Biokinetic coefficients determination of a MBR for Styrene and Ethylbenzene as substrate based on the Andrews model

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1 Advances n Envronmental Technology 4 (016) Advances n Envronmental Technology journal homepage: Boknetc coeffcents determnaton of a MBR for Styrene and Ethylbenzene as substrate based on the Andrews model Seyed Mojtaba Seyed 1, Hossen Hazrat,3,*, Jalal Shayegan 1 1 Department of Chemcal and Petroleum Engneerng, Sharf Unversty of Technology, Tehran, Iran Department of Chemcal Engneerng, Sahand Unversty of Technology, Tabrz, Iran 3 Envronmental Center of Sahand Unversty of Technology, Tabrz, Iran A R T I C L E I N F O Artcle hstory: Receved 15 January 017 Receved n revsed form 15 February 017 Accepted 8 February 017 Keywords: Boknetc coeffcent Hydraulc retenton tme Sold retenton tme Volatle organc compounds A B S T R A C T In ths study, a lab-scale membrane boreactor (MBR) was operated for a perod of more than 10 months to determne the boknetc coeffcents of the system under the hydraulc retenton tmes (HRT) of 0, 15 and 10 hrs and sludge retenton tmes (SRT) of 5-0 days. The results revealed that the bologcal removal effcency of styrene and ethylbenzene at a sold retenton tme of 0 day and a hydraulc retenton tme of 15 hr was hgher compared to a SRT of 10 day and at the same HRT. The results also showed that the yeld (Y), the endogenous decay coeffcent (k d), the maxmum specfc growth rate (μ max), and the saturaton constant (K s) for styrene and ethylbenzene as substrate were 0.60 and 0.60 mg/mg, 0.5 and 0.5 day 1, and h -1, and and.8 mg /l, respectvely. Furthermore, ethylbenzene was more approprate as a source of carbon to actvated sludge n the membrane boreactor than the styrene whch had a lower μ max than ethylbenzene. 1. Introducton Many dfferent chemcal ndustres n Iran produce wastewater that contans volatle organc compounds (VOCs). These materals are man-made and/or naturally occurrng hghly reactve hydrocarbons [1]. These contamnants not only have destructve effects on the envronment but also rase health concerns for workers. Wastewater treatment processes have been establshed to respond approprately and releve varous anxetes concernng publc health. These methods nclude physcal technques such as actvated carbon adsorpton [], chemcal procedures such as ozonaton [3], and bologcal methods such as conventonal actvated sludge process [4], rotaton bologcal contactor (RBC) processes [5], and stablzed boflm [6]. Among the bologcal methods, conventonal actvated sludge has been employed n many ndustres as well as n the petrochemcal ndustry. It was reported that bodegradaton, strppng, and absorpton are three man mechansms that facltate volatle organc compound removal n the conventonal actvated sludge systems [1]. Due to low mxed lquor suspended sold (MLSS) concentraton n conventonal actvated sludge plants (CASPs), they are one of the most sgnfcant VOC emsson sources. A membrane boreactor nvolves an actvated sludge process n whch the sedmentaton unt s replaced wth a membrane. The MBR process can acheve a hgher MLSS concentraton compared to CASP; therefore, these systems reduce VOC emssons. The man dsadvantage of a MBR s the reducton of output flux due to the cloggng of the membrane. Prevous works show that membrane foulng s related to operatng parameters such as hydraulc retenton tme, sludge retenton tme, and sludge specfcatons [6,7,8] HRT and SRT are the man parameters of bologcal processes such as CASP and MBR for wastewater treatment [9,10]. Further, defnng the degradaton knetcs of VOCs by bacteral populatons s one of the prncple steps to forecast and optmze the actvated sludge processes on an ndustral scale. Mathematcal models have been developed to evaluate the *Correspondng author. Tel: E-mal address: h.hazrat@sut.ac.r

2 08 S. M. Seyed et al. / Advances n Envronmental Technology 4 (016) bodegradaton rates of organc contamnants that nclude substrate utlzaton, bacteral growth and decay, and the utlzaton of electron acceptors. Hence, the employment of an approprate knetc model s necessary. For nstance, the models derved from Monod smulaton are employed for populaton growth studes n the case of the mcrobal growth knetcs [9]. Generally, the Monod knetc model s used n reports wth a pure culture, lmted substrate, and non-nhbtory bomass growth [10,11]. However, modfcatons of the Monod model, whch ncludes an nhbton term, are used when substances are above a certan concentraton. In addton, these models have been used to nvestgate the effects of substrate nhbton on bomass growth at hgh substrate concentratons [1, 13,14]. Up to now, there have been only a few nvestgatons concernng styrene and ethylbenzene removal va MBRs. The lterature survey showed that there s a lack of nformaton related to the boknetc coeffcents of membrane boreactors. Furthermore, no attempt has been made to determne the boknetc coeffcents of styrene and ethylbenzene n the MBR. The man goal of ths study was to determne the boknetc coeffcents of membrane boreactors for styrene, ethylbenzene and optmum HRT, whch would enable process engneers to determne the mnmum reactor volume and recognze the process control through reactor smulaton. In order to acheve ths am, a lab scale membrane boreactor was operated at varous HRTs and SRTs Boknetc coeffcents determnaton Basc equatons that descrbe the nteracton between the growth of mcroorgansms and the utlzaton of the growth lmtng substrate n actvated sludge processes are based on the Monod model; Monod was the frst to ntally suggest the dea of mcrobal growth knetcs controlled by an emprcal model (Eq. 1). μ = μ max S K s + S where the specfc growth rate of bomass s μ (h 1 ), μ max s the maxmum specfc growth rate of bomass (h 1 ), S s the substrate concentraton (mg/l), and K s s the substrate half-saturaton constant (.e. substrate concentraton at half μ max ). The Monod model presented the concept of a growth lmtng substrate through the parameters μ max and Ks [14]. However, ths model becomes unsatsfactory when a substrate prevents ts own bodegradaton. Therefore, a modfed verson of the Monod model, whch s named the Andrews model, was employed to delver an mproved fttng for the data acheved from the sole substrate tests. In ths case, the Andrews model, shown as Eq. (), was used for substrate nhbton [15]. Ths model was justfed and employed n ths study based on the satsfactory/relable results from prevous reports [15,16], the toxc nature of VOCs (n ths (1) case styrene and ethylbenzene), and the possblty of substrate nhbton. μ max S μ = K s + S + S () K I Agan, n Eq. (), μ max s the maxmum specfc growth rate (h 1 ), K s s the half velocty constant and K I s the substrate nhbton constant, whch quantfed the nfluence of a toxc compound on ts bodegradaton. The three knetc parameters,μ max, K s and K I could be estmated by fttng Eq. () to expermentally obtaned specfc growth rates as a functon of substrate concentraton. It s mportant to note that the Andrews model s nonlnear. The exstence of mxtures of chemcals n ndustral and muncpal wastewaters s a sgnfcant ssue n bodegradaton or boremedaton developments. The complexty of the degradaton models sharply ncreased based on the growng number of substrates caused by the nteracton between them. Furthermore, the knetc parameters for a sngle substrate were not able to llustrate the phenomena observed durng the mxture bodegradaton. Uncompettve nhbton, non-compettve nhbton and compettve nhbton are some nteractons that can occur when multple substrates are present [17]. Hence, several models have been establshed n order to defne the specfc growth rate through the degradaton of multple nteractng substrates. One of the most common types of these models s obtaned through the summaton of specfc growth rates. For nstance, durng compettve nhbton, substrates compete for bndng stes to be metabolzed by the mx culture; n ths envronment, a sum knetcs model ncorporatng purely compettve substrate knetcs s useful and s shown n Eq. 3 [18,]. μ max1 S 1 μ tot = μ 1 + μ = K s 1 + S 1 + ( K s1 K s )S μ max S (3) + K s + S + ( K s K 1 s 1)S Nevertheless, there s a model that accounts for substrate nteractons wthout drectly specfyng the type of nteracton [1]. Ths model s formulated by ncorporatng an nteracton parameter I.j as an unknown and s shown n Eq. 4. μ max1 S 1 μ tot = μ 1 + μ = K s 1 + S 1 + I.1 S μ max S (4) + K s + S + I 1. S 1 Ths model s known as sum knetcs wth nteracton parameters or SKIP. I.j Specfes the degree to whch substrate affects the bodegradaton of substrate j. Accordng to the model, the stronger nhbton has a drect relatonshp wth the large value of j [19]. The value of the nteracton parameter s calculated by fttng the SKIP model

3 S. M. Seyed et al. / Advances n Envronmental Technology 4 (016) to a bnary of mxture data sets. Therefore, the specfc growth rate of the bomass from the utlzaton of substrate can be expressed by: μ max S (5) μ = K s + S + I j. S j + I k. S k + Ths work employed three substrates as the carbon source: 1) styrene ) ethylbenzene and 3) ethanol. The bologcal removal of ethylbenzene and styrene are mportant due to ther low bodegradablty compared to ethanol; the value of the nteracton parameters for styrene and ethylbenzene are presented n Table 1. Table 1. The value of nteracton parameters for styrene and ethylbenzene Interacton parameter I 1, I 1,3 I,1 I,3 Value Not: Styrene: 1, Ethylbenzene: and Ethanol: 3 The equatons descrbng the performance of the reactor are the mass balance equatons of both the bomass and substrate. The bomass balance can be expressed by: V dx dt = μ X V k d X (6) V Q w X where V= reactor volume (m 3 ); X = bomass concentraton that s produced from the utlzaton of substrate n the reactor (mg/l); k d = bomass decay coeffcent for fracton (1/d); and Qw =wastage flow rate (m 3 /d); and t = tme (d). At steady-state condtons, dx = 0; hence, Eq. 6 becomes: dt μ k d = Q w (7) V Snce the SRT s defned as: SRT = θ c = VX Q w X = V Q w (8) Therefore, Eq. 7 becomes: μ = k d + 1 θ c (9) Substtutng the value of μ from Eq. 9 nto Eq. 5 yelds the followng equaton that descrbes the steady-state condton of the substrate concentraton n the reactor: (k d + 1 ) (K θ s S = c + I j.s j + I k. S k + ) μ max (k d + 1 ) θ c (10) On the other hand, the substrate balance can be expressed as: V ds dt = QS 0 μ XV S Y (Q Q w ) (11) Q w S where Y s the maxmum cell yeld for substrate ; S 0 = ntal concentraton of substrate (mg/l); and S s concentraton of substrate n the reactor. At the steady state, ds dt = 0, therefore : Q V = (S 0 S ) = μ X Y (1) Substtutng Eq. 9 nto Eq. 1 results n: Q(S 0 S ) VX = 1 Y ( 1 θ c ) + k d Y (13) Eq. 1 s plotted as Q(S 0 S ) 1 versus ; the boknetc VX θ c coeffcents k d and Y can be determned from the slope and the Y-ntercept of the equaton. To determne the boknetc coeffcents, μ max and K s, Eq. 10 can be rearranged to become: θ c 1 + k d θ = (K s + I j.s j + I k. S k + ) ( 1 ) + 1 c μ max S μ max (14) Substtutng the value of k d n Eq. (14) and plottng θ c 1+k d θ c versus 1, the boknetc coeffcents, μ S max and K s can also be examned from the slope and the Y-ntercept of the equaton. Smlarly, t could be appled to other substrates; vdelcetμ maxj, μ maxk K s j and K can also be determned by sk applyng the above method.. Materals and methods.1. Expermental setup The dmensons of the membrane boreactor for ths setup were cm (Fgure 1). The effectve volume n the reactor was 7 L. The membrane used n ths study was a Mcro-Fltraton (MF) type wth an effectve area of 0.1 m, a pore nomnal dameter of 0.4 µm, and an A4 sheet sze. The membrane was produced by the SINAP Company and was made of Poly-Ethylene (PE). The aeraton process n the MBR was done for two purposes: frst, to supply the oxygen needed for bologcal processes; and secondly, to clean the membrane surface and reduce the foulng rate. To acheve the second goal, a poly (methyl methacrylate) (PMMA) plate was used as a baffle to keep the ar bubbles near the membrane surface so that they can make proper tensons wth t and wash the sedments out of the surface. The aerobc sludge used n the MBR basn was suppled from the actvated sludge of the Tabrz Petrochemcal Company and then adapted wth synthetc feed for one month.

4 10 S. M. Seyed et al. / Advances n Envronmental Technology 4 (016) Fg. 1. Schematc of lab-scale expermental setup.. Influent wastewater The synthetc wastewater used n ths research was formulated to smulate petrochemcal ndustral wastewater n terms of chemcal oxygen demand (COD), styrene, and ethylbenzene concentratons whch were 100, and mg/l respectvely. Ethanol was used as a carbon source whch created a COD concentraton of about 100 mg/l. The synthetc wastewater compostons used n the present study are descrbed n Table. Table. The synthetc wastewater compostons Components Concentraton (mg/l) Ethanol Styrene (STR) Ethylbenzene (EB) NH 4Cl 560 K HPO 4 35 KH PO 4 45 MgSO 4.7H O 13 CaCl.H O 7 FeCl 3 5 ZnSO 4 NaHCO EDTA (C 10H 16N O 8) 7.3. Analytcal methods and operaton parameters The styrene and ethylbenzene concentratons were analyzed usng a gas chromatograph (GC). The GC (Young Ln, ACME-6100) was set wth a Flame Ionzng Detector (FID) and an attached slca capllary column (DB-5, 0.53 mm I.D., 30 m length, 1 mm flm thckness) that was desgned to be well suted for the analyss of volatle components. The carrer gas was helum flowng at 15 ml/mn. The oven temperature was mantaned at 70 ⁰C for a 1 mn duraton and then rased to 140 ⁰C. The temperatures of the njector and the detector were fxed at 00 and 40 ⁰C, respectvely. The styrene and ethylbenzene concentratons n the lqud phase were estmated usng the head-space method [0]. The gas flow rate from the boreactors headspace was measured usng a flow meter. The MLSS, MLVSS, and COD were estmated accordng to standard methods [1]. 3. Results and dscusson 3.1. Expermental results for MBR Styrene, ethylbenzene and COD removal effcency n HRT=0 hr and SRT= 0 d The styrene, ethylbenzene, and COD removal effcency s presented n Fgure. As t can be seen, n the steady state condton for a HRT of 0 hrs, the COD removal effcency n the reactor was around 98 percent; the styrene and ethylbenzene removal was more than 99 percent. In addton, the concentraton of ethylbenzene and styrene n the ext ar from the reactor was measured daly. Fgure 3 shows that n an HRT of 0 hrs, the concentratons of ethylbenzene and styrene n the reactor ext ar n a steadystate condton was 0.7 ppm (equal 1.16% strppng removal) and 1ppm (equal 1.65% strppng removal), respectvely. Ths fact ndcated that the mechansm of removal n the reactor was not a consequence of the volatlty of styrene and ethylbenzene. Also, the absorpton of a pollutant by a mcrobal culture can only be consdered as an mportant mechansm whenever the partton coeffcent of octanol - water (log Kow) was more than 4 [0],

5 S. M. Seyed et al. / Advances n Envronmental Technology 4 (016) whle ths coeffcent for styrene and ethylbenzene was about 3.15 and.85, respectvely [7,1]. Moreover, prevous studes revealed that the styrene absorpton by sludge as a removal mechansm was nsgnfcant [3]. Therefore, the removal mechansm n the reactor was manly through a bodegradaton mechansm. Fg.. Varatons of COD, styrene and ethylbenzene removal durng the operaton of the MBR (HRT of 0h (days 0-43th), 15h (days th ) and 10h (days th ) reduced, two parameters affected the removal effcency Styrene, Ethylbenzene and COD Removal Effcency n Frstly, organc loadng ncreased slghtly and as a HRT=15 & 10 h and SRT =0 d consequence, the amount of MLSS grew n the reactor. The In an HRT of 15 hr, except durng the frst few days (days 44 to 47th) n whch the removal effcency n the reactor was extremely reduced, the removal effcency of COD, styrene and ethylbenzene ncreased and after reachng a steady other parameter was the drop n contact tme between the contamnants and the sludge. It was obvous that an ncrease n MLSS had a postve effect and a reducton n the contact tme had a negatve nfluence on the bologcal state was 98, 99.9 and 99.9%, respectvely. Because of an removal effcency. Nevertheless, snce the MLSS unexpected boost n the amount of organc load that entered the system on the 44th day, the mcroorgansms were under shock and for ths reason, a declnng trend was observed. Followng ths stage, the mcroorgansms adapted themselves to the new condtons whch gradually ncreased the effcency of the system and eventually reached a steady state condton. After a change n the retenton tme from 0 to 15 hours, the strppng removal effcency of styrene concentraton n the reactor ncreased, the negatve effect of the contact tme was neutralzed and the removal effcency ncreased. When the HRT declned to 10 hrs, the organc load rate n the system ncreased; on the other hand, the contact tme between the actvated sludge and wastewater decreased sgnfcantly compared to the prevous states (e.g. n HRTs of 0 and 15 hrs). Therefore, the removal effcency n the reactor was reduced and ethylbenzene n the reactor slghtly decreased. Ths sgnfcantly. Under ths crcumstance, the removal could be attrbuted to styrene and ethylbenzene effcency of COD, styrene and ethylbenzene was 90, 99.9, concentratons n the ext ar whch decreased from 1 and 0.7 ppm to 0.8 and 0.5 ppm, respectvely. In a prevous study, t was also reported that HRT reducton decreased the removal effcency through volatlty [10]. After the change n the retenton tme from 0 to 15 hours, the bologcal removal effcency of styrene and ethylbenzene n and 99.9 percent, respectvely, but the bologcal removal for styrene and ethylbenzene was 93 and 94%. Further, the styrene and ethylbenzene concentraton n the ext ar of the reactor was 4 and 3 ppm, respectvely. It should be noted that the concentraton of ext gases also ncreased because of the fall of the MLSS value. Therefore, n an HRT the reactor ncreased. Thus, when the retenton tme was

6 Q(S0-S)/V.X SRT/(1+kd. SRT) 1 S. M. Seyed et al. / Advances n Envronmental Technology 4 (016) of 10 hr, the bologcal removal effcency n the reactor was reduced sgnfcantly compared to an HRT of 15 and 0 hr Styrene, ethylbenzene bologcal removal effcency n HRT=15 h and SRT = 10 day The bologcal removal effcency of styrene and ethylbenzene at a SRT of 10 and a HRT of 15 hrs was measured n a steady state. The bologcal removal of styrene and ethylbenzene was about 94.6% and 98.7% whle the SRT of 0d was about 99% and 99%, respectvely. Moreover, the concentraton of ethylbenzene and styrene n the reactor ext ar was measured daly under ths condton. The results showed that n the SRT of 10 day, the concentratons of ethylbenzene and styrene n the reactor ext ar was 3. and 4.9 ppm, respectvely. The compresson of bologcal removal effcency and VOCs concentraton for styrene and ethylbenzene n two SRTs (10 and 0) are presented n Table Determnaton of boknetc coeffcents The determnaton of the boknetc coeffcents at an MLSS concentraton of 4000 mg/l was ntated usng an SRT of 0 days. Durng the nvestgaton, the SRT was vared between 0 and 5 days. Table 4 shows the steady-state data obtaned at an MLSS concentraton of 4000 mg/l, whle Fgures 3 and 4 show the determnaton of the coeffcents usng Eqs. 13 and 14. The values of the boknetc coeffcents were found to be as follows: 1. for styrene substrate: Y = mg/mg, kd = 0.5 day 1, μmax = h 1, and Ks = mg/l; and. for ethylbenzene: Y = mg/mg, kd = 0.5 day 1, μmax = h 1, and Ks =.81 mg /L. Table 3. Comparson of bologcal removal effcency and VOCs concentraton n the ext ar for two SRTs SRT (d) HRT (hr) Bologcal removal of STR Bologcal removal of EB STR concentraton n ar EB concentraton n ar Note: Styrene (STR), Ethylbenzene (EB) Table 4. The steady-state data obtaned at an MLSS concentraton of about 4000 mg/l SRT (d) HRT (h) X (mg/l) Q (L/d) S 0 (mg/l) for STR & EB S (mg/l) for STR S (mg/l) for EB y = 1.668x R² = y = x R² = /SRT (1/d) 1/S (L/mg) a) (b) Fg. 3. Determnaton of (a) Y and kd (b) μ max and Ks for styrene substrate

7 SRT/(1+kd. SRT) Q(S0-S)/V.X S. M. Seyed et al. / Advances n Envronmental Technology 4 (016) y = 0.08x R² = y = 0.334x R² = /SRT (1/d) (a) /S (L/mg) (b) Fg.4. Determnaton of (a) Y and kd (b) μ max and Ks for ethylbenzene substrate The results showed that the yeld (Y) and the endogenous decay coeffcent (kd) was the same for styrene and ethylbenzene as the substrate, but the maxmum specfc growth rate (μ max ) for ethylbenzene was more than styrene. As can be seen n Table 5, n comparson to pror research that consdered the specal pure culture to evaluate the knetcs of bodegradaton, the boknetc coeffcents obtaned for both styrene and ethylbenzene n ths study were dfferent from the prevously ganed values [1,]. Ths clearly showed that the type of substrate and bacteral consortum can have a sgnfcant effect on the determnaton of the boknetc coeffcents. The μmax value showed the capablty of the mcrobal culture n MBR to use the specal pollutant as a source of carbon and energy. Although some mcroorgansms showed an excessve ablty to bodegradaton, the other culture cannot approprately use these components as a source of energy. Therefore, the abundance of the mcroorgansms led to a competton between the bacterum cultures for the common substrate [3,4]. Furthermore, t can be seen n ths case that the ethylbenzene was more approprate as a source of carbon to actvated sludge n the MBR than the styrene, whch had a lower μmax than the ethylbenzene. In addton, the dfferent values of μmax demonstrated dfferent pathways n order to completely catabolze the selected components usng the mcrobal speces pcked to attack and catabolzed the carbon sources []. 4. Conclusons The operaton of a MBR system for the bologcal removal of volatle organc compounds such as styrene and ethylbenzene demonstrated the 15-hour tme as an optmum HRT value at a SRT of 0 day. The operaton of a lab scale MBR confrmed that t can be a feasble procedure to reduce VOC emssons from petrochemcal wastewater. The results showed that the yeld (Y) and the endogenous decay coeffcent (kd) was the same for styrene and ethylbenzene as for the substrate, but the maxmum specfc growth rate (μmax) for ethylbenzene was more than the styrene. Further, the values of the boknetc coeffcents, except that of kd, were wthn the normal range reported for these components. Table 5. Comparson between knetc parameters estmated for the bodegradaton of styrene and ethylbenzene n dfferent studes Stran Substrate μ max (1/h) Y(mg/mg) K S (mg/l) k d (d -1 ) ph References Actvated sludge n MBR Styrene Ths study Exophala jeanselme Styrene [7] Actvated sludge n MBR Ethylbenzene Ths study Pseudomonas putda F1 Ethylbenzene [8] References [1] Harper, D. B. (1977). Mcrobal metabolsm of aromatc ntrles. Enzymology of C N cleavage by Nocarda sp. (Rhodochrous group) NCIB Bochemcal journal, 165(), [] Ln, C. K., Tsa, T. Y., Lu, J. C., Chen, M. C. (001). Enhanced bodegradaton of petrochemcal wastewater

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