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N ratio on process performance and membrane fouling in a submerged bioreactor
Desalination 285 (2012) 232–238
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The effect of COD/N ratio on process performance and membrane fouling in a submerged bioreactor Suping Feng a,⁎, Nannan Zhang a, Hanchao Liu a, Xiaolin Du a, Yongli Liu a, Hai Lin b a b
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan, 250100, China College of University of Electronic Science and Technology of China
a r t i c l e
i n f o
Article history: Received 10 July 2011 Received in revised form 3 September 2011 Accepted 4 October 2011 Available online 8 November 2011 Keywords: MBR COD/N ratio Membrane fouling EPS SMP NH4+
a b s t r a c t Influent chemical oxygen demand/nitrogen (COD/N) ratio is used to control fouling in membrane bioreactor (MBR) systems. However, COD/N also affects the physicochemical and biological properties of MBR biomass. The current study examined the relationship between COD/N ratio in feed wastewater and extracellular polymeric substances (EPS) production in MBRs. Two identical submerged MBRs with different COD/N ratios of 10:1 and 5:1 were operated in parallel. The cation concentration and floc-size of the sludge were measured. The composition and characteristics of bound EPS and soluble microbial products (SMP) under each COD/N ratio were also examined. Batch tests were conducted in 1000 mL bottles to study the process of the release of foulants from the sludge when 1 g of (NH4+-N)/L was added. Results showed that the influent COD/N ratio could change the physicochemical properties of EPS and SMP. Moreover, excessive NH4+ in the supernatant could facilitate the role of NH4+ as a monovalent cation, the replacement of the polyvalent cation in bound EPS, and even the extraction of EPS components from the surface of the sludge to form new SMP. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Membrane bioreactors (MBRs), where membrane and activated sludge processes are combined, have been increasingly used to treat various types of wastewaters [1-3]. However, membrane fouling is a major obstacle to the wide application of MBRs. Recent studies [4-6] have considered bound extracellular polymeric substances (EPS) and soluble microbial products (SMP) as two important factors that affect the membrane fouling potential of mixed liquor. Bound EPS is a complex mixture of macromolecular polyelectrolytes (e.g., carbohydrates, proteins, nucleic acids, and humic compounds) that determines most of the characteristics of sludge and has been shown to be directly related to the major fraction of fouling biocake [7,8]. SMP, believed to be identical to soluble EPS released into a solution from substrate metabolism and biomass decay [9], has been extensively studied in recent years. SMP can be absorbed onto the membrane surface and is capable of blocking membrane pores and forming a gel-like structure on the membrane surface, thereby facilitating hydraulic resistance to the permeate flow [5,6,10]. Thus, controlling the contents and properties of EPS and SMP is the most important issue in reducing membrane fouling [11,12]. For a given MBR process, membrane fouling control strategies can be classified into two groups, namely, operation conditions and feedwater characteristics [13]. Operation conditions include aeration [14],
⁎ Corresponding author. Tel./fax: + 86 531 88362819. E-mail address: [email protected] (S. Feng). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.10.008
flux [15], temperature, membrane relaxation, organic loading [16], and solid residence time (SRT) [17]. For example, membrane fouling is more severe under a higher monovalent to divalent (M/D) cation ratio [18]. All control strategies are based on changes in the properties of EPS or SMP. Despite the interest in EPS and SMP, most studies have focused on optimizing operating conditions. With regard to feedwater characteristics, only a few reports on the effect of carbon source and cations have been published. Chemical oxygen demand/nitrogen (COD/N) ratio, one of the most important parameters of microorganism growth, has been continually emphasized in numerous studies [19]. The COD/N ratio of raw wastewater always plays a significant role in the removal of nutrients and in the processes of nitrification and denitrification [20-22], because COD/N ratio not only affects functional microorganism populations, but also changes the structure of microbial communities. Several investigations have revealed that different removal rates of COD and NH4+-N were obtained under different COD/N ratios, regardless of whether traditional technologies or MBR was used [23,24]. Although the effect of COD/N ratio on membrane fouling has not been reported, several studies have considered the issue. Rosenberger et al. [25] reported that different fouling rates are observed only in the denitrification zone during the operation of two parallel MBRs. Unfortunately, the effect of COD/N ratio on membrane fouling was not emphasized. In addition, the heavy metal biosorption capability of sludge can be improved by determining a better COD/N ratio [26]. The adsorption process is accompanied by the release of different amounts of Ca, Mg, and carbohydrates that are attached to the surface of the cell as components of EPS. All these information indicate that the influent COD/N ratio can affect the characteristics of sludge.
S. Feng et al. / Desalination 285 (2012) 232–238
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Therefore, COD/N ratio can result in the identification of membrane fouling in MBRs. In the current study, the effect of the COD/N ratio on membrane fouling in a submerged MBR was investigated. Synthetic wastewater with different ammonia concentrations was used. Trans-membrane pressure (TMP), mixed-liquor characteristics, and SMP and EPS contents were monitored to provide a comprehensive understanding of their relationships with the COD/N ratio of feedwater. To determine the relationship between the COD/N ratio in feed wastewater and EPS production in MBRs, measurements of the cation concentration and floc-size measurements of the sludge, and the apparent molecular weight (MW) of SMP were also conducted.
fed into the two parallel MBRs with synthetic wastewater for more than a month. During each COD/N ratio experiment, only backwashing was used to recover the foulants. Hydraulic retention time (HRT) was maintained for 3 h. The reactor temperature was maintained at approximately 25 ± 2 °C using an electric heater. At the bottom of the reactor, air was continuously supplied at a flow rate of 200 L/h. Chemical cleaning of the membrane was conducted by soaking it in 2000 ppm of sodium hypochlorite solution for 2 h. The details are shown in Table 1.
2. Materials and methods
To study the effect of high concentrations of NH4+-N on the release of foulants from the sludge samples, a mixed-liquor supernatant was obtained from sludge samples after adding 1 g/L NH4+-N using a centrifuge (TG-II, Changsha China) operated at 3500 rpm for 15 min to remove the solids. The colloidal and soluble fractions were obtained after the supernatant was paper filtered (quantitative filter paper, 30–50 μm) and then membrane filtered (PTFE syringe filter, nominal pore size of 0.45 μm). The COD, carbohydrates, and proteins were separately measured to study the changes in the colloidal and soluble fractions released from the sludge sample over time.
2.1. Reactor setup Two identical submerged laboratory-scale MBRs (Fig. 1), each with a working volume of 13 L, were operated in parallel and fed with synthetic wastewater for a period of 50 days. The membrane modules (Bishuiyuan, China) comprised polypropylene hollow-fiber membranes with a pore size of 0.4 μm and an area of 0.3 m 2. The bioreactors were fed with raw wastewater through a constant-level tank, and the effluents were extracted directly using a suction pump based on a time sequence of 8 min on and 2 min off for each cycle. A manometer was fixed between the membrane module and the suction pump to monitor the TMP. 2.2. Simulated raw wastewater Synthetic wastewater, simulating municipal sewage, was used to ensure a stable feeding rate throughout the experiment. The synthetic influent contained glucose, NH4Cl, and NaH2PO4 as primary nutrients, whereas FeSO4·7H2O, CuSO4·5H2O, MnCl2·4H2O, MgCl2·6H2O, CaCl2·2H2O, H3BO3, and CoCl2. 6H2O comprised the trace nutrient solution. The influent concentrations were approximately 400 mg COD/L, 40– 80 mg TN/L for different COD/N ratios, and 6 mg phosphorus (PO43 −–P)/L. 2.3. Operating conditions Before the experiment was conducted, the seed sludge collected from a domestic sewage wastewater treatment plant in Jinan was
2.4. Fractionation
2.5. Chemical and physical analyses COD, MLSS, and mixed-liquor volatile solid (MLVSS) were determined according to standard methods. NO3−-N and NO2−-N were determined using an ion chromatograph (DX120). EPS and SMP were measured as total organic carbons (TOC), and their components were measured as carbohydrates and proteins using a cation exchange resin (Purolite C100E, Na + form) extraction method based on the procedure of Trussell et al. [27]. The carbohydrate and protein concentrations of the supernatant were measured using the anthrone [28] and Bradford methods [29], respectively. Bovine serum albumin and glucose were used as the respective protein and carbohydrate standards. Calcium, magnesium, aluminum, and iron in the effluent, as well as SMP and sludge, were measured using inductively coupled plasma (ICP) spectroscopy (IRIS Intrepid IIXSP, Thermo Electron Co. USA). To determine the multivalent cations, the sludge samples from MBRs under different COD/N ratios were digested according to the method used in an earlier study [30].
Temp. controller
Level controller
Effluent
Influent Pressure gauge
10L Balance Tank
Air Flowmeter
170L
Air Feed Tank
Membrane tank Fig. 1. Schematic diagram of the MBR experimental setup.
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Table 1 Operating conditions and performance of MBRs. Reactor
MBR1
MBR2
COD/N c Influent COD (mg/L) c Influent NH4 + -N (mg/L) SRT (day) HRT (hour) c Flux (L/m2h) c MLSS (g/L) c MLVSS/MLSS Average particle size (μm) c COD removal (%) c NH4 + -N removal (%) c TN removal (%) Nitrification rate a (mg N/g MLVSS·d) Denitrification rate b (mg N/g MLVSS·d)
10 398 ± 11 40 ± 1.5 20 3 14.3 ± 0.5 9.8 ± 0.3 0.90 ± 0.02 108.4 94.3 ± 5.5 74.8 ± 2.1 31.1 ± 11 508.6 ± 12.8 211.5 ± 15.1
5 400 ± 14 41 ± 2.1 20 3 14.3 ± 0.7 9.6 ± 0.2 0.91 ± 0.01 112.5 94.5 ± 7.2 54.5 ± 5.3 19.5 ± 9 618.5 ± 21.4 264.7 ± 18.9
a
þ Q in ðNHþ 4 N influent NH 4 Neffluent Þ MLVSS Q in ðTN influent TNeffluent Þ rate ¼ where MLVSS
Nitrification rate ¼
Denitrification Qin is the influent flux (L/d). Sample mean ± standard deviation, number of measurements: n = 25 (COD, NH4+-N, NO2-, NO3-, flux); n = 10 (MLSS, MLVSS). b c
The particle size of the sludge was determined using a BT-9300H laser particle size analyzer (Dandong Baite, China) in the measurement range of 0.1–340 μm. The MW distribution of SMP under each COD/N ratio was determined through gel-permeation chromatography (GPC) (PL-GPC 220) equipped with a refractive index (RI) detector and a column (PLaquagel-OH MIXED-H). Milli-Q water was used for the mobile phase. 2.6. Membrane resistance analyses The total filtration resistance values were primarily dependent on the suction pressure and permeate flux. At the end of each COD/N ratio experiment, the composition of resistance through the membrane, which reached the design maximum TMP (30 kPa), was analyzed. Eq. (1) was used to calculate the different values of filtration resistance. The filtration resistance was measured step by step, as shown in Fig. 2. The flux and TMP data on new membranes for the filtration of mixed liquor were recorded to measure intrinsic membrane resistance (Rm) under different COD/N ratios. The other filtration resistance values were calculated based on the data on suction pressure using Eq. (2). R¼
TMP μJ
ð1Þ
Rt ¼ Rm þ Rp þ Rc þ Rir
Fig. 2. Steps in measuring each filtration resistance.
ð2Þ
where R is the filtration resistance (m− 1), TMP denotes the transmembrane pressure (Pa), J is the permeated flux (L/m 2 h) converted to m3/(m 2 s), μ is the viscosity of the permeated water (Pa s), Rm is the resistance of the membrane itself (m− 1), Rc is the resistance of the cake layer (m− 1), Rp is the pore blocking, and Rir is the irreversible fouling resistance (m− 1). 2.7. Release of EPS at high NH4+-N Concentration To compare the effect of the release of EPS when the sludge was mixed with and without high concentrations of NH4+-N, the EPS release test was performed as follows: 1000 mL sludge from MBR1 was transferred to a beaker and placed in ice water. The suspension was stirred using a magnetic stirrer to mix the sludge fully. The suspension was cooled to below 4 °C to minimize biological activity. Subsequently, 1 g (NH4+-N)/L was added into the beaker, and samples of the suspension were collected after 0, 10, 20, 40, 60, 90, 150, and 240 min. The foulants released from the sludge were harvested, and the fractionation method presented earlier was applied to determine the colloidal and soluble COD, carbohydrates, and proteins. All operations were conducted in duplicate. 3. Results and discussion 3.1. Overall MBR performance under different COD/N ratios The performance of the MBR was evaluated based on COD, NH4+N, and TN removal (Table 1 and Fig. 3). All experiments were conducted under the same organic loading. However, different organic removal rates were observed and were attributed to different NH4+N concentrations in raw wastewater. During the operation, the average effluent COD concentrations were 23 and 22 mg/L with average efficiency levels of 94.3% and 94.5%, respectively, under COD/N ratios of 10 and 5. This result is in agreement with that of Ng et al. [31]. All MBRs performed equally well in terms of removal efficiency regardless of the operating conditions or time. In terms of NH4+-N removal, the nitrification and denitrification rates increased as the COD/N ratio decreased. When the COD/N ratio reached 5, the NH4+-N and NO3--N concentrations in the effluent significantly increased, which is attributed to the high influent NH4+-N and limited nitrification and denitrification capabilities. Therefore, the COD/N ratio is believed to be related to the treatment of wastewater in MBRs because this affected the removal not only of nitrogen, but also of COD. This result did not concur significantly with that of an earlier study, in which the COD of influent wastewater was 2000 mg/L [23]. The rate of increase in TMP is an important factor that affects membrane filterability in a submerged MBR system because it is directly related to the extent of membrane fouling. Fig. 3 also shows that the different values of TMP increase over time during the measurement of each COD/N ratio. The fouling profiles of the two MBRs in the present experiment exhibited a three-stage process. This process corresponded closely to the three-stage mechanism map for membrane fouling in MBRs [32]. In the proposed map, Stage 1 consisted of an initial short-term rapid rise in TMP attributable to the conditioning fouling of the membrane. Stage 2 reflected a weak long-term rise in TMP. Stage 3 showed the occurrence of a sharp increase in TMP, also known as the TMP jump. As evident in Fig. 3, both MBRs underwent all three membrane fouling stages. However, the phenomenon in MBR1 was clearer than that in MBR2. The pressure in MBR2 increased in the first few days of reactor operation and subsequently reached Stage 2 when the pressure increased slowly over the next 17 days. On day 17, the slope of the TMP increased at a significantly faster rate each day until the membranes had to be cleaned on day 25. These findings could be seen as the events of Stage 3. After membrane cleaning, a similar trend was observed in MBR2 for the
S. Feng et al. / Desalination 285 (2012) 232–238
Effluent COD (mg/l)
+
50
b
COD
NH 4-N
45
40
40
35
35
30
30
25
25
20
20
15
15
10
10
5
5 0
0
+
a
45
Effluent NH4 -N (mg/l)
50
235
30
TMP (Kpa)
MBR2
MBR1
25 20 15 10
chemical washing
5 0
5
10
15
20
25
30
35
40
back washing
45
50 0
5
10
15
20
Days
chemical washing
25
30
35
40
45
50
Days
Fig. 3. Overall MBR performance and normalized membrane suction pressure profile under different COD/N ratios; a: MBR1 with a COD/N ratio of 10; b: MBR2 with a COD/N ratio of 5.
second fouling cycle, except that the lasting time of Stage 2 decreased to 14 days or approximately 3 days before severe fouling occurred. MBR1, with a COD/N ratio of 10:1, successfully minimized the fouling rate in the current study. The TMP increased significantly more slowly in MBR1, and the duration for Stage 2 of over 30 days is nearly two times longer than that for MBR2. Therefore, the fouling rate can be controlled by regulating the COD/N ratios in MBR systems during daily operations. 3.2. SMP and EPS under different COD/N ratios The components of SMP and EPS in the mixed liquor were monitored separately because SMP and EPS have been widely accepted as major foulants in MBRs. In addition to TOC, which was used as the total SMP and EPS value, the contents of proteins and carbohydrates were also examined to obtain more detailed insights into the composition of SMP and EPS under each COD/N ratio. Table 2 indicates that the TOC values of SMP were similar for both COD/N ratios, but the concentrations of proteins and carbohydrates were different. The amount of carbohydrates in MBR2 with a COD/N ratio of 5 was 10.05 mg/L, which was more than two times higher than that in MBR1, whereas the protein content was slightly higher in MBR1 compared with that in MBR2. In particular, the carbohydrate/protein
ratios of SMP under various COD/N ratios were generally different. In recent years, a growing number of studies [17] have found that the hydrophilic neutrals (e.g., carbohydrates) and carbohydrate/protein ratios are most likely the primary factors responsible for the high fouling potentials of SMP. Table 3 shows that the EPS concentrations were 176.86 and 104.82 mg/g.VSS, corresponding to COD/N ratios of 10 and 5, respectively, which is an interesting observation. Table 3 shows that the TOC value of EPS and the concentrations of each composition of EPS in MBR1 were all higher than those in MBR2. However, these factors did not result in a higher fouling rate compared with the case of MBR2. This finding contradicts the results of other studies that indicate that high concentrations of EPS always result in a high membrane fouling rate. Miqueleto et al. [24] reported that a high COD/N ratio promotes the production of more EPS. EPS production is always considered to be governed by factors such as organic loading, carbon source, and limited nutrients. However, this result is not applicable for SMP in the present study. SMPs are always considered as the soluble EPS, the concentration of which is not only related to the total amount of EPS, but also depends on the properties of EPS. Under a high concentration of NH4+-N in feedwater with a COD/N ratio of 5, the NH4+-N concentration in the effluent was consistently higher than 40 mg/L. However, this value was only approximately 10 mg/L under a COD/ N of 10. A high content of NH4+-N in the supernatant could enhance the action of NH4+-N as monovalent cations and facilitate the replacement of the polyvalent cations in EPS. Moreover, the high NH4+-N
Table 2 Characteristics of mixed liquor supernatant under different COD/N ratios.a. COD/N TOC ratio (mg/L)
Carbohydrate Protein (mg/L) (mg/L)
Carbohydrate / SUV254 Protein (L/mg·TOC·m)
10
4.66 ± 0.42
2.28 ± 0.15
2.86 ± 0.16
6.48 ± 0.57
1.73 ± 0.41
5
10.14 ±1.21 11.57 ±2.65
10.05 ± 2.01
2.06 ± 1.18 1.55 ± 0.71
a Sample mean ± standard deviation, number of measurements: n = 15 (TOC, carbohydrate, protein).
Table 3 EPS concentration and composition under COD/N ratios.a COD/N ratio
TOC (mg/g·MLVSS)
Carbohydrate (mg/g·MLVSS)
Protein (mg/g·MLVSS)
Carbohydrate/ protein
10 5
176.86 ± 9.75 104.82 ± 7.30
7.23 ± 2.12 3.08 ± 1.03
50.09 ± 5.62 40.83 ± 4.93
0.14 ± 0.05 0.08 ± 0.02
a Sample mean ± standard deviation, number of measurements: n = 15 (TOC, carbohydrate, protein).
S. Feng et al. / Desalination 285 (2012) 232–238
content can help in the extraction of EPS components from the surface of sludge to form new SMPs. This finding may be the primary reason why a lower concentration of EPS in the MBR2, together with a higher membrane fouling rate, was observed.
3.3. Resistances through the membrane under each COD/N ratio To identify the primary contributor to membrane fouling under each COD/N ratio, hydraulic resistance analyses of the membranes were conducted at the end of each COD/N ratio experiment. Fig. 4 shows the values of filtration resistance, including intrinsic membrane resistance (Rm), cake resistance (Rc), pore blocking resistance (Rp), and irremovable fouling resistance (Rir). The biocake resistance accounted for 63% for the MBR1 and 57% for the MBR2. These results revealed that the value of Rc resulting from the formation of the biocake on the membrane surface was primarily responsible for the increase in TMP. Previous studies show a similar result [33,34]. Rm, Rp, and Rir appeared to be minor factors of the total resistance (Rt) compared with Rc. Fig. 4 shows that the ratio of biocake resistance of MBR1 was slightly greater than that of MBR2 for the same total resistance. However, the decrease of Rc of MBR2 was compensated with an increase in Rp and Rir, which is the reason why the Rp and Rir value of MBR2 is greater than that of MBR1. Although Rc constitutes a large part of Rt, it can be easily eliminated by the implementation of physical cleaning (e.g., backwashing). Moreover, Rc could form a secondary dynamic membrane on the surface of the membrane, which could prevent direct contact with the soluble or colloidal particles, as well as microorganisms, in the mixed liquor. Without Rc, membrane fouling would be more severe. Previous studies demonstrated that foulants, especially those comparable with or smaller than the membrane pore (e.g., soluble or colloidal particles and microorganisms), are the primary contributors to Rp and Rir, explaining the higher biocake resistance but lower fouling rate in MBR1 compared with that in MBR2. In recent years, studies have begun to focus on the possibility that the fouling attributable to Rp and Rir is the primary reason for the eventual loss of permeability in MBRs and that this fouling is directly linked to the pattern of TMP increase. In the present experiment, Rp of the two reactors were similar, but the Rir of MBR2 was nearly two times higher than that of MBR1. Only physical cleaning was conducted during each COD/N experiment. Portions of Rc and Rf, which can be removed by physical cleaning, did not grow continuously. However, the different backwashing levels had minimal effect on the Rir value. Therefore, the Rir value showed better correlation with the fouling profiles.
3.4. Effect of high contents of NH4+-N Fig. 5 shows that significant amounts of COD were released from the sludge flocs to the supernatant over time. However, after 240 min, more than 21.4% of colloidal COD was released from the sludge with NH4+-N compared with that released from the sludge without NH4+-N. For the soluble fraction, this difference was more pronounced, with a 34.5% higher release for high concentrations of NH4+-N amended sludge. The soluble COD was more sensitive than the colloidal COD under a high NH4+-N concentration in the MBRs. Table 4 presents the amount of extracted EPS and its compositions (i.e., proteins and carbohydrates). In both sludges, proteins, as well as carbohydrates, were released. Despite the similarity of the amounts of released soluble COD with respect to proteins and carbohydrates, a clear difference was observed for the colloidal COD. After 240 min, 84.8% and 88% colloidal carbohydrates and proteins were released from the NH4+-N amended sludge, whereas 47.5% and 47% were released from the sludge without NH4+-N, respectively. Fig. 6 shows the different concentrations of polyvalent cations (such as calcium, magnesium, aluminum, and iron) of the sludge from MBR1 and MBR2. The amounts of all four cations in the sludge from MBR1 were higher than those from MBR2. However, the average NH4+-N content in the effluent from MBR2 was 43.6 mg/L, which is four times higher than the content of 10.1 mg/L in the effluent from MBR1. Previous studies have demonstrated that polyvalent cations, especially divalent cations, in the sludge form bridges between the
300
Soluble COD
a
+
NH 4 -N
270
COD (mg/Ll)
236
+
no NH 4 -N
240 210 180 150 120 0
25
50
75
100 125 150 175 200 225 250
Time (min)
b
320
Colloidal COD +
Rc
63%
Rp R ir
57%
Resistance (m-1)
1.20E+009
Rm
1.00E+009 8.00E+008 6.00E+008
+
no NH 4 -N
280
COD (mg/l)
1.40E+009
NH 4 -N
300
260 240 220
23%
21%
200
4.00E+008 11% 2.00E+008 0.00E+000
5%
8%
11%
180 0
MBR1
MBR2
Fig. 4. Distribution of each filtration resistance value at the end of the operation.
25
50
75
100 125 150 175 200 225 250
Time (min) Fig. 5. Release of COD from sludge samples mixed with and without NH4 + -N.
S. Feng et al. / Desalination 285 (2012) 232–238
237
Table 4 Release of COD, protein, and carbohydrate from sludge with and without NH4 + -N amended. EPS extracted
From sludge without NH4 + -N (mg/L)
Extraction time (min) Colloidal COD Protein Carbohydrate Soluble COD Protein Carbohydrate
10 205 16.6 10.1 140 3.9 9.8
20 208 18.8 11.3 145 4.2 10.4
40 211 20.1 12.1 152 4.4 11.1
60 216 21.4 12.9 156 4.7 11.7
From sludge with NH4 + -N (mg/L) 90 208 22.6 13.6 168 5.1 12.5
150 221 23.2 14.2 178 5.3 13.2
polymers, which significantly affects sludge flocculation, settleability, and floc strength [35]. Moreover, the divalent cations are replaced by monovalent cations under a high M/D cation ratio. Consequently, the bridges are destroyed, and the foulants are released from the sludge to a certain extent [18]. Therefore, the high concentrations of NH4+-N in the reactor can be concluded to have acted as a monovalent cation, which resulted in the release of polyvalent cations and the formation of new SMPs from the sludge. Fig. 6 also shows that the release of Ca2 + and Fe3 + was more sensitive to the effect of high concentrations of NH4+-N than that of Mg2 + and Al3 +; the release values of Ca2 + and Fe3 + were 22.6% and 47.4%, respectively; the corresponding values for Mg 2 + and Al3 + were 5.2% and 11.4%, respectively. The polyvalent cations released in the supernatant and effluent were also measured. However, values of both Fe3 + and Al3 + were below the detection level, and very slight differences were observed for Mg2 + and Ca2 + because of high background. 3.5. Apparent MW distributions of SMP in supernatants and effluents at each COD/N ratio GPC was used to determine the effect of SMP under different COD/N ratios. Fig. 7 shows the apparent MW distributions of SMP at different COD/N ratios. All samples obtained three peaks, and Peak 3 (low molecular compound) was the largest fraction, which is consistent with other studies [17]. Fig. 7 also shows that the apparent MW distributions of SMP were slightly different for each COD/N ratio. All peak locations of the MW distribution for MBR2 were shifted to the right compared with MBR1, indicating that the large molecular compounds (e.g., protein, carbohydrates) could dissociate smaller molecular matters through interaction between the polyvalent cation and NH4+ in the reactor. This phenomenon seems beneficial in controlling membrane fouling as smaller molecules could easily pass through the membrane, even though previous studies have demonstrated that membrane blocking frequently occurred at a fraction of MW larger than 10 kDa. However, it should be noted that the excessive NH4+ in the reactor converted the larger molecules to smaller ones. The total amount of membrane foulants was not affected. The diagram shows that although the
240 217 24.4 14.9 190 5. 6 13.9
10 210 20.5 12.5 190 4.1 10.2
20 230 23.2 14.2 212 4.8 11.7
40 240 28.8 17.3 228 5.2 13.2
60 255 33.4 19.9 238 5.5 13.7
In the current study, particular attention has been paid to identify the role of influent COD/N ratio in controlling membrane fouling. The following specific conclusions are drawn: (1) The removal rate of nutrients could be affected by the COD/N ratio. Under the same organic loading, the removal rates of COD were similar and reached values greater than 90%. However, the removal rate of NH4+-N rapidly decreased from 74.8% to 54.5% when the COD/N ratio decreased from 10 to 5, respectively. This finding was primarily attributable to a high NH4+-N concentration in the feedwater and low nitrification growth rate in the reactors. (2) Higher COD/N ratio promoted the production of more EPS and higher carbohydrate proportion in EPS. SMP concentration was not positively correlated with EPS under a high content of NH4+ in the supernatant, which resulted in excessive NH4+, replacing polyvalent cation and extracting compositions from the EPS. Thus, high SMP and low EPS were observed in the system under a COD/N ratio of 5. At this particular instance, the membrane fouling rate was not only determined by the total amount of SMP, but also showed dependence on the properties (e.g., compositions, MW distribution, and hydrophobicity) of SMP. (3) GPC analyses indicated that the influent COD/N ratio changed the apparent MW distributions of SMP. However, the membrane fouling potential was generally related to the total amount and properties of SMP and seems to be slightly affected by the MW distribution of SMP.
Signal response (RI)
mg/g (MLSS)
0.5
0.0
peak 3 (low molecular)
120
1.0
240 276 38.6 23.1 290 6.3 15.6
4. Conclusions
MBR1 MBR2
1.5
150 270 37.8 22.8 268 5.8 14.6
fractions near Peak 1 (macromolecular compounds) for MBR2 were slightly lower than those for MBR1, others were considerably higher than those for MBR1. Therefore, the high contents of SMP played a significant role in membrane fouling. Similar results have been reported in other studies [36,37], where the SMP has a higher fouling potential compared with EPS under certain operating conditions.
140 2.0
90 266 36.2 21.7 250 5.7 14.3
SMP of MBR1 SMP of MBR2
100 80
peak 2 (mid molecular)
60 40
peak 1 (macromolecular)
20 0 -20 475 500 525 550 575 600 625 650 675 700 725 750 775 800
Ca
Mg
Al
Fe
Fig. 6. Concentration of polyvalent cations in sludges from two reactors.
Elution time (s) Fig. 7. Apparent MW distribution of SMP under each COD/N ratio.
238
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Desalination journal homepage: www.elsevier.com/locate/desal
The effect of COD/N ratio on process performance and membrane fouling in a submerged bioreactor Suping Feng a,⁎, Nannan Zhang a, Hanchao Liu a, Xiaolin Du a, Yongli Liu a, Hai Lin b a b
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan, 250100, China College of University of Electronic Science and Technology of China
a r t i c l e
i n f o
Article history: Received 10 July 2011 Received in revised form 3 September 2011 Accepted 4 October 2011 Available online 8 November 2011 Keywords: MBR COD/N ratio Membrane fouling EPS SMP NH4+
a b s t r a c t Influent chemical oxygen demand/nitrogen (COD/N) ratio is used to control fouling in membrane bioreactor (MBR) systems. However, COD/N also affects the physicochemical and biological properties of MBR biomass. The current study examined the relationship between COD/N ratio in feed wastewater and extracellular polymeric substances (EPS) production in MBRs. Two identical submerged MBRs with different COD/N ratios of 10:1 and 5:1 were operated in parallel. The cation concentration and floc-size of the sludge were measured. The composition and characteristics of bound EPS and soluble microbial products (SMP) under each COD/N ratio were also examined. Batch tests were conducted in 1000 mL bottles to study the process of the release of foulants from the sludge when 1 g of (NH4+-N)/L was added. Results showed that the influent COD/N ratio could change the physicochemical properties of EPS and SMP. Moreover, excessive NH4+ in the supernatant could facilitate the role of NH4+ as a monovalent cation, the replacement of the polyvalent cation in bound EPS, and even the extraction of EPS components from the surface of the sludge to form new SMP. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Membrane bioreactors (MBRs), where membrane and activated sludge processes are combined, have been increasingly used to treat various types of wastewaters [1-3]. However, membrane fouling is a major obstacle to the wide application of MBRs. Recent studies [4-6] have considered bound extracellular polymeric substances (EPS) and soluble microbial products (SMP) as two important factors that affect the membrane fouling potential of mixed liquor. Bound EPS is a complex mixture of macromolecular polyelectrolytes (e.g., carbohydrates, proteins, nucleic acids, and humic compounds) that determines most of the characteristics of sludge and has been shown to be directly related to the major fraction of fouling biocake [7,8]. SMP, believed to be identical to soluble EPS released into a solution from substrate metabolism and biomass decay [9], has been extensively studied in recent years. SMP can be absorbed onto the membrane surface and is capable of blocking membrane pores and forming a gel-like structure on the membrane surface, thereby facilitating hydraulic resistance to the permeate flow [5,6,10]. Thus, controlling the contents and properties of EPS and SMP is the most important issue in reducing membrane fouling [11,12]. For a given MBR process, membrane fouling control strategies can be classified into two groups, namely, operation conditions and feedwater characteristics [13]. Operation conditions include aeration [14],
⁎ Corresponding author. Tel./fax: + 86 531 88362819. E-mail address: [email protected] (S. Feng). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.10.008
flux [15], temperature, membrane relaxation, organic loading [16], and solid residence time (SRT) [17]. For example, membrane fouling is more severe under a higher monovalent to divalent (M/D) cation ratio [18]. All control strategies are based on changes in the properties of EPS or SMP. Despite the interest in EPS and SMP, most studies have focused on optimizing operating conditions. With regard to feedwater characteristics, only a few reports on the effect of carbon source and cations have been published. Chemical oxygen demand/nitrogen (COD/N) ratio, one of the most important parameters of microorganism growth, has been continually emphasized in numerous studies [19]. The COD/N ratio of raw wastewater always plays a significant role in the removal of nutrients and in the processes of nitrification and denitrification [20-22], because COD/N ratio not only affects functional microorganism populations, but also changes the structure of microbial communities. Several investigations have revealed that different removal rates of COD and NH4+-N were obtained under different COD/N ratios, regardless of whether traditional technologies or MBR was used [23,24]. Although the effect of COD/N ratio on membrane fouling has not been reported, several studies have considered the issue. Rosenberger et al. [25] reported that different fouling rates are observed only in the denitrification zone during the operation of two parallel MBRs. Unfortunately, the effect of COD/N ratio on membrane fouling was not emphasized. In addition, the heavy metal biosorption capability of sludge can be improved by determining a better COD/N ratio [26]. The adsorption process is accompanied by the release of different amounts of Ca, Mg, and carbohydrates that are attached to the surface of the cell as components of EPS. All these information indicate that the influent COD/N ratio can affect the characteristics of sludge.
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233
Therefore, COD/N ratio can result in the identification of membrane fouling in MBRs. In the current study, the effect of the COD/N ratio on membrane fouling in a submerged MBR was investigated. Synthetic wastewater with different ammonia concentrations was used. Trans-membrane pressure (TMP), mixed-liquor characteristics, and SMP and EPS contents were monitored to provide a comprehensive understanding of their relationships with the COD/N ratio of feedwater. To determine the relationship between the COD/N ratio in feed wastewater and EPS production in MBRs, measurements of the cation concentration and floc-size measurements of the sludge, and the apparent molecular weight (MW) of SMP were also conducted.
fed into the two parallel MBRs with synthetic wastewater for more than a month. During each COD/N ratio experiment, only backwashing was used to recover the foulants. Hydraulic retention time (HRT) was maintained for 3 h. The reactor temperature was maintained at approximately 25 ± 2 °C using an electric heater. At the bottom of the reactor, air was continuously supplied at a flow rate of 200 L/h. Chemical cleaning of the membrane was conducted by soaking it in 2000 ppm of sodium hypochlorite solution for 2 h. The details are shown in Table 1.
2. Materials and methods
To study the effect of high concentrations of NH4+-N on the release of foulants from the sludge samples, a mixed-liquor supernatant was obtained from sludge samples after adding 1 g/L NH4+-N using a centrifuge (TG-II, Changsha China) operated at 3500 rpm for 15 min to remove the solids. The colloidal and soluble fractions were obtained after the supernatant was paper filtered (quantitative filter paper, 30–50 μm) and then membrane filtered (PTFE syringe filter, nominal pore size of 0.45 μm). The COD, carbohydrates, and proteins were separately measured to study the changes in the colloidal and soluble fractions released from the sludge sample over time.
2.1. Reactor setup Two identical submerged laboratory-scale MBRs (Fig. 1), each with a working volume of 13 L, were operated in parallel and fed with synthetic wastewater for a period of 50 days. The membrane modules (Bishuiyuan, China) comprised polypropylene hollow-fiber membranes with a pore size of 0.4 μm and an area of 0.3 m 2. The bioreactors were fed with raw wastewater through a constant-level tank, and the effluents were extracted directly using a suction pump based on a time sequence of 8 min on and 2 min off for each cycle. A manometer was fixed between the membrane module and the suction pump to monitor the TMP. 2.2. Simulated raw wastewater Synthetic wastewater, simulating municipal sewage, was used to ensure a stable feeding rate throughout the experiment. The synthetic influent contained glucose, NH4Cl, and NaH2PO4 as primary nutrients, whereas FeSO4·7H2O, CuSO4·5H2O, MnCl2·4H2O, MgCl2·6H2O, CaCl2·2H2O, H3BO3, and CoCl2. 6H2O comprised the trace nutrient solution. The influent concentrations were approximately 400 mg COD/L, 40– 80 mg TN/L for different COD/N ratios, and 6 mg phosphorus (PO43 −–P)/L. 2.3. Operating conditions Before the experiment was conducted, the seed sludge collected from a domestic sewage wastewater treatment plant in Jinan was
2.4. Fractionation
2.5. Chemical and physical analyses COD, MLSS, and mixed-liquor volatile solid (MLVSS) were determined according to standard methods. NO3−-N and NO2−-N were determined using an ion chromatograph (DX120). EPS and SMP were measured as total organic carbons (TOC), and their components were measured as carbohydrates and proteins using a cation exchange resin (Purolite C100E, Na + form) extraction method based on the procedure of Trussell et al. [27]. The carbohydrate and protein concentrations of the supernatant were measured using the anthrone [28] and Bradford methods [29], respectively. Bovine serum albumin and glucose were used as the respective protein and carbohydrate standards. Calcium, magnesium, aluminum, and iron in the effluent, as well as SMP and sludge, were measured using inductively coupled plasma (ICP) spectroscopy (IRIS Intrepid IIXSP, Thermo Electron Co. USA). To determine the multivalent cations, the sludge samples from MBRs under different COD/N ratios were digested according to the method used in an earlier study [30].
Temp. controller
Level controller
Effluent
Influent Pressure gauge
10L Balance Tank
Air Flowmeter
170L
Air Feed Tank
Membrane tank Fig. 1. Schematic diagram of the MBR experimental setup.
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Table 1 Operating conditions and performance of MBRs. Reactor
MBR1
MBR2
COD/N c Influent COD (mg/L) c Influent NH4 + -N (mg/L) SRT (day) HRT (hour) c Flux (L/m2h) c MLSS (g/L) c MLVSS/MLSS Average particle size (μm) c COD removal (%) c NH4 + -N removal (%) c TN removal (%) Nitrification rate a (mg N/g MLVSS·d) Denitrification rate b (mg N/g MLVSS·d)
10 398 ± 11 40 ± 1.5 20 3 14.3 ± 0.5 9.8 ± 0.3 0.90 ± 0.02 108.4 94.3 ± 5.5 74.8 ± 2.1 31.1 ± 11 508.6 ± 12.8 211.5 ± 15.1
5 400 ± 14 41 ± 2.1 20 3 14.3 ± 0.7 9.6 ± 0.2 0.91 ± 0.01 112.5 94.5 ± 7.2 54.5 ± 5.3 19.5 ± 9 618.5 ± 21.4 264.7 ± 18.9
a
þ Q in ðNHþ 4 N influent NH 4 Neffluent Þ MLVSS Q in ðTN influent TNeffluent Þ rate ¼ where MLVSS
Nitrification rate ¼
Denitrification Qin is the influent flux (L/d). Sample mean ± standard deviation, number of measurements: n = 25 (COD, NH4+-N, NO2-, NO3-, flux); n = 10 (MLSS, MLVSS). b c
The particle size of the sludge was determined using a BT-9300H laser particle size analyzer (Dandong Baite, China) in the measurement range of 0.1–340 μm. The MW distribution of SMP under each COD/N ratio was determined through gel-permeation chromatography (GPC) (PL-GPC 220) equipped with a refractive index (RI) detector and a column (PLaquagel-OH MIXED-H). Milli-Q water was used for the mobile phase. 2.6. Membrane resistance analyses The total filtration resistance values were primarily dependent on the suction pressure and permeate flux. At the end of each COD/N ratio experiment, the composition of resistance through the membrane, which reached the design maximum TMP (30 kPa), was analyzed. Eq. (1) was used to calculate the different values of filtration resistance. The filtration resistance was measured step by step, as shown in Fig. 2. The flux and TMP data on new membranes for the filtration of mixed liquor were recorded to measure intrinsic membrane resistance (Rm) under different COD/N ratios. The other filtration resistance values were calculated based on the data on suction pressure using Eq. (2). R¼
TMP μJ
ð1Þ
Rt ¼ Rm þ Rp þ Rc þ Rir
Fig. 2. Steps in measuring each filtration resistance.
ð2Þ
where R is the filtration resistance (m− 1), TMP denotes the transmembrane pressure (Pa), J is the permeated flux (L/m 2 h) converted to m3/(m 2 s), μ is the viscosity of the permeated water (Pa s), Rm is the resistance of the membrane itself (m− 1), Rc is the resistance of the cake layer (m− 1), Rp is the pore blocking, and Rir is the irreversible fouling resistance (m− 1). 2.7. Release of EPS at high NH4+-N Concentration To compare the effect of the release of EPS when the sludge was mixed with and without high concentrations of NH4+-N, the EPS release test was performed as follows: 1000 mL sludge from MBR1 was transferred to a beaker and placed in ice water. The suspension was stirred using a magnetic stirrer to mix the sludge fully. The suspension was cooled to below 4 °C to minimize biological activity. Subsequently, 1 g (NH4+-N)/L was added into the beaker, and samples of the suspension were collected after 0, 10, 20, 40, 60, 90, 150, and 240 min. The foulants released from the sludge were harvested, and the fractionation method presented earlier was applied to determine the colloidal and soluble COD, carbohydrates, and proteins. All operations were conducted in duplicate. 3. Results and discussion 3.1. Overall MBR performance under different COD/N ratios The performance of the MBR was evaluated based on COD, NH4+N, and TN removal (Table 1 and Fig. 3). All experiments were conducted under the same organic loading. However, different organic removal rates were observed and were attributed to different NH4+N concentrations in raw wastewater. During the operation, the average effluent COD concentrations were 23 and 22 mg/L with average efficiency levels of 94.3% and 94.5%, respectively, under COD/N ratios of 10 and 5. This result is in agreement with that of Ng et al. [31]. All MBRs performed equally well in terms of removal efficiency regardless of the operating conditions or time. In terms of NH4+-N removal, the nitrification and denitrification rates increased as the COD/N ratio decreased. When the COD/N ratio reached 5, the NH4+-N and NO3--N concentrations in the effluent significantly increased, which is attributed to the high influent NH4+-N and limited nitrification and denitrification capabilities. Therefore, the COD/N ratio is believed to be related to the treatment of wastewater in MBRs because this affected the removal not only of nitrogen, but also of COD. This result did not concur significantly with that of an earlier study, in which the COD of influent wastewater was 2000 mg/L [23]. The rate of increase in TMP is an important factor that affects membrane filterability in a submerged MBR system because it is directly related to the extent of membrane fouling. Fig. 3 also shows that the different values of TMP increase over time during the measurement of each COD/N ratio. The fouling profiles of the two MBRs in the present experiment exhibited a three-stage process. This process corresponded closely to the three-stage mechanism map for membrane fouling in MBRs [32]. In the proposed map, Stage 1 consisted of an initial short-term rapid rise in TMP attributable to the conditioning fouling of the membrane. Stage 2 reflected a weak long-term rise in TMP. Stage 3 showed the occurrence of a sharp increase in TMP, also known as the TMP jump. As evident in Fig. 3, both MBRs underwent all three membrane fouling stages. However, the phenomenon in MBR1 was clearer than that in MBR2. The pressure in MBR2 increased in the first few days of reactor operation and subsequently reached Stage 2 when the pressure increased slowly over the next 17 days. On day 17, the slope of the TMP increased at a significantly faster rate each day until the membranes had to be cleaned on day 25. These findings could be seen as the events of Stage 3. After membrane cleaning, a similar trend was observed in MBR2 for the
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Effluent COD (mg/l)
+
50
b
COD
NH 4-N
45
40
40
35
35
30
30
25
25
20
20
15
15
10
10
5
5 0
0
+
a
45
Effluent NH4 -N (mg/l)
50
235
30
TMP (Kpa)
MBR2
MBR1
25 20 15 10
chemical washing
5 0
5
10
15
20
25
30
35
40
back washing
45
50 0
5
10
15
20
Days
chemical washing
25
30
35
40
45
50
Days
Fig. 3. Overall MBR performance and normalized membrane suction pressure profile under different COD/N ratios; a: MBR1 with a COD/N ratio of 10; b: MBR2 with a COD/N ratio of 5.
second fouling cycle, except that the lasting time of Stage 2 decreased to 14 days or approximately 3 days before severe fouling occurred. MBR1, with a COD/N ratio of 10:1, successfully minimized the fouling rate in the current study. The TMP increased significantly more slowly in MBR1, and the duration for Stage 2 of over 30 days is nearly two times longer than that for MBR2. Therefore, the fouling rate can be controlled by regulating the COD/N ratios in MBR systems during daily operations. 3.2. SMP and EPS under different COD/N ratios The components of SMP and EPS in the mixed liquor were monitored separately because SMP and EPS have been widely accepted as major foulants in MBRs. In addition to TOC, which was used as the total SMP and EPS value, the contents of proteins and carbohydrates were also examined to obtain more detailed insights into the composition of SMP and EPS under each COD/N ratio. Table 2 indicates that the TOC values of SMP were similar for both COD/N ratios, but the concentrations of proteins and carbohydrates were different. The amount of carbohydrates in MBR2 with a COD/N ratio of 5 was 10.05 mg/L, which was more than two times higher than that in MBR1, whereas the protein content was slightly higher in MBR1 compared with that in MBR2. In particular, the carbohydrate/protein
ratios of SMP under various COD/N ratios were generally different. In recent years, a growing number of studies [17] have found that the hydrophilic neutrals (e.g., carbohydrates) and carbohydrate/protein ratios are most likely the primary factors responsible for the high fouling potentials of SMP. Table 3 shows that the EPS concentrations were 176.86 and 104.82 mg/g.VSS, corresponding to COD/N ratios of 10 and 5, respectively, which is an interesting observation. Table 3 shows that the TOC value of EPS and the concentrations of each composition of EPS in MBR1 were all higher than those in MBR2. However, these factors did not result in a higher fouling rate compared with the case of MBR2. This finding contradicts the results of other studies that indicate that high concentrations of EPS always result in a high membrane fouling rate. Miqueleto et al. [24] reported that a high COD/N ratio promotes the production of more EPS. EPS production is always considered to be governed by factors such as organic loading, carbon source, and limited nutrients. However, this result is not applicable for SMP in the present study. SMPs are always considered as the soluble EPS, the concentration of which is not only related to the total amount of EPS, but also depends on the properties of EPS. Under a high concentration of NH4+-N in feedwater with a COD/N ratio of 5, the NH4+-N concentration in the effluent was consistently higher than 40 mg/L. However, this value was only approximately 10 mg/L under a COD/ N of 10. A high content of NH4+-N in the supernatant could enhance the action of NH4+-N as monovalent cations and facilitate the replacement of the polyvalent cations in EPS. Moreover, the high NH4+-N
Table 2 Characteristics of mixed liquor supernatant under different COD/N ratios.a. COD/N TOC ratio (mg/L)
Carbohydrate Protein (mg/L) (mg/L)
Carbohydrate / SUV254 Protein (L/mg·TOC·m)
10
4.66 ± 0.42
2.28 ± 0.15
2.86 ± 0.16
6.48 ± 0.57
1.73 ± 0.41
5
10.14 ±1.21 11.57 ±2.65
10.05 ± 2.01
2.06 ± 1.18 1.55 ± 0.71
a Sample mean ± standard deviation, number of measurements: n = 15 (TOC, carbohydrate, protein).
Table 3 EPS concentration and composition under COD/N ratios.a COD/N ratio
TOC (mg/g·MLVSS)
Carbohydrate (mg/g·MLVSS)
Protein (mg/g·MLVSS)
Carbohydrate/ protein
10 5
176.86 ± 9.75 104.82 ± 7.30
7.23 ± 2.12 3.08 ± 1.03
50.09 ± 5.62 40.83 ± 4.93
0.14 ± 0.05 0.08 ± 0.02
a Sample mean ± standard deviation, number of measurements: n = 15 (TOC, carbohydrate, protein).
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content can help in the extraction of EPS components from the surface of sludge to form new SMPs. This finding may be the primary reason why a lower concentration of EPS in the MBR2, together with a higher membrane fouling rate, was observed.
3.3. Resistances through the membrane under each COD/N ratio To identify the primary contributor to membrane fouling under each COD/N ratio, hydraulic resistance analyses of the membranes were conducted at the end of each COD/N ratio experiment. Fig. 4 shows the values of filtration resistance, including intrinsic membrane resistance (Rm), cake resistance (Rc), pore blocking resistance (Rp), and irremovable fouling resistance (Rir). The biocake resistance accounted for 63% for the MBR1 and 57% for the MBR2. These results revealed that the value of Rc resulting from the formation of the biocake on the membrane surface was primarily responsible for the increase in TMP. Previous studies show a similar result [33,34]. Rm, Rp, and Rir appeared to be minor factors of the total resistance (Rt) compared with Rc. Fig. 4 shows that the ratio of biocake resistance of MBR1 was slightly greater than that of MBR2 for the same total resistance. However, the decrease of Rc of MBR2 was compensated with an increase in Rp and Rir, which is the reason why the Rp and Rir value of MBR2 is greater than that of MBR1. Although Rc constitutes a large part of Rt, it can be easily eliminated by the implementation of physical cleaning (e.g., backwashing). Moreover, Rc could form a secondary dynamic membrane on the surface of the membrane, which could prevent direct contact with the soluble or colloidal particles, as well as microorganisms, in the mixed liquor. Without Rc, membrane fouling would be more severe. Previous studies demonstrated that foulants, especially those comparable with or smaller than the membrane pore (e.g., soluble or colloidal particles and microorganisms), are the primary contributors to Rp and Rir, explaining the higher biocake resistance but lower fouling rate in MBR1 compared with that in MBR2. In recent years, studies have begun to focus on the possibility that the fouling attributable to Rp and Rir is the primary reason for the eventual loss of permeability in MBRs and that this fouling is directly linked to the pattern of TMP increase. In the present experiment, Rp of the two reactors were similar, but the Rir of MBR2 was nearly two times higher than that of MBR1. Only physical cleaning was conducted during each COD/N experiment. Portions of Rc and Rf, which can be removed by physical cleaning, did not grow continuously. However, the different backwashing levels had minimal effect on the Rir value. Therefore, the Rir value showed better correlation with the fouling profiles.
3.4. Effect of high contents of NH4+-N Fig. 5 shows that significant amounts of COD were released from the sludge flocs to the supernatant over time. However, after 240 min, more than 21.4% of colloidal COD was released from the sludge with NH4+-N compared with that released from the sludge without NH4+-N. For the soluble fraction, this difference was more pronounced, with a 34.5% higher release for high concentrations of NH4+-N amended sludge. The soluble COD was more sensitive than the colloidal COD under a high NH4+-N concentration in the MBRs. Table 4 presents the amount of extracted EPS and its compositions (i.e., proteins and carbohydrates). In both sludges, proteins, as well as carbohydrates, were released. Despite the similarity of the amounts of released soluble COD with respect to proteins and carbohydrates, a clear difference was observed for the colloidal COD. After 240 min, 84.8% and 88% colloidal carbohydrates and proteins were released from the NH4+-N amended sludge, whereas 47.5% and 47% were released from the sludge without NH4+-N, respectively. Fig. 6 shows the different concentrations of polyvalent cations (such as calcium, magnesium, aluminum, and iron) of the sludge from MBR1 and MBR2. The amounts of all four cations in the sludge from MBR1 were higher than those from MBR2. However, the average NH4+-N content in the effluent from MBR2 was 43.6 mg/L, which is four times higher than the content of 10.1 mg/L in the effluent from MBR1. Previous studies have demonstrated that polyvalent cations, especially divalent cations, in the sludge form bridges between the
300
Soluble COD
a
+
NH 4 -N
270
COD (mg/Ll)
236
+
no NH 4 -N
240 210 180 150 120 0
25
50
75
100 125 150 175 200 225 250
Time (min)
b
320
Colloidal COD +
Rc
63%
Rp R ir
57%
Resistance (m-1)
1.20E+009
Rm
1.00E+009 8.00E+008 6.00E+008
+
no NH 4 -N
280
COD (mg/l)
1.40E+009
NH 4 -N
300
260 240 220
23%
21%
200
4.00E+008 11% 2.00E+008 0.00E+000
5%
8%
11%
180 0
MBR1
MBR2
Fig. 4. Distribution of each filtration resistance value at the end of the operation.
25
50
75
100 125 150 175 200 225 250
Time (min) Fig. 5. Release of COD from sludge samples mixed with and without NH4 + -N.
S. Feng et al. / Desalination 285 (2012) 232–238
237
Table 4 Release of COD, protein, and carbohydrate from sludge with and without NH4 + -N amended. EPS extracted
From sludge without NH4 + -N (mg/L)
Extraction time (min) Colloidal COD Protein Carbohydrate Soluble COD Protein Carbohydrate
10 205 16.6 10.1 140 3.9 9.8
20 208 18.8 11.3 145 4.2 10.4
40 211 20.1 12.1 152 4.4 11.1
60 216 21.4 12.9 156 4.7 11.7
From sludge with NH4 + -N (mg/L) 90 208 22.6 13.6 168 5.1 12.5
150 221 23.2 14.2 178 5.3 13.2
polymers, which significantly affects sludge flocculation, settleability, and floc strength [35]. Moreover, the divalent cations are replaced by monovalent cations under a high M/D cation ratio. Consequently, the bridges are destroyed, and the foulants are released from the sludge to a certain extent [18]. Therefore, the high concentrations of NH4+-N in the reactor can be concluded to have acted as a monovalent cation, which resulted in the release of polyvalent cations and the formation of new SMPs from the sludge. Fig. 6 also shows that the release of Ca2 + and Fe3 + was more sensitive to the effect of high concentrations of NH4+-N than that of Mg2 + and Al3 +; the release values of Ca2 + and Fe3 + were 22.6% and 47.4%, respectively; the corresponding values for Mg 2 + and Al3 + were 5.2% and 11.4%, respectively. The polyvalent cations released in the supernatant and effluent were also measured. However, values of both Fe3 + and Al3 + were below the detection level, and very slight differences were observed for Mg2 + and Ca2 + because of high background. 3.5. Apparent MW distributions of SMP in supernatants and effluents at each COD/N ratio GPC was used to determine the effect of SMP under different COD/N ratios. Fig. 7 shows the apparent MW distributions of SMP at different COD/N ratios. All samples obtained three peaks, and Peak 3 (low molecular compound) was the largest fraction, which is consistent with other studies [17]. Fig. 7 also shows that the apparent MW distributions of SMP were slightly different for each COD/N ratio. All peak locations of the MW distribution for MBR2 were shifted to the right compared with MBR1, indicating that the large molecular compounds (e.g., protein, carbohydrates) could dissociate smaller molecular matters through interaction between the polyvalent cation and NH4+ in the reactor. This phenomenon seems beneficial in controlling membrane fouling as smaller molecules could easily pass through the membrane, even though previous studies have demonstrated that membrane blocking frequently occurred at a fraction of MW larger than 10 kDa. However, it should be noted that the excessive NH4+ in the reactor converted the larger molecules to smaller ones. The total amount of membrane foulants was not affected. The diagram shows that although the
240 217 24.4 14.9 190 5. 6 13.9
10 210 20.5 12.5 190 4.1 10.2
20 230 23.2 14.2 212 4.8 11.7
40 240 28.8 17.3 228 5.2 13.2
60 255 33.4 19.9 238 5.5 13.7
In the current study, particular attention has been paid to identify the role of influent COD/N ratio in controlling membrane fouling. The following specific conclusions are drawn: (1) The removal rate of nutrients could be affected by the COD/N ratio. Under the same organic loading, the removal rates of COD were similar and reached values greater than 90%. However, the removal rate of NH4+-N rapidly decreased from 74.8% to 54.5% when the COD/N ratio decreased from 10 to 5, respectively. This finding was primarily attributable to a high NH4+-N concentration in the feedwater and low nitrification growth rate in the reactors. (2) Higher COD/N ratio promoted the production of more EPS and higher carbohydrate proportion in EPS. SMP concentration was not positively correlated with EPS under a high content of NH4+ in the supernatant, which resulted in excessive NH4+, replacing polyvalent cation and extracting compositions from the EPS. Thus, high SMP and low EPS were observed in the system under a COD/N ratio of 5. At this particular instance, the membrane fouling rate was not only determined by the total amount of SMP, but also showed dependence on the properties (e.g., compositions, MW distribution, and hydrophobicity) of SMP. (3) GPC analyses indicated that the influent COD/N ratio changed the apparent MW distributions of SMP. However, the membrane fouling potential was generally related to the total amount and properties of SMP and seems to be slightly affected by the MW distribution of SMP.
Signal response (RI)
mg/g (MLSS)
0.5
0.0
peak 3 (low molecular)
120
1.0
240 276 38.6 23.1 290 6.3 15.6
4. Conclusions
MBR1 MBR2
1.5
150 270 37.8 22.8 268 5.8 14.6
fractions near Peak 1 (macromolecular compounds) for MBR2 were slightly lower than those for MBR1, others were considerably higher than those for MBR1. Therefore, the high contents of SMP played a significant role in membrane fouling. Similar results have been reported in other studies [36,37], where the SMP has a higher fouling potential compared with EPS under certain operating conditions.
140 2.0
90 266 36.2 21.7 250 5.7 14.3
SMP of MBR1 SMP of MBR2
100 80
peak 2 (mid molecular)
60 40
peak 1 (macromolecular)
20 0 -20 475 500 525 550 575 600 625 650 675 700 725 750 775 800
Ca
Mg
Al
Fe
Fig. 6. Concentration of polyvalent cations in sludges from two reactors.
Elution time (s) Fig. 7. Apparent MW distribution of SMP under each COD/N ratio.
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