- Email: [email protected]
M conditions
Bioresource Technology 279 (2019) 189–194
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Organic degradation and extracellular products of pure oxygen aerated activated sludge under different F/M conditions Hong-Ling Zhanga,c, Wei-Li Jiangc, Rong Liub,d, Ying Zhoub, Yong Zhangb,d,e,f,
T
⁎
a
Nanjing Institute of Environmental Science, MEP, Nanjing 210000, China School of the Environment, Nanjing Normal University, Nanjing 210023, China c Jiangsu Provincial Key Laboratory of Environmental Engineering, Nanjing 210000, China d Key Laboratory of Virtual Geographic Environment (Nanjing Normal University), Ministry of Education, Nanjing 210023, China e State Key Laboratory Cultivation Base of Geographical Environment Evolution (Jiangsu Province), Nanjing 210023, China f Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210023, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Pure oxygen aeration Activated sludge Food to microorganisms rate (F/M) Extracellular polymeric substances (EPS) Soluble microbial products (SMP)
This study aimed to investigate the effect of food to microorganisms rate (F/M) on organic removal, extracellular polymeric substances (EPS) and soluble microbial products (SMP) of the pure oxygen aerated activated sludge running in batch mode. The F/M rates were controlled by adjusting the MLSS concentrations (2000, 5000, 8000 mg/L) and/or the initial TOC concentrations (100, 500 mg/L). Results showed that at high F/M rate (0.25 kg TOC/kg MLSS), the substrate degradation rate in the oxygen aerated reactor could reach 1.347 mg TOC/(L·min)), much higher than that in the air aerated reactor (0.640 mg TOC/(L·min)). The SMP concentrations with oxygen aeration were also higher than those with air aeration under high F/M conditions. The total EPS contents in the pure oxygen aerated sludge were significantly lower regardless of the different F/M rates. High F/M condition would lead to more amount of polysaccharides synthesis rather than proteins synthesis in EPS.
1. Introduction Biological treatment is an effective and economical method to remove organic pollutants from wastewater. As a conventional secondary treatment technology, aerobic activated sludge process is widely used in many wastewater treatment plants over the world, to which oxygen supply is a key factor to ensure the normal metabolism of aerobic bacteria in activated sludge (Holenda et al., 2008). Common oxygen supply technologies could generally satisfy the oxygen demand of the biological system when treating low intensity wastewaters. However, when treating high concentration wastewaters, oxygen supply always became a typical bottleneck of the aerobic activated sludge process. Aimed to solve this problem especially when the objects were high density and refractory industrial wastewaters, several intensified methods were employed, e.g., deep shaft technology, pressurized aeration, and pure oxygen aeration (Niu et al., 2013; Zhang et al., 2016; Zhuang et al., 2016). The pure oxygen aeration replaces air with oxygen in the aeration system to maintain high DO concentration in aerobic system. The technology was put into commercial use in 1970 (Shammas and Wang, 2009). Dozens of industrial wastewaters have been reported
⁎
to be successfully treated by pure oxygen aerated biological process (Zhuang et al., 2016; Zhu et al., 1999). Several operating parameters regularly affected the treating efficiency of the activated sludge process, and reasonable regulation and control of these parameters would ensure high-efficiency running of the process. Food to microorganisms rate (F/M) is one of the most important parameters which plays important role in the stability and performance of biological treatments (Khoshfetrat et al., 2011). In anaerobic bioreactor, F/M would affect the volatile fatty acid (VFA) and biogas productions significantly (Slezak et al., 2017). Generally, increasing the organic load could save reactor volume of the conventional activated sludge processes (Dionisi and Rasheed, 2018). However, decrease of COD removal efficiency was always observed with the increase of F/M in most aerobic treatments. Besides, there were also close links between F/M and membrane biofouling, microbial community composition of the biosystem, etc. (Shariati et al., 2013; Sato et al., 2016). With regard to the intensified aeration technology, Xu et al. (2016) reported that the degradation rate of the pressurized aerated reactor was related to the organic load of the system. Nevertheless, it still deserves to discover the organic degradation profile of the pure
Corresponding author: School of Environment, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, China. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.biortech.2019.01.130 Received 18 December 2018; Received in revised form 26 January 2019; Accepted 28 January 2019 Available online 29 January 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
2.2. Reactors and experimental design
oxygen aerated activated sludge at different F/M rates. Extracellular polymeric substances (EPS) and soluble microbial products (SMP) are two important bio-products of the activated sludge, transferring and relating each other during the biodegradation process. Many physicochemical properties of activated sludge such as flocculation, sludge surface charge, sedimentation performance were significantly affected by EPS (Liu et al., 2010; Wang et al., 2013). Meanwhile, the effluent quality was largely determined by SMP which contributed to the majority of the effluent COD (Zhang et al., 2015). Thus it is essential to understand the dynamic status of these bio-products in biochemical systems. Our previous studies have demonstrated that EPS content of the pressurized aerated sludge was lower than that of the conventional one (Xu et al., 2016). It was also reported that EPS content and composition would change at different organic loads (Rusanowska et al., 2019), while SMP was more abundant at higher organic loads in membrane bioreactors (Maqbool et al., 2017). However, there were few papers regarding the relationship between the EPS/SMP and the organic load in the pure oxygen aerated activated sludge process. The aim of this work was to evaluate the influence of the F/M on EPS and SMP of the pure oxygen aerated activated sludge. The experiments were conducted in a pure oxygen aerated reactor running in batch mode. Another reactor with the same shape and size was aerated with air as the control reactor. The F/M rates were adjusted by changing the influent concentrations or/and mixed liquor suspended solids (MLSS). Dynamic changes of organic concentrations, EPS and SMP were determined and compared among different F/M rates. This study intended to gain a better understanding of the organic wastewater treatment by activated sludge with pure oxygen aeration.
The experiments were conducted in two identical reactors with the diameter of 200 mm, the height of 260 mm and the working volume of 7.8 L. The aeration rate of each bioreactor was controlled at 15 L/h by a flowmeter. An agitator kept running in each reactor to prevent sludge precipitation. Both reactors were running in batch mode (6 h-cycle). The experiments could be divided into two stages. In the first stage, the MLSS was kept at 2000 mg/L, approximately. The initial TOC concentrations were controlled at 100 mg/L and 500 mg/L to represent the low F/M status and the high F/M status, respectively. TOC degradation, EPS and SMP contents were compared between the two F/M status. In the second stage, the initial TOC concentration was kept at about 500 mg/L, and the MLSS was controlled at 2000, 5000 and 8000 mg/L, approximately. The operating temperature was maintained at 25 ± 2 °C.
2.3. EPS and SMP extraction EPS extraction: The heat extraction method (Li and Yang, 2007) was modified to extract EPS from the activated sludge. Firstly, 50 mL sludge suspension was centrifuged at 4000 rpm for 5 min and the supernatant was separated for SMP determination. Then, the mud residue in the centrifuge tube was re-suspended, centrifuged, and washed with 50 mL NaCl solution (0.9%) for three times. Finally, the remaining sludge pellet in the centrifuge tube was re-suspended in 50 mL NaCl solution (0.9%), mixed for one minute by a vortex mixer, heated to 80 °C in a water bath for 5 min, and then centrifuged again at 4000 rpm for 30 min. The supernatant was regarded as the solution of EPS for further detection of polysaccharides (PS) and proteins (PN). SMP extraction: The supernatant collected from the first step of the EPS extraction was filtered through 0.45 μm mixed cellulose esters filters by vacuum filter. The filtrate was regarded as the SMP solution for further analysis of PS and PN.
2. Materials and methods 2.1. Wastewater and activated sludge Synthetic wastewater was employed as the experimental influent. The composition of the synthetic wastewater used in the high F/M experiments was as follows: CH3COONa: 2000 mg/L; NH4Cl: 340 mg/L; KH2PO4: 40 mg/L; NaHCO3: 80 mg/L; CaCl2: 80 mg/L; MgSO4·7H2O: 328 mg/L. The characteristics of the wastewater were total organic carbon (TOC) 450–600 mg/L, ammonia nitrogen (NH3-N) 75–90 mg/L, total phosphorus (TP) 7–9 mg/L, and pH 6.5–7.5. The wastewater used in the low organic load experiments was the dilution of the synthetic wastewater with the dilution multiple of 5, approximately. The activated sludge was obtained from the secondary sedimentation tank in a municipal wastewater treatment plant located in Nanjing, China. The seed sludge was then cultured by the synthetic wastewater in sequencing mode, and each cycle (6 h) consisted of 0.5 h feeding, 3 h aeration, 1 h settling, 0.5 h decanting and 1 h idling. Detailed introduction about the culture process could be seen in Zhang et al. (2017).
2.4. Analytical methods TOC was measured by a TOC analyzer (TOC-VCPH, Shimadzu). DO concentration was detected by a dissolved oxygen meter (JPBJ-608, China). MLSS was determined according to the standard method (SEPA, 2002). The PN contents in EPS and SMP extractions were determined based on the method of modified Lowry method, employing bovine serum albumin as the standard. The PS contents in EPS and SMP extractions were determined by the phenol-sulphuric acid method, referring to Kim et al. (2001).
Fig. 1. Organic degradation comparison between the oxygen aeration and the air aeration with the initial TOC concentrations of 100 mg/L (a) and 500 mg/L (b). 190
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
3. Results and discussion
10.5 and 9.8 mg/g MLSS of the air aerated sludge and the pure oxygen aerated sludge, respectively. EPS was reported to increase in the substrate utilization phase, while it would decrease in the endogenous phase (Ni et al., 2009). The laws could be explained by the fact that EPS of activated sludge would be consumed by microorganism as carbon source at low F/M conditions (Wang et al., 2006; Ding et al., 2015), which led to the decrease of EPS after 120 min. In the batch reactions, organic matters were consumed rapidly under low initial TOC concentration conditions. As shown in Fig. 1(a), the residual organic concentrations (TOC) in the air aerated reactor and the pure oxygen aerated reactor were 45.2 and 2.0 mg/L, respectively. When the TOC concentrations were increased to 500 mg/L (Fig. 2(b)), the similar EPS changing trend was kept in the pure oxygen aerated reactor. However, the EPS content in the air aerated reactor was still very high at the end of the batch reaction due to the high residual TOC concentration (389.9 mg/L) after 180 min. Janga et al. (2007) also found that the EPS concentrations increased with the increase of F/M in an air aerated membrane bioreactor. The results were consistent with the lower substrate degradation rate of the air aerated reactor in Fig. 1(b). Fig. 2 also shows that the EPS contents in the pure oxygen aerated reactor were always lower than those in the air aerated reactor, regardless of the different initial TOC concentrations. Adav et al. (2007) studied the effect of aeration intensity on EPS of the aerobic granule and also found that higher aeration intensities could increase EPS contents. In an intensified aeration system by pressurized aeration, similar difference of EPS content between the air aeration and the pressurized aeration was also observed (Xu et al., 2016). The results indicated that higher oxygen supply efficiency achieved by the pure oxygen aeration not only increased the TOC removal rate (Fig. 1(b)), but also promoted the consumption of EPS (Xu et al., 2016). The SMP concentrations of the two aeration systems are shown in Fig. 3. The MLSS concentration of each reactor was controlled at 2000 mg/L during the experiments. It was obvious that the SMP concentrations under high F/M condition (Fig. 3(b)) were significantly higher than those under low F/M condition (Fig. 3(a)). Besides, the SMP concentrations at low F/M condition were basically stable during the batch reaction, while an increasing trend of SMP concentration was presented in Fig. 3(b) when at high F/M condition. SMP could be divided into two parts: the utilization-associated products (UAP) produced in the substrate-utilization process and the biomass-associated products (BAP) produced associated with biomass decay (Ni et al., 2010). Xu et al. (2011) found that the substrate concentration is an important running parameter which would influence SMP production by activated sludge, prior to many other parameters, e. g. aeration rate. At high F/M condition, large amount of UAP would be generated and format the main fraction of the SMP (Xie et al., 2010). Therefore, UAP would attribute to the increasing SMP in Fig. 3(b). There was no significant difference of SMP concentrations between the two aeration systems when the reactors were running under low F/ M condition. At the end of the reaction, the SMP of the pure oxygen aerated effluent was even lower than that of the air aerated one (Fig. 3(a)). However, the SMP concentrations in the pure oxygen aerated reactor were always higher than those in the air aerated reactor under high F/M condition. The reason could probably owe to the improved biomass activity by the high oxygen transfer impetus with the pure oxygen aeration (Lee and Kim, 2013). Zhuang et al. (2016) also found that the pure oxygen aeration could provide sufficient electron donor and thus significantly improve the enzymatic activity of the activated sludge. Higher biomass activity by the pure oxygen aerated sludge thus decreased the BAP released from endogenous decay of dead cells at low F/M condition (Fig. 3(a)) as well as increased UAP produced in rapid organic degradation at high F/M condition (Fig. 3(b)) (Ni et al., 2010; Laspidou and Rittmann, 2002).
3.1. TOC degradation by activated sludge at different initial TOC concentrations Fig. 1 shows the dynamic changes of DO and TOC concentrations in the pure oxygen aerated reactor and the air aerated reactor at different initial TOC concentrations. The MLSS concentrations in both reactors were 2000 mg/L. The initial TOC concentration of Fig. 1(a) was controlled at 100 mg/L, representing the low F/M rate of 0.05 kg TOC/kg MLSS. The initial TOC concentration of Fig. 1(b) was 500 mg/L, representing the high F/M of 0.25 kg TOC/kg MLSS. The DO concentrations presented quite different variations between the two aeration methods at low F/M (Fig. 1(a)). DO concentrations in the air aerated reactor were stable within the range between 2.7 and 3.6 mg/L. Otherwise, DO concentrations in the pure oxygen aerated reactor were far higher than those in the air aerated reactor, especially in the later period of the batch reaction. On the one hand, the pure oxygen aeration could increase the oxygen transfer impetus significantly and thus raised the DO concentrations in water (Rodríguez et al., 2012). On the other hand, the phenomenon was also associated with the difference of the organic matter degradation process between the two reactors. As shown in Fig. 1(a), the TOC concentration in the pure oxygen aerated reactor decreased dramatically from initial 103 mg/L to 13 mg/L after 60 min, much lower than that in the air aerated reactor (66 mg/L after 60 min). It also demonstrated that, compared to the conventional air aerated system, the pure oxygen aerated activated sludge could improve degradation rate and thus obviously lower required tank volume. Less residual organic matter in the pure oxygen aerated reactor also reduced oxygen consumption by the activated sludge and increased the DO concentrations in the reactor (Xu et al., 2016). Therefore, the change of DO concentration was in consistent with the change of remaining TOC concentration in Fig. 1(a). In Fig. 1(b), DO concentrations fluctuated in narrow ranges from 1.0 mg/L to 1.5 mg/L in the air aerated reactor and from 3.1 mg/L to 4.2 mg/L in the pure oxygen aerated reactor. No obvious increasing trend of DO concentration was observed at the later stage of the batch reaction in the pure oxygen aerated reactor at high organic load. This was due to the high concentration of residual organic matters which improved the oxygen consumption of the activated sludge. As shown in Fig. 1(b), the TOC concentration in the pure oxygen aerated reactor still reached 160.8 mg/L. The linear fits of TOC concentrations of the two reactors were also shown in Fig. 1(b). The correlation coefficients of the pure oxygen aerated reactor and the air aerated reactor were 0.915 and 0.988, respectively, illustrating the reliability of the linear fitting. According to the Monod equation (Arnaldos et al., 2015), the substrate degradation rate and the substrate concentration would follow a zeroorder reaction when the half-saturation constant (Ks) was far below the substrate concentration. The TOC concentrations in Fig. 1(b) were well above the general Ks of activated sludge in sewage treatment (Qasim, 1999). In the circumstances, the slope of the fitting line could represent the substrate degradation rate, which were 1.347 mg TOC/(L·min) in the pure oxygen aerated reactor, much higher than 0.640 mg TOC/ (L·min) in the air aerated reactor. The results demonstrated that the pure oxygen aeration could significantly improve the substrate degradation rate at high F/M. Similar regularity has also been found in the wastewater treatment by activated sludge with pressurized aeration technology (Xu et al., 2016). 3.2. EPS and SMP of activated sludge at different initial TOC concentrations Fig. 2 shows the changes of EPS over time in one batch reaction with different initial TOC concentrations. At low initial TOC concentrations (Fig. 2(a)), similar laws of first increasing and then decreasing were observed in both aeration systems. The peaks of EPS at 120 min were 191
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
Fig. 2. EPS of the oxygen (air) aerated activated sludge with the initial TOC concentrations of 100 mg/L (a) and 500 mg/L (b).
3.3. TOC and EPS profile of the pure oxygen aerated activated sludge with different MLSS TOC removal rates of activated sludge with different MLSS concentrations are shown in Fig. 4. The initial TOC was 500 mg/L, approximately. MLSS concentrations were controlled at 2000, 5000 and 8000 mg/L, with the corresponding F/M rates of 0.25, 0.10 and 0.06 kg TOC/kg MLSS, respectively. As shown in Fig. 4, TOC removal rates of the pure oxygen aerated reactor were generally higher than those of the air aerated one. It was interesting that the gap of the removal rates of the two reactors increased with the increase of MLSS concentrations. For example, the gap was 16.8% between the pure oxygen aerated reactor (61.4%) and the air aerated reactor (44.6%) when the MLSS concentration was 2000 mg/L, while it widened to 76.5% between the pure oxygen aeration (94.9%) and the air aeration (18.4%) when the MLSS concentration increased to 4000 mg/L. On the one hand, aeration efficiency decreased with the increase of MLSS (Rodríguez et al., 2012). On the other hand, more oxygen should be needed for extra microbial endogenous respiration at higher MLSS concentrations (Rodríguez et al., 2012). The results indicated that the pure oxygen aeration would enhance the treating efficiencies of aerobic systems more significantly at higher MLSS concentrations. The effects of MLSS on the two aeration systems could be further analyzed by comparing Fig. 4 with Fig. 1. When at low F/M rates, the TOC removal rate of the air aerated reactor (F/M = 0.05 kg TOC/kg MLSS) was 74.4% (Fig. 1(a)) at 2000 mg/L MLSS, while it sharply decreased to 18.4% (Fig. 4) at 8000 mg/L MLSS even with the close F/M rate of 0.06 kg TOC/kg MLSS. With regard to the pure oxygen aeration, the TOC removal rate was 68.0% (Fig. 1(b)) at high F/M (0.25 TOC/kg MLSS) when MLSS concentration was 2000 mg/L, while it increased rapidly to 94.9% (Fig. 4) at the same F/M rate (0.25 TOC/kg MLSS)
Fig. 4. TOC removal rates by the oxygen (air) aerated activated sludge with different MLSS.
when MLSS increased to 8000 mg/L. Molina-Muñoz et al. (2007) observed a decay when threshold values of VSS concentration were reached in an air aerated aerobic treatment system. Thus a large amount of SMP was released into the effluent during the decay (Ni et al., 2010) and violently decreased the TOC removal rates of the air aerated system. Nevertheless, biomass decay of aerobic activated sludge with high MLSS concentrations could be effectively inhibited by sufficient oxygen supply with the pure oxygen aeration. The results also indicated that the pure oxygen aeration would be more effective when
Fig. 3. SMP comparison between the oxygen aeration and the air aeration at the initial TOC concentrations of 100 mg/L (a) and 500 mg/L (b). 192
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
Fig. 5. Main chemical compositions in EPS of the oxygen (air) aerated activated sludge at MLSS of (a) 2000 mg/L, (b) 5000 mg/L and (c) 8000 mg/L.
et al. (2007) studied the biodegradability of EPS and found that 50% of EPS could be utilized by their producers under aerobic starvation condition. Therefore, it suggested that the pure oxygen aeration could not only accelerate the substrate degradation (Fig. 1) and thus increase the TOC removal rates of activated sludge (Fig. 4), but also promote the decomposition of EPS especially at the later stage of the batch reaction (Fig. 2, Fig. 5). As a result, more EPS consumption may be an important reason for the advantage of sludge reduction by the intensified aeration method (Novak et al., 2007).
it was applied in high MLSS concentration biological treatments. The dynamic changes of the chemical components in EPS at different MLSS concentrations were further determined, as shown in Fig. 5. The initial TOC concentration was controlled at 500 mg/L, and the corresponding F/M rates with MLSS of 2000, 5000 and 8000 mg/L were 0.25 (Fig. 5(a)), 0.10 (Fig. 5(b)) and 0.06 kg TOC/kg MLSS (Fig. 5(c)), respectively. The total EPS presented a changing rule of first increase and then decrease during the batch reaction in most cases except for the air aerated reactor with the highest F/M rate, where a continuing growth of total EPS was observed in the whole 180 min batch reaction, as shown in Fig. 5(a). The reason may be that the generation of EPS is connected with substrate utilization. The total EPS content would decrease rapidly when the growth stage of microorganisms transferred from the logarithmic phase to the stationary phase with the depletion of substrates (Sheng and Yu, 2006). The results were consistent with Duan et al. (2014) reporting that EPS of activated sludge in a submerged membrane bioreactor increased with the increase of F/M rate. It was interesting that at the highest F/M rate (Fig. 5(a)), the PS contents of both aeration methods increased over time during the whole cycle. At the end of the batch reaction, the PS contents in the activated sludge with MLSS of 2000 mg/L were pretty higher than those with MLSS of 5000 mg/L and 8000 mg/L. The results indicated that high F/ M condition would lead to more amount of PS synthesis rather than PN synthesis in EPS. Li et al. (2016) also found that PS would obviously increase at high F/M via decreasing the empty bed residence time in a suspended biofilter, while PN was less affected. The total EPS contents of the pure oxygen aeration were significantly lower than those of the air aeration, regardless of the different F/M rates. The results were also accordance with Fig. 2. Wang
4. Conclusions The substrate degradation rate was significantly improved by the pure oxygen aeration. SMP concentrations under high F/M condition were significantly higher than those under low F/M condition. At high F/M, the SMP concentrations with oxygen aeration were always higher than those with air aeration. The oxygen aerated reactor could achieve higher TOC removal rate with higher MLSS concentration at the same F/M condition. High F/M condition would lead to more amount of PS synthesis in EPS by activated sludge rather than PN synthesis. Total EPS contents of the oxygen aeration were significantly lower regardless of the different F/M rates. Acknowledgments This work was financially supported by the Open Research Fund of Jiangsu Province Key Laboratory of Environmental Engineering (KF2018003), the Natural Science Foundation of Jiangsu Province of China (BK20171478), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (164320H116). 193
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
References
Hazard. Mater. 252–253, 250–257. Novak, J.T., Chon, D.H., Curtis, B.A., Doyle, M., 2007. Biological solids reduction using the cannibal process. Water Environ. Res. 79, 2380–2386. Qasim, S.R., 1999. Wastewater Treatment Plants: Planning, Design, and Operation, second ed. Technomic Publishing Company Inc, Lancaster, Pennsylvania, USA. Rodríguez, F.A., Reboleiro-Rivas, P., Osorio, F., Martínez-Toledo, M.V., Hontoria, E., Poyatos, J.M., 2012. Influence of mixed liquid suspended solids and hydraulic retention time on oxygen transfer efficiency and viscosity in a submerged membrane bioreactor using pure oxygen to supply aerobic conditions. Biochem. Eng. J. 60, 135–141. Rusanowska, P., Cydzik-Kwiatkowska, A., Świątczak, P., Wojnowska-Baryła, I., 2019. Changes in extracellular polymeric substances (EPS) content and composition in aerobic granule size-fractions during reactor cycles at different organic loads. Bioresour. Technol. 272, 188–193. Sato, Y., Hori, T., Navarro, R.R., Habe, H., Yanagishita, H., Ogata, A., 2016. Fine-scale monitoring of shifts in microbial community composition after high organic loading in a pilot-scale membrane bioreactor. J. Biosci. Bioeng. 121, 550–556. SEPA, 2002. Water and Wastewater Monitoring Methods, fourth ed. Chinese Environmental Science Publishing House, Beijing, China. Shammas, N.K., Wang, L.K., 2009. Pure oxygen activated sludge process. Handbook of Environmental Engineering, vol. 8 Springer. Shariati, F.P., Mehrnia, M.R., Sarrafzadeh, M.H., Rezaee, S., Grasmick, A., Heran, M., 2013. Fouling in a novel airlift oxidation ditch membrane bioreactor (AOXMBR) at different high organic loading rate. Sep. Purif. Technol. 105, 69–78. Sheng, G.P., Yu, H.Q., 2006. Relationship between the extracellular polymeric substances and surface characteristics of Rhodopseudomonas acidophila. Appl. Microbiol. Biotechnol. 72, 126–131. Slezak, R., Grzelak, J., Krzystek, L., Ledakowicz, S., 2017. The effect of initial organic load of the kitchen waste on the production of VFA and H2 in dark fermentation. Waste Manage. 68, 610–617. Wang, Z., Liu, L., Yao, J., Cai, W., 2006. Effects of extracellular polymeric substances on aerobic granulation in sequencing batch reactors. Chemosphere 63, 1728–1735. Wang, Z., Gao, M., Wang, Z., She, Z., Chang, Q., Sun, C., Zhang, I., Ren, Y., Yang, N., 2013. Effect of salinity on extracellular polymeric substances of activated sludge from an anoxic-aerobic sequencing batch reactor. Chemosphere 93, 2789–2795. Wang, Z.W., Liu, Y., Tay, J.H., 2007. Biodegradability of extracellular polymeric substances produced by aerobic granules. Appl. Microbiol. Biotechnol. 74, 462–466. Xie, W.M., Ni, B.J., Zeng, R.J., Sheng, G.P., Yu, H.Q., Song, J., Le, D.Z., Bi, X.J., Liu, C.P., Yang, M., 2010. Formation of soluble microbial products by activated sludge under anoxic conditions. Appl. Microbiol. Biotechnol. 87, 373–382. Xu, J., Sheng, G.P., Luo, H.W., Fang, F., Li, W.W., Zeng, R.J., Tong, Z.H., Yu, H.Q., 2011. Evaluating the influence of process parameters on soluble microbial products formation using response surface methodology coupled with grey relational analysis. Water Res. 45, 674–680. Xu, R.X., Li, B., Zhang, Y., Si, L., Zhang, X.Q., Xie, B., 2016. Response of biodegradation characteristics of unacclimated activated sludge to moderate pressure in a batch reactor. Chemosphere 148, 41–46. Zhang, B., Xian, Q., Zhu, J., Li, A., Gong, T., 2015. Characterization, DBPs formation, and mutagenicity of soluble microbial products (SMPs) in wastewater under simulated stressful conditions. Chem. Eng. J. 279, 258–263. Zhang, Y., Jiang, W.L., Xu, R.X., Wang, G.X., Xie, B., 2017. Effect of short-term salinity shock on unacclimated activated sludge with pressurized aeration in a sequencing batch reactor. Sep. Purif. Technol. 178, 200–206. Zhang, Y., Li, B., Xu, R.X., Wang, G.X., Zhou, Y., Xie, B., 2016. Effects of pressurized aeration on organic degradation efficiency and bacterial community structure of activated sludge treating saline wastewater. Bioresour. Technol. 222, 182–189. Zhu, H., Keener, T.C., Bishop, P.L., Orton, T.L., Wang, M., Siddiqui, K.F., 1999. Aeration recirculation in air and high purity oxygen systems for control of VOC emissions from wastewater aeration basins. Environ. Prog. Sustain. 18, 101–106. Zhuang, H., Hong, X., Han, H., Shan, S., 2016. Effect of pure oxygen fine bubbles on the organic matter removal and bacterial community evolution treating coal gasification wastewater by membrane bioreactor. Bioresour. Technol. 221, 262–269.
Adav, S.S., Lee, D.J., Lai, J.Y., 2007. Effects of aeration intensity on formation of phenolfed aerobic granules and extracellular polymeric substances. Appl. Microbiol. Biotechnol. 77, 175–182. Arnaldos, M., Amerlinck, Y., Rehman, U., Maere, T., van Hoey, S., Naessens, W., Nopens, I., 2015. From the affinity constant to the half-saturation index: understanding conventional modeling concepts in novel wastewater treatment processes. Water Res. 70, 458–470. Ding, Z., Bourven, I., Guibaud, G., van Hullebusch, E.D., Panico, A., Pirozzi, F., Esposito, G., 2015. Role of extracellular polymeric substances (EPS) production in bioaggregation: application to wastewater treatment. Appl. Microbiol. Biotechnol. 99, 9883–9905. Dionisi, D., Rasheed, A.A., 2018. Maximisation of the organic load rate and minimisation of oxygen consumption in aerobic biological wastewater treatment processes by manipulation of the hydraulic and solids residence time. J. Water Process Eng. 22, 138–146. Duan, L., Song, Y., Yu, H., Xia, S., Hermanowicz, S.W., 2014. The effect of solids retention times on the characterization of extracellular polymeric substances and soluble microbial products in a submerged membrane bioreactor. Bioresour. Technol. 163, 395–398. Holenda, B., Domokos, E., Redey, A., Fazakas, J., 2008. Dissolved oxygen control of the activated sludge wastewater treatment process using model predictive control. Comput. Chem. Eng. 32, 1270–1278. Janga, N., Ren, X., Kim, G., Ahn, C., Cho, J., Kim, I.S., 2007. Characteristics of soluble microbial products and extracellular polymeric substances in the membrane bioreactor for water reuse. Desalination 202, 90–98. Khoshfetrat, A.B., Nikakhtari, H., Sadeghifar, M., Khatibi, M.S., 2011. Influence of organic loading and aeration rates on performance of a lab-scale upflow aerated submerged fixed-film bioreactor. Process Saf. Environ. 89, 193–197. Kim, J.S., Lee, C.H., Chang, I.S., 2001. Effect of pump shear on the performance of a crossflow membrane bioreactor. Water Res. 35, 2137–2144. Laspidou, C.S., Rittmann, B.E., 2002. A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass. Water Res. 36, 2711–2720. Lee, S., Kim, M., 2013. Fouling characteristics in pure oxygen MBR process according to MLSS concentrations and COD loadings. J. Membr. Sci. 428, 323–330. Li, H., Huang, S., Zhou, S., Chen, P., Zhang, Y., 2016. Study of extracellular polymeric substances in the biofilms of a suspended biofilter for nitric oxide removal. Appl. Microbiol. Biotechnol. 100, 9733–9743. Li, X.Y., Yang, S.F., 2007. Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge. Water Res. 41, 1022–1030. Liu, X.M., Sheng, G.P., Luo, H.W., Zhang, F., Yuan, S.J., Xu, J., Zeng, R.J., Wu, J.G., Yu, H.Q., 2010. Contribution of extracellular polymeric substances (EPS) to the sludge aggregation. Environ. Sci. Technol. 44, 4355–4360. Maqbool, T., Cho, J., Hur, J., 2017. Dynamic changes of dissolved organic matter in membrane bioreactors at different organic loading rates: evidence from spectroscopic and chromatographic methods. Bioresour. Technol. 234, 131–139. Molina-Muñoz, M., Poyatos, J.M., Vílchez, R., Hontoria, E., Rodelas, B., González-López, J., 2007. Effect of the concentration of suspended solids on the enzymatic activities and biodiversity of a submerged membrane bioreactor for aerobic treatment of domestic wastewater. Appl. Microbiol. Biotechnol. 73, 1441–1451. Ni, B.J., Fang, F., Xie, W.M., Sun, M., Sheng, G.P., Li, W.H., Yu, H.Q., 2009. Characterization of extracellular polymeric substances produced by mixed microorganisms in activated sludge with gel-permeating chromatography, excitation–emission matrix fluorescence spectroscopy measurement and kinetic modeling. Water Res. 43, 1350–1358. Ni, B.J., Zeng, R.J., Fang, F., Xie, W.M., Sheng, G.P., Yu, H.Q., 2010. Fractionating soluble microbial products in the activated sludge process. Water Res. 44, 2292–2302. Niu, J., Zhang, T., He, Y., Zhou, H., Zhao, A., Zhao, Y., 2013. Pretreatment of landfill leachate using deep shaft aeration bioreactor (DSAB) in cold winter season. J.
194
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Organic degradation and extracellular products of pure oxygen aerated activated sludge under different F/M conditions Hong-Ling Zhanga,c, Wei-Li Jiangc, Rong Liub,d, Ying Zhoub, Yong Zhangb,d,e,f,
T
⁎
a
Nanjing Institute of Environmental Science, MEP, Nanjing 210000, China School of the Environment, Nanjing Normal University, Nanjing 210023, China c Jiangsu Provincial Key Laboratory of Environmental Engineering, Nanjing 210000, China d Key Laboratory of Virtual Geographic Environment (Nanjing Normal University), Ministry of Education, Nanjing 210023, China e State Key Laboratory Cultivation Base of Geographical Environment Evolution (Jiangsu Province), Nanjing 210023, China f Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210023, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Pure oxygen aeration Activated sludge Food to microorganisms rate (F/M) Extracellular polymeric substances (EPS) Soluble microbial products (SMP)
This study aimed to investigate the effect of food to microorganisms rate (F/M) on organic removal, extracellular polymeric substances (EPS) and soluble microbial products (SMP) of the pure oxygen aerated activated sludge running in batch mode. The F/M rates were controlled by adjusting the MLSS concentrations (2000, 5000, 8000 mg/L) and/or the initial TOC concentrations (100, 500 mg/L). Results showed that at high F/M rate (0.25 kg TOC/kg MLSS), the substrate degradation rate in the oxygen aerated reactor could reach 1.347 mg TOC/(L·min)), much higher than that in the air aerated reactor (0.640 mg TOC/(L·min)). The SMP concentrations with oxygen aeration were also higher than those with air aeration under high F/M conditions. The total EPS contents in the pure oxygen aerated sludge were significantly lower regardless of the different F/M rates. High F/M condition would lead to more amount of polysaccharides synthesis rather than proteins synthesis in EPS.
1. Introduction Biological treatment is an effective and economical method to remove organic pollutants from wastewater. As a conventional secondary treatment technology, aerobic activated sludge process is widely used in many wastewater treatment plants over the world, to which oxygen supply is a key factor to ensure the normal metabolism of aerobic bacteria in activated sludge (Holenda et al., 2008). Common oxygen supply technologies could generally satisfy the oxygen demand of the biological system when treating low intensity wastewaters. However, when treating high concentration wastewaters, oxygen supply always became a typical bottleneck of the aerobic activated sludge process. Aimed to solve this problem especially when the objects were high density and refractory industrial wastewaters, several intensified methods were employed, e.g., deep shaft technology, pressurized aeration, and pure oxygen aeration (Niu et al., 2013; Zhang et al., 2016; Zhuang et al., 2016). The pure oxygen aeration replaces air with oxygen in the aeration system to maintain high DO concentration in aerobic system. The technology was put into commercial use in 1970 (Shammas and Wang, 2009). Dozens of industrial wastewaters have been reported
⁎
to be successfully treated by pure oxygen aerated biological process (Zhuang et al., 2016; Zhu et al., 1999). Several operating parameters regularly affected the treating efficiency of the activated sludge process, and reasonable regulation and control of these parameters would ensure high-efficiency running of the process. Food to microorganisms rate (F/M) is one of the most important parameters which plays important role in the stability and performance of biological treatments (Khoshfetrat et al., 2011). In anaerobic bioreactor, F/M would affect the volatile fatty acid (VFA) and biogas productions significantly (Slezak et al., 2017). Generally, increasing the organic load could save reactor volume of the conventional activated sludge processes (Dionisi and Rasheed, 2018). However, decrease of COD removal efficiency was always observed with the increase of F/M in most aerobic treatments. Besides, there were also close links between F/M and membrane biofouling, microbial community composition of the biosystem, etc. (Shariati et al., 2013; Sato et al., 2016). With regard to the intensified aeration technology, Xu et al. (2016) reported that the degradation rate of the pressurized aerated reactor was related to the organic load of the system. Nevertheless, it still deserves to discover the organic degradation profile of the pure
Corresponding author: School of Environment, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, China. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.biortech.2019.01.130 Received 18 December 2018; Received in revised form 26 January 2019; Accepted 28 January 2019 Available online 29 January 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
2.2. Reactors and experimental design
oxygen aerated activated sludge at different F/M rates. Extracellular polymeric substances (EPS) and soluble microbial products (SMP) are two important bio-products of the activated sludge, transferring and relating each other during the biodegradation process. Many physicochemical properties of activated sludge such as flocculation, sludge surface charge, sedimentation performance were significantly affected by EPS (Liu et al., 2010; Wang et al., 2013). Meanwhile, the effluent quality was largely determined by SMP which contributed to the majority of the effluent COD (Zhang et al., 2015). Thus it is essential to understand the dynamic status of these bio-products in biochemical systems. Our previous studies have demonstrated that EPS content of the pressurized aerated sludge was lower than that of the conventional one (Xu et al., 2016). It was also reported that EPS content and composition would change at different organic loads (Rusanowska et al., 2019), while SMP was more abundant at higher organic loads in membrane bioreactors (Maqbool et al., 2017). However, there were few papers regarding the relationship between the EPS/SMP and the organic load in the pure oxygen aerated activated sludge process. The aim of this work was to evaluate the influence of the F/M on EPS and SMP of the pure oxygen aerated activated sludge. The experiments were conducted in a pure oxygen aerated reactor running in batch mode. Another reactor with the same shape and size was aerated with air as the control reactor. The F/M rates were adjusted by changing the influent concentrations or/and mixed liquor suspended solids (MLSS). Dynamic changes of organic concentrations, EPS and SMP were determined and compared among different F/M rates. This study intended to gain a better understanding of the organic wastewater treatment by activated sludge with pure oxygen aeration.
The experiments were conducted in two identical reactors with the diameter of 200 mm, the height of 260 mm and the working volume of 7.8 L. The aeration rate of each bioreactor was controlled at 15 L/h by a flowmeter. An agitator kept running in each reactor to prevent sludge precipitation. Both reactors were running in batch mode (6 h-cycle). The experiments could be divided into two stages. In the first stage, the MLSS was kept at 2000 mg/L, approximately. The initial TOC concentrations were controlled at 100 mg/L and 500 mg/L to represent the low F/M status and the high F/M status, respectively. TOC degradation, EPS and SMP contents were compared between the two F/M status. In the second stage, the initial TOC concentration was kept at about 500 mg/L, and the MLSS was controlled at 2000, 5000 and 8000 mg/L, approximately. The operating temperature was maintained at 25 ± 2 °C.
2.3. EPS and SMP extraction EPS extraction: The heat extraction method (Li and Yang, 2007) was modified to extract EPS from the activated sludge. Firstly, 50 mL sludge suspension was centrifuged at 4000 rpm for 5 min and the supernatant was separated for SMP determination. Then, the mud residue in the centrifuge tube was re-suspended, centrifuged, and washed with 50 mL NaCl solution (0.9%) for three times. Finally, the remaining sludge pellet in the centrifuge tube was re-suspended in 50 mL NaCl solution (0.9%), mixed for one minute by a vortex mixer, heated to 80 °C in a water bath for 5 min, and then centrifuged again at 4000 rpm for 30 min. The supernatant was regarded as the solution of EPS for further detection of polysaccharides (PS) and proteins (PN). SMP extraction: The supernatant collected from the first step of the EPS extraction was filtered through 0.45 μm mixed cellulose esters filters by vacuum filter. The filtrate was regarded as the SMP solution for further analysis of PS and PN.
2. Materials and methods 2.1. Wastewater and activated sludge Synthetic wastewater was employed as the experimental influent. The composition of the synthetic wastewater used in the high F/M experiments was as follows: CH3COONa: 2000 mg/L; NH4Cl: 340 mg/L; KH2PO4: 40 mg/L; NaHCO3: 80 mg/L; CaCl2: 80 mg/L; MgSO4·7H2O: 328 mg/L. The characteristics of the wastewater were total organic carbon (TOC) 450–600 mg/L, ammonia nitrogen (NH3-N) 75–90 mg/L, total phosphorus (TP) 7–9 mg/L, and pH 6.5–7.5. The wastewater used in the low organic load experiments was the dilution of the synthetic wastewater with the dilution multiple of 5, approximately. The activated sludge was obtained from the secondary sedimentation tank in a municipal wastewater treatment plant located in Nanjing, China. The seed sludge was then cultured by the synthetic wastewater in sequencing mode, and each cycle (6 h) consisted of 0.5 h feeding, 3 h aeration, 1 h settling, 0.5 h decanting and 1 h idling. Detailed introduction about the culture process could be seen in Zhang et al. (2017).
2.4. Analytical methods TOC was measured by a TOC analyzer (TOC-VCPH, Shimadzu). DO concentration was detected by a dissolved oxygen meter (JPBJ-608, China). MLSS was determined according to the standard method (SEPA, 2002). The PN contents in EPS and SMP extractions were determined based on the method of modified Lowry method, employing bovine serum albumin as the standard. The PS contents in EPS and SMP extractions were determined by the phenol-sulphuric acid method, referring to Kim et al. (2001).
Fig. 1. Organic degradation comparison between the oxygen aeration and the air aeration with the initial TOC concentrations of 100 mg/L (a) and 500 mg/L (b). 190
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
3. Results and discussion
10.5 and 9.8 mg/g MLSS of the air aerated sludge and the pure oxygen aerated sludge, respectively. EPS was reported to increase in the substrate utilization phase, while it would decrease in the endogenous phase (Ni et al., 2009). The laws could be explained by the fact that EPS of activated sludge would be consumed by microorganism as carbon source at low F/M conditions (Wang et al., 2006; Ding et al., 2015), which led to the decrease of EPS after 120 min. In the batch reactions, organic matters were consumed rapidly under low initial TOC concentration conditions. As shown in Fig. 1(a), the residual organic concentrations (TOC) in the air aerated reactor and the pure oxygen aerated reactor were 45.2 and 2.0 mg/L, respectively. When the TOC concentrations were increased to 500 mg/L (Fig. 2(b)), the similar EPS changing trend was kept in the pure oxygen aerated reactor. However, the EPS content in the air aerated reactor was still very high at the end of the batch reaction due to the high residual TOC concentration (389.9 mg/L) after 180 min. Janga et al. (2007) also found that the EPS concentrations increased with the increase of F/M in an air aerated membrane bioreactor. The results were consistent with the lower substrate degradation rate of the air aerated reactor in Fig. 1(b). Fig. 2 also shows that the EPS contents in the pure oxygen aerated reactor were always lower than those in the air aerated reactor, regardless of the different initial TOC concentrations. Adav et al. (2007) studied the effect of aeration intensity on EPS of the aerobic granule and also found that higher aeration intensities could increase EPS contents. In an intensified aeration system by pressurized aeration, similar difference of EPS content between the air aeration and the pressurized aeration was also observed (Xu et al., 2016). The results indicated that higher oxygen supply efficiency achieved by the pure oxygen aeration not only increased the TOC removal rate (Fig. 1(b)), but also promoted the consumption of EPS (Xu et al., 2016). The SMP concentrations of the two aeration systems are shown in Fig. 3. The MLSS concentration of each reactor was controlled at 2000 mg/L during the experiments. It was obvious that the SMP concentrations under high F/M condition (Fig. 3(b)) were significantly higher than those under low F/M condition (Fig. 3(a)). Besides, the SMP concentrations at low F/M condition were basically stable during the batch reaction, while an increasing trend of SMP concentration was presented in Fig. 3(b) when at high F/M condition. SMP could be divided into two parts: the utilization-associated products (UAP) produced in the substrate-utilization process and the biomass-associated products (BAP) produced associated with biomass decay (Ni et al., 2010). Xu et al. (2011) found that the substrate concentration is an important running parameter which would influence SMP production by activated sludge, prior to many other parameters, e. g. aeration rate. At high F/M condition, large amount of UAP would be generated and format the main fraction of the SMP (Xie et al., 2010). Therefore, UAP would attribute to the increasing SMP in Fig. 3(b). There was no significant difference of SMP concentrations between the two aeration systems when the reactors were running under low F/ M condition. At the end of the reaction, the SMP of the pure oxygen aerated effluent was even lower than that of the air aerated one (Fig. 3(a)). However, the SMP concentrations in the pure oxygen aerated reactor were always higher than those in the air aerated reactor under high F/M condition. The reason could probably owe to the improved biomass activity by the high oxygen transfer impetus with the pure oxygen aeration (Lee and Kim, 2013). Zhuang et al. (2016) also found that the pure oxygen aeration could provide sufficient electron donor and thus significantly improve the enzymatic activity of the activated sludge. Higher biomass activity by the pure oxygen aerated sludge thus decreased the BAP released from endogenous decay of dead cells at low F/M condition (Fig. 3(a)) as well as increased UAP produced in rapid organic degradation at high F/M condition (Fig. 3(b)) (Ni et al., 2010; Laspidou and Rittmann, 2002).
3.1. TOC degradation by activated sludge at different initial TOC concentrations Fig. 1 shows the dynamic changes of DO and TOC concentrations in the pure oxygen aerated reactor and the air aerated reactor at different initial TOC concentrations. The MLSS concentrations in both reactors were 2000 mg/L. The initial TOC concentration of Fig. 1(a) was controlled at 100 mg/L, representing the low F/M rate of 0.05 kg TOC/kg MLSS. The initial TOC concentration of Fig. 1(b) was 500 mg/L, representing the high F/M of 0.25 kg TOC/kg MLSS. The DO concentrations presented quite different variations between the two aeration methods at low F/M (Fig. 1(a)). DO concentrations in the air aerated reactor were stable within the range between 2.7 and 3.6 mg/L. Otherwise, DO concentrations in the pure oxygen aerated reactor were far higher than those in the air aerated reactor, especially in the later period of the batch reaction. On the one hand, the pure oxygen aeration could increase the oxygen transfer impetus significantly and thus raised the DO concentrations in water (Rodríguez et al., 2012). On the other hand, the phenomenon was also associated with the difference of the organic matter degradation process between the two reactors. As shown in Fig. 1(a), the TOC concentration in the pure oxygen aerated reactor decreased dramatically from initial 103 mg/L to 13 mg/L after 60 min, much lower than that in the air aerated reactor (66 mg/L after 60 min). It also demonstrated that, compared to the conventional air aerated system, the pure oxygen aerated activated sludge could improve degradation rate and thus obviously lower required tank volume. Less residual organic matter in the pure oxygen aerated reactor also reduced oxygen consumption by the activated sludge and increased the DO concentrations in the reactor (Xu et al., 2016). Therefore, the change of DO concentration was in consistent with the change of remaining TOC concentration in Fig. 1(a). In Fig. 1(b), DO concentrations fluctuated in narrow ranges from 1.0 mg/L to 1.5 mg/L in the air aerated reactor and from 3.1 mg/L to 4.2 mg/L in the pure oxygen aerated reactor. No obvious increasing trend of DO concentration was observed at the later stage of the batch reaction in the pure oxygen aerated reactor at high organic load. This was due to the high concentration of residual organic matters which improved the oxygen consumption of the activated sludge. As shown in Fig. 1(b), the TOC concentration in the pure oxygen aerated reactor still reached 160.8 mg/L. The linear fits of TOC concentrations of the two reactors were also shown in Fig. 1(b). The correlation coefficients of the pure oxygen aerated reactor and the air aerated reactor were 0.915 and 0.988, respectively, illustrating the reliability of the linear fitting. According to the Monod equation (Arnaldos et al., 2015), the substrate degradation rate and the substrate concentration would follow a zeroorder reaction when the half-saturation constant (Ks) was far below the substrate concentration. The TOC concentrations in Fig. 1(b) were well above the general Ks of activated sludge in sewage treatment (Qasim, 1999). In the circumstances, the slope of the fitting line could represent the substrate degradation rate, which were 1.347 mg TOC/(L·min) in the pure oxygen aerated reactor, much higher than 0.640 mg TOC/ (L·min) in the air aerated reactor. The results demonstrated that the pure oxygen aeration could significantly improve the substrate degradation rate at high F/M. Similar regularity has also been found in the wastewater treatment by activated sludge with pressurized aeration technology (Xu et al., 2016). 3.2. EPS and SMP of activated sludge at different initial TOC concentrations Fig. 2 shows the changes of EPS over time in one batch reaction with different initial TOC concentrations. At low initial TOC concentrations (Fig. 2(a)), similar laws of first increasing and then decreasing were observed in both aeration systems. The peaks of EPS at 120 min were 191
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
Fig. 2. EPS of the oxygen (air) aerated activated sludge with the initial TOC concentrations of 100 mg/L (a) and 500 mg/L (b).
3.3. TOC and EPS profile of the pure oxygen aerated activated sludge with different MLSS TOC removal rates of activated sludge with different MLSS concentrations are shown in Fig. 4. The initial TOC was 500 mg/L, approximately. MLSS concentrations were controlled at 2000, 5000 and 8000 mg/L, with the corresponding F/M rates of 0.25, 0.10 and 0.06 kg TOC/kg MLSS, respectively. As shown in Fig. 4, TOC removal rates of the pure oxygen aerated reactor were generally higher than those of the air aerated one. It was interesting that the gap of the removal rates of the two reactors increased with the increase of MLSS concentrations. For example, the gap was 16.8% between the pure oxygen aerated reactor (61.4%) and the air aerated reactor (44.6%) when the MLSS concentration was 2000 mg/L, while it widened to 76.5% between the pure oxygen aeration (94.9%) and the air aeration (18.4%) when the MLSS concentration increased to 4000 mg/L. On the one hand, aeration efficiency decreased with the increase of MLSS (Rodríguez et al., 2012). On the other hand, more oxygen should be needed for extra microbial endogenous respiration at higher MLSS concentrations (Rodríguez et al., 2012). The results indicated that the pure oxygen aeration would enhance the treating efficiencies of aerobic systems more significantly at higher MLSS concentrations. The effects of MLSS on the two aeration systems could be further analyzed by comparing Fig. 4 with Fig. 1. When at low F/M rates, the TOC removal rate of the air aerated reactor (F/M = 0.05 kg TOC/kg MLSS) was 74.4% (Fig. 1(a)) at 2000 mg/L MLSS, while it sharply decreased to 18.4% (Fig. 4) at 8000 mg/L MLSS even with the close F/M rate of 0.06 kg TOC/kg MLSS. With regard to the pure oxygen aeration, the TOC removal rate was 68.0% (Fig. 1(b)) at high F/M (0.25 TOC/kg MLSS) when MLSS concentration was 2000 mg/L, while it increased rapidly to 94.9% (Fig. 4) at the same F/M rate (0.25 TOC/kg MLSS)
Fig. 4. TOC removal rates by the oxygen (air) aerated activated sludge with different MLSS.
when MLSS increased to 8000 mg/L. Molina-Muñoz et al. (2007) observed a decay when threshold values of VSS concentration were reached in an air aerated aerobic treatment system. Thus a large amount of SMP was released into the effluent during the decay (Ni et al., 2010) and violently decreased the TOC removal rates of the air aerated system. Nevertheless, biomass decay of aerobic activated sludge with high MLSS concentrations could be effectively inhibited by sufficient oxygen supply with the pure oxygen aeration. The results also indicated that the pure oxygen aeration would be more effective when
Fig. 3. SMP comparison between the oxygen aeration and the air aeration at the initial TOC concentrations of 100 mg/L (a) and 500 mg/L (b). 192
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
Fig. 5. Main chemical compositions in EPS of the oxygen (air) aerated activated sludge at MLSS of (a) 2000 mg/L, (b) 5000 mg/L and (c) 8000 mg/L.
et al. (2007) studied the biodegradability of EPS and found that 50% of EPS could be utilized by their producers under aerobic starvation condition. Therefore, it suggested that the pure oxygen aeration could not only accelerate the substrate degradation (Fig. 1) and thus increase the TOC removal rates of activated sludge (Fig. 4), but also promote the decomposition of EPS especially at the later stage of the batch reaction (Fig. 2, Fig. 5). As a result, more EPS consumption may be an important reason for the advantage of sludge reduction by the intensified aeration method (Novak et al., 2007).
it was applied in high MLSS concentration biological treatments. The dynamic changes of the chemical components in EPS at different MLSS concentrations were further determined, as shown in Fig. 5. The initial TOC concentration was controlled at 500 mg/L, and the corresponding F/M rates with MLSS of 2000, 5000 and 8000 mg/L were 0.25 (Fig. 5(a)), 0.10 (Fig. 5(b)) and 0.06 kg TOC/kg MLSS (Fig. 5(c)), respectively. The total EPS presented a changing rule of first increase and then decrease during the batch reaction in most cases except for the air aerated reactor with the highest F/M rate, where a continuing growth of total EPS was observed in the whole 180 min batch reaction, as shown in Fig. 5(a). The reason may be that the generation of EPS is connected with substrate utilization. The total EPS content would decrease rapidly when the growth stage of microorganisms transferred from the logarithmic phase to the stationary phase with the depletion of substrates (Sheng and Yu, 2006). The results were consistent with Duan et al. (2014) reporting that EPS of activated sludge in a submerged membrane bioreactor increased with the increase of F/M rate. It was interesting that at the highest F/M rate (Fig. 5(a)), the PS contents of both aeration methods increased over time during the whole cycle. At the end of the batch reaction, the PS contents in the activated sludge with MLSS of 2000 mg/L were pretty higher than those with MLSS of 5000 mg/L and 8000 mg/L. The results indicated that high F/ M condition would lead to more amount of PS synthesis rather than PN synthesis in EPS. Li et al. (2016) also found that PS would obviously increase at high F/M via decreasing the empty bed residence time in a suspended biofilter, while PN was less affected. The total EPS contents of the pure oxygen aeration were significantly lower than those of the air aeration, regardless of the different F/M rates. The results were also accordance with Fig. 2. Wang
4. Conclusions The substrate degradation rate was significantly improved by the pure oxygen aeration. SMP concentrations under high F/M condition were significantly higher than those under low F/M condition. At high F/M, the SMP concentrations with oxygen aeration were always higher than those with air aeration. The oxygen aerated reactor could achieve higher TOC removal rate with higher MLSS concentration at the same F/M condition. High F/M condition would lead to more amount of PS synthesis in EPS by activated sludge rather than PN synthesis. Total EPS contents of the oxygen aeration were significantly lower regardless of the different F/M rates. Acknowledgments This work was financially supported by the Open Research Fund of Jiangsu Province Key Laboratory of Environmental Engineering (KF2018003), the Natural Science Foundation of Jiangsu Province of China (BK20171478), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (164320H116). 193
Bioresource Technology 279 (2019) 189–194
H.-L. Zhang et al.
References
Hazard. Mater. 252–253, 250–257. Novak, J.T., Chon, D.H., Curtis, B.A., Doyle, M., 2007. Biological solids reduction using the cannibal process. Water Environ. Res. 79, 2380–2386. Qasim, S.R., 1999. Wastewater Treatment Plants: Planning, Design, and Operation, second ed. Technomic Publishing Company Inc, Lancaster, Pennsylvania, USA. Rodríguez, F.A., Reboleiro-Rivas, P., Osorio, F., Martínez-Toledo, M.V., Hontoria, E., Poyatos, J.M., 2012. Influence of mixed liquid suspended solids and hydraulic retention time on oxygen transfer efficiency and viscosity in a submerged membrane bioreactor using pure oxygen to supply aerobic conditions. Biochem. Eng. J. 60, 135–141. Rusanowska, P., Cydzik-Kwiatkowska, A., Świątczak, P., Wojnowska-Baryła, I., 2019. Changes in extracellular polymeric substances (EPS) content and composition in aerobic granule size-fractions during reactor cycles at different organic loads. Bioresour. Technol. 272, 188–193. Sato, Y., Hori, T., Navarro, R.R., Habe, H., Yanagishita, H., Ogata, A., 2016. Fine-scale monitoring of shifts in microbial community composition after high organic loading in a pilot-scale membrane bioreactor. J. Biosci. Bioeng. 121, 550–556. SEPA, 2002. Water and Wastewater Monitoring Methods, fourth ed. Chinese Environmental Science Publishing House, Beijing, China. Shammas, N.K., Wang, L.K., 2009. Pure oxygen activated sludge process. Handbook of Environmental Engineering, vol. 8 Springer. Shariati, F.P., Mehrnia, M.R., Sarrafzadeh, M.H., Rezaee, S., Grasmick, A., Heran, M., 2013. Fouling in a novel airlift oxidation ditch membrane bioreactor (AOXMBR) at different high organic loading rate. Sep. Purif. Technol. 105, 69–78. Sheng, G.P., Yu, H.Q., 2006. Relationship between the extracellular polymeric substances and surface characteristics of Rhodopseudomonas acidophila. Appl. Microbiol. Biotechnol. 72, 126–131. Slezak, R., Grzelak, J., Krzystek, L., Ledakowicz, S., 2017. The effect of initial organic load of the kitchen waste on the production of VFA and H2 in dark fermentation. Waste Manage. 68, 610–617. Wang, Z., Liu, L., Yao, J., Cai, W., 2006. Effects of extracellular polymeric substances on aerobic granulation in sequencing batch reactors. Chemosphere 63, 1728–1735. Wang, Z., Gao, M., Wang, Z., She, Z., Chang, Q., Sun, C., Zhang, I., Ren, Y., Yang, N., 2013. Effect of salinity on extracellular polymeric substances of activated sludge from an anoxic-aerobic sequencing batch reactor. Chemosphere 93, 2789–2795. Wang, Z.W., Liu, Y., Tay, J.H., 2007. Biodegradability of extracellular polymeric substances produced by aerobic granules. Appl. Microbiol. Biotechnol. 74, 462–466. Xie, W.M., Ni, B.J., Zeng, R.J., Sheng, G.P., Yu, H.Q., Song, J., Le, D.Z., Bi, X.J., Liu, C.P., Yang, M., 2010. Formation of soluble microbial products by activated sludge under anoxic conditions. Appl. Microbiol. Biotechnol. 87, 373–382. Xu, J., Sheng, G.P., Luo, H.W., Fang, F., Li, W.W., Zeng, R.J., Tong, Z.H., Yu, H.Q., 2011. Evaluating the influence of process parameters on soluble microbial products formation using response surface methodology coupled with grey relational analysis. Water Res. 45, 674–680. Xu, R.X., Li, B., Zhang, Y., Si, L., Zhang, X.Q., Xie, B., 2016. Response of biodegradation characteristics of unacclimated activated sludge to moderate pressure in a batch reactor. Chemosphere 148, 41–46. Zhang, B., Xian, Q., Zhu, J., Li, A., Gong, T., 2015. Characterization, DBPs formation, and mutagenicity of soluble microbial products (SMPs) in wastewater under simulated stressful conditions. Chem. Eng. J. 279, 258–263. Zhang, Y., Jiang, W.L., Xu, R.X., Wang, G.X., Xie, B., 2017. Effect of short-term salinity shock on unacclimated activated sludge with pressurized aeration in a sequencing batch reactor. Sep. Purif. Technol. 178, 200–206. Zhang, Y., Li, B., Xu, R.X., Wang, G.X., Zhou, Y., Xie, B., 2016. Effects of pressurized aeration on organic degradation efficiency and bacterial community structure of activated sludge treating saline wastewater. Bioresour. Technol. 222, 182–189. Zhu, H., Keener, T.C., Bishop, P.L., Orton, T.L., Wang, M., Siddiqui, K.F., 1999. Aeration recirculation in air and high purity oxygen systems for control of VOC emissions from wastewater aeration basins. Environ. Prog. Sustain. 18, 101–106. Zhuang, H., Hong, X., Han, H., Shan, S., 2016. Effect of pure oxygen fine bubbles on the organic matter removal and bacterial community evolution treating coal gasification wastewater by membrane bioreactor. Bioresour. Technol. 221, 262–269.
Adav, S.S., Lee, D.J., Lai, J.Y., 2007. Effects of aeration intensity on formation of phenolfed aerobic granules and extracellular polymeric substances. Appl. Microbiol. Biotechnol. 77, 175–182. Arnaldos, M., Amerlinck, Y., Rehman, U., Maere, T., van Hoey, S., Naessens, W., Nopens, I., 2015. From the affinity constant to the half-saturation index: understanding conventional modeling concepts in novel wastewater treatment processes. Water Res. 70, 458–470. Ding, Z., Bourven, I., Guibaud, G., van Hullebusch, E.D., Panico, A., Pirozzi, F., Esposito, G., 2015. Role of extracellular polymeric substances (EPS) production in bioaggregation: application to wastewater treatment. Appl. Microbiol. Biotechnol. 99, 9883–9905. Dionisi, D., Rasheed, A.A., 2018. Maximisation of the organic load rate and minimisation of oxygen consumption in aerobic biological wastewater treatment processes by manipulation of the hydraulic and solids residence time. J. Water Process Eng. 22, 138–146. Duan, L., Song, Y., Yu, H., Xia, S., Hermanowicz, S.W., 2014. The effect of solids retention times on the characterization of extracellular polymeric substances and soluble microbial products in a submerged membrane bioreactor. Bioresour. Technol. 163, 395–398. Holenda, B., Domokos, E., Redey, A., Fazakas, J., 2008. Dissolved oxygen control of the activated sludge wastewater treatment process using model predictive control. Comput. Chem. Eng. 32, 1270–1278. Janga, N., Ren, X., Kim, G., Ahn, C., Cho, J., Kim, I.S., 2007. Characteristics of soluble microbial products and extracellular polymeric substances in the membrane bioreactor for water reuse. Desalination 202, 90–98. Khoshfetrat, A.B., Nikakhtari, H., Sadeghifar, M., Khatibi, M.S., 2011. Influence of organic loading and aeration rates on performance of a lab-scale upflow aerated submerged fixed-film bioreactor. Process Saf. Environ. 89, 193–197. Kim, J.S., Lee, C.H., Chang, I.S., 2001. Effect of pump shear on the performance of a crossflow membrane bioreactor. Water Res. 35, 2137–2144. Laspidou, C.S., Rittmann, B.E., 2002. A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass. Water Res. 36, 2711–2720. Lee, S., Kim, M., 2013. Fouling characteristics in pure oxygen MBR process according to MLSS concentrations and COD loadings. J. Membr. Sci. 428, 323–330. Li, H., Huang, S., Zhou, S., Chen, P., Zhang, Y., 2016. Study of extracellular polymeric substances in the biofilms of a suspended biofilter for nitric oxide removal. Appl. Microbiol. Biotechnol. 100, 9733–9743. Li, X.Y., Yang, S.F., 2007. Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge. Water Res. 41, 1022–1030. Liu, X.M., Sheng, G.P., Luo, H.W., Zhang, F., Yuan, S.J., Xu, J., Zeng, R.J., Wu, J.G., Yu, H.Q., 2010. Contribution of extracellular polymeric substances (EPS) to the sludge aggregation. Environ. Sci. Technol. 44, 4355–4360. Maqbool, T., Cho, J., Hur, J., 2017. Dynamic changes of dissolved organic matter in membrane bioreactors at different organic loading rates: evidence from spectroscopic and chromatographic methods. Bioresour. Technol. 234, 131–139. Molina-Muñoz, M., Poyatos, J.M., Vílchez, R., Hontoria, E., Rodelas, B., González-López, J., 2007. Effect of the concentration of suspended solids on the enzymatic activities and biodiversity of a submerged membrane bioreactor for aerobic treatment of domestic wastewater. Appl. Microbiol. Biotechnol. 73, 1441–1451. Ni, B.J., Fang, F., Xie, W.M., Sun, M., Sheng, G.P., Li, W.H., Yu, H.Q., 2009. Characterization of extracellular polymeric substances produced by mixed microorganisms in activated sludge with gel-permeating chromatography, excitation–emission matrix fluorescence spectroscopy measurement and kinetic modeling. Water Res. 43, 1350–1358. Ni, B.J., Zeng, R.J., Fang, F., Xie, W.M., Sheng, G.P., Yu, H.Q., 2010. Fractionating soluble microbial products in the activated sludge process. Water Res. 44, 2292–2302. Niu, J., Zhang, T., He, Y., Zhou, H., Zhao, A., Zhao, Y., 2013. Pretreatment of landfill leachate using deep shaft aeration bioreactor (DSAB) in cold winter season. J.
194