Removal of selected nitrogenous heterocyclic compounds in biologically pretreated coal gasification wastewater (BPCGW) using the catalytic ozonation process combined with the two-stage membrane bioreactor (MBR)

Accepted Manuscript Removal of selected nitrogenous heterocyclic compounds in biologically pretreated coal gasification wastewater (BPCGW) using the catalytic ozonation process combined with the two-stage membrane bioreactor (MBR) Hao Zhu, Yuxing Han, Wencheng Ma, Hongjun Han, Weiwei Ma PII: DOI: Reference:

S0960-8524(17)31577-8 http://dx.doi.org/10.1016/j.biortech.2017.09.029 BITE 18857

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

12 July 2017 3 September 2017 4 September 2017

Please cite this article as: Zhu, H., Han, Y., Ma, W., Han, H., Ma, W., Removal of selected nitrogenous heterocyclic compounds in biologically pretreated coal gasification wastewater (BPCGW) using the catalytic ozonation process combined with the two-stage membrane bioreactor (MBR), Bioresource Technology (2017), doi: http://dx.doi.org/ 10.1016/j.biortech.2017.09.029

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Removal of selected nitrogenous heterocyclic compounds in biologically pretreated coal gasification wastewater (BPCGW) using the catalytic ozonation process combined with the two-stage membrane bioreactor (MBR)

Hao Zhu1, Yuxing Han2, Wencheng Ma1, Hongjun Han1,*, Weiwei Ma1

1

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute

of Technology, Harbin 150090, China 2

School of Engineering, South China Agriculture University, Guangzhou 510642,

China

1

*Corresponding author: E-mail: [email protected], Tel: +86 13946003379,

Fax: +86 451 87649777 1

Abstract Three identical anoxic-aerobic membrane bioreactors (MBRs) were operated in parallel for 300 consecutive days for raw (R1), ozonated (R2) and catalytic ozonated (R3) biologically pretreated coal gasification wastewater (BPCGW) treatment. The results demonstrated that catalytic ozonation process (COP) applied as a pretreatment remarkably improved the performance of the unsatisfactory single MBR. The overall removal efficiencies of COD, NH3-N and TN in R3 were 92.7%, 95.6% and 80.6%, respectively. In addition, typical nitrogenous heterocyclic compounds (NHCs) of quinoline, pyridine and indole were completely removed in the integrated process. Moreover, COP could alter sludge properties and reshape microbial community structure, thus delaying the occurrence of membrane fouling. Finally, the total cost for this integrated process was estimated to be lower than that of single MBR. The results of this study suggest that COP is a good option to enhance pollutants removal and alleviate membrane fouling in the MBR for BPCGW treatment. Keywords Nitrogenous heterocyclic compounds; Catalytic ozonation; Membrane bioreactors; Coal gasification wastewater; Microbial community structure

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1. Introduction The biologically pretreated coal gasification wastewater (BPCGW) contains large number of toxic and refractory compounds, such as phenolic compounds, polycyclic aromatic hydrocarbons, nitrogenous heterocyclic compounds (NHCs), long-chain hydrocarbons, ammonia, etc., along with low biodegradability and unsatisfactory effluent quality (Zhuang et al., 2014). This wastewater treatment has become a bottleneck for the development of coal gasification industry in China which has played a key role in green and renewable energy market in recent years (Wang and Han, 2012). Among various pollutants, NHCs such as quinoline, pyridine and indole are typical refractory biodegradable substances (Chen et al., 2013; Jing et al., 2013; Padoley et al., 2011). Heterocyclic structures of them improve their solubility compared with that of homocyclic analogues, making them easier to get into the soil and ground water (Padoley et al., 2008). Once released into the environment, they present high toxicity, mutagenicity and carcinogenicity among various life forms (Chang et al., 2014; Kamath and Vaidyanathan, 1990; Zhu et al., 2016). Therefore, it is necessary to develop efficient treatment techniques to eliminate the target pollutants such as quinoline, pyridine and indole before they enter into the environment. Various technologies have been applied for the treatment of wastewater containing quinoline, pyridine and indole, such as biological methods (Chen et al., 2013; Liu et al., 2015; Luo et al., 2010; Sun et al., 2011; Zhang et al., 2009), Fenton-like oxidation (Li et al., 2014), catalytic oxidation (Singh and Shang, 2016), ozonation (Tekle-Röttering et al., 2016), adsorption (Zhu et al., 2016), wet oxidation 3

(Thomsen and Kilen, 1998), O3/UV (Wang et al., 2004), electrochemical oxidation (Wang et al., 2016) and UV photolysis (Yan et al., 2013). For biodegradable organic pollutants, biological processes are economical and environmentally friendly technologies (Yan et al., 2013), while NHCs have toxic and inhibitory impact on the biological processes at high concentrations (Chen et al., 2013; Jing et al., 2013; Padoley et al., 2011). It has been reported that pretreatment of quinoline in a catalytic ozonation process leads to partial degradation of the toxic compounds and thus the organic load applied on the biological treatment process is reduced (Zhu et al., 2017). It in turn results in enhancing the subsequent biodegradation and thus of the overall efficiency (Dai et al., 2014). In other words, chemical oxidation as a pretreatment process could be used to reduce toxicity and to increase the biodegradability of wastewater, which would favor the subsequent biological process and improve overall effectiveness (Moussavi et al., 2014). Heterogeneous catalytic ozonation was found to be very effective for eliminating toxic pollutants in wastewater (Bai et al., 2016). In this process, a catalyst is applied to accelerate ozone decomposition to generate hydroxyl radicals, which are capable of oxidizing toxic pollutants to convert them into readily biodegradable compounds. Among various catalysts, nano-MgO presented good catalytic performance for the degradation of several types of water and wastewater pollutants due to its high efficient activity, structural stability, environmental friendliness and low toxicity (Moussavi et al., 2010; Moussavi and Mahmoudi, 2009). While high cost of single catalytic ozonation process (COP) has restricted its wide application. Therefore, there 4

was a great advantage of combining COP with biological process to achieve a more efficient and cost-effective performance for treating low biodegradability and high toxicity wastewater (Aghapour et al., 2015). Especially, nitrogenous compounds were difficult to remove in COP, further indicating the necessity of subsequent biological treatment (Zhuang et al., 2014). It was reported that the two-stage MBR had been developed and successfully applied for the treatment of wastewater (Boonyaroj et al., 2012; Chiemchaisri et al., 2011). The first stage anoxic reactor followed by an aerobic reactor with a submerged membrane module for solid-liquid separation was attractive for the removal of biological nitrogen through pre-denitrification and aerobic nitrification process. In particular, the system has been proved to be feasible in the removal of emerging contaminants (Boonnorat et al., 2014). Therefore, it is a good option for the treatment of BPCGW. To our best knowledge, no study could be found on heterogeneous catalytic ozonation with nano-MgO as a catalyst combined with the two-stage MBR for the treatment of BPCGW. The primary objects of this study were as follows: (1) to compare the performance of the two-stage MBR for the treatment of BPCGW with and without oxidized pretreatment. (2) to explore the role of COP in the combined system in terms of sludge properties, membrane fouling and microbial community structure. (3) to analyze the energy consumption in MBR and COP combined with MBR system. 2. Materials and methods 2.1. Characteristics of wastewater and inoculation sludge The real BPCGW was collected from the effluent of secondary settling tank in 5

the full-scale wastewater treatment facility. Raw wastewater had a relatively stable composition of 150-180 mg L-1 of chemical oxygen demand (COD), 0.05-0.08 of BOD5/COD ratio (B/C), 60-70 mg L-1 of total organic carbon (TOC), 50-60 mg L-1 of total nitrogen (TN), 20-30 mg L-1 of ammonia nitrogen (NH3-N) and pH value of 7.5-8.5. It was stored in 50-L polyethylene barrels in dark at 4 ℃ before use. The inoculation sludge was taken from aerobic tank in full-scale wastewater treatment facility. The aerobic tank had been operating for over 4 years and the sludge was grey-black with good settlement property. 2.2. Experimental set-up and operating conditions The COP set-up and operation were described in detail in the previous published report (Zhu et al., 2017). Schematic diagram of the experiment set-up was shown in Fig. 1. Ozone was produced in dry air by an ozone generator (ANSEROS, COM-AD-01) and was introduced continuously into the solution at the bottom of the reactor through a porous glass diffuser with a desired gas flow rate of 40 L h-1 in the experimental process. When the gas flow rate was constant, the influent ozone concentration was controlled at 4 mg L-1. 0.2 g of the catalyst was put into the reactor. All experiments were carried out at room temperature. The COP reactor was operated in batch mode using nano-MgO as the catalyst. The preparation method and the characteristics of the catalyst were presented in detail elsewhere (Zhu et al., 2017). In brief, 20 g MgCl2•6H2O was firstly dissolved in 1 L deionized water. Then, 0.2 g polyving akohol was put into the solution and magnetically stirred at 200 rpm for 1 h. 30 ml of NaOH (6 M) was slowly dripped into 6

the prepared solution and the solution was aged for 24 h at room temperature. Subsequently, the precipitates were centrifuged at 4000 rpm for 10 min and washed with deionized water and ethanol for three times. The washed precipitates were dried at 120 ℃ for 12 h. Finally, the power was calcined at 500 ℃ for 2 h under air atmosphere. In each batch, the BPCGW was fed to the COP reactor, which was operated under the conditions of influent ozone concentration of 4 mg L-1, catalyst dosage of 0.2 g L-1 and reaction time of 30 min. The effluent concentration of COD, NH3-N, TN, quinoline, pyridine and indole after COP were 122.5 mg L-1, 28.4 mg L-1, 56.1 mg L-1, 0.58 mg L-1, 3.2 mg L-1 and not detected, respectively. At the end of the specific batch, the catalyst was separated from the solution by filtration and the filtrate was transferred into the inlet tank of the two-stage MBR after regulating the pH value from around 5.5 to 7.5 by adding sodium hydroxide solution. Ozonated experiment was performed as the above procedure except the addition of catalyst. Oxidized BPCGW was also preserved in a 50-L polyethylene barrel in dark at 4 ℃ prior to use. Three identical anoxic-aerobic membrane bioreactors were operated in parallel for raw (R1), ozonated (R2) and catalytic ozonated (R3) BPCGW treatment, respectively. The two-stage MBR consisted of an anoxic reactor followed by an aerobic membrane bioreactor, the working volumes of which were 0.75 L (Diameter×height: 6×26.5 cm) and 2.25 L (Length×width×height: 15×5×30 cm), respectively. The two-stage MBR reactors were performed in a sequential mode. An agitator was applied in the anoxic reactor and a hollow fiber membrane (Tianjin 7

Motian Membrane Technology Co., LTD) with a pore size of 0.2 um as well as a total membrane surface area of 0.2 m2 was submerged in the aerobic reactor. An air pump was installed at the bottom of the membrane module to provide dissolved oxygen (DO). DO concentrations in the anoxic tank and aerobic tank were maintained at 0.3-0.5 mg L-1 and 4-5 mg L-1, respectively. A recirculation with a ratio of 2 was employed from the aerobic reactor to the anoxic reactor. The system was carried out in an intermittent suction mode with 9.5 min of membrane filtration followed by a 0.5 min backwashing. Chemical cleaning (0.5% sodium hypochlorite for 1 h followed by tap water rinsing) was conducted when the trans-membrane pressure exceeded 40 kPa. The temperature was maintained at 28-32 ℃ using electrical heating system and transmembrane pressure (TMP) was measured by vacuum manometer to control the permeate flow. At the startup and acclimation stage, the volume of BPCGW increased stepwise from 20 % to 100 % with the addition of sodium acetate as external carbon source to make the final COD concentration of around 180 mg L-1. Meanwhile, nitrate, ammonium, phosphate and trace metals were added into the MBR system for the microbial growth. During the stable stage, there was no sludge discharge from the system during the long term operation except a small amount for analytical purpose. The MBRs were operated in two experimental stages for throughout 300 days. In the first stage, the three MBRs started up with the increase of BPCGW volume percentage at hydraulic retention time (HRT) of 24 h for 150 days until stable period. Then, performance of the MBRs was investigated at HRT of 12h on days 151-210. In 8

the second stage, one of the MBRs (R1) was still used to treat BPCGW. The other two were applied to treat the oxidized (R2 and R3) BPCGW at HRT of 12 h on days 211-300 to study their performance in terms of COD, NH3-N, TN and NHCs. In addition, comparison in sludge properties, membrane fouling and microbial community structure between R1 and R3 was performed to evaluate the role of catalytic ozonation pretreatment. 2.3. Analytical methods COD, NH3-N, TN, mixed liquor volatile suspended solids (MLVSS) and mixed liquor solids concentration (MLSS) were analyzed according to the standard methods for the examination of water and wastewater (APHA, 1998). The concentration of quinoline, pyridine and indole was analyzed using a liquid chromatograph (HPLC, LC-1200, Agilent, USA) with an Agilent C18 (150 mm×4.6 mm). The mobile phase was 60% methanol and 40% water and the wavelength of quinoline, pyridine and indole was 313 nm, 253 nm and 287 nm, respectively. Ozone concentration in the gas was monitored by ozone gas detection instrument. DO concentration and pH were determined by a hybrid meter (HACH 30 d). Particle size distribution of sludge was measured by Nano-s (Malvern, England). Extraction of extracellular polymeric substances (EPS) and soluble microbial products (SMP) as well as measurement of protein and polysaccharide derived from the method in previous study (Tang et al., 2016). The sludge samples from aerobic reactor and membrane module of R1 and R3 on day 300 were collected to analyze the microbial community via high-throughput 16S 9

rRNA pyrosequencing. The detailed methods of DNA extraction, PCR and sequencing were mainly based on the studies of Wang et al., (2017). 3. Results and discussion 3.1. Performance of MBR Fig. 2 showed the removal of COD, NH3-N, TN and selected NHCs with different BPCGW ratios in feed during MBR operation. As revealed in Fig. 2a, average COD removals of 94.7%, 79.4%, 67.2% and 52.3% were observed when the volumetric ratio of BPCGW was 20%, 50%, 80% and 100%, respectively. COD removal efficiency sharply decreased to 40.8% when the HRT was reduced from 24 h to 12 h. With the increased BPCGW loading due to the increased volumetric percentage or reduced HRT in feed, microbial activity in MBR system was inhibited due to the increased toxic and refractory compounds, which would decrease the COD removal efficiency. In a previous study, Jia et al., (2014) also observed that increased organic loading rate (OLR) led to the reduction of COD removal in a MBR reactor for the treatment of coal gasification wastewater (CGW). As revealed in Fig. 2b, 86.1% of NH3-N (influent concentration of 25 mg L-1) was removed with the effluent all below 5 mg L-1 during the entire experimental period, indicating that the removal of NH3-N was not affected by the BPCGW loading. While it was worth noting that the effluent NH3-N concentration increased suddenly on the 60st and 150st day. The most possible reason was believed to be that increased BPCGW loading caused by BPCGW ratio rise or HRT reduction restrained the bioactivity of autotrophic nitrifying bacteria and nitrification process (Sun et al., 10

2011). As shown in Fig. 2c, when the concentration of influent TN was 56.1 mg L-1, removal efficiency of 73.1% and 66.1% was found from the stage of 20% BPCGW to 50% BPCGW, while higher effluent TN concentration was observed later (80% BPCGW and 100% BPCGW), implying that the adaptability of the system to a high-loading BPCGW impact was not good. Previous research had shown that TN removal efficiency was correlated positively with HRT (Kim et al., 2008), which explained the decreased TN removal efficiency at shorter HRT of 12 h in this study. Fig. 2d was made to illustrate the selected NHCs concentrations in raw wastewater and MBR permeate at various volumetric ratios of BPCGW. It was found that removal rates decreased with the increasing of BPCGW ratio, which might be due to the bio-inhibitory impact of NHCs on microbial activity (Chen et al., 2013; Jing et al., 2013; Padoley et al., 2011). Besides, removal of quinoline, pyridine and indole declined at shorter HRT of 12 h and they could be still detected after the treatment. The phenomena was also found in COD and TN removal and the reasons were similar. Based on the above results, it was revealed that a single MBR system could not be effective in treating BPCGW. Therefore, it was decided to combine COP with the MBR to improve its performance. 3.2. Performance of combination of the catalytic ozonation and MBR As shown in Fig. 3, the performance of the MBR in the combined system started to increase immediately after introduction of the oxidized BPCGW and reached to the steady-state level after the combination for 6-10 days. It was found that the removal 11

efficiencies of COD, NH3-N and TN in R3 reached 92.7%, 95.6% and 80.6%, respectively, which were higher than those of R2 (68.1% of COD, 88.8% of NH3-N and 71.0% of TN). In the previous literatures, physicochemical pretreatment combined with biological processes were proved to be very effective methods to treat the BPCGW (Hou et al., 2015; Jia et al., 2015; Zhuang et al., 2014). For a typical case, Zhuang et al., (2014) investigated the treatment of BPCGW by heterogeneous catalytic ozonation integrated with anoxic moving bed biofilm reactor and biological aerated filter process. After pretreatment of raw wastewater during 30 min of COP, the biological process acquired a high COD, TOC, NH3-N, TN and TPh removal efficiency, up to 87.5%, 85.5%, 90.9%, 80.6% and 98.7%, respectively. Additionally, as shown in Table 1, the concentrations of quinoline, pyridine and indole were 7.5, 16.2 and 7.1 mg L-1, respectively after single MBR treatment. Although both pretreatment of ozonation and catalytic ozonation improved the removal efficiency of NHCs, it was found that pyridine (2.1 mg L-1) could be still detected in R2. Better performance was achieved in R3, which could be attributed to the detoxification of BPCGW in the COP. The reduction of refractory pollutants resulted in comfortable conditions for subsequent biodegradation. Indeed, the combined system of catalytic ozonation and MBR could efficiently treat the BPCGW during a total time of 12.5 h (30 min in the COP and 12 h in the MBR). Accordingly, the COP with nano-MgO as a catalyst is an appropriate pretreatment option for improving the performance of the overloaded biological processes. 3.3. The role of COP in the combined system 12

According to the above results, COP pretreatment could benefit the performance of MBR. In order to better understand the role of COP in the combined system, comparison in terms of sludge properties, membrane fouling and microbial community between R1 and R3 was investigated. 3.3.1. Effect of COP on sludge properties Fig. 4a illustrated MLVSS and the MLVSS/MLSS ratio over the period of days 210-300 in each MBR. Slight difference of MLVSS was observed between R1 and R3. Generally, MLVSS/MLSS ratio served as an indicator of viable sludge fractions in a biological system. According to the previous report (Xue et al., 2016), VSS fraction of total suspended solids was about 0.85 and the ratio could be as low as 0.60 under certain circumstances. It was found that the MLVSS/MLSS ratios in R1 and R3 were ~0.56 and ~0.65, respectively, notwithstanding some minor fluctuations over time. The MLVSS/MLSS ratio of R3 was higher than that of R1, suggesting a lower amount of inert solids in R3 (Xue et al., 2016), which might contribute to the higher removal efficiency of R3. EPS and SMP of the two MBRs during the period of days 210-300 were depicted in Fig. 4b. As shown in Fig. 4b, R1 had higher EPS-protein, EPS-carbohydrate, SMP-protein and SMP-carbohydrate concentrations than those of R3. The EPS-protein, EPS-carbohydrate, SMP-protein and SMP-carbohydrate of R1 were 182.8, 35.6, 21.3 and 4.6 mg g-1 MLVSS-1, respectively; while the corresponding concentrations of R3 were 84.5, 26.7, 10.8 and 3.2 mg g-1 MLVSS-1, respectively. The lower concentrations of EPS and SMP of R3 could be attributed to more active biomass, which prevented 13

the release of more EPS and SMP produced by cell lysis to the bulk liquid. In addition, the EPS-protein and SMP-protein were more abundant than the EPS-carbohydrate and SMP-carbohydrate in the two MBRs, indicating that proteins of EPS and SMP were the dominant components and played an important role in the removal efficiency of the two MBRs, which were consistent with the previous study (Ng et al., 2015). The relationship between EPS and SMP fractions of sludge in the reactor and cake sludge in the fouled membrane surface will be discussed in the future work. Fig. 4c showed the particle size distribution (PSD) of the two MBRs. It was observed that R1 had a slightly larger median particle size (46.6 um) than that (36.0 um) of R3. As reported by Xue et al., (2016), the MBR treating oxidized wastewater owned a smaller representative particle size compared with that of MBR treating raw wastewater, which was similar to this investigation. It was suggested that the concentration of EPS and SMP was positively related to the membrane fouling tendency in the MBRs. Regarding the correlation between sludge PSD and EPS as well as SMP, R1 showed higher PSD and higher sludge EPS and SMP concentration. 3.3.2. Effect of COP on membrane fouling Fig. 5 described the TMP values of R1 and R3 over the operating period of days 210-300. TMP values of the two parallel MBRs before day 210 increased slowly and the values were no more than 8 kPa (data not shown). Throughout 300 days of continuous operation, the TMP value of R1 exceeded the critical TMP (40 kPa) on day 280, while the occurrence of membrane fouling of R3 was delayed. Maximum TMP of 22.3 kPa was observed in R3 throughout 300 days of continuous operation. It 14

indicated that catalytic ozonation pretreatment could improve mitigation of membrane fouling. On one hand, more inert solids of R1 might be positively related to membrane fouling tendency. On the other hand, the higher EPS and SMP concentrations of R1 would lead to severe membrane fouling because they could block the membrane pores and/or form a membrane cake layer, resulting in an increase in membrane filtration resistance (Ng et al., 2015). In addition, higher protein/carbohydrate ratio (5.1 for EPS and 4.6 for SMP) of R1 compared with that (3.2 for EPS and 3.4 for SMP) of R3 might be another reason for its rapidly increased TMP value. What’s more, according to Xue et al., (2016), higher sludge particle size contributed to severer fouling and the phenomenon was also observed in the current study. 3.3.3. Effect of COP on microbial community The bacterial communities in the aerobic sludge and cake layer of R1 as well as those of R3 were analyzed through high-throughput 16S rRNA pyrosequencing. Summary of the sequencing was shown in Table 2. The observed species of aerobic sludge and cake layer of R1 were 2849 and 3124 while those values for R3 were 3567 and 3953. As a parameter represented for richness, the Chao1 were 5534 and 5778 in aerobic sludge and cake layer of R3, which were higher than those of R1 (4235 and 4529). The Shannon indexes, representing community diversity, were 4.16 and 4.67 in aerobic sludge and cake layer of R1, which were lower than those of R3 (5.22 and 5.86). The above indexes indicated that bacterial community in aerobic sludge and cake layer became relatively richer by catalytic ozonation pretreatment. It has been proved that the high biodiversity could protect the ecosystem against its function 15

decline and allowed it to adapt to conditions change (Wang et al., 2017). To get an insight into microbial community structure in four sludge samples, the sequences from the dominant bacterial communities were analyzed on the genus level (Fig. 6). As revealed in Fig. 6, it was found that the most abundant genus in R1 (aerobic sludge vs cake layer) were Acidovorax (11.5% vs 14.5%), followed by Thauera (10.3% vs 12.6%), Dechloromonas (8.5% vs 10.4%), Thiothrix (6.3% vs 8.2%), Pseudomonas (6.2% vs 8.6%), Afipia (4.1% vs 6.3%), Comamonas (3.1% vs 5.2%), Zoogloea (2.9% vs 4.9%), Bdellovibrio (2.8% vs 4.3%) and Ferruginibacter (1.2% vs 1.8%). While the predominant species in R3 (aerobic sludge vs cake layer) were Acidovorax (17.3% vs 21.2%), followed by Pseudomonas (9.3% vs 11.2%), Thauera (7.3% vs 9.8%), Afipia (6.1% vs 8.8%), Dechloromonas (5.9% vs 9.3%), Comamonas (5.9% vs 7.7%), Zoogloea (5.9% vs 7.2%), Bdellovibrio (4.8% vs 6.7%), Ferruginibacter (1.9% vs 2.5%) and Thiothrix (1.1% vs 2.3%). Acidovorax and Zoogloea were reported to be likely related with denitrification process (Grijalbo et al., 2015). Meanwhile, Comamonas and Ferruginibacter have been demonstrated to have capacity to remove aromatic pollutants such as quinoline, pyridine and phenol (Jiang et al., 2017; Tao et al., 2017). In addition, it was reported that Bdellovibrio could degrade benzene, toluene, ethylbenzene and xylene (Li and Goel, 2012). The substantially higher abundances of these genera in R3 might contribute to the high removal of organic compounds and nitrogen. On the other hand, previous studies have revealed that Comamonas and Pseudomonas promoted the membrane fouling alleviation (Jia et al., 2017; Jiang et al., 2017), which explained the lower TMP value 16

of R3 during the period of days 210-300. Additionally, some genera (e.g., Thauera, Dechloromonas and Thiothrix) were EPS producers (Huang et al., 2017; Xue et al., 2016), suggesting that higher abundance of them in R1 might result in the server membrane fouling. What’s more, the microbial diversity of the cake layer community was higher compared with the suspended biomass in each MBR, which was consistent with the previous study (Xue et al., 2016). The results of this section indicated that catalytic ozonation pretreatment could enhance the overall removal efficiency and improve the MBR's anti-fouling performance by reshaping the microbial community structure. 3.4. Energy assessment In order to assess the applicability of the combined system for the treatment of BPCGW, energy consumption studies were evaluated based on above results. For catalytic ozonation experiments, the energy consumption was 12.5 kWh/(kg O3) according to the manufacture. Under the conditions of ozone dosage of 4 mg L-1, influent flow of 40 L h-1 and reaction of 30 min for treatment of 1 L BPCGW, it was estimated that energy consumption for 1 m3 wastewater worked out to be 1 kWh m-3. The energy cost in China, though varies from place to place, but the average is 0.06 EUR kWh-1. Thus, as shown in Table 3, the energy cost for 1 m3 BPCGW treatment by catalytic ozonation would be 0.06 EUR m-3. Average energy consumption for MBR is 0.7 kWh m-3 in terms of peristaltic pumps, aeration pumps and agitator. According to the reagents cost for membrane cleaning, the total cost for this integrated process is estimated to be 0.214 EUR m-3, which was lower than that of 17

single MBR (0.258 EUR m-3). This can be linked to the reduction in COD as well. Considering the concentrations of COD removed, the cost for reduction in COD would be 1.28 EUR kg-1 of COD for combined system of catalytic ozonation and MBR, almost one third of that of single MBR. 4. Conclusions The performance of COP as a pretreatment of the two-stage MBR with nano-MgO as a catalyst was examined for the treatment of BPCGW. The results revealed that the combined system achieved high removal efficiencies of COD, NH3-N and TN of 92.7%, 95.6% and 80.6%, respectively. The lower concentration of EPS (111.2 mg g-1 MLVSS-1) and SMP (14.0 mg g-1 MLVSS-1) as well as TMP value (maximum of 22.3 KPa) was found in R3. The higher removal efficiency and better fouling control performance signalled that the combined process was a promising technology for BPCGW treatment in large-scale in the future. Acknowledgements This work was supported by Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No.QA201611-01).

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21

Figures Captions Fig. 1. Schematic diagram of the experimental set-up. Fig. 2. Concentrations and removal of COD (a), NH3-N (b), TN (c) and selected NHCs (d) with different BPCGW ratios in feed during MBR operation. Fig. 3. Concentrations and removal of COD (a), NH3-N (b) and TN (c) in R2 and R3. Fig. 4. MLVSS, MLVSS/MLSS, EPS, SMP and particle size distribution of R1 and R3. Fig. 5. TMP of R1 and R3 over time. Fig. 6. Bacterial community structure of aerobic sludge and cake layer in R1 and R3. Genus level with relative abundance lower than 1.00% were classified into group “others”.

Table Captions Table 1 Concentrations of nitrogenous heterocyclic compounds in each process. Table 2 Alpha diversity analyses. Table 3 Economical assessment of R1 and R3.

22

Fig. 1. Schematic diagram of the experimental set-up.

23

75

100

50 HRT=24 h

HRT=12 h

50

25

0 0

30

60

90

120

150

180

30

75

25 Influent NH3-N

20

20% 50% 80%

15 10

30

100%

100%

60

(d)

50

25 HRT=24 h

HRT=12 h

0 60

90

120

150

180

-1

Concentration (mg L )

Influent TN Effluent TN TN removal efficiency

TN removal efficiency (%)

TN concentration (mg L-1)

45

30

120

180

90

150

25

0 210

HRT=12h

HRT=24h

100

75

0

HRT=12 h

Time (d)

40

0h

0h

Quinoline Pyridine 0h Indole

50

15

HRT=24 h

5 0

60

30

50

Effluent NH3-N NH3-N removal efficiency

0

0 210

100

35

Time (d)

(c)

100%

NH3-N removal efficiency (%)

Influent COD Effluent COD COD removal efficiency

150

100%

20% 50% 80%

(b) 40

100

NH3-N concentration (mg L-1)

COD concentration (mg L-1)

100%

COD removal efficiency (%)

100%

50% 80% (a) 200 20%

12h

30

24h

0h

20

24h 0h

24h

10 24h

0 210

0 20%

Time (d)

100% 80% 50% Volumetric BPCGW ratio

100%

Fig. 2. Concentrations and removal of COD (a), NH3-N (b), TN (c) and selected NHCs (d) with different BPCGW ratios in feed during MBR operation.

24

100 120 80

100 80

60

Influent COD of R3

60 40

Effluent COD of R3 COD removal of R3

40

COD removal of R2

20

20 0 210

240

270

COD removal efficiency (%)

COD concentration (mg L-1)

(a)

0 300

Time (d)

100

30

80

25 Influent NH3-N of R3

20

60

Effluent NH3-N of R3 NH3-N removal of R3

15

NH3-N removal of R2

40

10 20

5 0 210

240

270

NH3-N removal efficiency (%)

NH3-N concentration (mg L-1)

(b) 35

0 300

Time (d)

100

50

80

40 60

Influent TN of R3

30

Effluent TN of R3 TN removal of R3

20

40

TN removal of R2

20

10 0 210

240

270

TN removal efficiency (%)

TN concentration (mg L-1)

(c) 60

0 300

Time (d)

Fig. 3. Concentrations and removal of COD (a), NH3-N (b) and TN (c) in R2 and R3. 25

1.0

(a) 5000

0.6

3000

0.4

2000

R1 (MLVSS)

1000

R1 (MLVSS/MLSS)

MLVSS/MLSS

MLVSS (mg L-1)

0.8 4000

R3 (MLVSS)

0.2

R3 (MLVSS/MLSS)

0 210

240

0.0 300

270 Time (d)

EPS and SMP (mg g-1 MLVSS-1)

(b) 200

EPS Protein EPS Carbohydrate SMP Protein

150

SMP Carbohydrate

100

50

0 R3

R1

(c)

8

Volume percentage (%)

R1 R3

6

4

2

0 0

50

100

150

200

250

Particle size (um)

Fig. 4. MLVSS, MLVSS/MLSS, EPS, SMP and particle size distribution of R1 and R3. 26

Chemical cleaning

40

R1

TMP (kPa)

R3

30

20

10 0 210

240

270 Time (d)

300

Fig. 5. TMP of R1 and R3 over time.

27

Afipia Thiothrix

R3 cake

Dechloromonas

R1 cake

R3 aerobic R1 aerobic

Thauera Pseudomonas Bdellovibrio Ferruginibacter Comamonas Zoogloea Acidovorax 0.00

0.05

0.10

0.15

0.20

Relative abundance

Fig. 6. Bacterial community structure of aerobic sludge and cake layer in R1 and R3. Genus level with relative abundance lower than 1.00% were classified into group “others”.

28

Table 1 Concentrations of nitrogenous heterocyclic compounds in each process. Compounds

R1 Raw

Quinoline Pyridine Indole

20 20 10

a

Effluent 7.5 16.2 7.1

R2 Influent 3.1 8.2 ND

R3 Effluent ND 2.1 ND

b

Influent

Effluent

0.58 3.2 ND

ND ND ND

Raw, real BPCGW; Influent, treated wastewater after ozonation or catalytic ozonation; Effluent, treated wastewater after MBR. a Values represent the average concentration of NHCs. b ND, not detected.

29

Table 2 Alpha diversity analyses. Sample

Observed species a

Chao1 b

Shannon c

Coverage d

R1 aerobic R1 cake R3 aerobic R3 cake

2849 3124 3567 3953

4235 4529 5534 5778

4.16 4.67 5.22 5.86

0.96 0.96 0.96 0.96

a

Observed species: The count of the unique OTUs in the sample. Chao1: community richness. A higher number indicates more richness. c Shannon: community diversity. A higher number indicates more diversity. d Coverage: The coverage fraction of sample library. b

30

Table 3 Economical assessment of R1 and R3. Reactor

R1 R3

Energy cost (EUR m-3) Ozone generator

Peristaltic pump

Aeration pump

Agitator

0.06

0.018 0.018

0.016 0.016

0.008 0.008

31

Reagent cost (EUR m-3)

Total cost (EUR m-3)

Total cost COD-1 (EUR kg-1)

0.216 0.112

0.258 0.214

3.51 1.28

Highlights 1. Catalytic ozonation process (COP) reduced the pollutants and enhanced the subsequent biodegradation. 2. COP could delay the occurrence of membrane fouling by altering sludge properties and reshaping microbial community structure. 3. Acidovorax was the most dominant bacterial genus in the MBR. 4. The total cost for the integrated process was estimated to be lower than that of single MBR.

32