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Nov 22, 2018 - Fouling Mitigation and Wastewater Treatment ... Electrochemical processes such as electrocoagulation, electro-osmosis and electrophoresis.

membranes Article

Fouling Mitigation and Wastewater Treatment Enhancement through the Application of an Electro Moving Bed Membrane Bioreactor (eMB-MBR) Jessa Marie J. Millanar-Marfa 1 , Laura Borea 2, *, Mark Daniel G. de Luna 1,3 , Florencio C. Ballesteros Jr. 1,3 , Vincenzo Belgiorno 2 and Vincenzo Naddeo 2 1

2 3

*

Environmental Engineering Program, National Graduate School of Engineering, University of the Philippines, 1101 Diliman, Quezon City, Philippines; [email protected] (J.M.J.M.-M.); [email protected] (M.D.G.d.L.); [email protected] (F.C.B.J.) Sanitary Environmental Engineering Division (SEED), Department of Civil Engineering, University of Salerno, 84084 Fisciano, Italy; [email protected] (V.B.); [email protected] (V.N.) Department of Chemical Engineering, University of the Philippines, 1101 Diliman, Quezon City, Philippines Correspondence: [email protected]; Tel.: +39-089-969-301

Received: 15 October 2018; Accepted: 16 November 2018; Published: 22 November 2018

 

Abstract: High operational cost due to membrane fouling propensity remains a major drawback for the widespread application of membrane bioreactor (MBR) technology. As a result, studies on membrane fouling mitigation through the application of integrated processes have been widely explored. In this work, the combined application of electrochemical processes and moving bed biofilm reactor (MBBR) technology within an MBR at laboratory scale was performed by applying an intermittent voltage of 3 V/cm to a reactor filled with 30% carriers. The treatment efficiency of the electro moving bed membrane bioreactor (eMB-MBR) technology in terms of ammonium nitrogen (NH4 -N) and orthophosphate (PO4 -P) removal significantly improved from 49.8% and 76.7% in the moving bed membrane bioreactor (MB-MBR) control system to 55% and 98.7% in the eMB-MBR, respectively. Additionally, concentrations of known fouling precursors and membrane fouling rate were noticeably lower in the eMB-MBR system as compared to the control system. Hence, this study successfully demonstrated an innovative and effective technology (i.e., eMB-MBR) to improve MBR performance in terms of both conventional contaminant removal and fouling mitigation. Keywords: electrochemical processes; fouling precursors; moving bed biofilm reactor; MBBR; voltage gradient

1. Introduction Membrane bioreactor (MBR) technology which involves the integration of membrane filtration with biological treatment has been widely investigated and deemed as a promising alternative to conventional wastewater treatment due to its numerous advantages [1–3]. However, the hydrophobic nature of commonly manufactured membranes makes it prone to membrane fouling which negatively impacts MBR efficiency [4] and increases operational and maintenance (O&M) costs. Approximately 34% of O&M costs are attributed to the energy requirement, mainly for pumping and the aeration of MBR for fouling mitigation, and roughly 28% of the costs are attributed to membrane replacement [2,5]. Because of the above, an effective process for fouling mitigation in MBR technology is necessary to expand its applications [6,7]. Numerous methods have been explored by research studies to address membrane fouling in MBRs. Some of these include membrane surface modification [8], the addition of adsorbents and Membranes 2018, 8, 116; doi:10.3390/membranes8040116

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coagulants [9–11], the addition of bio-carriers [12], and the application of an electric field and the application of ultrasound [7,13–15]. Among these, the application of an electric field to MBR reactors [4,16–18] and the addition of carriers to mimic a moving bed biofilm reactor (MBBR) [19–22] have gained increasing interest for membrane fouling control and wastewater treatment performance enhancement since these methods do not involve the addition of chemicals that may alter the activity inside the bioreactor and can be easily done and controlled in situ. Electrochemical processes such as electrocoagulation, electro-osmosis and electrophoresis arising from electric field application in MBRs have been found effective in fouling mitigation and the enhancement of nutrient and contaminant removal from wastewater [4,23,24]. A study by Ibeid et al. [25] found that the application of an electric field on a pilot scale MBR could reduce the fouling rate up to x3-fold, whereas Hua et al. [4] observed a 7.8-fold reduction in the membrane fouling rate. Another study by Liu et al. [13] observed an increase in the filtration cycle from 11 days to 24 days when 1 mA current was applied. In terms of effluent quality, Bani-Melhem and Elektorowicz [26] noted an improvement in the chemical oxygen demand (COD) and PO4 -P removal of 96% and 98%, respectively, upon intermittent electric field application, whereas Tafti et al. [27] noted 4% and 43% increases in COD and phosphate removals, respectively. The reduction of membrane fouling precursors, the improvement of sludge settleability and filterability, the limitation of growth of filamentous bacteria, and the promotion of conditions that favor nutrient and organic contaminant removal are some of the observed effects of electric field application that contribute to its better performance compared to a conventional MBR [4,9,23,28]. On the other hand, the moving bed membrane bioreactor (MB-MBR) has improved the treatment efficiency of the MBR by providing a higher surface area where growth and diversity of microorganisms are favored [19,21]. Furthermore, the addition of media in the form of carriers has promoted nutrient removal by allowing both aerobic, anoxic, and anaerobic conditions to spontaneously occur inside the carrier [29]. Mannina et al. [22] obtained high COD, nitrogen, and phosphorus removals of 98%, 53–69%, and 67–87%, respectively, for an MB-MBR system with C/N ratio of 10 and 5, whereas Subtil et al. [30] obtained 98% and 73% removal of ammonia and total nitrogen, respectively. Furthermore, a decrease of 6% in the membrane fouling rate was also noted in their study through the reduction of suspended solids in the reactor and the reduction of solids accumulation on the membrane surface by the attachment of biomass to the added carriers [12]. A review by Kawan et al. [21] noted that approximately 90% of solids in the reactor could be attached to the carrier which led to a decrease in membrane fouling and promoted a better environment for microbial growth [20,21,31]. With the known advantages of electric field application and the MBBR process, this work aimed to propose an innovative technology for fouling mitigation and treatment performance improvement of MBR through the integration of electric field application within an MBBR into an MBR at laboratory scale. The performance of this new technology, called electro moving bed membrane bioreactor or eMB-MBR, will be evaluated and compared to the MB-MBR control system. 2. Materials and Methods 2.1. Reactor and Materials The experimental setup used in this study is shown in Figure 1. The reactor worked in two successive runs, each lasting for approximately 30 days: in the first run as an MB-MBR reactor and in the second run as an eMB-MBR reactor with the application of electrochemical processes. The membrane module used for all the runs was the ZeeWeed® -1 (ZW-1, Zenon Europe Kft, Oroszlany, Hungary) submerged polyvinylidene flouride (PVDF) hollow fiber ultrafiltration membrane module with a nominal pore diameter of 0.04 µm and an effective membrane surface area of 0.047 m2 placed in the center of the reactor. The MB-MBR setup was obtained by using a cylindrical reactor with a working volume of 13 L filled with BIOMASTER BCN 012 KLS Amitec® carriers (Amitec, Cernusco sul Naviglio, Italy), with a filling ratio (FR) of 30% and a net surface area of 500 m2 /m3 . This setup was

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sul Naviglio, Italy), with a filling ratio (FR) of 30% and a net surface area of 500 m2/m3. This setup referred to as to theascontrol system. On the hand,hand, the eMB-MBR setupsetup usedused the same reactor and was referred the control system. Onother the other the eMB-MBR the same reactor membrane. A cylindrical aluminum anode anode and a stainless steel cathode connected to a digital and membrane. A cylindrical aluminum and a stainless steel cathode connected to aexternal digital DC power TTi, 0–60TTi, V, 0–20 placed the membrane modulemodule with a external DCsupply power(CPX400, supply (CPX400, 0–60 A) V, were 0–20 A) werearound placed around the membrane radial of 6 cm from eachfrom other. Dissolved oxygen (DO) concentration inside the bioreactor with adistance radial distance of 6 cm each other. Dissolved oxygen (DO) concentration inside the was maintained using aerators placed at the bottom. Good mixing andmixing slight and air scouring also bioreactor was maintained using aerators placed at the bottom. Good slight airwere scouring promoted the installed aeration system, as issystem, shownas in is Figure 1. in Synthetic were also by promoted by the installed aeration shown Figure municipal 1. Syntheticwastewater municipal with characteristics described in described Borea et al.in[32] was fed to both bioreactors treated wastewater with characteristics Borea etcontinuously al. [32] was continuously fed to bothand bioreactors water was pumped at apumped constant of 15 LMH. liquor volatile suspended solids (MLVSS) and treated water was atflux a constant flux ofMixed 15 LMH. Mixed liquor volatile suspended solids during theduring experiment were 2700 were to 4000 mg/L, whereas hydraulic retention time (HRT) used was (MLVSS) the experiment 2700 to 4000 mg/L,the whereas the hydraulic retention time (HRT) 18 h and (SRT)time was (SRT) approximately 40 days. 40 days. used wasthe 18 solids h and retention the solidstime retention was approximately The The control control system system (MB-MBR) (MB-MBR) was was operated operated with with the the electrodes electrodes disconnected disconnected from the the power power supply, supply,whereas whereasthe theeMB-MBR eMB-MBRsystem systemwas wasoperated operatedwith withan anintermittent intermittentapplication applicationofof33V/cm V/cm at at an an operation mode of 5 min ON and 20 min OFF using an electronic controller referred to in previous operation min ON and 20 min OFF using an electronic controller referred to in previous studies studies [23–25,32]. [23–25,32].

Figure Figure 1. 1. Experimental Experimental setup setup of of the the electro electro moving moving bed bed membrane membrane bioreactor bioreactor (eMB-MBR) (eMB-MBR) system. system.

2.2. Analytical Methods 2.2. Analytical Methods The pH, dissolved oxygen (DO) concentration, temperature, and redox potential inside the The pH, dissolved oxygen (DO) concentration, temperature, and redox potential inside the bioreactor were monitored using a multiparametric probe (Hanna Instruments, Padova, Italy, HI2838). bioreactor were monitored using a multiparametric probe (Hanna Instruments, Padova, Italy, Samples were obtained from the influent, effluent, and supernatant. Concentrations of COD and HI2838). Samples were obtained from the influent, effluent, and supernatant. Concentrations of COD nutrients (ammonium nitrogen (NH4 -N), nitrate nitrogen (NO3 -N), nitrite nitrogen (NO2 -N) and and nutrients (ammonium nitrogen (NH4-N), nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N) and orthophosphate (PO4 -P)) from each sample were measured according to the standard methods [33]. orthophosphate (PO4-P)) from each sample were measured according to the standard methods [33]. Anions in effluent and the bioreactor were determined by filtering samples from the permeate and Anions in effluent and the bioreactor were determined by filtering samples from the permeate and mixed liquor, respectively, and using an ion chromatograph. The percent removals of COD, NH4 -N, mixed liquor, respectively, and using an ion chromatograph. The percent removals of COD, NH4-N, and PO4 -P were analyzed as removals from biodegradation, filtration, and total removal. Total percent and PO4-P were analyzed as removals from biodegradation, filtration, and total removal. Total removal was computed using Equation (1), whereas biodegradation removal for the MB-MBR and percent removal was computed using Equation (1), whereas biodegradation removal for the MBbiodegradation with electrocoagulation removal for the eMB-MBR were computed from the difference MBR and biodegradation with electrocoagulation removal for the eMB-MBR were computed from in influent and reactor concentrations using Equation (2). Filtration removal was the difference between the difference in influent and reactor concentrations using Equation (2). Filtration removal was the the total removal and biodegradation removal computed by using Equation (3). difference between the total removal and biodegradation removal computed by using Equation (3). CC − - CCe Total × 100% ] == i i Totalremoval removal[%% × 100% C Cii

(1) (1)

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Biodegradation or Biodegradation + Electrocoagulation removal % Ci - Cr × 100% = Ci + Electrocoagulation removal [%] = CiC−Cr ×100% Biodegradation or Biodegradation

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i

Filtration removal removal [% %]== Total Total removal removal [% %]−- Biodegradation Biodegradation removal removal[[%] Filtration %]

(2) (2) (3) (3)

Mixed liquor liquorsuspended suspendedsolids solids (MLSS) mixed liquor volatile suspended (MLVSS) Mixed (MLSS) andand mixed liquor volatile suspended solidssolids (MLVSS) inside inside the bioreactor were measured using standard methods [33]. Biofilm (BS)measured were measured the bioreactor were measured using standard methods [33]. Biofilm solids solids (BS) were using using Plattes et al. [34] as reference. Membrane fouling inside the bioreactor was monitored by Plattes et al. [34] as reference. Membrane fouling inside the bioreactor was monitored by measuring the measuring the transmembrane pressure variation over time, through a pressure transducer (PX409transmembrane pressure variation over time, through a pressure transducer (PX409-0-15VI, Omega, 0-15VI, Omega, Sunbury, OH, USA) connected to a datalogger (34972A LXI Data Acquisition/Switch Sunbury, OH, USA) connected to a datalogger (34972A LXI Data Acquisition/Switch unit, Agilent, unit, Agilent, Malaysia), the concentrations of known membrane fouling precursors, namely Malaysia), the concentrations of known membrane fouling precursors, namely extracellular polymeric extracellular polymeric substances (EPS), soluble microbial products (SMP), and transparent substances (EPS), soluble microbial products (SMP), and transparent exopolymeric particles (TEP). exopolymeric particles (TEP).into EPS and SMP were classified into proteins(EPSc, (EPSp, SMPp) and EPS and SMP were classified proteins (EPSp, SMPp) and carbohydrates SMPc) [35–37]. carbohydrates (EPSc, SMPc) [35–37]. A heating method was used to separately obtain EPS and SMP A heating method was used to separately obtain EPS and SMP samples from sludge flocs. Mixed liquor samples from flocs. liquorobtained samples was werereferred filtered, to and obtained was referred samples were sludge filtered, andMixed the filtrate asthe thefiltrate SMP. After SMP extraction, to as the SMP. After SMP extraction, the remaining samples were filled with deionized water ◦ C. the remaining samples were filled with deionized water and heated inside an oven at 80 and heated inside an oven at 80 °C. These samples were once again filtered and the filtrate from this These samples were once again filtered and the filtrate from this process was referred to as the process wasPhotometric referred to as the EPSfrom [6,38].Frølund Photometric from al. and DuBois et al. EPS [6,38]. methods et al. methods and DuBois etFrølund al. wereetthen used to analyze were then used to analyze both the protein and carbohydrates components of EPS and SMP both the protein and carbohydrates components of EPS and SMP [6,35,36,39], respectively, using [6,35,36,39], respectively, using bovine serum albumin (BSA) (Sigma, St. Louis, MO, USA) and Dbovine serum albumin (BSA) (Sigma, St. Louis, MO, USA) and D-glucose (Sigma, St. Louis, MO, glucose St. Louis, MO, USA) standards. A method used and developed in aused previous study USA) as(Sigma, standards. A method usedas and developed in a previous study [23] was to analyze [23] was used to analyze the TEP concentration. The concentration of TEP, EPS, and SMP, in terms of the TEP concentration. The concentration of TEP, EPS, and SMP, in terms of protein (EPSp, SMPp) protein (EPSp, SMPp) andSMPc) carbohydrate (EPSc, SMPc) was normalized by the MLVSS content. and carbohydrate (EPSc, was then normalized by then the MLVSS content. Fouling precursor Fouling precursor concentrations and transmembrane pressure (TMP) values were then correlated. concentrations and transmembrane pressure (TMP) values were then correlated. 3. Results and Discussion 3.1. COD, Nutrients, and Fouling Precursor Behavior inside inside the the Bioreactor Bioreactor 3.1.1. MB-MBR System 3.1.1. MB-MBR System Figure Figure 2a 2a shows shows the the behavior behavior of of COD, COD, NH NH44-N, -N, NO NO33-N, -N, and and PO PO44-P -P concentrations, concentrations, whereas whereas Figure Figure 2b 2b shows shows the the trends trends of of MLSS MLSS and and biofilm biofilm solid solid concentration concentration on on the the carriers carriers (BSc) (BSc) over over time time inside the MB-MBR system. inside the MB-MBR system.

(a)

(b)

Figure 2. (a) Chemical Chemical oxygen demand (COD), nutrient, and (b) solid concentration inside the moving bioreactor (MB-MBR). (MB-MBR). bed membrane bioreactor

These plots can be divided into five five parts. parts. During the first part (days 3 to 7), it was observed that the setup in the stabilization stagestage wherewhere microorganisms started to acclimatize [40]. However, setupwas wasstill still in the stabilization microorganisms started to acclimatize [40].

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However, a high decrease in COD could be due to the high concentration of readily biodegradable the feed NHto4-N during this part was limited by slow growing aCOD high in decrease inwastewater COD could [23]. be due thenitrification high concentration of readily biodegradable COD in the nitrifying bacteria [41]; the addition of influent NH 4-N resulted in an increase in NH4-N feed wastewater [23]. NHthus, -N nitrification during this part was limited by slow growing nitrifying 4 concentration inside the bioreactor. At NH this4 -N stage, MLSS BSc also did 4not undergo significant bacteria [41]; thus, the addition of influent resulted inand an increase in NH -N concentration inside changes in concentration which indicated that there was no remarkable increase in microbial the bioreactor. At this stage, MLSS and BSc also did not undergo significant changes in concentration community or activity inside bioreactor, justifying occurrence of stabilization inside the which indicated that there was the no remarkable increase inthe microbial community or activity inside bioreactor [20]. During the second part (days 7 to 9), no significant increase was observed with MLSS the bioreactor, justifying the occurrence of stabilization inside the bioreactor [20]. During the second and(days BSc concentration inside the bioreactor. Additionally, increasing concentrations ofinside COD the and part 7 to 9), no significant increase was observed with MLSS and BSc concentration NH4-N were observed due to theconcentrations additional influent COD -N, indicating a minimal bioreactor. Additionally, increasing of COD andand NH4NH -N 4were observed due to the biodegradation that could occur during the stabilization stage. An upward trend PO4-P additional influent COD and NH4 -N, indicating a minimal biodegradation that could occurinduring concentration signaled phosphate organism (PAO) activity the reactor that could the stabilization stage. An upwardaccumulating trend in PO4 -P concentration signaledinside phosphate accumulating suggest a slight change in operating conditions inside the bioreactor or inside the carriers [29]. At organism (PAO) activity inside the reactor that could suggest a slight change in operating conditions days 9 to 14 (third part), the setup started to become more active. Although there was a decrease inside the bioreactor or inside the carriers [29]. At days 9 to 14 (third part), the setup started to becomein MLSSactive. concentration atthere this stage, sharp increase in BSc was observed. The decrease in MLSS in the more Although was aadecrease in MLSS concentration at this stage, a sharp increase in reactor could be attributed to the number of solids that were attached to the carriers [20]. In addition, BSc was observed. The decrease in MLSS in the reactor could be attributed to the number of solids that higher microbial community andInactivity was indicated by the increase in BScactivity [30]. This were attached to the carriers [20]. addition, higher microbial community and wasresulted indicatedin thethe decrease NH4This -N and PO4-Pinconcentrations. of the supposedly high microbial by increaseininCOD, BSc [30]. resulted the decrease inBecause COD, NH 4 -N and PO4 -P concentrations. activity and growth in the previous stage, and as the system approached days 14 to 17as (fourth part), Because of the supposedly high microbial activity and growth in the previous stage, and the system a sharp decrease in COD was observed, that theinmajority of observed, available DO was consumed approached days 14 to 17 (fourth part), aimplying sharp decrease COD was implying that the by COD. This resulted in a low nitrification rate and a higher PO 4-P concentration. MLSS and BSc majority of available DO was consumed by COD. This resulted in a low nitrification rate and a higher plots appeared flat at this stage, signaling that there was no significant microbial activity brought PO 4 -P concentration. MLSS and BSc plots appeared flat at this stage, signaling that there was no about by population shift (e.g., aerobic to by anaerobic organisms). 17 onwards (fifth significant microbial activity brought about population shift (e.g.,Finally, aerobicattodays anaerobic organisms). part), the decreasing trend of COD concentration was not as sharp as days 14 to 17, implying a lower Finally, at days 17 onwards (fifth part), the decreasing trend of COD concentration was not as sharp biodegradation The nitrification process was also These process could bewas attributed to anoxic as days 14 to 17, rate. implying a lower biodegradation rate.hampered. The nitrification also hampered. conditions by the inner part of the exhibited carrier with biofilm [42]. After thisdeposited stage, the These could exhibited be attributed to anoxic conditions by deposited the inner part of the carrier with bioreactor started to stabilize, and no significant changes were noted. biofilm [42]. After this stage, the bioreactor started to stabilize, and no significant changes were noted. Since fouling fouling precursors precursors are are products products of of microbial microbial activity activity and and decay, decay, fluctuation fluctuation in in the the Since concentrations of these compounds might signal that an operating condition-driven change concentrations of these compounds might signal that an operating condition-driven change in microbialin microbial activity occurred Figure shows the plots EPS, and TEP concentration activity occurred [9]. Figure 3[9]. shows the3plots of EPS, SMP,of and TEPSMP, concentration over time withover its time with its changing trend complementing the earlier findings. changing trend complementing the earlier findings.

Figure 3. Fouling precursor concentration inside the MB-MBR. Figure 3. Fouling precursor concentration inside the MB-MBR.

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3.1.2. eMB-MBR System 3.1.2. eMB-MBR System Figure 4 shows the plots for COD, nutrients, and solids behavior inside the eMB-MBR system. Figure 4 shows the plots for COD, nutrients, and solids behavior inside the eMB-MBR system. From Figure 4a, COD, PO4 -P, and NH4 -N all have almost no change in concentrations. This could be due From Figure 4a, COD, PO4-P, and NH4-N all have almost no change in concentrations. This could be to the impact of the electric field application on the system. The COD content of the synthetic wastewater due to the impact of the electric field application on the system. The COD content of the synthetic used was readily biodegradable; thus, it already had high removal even at the start of the experiment [23]. wastewater used was readily biodegradable; thus, it already had high removal even at the start of the Additionally, PO4 -P also had a high removal rate due to the combined effects of biodegradation and experiment [23]. Additionally, PO4-P also had a high removal rate due to the combined effects of electrocoagulation NH4 -N removal still dominated by biodegradation the alternating biodegradation [24], and whereas electrocoagulation [24], was whereas NH4-N removal was still via dominated by aerobic and anaerobic process with minimal effects of electrocoagulation [23]. MLSS and BSc biodegradation via the alternating aerobic and anaerobic process with minimal concentration effects of plots followed an upward trend since energy is generated during substrate catabolism electrocoagulation [23]. MLSS and BSc concentration plots followed an upward trendunder sinceanaerobic energy is or aerobic conditions generated during[22]. substrate catabolism under anaerobic or aerobic conditions [22].

(a)

(b)

Figure4.4.(a) (a)COD, COD,nutrient, nutrient, and and (b) solid Figure solid concentration concentrationinside insidethe theeMB-MBR. eMB-MBR.

The eMB-MBRsystem system was divided parts. TheThe firstfirst partpart (days 3 to 9)3 corresponded to a The eMB-MBR dividedinto intofour four parts. (days to 9) corresponded increase in MLSS and BSc, the occurrence of a stabilization stage insidestage the bioreactor to slow a slow increase in MLSS andindicating BSc, indicating the occurrence of a stabilization inside the [40]. It is [40]. proposed the reactions in reactions the bioreactor were dominated by the effect caused by the bioreactor It is that proposed that the in the bioreactor were dominated by the effect electric field application, indicated by the removal of COD and NH 4-N with the increase in NO3-N caused by the electric field application, indicated by the removal of COD and NH4 -N with the even during stabilization stage. No increase in PO4-P concentration was observed owing to increase in NOthe 3 -N even during the stabilization stage. No increase in PO4 -P concentration was reduced activity of PAO under aerobic conditions. The same result was by García-Gómez observed owing to reduced activity of PAO under aerobic conditions. Theobserved same result was observed by et al. [14]. PO4-P removal was attributed to electrocoagulation [43]. During days 9 to 11 (second part), García-Gómez et al. [14]. PO4 -P removal was attributed to electrocoagulation [43]. During days 9 to 11 a lower MLSS concentration and an almost constant BSc concentration were noted. Lower energy (second part), a lower MLSS concentration and an almost constant BSc concentration were noted. Lower was harnessed by microorganisms under the governing conditions, resulting in a lower cellular mass energy was harnessed by microorganisms under the governing conditions, resulting in a lower cellular being synthesized and leading to the increase in COD, PO4-P, and NH4-N concentrations and the mass being synthesized and leading to the increase in COD, PO4 -P, and NH4 -N concentrations and the decrease in NO3-N concentration [44]. It is proposed that, at that time, the bioreactor approached an decrease in NO3 -N concentration [44]. It is proposed that, at that time, the bioreactor approached an anoxic stage induced by the electric field application. This condition could be seen from the activity anoxic stage induced by the electric field application. This conditiondays could from part) the activity of PAO and the inhibition of the nitrification process. Moreover, 11be toseen 14 (third had a of PAO and the inhibition of the nitrification process. Moreover, days 11 to 14 (third part) had a noticeable noticeable increase in MLSS and BSc concentrations, as is shown in Figure 4b. The combination of the increase MLSS and BSc concentrations, as is shown in Figureled 4b.toThe combination of the aerobicinbiodegradation process with the electric field application a high COD removal rateaerobic and biodegradation process with the electric field application led to a high COD removal rate and a lower a lower NH4-N and PO4-P concentration. Finally, at day 16 onwards (fourth part), an increase in MLSS NH -N and PO -P concentration. Finally, at day 16 onwards (fourth part), an increase in MLSS and and was still observed. However, the high removal rate of COD observed in days 4 BSc concentrations 4 BSc11concentrations was still observed. However, the then highthe removal of COD observed inshowing days 11 to to 14 might have consumed the available DO and NO3-Nrate concentration flattened, 14 very might have consumed available DO and then the NO3 -N concentration flattened, showing very little occurrence of the nitrification. The proposed shift in the operating conditions caused by the electric field application might little occurrence of nitrification. have in the change EPSp, SMP and TEP caused concentrations shown field in Figure 5. Theresulted proposed shift in theinoperating conditions by the electric application might have the observations made about MB-MBR and eMB-MBR systems’ resultedBased in theonchange in EPSp, SMP and TEP the concentrations shown in Figure 5. performance, it was established that microorganisms significantly affectand MB-MBR and systems’ eMB-MBR systems. These Based on the observations made about the MB-MBR eMB-MBR performance, it was microbial communities are affected by operating conditions inside the bioreactor. future established that microorganisms significantly affect MB-MBR and eMB-MBR systems. Hence, These microbial

communities are affected by operating conditions inside the bioreactor. Hence, future studies should

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note the19-docosahexaenoic acid (DHA) and the specific oxygen uptake rate (SOUR) to have a better studies should note the19-docosahexaenoic acid (DHA) and the specific oxygen uptake rate (SOUR) understanding of theunderstanding microbial community behavior insidebehavior the bioreactor. to have a better of the microbial community inside the bioreactor.

Figure 5. Fouling precursor concentration concentration inside the the eMB-MBR. Figure 5. Fouling precursor inside eMB-MBR.

3.2. Removal of Nutrients Chemical Oxygen Demand Demand 3.2. Removal of Nutrients andand Chemical Oxygen The combination of the electric field application with freely moving carriers inside the bioreactor The combination of the electric field application with freely moving carriers inside the bioreactor enhanced the nutrient removal. As can be seen in Figure 6, an improvement of 5.2% and 22% was enhanced the nutrient removal. As can be seen in Figure 6, an improvement of 5.2% and 22% was observed for NH4-N and PO4-P removal efficiencies, respectively, in the eMB-MBR system compared observed forMB-MBR NH4 -N system. and PO4 -P removal efficiencies, respectively, in the eMB-MBR system compared to the + to the MB-MBR NH4 system. removal in MB-MBR systems can be limited by the concentrations and diffusion rates of + removal + removal in the MBin the bed the low NH4and both dissolved oxygen (DO) and NH4+ can NH in MB-MBR systems bemoving limited by[29]. theThus, concentrations diffusion rates of both 4 MBR system(DO) can and be attributed insufficient aerobic inside Nevertheless, dissolved oxygen NH4 + intothe moving bed [29].sites Thus, the the lowcarriers. NH4 + removal in thethe MB-MBR applied current in the eMB-MBR system controlled several electrochemical mechanisms inside the system can be attributed to insufficient aerobic sites inside the carriers. Nevertheless, the applied bioreactor. At the anode side of the reactor, NH4-N concentration decreased due to the oxidation to currentNO in 3the eMB-MBR system controlled several electrochemical mechanismsThese inside the bioreactor. -N [45,46], whereas at the cathode side, reductive reactions predominated. reactions At the anode sideDO of and the subsequently reactor, NH4induced -N concentration decreased to the oxidation to NO3 -N [45,46], consumed anoxic conditions inside due the bioreactor with the application whereas cathode side, reductiveanoxic reactions predominated. These consumed of at thethe electric field [32]. Alternating and aerobic conditions inside thereactions carriers and induced byDO and electric field application favored the occurrence of the nitrification process, causing higher subsequently induced anoxic conditions inside the bioreactor with the application ofNH the4+ electric removals in the anoxic eMB-MBR corresponds with the results from previous studiesby [46,47]. field [32]. Alternating andwhich aerobic conditions inside the carriers and induced electric field Additionally, since DO concentration has a significant impact on microbial activity, the + alternation application favored the occurrence of the nitrification process, causing higher NH4 removals in the of aerobic and anoxic conditions enhanced the NH4-N removal in the eMB-MBR in terms of eMB-MBR which corresponds with the results previous studiesshown [46,47]. Additionally, since DO biodegradation following the nitrification and from denitrification equations in Equations (4) and concentration has a significant impact on microbial activity,from thethe alternation aerobic and anoxic (5). The occurrence of the nitrification process was observed increase inof NO 3-N average concentrations from mg/L in the influent to 1.61 mg/L in the reactor, whereas the occurrence of conditions enhanced the0.10 NH -N removal in the eMB-MBR in terms of biodegradation following 4 the denitrification process was noted from the higher NO 3-N average concentration (1.61 mg/L) inside the nitrification and denitrification equations shown in Equations (4) and (5). The occurrence of the the reactor compared to the effluent average NO3-N concentration (1.19 mg/L), signifying that nitrate nitrification process was observed from the increase in NO3 -N average concentrations from 0.10 mg/L was reduced to nitrogen gas. On the other hand, the higher attraction of sludge particles to the in the influent to 1.61 mg/L in the reactor, whereas the occurrence of the denitrification process was membrane module due to the absence of an electric field in the MB-MBR system facilitated the noted from the higher NO3 -Nlayer average concentration (1.61 mg/L) insideserve the as reactor formation of a secondary that was both biologically active and could a goodcompared filtration to the effluentmedium. averageThus, NO3higher -N concentration (1.19 mg/L), signifying nitrate wasisreduced totonitrogen NH4-N removal in terms of filtration in thethat MB-MBR system attributed the additional filtration and biodegradation that might have occurred when water passed through gas. On the other hand, the higher attraction of sludge particles to the membrane module due to the layer. absencethis of secondary an electric field in the MB-MBR system facilitated the formation of a secondary layer that was both biologically active and could serve as a good filtration medium. Thus, higher NH4 -N removal in terms of filtration in the MB-MBR system is attributed to the additional filtration and biodegradation that might have occurred when water passed through this secondary layer.

NH4 + + 2O2 → NO3 − + 2H+ + H2 O

(4)

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→NO2 − → NO → N2 O → N2(g)

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(5)

In addition, the electric current flowing in the eMB-MBR system released Al3+ and Al6 (OH)15 3+ NH 4+ + 2O2 → NO3− + 2H+ + H2O (4) side. ions from the dissolution of aluminum anode and formed H2 and OH− from water at the cathode − − These chemical species combined with ions toN form AlPO4(s) and [Al6 (OH)15 NO3 orthophosphate →NO2 → NO → N 2O → 2(g) (5)]PO4(s) compounds according to Equations (6) and (7) [16,38]. Moreover, as compared to the Fe 3+ In addition, the electric current flowing in the eMB-MBR system released Al3+ and Al6(OH)15anode, the aluminum hydroxide species formedanode fromand theformed aluminum anode were reported to haveside. a higher ions from the dissolution of aluminum H2 and OH− from water at the cathode surface areachemical and were more efficientwith in terms of particleions entrapment and4(s)adsorption then the These species combined orthophosphate to form AlPO and [Al6(OH) 15]PO 4(s) iron hydroxide formed from the Fe anode(6)[16]. Thus, 98.7Moreover, ± 0.2% average total -P anode, removal compounds according to Equations and (7) [16,38]. as compared to PO the 4Fe the was aluminum formed the14.1% aluminum anode were to have a higher achieved in the hydroxide eMB-MBRspecies compared to from 76.7 ± average total POreported in the MB-MBR 4 -P removal surface area and were more efficient in terms of particle entrapment and adsorption then iron in system. The occurrence of the electrocoagulation process was corroborated from the the decrease hydroxide formed from the Fe anode 98.7of± the 0.2% average total PO4-P removal was weight of the aluminum anode from 349 [16]. g at Thus, the start experiment to 306 g at the end of the achieved in the eMB-MBR compared to 76.7 ± 14.1% average total PO4-P removal in the MB-MBR experiment. Nevertheless, the PO4 -P filtration performance of the MB-MBR was higher than that of the system. The occurrence of the electrocoagulation process was corroborated from the decrease in eMB-MBR, having filtration removals of 19.2 ± 13.4% and 3.9 ± 10.9%, respectively. The reason was weight of the aluminum anode from 349 g at the start of the experiment to 306 g at the end of the that only a smallNevertheless, concentration PO was available for filtration since most of it was 4 -P experiment. theofPO 4-P filtration performance of the MB-MBR was higher thanremoved that of by the combination of biological and electrocoagulation processes in the eMB-MBR system. Additionally, the eMB-MBR, having filtration removals of 19.2 ± 13.4% and 3.9 ± 10.9%, respectively. The reason as discussed NHa4small -N removal, the newly dynamic membrane might contributed was thatin only concentration of POproduced 4-P was available for filtration since most have of it was removed to a 3 betterby filtration performance of the remaining PO4 -P in the MB-MBR the combination of biological and electrocoagulation processessystem. in the eMB-MBR system. Additionally, as discussed in NH4-N removal, the newly produced dynamic membrane might have − remaining PO43-P in the MB-MBR system. contributed to a better filtration performance Al3+ + POof4 3the → AlPO4(s) (6) Al3+ + PO43− → AlPO4(s)

(6)

Al6(OH)153+ + PO43− → [Al6(OH)15]PO4(s)

(7)

Al6 (OH)15 3+ + PO4 3− → [Al6 (OH)15 ]PO4(s)

(7)

From Figure 6, high total COD removals were noted for both systems at 98.6% and 98.7% for From Figure 6, high total COD removals were noted for both systems at 98.6% and 98.7% for the the MB-MBR and eMB-MBR systems, respectively. Of the COD removal efficiencies presented, MB-MBR and eMB-MBR systems, respectively. Of the COD removal efficiencies presented, approximately 4–7% was attributed to filtration and more than 90% to biodegradation. The high approximately 4–7% was attributed to filtration and more than 90% to biodegradation. The high COD COD removal removalefficiencies efficiencies of the systems are directly to the highly biodegradable of the two two MBRMBR systems are directly linked tolinked the highly biodegradable sucrose sucrose glucose content the synthetic municipal wastewater. andand glucose content of theofsynthetic municipal wastewater.

+ and 3−3− removal efficiencies 6. COD, 4+ and and eMB-MBR configurations. FigureFigure 6. COD, NH4NH POPO efficienciesby byMB-MBR MB-MBR and eMB-MBR configurations. 4 4 removal

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3.3. Control of Membrane Fouling 3.3. Control of Membrane Fouling Membrane fouling is one of the major challenges in MBR technology implementation. Fouling Membrane fouling is one of the challenges in flux MBRdecline technology implementation. Fouling is characterized by a rising TMP andmajor the simultaneous resulting from the build-up of is characterized by a rising TMP and the simultaneous flux decline resulting from the build-up of extracellular organics, microbial cells, solid particles, and other inorganic materials on the membrane extracellular organics, microbial cells, particles, other inorganic materials the membrane surface [17,35]. One simple method tosolid control foulingand is by chemical cleaning of theon membrane; thus, surface [17,35]. One simple method to control fouling is by chemical cleaning of the membrane; thus, each sudden drop in TMP corresponds to the application of chemical cleaning on the fouled each sudden drop in TMP corresponds to the application of chemical cleaning on the fouled membrane. membrane. In this study, it was observed that less frequent chemical cleaning was required for the In this study, it wascompared observed to that frequent chemical cleaning was required for thecycle eMB-MBR eMB-MBR system theless MB-MBR system. Additionally, a longer filtration and a system compared to the MB-MBR system. Additionally, a longer filtration cycle and a significant significant decrease in membrane fouling rate (60%) were obtained when an electric field was decrease in is membrane (60%) obtained when an electric field was applied,affects as is applied, as shown in fouling Figure 7.rate This resultwere suggests that electric field application positively shown in Figure 7. mitigation This resultinside suggests that electricThis field application positively membrane membrane fouling the bioreactor. finding is consistent withaffects those reported in fouling mitigation inside the bioreactor. This finding is consistent with those reported in previous previous studies whereby the application of an electric current to the bioreactor caused lower fouling studies whereby the application of an electric current to the bioreactor caused lower rates and rates and subsequently less frequent membrane cleaning [4,23,25,29,45,48]. Thus, fouling effective fouling subsequently less frequent membrane cleaning [4,23,25,29,45,48]. Thus, effective fouling control in the control in the eMB-MBR system enabled the membrane to operate at longer periods prior to chemical eMB-MBR system enabled the membrane to operate at longer periods prior to chemical cleaning. cleaning.

Figure 7. 7. Comparison Comparisonofof∆Δtransmembrane transmembrane pressure (TMP) of MB-MBR the MB-MBR andeMB-MBR the eMB-MBR over Figure pressure (TMP) of the and the over time. time.

As MBR technology mainly relies on microbial activity for biodegradation, microorganisms are considered a vital keymainly to MBR applications. Nonetheless, activity such as substrate As MBR technology relies on microbial activity for microbial biodegradation, microorganisms are metabolism microbial produceNonetheless, fouling precursors inside the membrane considered and a vital key todecomposition MBR applications. microbial activity such as bioreactor. substrate These includeand EPSmicrobial and SMPdecomposition that are principally made of proteins and high molecular weight metabolism produce fouling precursors inside the membrane carbohydrates andinclude TEP. Based on some commonly non-covalent networks bioreactor. These EPS and SMP studies, that are proteins principally made of form proteins and high molecular that cause severe membrane fouling, molecular carbohydrates such as weight carbohydrates and TEP. Based whereas on some high studies, proteinsweight commonly form non-covalent polysaccharides are difficult to mineralize andwhereas tend to high behave like gelweight at acidic and neutralsuch pH. networks that cause severe membrane fouling, molecular carbohydrates Moreover, TEP are gel-like biopolymers that easily adsorb on thelike membrane surface increase as polysaccharides are difficult to mineralize and tend to behave gel at acidic andand neutral pH. sludge viscosity [23,24,47]. to optimize role adsorb of microorganisms, it is noteworthy to monitor Moreover, TEP are gel-likeThus, biopolymers thatthe easily on the membrane surface and increase the concentrations of theseThus, fouling precursors TMP and to itminimize their to respective sludge viscosity [23,24,47]. to optimize thealong role ofwith microorganisms, is noteworthy monitor concentrations inside system. the concentrations ofthe these fouling precursors along with TMP and to minimize their respective The effect of electric application was investigated through the concentrations of major fouling concentrations inside thefield system. precursors such as SMP, andapplication TEP. Figure was 8 represents a box through plot with the maximum fouling precursor The effect of EPS, electric field investigated concentrations of major concentration and standard deviation overTEP. the period operation.aFrom the with plotsmaximum in Figure 8,fouling it can fouling precursors such as EPS, SMP, and Figure of 8 represents box plot be observed that lower concentrations EPS, SMP, and TEP wereofobtained in the eMB-MBR as precursor concentration and standardof deviation over the period operation. From the plotssystem in Figure 8, it can beto observed that lower concentrations of EPS, SMP, and TEP were obtained in the eMB-MBR compared the MB-MBR. system as compared to the MB-MBR.

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Electric field application decreased the membrane fouling rate and decreased the transmembrane risetheinmembrane the eMB-MBR via and electrochemical The Electric field pressure application(TMP) decreased fouling rate decreased the processes. transmembrane electrocoagulation charged metal ions resulting in foulant attraction, pressure (TMP) riseprocess in the produced eMB-MBR positively via electrochemical processes. The electrocoagulation process 3+ and Al6(OH)153+ ions neutralization, and destabilization. Additionally, positively charged Al produced positively charged metal ions resulting in foulant attraction, neutralization, and destabilization. reduced repulsive forces between leading to3+an increase in floc size and a lesser floc deposition Additionally, positively charged Al3+flocs, and Al 6 (OH)15 ions reduced repulsive forces between flocs, leading on membrane surface. electrophoresis and electro-osmosis processes drove the to anthe increase in floc size and aMoreover, lesser floc deposition on the membrane surface. Moreover, electrophoresis negatively charged foulants away from the membrane and moved the positively charged bulk liquid and electro-osmosis processes drove the negatively charged foulants away from the membrane and moved toward the cathode and, thus, thetoward membrane [17,38,46]. the positively charged bulk liquid the cathode and, thus, the membrane [17,38,46].

Fouling precursor precursor concentrations in the MB-MBR and eMB-MBR systems. Figure 8. Fouling

Moreover, an illustrated in in Figures Figures 22 and and 44 Moreover, an increase increase in in the the biosolid biosolid concentration concentration on on the the carriers carriers illustrated suggest that the carriers inside the bioreactor serve as attachment media to decrease the amount of suggest that the carriers inside the bioreactor serve as attachment media to decrease the amount of biosolids that may accumulate on the membrane surface, resulting in a lower filtration resistance. biosolids that may accumulate on the membrane surface, resulting in a lower filtration resistance. The same The same was was reported reported by by Chen Chen et et al. al. [12] [12] Humic substances are strongly hydrophobic substances substances frequently frequently associated associated with with EPS EPS that that are are Humic substances are strongly hydrophobic contributors to to membrane membrane fouling. fouling. Based Based on on some some studies, studies, these these substances modify the the membrane contributors substances modify membrane surface through their combination with proteins and polysaccharides via hydrophobic and electrostatic surface through their combination with proteins and polysaccharides via hydrophobic and interactions interactions and adsorption on the membrane surface [13,49]. Thus, lowThus, concentrations of these electrostatic and adsorption on the membrane surface [13,49]. low concentrations substances are preferred in the system. this experiment, the average of the difference in influent of these substances are preferred in the For system. For this experiment, the average of the difference in and effluent humic substance concentration was expressed as the removal. A higher removal of humic influent and effluent humic substance concentration was expressed as the removal. A higher removal substances computedcomputed from UV254 absorbance was obtained for the eMB-MBR system (92.36 ± (92.36 12.6% of humic substances from UV254 absorbance was obtained for the eMB-MBR system than for than the MB-MBR system (86.8 ± 40.2% ±removal) 12.6% removal) for the MB-MBR system (86.8 ±removal). 40.2% removal). The concentrations of EPS and SMP were The concentrations of EPS and SMP were investigated investigated using using protein protein and and carbohydrate carbohydrate concentrations. The The results results shown shown in in Figure Figure 88 illustrate illustrate the the positive positive impact impact of of electric electric field field application application concentrations. in the reduction of fouling precursor concentrations. EPS and SMP concentrations expressed as proteins in the reduction of fouling precursor concentrations. EPS and SMP concentrations expressed as and carbohydrates were noticeably lower in the eMB-MBR than in the MB-MBR. The application an proteins and carbohydrates were noticeably lower in the eMB-MBR than in the MB-MBR.ofThe electric fieldof induced the electrochemical of water and favoredof thewater formation of hydroxyl application an electric field induced oxidation the electrochemical oxidation and favored the radicals. This resulted in the conversion of these fouling precursors into lower molecular weight formation of hydroxyl radicals. This resulted in the conversion of these fouling precursors into lower compounds weight that are easier to mineralize Furthermore, electrocoagulation the molecular compounds that [35,46,47]. are easier to mineralize [35,46,47]. promoted Furthermore, destabilization of foulants in suspension. Consequently, the reduction of SMP and Consequently, TEP concentrations electrocoagulation promoted the destabilization of foulants in suspension. the led to a less viscous sludge and cake layer, resulting in a higher filterability and a lower membrane reduction of SMP and TEP concentrations led to a less viscous sludge and cake layer, resulting in a resistance [25]. Lower precursor concentrations the eMB-MBR suggestconcentrations that the applied higher filterability and afouling lower membrane resistance [25]. in Lower fouling precursor in electric field and the generated metal ions were not detrimental to microorganisms inside the system. the eMB-MBR suggest that the applied electric field and the generated metal ions were not

detrimental to microorganisms inside the system.

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4. Conclusions The results of this study showed that an effective and innovative technology called eMB-MBR was successfully obtained from the integration of electrochemical processes within an MBBR into an MBR reactor. This new technology was evaluated and compared to a control system (MB-MBR). An improvement in treatment efficiency especially in terms of nutrient removal and a significant reduction in fouling of approximately 60% were obtained for the eMB-MBR compared to the MB-MBR. Thus, eMB-MBR technology can be considered a promising new technology to further improve MBR and MBBR systems for wastewater treatment in terms of conventional contaminant removal and fouling mitigation. Author Contributions: V.N. and V.B. developed the research idea and planned the research activities, L.B. carried out the research activities, J.M.J.M.-M., M.D.G.-d.L., and F.C.B. analyzed the data and prepared the manuscript, and V.N. and V.B. reviewed the final draft of the manuscript. Funding: The research activities were partially funded by the University of Salerno under Project Nos. ORSA167105 and ORSA15425 and under Project No. IN17GR09/INT/Italy/P-17/2016 (SP) funded by the Ministry of Foreign Affairs and International Cooperation, Government of Italy, and the Department of Science and Technology, Ministry of Science and Technology, Government of India. Research activities are also linked to Project No. EG16MO01 funded by the Ministry of Foreign Affairs and International Cooperation, Government of Italy. Acknowledgments: The authors are grateful to the Sanitary Environmental Engineering Division (SEED) Laboratory of the University of Salerno for providing the facilities and research fund. The authors also gratefully thank SUEZ WTS Italy S.r.l. for donating the membrane modules used in the laboratory scale plant and Anna Conte, Paolo Napodano, and Anna Farina for the cooperation and the precious help given during the research activity. They also highly acknowledge the University of the Philippines—Diliman and the Engineering Research and Development for Technology (ERDT) through the Department of Science and Technology, Philippines for the Ph.D. Scholarship Grant and Sandwich Program awarded to J.M.J. Millanar-Marfa. Conflicts of Interest: The authors declare no competing financial interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.

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