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submerged membrane electro-bioreactor (SMEBR(QQ)) hybrid system
Biomass and Bioenergy 128 (2019) 105329
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Research paper
Quorum sensing control and wastewater treatment in quorum quenching/ submerged membrane electro-bioreactor (SMEBR(QQ)) hybrid system
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Maham Khana, Sher J. Khana,**, Shadi W. Hasanb,* a Institute of Environmental Sciences and Engineering, School of Civil and Environmental Engineering, National University of Sciences and Technology (NUST), Islamabad, Pakistan b Center for Membrane and Advanced Water Technology (CMAT), Department of Chemical Engineering, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
A R T I C LE I N FO
A B S T R A C T
Keywords: Submerged membrane electro-bioreactor (SMEBR) Quorum sensing Quorum quenching Electrokinetics Wastewater
Membrane biofouling, in terms of quorum sensing (QS), reduction with hollow cylindrical quorum quenching (QQ) beads in submerged membrane bioreactor (SMBR(QQ)) and submerged membrane electro-bioreactor (SMEBR(QQ)) was investigated. Results showed that the rate of pressure increase (i.e. dTMP dt−1) was 0.7 and 0.5 kPa d−1 in SMBR(QQ) and SMEBR(QQ), respectively. Furthermore, sludge physicochemical and biological characteristics were analyzed in terms of mixed liquor suspended solids (MLSS), sludge volume index (SVI), time to filter (TTF), and mean sludge particle size (PSD). Results showed that the presence of electric field reduced SVI and TTF in the SMEBR(QQ) by 47 and 26%, respectively. Also, the concentration profiles of loosely bound extracellular polymeric substances (LB-EPS) and tightly bound EPS (TB-EPS) showed superiority in the performance of SMEBR(QQ) over SMBR(QQ). Chemical oxygen demand (COD) and nutrient removals were higher in the SMEBR(QQ) reporting 97, 90 and 90% removal of COD, ammonium-N (NH4+-N) and phosphorus-P (PO43-P), respectively. The synergetic effects of QQ and electric field have shown significant improvements of conventional MBRs and could be further developed as an efficient wastewater treatment and water reuse.
1. Introduction Numerous technologies such as conventional activated sludge process (CAS), trickling filters, oxidation ditches, anaerobic digestion, and lagoons have been utilized for wastewater treatment, biomass remediation and bioenergy production. In comparison to those conventional technologies; membrane bioreactors (MBRs) are considered the most advanced and effective having several advantages such as lesser footprints, greater effluent quality, lesser sludge production and energy consumption. Besides many advantages of MBRs, membrane fouling is a critical issue that limits its application into full scale practical function and greatly compromises the efficiency of treatment process. Best indicators of membrane fouling are the transmembrane pressure (TMP) and permeate flux. Membrane fouling causes a significant increase in the hydraulic resistance; demonstrated as permeate flux decline or TMP increase when the process is operated under constant-TMP or constantflux conditions, respectively. Many research studies have been conducted to reduce membrane fouling. These included the use of different adsorbents and moving media, backwashing and periodic relaxation,
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however, the outcomes revealed slight improvements towards the reduction of membrane fouling. More specifically, membrane biofouling which refers to the blockage of membrane pores during filtration due to growth of biofilm on the surface of the membrane is a major challenge hindering the energy-efficient operation and maintenance of MBRs [1,2]. Bacteria behave in a linked manner by cell to cell interaction and communicate or connect with each other by signal molecules called autoinducers. The term quorum sensing (QS) is used to illustrate an environmental system through which bacteria syncronizes their physilogical behavior dependently. This occurs via the intracellular synthesis of the small-molecule signal autoinducers which are actively exchanged with the surrounding environment. It should be noted that the accumulation of signals is directly proportional to the bacteria population. The binding between the signals and the cognate receprtors happens at the threshold concentration of the signals leading to aletration in the bacterial population gene experession [3]. Quroum quenching (QQ) is a process of QS inhebition through which autoinducers QS are disrupted to prevent bacteria gene expression and therefore slowing down their
Corresponding authors. Corresponding author. E-mail addresses: [email protected] (M. Khan), [email protected] (S.J. Khan), [email protected] (S.W. Hasan).
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https://doi.org/10.1016/j.biombioe.2019.105329 Received 7 June 2019; Received in revised form 30 July 2019; Accepted 1 August 2019 Available online 07 August 2019 0961-9534/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematics of lab scale SMEBR(QQ) and SMBR(QQ) set-ups.
less membrane biofouling as compared to the condition in which microporous membrane was placed in a separated bio-tank with recirculated sludge. Kim et al. [8] investigated the impact of QQ on microbial dynamics in MBR. They observed that QQ bacteria reduced the auto inducer producing microbial species which ultimately reduced the EPS production and resulted in less biofouling. On the other hand, advanced and hybrid wastewater treatment technologies were recently developed [9–11]. For example, submerged membrane electro-bioreactor (SMEBR) has shown to be effective in terms of water quality, membrane fouling and sludge characteristics. The incorporation of electric field in MBRs reported significant changes on the physicochemical and biological characteristics of the sludge leading to better system performance and less membrane fouling when compared to existing conventional wastewater treatment technologies such as MBRs [12–15]. Nonetheless, the SMEBR continuous development and system improvement have been attracting several researchers recently. To the best of authors’ knowledge; the incorporation of QQ method in SMEBR was never reported in literature. Consequently, the main objective of this research study was to investigate the impact of incorporating QQ bacteria in the SMEBR and monitor the changes in the rate of TMP increase, sludge settleability and filterability.
activivties. Several signal molecules in bacteria communitites were identified. These include modified oligopeptides or autoinducing peptides (AIP) generally employed by Gram-positive bacteria, acylhomoserine lactones (AHLs) produced by Gram-negative bacteria, and autoinducer-2 (AI-2) used by both Gram-negative and Gram-positive bacteria for interspecies. AHLs based QS is the most common in gram negative bacteria in wastewater where more than twenty five species were identified, and more than half dozens of QS types have been described in bacteria [4]. Microorganisms release gooey gel-like sticky materials called extracellular polymeric substances (EPS) and soluble microbial products (SMP) which serve as a proper medium for biofilm creation [2]. EPS and SMP composed of different compunds such as proteins, polysaccharides, nucleic acid and humic acid. SMP were found to form a gel film on the surface of membranes, whereas EPS reported a strong capacity to form a hydrated gel matrix. Both EPS and SMP are known to provide nutrients to microbial communities in the activated sludge matrix thus facilitating membrane biofouling in MBRs. EPS consists of several compounds such as SMP, loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) [3,4]. Novel QQ strategies were recently adapted and several research studies were carried out in order to control membrane biofouling in MBRs for wastewater treatment. For example, Yeon [5] deactivated the AHLs by hydrolyzing at lactone ring by lactonase and at acyl-amide linkage by acylase. They used porcine kidney enzymes I (EC 3.5.1.14) and reported a reduction in the AHLs concentration, EPS production and delayed TMP increase in the MBR which had acylase. Also, Oh et al. [6] investigated the isolation of QQ bacteria which produce QQ enzymes and reported four species out of which Rhodococcus and Panibaccilus stains were found to be more effective. They encapsulated the Rhodococcus sp.BH4 in micro-porous MBR through which they reported a significant reduction in the rate of TMP increase when compared to the rate of TMP increase in the control MBR which had no QQ bacteria. Jahangir et al. [7] encapsulated QQ bacteria and their results revealed
2. Materials and methods 2.1. Preparation of synthetic wastewater and SMEBR(QQ) bioreactor design Synthetic wastewater was prepared and used in this study. The chemicals used with respective concentrations include C6H12O6 (100 g m−3), NH4Cl (382 g m−3), KH2PO4 (47.7 g m−3), CaCl2 (9.7 g m−3), MgSO4.7H2O (9.7 g m−3), FeCl3 (1.0 g m−3), and NaHCO3 (120 g m−3). The concentrations of chemical oxygen demand (COD), ammonium-N (NH4+-N) and phosphorus-P (PO43--P) in the feed synthetic wastewater was 1000 ± 100, 80 ± 5 and 5 ± 0.5 g m−3, 2
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contributed to the increase in MLSS in the SMEBR(QQ) [14]. It was also observed that the rate of MLVSS increase over 40 d of operation was higher when compared to the rate of MLVSS increase in the SMBR(QQ). This trend was expected due to the positive stimulation impact of electric field, reported in the same range of the applied current density in this research study (i.e. 12 A m−2), on the growth of microorganisms [12,16,19–21]. It was also correlated to the drop in the mixed liquor dissolved oxygen (DO) reported after 40 d of operation. However, MLVSS concentration showed a slight drop after 40 d in the SMEBR (QQ) which could be attributed to the substrate bioavailability necessary for the microbial growth as reported in Giwa et al. [13]. MLVSS:MLSS ratio of 0.76 and 0.64 were reported in the SMBR(QQ) and SMEBR(QQ), respectively.
respectively; maintaining a COD: N: P of 100:10:1. Fresh activated sludge having an MLSS of 3700 ± 1100 g m−3 was collected from a full scale MBR plant at the National University of Sciences and Technology (NUST) in Islamabad (Pakistan) and acclimatized prior to experimentation. The SMEBR, containing 0.5 wt% hollow cylindrical quorum beads (HC QQ) having Rhodococcus sp. (BH4) specie, had an effective volume of 0.019 m3 and operated at a permeate flux of 0.02 m3 m−2 h−1 Fig. 1. Hydraulic retention time (HRT) and sludge retention time (SRT) of 13.5 h and 20 d were adjusted, respectively. The out-in filtration mode was operated at an optimized 8-min filtration and 2-min relaxation. A hollow fiber polyvinyl difluoride (PVDF) microfiltration (MF) membrane (Mitsubishi Rayon Engineering Co. Ltd., Japan) having a pore size of 0.05 μm, effective filtration area of 0.07 m2, and a maximum TMP of 30 kPa was submerged in the bioreactor and used in this research study. Aluminum anode and stainlesssteel cathode were used as electrodes aligned in an anode-cathodemembrane (A-C-M) configuration. Both electrodes were connected to a direct current (DC) power supply through which the bioreactor operated at a constant current density of 12 A m−2 which was selected according to literature [14,16]. An electric timer was used to supply electric field in intermittent mode of 5 min ON: 30 min OFF. This mode was selected as reported in other research study [14]. Aeration was supplied in the bioreactor at a flow rate of 0.13 × 10−3 m3 s−1 and distributed uniformly through fine bubble air diffusers located at the bottom of the bioreactor. Coarse aeration was also ensured for membrane scouring. The level of water in the bioreactor was controlled through water level sensors. A control submerged membrane bioreactor (SMBR(QQ)), having no electrodes, was operated in parallel for comparative analysis.
3.2. Variation of SVI, TTF and PSD in SMEBR(QQ) and SMBR(QQ) The presence of electro-kinetics has an impact on the quality and the volumes of the waste sludge which will affect both settleablity and filterability. The changes in the sludge physico-chemical properties in the SMEBR(QQ) and SMBR(QQ) were monitored via measuring the SVI, TTF and PSD. Fig. 3 (inset) shows that the SVI of 95 and 77 mL g−1 was reported in SMBR(QQ) and SMEBR(QQ), respectively. It was evident that the incorporation of electric field along with QQ bacteria in the SMEBR(QQ) had improved the sludge settleability via producing denser flocs when compared to the flocs generated in the SMBR(QQ). A further reduction in SVI in the SMEBR(QQ) to 53 mL g−1 was reported after 60 d. This was also confirmed by the significant reduction in the TTF (Fig. 3) in the SMEBR(QQ) when compared to the sludge dewatering ability in the SMBR(QQ) (i.e., from 108 to 80 s, and from 130 to 87 s, respectively). The significant improvement in the sludge settleability and filterability was due to the presence of electro-osmosis phenomenon in the SMEBR(QQ) that resulted in producing denser flocs with less bound water as reported and thoroughly discussed in other research studies [13,14,22]. Furthermore, the results of the PSD (shown in Fig. 4) in the SMEBR(QQ) revealed the interaction between electrocoagulation and electro-osmosis processes. For example, the PSD was initially increased to 11.3 μm in the first 15th d, yet has decreased to 4.2 μm after 50 d of operation. This suggests that electro-coagulation was predominant in the first two weeks of operation, whereas electroosmosis took over coagulation and flocculation thereafter. The decrease in the mean sludge particle diameter provided even better sludge management opportunities, as it contributed to the reduction of the overall sludge volume. These results were in line with other studies reported in literature [13,22] in which they observed a PSD increase in the first 19th d followed by a continuous reduction until the end of operation.
2.2. Sampling and analytical methods Fresh activated sludge samples were collected from SMEBR(QQ) and SMBR(QQ) and immediately characterized for MLSS, mixed liquor volatile suspended solids (MLVSS), sludge volume index (SVI), mean particle size diameter (PSD), time to filter (TTF), and EPS. MLSS, MLVSS, COD, NH4+-N and PO43--P analyses followed standard methods [17]. SVI was measure via allowing 0.001 m3 of sludge to settle for 35 min while PSD was measured using the laser particle size distribution analyzer (Horiba Scientific, Japan). Sludge samples were filtered through Whatman fritted glass filtration assembly to determine TTF as reported in Ref. [17]. EPS from MBRs sludge was extracted according to the method developed by Froelund et al. [18] with some modification using Dowex cations exchange resins (Sigma-aldrich) for which 0.05 × 10−3 m3 activated sample was collected from each MBR and centrifuged using refrigerated centrifuge (K2015R, Pro-Reseearch, Britain) at 4000 g and 4 °C for 15 min. Supernatant having soluble EPS (i.e. SMP) was separated from the biomass pellet having bound EPS. For LB-EPS extraction, biomass pellets were suspended in phosphoric buffer solution, stirred on magnetic stirrer for 1 h, centrifuged at 4000 g and 4 °C for 15 min, and lastly supernatant was removed for LB-EPS analysis. Supernatant containing TB-EPS was extracted by re-suspending the sludge pellet in the buffer solution while adding Dowex cations exchange resins followed by a 1 h stirring and centrifugation (see Fig. 1).
3.3. Impact on SMP, LB-EPS and TB-EPS The negatively charged EPS are released during cell lyses or lost during syntheses. EPS act as a platform and provide an environment for microorganism to agglomerate by polymer tangle on surface of membrane. EPS have a crucial role within bacterial consortia and play a key role in the physico-chemical characteristics of sludge flocs. EPS consists of several compounds such as SMP, loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS). Cell-bound EPS is divided into TB-EPS and LB-EPS which are found in the inner and the outer surface of the cell, respectively. Results shown in Fig. 5 revealed that the concentration of SMP were nearly similar towards the end of operation (Fig. 5a). Also, the concentration profiles of LB-EPS and TB-EPS showed superiority in the performance of SMEBR(QQ) over SMBR(QQ). Radaideh et al. [23] investigated that the floc size had significant impact on sludge dewaterability as it affects the total particle surface area. It was reported by Lin et al. [24] that small flocs excrete more EPS than agglomerated flocs. The results shown in Fig. 5b were in line with such findings through which the LB-EPS profile in SMBR(QQ) reported less
3. Results and discussion 3.1. Variation of MLSS:MLVSS ratio in SMEBR(QQ) and SMBR(QQ) Fig. 2 shows the concentration profile of MLSS and MLVSS in SMEBR(QQ) and SMBR(QQ) over tested duration. Results showed that MLSS increased from 3700 to 8400 g m−3, and from 3300 to 5100 g m−3 in SMEBR(QQ) and SMBR(QQ), respectively. The chemical sludge which resulted from the electro-oxidation of the sacrificial anode and the generation of the aluminum coagulants in the bulk solution has 3
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Fig. 2. Variation of MLSS and MLVSS (inset) in SMEBR(QQ) and SMBR(QQ).
Fig. 3. TTF and SVI (inset) profiles in SMEBR(QQ) and SMBR(QQ).
concentrations when compared to those reported in the SMEBR(QQ). However, the impact of electric field in enhancing the extraction of the bound water from the inner surfaces of the sludge flocs was evident as shown in the concentration profile of the TB-EPS reported in Fig. 5c. The removal of EPS, in general, was promoted through which the positively charged aluminum coagulants have neutralized the negatively charged EPS. Those results agreed with the TMP profiles (shown in Fig. 6) reporting less membrane (bio)fouling in SMEBR(QQ) when compared to SMBR(QQ).
3.4. TMP profile in SMEBR(QQ) and SMBR(QQ) Fig. 6 shows the TMP profiles reported in SMEBR(QQ) and SMBR (QQ). It was observed that the rate of pressure increase (i.e. dTMP dt−1) was 0.7 and 0.5 kPa d−1 in SMBR(QQ) and SMEBR(QQ), respectively.
Fig. 4. Variation of PSD in SMEBR(QQ) and SMBR(QQ).
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Fig. 5. Profiles of a) SMP b) LB-EPS and c) TB-EPS concentrations in SMEBR(QQ) and SMBR(QQ).
Fig. 6. TMP profile in SMEBR(QQ) and SMBR(QQ).
The incorporation of electric field further delayed the TMP increase in the SMEBR(QQ) by 15 d. The increase in TMP in the SMBR was observed after 50 d of operation. Previous studies showed the effectiveness of adding QQ bacteria in reducing membrane fouling in
conventional SMBRs. For instance, Pervez et al. [25] reported a TMP increase in a conventional SMBR after 12 d of operation whereas the addition of the QQ bacteria extended the lifetime of the membrane reporting a TMP increase only after 47 d. The results reported here were 5
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Fig. 7. Histographs of (a) COD, (b) NH4+-N, and (c) PO43--P removal in SMEBR(QQ) and SMBR(QQ).
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efforts presented in this research study will open the doors towards future SMEBR(QQ) developments and system scale up for potential implementation within existing or future wastewater plant. The interaction between the QQ biomass and electric field can be another interesting research topic in the near future.
in agreement with the EPS results shown earlier in Fig. 5c through which electro-kinetics contributed significantly in producing smaller flocs yet with less bound water which were hindered from depositing onto the surface of the membrane. Also, it could be deduced that the QQ bacteria required less time of acclimation in the presence of electric field [14,16] which was also confirmed by the high removal of organics and nutrients as in Fig. 7.
Acknowledgement The authors appreciate the financial support provided by the MS research grant of the National University of Sciences and Technology, Islamabad, Pakistan.
3.5. Water quality and removal of COD, NH4+-N and PO43--P Fig. 7 shows the histographs of COD, NH4+-N and PO43--P over tested operation. Initial fluctuations in the data were due to the bacterial acclimation in both reactors, yet stability in the results started to appear after 40 d. Fig. 7a shows an average COD removal in the range of 93–94% and 95–97% reported in the SMBR(QQ) and SMEBR(QQ), respectively. Furthermore, Fig. 7b shows an average NH4+-N removal of 85 and 90% reported in the SMBR(QQ) and SMEBR(QQ), respectively. Interestingly, the SMEBR(QQ) showed superiority in PO43--P removal reporting more than 90% when compared to 64% removal obtained in the SMBR(QQ). In SMEBR(QQ), electro-kinetics enhanced the interaction between organic pollutants and nutrients (i.e. NH4+-N and PO43--P) with the electrically generated aluminum coagulants. The electro-oxidation of anode and water electro-reduction at the cathode contributed to the formation and precipitation of AlPO4 and Al(OH)3 (Eq. (1)) in the bulk solution leading to higher rates of adsorption PO43-P onto the surface of the flocs [26]. Furthermore, the generation of H2 and O2 gases near the surfaces of the cathode and anode, respectively through which continuous oxidation of COD and NH4+-N could enhance the removal efficiency. The enhancement of COD, NH4+-N and PO43--P removal in electrically enhanced MBRs was further investigated by ElNaker et al. [19–21] who carried out a functional microbial characterization under electric field. Their results revealed that certain functional bacterial communities such as Nitrospira, Nitrospiraceae, Dechloromonas sp., Rhodocyclaceae and Rhodobacteraceae were increased in the presence of electric field indicating the higher removal of nutrients and COD in the SMEBR(QQ). Al3+ + HnPO43−n ↔ ALPO4 + nH+
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(1)
The impact of synergetic impacts of QQ and electric field in SMEBR (QQ) has shown better system performance and less maintenance when compared to SMBR(QQ). The results obtained in this study were comparable with those reported in literature. For example, Naghizadeh et al. [27] reported > 96% of COD using a hollow fiber MBR while Khazaei et al. [28] reported 50, 54, 63, and 68% for total Kjeldahl nitrogen, total phosphorous, TSS, and COD, respectively using a horizontal roughing filter. Furthermore, Mesdaghinia et al. [29] reported 85.7% COD removal using upflow sludge blanket filtration system. 4. Conclusion This research study aimed at investigating a novel hybrid system that consisted of a recently developed SMEBR with the QQ embedded within the system. The new SMEBR(QQ) hybrid system is another level of advancement to the SMEBR technology. Nonetheless, the incorporation of QQ was extensively demonstrated especially in the area of QS fouling control. The introduction of electric current along with HC QQ beads with entrapped Rhodococcus sp. enhanced the sludge and microbial performance by minimizing nutrient concentrations and improving sludge settleability and dewaterability. The addition of electric field in the SMEBR(QQ) also reduced membrane biofouling reporting a fouling rate 1.36 times lesser than SMBR(QQ). It could be depicted from these results that the electric current could have activated the QQ bacteria, similar to those families investigated by ElNaker et al., which in conjunction to electro-kinetics improved water effluent quality, enhanced sludge physico-chemical and biological characteristics. The 7
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[26] Ö.K. Apaydin, An investigation on the treatment of tannery wastewater by electrocoagulation, Global NEST J 11 (2009) 546–555. [27] A. Naghizadeh, A.H. Mahvi, A.R. Mesdaghinia, M. Alimohammadi, Application of MBR technology in municipal wastewater treatment, Arabian J. Sci. Eng. 36 (1) (2011) 3–10. [28] M. Khazaei, R. Nabizadeh, A.H. Mahvi, H. Izanloo, R.A. Tadi, F. Gharagazloo, Nitrogen and Phosphorous Removal from Aerated Lagoon Effluent Using Horizontal Roughing Filter (HRF), Desalin Water Treat, 2015, pp. 1–10. [29] A.R. Mesdaghinia, A.H. Mahvi, R. Saeedi, H. Pishrafti, Upflow sludge blanket filtration (USBF): an innovative technology in activated sludge process, Iran. J. Public Health 39 (2) (2010) 7–12.
[22] S. Ibeid, M. Elektorowicz, J.A. Oleszkiewicz, Novel electrokinetic approach reduces membrane fouling, Water Res. 47 (16) (2013) 6358–6366. [23] J.A. Radaideh, B.Y. Ammary, K.K. Al-Zboon, Dewaterability of sludge digested in extended aeration plants using conventional sand drying beds, Afr. J. Biotechnol. 9 (29) (2010) 4578–4583. [24] H.J. Lin, W.J. Gao, K.T. Leung, B.Q. Liao, Characteristics of different fractions of microbial flocs and their role in membrane fouling, Water Sci. Technol. 63 (2) (2011) 262–269. [25] S. Pervez, S.J. Khan, H. Waheed, I. Hashmi, C.H. Lee, Impact of quorum quenching bacteria on biofouling retardation in submerged membrane bioreactor (SMBR), Membr. Water Treat. 9 (4) (2018) 279–284.
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Biomass and Bioenergy journal homepage: www.elsevier.com/locate/biombioe
Research paper
Quorum sensing control and wastewater treatment in quorum quenching/ submerged membrane electro-bioreactor (SMEBR(QQ)) hybrid system
T
Maham Khana, Sher J. Khana,**, Shadi W. Hasanb,* a Institute of Environmental Sciences and Engineering, School of Civil and Environmental Engineering, National University of Sciences and Technology (NUST), Islamabad, Pakistan b Center for Membrane and Advanced Water Technology (CMAT), Department of Chemical Engineering, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
A R T I C LE I N FO
A B S T R A C T
Keywords: Submerged membrane electro-bioreactor (SMEBR) Quorum sensing Quorum quenching Electrokinetics Wastewater
Membrane biofouling, in terms of quorum sensing (QS), reduction with hollow cylindrical quorum quenching (QQ) beads in submerged membrane bioreactor (SMBR(QQ)) and submerged membrane electro-bioreactor (SMEBR(QQ)) was investigated. Results showed that the rate of pressure increase (i.e. dTMP dt−1) was 0.7 and 0.5 kPa d−1 in SMBR(QQ) and SMEBR(QQ), respectively. Furthermore, sludge physicochemical and biological characteristics were analyzed in terms of mixed liquor suspended solids (MLSS), sludge volume index (SVI), time to filter (TTF), and mean sludge particle size (PSD). Results showed that the presence of electric field reduced SVI and TTF in the SMEBR(QQ) by 47 and 26%, respectively. Also, the concentration profiles of loosely bound extracellular polymeric substances (LB-EPS) and tightly bound EPS (TB-EPS) showed superiority in the performance of SMEBR(QQ) over SMBR(QQ). Chemical oxygen demand (COD) and nutrient removals were higher in the SMEBR(QQ) reporting 97, 90 and 90% removal of COD, ammonium-N (NH4+-N) and phosphorus-P (PO43-P), respectively. The synergetic effects of QQ and electric field have shown significant improvements of conventional MBRs and could be further developed as an efficient wastewater treatment and water reuse.
1. Introduction Numerous technologies such as conventional activated sludge process (CAS), trickling filters, oxidation ditches, anaerobic digestion, and lagoons have been utilized for wastewater treatment, biomass remediation and bioenergy production. In comparison to those conventional technologies; membrane bioreactors (MBRs) are considered the most advanced and effective having several advantages such as lesser footprints, greater effluent quality, lesser sludge production and energy consumption. Besides many advantages of MBRs, membrane fouling is a critical issue that limits its application into full scale practical function and greatly compromises the efficiency of treatment process. Best indicators of membrane fouling are the transmembrane pressure (TMP) and permeate flux. Membrane fouling causes a significant increase in the hydraulic resistance; demonstrated as permeate flux decline or TMP increase when the process is operated under constant-TMP or constantflux conditions, respectively. Many research studies have been conducted to reduce membrane fouling. These included the use of different adsorbents and moving media, backwashing and periodic relaxation,
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however, the outcomes revealed slight improvements towards the reduction of membrane fouling. More specifically, membrane biofouling which refers to the blockage of membrane pores during filtration due to growth of biofilm on the surface of the membrane is a major challenge hindering the energy-efficient operation and maintenance of MBRs [1,2]. Bacteria behave in a linked manner by cell to cell interaction and communicate or connect with each other by signal molecules called autoinducers. The term quorum sensing (QS) is used to illustrate an environmental system through which bacteria syncronizes their physilogical behavior dependently. This occurs via the intracellular synthesis of the small-molecule signal autoinducers which are actively exchanged with the surrounding environment. It should be noted that the accumulation of signals is directly proportional to the bacteria population. The binding between the signals and the cognate receprtors happens at the threshold concentration of the signals leading to aletration in the bacterial population gene experession [3]. Quroum quenching (QQ) is a process of QS inhebition through which autoinducers QS are disrupted to prevent bacteria gene expression and therefore slowing down their
Corresponding authors. Corresponding author. E-mail addresses: [email protected] (M. Khan), [email protected] (S.J. Khan), [email protected] (S.W. Hasan).
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https://doi.org/10.1016/j.biombioe.2019.105329 Received 7 June 2019; Received in revised form 30 July 2019; Accepted 1 August 2019 Available online 07 August 2019 0961-9534/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematics of lab scale SMEBR(QQ) and SMBR(QQ) set-ups.
less membrane biofouling as compared to the condition in which microporous membrane was placed in a separated bio-tank with recirculated sludge. Kim et al. [8] investigated the impact of QQ on microbial dynamics in MBR. They observed that QQ bacteria reduced the auto inducer producing microbial species which ultimately reduced the EPS production and resulted in less biofouling. On the other hand, advanced and hybrid wastewater treatment technologies were recently developed [9–11]. For example, submerged membrane electro-bioreactor (SMEBR) has shown to be effective in terms of water quality, membrane fouling and sludge characteristics. The incorporation of electric field in MBRs reported significant changes on the physicochemical and biological characteristics of the sludge leading to better system performance and less membrane fouling when compared to existing conventional wastewater treatment technologies such as MBRs [12–15]. Nonetheless, the SMEBR continuous development and system improvement have been attracting several researchers recently. To the best of authors’ knowledge; the incorporation of QQ method in SMEBR was never reported in literature. Consequently, the main objective of this research study was to investigate the impact of incorporating QQ bacteria in the SMEBR and monitor the changes in the rate of TMP increase, sludge settleability and filterability.
activivties. Several signal molecules in bacteria communitites were identified. These include modified oligopeptides or autoinducing peptides (AIP) generally employed by Gram-positive bacteria, acylhomoserine lactones (AHLs) produced by Gram-negative bacteria, and autoinducer-2 (AI-2) used by both Gram-negative and Gram-positive bacteria for interspecies. AHLs based QS is the most common in gram negative bacteria in wastewater where more than twenty five species were identified, and more than half dozens of QS types have been described in bacteria [4]. Microorganisms release gooey gel-like sticky materials called extracellular polymeric substances (EPS) and soluble microbial products (SMP) which serve as a proper medium for biofilm creation [2]. EPS and SMP composed of different compunds such as proteins, polysaccharides, nucleic acid and humic acid. SMP were found to form a gel film on the surface of membranes, whereas EPS reported a strong capacity to form a hydrated gel matrix. Both EPS and SMP are known to provide nutrients to microbial communities in the activated sludge matrix thus facilitating membrane biofouling in MBRs. EPS consists of several compounds such as SMP, loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) [3,4]. Novel QQ strategies were recently adapted and several research studies were carried out in order to control membrane biofouling in MBRs for wastewater treatment. For example, Yeon [5] deactivated the AHLs by hydrolyzing at lactone ring by lactonase and at acyl-amide linkage by acylase. They used porcine kidney enzymes I (EC 3.5.1.14) and reported a reduction in the AHLs concentration, EPS production and delayed TMP increase in the MBR which had acylase. Also, Oh et al. [6] investigated the isolation of QQ bacteria which produce QQ enzymes and reported four species out of which Rhodococcus and Panibaccilus stains were found to be more effective. They encapsulated the Rhodococcus sp.BH4 in micro-porous MBR through which they reported a significant reduction in the rate of TMP increase when compared to the rate of TMP increase in the control MBR which had no QQ bacteria. Jahangir et al. [7] encapsulated QQ bacteria and their results revealed
2. Materials and methods 2.1. Preparation of synthetic wastewater and SMEBR(QQ) bioreactor design Synthetic wastewater was prepared and used in this study. The chemicals used with respective concentrations include C6H12O6 (100 g m−3), NH4Cl (382 g m−3), KH2PO4 (47.7 g m−3), CaCl2 (9.7 g m−3), MgSO4.7H2O (9.7 g m−3), FeCl3 (1.0 g m−3), and NaHCO3 (120 g m−3). The concentrations of chemical oxygen demand (COD), ammonium-N (NH4+-N) and phosphorus-P (PO43--P) in the feed synthetic wastewater was 1000 ± 100, 80 ± 5 and 5 ± 0.5 g m−3, 2
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contributed to the increase in MLSS in the SMEBR(QQ) [14]. It was also observed that the rate of MLVSS increase over 40 d of operation was higher when compared to the rate of MLVSS increase in the SMBR(QQ). This trend was expected due to the positive stimulation impact of electric field, reported in the same range of the applied current density in this research study (i.e. 12 A m−2), on the growth of microorganisms [12,16,19–21]. It was also correlated to the drop in the mixed liquor dissolved oxygen (DO) reported after 40 d of operation. However, MLVSS concentration showed a slight drop after 40 d in the SMEBR (QQ) which could be attributed to the substrate bioavailability necessary for the microbial growth as reported in Giwa et al. [13]. MLVSS:MLSS ratio of 0.76 and 0.64 were reported in the SMBR(QQ) and SMEBR(QQ), respectively.
respectively; maintaining a COD: N: P of 100:10:1. Fresh activated sludge having an MLSS of 3700 ± 1100 g m−3 was collected from a full scale MBR plant at the National University of Sciences and Technology (NUST) in Islamabad (Pakistan) and acclimatized prior to experimentation. The SMEBR, containing 0.5 wt% hollow cylindrical quorum beads (HC QQ) having Rhodococcus sp. (BH4) specie, had an effective volume of 0.019 m3 and operated at a permeate flux of 0.02 m3 m−2 h−1 Fig. 1. Hydraulic retention time (HRT) and sludge retention time (SRT) of 13.5 h and 20 d were adjusted, respectively. The out-in filtration mode was operated at an optimized 8-min filtration and 2-min relaxation. A hollow fiber polyvinyl difluoride (PVDF) microfiltration (MF) membrane (Mitsubishi Rayon Engineering Co. Ltd., Japan) having a pore size of 0.05 μm, effective filtration area of 0.07 m2, and a maximum TMP of 30 kPa was submerged in the bioreactor and used in this research study. Aluminum anode and stainlesssteel cathode were used as electrodes aligned in an anode-cathodemembrane (A-C-M) configuration. Both electrodes were connected to a direct current (DC) power supply through which the bioreactor operated at a constant current density of 12 A m−2 which was selected according to literature [14,16]. An electric timer was used to supply electric field in intermittent mode of 5 min ON: 30 min OFF. This mode was selected as reported in other research study [14]. Aeration was supplied in the bioreactor at a flow rate of 0.13 × 10−3 m3 s−1 and distributed uniformly through fine bubble air diffusers located at the bottom of the bioreactor. Coarse aeration was also ensured for membrane scouring. The level of water in the bioreactor was controlled through water level sensors. A control submerged membrane bioreactor (SMBR(QQ)), having no electrodes, was operated in parallel for comparative analysis.
3.2. Variation of SVI, TTF and PSD in SMEBR(QQ) and SMBR(QQ) The presence of electro-kinetics has an impact on the quality and the volumes of the waste sludge which will affect both settleablity and filterability. The changes in the sludge physico-chemical properties in the SMEBR(QQ) and SMBR(QQ) were monitored via measuring the SVI, TTF and PSD. Fig. 3 (inset) shows that the SVI of 95 and 77 mL g−1 was reported in SMBR(QQ) and SMEBR(QQ), respectively. It was evident that the incorporation of electric field along with QQ bacteria in the SMEBR(QQ) had improved the sludge settleability via producing denser flocs when compared to the flocs generated in the SMBR(QQ). A further reduction in SVI in the SMEBR(QQ) to 53 mL g−1 was reported after 60 d. This was also confirmed by the significant reduction in the TTF (Fig. 3) in the SMEBR(QQ) when compared to the sludge dewatering ability in the SMBR(QQ) (i.e., from 108 to 80 s, and from 130 to 87 s, respectively). The significant improvement in the sludge settleability and filterability was due to the presence of electro-osmosis phenomenon in the SMEBR(QQ) that resulted in producing denser flocs with less bound water as reported and thoroughly discussed in other research studies [13,14,22]. Furthermore, the results of the PSD (shown in Fig. 4) in the SMEBR(QQ) revealed the interaction between electrocoagulation and electro-osmosis processes. For example, the PSD was initially increased to 11.3 μm in the first 15th d, yet has decreased to 4.2 μm after 50 d of operation. This suggests that electro-coagulation was predominant in the first two weeks of operation, whereas electroosmosis took over coagulation and flocculation thereafter. The decrease in the mean sludge particle diameter provided even better sludge management opportunities, as it contributed to the reduction of the overall sludge volume. These results were in line with other studies reported in literature [13,22] in which they observed a PSD increase in the first 19th d followed by a continuous reduction until the end of operation.
2.2. Sampling and analytical methods Fresh activated sludge samples were collected from SMEBR(QQ) and SMBR(QQ) and immediately characterized for MLSS, mixed liquor volatile suspended solids (MLVSS), sludge volume index (SVI), mean particle size diameter (PSD), time to filter (TTF), and EPS. MLSS, MLVSS, COD, NH4+-N and PO43--P analyses followed standard methods [17]. SVI was measure via allowing 0.001 m3 of sludge to settle for 35 min while PSD was measured using the laser particle size distribution analyzer (Horiba Scientific, Japan). Sludge samples were filtered through Whatman fritted glass filtration assembly to determine TTF as reported in Ref. [17]. EPS from MBRs sludge was extracted according to the method developed by Froelund et al. [18] with some modification using Dowex cations exchange resins (Sigma-aldrich) for which 0.05 × 10−3 m3 activated sample was collected from each MBR and centrifuged using refrigerated centrifuge (K2015R, Pro-Reseearch, Britain) at 4000 g and 4 °C for 15 min. Supernatant having soluble EPS (i.e. SMP) was separated from the biomass pellet having bound EPS. For LB-EPS extraction, biomass pellets were suspended in phosphoric buffer solution, stirred on magnetic stirrer for 1 h, centrifuged at 4000 g and 4 °C for 15 min, and lastly supernatant was removed for LB-EPS analysis. Supernatant containing TB-EPS was extracted by re-suspending the sludge pellet in the buffer solution while adding Dowex cations exchange resins followed by a 1 h stirring and centrifugation (see Fig. 1).
3.3. Impact on SMP, LB-EPS and TB-EPS The negatively charged EPS are released during cell lyses or lost during syntheses. EPS act as a platform and provide an environment for microorganism to agglomerate by polymer tangle on surface of membrane. EPS have a crucial role within bacterial consortia and play a key role in the physico-chemical characteristics of sludge flocs. EPS consists of several compounds such as SMP, loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS). Cell-bound EPS is divided into TB-EPS and LB-EPS which are found in the inner and the outer surface of the cell, respectively. Results shown in Fig. 5 revealed that the concentration of SMP were nearly similar towards the end of operation (Fig. 5a). Also, the concentration profiles of LB-EPS and TB-EPS showed superiority in the performance of SMEBR(QQ) over SMBR(QQ). Radaideh et al. [23] investigated that the floc size had significant impact on sludge dewaterability as it affects the total particle surface area. It was reported by Lin et al. [24] that small flocs excrete more EPS than agglomerated flocs. The results shown in Fig. 5b were in line with such findings through which the LB-EPS profile in SMBR(QQ) reported less
3. Results and discussion 3.1. Variation of MLSS:MLVSS ratio in SMEBR(QQ) and SMBR(QQ) Fig. 2 shows the concentration profile of MLSS and MLVSS in SMEBR(QQ) and SMBR(QQ) over tested duration. Results showed that MLSS increased from 3700 to 8400 g m−3, and from 3300 to 5100 g m−3 in SMEBR(QQ) and SMBR(QQ), respectively. The chemical sludge which resulted from the electro-oxidation of the sacrificial anode and the generation of the aluminum coagulants in the bulk solution has 3
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Fig. 2. Variation of MLSS and MLVSS (inset) in SMEBR(QQ) and SMBR(QQ).
Fig. 3. TTF and SVI (inset) profiles in SMEBR(QQ) and SMBR(QQ).
concentrations when compared to those reported in the SMEBR(QQ). However, the impact of electric field in enhancing the extraction of the bound water from the inner surfaces of the sludge flocs was evident as shown in the concentration profile of the TB-EPS reported in Fig. 5c. The removal of EPS, in general, was promoted through which the positively charged aluminum coagulants have neutralized the negatively charged EPS. Those results agreed with the TMP profiles (shown in Fig. 6) reporting less membrane (bio)fouling in SMEBR(QQ) when compared to SMBR(QQ).
3.4. TMP profile in SMEBR(QQ) and SMBR(QQ) Fig. 6 shows the TMP profiles reported in SMEBR(QQ) and SMBR (QQ). It was observed that the rate of pressure increase (i.e. dTMP dt−1) was 0.7 and 0.5 kPa d−1 in SMBR(QQ) and SMEBR(QQ), respectively.
Fig. 4. Variation of PSD in SMEBR(QQ) and SMBR(QQ).
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Fig. 5. Profiles of a) SMP b) LB-EPS and c) TB-EPS concentrations in SMEBR(QQ) and SMBR(QQ).
Fig. 6. TMP profile in SMEBR(QQ) and SMBR(QQ).
The incorporation of electric field further delayed the TMP increase in the SMEBR(QQ) by 15 d. The increase in TMP in the SMBR was observed after 50 d of operation. Previous studies showed the effectiveness of adding QQ bacteria in reducing membrane fouling in
conventional SMBRs. For instance, Pervez et al. [25] reported a TMP increase in a conventional SMBR after 12 d of operation whereas the addition of the QQ bacteria extended the lifetime of the membrane reporting a TMP increase only after 47 d. The results reported here were 5
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Fig. 7. Histographs of (a) COD, (b) NH4+-N, and (c) PO43--P removal in SMEBR(QQ) and SMBR(QQ).
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efforts presented in this research study will open the doors towards future SMEBR(QQ) developments and system scale up for potential implementation within existing or future wastewater plant. The interaction between the QQ biomass and electric field can be another interesting research topic in the near future.
in agreement with the EPS results shown earlier in Fig. 5c through which electro-kinetics contributed significantly in producing smaller flocs yet with less bound water which were hindered from depositing onto the surface of the membrane. Also, it could be deduced that the QQ bacteria required less time of acclimation in the presence of electric field [14,16] which was also confirmed by the high removal of organics and nutrients as in Fig. 7.
Acknowledgement The authors appreciate the financial support provided by the MS research grant of the National University of Sciences and Technology, Islamabad, Pakistan.
3.5. Water quality and removal of COD, NH4+-N and PO43--P Fig. 7 shows the histographs of COD, NH4+-N and PO43--P over tested operation. Initial fluctuations in the data were due to the bacterial acclimation in both reactors, yet stability in the results started to appear after 40 d. Fig. 7a shows an average COD removal in the range of 93–94% and 95–97% reported in the SMBR(QQ) and SMEBR(QQ), respectively. Furthermore, Fig. 7b shows an average NH4+-N removal of 85 and 90% reported in the SMBR(QQ) and SMEBR(QQ), respectively. Interestingly, the SMEBR(QQ) showed superiority in PO43--P removal reporting more than 90% when compared to 64% removal obtained in the SMBR(QQ). In SMEBR(QQ), electro-kinetics enhanced the interaction between organic pollutants and nutrients (i.e. NH4+-N and PO43--P) with the electrically generated aluminum coagulants. The electro-oxidation of anode and water electro-reduction at the cathode contributed to the formation and precipitation of AlPO4 and Al(OH)3 (Eq. (1)) in the bulk solution leading to higher rates of adsorption PO43-P onto the surface of the flocs [26]. Furthermore, the generation of H2 and O2 gases near the surfaces of the cathode and anode, respectively through which continuous oxidation of COD and NH4+-N could enhance the removal efficiency. The enhancement of COD, NH4+-N and PO43--P removal in electrically enhanced MBRs was further investigated by ElNaker et al. [19–21] who carried out a functional microbial characterization under electric field. Their results revealed that certain functional bacterial communities such as Nitrospira, Nitrospiraceae, Dechloromonas sp., Rhodocyclaceae and Rhodobacteraceae were increased in the presence of electric field indicating the higher removal of nutrients and COD in the SMEBR(QQ). Al3+ + HnPO43−n ↔ ALPO4 + nH+
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The impact of synergetic impacts of QQ and electric field in SMEBR (QQ) has shown better system performance and less maintenance when compared to SMBR(QQ). The results obtained in this study were comparable with those reported in literature. For example, Naghizadeh et al. [27] reported > 96% of COD using a hollow fiber MBR while Khazaei et al. [28] reported 50, 54, 63, and 68% for total Kjeldahl nitrogen, total phosphorous, TSS, and COD, respectively using a horizontal roughing filter. Furthermore, Mesdaghinia et al. [29] reported 85.7% COD removal using upflow sludge blanket filtration system. 4. Conclusion This research study aimed at investigating a novel hybrid system that consisted of a recently developed SMEBR with the QQ embedded within the system. The new SMEBR(QQ) hybrid system is another level of advancement to the SMEBR technology. Nonetheless, the incorporation of QQ was extensively demonstrated especially in the area of QS fouling control. The introduction of electric current along with HC QQ beads with entrapped Rhodococcus sp. enhanced the sludge and microbial performance by minimizing nutrient concentrations and improving sludge settleability and dewaterability. The addition of electric field in the SMEBR(QQ) also reduced membrane biofouling reporting a fouling rate 1.36 times lesser than SMBR(QQ). It could be depicted from these results that the electric current could have activated the QQ bacteria, similar to those families investigated by ElNaker et al., which in conjunction to electro-kinetics improved water effluent quality, enhanced sludge physico-chemical and biological characteristics. The 7
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[26] Ö.K. Apaydin, An investigation on the treatment of tannery wastewater by electrocoagulation, Global NEST J 11 (2009) 546–555. [27] A. Naghizadeh, A.H. Mahvi, A.R. Mesdaghinia, M. Alimohammadi, Application of MBR technology in municipal wastewater treatment, Arabian J. Sci. Eng. 36 (1) (2011) 3–10. [28] M. Khazaei, R. Nabizadeh, A.H. Mahvi, H. Izanloo, R.A. Tadi, F. Gharagazloo, Nitrogen and Phosphorous Removal from Aerated Lagoon Effluent Using Horizontal Roughing Filter (HRF), Desalin Water Treat, 2015, pp. 1–10. [29] A.R. Mesdaghinia, A.H. Mahvi, R. Saeedi, H. Pishrafti, Upflow sludge blanket filtration (USBF): an innovative technology in activated sludge process, Iran. J. Public Health 39 (2) (2010) 7–12.
[22] S. Ibeid, M. Elektorowicz, J.A. Oleszkiewicz, Novel electrokinetic approach reduces membrane fouling, Water Res. 47 (16) (2013) 6358–6366. [23] J.A. Radaideh, B.Y. Ammary, K.K. Al-Zboon, Dewaterability of sludge digested in extended aeration plants using conventional sand drying beds, Afr. J. Biotechnol. 9 (29) (2010) 4578–4583. [24] H.J. Lin, W.J. Gao, K.T. Leung, B.Q. Liao, Characteristics of different fractions of microbial flocs and their role in membrane fouling, Water Sci. Technol. 63 (2) (2011) 262–269. [25] S. Pervez, S.J. Khan, H. Waheed, I. Hashmi, C.H. Lee, Impact of quorum quenching bacteria on biofouling retardation in submerged membrane bioreactor (SMBR), Membr. Water Treat. 9 (4) (2018) 279–284.
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