Successful application of Shirasu porous glass (SPG) membrane system for microbubble aeration in a biofilm reactor treating synthetic wastewater

Separation and Purification Technology 103 (2013) 53–59

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Successful application of Shirasu porous glass (SPG) membrane system for microbubble aeration in a biofilm reactor treating synthetic wastewater Chun Liu a,⇑, Hiroshi Tanaka b, Jing Zhang a, Lei Zhang a, Jingliang Yang a, Xia Huang c, Nobuhiko Kubota b a

School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China Technical Development and Engineering Center, IHI Corporation, Shin-Nakahara-cho, Isogo-ku,Yokohama 235-8501, Japan c State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China b

a r t i c l e

i n f o

Article history: Received 8 August 2012 Received in revised form 15 October 2012 Accepted 15 October 2012 Available online 26 October 2012 Keywords: Microbubble aeration SPG membrane Biofilm reactor Oxygen utilization Energy consumption

a b s t r a c t Microbubble aeration is supposed to be able to provide potential advantage for aerobic biological wastewater treatment due to enhancement of oxygen mass transfer. Then SPG membrane microbubble generation system was used to aerate a biofilm reactor treating a synthetic wastewater. Successfully long-term operation of the experimental system demonstrated the application feasibility of microbubble aeration in aerobic biological wastewater treatment. The air permeation of SPG membrane depended on its surface property and pore size, rather than membrane fouling which influenced microbubble generation and subsequent oxygen transfer. Stable and efficient COD removal was achieved but the ammonia removal became inefficient at high organic loading rate due to DO limitation. The suitable SPG membrane area-based COD removal capacity should be controlled around 6.69 kgCOD/(m2 d), considering the stable DO concentration and efficient contaminant removal. Microbubble floatation also provided contribution to suspending and colloidal contaminant removal. The oxygen utilization was estimated as high as almost close to 100% under certain conditions based on contaminant removal, but the energy consumption of this microbubble aeration system was also very high. To use hydrophobic SPG membranes and increase their air supply capacity could contribute to energy-saving. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In wastewater treatment plants, the aerobic bioreactor is the most important process, where the aerobic contaminant digestion depends on the availability of sufficient dissolved oxygen (DO) and high DO concentration could accelerate organic contaminants removal [1,2]. To promote the aerobic biochemical reaction, the oxygen supply rate into microorganisms has to be fast because of oxygen feed limitation; as a result, a large amount of energy consumption is required for oxygen supply [3,4]. Therefore, the highly efficient oxygen supply methods are expected. Nowadays, microbubble technology has been explored for applications in wastewater treatment [5], which provides a promising solution for efficient oxygen supply in aerobic biological wastewater treatment. A microbubble is defined as a small bubble with a diameter range of 10–50 lm. Microbubbles have useful characteristics, such as a large gas–liquid interfacial area, long residence time in the liquid phase and fast dissolution rate so that they have an advantage to dissolve the oxygen gas in air into water.

⇑ Corresponding author. Tel.: +86 311 81668436; fax: +86 311 81668428. E-mail address: [email protected] (C. Liu). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.10.023

The present applications of microbubble technology for wastewater treatment focus on physical–chemical processes and enhancement of gas–liquid mass transfer has been confirmed, including oxygen. Terasaka et al. [6] demonstrated better oxygen transfer rates of microbubble aerator than the typical gas distributors. Liu et al. [7] found that microbubble enhanced oxygen transfer rate and contaminant removal in a coagulation floatation process of dyeing wastewater. Chu et al. [8,9] also found that the production of microbubbles helped to improve the ozone mass transfer efficiency and further enhanced the soluble contaminant removal of simulated dyestuff wastewater and practical textile wastewater. Few application of microbubble technology has been reported in continuously running bioreactors for wastewater treatment until now, although some microbubble aerators have been developed [6]. The widely used methods of microbubble generation are based on gas–water circulation [8–12], decompression [7,13] and gas–water dispersion process using a certain medium, such as SPG membrane [14–19]. SPG membrane as a kind of porous glass membrane is prepared based on phase separation of primary Na2OACaOAMgOAAl2O3AB2O3ASiO2 type glasses and subsequent acid leaching [20]. SPG membranes have uniform-sized cylindrical, tortuous pores, which together form a three-dimensional interconnected network. With gas dispersion process, gas phase at high pressure is forced through an SPG membrane into the liquid phase

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to form microbubbles. The very small and uniform-sized microbubbles can be generated using SPG membrane, compared to porous ceramic membranes which generate polydispersed larger bubbles [21]. Another advantage of this technique is that the resultant bubble size and void fraction are mainly determined by the membrane pore size and membrane area, respectively. This indicates that bubble size and void fraction can be optimized for a large-scale application. SPG membrane has been used for microbubble aeration to enhance ozone gas–liquid mass transfer [21]. SPG membrane surface is naturally hydrophilic due to the presence of hydroxyl groups such as silanol groups on its surface [17]. On the other hand, the surface of SPG membranes can be hydrophobized by chemical modification with organosilane compounds [19,22]. The performance of SPG membrane using for microbubble aeration is influenced by its surface wettability, such as microbubble formation [17] and gas–water transfer resistance [21]. However, there are some problems for microbubble aeration applied in activated sludge-based wastewater treatment processes. For example, the sludge floatation caused by attachment of microbubbles to sludge flocs results in poor sludge settleability. Furthermore, almost all microbubble generators require mechanical moving parts, e.g. a pump, where a strong shear force acts on a liquid. A liquid circulation pump is also necessary for microbubble generation when using SPG membrane system. The suspended activated sludge flocs are broken easily when they flow into the circulation pump, and as a result, their activity decreases and the performance of activated sludge process is influenced significantly [6]. A solution to these problems is to fix the sludge (microbes) on the carrier to avoid negative effects of microbubble aeration on activated sludge, for example a biofilm reactor where the biofilm is fixed on the carrier and will not experience microbubble generation process. In this study, a SPG membrane system was used for microbubble aeration in a flexible fiber biofilm reactor. The performance of contaminant removal in the biofilm reactor was investigated during continuously long-term operation. The oxygen utilization was evaluated based on contaminant removal. The energy consumption by the experimental system was determined and the possible strategy for energy saving was suggested. The effects of membrane pore size and membrane wettability on the performance of the experimental system were also discussed.

Fig. 1. Schematic diagram of the experimental apparatus.

range of 0.58–1.06 m/s, while air was introduced on the outside and forced through the membrane pores. The air flow rate was controlled at a range of 30–40 mL/min by regulating transmembrane pressure (difference between outside air pressure and inside liquid pressure of SPG membrane) and the corresponding air flux ranged from 1.15 to 1.51 m3/(m2 h). The microbubbles generated by SPG membrane in clean water are shown in Fig. 2 under these conditions, and their diameter range is determined as 20–50 lm based on microscope observation and measurement [8]. The laboratory scale reactor was a transparent plexiglass tank with a diameter of 250 mm and a depth of 600 mm. The work volume of the reactor was 15 L. A common commercial flexible fiber carrier was used in the reactor. The flexible fibers were bundled and circularly attached to a circular plastic disk with a diameter of 80 mm. Four carriers were supported by a rope at intervals of about 80 mm and the rope was fixed vertically in the center of the reactor.

2.3. Experimental procedure 2. Materials and methods 2.1. SPG membrane Three types of tubular SPG membranes obtained from SPG Technology Co. Ltd. (Miyazaki, Japan) were used in this study, including type I (hydrophilic, 0.8 lm of membrane pore size), type II (hydrophobic, 0.6 lm of membrane pore size) and type III (hydrophilic, 0.6 lm of membrane pore size). The detailed preparation procedures of SPG membrane have been described elsewhere [20].

The activated sludge from a municipal wastewater treatment plant was inoculated into the reactor at a concentration of 0.8 g/L to enhance biofilm formation on the fibers at the start-up period. SPG membrane microbubble aeration system was applied in the

2.2. Experimental set-up Fig. 1 depicts schematic of a biofilm reactor combined with SPG membrane microbubble generation system for aeration. The three types of tubular SPG membranes were used to generate air microbubbles. The gas–liquid dispersion system for microbubble generation consisted of air as the dispersed gaseous phase and mixed liquor in the bioreactor as the continuous liquid phase. An air compressor was used to provide compressed air with pressure range of 0.6–0.8 MPa. A continuous liquid phase was flowed inside the membrane using a canned type circulation pump at a velocity

Fig. 2. Observation of microbubbles generated by SPG membrane in clean water.

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bioreactor after the biofilm on the carriers was stable at about dry weight biomass of 3.0 g per carrier. A synthetic wastewater was treated in the biofilm reactor. Glucose, starch, peptone, NH4Cl and KH2PO4 were the main constituents in the synthetic wastewater to provide carbon, nitrogen and phosphorus sources [23]. The synthetic wastewater had a COD of 331.7 ± 83.4 mg/L and ammonia of 48.0 ± 8.2 mg/L. Feed wastewater was pumped to the reactor continuously and the effluent from the bottom of the reactor was controlled by a valve. The reactor was operated at room temperature but the water temperature reached to 35 ± 2 °C due to pump circulation. The stable continuous operation of the biofilm reactor included five phases. The operating conditions in the five phases were shown in Table 1. In order to control SPG membrane fouling, offline cleaning methods were used in Phase 2, including step 1, ultrasonic treatment (20 kHz and 1000 W for 20 min) and subsequent fresh water washing to remove fouling layer; step 2, air-drying of SPG membrane at room temperature; step 3, thermal treatment at 550 °C for 2 h to remove the remaining organic foulants and step 4, acid treatment (hydrochloric acid solution of 0.5 mol/L, for 4 h at room temperature) to remove deposited inorganic foulants. In addition, online chemical membrane cleaning was utilized from Phase 3 to Phase 5 at a frequency of one time per 2 days, including sodium hypochlorite cleaning (1000 mg/L, 30 min) to control organic adsorption and biofilm growth, and hydrochloric acid cleaning (0.5 mol/L, 30 min) to control inorganic deposition. A single-phase kilowatt hour meter was fitted to the electricity supply to measure the power consumptions by the air compressor, the circulation pump, the influent pump and the online cleaning pump. 2.4. Analytical methods The DO concentration was measured four times per day with an electrochemical membrane electrode (WTW cellOx 325, Germany) and a digital DO meter, to determine the average DO value. Both outside air pressure and inside liquid pressure of SPG membrane were measured four times per day using pressure gauges to determine the transmembrane pressure, and the air flow rate was determined by a gas flowmeter. The influent and effluent water testing was conducted on a daily basis for the duration of the experiment. Total COD, ammonia and nitrate were measured in accordance with the standard method. The turbidity was determined using a digital turbidity meter (WGZ-1, Xinrui, China). The concentrations of total organic carbon (TOC), colloidal organic carbon (COC) and soluble organic carbon (SOC) of effluent and surface foam layer samples were determined according to Bouhabila et al. [24] with a TOC analyzer (TOC-V CPN, Shimadzu, Japan). 3. Results and discussion The flexible fiber biofilm reactor combined with SPG membrane system for microbubble aeration has been running for 99 d

without any severe problems, demonstrating the successful application of microbubble technology for aerobic biological wastewater treatment. The results are classified in five operation phases according to the different operating conditions (see Table 1). 3.1. Air permeation and DO concentration The air flow rate and transmembrane pressure were monitored to investigate the air permeation of SPG membrane. Fig. 3 shows the variation of transmembrane pressure during continuous operation of the bioreactor when air flow rate is controlled at 33.0 ± 3.6 mL/ min. The transmembrane pressure of type I SPG membrane (hydrophilic) was almost stable at 131.6 ± 4.4 kPa in Phase 1 even after the membrane external fouling happened. When type III SPG membrane (hydrophilic) was used, the higher transmembrane pressure was observed in Phase 3, due to its smaller pore size. On the other hand, the transmembrane pressure of type II SPG membrane (hydrophobic) used in Phase 2 was much lower than that of hydrophilic SPG membrane. The wetting and the capillary effect of the hydrophilic SPG membranes are considered to be responsible for their higher transmembrane pressure [21], compared to hydrophobic SPG membranes. In addition, SPG membrane fouling happened in Phase 1 due to no membrane cleaning, but the transmembrane pressure was not influenced. These results imply that the transmembrane pressure of SPG membrane depends on both surface property and pore size at a certain air flux, and could not be used as a sensitive indicator for membrane fouling. When the three types of SPG membrane were used for microbubble aeration in tap water under the similar conditions of the bioreactor, the total volumetric oxygen mass transfer coefficient was determined as 4.32–4.88 h 1 and the corresponding oxygen transfer efficiency was approximately 70–80%, much higher than 15–40% of conventional coarse bubble aeration systems [25]. The hydrophobic SPG membrane showed more efficient oxygen transfer due to its lower resistance [21], but not significantly. The DO concentration in the bioreactor depends on oxygen gas– liquid transfer and oxygen utilization for aerobic digestion. Fig. 4 shows the variation of DO concentration in the bioreactor during continuous operation. In Phase 1 microbubble aeration presented highly efficient oxygen transfer at the initial time and the DO concentration in the bioreactor was above 5 mg/L even at low air flow rate. However, the microbubble formation was influenced by SPG membrane fouling, resulting in larger bubbles generated. As a result, the oxygen mass transfer deteriorated and the DO concentration decreased. The significant reduction in DO concentration was also observed in Phase 2 due to SPG membrane fouling, but both microbubble generation and efficient oxygen transfer recovered after offline membrane cleaning. The SPG membrane fouling was controlled effectively by online cleaning and the DO concentration was relatively stable in Phase 3. These results indicate that SPG membrane fouling influenced microbubble generation and consequent oxygen mass transfer mainly, rather than membrane air permeation. An overall downward trend of DO concentration was also observed from Phase 1 to Phase 4 due to the increased organic

Table 1 Operating conditions.

a

Item

Phase 1

Operation days/d SPG membrane typea SPG membrane area/m2 Hydraulic retention time (HRT)/h Average influent organic loading rate/kgCOD/(m3 d) SPG membrane cleaning

1–16 Type I 1.57  10 24 0.25 None

Phase 2

Phase 3

Phase 4

Phase 5

18–45 Type II

46–69 Type III

70–83 Type II

84–99 Type II 3.14  10

16 0.49 Offline

12 0.74 Online

8 1.16

3

3

Type I (hydrophilic, 0.8 lm of membrane pore size), type II (hydrophobic, 0.6 lm of membrane pore size) and type III (hydrophilic, 0.6 lm of membrane pore size).

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Phase 1

210

COD concentration (mg/L)

180 150 120 90

Phase 1

Phase 2

Phase 3

60 30 0 -30 -60

800

Phase 2

Phase 4

Phase 5

700 600 500 400 300 200 100 0

0

10

20

30

40

50

60

0

70

10

20

30

Fig. 3. Variation of transmemebrane pressure and air flow rate of SPG membranes. () Transmembrane pressure; (h) air flow rate.

Phase 2

Phase 3

Phase 4 Phase 5

7 Online cleaning

Offline cleaning

6 5 4 3

Phase 1 120

COD removal efficiency (%)

Phase 1

40

50

60

70

80

90

100

Running time (d)

Running time (d)

DO concentration (mg/L)

Phase 3

(a)

Phase 2

Phase 3

Phase 4

Phase 5 2

(b)

100

1.6

80 1.2 60 0.8 40 0.4

20

0

0

2

Organic loading rate (kg/m3d)

Transmembrane pressure (kPa) and air flow rate (mL/min)

240

0

10

20

30

40

50

60

70

80

90

100

Running time (d)

1 0 0

10

20

30

40

50

60

70

80

90

100

Running time (d) Fig. 4. Variation of dissolved oxygen (DO) concentration in the biofilm reactor using SPG membranes for microbubble aeration.

loading rate. The DO concentration was almost close to zero in Phase 4 at a high organic loading rate, indicating that air supply capacity reached its limitation for aerobic digestion. Then the SPG membrane area was enlarged to increase air supply capacity at a certain air flux in Phase 5 and the DO concentration increased to about 4 mg/L again. 3.2. Performance of contaminant removal 3.2.1. COD removal Fig. 5a shows the COD values of the influent and effluent of the biofilm reactor. The average COD values of the effluent were 47.2 ± 18.4 mg/L, 40.5 ± 23.2 mg/L, 37.2 ± 25.8 mg/L, 112.1 ± 25.3 mg/L and 76.7 ± 21.7 mg/L in Phases 1, 2, 3, 4 and 5, respectively. Fig. 5b shows the COD removal efficiency and organic loading rate removed of the biofilm reactor. The average COD removal efficiencies were 80.0 ± 7.8%, 86.4 ± 8.2%, 90.9 ± 6.4%, 69.4 ± 8.9% and 77.2 ± 9.4% in Phases 1, 2, 3, 4 and 5, respectively. The average organic loading rates removed were 0.18 ± 0.03 kgCOD/(m3 d), 0.42 ± 0.08 kgCOD/(m3 d), 0.70 ± 0.14 kgCOD/(m3 d), 1.07 ± 0.17 kgCOD/(m3 d) and 0.92 ± 0.20 kgCOD/(m3 d) in Phases 1, 2, 3, 4 and 5, respectively. The corresponding SPG membrane area-based COD removal capacities were 1.72 kgCOD/(m2 d), 4.01 kgCOD/(m2 d), 6.69 kgCOD/ (m2 d), 10.22 kgCOD/(m2 d) and 4.39 kgCOD/(m2 d) in Phases 1, 2, 3, 4 and 5, respectively. These results indicate the proper performance of the prolonged biofilm reactor combined with SPG membrane system for microbubble aeration. The COD removal was affected by organic loading rate due to available DO for aerobic digestion. The DO provided by microbubble aeration seemed inadequate for COD removal at high organic loading rate in Phase 4 and as a result, the COD removal became

Fig. 5. (a) Time course of COD concentration in the influent and the effluent, and (b) time course of COD removal efficiency and organic loading rate removed. () Influent; (h) effluent; (N) COD removal efficiency; (s) organic loading rate removed.

inefficient. When air supply capacity increased in Phase 5, the COD removal was also improved. Then 10.22 kgCOD/(m2 d) obtained in Phase 4 could be estimated as the maximum SPG membrane area-based COD removal capacity under the experimental conditions. COD removal capacity depends on available DO which is determined by air supply capacity (or air flux) of SPG membrane and oxygen transfer efficiency. On the other hand, an increase in air flux of SPG membrane would increase microbubble size [14], resulting in a reduction in oxygen transfer efficiency. Then the air flux of SPG membrane should be optimized to maximize air supply capacity and oxygen transfer efficiency simultaneously. The theoretically maximum SPG membrane area-based COD removal capacity should be achieved at the optimal air flux of SPG membrane. 3.2.2. Ammonia removal Fig. 6a shows the ammonia values of the influent and effluent of the biofilm reactor. The significant fluctuation of ammonia removal was observed, compared to relatively stable COD removal. The average ammonia values of the effluent were 22.9 ± 16.1 mg/L, 24.4 ± 13.2 mg/L, 15.3 ± 6.9 mg/L, 29.9 ± 8.7 mg/L and 39.6 ± 4.7 mg/L in Phases 1, 2, 3, 4 and 5, respectively. Fig. 6b shows the ammonia removal efficiency and loading rate removed of the biofilm reactor. The average ammonia removal efficiencies were 54.8 ± 29.8%, 46.4 ± 28.7%, 71.3 ± 12.5%, 42.4 ± 18.1% and 26.5 ± 6.4% in Phases 1, 2, 3, 4 and 5, respectively. The average ammonia loading rates removed were 0.026 ± 0.013 kgNH3AN/(m3 d), 0.039 ± 0.020 kgNH3AN/ (m3 d), 0.072 ± 0.016 kgNH3AN/(m3 d), 0.070 ± 0.032 kgNH3 AN/ (m3 d) and 0.042 ± 0.011 kgNH3AN/(m3 d) in Phases 1, 2, 3, 4 and 5, respectively. The ammonia removal was also affected by available DO in the bioreactor. In Phase 2, the ammonia removal deteriorated

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Phase 1

Phase 2

Phase 3

140

Phase 4 Phase 5

(a)

CDOconcentration (mg/L) and Turbidity (NTU)

NH 3 -N concentration (mg/L)

80

60

40

20

(a)

Surface foam

120

Effluent

100 80 60 40 20

0 0

10

20

30

40

50

60

70

80

90

100

0

Running time (d)

Phase 3

Phase 4 Phase 5 0.2

(b)

80

0.15

60 0.1 40 0.05

20 0 0

10

20

30

40

50

60

70

80

90

0 100

Running time (d)

Effluent nitrate concentration (mg/L)

Phase 1 40

Phase 2

Phase 3

20

10

0 20

30

40

50

60

(b)

30

Surface foam 25

Effluent

20 15 10 5 0

TOC

COC

SOC

Phase 4 Phase 5

30

10

35

COD

Fig. 7. Comparisons of turbidity and COD: (a) and organic carbon concentration, and (b) between surface foam layer and the effluent.

(c)

0

Organic carbon concentration (mg/L)

100

Phase 2

NH 3 -N loading rate (kg/m3d)

NH 3-N removal efficiency (%)

Phase 1

Turbidity

70

80

90

100

Running time (d) Fig. 6. (a) Time course of ammonia concentration in the influent and the effluent, (b) time course of ammonia removal efficiency and ammonia loading rate removed, and (c) time course of nitrate concentration in the effluent. () Ammonia influent; (h) ammonia effluent; (N) ammonia removal efficiency; (s) ammonia loading rate removed; (j) nitrate effluent.

significantly due to decreased DO concentration after SPG membrane fouling. After offline membrane cleaning and subsequent recovery of DO concentration, the ammonia removal was also improved. The relatively stable and efficient ammonia removal was achieved in Phase 3 due to relatively stable DO concentration. When organic loading rate increased and more DO was required for organic decomposition, the ammonia removal became inefficient again in Phase 4 due to DO limitation. Although the DO concentration recovered by increasing air supply in Phase 5, the ammonia removal deteriorated further, indicating that available DO was still insufficient for ammonia removal even when DO concentration was above 2 mg/L. The possible reason is that the biofilm growth was enhanced at high organic loading rate in Phase 4 and Phase 5, resulting in an increase in biofilm mass transfer resistance and subsequent DO diffusion limitation [26].

The nitrate values in the effluent are shown in Fig. 6c to present nitrification and denitrification processes for nitrogen removal. In Phases 1 and 2, the nitrate values in the effluent depended on the ammonia removal efficiency, indicating that nitrification process was mainly responsible for ammonia removal. From the late period of Phase 2 to Phase 5, the ammonia removal remained and the nitrate in the effluent reduced to a very low level simultaneously, implying that simultaneous nitrification and denitrification occurred due to the anoxic environment inside the biofilm caused by DO diffusion limitation. In comparison, the suitable SPG membrane area-based COD removal capacity should be controlled around 6.69 kgCOD/(m2 d) obtained in Phase 3, considering the stable DO concentration and efficient COD and ammonia removal. 3.2.3. Effect of microbubble floatation on contaminant removal A thin foam layer was observed on the surface of the bioreactor due to microbubble floatation, and Fig. 7 shows the comparisons of contaminant concentrations between the surface foam layer and the effluent from the bottom. Both turbidity and COD in the surface foam layer were much higher than these in the bottom effluent. In addition, the COC concentration in the surface foam layer was also much higher than that in the bottom effluent. These results reveal that the suspended and colloidal contaminants were accumulated in the surface foam layer due to the microbubble floatation [7,27]. So the bottom effluent instead of surface overflow is better to improve effluent quality. 3.3. Estimation of oxygen utilization based on contaminant removal The theoretical maximum oxygen supply capacity was 13.51 g/ d in the bioreactor when the air flow rate of microbubble aeration was controlled at 33.0 ± 3.6 mL/min at room temperature. For simplification, only organic decomposition and ammonia nitrification

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Navicula sp.

Pandorina sp.

Fig. 8. Observation of two main kinds of algae in the biofilm.

was considered for DO consumption. As a result, the estimated oxygen utilization efficiencies were 30.8%, 62.9%, 107.8% and 148.0% in Phases 1, 2, 3 and 4, respectively, based on COD and ammonia loading rate removed. Obviously, an increase in organic loading rate resulted in a higher oxygen utilization. On the other hand, the oxygen utilization efficiency was obviously overestimated. One possible reason was that part of COD and ammonia was not removed by aerobic digestion but by other processes, for example, assimilation of heterotrophic microbes and swallowing of protozoa. Moreover, two main kinds of algae were observed in the biofilm, including Navicula sp. and Pandorina sp. (see Fig. 8). These algae provided additional DO for contaminant removal due to photosynthesis, which was another reason for overestimation of oxygen utilization. In a control biofilm reactor without any illumination, the estimated oxygen utilization efficiency was 108.3% under the same conditions in Phase 4. Therefore, the contribution of algae to oxygen supply might be estimated as about 40% of microbubble aeration under these conditions. The algal–bacterial symbiotic system in this experimental system should be investigated further. Anyway, these results demonstrated that very high oxygen utilization could be achieved in the experimental system due to the merit of microbubble aeration, even being close to 100% under certain conditions. 3.4. Energy consumption The power consumption of the existing microbubble generators is convinced to be greater than that of the usual gas distributors [6]; therefore, it is important to investigate energy consumption of SPG membrane microbubble aeration system for the purpose of energy-saving in its future application. The electricity consumption of this microbubble aeration system included two aspects: air supply by an air compressor (1.5 kW of input power) and liquid circulation by a pump (0.18 kW of input power). Other energy con-

sumption parts of the experimental system include wastewater feeding by a pump and online chemical cleaning by a pump. The power consumption of various parts and their proportions in the anterior three operation phases were listed in Table 2. Obviously, the energy consumption of SPG membrane microbubble aeration system was much higher than 0.79 kW h/d of a common coarse bubble aerator (0.15 kW of input power) tested in this study. In addition, more than 99% energy was consumed for microbubble aeration in the experimental system. The liquid circulation pump accounted for the largest fraction of energy consumption in the experimental system, which provided a certain liquid phase velocity inside the SPG membrane required for microbubble generation. Theoretically, the pump power decreased with a decrease in liquid flow rate. In this study, the energy consumption of circulation pump only decreased 0.7% when the liquid flow rate decreased from 460 L/h to 120 L/h and the corresponding circulation velocity decreased from 1.67 m/s to 0.44 m/s. Then the contribution of decreasing circulation velocity to energy conservation seemed very limited. The energy consumption of air supply depended on air permeation of SPG membrane and air supply capacity. The energy consumption of air compressor was decreased by 26.9% using hydrophobic SPG membrane due to its better air permeation, compared to hydrophilic SPG membrane with the same pore size. The larger pore size of SPG membrane seemed also helpful for energysaving when the energy consumption of Phase 1 was compared to that of Phase 3. On the other hand, the energy consumption of air compressor only increased by 8.3% when air supply capacity was doubled by increasing SPG membrane area in Phase 5, compared to Phase 2. As a result, the energy consumption of specific air supply of the experimental system decreased by 48.9% in Phase 5. This implies that an increase in SPG membrane area served by the circulation pump to obtain larger air supply capacity is effective for energysaving, since the main purpose of this microbubble aeration system is to provide sufficient air for aerobic contaminant digestion. In addition, to maximize air transmembrane flux is another feasible way to increase air supply capacity, if ensuring microbubble generation simultaneously. For the future application of SPG membrane microbubble aeration system in aerobic biological wastewater treatment, hydrophobic SPG membranes show some potential advantages, including better air permeation and consequent energy-saving, faster oxygen transfer, and easier membrane fouling control. The adsorption of some foulants, such as proteins, on the SPG membrane surface could be reduced probably if SPG membrane surface is hydrophobically modified. In addition, it might be easier to remove these foulants from hydrophobic SPG membranes due to weaker adsorption, compared to hydrophilic SPG membranes. 4. Conclusions SPG membranes were applied for microbubble aeration in a long-term running biofilm reactor successfully. The performance

Table 2 Energy consumption of various parts and their proportions in each operation phase. Item

Air supply Liquid circulation Wastewater feeding Online chemical cleaning Total

Phase 1

Phase 2

Phase 3

Energy consumption (kW h/d)

Proportion (%)

Energy consumption (kW h/d)

Proportion (%)

Energy consumption (kW h/d)

Proportion (%)

1.19 2.22 0.0075 – 3.41

34.8 65.0 0.2 – 100

0.87 2.22 0.0075 – 3.01

28.1 71.6 0.3 – 100

1.27 2.22 0.0075 0.015 3.51

36.3 63.1 0.2 0.4 100

C. Liu et al. / Separation and Purification Technology 103 (2013) 53–59

of contaminant removal in the biofilm reactor was investigated and the oxygen utilization was estimated roughly based on contaminant removal. The energy consumption of the experimental system was determined and the possible strategy for energy saving was suggested. The main conclusions are as follows: (1) When three types of SPG membranes were used, the air permeation of SPG membrane depended on its surface property and pore size, rather than SPG membrane fouling which influenced microbubble generation and subsequent oxygen transfer. Hydrophobic SPG membranes showed better air permeation. (2) Stable and efficient COD removal was achieved but the ammonia removal was inefficient at high organic loading rate due to DO diffusion limitation. The suitable SPG membrane area-based COD removal capacity should be controlled around 6.69 kgCOD/(m2 d), considering the stable DO concentration and efficient COD and ammonia removal. Microbubble floatation also provided contribution to suspending and colloidal contaminant removal. (3) The oxygen utilization was estimated as high as almost close to 100% under certain conditions based on contaminant removal, but the energy consumption of this microbubble aeration system was also very high. To use hydrophobic SPG membranes and increase their air supply capacity could contribute to energy-saving.

Acknowledgments The authors acknowledge the financial supports from IHI Corporation, Japan, National Natural Science Foundation of China (51008111) and Hebei Science and Technology Agency, China (11966726D).

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