Anaerobic Treatment of Oily Wastewater Using A Bio lm-Electrode Reactor: A Kinetic Study And Energy Consumption

10 The present study introduced a laboratory-scale, anaerobic treatment system for the removal of oil 11 from synthetic wastewr using a biofilm-electrode reactor (BER). The operating parameters of 12 current intensity, initial concentration, reaction time, and supporting electrolyte were investigated. 13 The results of the present study showed that the optimal conditions were: a current intensity of 15 14 mA, COD concentration of 1500 mg/L, a reaction time of three days, and a supporting electrolyte 15 (NaCl) of 150 mg/L. The highest efficiency for the removal of COD was 86.7% using the 16 introduced method, while it was 65.9% using biological processes. Increased efficiency was 17 attributed to the employment of the proposed bioelectrochemical system that stimulated bacterial 18 growth. In the present study, the energy consumed by the bioelectrochemical system was 1.914 19 kWh/m. The kinetic study indicated that the removal reaction was more consistent with the 20


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Recently, oil-contaminated wastewater is considered one of the environmental problems. The large 27 quantity of these wastewaters is generated by various sources due to the rapid growth of  (Sharma et al., 2020). Oil, at higher levels, harms aquatic 36 life, creates obnoxious odors and unpleasant sights, reduces tourism activities, and causes 37 economic damage (Brillas et al., 2020). Various physical, chemical, and biological methods are 38 introduced for the removal of oil from wastewater, of which biological systems are significantly 39 used due to their advantages, of which more compatibility with the environment is noteworthy. 40 Moreover, no environmentally harmful chemicals are used in biological systems; therefore, their 41 effluent and sludge are less hazardous to the receiving waters than the chemical systems. These features make bioelectrochemical systems a cost-effective and environmental-friendly approach. 43 Biodegradation of organic matter is triggered by supporting microbial growth and creating 44 optimum environmental conditions to turn pollutants into carbon dioxide and other gases, 45 inorganic matter, water, safe and stable substances, and biomass. Among biological methods, 46 biofilm reactors have advantageous properties. It is well understood that biofilm reactors are 47 suitable for the treatment of effluents containing poorly biodegradable compounds and 48 decomposable organic matter. Biofilm, formed by the fixation on active microorganisms, increases 49 biomass and improves organic and hydraulic loading (Karadag et al.,2015). Fixed biofilm is 50 effective in reducing toxic compounds and the treatment of wastewater containing biodegradable 51 matter (Zhang et al., 2015). Other advantages of biofilm systems are longer shelf life, more diverse 52 microbial species, and increased stability (Tan et al., 2018). Numerous researchers tried to treat 53 oily effluent in anaerobic systems (Chan, et al., 2010;Wang et al., 2016). The anaerobic process, 54 widely used for industrial wastewater treatment, has advantages such as the generation of less 55 sludge, lower costs, less energy consumption, no need for aeration, low nutritional requirements, 56 high organic load tolerance, high shock loading resistance, and production of methane gas. The 57 improvement and repair of this process are faster if it is used in industries closed for a period in 58 the year with no utilization. It also has disadvantages as the quality of wastewater is not high and 59 does not meet the effluent disposal standards (Kong, et al., 2019). Therefore, an anaerobic system 60 needs further measures to be improved and upgraded, for example, increasing the activity of 61 bacteria by electrostimulation (Adibzadeh, et al., 2016). Bioelectrochemical systems (BESs) or 62 biofilm electrode reactors (BERs), as a relatively new technology, are appropriate and promising 63 methods with great potentials for wastewater treatment and are considered clean technology. In 64 this technology, microorganisms are used as an electrode-attached biofilm. In this method, 65 oxidation-reduction (redox) reactions are catalyzed by the interaction between the electrode and 66 the biofilm. In recent years, wastewater treatment by BESs is critically considered by 67 researchers (Cao, et al., 2018). Evidence suggests that induced current can stimulate metabolism 68 and enhance biochemical function in bacteria. The applied current increases the rate of ion 69 migration and reactions on the surface of the electrodes. Lower voltages are used in these systems, 70 which can overcome some problems such as corrosion of the anode and high energy consumption 71 observed in the electrochemical method. The induced current must be adequate, otherwise inverse 72 results are obtained, and the activity of microorganisms is restrained . Due to the 73 increasing generation of oil-contaminated wastewater and the benefits of applying the anaerobic 74 method, and that this method needs upgrading and improvement of efficiency, the present study   It contained stainless steel electrodes fixed by a holder. It was covered to provide anaerobic 87 conditions. The anode electrode was steel mesh, and its cavities were so small to facilitate the 88 loading of biomass. A magnetic stirrer (Alfa, HS-860, Iran) was used throughout the experiments 89 for gently mixing and homogenizing the reactor contents. Direct current was supplied by a power 90 supply (ATTEN APS3005S-3D, China). The electrodes were washed with HCl, rubbed with a 91 sponge, and rinsed with distilled water in order to be prepared.  The current experimental, laboratory-scale study was performed as a batch system in a BER under 97 anaerobic conditions. The seed sludge was taken from a wastewater treatment plant (Tehran, Iran), 98 washed three times with tap water to remove impurities, and used as a microbial inoculant. All

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COD concentration was measured by the closed reflux method, described in the standard method.

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The samples were analyzed using a spectrophotometer (Rayleigh, Vis-7220 / UV-9200). The   Assuming that the first-order kinetic reactor is predominant, the changes in substrate concentration 124 in a completely-mixed system were expressed as follows: In a biological reactor, under steady-state conditions, changes in the removal of substrate where KB and Umax represent saturation constant (mg/L.day) and maximum substrate removal rate 154 (mg/L.day), respectively. The linear form of this equation is expressed as Equation 9 : Plotting V/[Q (S0 -Se)] versus (V/QS0) creates a straight line that gives the intercept and slope of electric current. Oil-contaminated compounds usually contain substances that are not easily 163 degraded by bacteria in nature, so the bacteria should be acclimatized to the environment. The 164 gradual increase in pollutant concentration is a method that can be used for better adaptation. This 165 strategy prevents severe shocks, and by gradually adding oil-contaminated wastewater, the bacteria 166 are given the chance to adapt to the environment. Figure 1 shows the biomass adaptation results.

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COD concentration at this stage was 1500 mg/L, and HRT was considered three constant days.

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Glucose was first used to acclimatize the bacteria to oil-contaminated wastewater as it is a palatable 169 organic matter for them. During the experiment, glucose levels were reduced and the oil-   Evaluation of the effect of changes in the applied current on the removal efficiency using BES 184 indicated that by applying a current intensity of 5 mA, the removal efficiency was 72.3%, and the applied current intensity increased after reaching stable conditions. With increasing current 186 intensity above 15 mA, the removal efficiency decreased, and the decreasing trend even continued 187 by increasing the current intensity. In other words, a very high increase in current intensity reduced 188 efficiency. As shown in Figure 2b, maximum efficiency was obtained at 15 mA that was 83.4%.

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Therefore, it was selected as the optimum current and used in the experiment. The obtined results

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show that the optimum current has a beneficial effect on bacteria and their enzymatic activity.

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When an electric field is applied to the microbial system, the permeability of the cytoplasmic   the conditions for electrostimulation are more suitable at higher HRTs than shorter times in BESs.

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That is, the removal efficiency is higher at higher HRTs. They also found that the oil and grease 274 removal efficiencies were significantly higher than that of COD, which could be due to the Therefore, researchers believe that higher HRTs are beneficial for the removal of poorly 296 degradable or biodegradation-resistant compounds. In a bioreactor that refines poorly-degradable 297 materials, a longer HRT can help to achieve high biodegradation efficiencies. Increased HRT gives 298 more chance to substances to better contact microorganisms that increases the biodegradation rate.

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In lower HRTs, organic loading increases, leading to incomplete biodegradation so that the 300 degradation process remains incomplete. In addition to influencing the efficiency of the process,

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HRT affects reactor volume and manufacturing costs. Therefore, determining the optimum time 302 for an acceptable and satisfactory efficiency is one of the main stages in the bioreactor design, 303 considering the minimum bioreactor volume required (Shi et al., 2017). To prevent the loss of 304 system biomass, resulting from shock loading, and improve the reactor performance, HRT 305 increased at the initial stage (three days).    The steady-state data of each stage were used to determine the kinetic coefficients. As shown in 467 Figure 10a and Table 1, the first-order kinetic constant (K1) was 0.485 (1/d). Also, the correlation 468 coefficient (R 2 ) was 0.85.  In the present study, Umax was 1.106 and KB 0.767 g/ L.d. Both KB and Umax play a pivotal role in 476 determining the volume of a bioreactor. The results showed that among the three proposed models, the R 2 of the modified Stover-Kincannon model was higher (0.975), indicating that the model was 478 more fitted with bioreactor performance and the COD removal reaction was more consistent with 479 it. Therefore, the modified Stover-Kincannon model can be used for the accurate prediction of the 480 removal of biodegradable organic matter.             Comparison of COD removal e ciency with and without electric eld.

Figure 8
Kinetic plots of the COD removal through the BER process: (a) the rst-order model; (b) the second-order (Grau) model; (c) the modi ed Stover-Kincannon model.