performance. Parameter
Values
Mg/Ca (mequiv./L) M/D (mequiv./L) Notation Na+ (mg/L) Ca2+ (mg/L) Mg2+ (mg/L) Total divalent (mequiv./L) Floc size (m) Fouling rate (LMH/bar day) TSS (g/L) VSS (g/L) COD removal (%) NH4 removal (%)
1:5 3:1 MBR3,0.2 140 38 5 2.3 37 ± 9 (10) 4.0 ± 0.6 (8) 10 ± 1.3 (10) 7.4 ± 1.2 (10) 97 97
1:5 1:1 MBR1,0.2 170 150 18 9.5 45 ± 5 (7) 8.6 ± 1.7 (7) 8 ± 0.6 (6) 5 ± 0.5 (6) 98 99
5:1 3:1 MBR3,5 645 36 96 9.8 34 ± 5 (7) 12.5 ± 2.4 (10) 7.8 ± 0.5 (6) 4.8 ± 0.3 (6) 96 99
5:1 1:1 MBR1,5 140 36 96 9.8 58 ± 8 (10) 1.9 ± 0.3 (5) 9 ± 1.4 (12) 6.5 ± 1.1 (8) 97 99
1:1 2:1 MBR2,1 140 36 21 3.5 43 ± 7 (10) 3.7 ± 0.8 (5) 10.2 ± 1 (10) 6.7 ± 0.7(10) 98 98
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[18] as confirmed by around 40% lower floc size in MBR3,5 compared to MBR1,5 . It is known that smaller flocs in activated sludge increase cake resistance and membrane fouling rate [6]. The performance of MBR3,5 (M/D ratio of 3:1 and Mg/Ca ratio of 5:1) with fouling rate of 12.5 ± 2.4 LMH/bar day could be compared with the results of another research study by Kim and Jang [27] using submerged MBR employing a hydrophilic membrane. In the abovementioned study, a MBR was operated at Mg/Ca ratio of 5:1 and M/D ratio of 33, exhibiting a fouling rate of 13.4 LMH/bar day. It is concluded that at the high Mg/Ca ratio of 5:1, increasing the M/D ratio has detrimental impact on membrane permeability for the MBRs.
Fig. 2. Metal concentrations in the influent and effluent for the MBR3,5 .
Temperature corrected permeability to 20 ◦ C (LP20 from Eq. (3) [26] as follows: LP20
◦C
=
Jp e−0.0239(T −20) pTM
[LMH/bar]
◦C
) was found
(3)
where pTM is the transmembrane pressure (averaged over a day, using three data points). Fouling rate is calculated as the slope of permeability versus time graph. 3. Results and discussion
3.1.3. Impact of Mg/Ca at high M/D ratio As apparent from Table 2, for the MBRs at the high M/D ratio of 3:1, the fouling rate was almost three times higher at Mg/Ca ratio of 5:1 than the 1:5. The floc sizes for the two aforementioned sludges were found to be almost equal (Table 2). Analysis of cation concentrations presented in Table 2 shows that both sodium and magnesium were higher for MBR3,5 compared to MBR1,5 with 4.5 times higher sodium and almost 19 times magnesium concentration. 3.1.4. Impact of Mg/Ca at low M/D ratio At the lower M/D ratio of 1:1, the MBR at lower Mg/Ca ratio (1:5) showed a 4.5 times higher fouling rate than the MBR at Mg/Ca of 5:1. The sodium concentration was equal for both MBRs; however,
3.1. MBR performance The steady-state performance data of the three MBRs is summarized in Table 2. Steady-sate data was collected after five turnovers of the mean SRT. As apparent from Table 2, throughout the experimental period COD and ammonia removal efficiencies were almost equal in the MBRs. It must be noted that the concentration of inorganic suspended solids was on average around 2.9 ± 0.4 g/L for the five MBRs which shows that the inorganic content of the mixed liquor in the MBRs was stable and no inorganic solids were accumulated in the MBRs to contribute to inorganic fouling (except for the calcium in MBR1,0.2 as shown in Section 3.5). Fig. 2 shows the typical temporal variation of influent and effluent calcium, magnesium, and sodium concentrations in the MBRs for MBR3,5 as an example. As apparent from Fig. 2, the influent and effluent concentrations of calcium and magnesium were almost constant in the MBR proving that the inorganic content of the MBR was constant and no inorganic precipitation occurred. 3.1.1. Impact of M/D at low Mg/Ca ratio Fig. 3(a)–(c) shows the temporal variation of permeability for the five MBRs throughout the operation. Fouling rate data is presented in Table 2. At the lower Mg/Ca ratio (1:5), the MBR at M/D ratio of 1:1 showed twice higher fouling rate than the MBR at M/D ratio of 3:1 despite 20% lower MLSS concentration. Comparing the floc sizes for MBR1,0.2 and MBR3,0.2 shows that the floc size was 21% higher in the MBR1,0.2 compared to MBR3,0.2 . This might be due to higher calcium concentration in MBR1,0.2 which enhances bioflocculation and increases the floc size. Analysis of the supernatant and hydrophobicity of EPS was performed to find the reason. 3.1.2. Impact of M/D at high Mg/Ca ratio The fouling rates data (presented in Table 2) show that at the higher Mg/Ca ratio (5:1), the fouling rate in the MBR at an M/D ratio of 3:1, was 6.6 times higher than at an M/D ratio of 1:1. The concentration of both calcium and magnesium was equal in MBR3,5 and MBR1,5 except for 4.5 times higher sodium concentration for MBR3,5 . Higher fouling rate for MBR3,5 is attributed to higher sodium concentration in the feed which deteriorates floc properties
Fig. 3. Time variation of permeability for: (a) MBR3,0.2 and MBR 1,0.2 ; (b) MBR1,5 and MBR3,5 ; (c) MBR2,1 .
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calcium concentration was four times higher for MBR1,0.2 compared to MBR1,5 while magnesium concentration was five times higher in MBR1,5 compared to MBR1,0.2 . It must be noted that the total divalent cation concentrations were equal (around 9.8 mequiv./L) in both reactors. Thus, the higher magnesium concentration in the feed appears to be beneficial for fouling mitigation through enhanced bioflocculation of EPS, as reflected by a 28% increase in floc size (Table 2). The optimum conditions for the cations in the feed found in this study is Mg/Ca ratio of 5:1 and M/D of 1:1 (MBR1,5 ), as reflected by the observed minimum fouling rate of 1.9 LMH/bar day. The MBR at Mg/Ca ratio of 1:1 and M/D of 2:1, chosen according to the critical ratios proposed by Higgins and Novac [18] for settling and dewatering of activated sludge, showed a fouling rate of 3.7 ± 0.8 LMH/bar day, very close to 4 LMH/bar day observed in MBR3,1 at an M/D ratio of 3:1 and Mg/Ca ratio of 1:5 but almost double the minimum fouling rate. The analysis of cations for the abovementioned MBRs (MBR3,0.2 and MBR2,1 ) showed that both MBRs have the same calcium and sodium concentrations (Table 2) while the magnesium concentration was almost four times higher in the MBR2,1 . Comparing MBR2,1 with the MBR1,5 (lowest fouling rate achieved) also shows that both MBRs had equal sodium and calcium concentration while the MBR at optimum conditions (MBR1,5 ) had 4.5 times higher influent magnesium concentration than MBR2,1 . The floc sizes presented in Table 2 also show that the activated sludge floc size increases from 43 m in MBR2,1 to 58 m in MBR1,5 , thus emphasizing the beneficial role played by magnesium in reducing fouling rate by bridging the EPS and increasing the floc size. 3.1.5. Impact of total divalent cations Although according to the divalent cation theory, bioflocculation is positively impacted by the total divalent cations [7], the findings of this study are not completely consistent with the DCB theory. Comparing the fouling rates in MBR1,5 and MBR1,0.2 both with comparable total divalent cation concentrations of 9.8 mequiv./L (MBR1,5 ) and 9.5 mequiv./L for (MBR1,0.2 ), shows that not only was the fouling rate 4.5 times higher in MBR1,0.2 than MBR1,5 , but also the floc size was 30% smaller. MBR1,5 had 5.3 times the magnesium concentration of MBR1,0.2 , but only 24% of the calcium (Table 2) clearly emphasizing the superiority of magnesium over calcium in fouling mitigation. Additionally, MBR1,5 (lowest fouling rate) and MBR3,5 (highest fouling rate) both with the same amount of total divalent cation concentration of 9.8 mequiv./L exhibited very different behavior, with the fouling rate at the high sodium concentration (MBR3,5 ) 6.6 times higher and floc size 40% smaller than MBR1,5 . Furthermore, comparing MBR3,0.2 (2.3 mequiv./L total divalent cation concentration) and MBR1,0.2 (9.5 mequiv./L total divalent cation concentration) in terms of fouling indicates that the fouling rate at the low divalent concentration was about half of that at the higher concentration despite a 20% reduction in floc size. While the aforementioned three comparisons clearly refute the principles of the DCB, a comparison of the fouling rates and floc sizes in MBR3,0.2 and MBR2,1 emphasizes that increasing the total divalent cation concentrations from 2.3 mequiv./L in MBR3,0.2 to 3.5 mequiv./L in MBR2,1 in fact reduced the fouling rate by 7.5% and simultaneously increased floc size by 15%. While the abovementioned comparisons emphatically demonstrate the beneficial role of magnesium in mitigating fouling and bioflocculation, it unambiguously corroborates that the total divalent cation concentration is not the sole factor impacting bioflocculation.
Table 3 Fouling characteristics for the MBRs. MFI (×103 s/L2 )
Sol
MBR3,0.2 MBR1,0.2 MBR2,1 MBR1,5 MBR3,5
4 4.5 3 1.5 5.7
SS ± ± ± ± ±
1.2 (6) 1.5 (6) 0.8 (6) 0.5 (6) 2.2 (6)
3 4.7 2.2 1 7.4
± ± ± ± ±
suspended solids (SS) MFI for the MBRs. Fig. 4 proves the correlation between the total MFI (suspended plus soluble) values and the reactors fouling rate despite the different filtration conditions of constant flux mode in the MBR versus constant pressure mode in the batch tests for MFI, and dead-end (in MFI test) versus the hollow fiber outside–in Zenon (MBR). Statistical analysis using MINITAB (Version 14, Minitab, State College, PA) was performed on all the data to assess the significance of the differences in means (averages) between the parameters in the MBRs. Prior to the t-test analysis, normal data distribution has been investigated using the Anderson–Darling test. 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 suspended and soluble MFI values have been confirmed to be statistically significant at the 95% confidence level using paired t-test. As apparent from Table 3, the highest and lowest MFI values were attributed to MBR3,5 and MBR1,5 , respectively. Soluble MFI was higher for all MBRs except MBR1,0.2 which had almost equal SS and Sol MFI and MBR3,5 where SS MFI was higher due to deterioration of floc properties as a result of the high Na concentration in the feed. Both SS and Sol MFI trends were consistent with the fouling rate trend from the highest to the lowest as follows: MBR3,5 > MBR1,0.2 > MBR3,0.2 > MBR2,1 > MBR1,5 . The changes in the EPS and SMP and their relative hydrophobicities were investigated to rationalize the MFI results. 3.3. EPS Table 4 shows the concentration and relative hydrophobicity of EPS in the MBRs. The highest and lowest EPS concentrations correspond to MBR1,5 and MBR3,5 , respectively, which experienced the lowest and highest fouling rates, respectively. Similarly, MBR1,5 and MBR3,5 showed the largest and smallest floc sizes (Table 2). Carbohydrate EPS concentrations in ascending order are as follows: MBR3,5 < MBR1,0.2 < MBR3,0.2 < MBR2,1 < MBR1,5 , which is exactly the same as the descending order with respect to both the SS MFI (Table 3) and the fouling rates (Table 2), clearly suggesting that carbohydrate EPS concentrations may be inversely related to cake resistance. Total EPS concentrations arranged in ascending order were similar, though not identical, to the carbohydrate concentrations, i.e., MBR3,5 < MBR3,0.2 < MBR1,0.2 < MBR2,1 < MBR1,5 .
3.2. Mixed liquor hydraulic resistance The hydraulic resistance attributed to the individual components of the mixed liquor (ML) was determined in batch ultrafiltration experiments. Table 3 shows the soluble (Sol) and
1.1 (6) 2 (6) 0.9 (6) 0.3 (6) 3.1 (6)
Fig. 4. Correlation between fouling rate and total MFI for the MBRs.
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Table 4 EPS concentration and composition for the MBRs in mg/g VSS. Parameter
Values
Mg/Ca M/D
1:5 3:1
1:5 1:1
5:1 3:1
5:1 1:1
1:1 2:1
EPSp EPSc EPSt RH (%)
6.4 ± 1.6 (8) 4.7 ± 1.2 (8) 11.6 ± 2.3 (8) 17 ± 7 (7)
7.5 ± 0.8 (5) 4.5 ± 0.5 (5) 11.9 ± 0.5 (5) 20 ± 8 (5)
4.5 ± 0.6 (5) 3.7 ± 0.6 (5) 8.3 ± 1.1 (5) 15 ± 7 (5)
9.4 ± 1 (8) 7.3 ± 0.8 (8) 16.4 ± 1.3 (8) 9 ± 6 (7)
7.2 ± 0.9 (8) 6.4 ± 2.2 (8) 13.6 ± 2.1 (8) 13 ± 5 (7)
Table 5 SMP concentration and composition for the MBRs in mg/L. Parameter
Values
Mg/Ca M/D
1:5 3:1
1:5 1:1
5:1 3:1
5:1 1:1
1:1 2:1
SMPp SMPc SMPt RH (%)
36 ± 4 (6) 57 ± 12 (6) 93 ± 12 (6) 19 ± 4 (7)
30 ± 3 (5) 40 ± 5 (5) 70 ± 6 (5) 22 ± 9 (5)
45 ± 5 (5) 65 ± 6 (5) 111 ± 11 (5) 28 ± 8 (5)
20 ± 3 (6) 39 ± 7 (6) 60 ± 9 (6) 10 ± 3 (7)
32 ± 7 (6) 50 ± 6.5 (6) 82 ± 6 (6) 12 ± 7 (7)
It was also found that the EPS protein concentration increased as the floc size increased in the order as follows: MBR3,5 < MBR3,0.2 < MBR2,1 < MBR1,0.2 < MBR1,5 . This shows that there is a correlation between the protein content of the EPS and the floc size. In general, EPS concentration tend to be lower at higher M/D ratio of 3:1 compared to M/D ratio of 1:1 which is due to presence of higher monovalent cations in the reactor and their replacement with divalent cations that leads to decrease in the floc size and strength. Examination of the relative hydrophobicity data presented in Table 4 shows that the lowest RH of EPS was observed in sludge from MBR1,5 with the lowest fouling rate and highest EPS concentration. Generally higher relative hydrophobicities were found for the MBRs at lower Mg/Ca ratios which indicate that in the absence of magnesium, more hydrophobic EPS is released or in other words, magnesium preferentially binds and bridges with the hydrophobic EPS than hydrophilic EPS.
3.4. SMP Table 5 presents the concentrations and relative hydrophobicities of SMPs in the MBRs. Statistical analysis using paired t-tests indicated that the differences between averages of SMPs for the MBRs were significant at the 95% confidence level except for the carbohydrate SMP in MBR1,0.2 and MBR1,5 . SMP concentrations as both protein and carbohydrate were higher for the MBRs at M/D ratio of 3:1 for both Mg/Ca ratio of 5:1 and 1:5, than at M/D ratio of 1:1. The MBR at optimum conditions (MBR1,5 ) showed the lowest SMP concentration and RH. The higher RH of SMP for MBR1,0.2 and MBR3,5 resulted in a higher soluble MFI as shown in Table 5. Comparing the MBRs at the lower M/D ratio of 1:1, carbohydrate SMP concentrations remained constant at 40 mg/L for the Mg/Ca ratio of 5:1 and 1:5 while protein SMP decreased by 33% from Mg/Ca ratio of 1:5 to 5:1. In general, SMP concentrations were higher for the MBRs at higher M/D ratio which is consistent with the findings of other studies showing that introduction of sodium caused a release of soluble proteins [17]. This shows that, at the low M/D ratio of 1:1, higher magnesium concentration (Mg/Ca of 5:1) was beneficial in reducing protein SMP compared to the MBR at higher calcium concentration (Mg/Ca of 1:5). Conversely, at the high M/D ratio of 3:1, increasing magnesium increased both carbohydrate and protein SMPs, resulting in a 40% increase in soluble MFI (Table 3).
3.5. Correlation of fouling rates with individual cation concentrations Statistical analysis was performed to study the influence of each individual cation concentration (Na, Ca, and Mg) on membrane fouling (SPSS for Windows, Version 14). Multiple regression analysis was chosen to evaluate which of the three independent variables (Na, Ca, and Mg concentration) exerted the greatest effect on the dependent variable (membrane fouling rate). The data used in the multiple regression analysis includes the five MBRs operated in the present study and two MBRs, previously reported by the authors [28], operating at Mg, and Na concentrations of 5 and 140 mg/L and two Ca concentrations of 36 and 250 mg/L, respectively. Modified or standardized regression coefficient, called the Beta coefficients were used to interpret the regression model. They have a common unit of measurement and so allow direct comparison of the impact of each independent variable [29]. They show the sensitivity of the dependent variable to each of the independent variables. A P value of 0.05, i.e., a 95% confidence level, was used as the cut-off for statistical significance. The fouling rate variations can be explained by following equation: Fouling rate = 0.517 + 0.237 Ca + 1.129 Na − 0.248 Mg (R2 = 0.96)
(4)
Ca and Na concentration had statistically positive impact on membrane fouling rate, i.e., an increase in dependent variable increased the fouling rate. The effect of Na on the membrane fouling rate was noticeably more pronounced than the effect of the other two parameters, i.e., Ca and Mg; Ca and Mg concentration affected the permeability with almost the same significance. Mg concentration showed a negative correlation with fouling rate while Ca showed a positive correlation with fouling rate. The Pearson’s product momentum correlation coefficients (rp ) were used for linear correlations between two parameters (Table 6). Table 6 Pearson’s (rp ) coefficient for linear correlations between fouling rate, Mg/Ca ratio, M/D ratio and the product of the two.
Fouling rate Mg/Ca M/D (Mg/Ca) × (M/D)
Fouling rate
Mg/Ca
M/D
(Mg/Ca) × (M/D)
1 0.339733 0.229331 0.77768
1 −0.02194 0.803966
1 0.329962
1
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The Pearson’s (rp ) coefficient is always between −1 and +1 where −1 means a perfect negative correlation and +1 a perfect positive correlation while 0 means absence of relationship. The data analysis showed that there was a relatively significant correlation between the fouling rate and the product of Mg/Ca and M/D ratios with no significant correlation found with the two independent parameters. This indicates that the two parameters must be simultaneously taken into consideration for the optimization of cation concentrations for the improvement of filtration performance in MBRs. 3.6. Impact of individual cations MBR3,0.2 , MBR2,1 and MBR1,5 are compared to study the impact of magnesium on membrane fouling since they have the same influent calcium and sodium concentrations. Magnesium concentration increased from 5 mg/L in MBR3,0.2 to 21 mg/L in MBR2,1 to 96 mg/L in MBR1,5 . Fig. 5(a)–(d) compares the three MBRs with respect to fouling rate, floc size, EPS, and SMP concentrations, respectively.
Fig. 6. Comparison of MBR characteristics with respect to sodium concentration.
Fig. 5. Comparison of MBR characteristics with respect to magnesium concentration.
As apparent from Fig. 5(a), fouling rate tends to decrease with increasing Mg concentration which is consistent with the results of statistical analysis discussed in Section 3.5. Fouling rate was almost constant at around 4 LMH/bar day for MBR3,0.2 and MBR2,1 but decreased to 1.9 LMH/bar day for MBR1,5 . The floc size and EPS concentration increased as the magnesium concentration increased which proves that magnesium was beneficial in reducing membrane fouling by increasing the floc size and hence lowering the cake filtration resistance. Fig. 5(d) presents that the SMP concentrations decreased by 11% upon increasing influent Mg concentration from 5 to 21 mg/L and by a further 26% from 21 mg/L to 96 mg/L. Therefore, increase of influent magnesium concentrations resulted in lower SMP concentrations in the reactors and decreased fouling rates. MBR1,5 (Na = 140 mg/L) and MBR3,5 (Na = 645 mg/L), both with the same calcium and magnesium, are also compared to exam-
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ine the impact of sodium concentration on membrane fouling. Fig. 6(a)–(d) compares the two MBRs with respect to fouling rate, floc size, EPS, and SMP concentrations, respectively. As apparent from Fig. 6(a), fouling rate increased by increasing sodium concentration from 140 mg/L in MBR1,5 to 645 mg/L in MBR3,5 which is consistent with the results of statistical analysis in Section 3.5. Fouling rate was around 6.6 times higher for the MBR3,5 at the Na concentration of 645 mg/L compared to MBR1,5 at Na concentration of 140 mg/L. Fig. 6(b) shows that the floc size was 41% lower at the higher sodium concentration which rationalize the higher suspended solids MFI for the MBR3,5 . EPS concentration decreased by about 50% while the SMP concentration almost doubled with the increase in influent sodium concentration from 140 to 645 mg/L (shown in Fig. 6(c) and (d)). This shows that the EPS bound to the flocs were released to the solution to form soluble EPS (SMP) as a result of the exchange of sodium with divalent cations in the floc structure which increased membrane fouling rate. The examination of the MFI data (Table 3) emphasizes the deterioration of both soluble and suspended solids filterability with the rise in sodium. Filtration performance can be compared with respect to calcium concentration in MBR1,0.2 and MBR2,1 where sodium and magnesium concentrations are close and calcium concentration is four times higher in MBR1,0.2 . Preliminary analysis of data showed that the fouling rate increased by increasing Ca concentration from 36 mg/L in MBR2,1 to 150 mg/L in MBR1,0.2 which is consistent with the results of statistical analysis (Section 3.5). However, cation analysis using MINTEQ program (MINTQA2, version 3.11) showed that calcium carbonate precipitated in the MBR1,0.2 and therefore the impact of calcium on bioflocculation cannot be assessed comparing the two MBRs. 4. Summary and conclusions The findings of this study illustrate the significant role of cations in membrane fouling expressed as Mg/Ca and M/D ratio. (1) Within the concentration ranges used in this research, Ca concentration of 36–150 mg/L, Mg of 5–96 mg/L, and Na of 140–645 mg/L, the optimum condition found was M/D of 1:1 and Mg/Ca of 5:1 in MBR1,5 , with Ca, Mg, and Na concentrations of 36, 96, and 140 mg/L, respectively. At the lower M/D ratio of 1:1, higher Mg/Ca was beneficial in reducing membrane fouling while at the higher M/D ratio of 3:1, a lower Mg/Ca showed better performance than the higher Mg/Ca ratio. (2) High sodium concentration of 645 mg/L at M/D of 3:1 resulted in a decrease in the floc size and increased the fouling rate. (3) The optimum ratios of M/D and Mg/Ca of 2:1 and 1:1 observed in the literature for sludge settling and dewatering did not result in the lowest membrane fouling rates or membrane filtrations resistance. The MBR (MBR1,5 ) at M/D and Mg/Ca ratios of 1:1 and 1:5, respectively not only exhibited the least fouling rate but also the largest floc size, highest EPS, the lowest SMP concentration and RH. The higher RH of SMP for MBR1,0.2 and MBR3,5 contributed to higher soluble MFI. (4) At the low M/D ratio of 1:1 and a total divalent cation concentration of about 9.5 mequiv., higher Mg concentration of 96 mg/L was beneficial in reducing SMP compared to the MBR at higher calcium concentration of 150 mg/L (Mg/Ca of 1:5) while at an M/D ratio of 3:1, higher magnesium increased both carbohydrate and protein SMPs, thus increasing both soluble and suspended solids MFI. (5) At constant influent calcium and sodium concentrations of 36 and 170 mg/L, respectively, increased magnesium concentration in the range of 5–96 mg/L simultaneously reduced fouling rate, increased floc size and EPS, and decreased SMPs.
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(6) At a constant influent calcium and magnesium concentrations of 36 and 96 mg/L, increasing sodium from 140 to 645 mg/L increased fouling rate by around six times, decreased both the floc size and EPS by 41% and 50%, respectively, while increasing SMP by 100%. (7) Statistical analysis of the impact of each individual cation (Na, Mg, and Ca) on membrane fouling rates revealed that Ca and Na increased membrane fouling rate while Mg concentration had a negative correlation with fouling rate. The Pearson’s product momentum correlation coefficient (rp ) showed that Mg/Ca and M/D ratios must be simultaneously taken into consideration for optimization of cation concentrations for improvement of filtration performance in MBRs. Acknowledgment This work was fully funded by Natural Sciences and Engineering Research Council of Canada (NSERC). References [1] P. Le Clech, V. Chen, T.A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284 (2006) 17–53. [2] I.S. Chang, C.H. Lee, Membrane filtration characteristics in membrane-coupled activated sludge system—the effect of physiochemical states of activated sludge on membrane fouling, Desalination 120 (1998) 221–233. [3] B.Q. Liao, D.M. Bagley, H.E. Kraemer, G.G. Leppard, S.N. Liss, A review of biofouling and its control in membrane separation bioreactors, Water Environ. Res. 76 (2004) 425–436. [4] Y. Liu, H.H.P. Fang, Influence of intracellular polymeric substances (EPS) on flocculation, settling, and dewatering of activated sludge, Crit. Rev. Environ. Sci. Technol. 33 (2003) 237–273. [5] V. Urbain, J.C. Block, J. Manem, Bioflocculation in activated sludge: an analytical approach, Water Res. 27 (1993) 829–838. [6] C. Wisniewski, A. Grasmick, Floc size distribution in a membrane bioreactor and consequences for membrane fouling, Colloid Surf. A: Physiochem. Eng. Asp. 138 (1998) 403–411. [7] D.C. Sobeck, M.J. Higgins, Examination of three theories for mechanisms of cation-induced bioflocculation, Water Res. 36 (2002) 527–538. [8] F.D. Sanin, P.A. Vesilind, Bioflocculation of activated sludge: the role of calcium ion and extracellular polymers, Environ. Technol. 21 (2000) 1405–1412. [9] B.M. Wilen, B. Jin, P. Lant, The influence of key chemical constituents in activated sludge on surface and flocculating properties, Water Res. 37 (2003) 2127–2139. [10] T. Itonaga, K. Kimura, Y. Watanabe, Influence of suspension viscosity and colloidal particles on permeability of membrane used in membrane bioreactor (MBR), Water Sci. Technol. 50 (2004) 301–309. [11] N. Cicek, M.T. Suidan, P. Ginestet, J.-M. Audic, Impact of soluble organic compounds on permeate flux in and aerobic membrane bioreactor, Environ. Technol. 24 (2003) 249–256. [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] F. Saravia, C. Zwiener, F.H. Frimmel, Interactions between membrane surface, dissolved organic substances and ions in submerged membrane filtration, Desalination 192 (2006) 280–287. [14] M.B. Müller, D. Schmitt, F.H. Frimmel, Fractionation of natural organic matter by size exclusion chromatography—properties and stability of fractions, Environ. Sci. Technol. 34 (2000) 4867–4872. [15] C. Jucker, M.M. Clark, Adsorption of aquatic humic substances on hydrophobic ultrafiltration membranes, J. Membr. Sci. 97 (1994) 37–52. [16] R.S. Swift, Molecular weight, size, shape, and charge characteristics of humic substances: some basic considerations, in Humic Substances II—in search of structure, Wiley VCH, 1989, pp. 449–465. [17] S.N. Murthy, J.T. Novak, R.D. De Haas, Monitoring cations to predict and improve activated sludge settling and dewatering properties of industrial wastewaters, Water Sci. Technol. 38 (1998) 119–126. [18] M.J. Higgins, J.T. Novak, Dewatering and settling of activated sludge: the case for using cation analysis, Water Environ. Res. 69 (1997) 225–232. [19] M.J. Higgins, L.A. Tom, D.C. Sobeck, Case study I: application of the divalent cation bridging theory to improve biofloc properties and industrial activated sludge system performance-direct addition of divalent cations, Water Environ. Res. 76 (2004) 344–352. [20] J.H. Bruus, P.H. Nielsen, K. Keiding, On the stability of activated sludge flocs with implications to dewatering, Water Res. 26 (1992) 1597–1604. [21] B. Jin, B.M. Wilen, P. Lant, Impacts of morphological, physical and chemical properties of sludge flocs on dewaterability of activated sludge, Chem. Eng. J. 98 (2004) 115–126.
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