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International Biodeterioration & Biodegradation 146 (2020) 104810
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Enhanced simultaneous nitrification and denitrification performance in a fixed-bed system packed with PHBV/PLA blends
T
Haimeng Suna, Zhongchen Yanga, Feifei Yanga, Weizhong Wua,∗, Jianlong Wangb,∗∗ a b
Department of Environmental Science, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, PR China Laboratory of Environmental Technology, INET, Tsinghua University, Beijing, 100084, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen Simultaneous nitrification and denitrification Biodegradable polymers Microbial community
In the present study, simultaneous nitrification and denitrification of sewage treatment plant effluent was evaluated using an up-flow fixed-bed system packed with poly (3-hydroxybutyrate-hydroxyvalerate)/polylactide (PHBV/PLA) blends used in a dual role as carbon source and biofilms carrier. 98.1 ± 2.9% and 87.2 ± 6.8% of influent NH4+-N and NO3−-N was removed from the synthetic wastewater. TN removal efficiency was 89.3 ± 6.3% with the average effluent TN concentration of 1.6 ± 0.9 mg/L during the stable period indicating that simultaneous nitrification and denitrification occurred. An initial high release of DOC in the effluent eventually stabilized at average of 9.0 ± 3.4 mg/L. Simultaneous nitrification and denitrification occurred in the first 5 cm, and denitrification only in higher column sections. The PHBV/PLA supported system is a promising technology which could be applied for post-treatment of wastewater with low C/N ratios.
1. Introduction Nitrogen discharge of sewage treatment plants (STPs) is a major threat to the aquatic environment (Carey and Migliaccio, 2009). In China, for example, the average nitrogen concentration released to environment was 14.3–16.5 mg N/L in the effluent of STPs (Yu et al., 2019), which significantly exceeds the threshold of the total nitrogen concentration of 0.5–1.2 mg/L that causes eutrophication (Lewis et al., 2011; Xu et al., 2014). Appropriate strategies are in urgent need to realize the effective nitrogen removal from the effluent of STPs. Biological nitrogen removal is recognized as the preferred method because it can covert dissolved nitrogen to harmless dinitrogen gas effectively and economically (Sun et al., 2010). Aerobic autotrophic nitrification with subsequent heterotrophic denitrification is the most common process in large-scale STPs (Pynaert et al., 2004). However, the low C/N ratios in the effluent of STPs become a limiting factor for advanced nitrogen removal due to the lack of carbon sources (Sheng et al., 2018). Heterotrophic denitrification is difficult to achieve because of the insufficient electron donors generally supplied by carbon sources. To improve the denitrification rate in wastewater with low C/N ratios, one way is to introduce autotrophic denitrification (Capua et al., 2019). Hydrogen gas and reduced sulfur compounds are the most common electron donors for autotrophic denitrification (Capua et al., 2019; Wu et al., 2018). However, the issues of safety, low ∗
denitrification rate and high costs of hydrogen and the troubles of sulfate limit the application of autotrophic denitrification (Zhao et al., 2011). The most common option applied in practice is the addition of external carbon sources such as methanol, ethanol, or acetate, yet the risks of undersupply or overdosing may negatively impact effluent quality (Chu and Wang, 2016). Furthermore, the dosing systems of the soluble carbons will add extra expense (Chu and Wang, 2016). In recent years, solid-phase denitrification (SPD) systems, in which synthetic biodegradable polymers (BDPs), such as polybutylene succinate (PBS), polycaprolactone (PCL), poly(3-hydroxybutyrate-hydroxyvalerate) (PHBV) and poly (3-hydroxybutyrate-hydroxyvalerate)/ polylactide (PHBV/PLA) are used as carbon sources and biofilm carrier, have been developed in nitrogen removal for wastewater with low C/N ratios (Wang and Chu, 2016). Small molecule soluble organic carbons could be released by the polymers “on-demand” by the attack of extracellular hydrolytic enzymes excreted by bacteria responding to nitrate levels. This characteristic could avoid the risk of insufficient dose or overdosing of the external organic carbons (Wang and Chu, 2016). According to previous studies, PHBV/PLA blends are attractive carbon sources due to the high nitrate removal efficiencies and low cost (Wu et al., 2012; Xu et al., 2018a). Xu et al. (2011) compared the denitrification performance in systems supported by PHBV/PLA blends and PCL, respectively. The results showed that denitrification rate in PHBV/ PLA supported system (6.63 mg N/(L·h)) was higher than that in the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (W. Wu), [email protected] (J. Wang).
∗∗
https://doi.org/10.1016/j.ibiod.2019.104810 Received 20 March 2019; Received in revised form 27 September 2019; Accepted 27 September 2019 0964-8305/ © 2019 Elsevier Ltd. All rights reserved.
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single PCL supported system (6.34 mg N/(L·h)). Moreover, the average denitrification removal efficiency could reach 97.0% in the PHBV/PLA supported reactor under favorable condition (Xu and Chai, 2017), and the performance was better than that in most studies reviewed by Wang and Chu (2016). At present, PHBV/PLA supported systems have been applied successfully for nitrate removal (Qiu et al., 2017; Wu et al., 2012; Xu et al., 2018a, 2018b). Effluents of aerobic or anaerobic effluent treatment stages usually contain ammonium (He et al., 2017; Sun et al., 2016), and there is little literature focused on the simultaneous removal of ammonium and nitrate in a single PHBV/PLA supported system through a simultaneous nitrification and denitrification (SND). Appropriately, in such attached-growth systems packed with PHBV/PLA as carbon source and biofilm carrier, the potential interdependent aerobic and anoxic micro-ecological environment of the denser biofilms is thought to be conducive for SND (Modin et al., 2007). However, potential problems may also arise. For example, denitrification might be suppressed because dissolved oxygen (DO) is a more active electron acceptor than NO3−. Furthermore, in the presence of DO, the consumption rate of the solid carbon sources could increase due to the consumption of organics by aerobic bacteria (Gutierrez-Wing et al., 2012). This may affect the sustainable supply of the soluble carbons released by PHBV/PLA blends, especially considering that PLA is not available as carbon source since its main function is to maintain the mechanical integrity of the framework of the blends (Xu and Chai, 2017). Therefore, it is of significant importance to evaluate the nitrogen removal performance in PHBV/PLA supported system in SND prior to the practical application. In the present study, a fixed-bed system packed with PHBV/PLA blends as post-treatment of effluent from STPs was employed in order to achieve simultaneous ammonium and nitrate removal from the simulated wastewater. The objectives of the study were: (1) to evaluate the SND performance in a fixed-bed system packed with PHBV/PLA blends; (2) to get more insights into the nitrogen removal mechanisms through the nitrogen transformation along the flow direction in the substrate; (3) to evaluate the potential large-scale application prospects of the system.
Fig. 1. Schematic diagram of the experimental system.
settlement. The nutrient solution was replenished when the NH4+-N and NO3−-N dropped below 1.0 and 2.0 mg/L, respectively. 2.3. Water sampling and analytical methods All water samples were analyzed for NH4+-N, NO2−-N, NO3−-N and TN according to standard methods (APHA, 2005). DOC in the effluent was measured with a TOC analyzer (HACH, IL530 TOC-TN, USA). DO was determined with a HQ 40d 15 53LED™ HACH (USA) DO analyzer and pH was measured with a PHB-4 pH meter (INESA, Shanghai, China). Furthermore, hydraulic loading rate (HLR, m/d) and nitrogen volumetric removal rate (NVRR, g N/(m3·d)) were estimated as follows:
HLR =
2. Materials and methods
0.001Q πd2
Q×(CNH+4 − Nin + CNO−2 − Nin + CNO−3 − Nin − CNH+4 − Nef − CNO−2 − Nef
2.1. Experiment setup
− CNO−3 − Nef )
The fixed-bed bioreactor (Æ5 cm and 55 cm height) made of plexiglas contained a perforated plate as supporting layer 5 cm above the bottom and the outlet was placed 5 cm below the top (Fig. 1). The working volume was 0.88 L and the PHBV/PLA blends were packed to a height of 25 cm (Fig. 1). PHBV/PLA blends were cylindrical in shape with weight ratio PHBV: PLA of 1 : 1 (Tian'an Biopolymer co. LTD, Zhejiang province, China).
NVRR =
2.2. Experimental procedure
2.4. Statistical analysis
The synthetic wastewater was injected into the column at the bottom at a flow rate of 0.44 L/h and aeration rate of 3–5 mL/min. The column was percolated for 134 days at an indoor air temperature average of 25.9 ± 1.6 °C. The empty bed hydraulic retention time (HRT) was 2 h corresponding to the hydraulic loading rate (HLR) of 5.4 m/d. The influent nitrogen concentration of 5 mg/L NH4+-N and 10 mg/L NO3−-N was in accordance with the Class I (A) of the Wastewater Discharge Standard (GB 18918-2002, China) (Sun et al., 2019). The synthetic wastewater was prepared by mixing 19.1 mg/L NH4Cl and 60.7 mg/L NaNO3 with tap water. Reactors were inoculated with 0.28 L high nutrient solution (25 mg/ L NH4+-N and 50 mg/L NO3−-N, respectively) mixed with biomass from a secondary settling tank of an activated sludge wastewater plant (v: v = 1:1). During one week, the reactor was subjected to cycles of intermittent aeration at 3–5 mL/min for 12 h followed by 12 h
Statistical analyses were performed using PASW Statistics 18.0 (IBM Corporation, Route 100, Somers, NY 10589, USA). Student's t-test was used to determine the difference of the sample data under different conditions. F-test was used to provide volatility test of the effluent water quality between different periods. p < 0.05 were taken to indicate statistical significance at a 95.0% confidence level.
v
where,Q(L/d)itsheinflowrate;d(m)itsheinnerdiameter;CNH4+ -Nin, − + − CNO2− -Nin and CNO3-Nin are the influent concentration of NH4 -N, NO2 − + − − N and NO3 -N, all mg/L and CNH4-Nout, CNO2-Nout and CNO3-Nout are the effluent concentration of NH4+-N, NO2−-N and NO3−-N all mg/L; v (L) is the working volume of the system.
3. Results and discussion 3.1. Nitrogen removal performance of the fixed-bed system Fig. 2 showed the nitrogen removal performance in the present system. Effluent NH4+-N concentration dropped sharply to 3.6 mg/L on the first day, indicating that the rapid start-up of nitrification was obtained in the present system (Fig .2a). The nitrifying bacteria were 2
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Fig. 2. Removal performance of NH4+-N (a), NO3−-N (b), TN (c) and the variation of effluent NO2−-N (b) during the operation period in the experimental system.
inner layer in the attached biofilms (Nerenberg, 2016). In the present system, soluble organics were supplied from the location of the enzymatic attack on the PHBV/PLA blends that have a dual role as electron donor and biofilm carrier. Therefore, since DO was possibly consumed by the aerobic bacteria in the outer biofilm layer (like nitrification bacteria), denitrification bacteria might experience high soluble organic and low DO concentration simultaneously in the inner layer. This kind of “counter-diffusional” biofilms probably further enhanced the SND in the present system. As a result, satisfactory TN removal performance was obtained during the stable period, with average removal efficiency of 89.3 ± 6.3% and average effluent concentration of 1.6 ± 0.9 mg/L without significant accumulation of NO2−-N (Fig. 2 b and c) (p < 0.05). During the initial period, the contaminant transformation fluxes were low due to the thinner biofilms. As the biofilm thickness increased, the fluxes increased with the higher mass transfer resistance in the biofilm. Finally, the biofilms were stable when the decay and detachment was balanced. This dynamic equilibrium process took time to establish. This may be a factor leading to the fluctuation of effluent concentration of NH4+-N, NO3−-N and TN before 90 d (p < 0.05).
present in the inoculum. Furthermore, DO was easily distributed throughout the thin biofilm in the initial period (Modin et al., 2007). Whereafter, NH4+-N removal performance was variable from 5 to 90 d, and subsequently stabilized in the period from 91 to 134 d (p < 0.05). The average effluent NH4+-N concentration was 0.1 ± 0.1 mg/L, with average removal efficiency of 98.1 ± 2.9% during the stable period. These results indicated that complete nitrification could occur in SPD systems without being inhibited. As can be seen in Fig. 2b, NO3−-N removal could be divided into three phases: start-up period for days 1–26 (phase Ⅰ), fluctuation period for days 27–90 (phase Ⅱ) and stable period for days 91–134 (phase Ⅲ). Average removal efficiencies in phase Ⅰ to Ⅲ were 5.5 ± 11.3, 65.3 ± 14.0 and 87.2 ± 6.8%, with average effluent NO3−-N concentration of 10.0 ± 1.1, 4.0 ± 1.5 and 1.3 ± 0.7 mg/L, respectively. Unlike the rapid start-up of nitrification, a lag-time of NO3−-N removal presented during the start-up period. Xu and Chai (2017) reported that the longest lag-time was merely 13 days in the SPD system with different weight ratios of PHBV/PLA, which was shorter than that in the present study. It was observed that volatile fatty acids (VFAs), such as acetic acid and n-butyric acid, were the main available soluble carbon sources released by PHBV (Xu et al., 2019). The inoculum was offered sodium acetate as external carbon source during the denitrification. The denitrification bacteria needed to take some time to adapt the soluble carbon sources released by the PHBV/PLA blends. Furthermore, aeration was supplied in the present system for nitrification. In the initial stage, DO was easily distributed throughout the biomass (Modin et al., 2007). The denitrification relied on the inner anoxic area, which could be provided after the formation of the thicker biofilms. This probably led to the extension of the lag-time of the denitrification. Previously reported biological removals of NO3−-N in PHBV/PLA supported SPD systems optimized for denitrification were all higher than 90% at proper operated parameters (Qiu et al., 2017; Wu et al., 2012; Xu et al., 2018a, 2018b). These removal efficiencies were higher than the removal efficiency observed during the stable period in this study. NO3−-N would be produced as the terminal product of nitrification. Therefore, the removed NO3−-N in the system included two parts: NO3−-N in the influent and that produced in the reactor by NH4+-N oxidation. However, the calculation of NO3−-N removal efficiency was merely based on the influent NO3−-N concentration. The calculated removal efficiency of NO3−-N was certainly lower than the real removal efficiency. On account of this, denitrification performance in this aerated system still maintained comparable level with that in other studies. Virdis et al. (2011) reported that biofilms in the SND systems were divided into two layers due to the oxygen gradient with the outer layers occupied by nitrifying bacteria and the inner layers occupied by denitrifying bacteria. In most attached-growth systems, the biofilms are established on inert substrates, such as zeolite, plastic foams, or ceramsite. The electron donors (soluble organics) and electron acceptors (NO3−-N, DO) diffuse in the same direction from the outer layer to the
3.2. Variation of DOC, pH and DO As depicted in Fig. 3a, DOC increased rapidly in the first 70 d up to the maximum concentration of 36.4 mg/L. A certain degree of organic loading was produced by PHBV/PLA blends considering that there was no DOC in the influent. During this period, biofilms were not fully mature and the consumption of the soluble organic matter was less than the production, which led to the increase of effluent DOC (Wang and Chu, 2016). During 70–90 d, DOC decreased corresponding with the decline of effluent NO3−-N (Figs. 2b and 3a). On the one hand, more available soluble organic matter was utilized by the attached increased biomass with the demand for assimilation or respiration. On the other hand, the less degradable regions of PHBV were exposed as operation time progressed (Timmins et al., 1997). With the maturation of biofilms and the stability of hydrolysis rate of PHBV, the system achieved the approximate balance between the production of DOC and the sum of the consumption of DOC and effluent DOC after 90 d. Effluent DOC maintained stability during the stable period with the average of 9.0 ± 3.4 mg/L, which was in accordance with the stable nitrogen removal during this period. Xu et al. (2018a) reported that the effluent DOC was 7.0–55.6 mg/L under different operation conditions in PHBV/ PLA blends supported SPD system. Chu and Wang (2016) compared the denitrification performance in the SPD systems filled with three kinds of biopolymers, PHBV, PHBV/starch and PHBV/bamboo powder (BP) blends. Effluent DOC was 10.5 mg/L observed in PHBV/BP system, which was considered to minimize the negative impact of external carbons. The present system further reduced the risk of DOC secondary pollution produced by PHBV/PLA during the stable period, which indicated that DOC leaching of the PHBV/PLA blends showed 3
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Fig. 3. Variation of DOC (a), pH and DO (b) during the operation period in the experimental system.
Fig. 4. Nitrogen transformation (a) and DO concentration (b) along the flow direction. Table 1 Cost evaluation of different carbon sources for nitrate removal. Substrate
Price of substrate ($/kg)
Consumption of substrate (kg/kg NO3−-N)
Cost of denitrification ($/kg NO3−-N)
References
PHBV/PLA PHBV PCL Methanol Ethanol Sodium acetate
3.05 4.37 3.93 0.33 0.84 1.03
1.4 1.5–1.7 1.6–3.7 2.0–3.2 2.0–3.3 3.5–4.3
4.27 6.56–7.43 6.29–14.54 0.66–1.06 1.68–2.77 3.61–4.43
This study Chu & Wang (2016) Chu & Wang (2013) (Kim et al., 2017; Mohseni-Bandpi et al., 2013) (Kim et al., 2017; Mohseni-Bandpi et al., 2013) (Kim et al., 2017; Mohseni-Bandpi et al., 2013)
the influent (7.5 ± 0.1, Fig. 3b) (p < 0.05). This is different from most previous studies on SPD where effluent pH generally decreased compared to influent pH (Shen et al., 2013; Xie et al., 2017; Xu and Chai, 2017; Xu et al., 2018a, 2018b). Soluble organic acids could be released through the degradation of solid carbon sources, and these acidic substances could provide enough acidity to offset the alkalinity produced by denitrification (Wang and Chu, 2016). Therefore, DOC concentration in the SPD system is a major determinant of pH decrease or increase. In the present system, effluent DOC concentration was at relative low level compared with the previous studies, so slight increase of pH was observed in the effluent. Average DO was 4.9 ± 0.7 mg/Lin the effluent and 3.2 ± 0.9 mg/ L in the effluent. These DO levels were similar to those recorded in our previous study in which satisfactory nitrogen removal performance was achieved by SND in a constructed wetland filled with PHBV (Sun et al., 2018). As mentioned above, denitrification bacteria could accumulate in the inner zones of the attached biofilms, and the oxygen might be consumed by aerobic or facultative bacteria in the outer layer. Therefore, from the point of view of nitrogen removal performance, the
Table 2 Comparison of NVRRs by using various carbon sources for nitrogen removal. Carbon sources
Types of system
Optimal NVRRs
References
PHBV/PLA Methanol
Fixed bed PFRa SBRb MBRc PFR SBR MBR PFR SBR MBR
162.2 83.8 147.5 107.0 85.3 175.5 56.4 203.8 183.2 70.0–412.5
This study Nyberg et al. (1996) Peng et al. (2007) Ivanovic & Leiknes (2011) Nyberg et al. (1996) Puig et al. (2007) Ylinen (2013) Yoo et al. (1999) Lim et al. (2008) Kumar et al. (2012)
Ethanol
Sodium acetate
a b c
Plug flow reactor. Sequencing batch reactor. Membrane bioreactor.
controllable and sustainable characteristics without adverse effect during SND. The average effluent pH (7.6 ± 0.1) was slightly higher than that in 4
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The other point that should not be ignored is that soluble carbon such as sodium acetate are not suitable for advanced nitrogen removal, e.g., nitrogen removal in sensitive water environment or sewage plant effluent, due to the risk of overdosing. We conclude that the PHBV/PLA supported system is a competitive technology for nitrogen removal in large-scale applications, especially in the field of advanced nitrogen removal.
observed DO level was in a reasonable range favoring SND in the present system. Furthermore, the oxygen-rich environment further strengthened the control of effluent DOC for that some soluble organics could be utilized by the aerobic bacteria for respiration in the presence of adequate DO (Gutierrez-Wing et al., 2012). 3.3. Nitrogen removal profile in the column
4. Conclusion
In order to further understand SND dynamics in the reactor, nitrogen removal performance along the flow direction was evaluated. NH4+-N was nearly completely removed in the first 5 cm height of the substrate (Fig. 4a). In contrast, NO3−-N concentration increased at 5 cm height, and then gradually decreased along the flow direction. DO dropped from 5.3 ± 0.5 to 3.4 ± 0.2 mg/L in the first 5 cm, and then varied in the range of 3.0–3.5 mg/L in the remainder of the column (Fig. 4b). As the terminal product of nitrification, the net accumulation of NO3−-N at 5 cm depth was attributed to the oxidation of NH4+-N, which could also explain the DO decline from 0 to 5 cm. TN was removed along the flow direction (Fig. 4a). In the first 5 cm, the TN concentration declined from 15.2 ± 0.3 to 12.3 ± 0.6 mg/L, even though the NO3−-N concentration increased at 5 cm depth (Fig. 4a). This also indicated that SND occurred in this region. Denitrification accounted for most of the TN removal in the remaining sections of the column (Fig. 4a). The zero-order model was applied to describe the TN removal after the conversion of height to HRT, which fitted well in the present study (Fig. 4a). The zero-order constant was 10.9 mg N/(L·h) (Fig. 4a), which was acceptable compared with previous studies (Wang and Chu, 2016).
The SND fixed-bed system packed with PHBV/PLA blends as carbon source and biofilm carrier was verified as an effective technology for nitrogen removal from sewage plant effluent. Average TN removal efficiency of 89.3 ± 6.3% could be achieved with controllable effluent DOC concentration of 9.0 ± 3.4 mg/L during the stable period. The oxygen-rich environment is beneficial to decrease the risk of DOC excess to the extreme extent in the present study. SND dominated in the first 5 cm and denitrification occurred in the remainder of the column. Furthermore, the potential large-scale application of the present system was evaluated from the perspective of cost and volumetric removal rate. The cost of nitrate removal using PHBV/PLA was estimated at 4.27 $/kg NO3−-N, while the NVRR was 162.2 g N/(m3·d). These two data indicated competitive performance to those of conventional systems with soluble carbon sources. Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
3.4. Evaluation of the potential large-scale application Acknowledgement Cost and the volumetric removal rate are two important indicators to evaluate the system for potential large-scale application (Zhang et al., 2016). The two indicators of the present system were calculated and then compared with those of conventional systems (Tables 1 and 2). Table 1 presented the denitrification costs by using PHBV/PLA and other carbon sources. The cost of the carbon source for nitrate removal was calculated according to the consumption of the substrate and the unit price of the substrate. The results showed that the PHBV/PLA (4.27 $/kg NO3−-N) was more economical than PHBV and PCL, indicating that the blends improved the practical applicability of solid carbon sources. When compared with soluble carbon source, the cost of using PHBV/PLA was 4.0–6.5 and 1.5–2.5 times higher than that using methanol and ethanol, respectively, and at the same level with sodium acetate. However, in addition to the cost of feedstock, additional expenses are incurred due to the need of auxiliary facilities such as control systems and dosing pumps (Wang and Chu, 2016). Therefore, the economic performance of the PHBV/PLA for denitrification is comparable to that of conventional carbon sources. Table 2 summarized the NVRRs in the present system and other traditional systems. It should be noted that, all systems were capable of simultaneous removal of NH4+-N and NO3−-N. Furthermore, some NVRRs were obtained after calculation according to the results reported by the referenced articles. As can be seen in Table 2, PHBV/PLA provided higher NVRR (162.2 g N/(m3·d)) than that in most systems with methanol and ethanol as carbon sources. Compared with methanol and ethanol, sodium acetate could be easily metabolized and directly utilized by bacteria (Fowdar et al., 2015). It is mainly for this reason that NVRR of the present system was slightly lower than that of the activated sludge systems with sodium acetate as carbon source. However, in such suspended-growth systems, nitrification and denitrification need to be partitioned based on space or time sequence due to the uniformly distributed DO concentration in the aerobic region. This will certainly bring higher construction costs or more operational troubles.
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Enhanced simultaneous nitrification and denitrification performance in a fixed-bed system packed with PHBV/PLA blends
T
Haimeng Suna, Zhongchen Yanga, Feifei Yanga, Weizhong Wua,∗, Jianlong Wangb,∗∗ a b
Department of Environmental Science, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, PR China Laboratory of Environmental Technology, INET, Tsinghua University, Beijing, 100084, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen Simultaneous nitrification and denitrification Biodegradable polymers Microbial community
In the present study, simultaneous nitrification and denitrification of sewage treatment plant effluent was evaluated using an up-flow fixed-bed system packed with poly (3-hydroxybutyrate-hydroxyvalerate)/polylactide (PHBV/PLA) blends used in a dual role as carbon source and biofilms carrier. 98.1 ± 2.9% and 87.2 ± 6.8% of influent NH4+-N and NO3−-N was removed from the synthetic wastewater. TN removal efficiency was 89.3 ± 6.3% with the average effluent TN concentration of 1.6 ± 0.9 mg/L during the stable period indicating that simultaneous nitrification and denitrification occurred. An initial high release of DOC in the effluent eventually stabilized at average of 9.0 ± 3.4 mg/L. Simultaneous nitrification and denitrification occurred in the first 5 cm, and denitrification only in higher column sections. The PHBV/PLA supported system is a promising technology which could be applied for post-treatment of wastewater with low C/N ratios.
1. Introduction Nitrogen discharge of sewage treatment plants (STPs) is a major threat to the aquatic environment (Carey and Migliaccio, 2009). In China, for example, the average nitrogen concentration released to environment was 14.3–16.5 mg N/L in the effluent of STPs (Yu et al., 2019), which significantly exceeds the threshold of the total nitrogen concentration of 0.5–1.2 mg/L that causes eutrophication (Lewis et al., 2011; Xu et al., 2014). Appropriate strategies are in urgent need to realize the effective nitrogen removal from the effluent of STPs. Biological nitrogen removal is recognized as the preferred method because it can covert dissolved nitrogen to harmless dinitrogen gas effectively and economically (Sun et al., 2010). Aerobic autotrophic nitrification with subsequent heterotrophic denitrification is the most common process in large-scale STPs (Pynaert et al., 2004). However, the low C/N ratios in the effluent of STPs become a limiting factor for advanced nitrogen removal due to the lack of carbon sources (Sheng et al., 2018). Heterotrophic denitrification is difficult to achieve because of the insufficient electron donors generally supplied by carbon sources. To improve the denitrification rate in wastewater with low C/N ratios, one way is to introduce autotrophic denitrification (Capua et al., 2019). Hydrogen gas and reduced sulfur compounds are the most common electron donors for autotrophic denitrification (Capua et al., 2019; Wu et al., 2018). However, the issues of safety, low ∗
denitrification rate and high costs of hydrogen and the troubles of sulfate limit the application of autotrophic denitrification (Zhao et al., 2011). The most common option applied in practice is the addition of external carbon sources such as methanol, ethanol, or acetate, yet the risks of undersupply or overdosing may negatively impact effluent quality (Chu and Wang, 2016). Furthermore, the dosing systems of the soluble carbons will add extra expense (Chu and Wang, 2016). In recent years, solid-phase denitrification (SPD) systems, in which synthetic biodegradable polymers (BDPs), such as polybutylene succinate (PBS), polycaprolactone (PCL), poly(3-hydroxybutyrate-hydroxyvalerate) (PHBV) and poly (3-hydroxybutyrate-hydroxyvalerate)/ polylactide (PHBV/PLA) are used as carbon sources and biofilm carrier, have been developed in nitrogen removal for wastewater with low C/N ratios (Wang and Chu, 2016). Small molecule soluble organic carbons could be released by the polymers “on-demand” by the attack of extracellular hydrolytic enzymes excreted by bacteria responding to nitrate levels. This characteristic could avoid the risk of insufficient dose or overdosing of the external organic carbons (Wang and Chu, 2016). According to previous studies, PHBV/PLA blends are attractive carbon sources due to the high nitrate removal efficiencies and low cost (Wu et al., 2012; Xu et al., 2018a). Xu et al. (2011) compared the denitrification performance in systems supported by PHBV/PLA blends and PCL, respectively. The results showed that denitrification rate in PHBV/ PLA supported system (6.63 mg N/(L·h)) was higher than that in the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (W. Wu), [email protected] (J. Wang).
∗∗
https://doi.org/10.1016/j.ibiod.2019.104810 Received 20 March 2019; Received in revised form 27 September 2019; Accepted 27 September 2019 0964-8305/ © 2019 Elsevier Ltd. All rights reserved.
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single PCL supported system (6.34 mg N/(L·h)). Moreover, the average denitrification removal efficiency could reach 97.0% in the PHBV/PLA supported reactor under favorable condition (Xu and Chai, 2017), and the performance was better than that in most studies reviewed by Wang and Chu (2016). At present, PHBV/PLA supported systems have been applied successfully for nitrate removal (Qiu et al., 2017; Wu et al., 2012; Xu et al., 2018a, 2018b). Effluents of aerobic or anaerobic effluent treatment stages usually contain ammonium (He et al., 2017; Sun et al., 2016), and there is little literature focused on the simultaneous removal of ammonium and nitrate in a single PHBV/PLA supported system through a simultaneous nitrification and denitrification (SND). Appropriately, in such attached-growth systems packed with PHBV/PLA as carbon source and biofilm carrier, the potential interdependent aerobic and anoxic micro-ecological environment of the denser biofilms is thought to be conducive for SND (Modin et al., 2007). However, potential problems may also arise. For example, denitrification might be suppressed because dissolved oxygen (DO) is a more active electron acceptor than NO3−. Furthermore, in the presence of DO, the consumption rate of the solid carbon sources could increase due to the consumption of organics by aerobic bacteria (Gutierrez-Wing et al., 2012). This may affect the sustainable supply of the soluble carbons released by PHBV/PLA blends, especially considering that PLA is not available as carbon source since its main function is to maintain the mechanical integrity of the framework of the blends (Xu and Chai, 2017). Therefore, it is of significant importance to evaluate the nitrogen removal performance in PHBV/PLA supported system in SND prior to the practical application. In the present study, a fixed-bed system packed with PHBV/PLA blends as post-treatment of effluent from STPs was employed in order to achieve simultaneous ammonium and nitrate removal from the simulated wastewater. The objectives of the study were: (1) to evaluate the SND performance in a fixed-bed system packed with PHBV/PLA blends; (2) to get more insights into the nitrogen removal mechanisms through the nitrogen transformation along the flow direction in the substrate; (3) to evaluate the potential large-scale application prospects of the system.
Fig. 1. Schematic diagram of the experimental system.
settlement. The nutrient solution was replenished when the NH4+-N and NO3−-N dropped below 1.0 and 2.0 mg/L, respectively. 2.3. Water sampling and analytical methods All water samples were analyzed for NH4+-N, NO2−-N, NO3−-N and TN according to standard methods (APHA, 2005). DOC in the effluent was measured with a TOC analyzer (HACH, IL530 TOC-TN, USA). DO was determined with a HQ 40d 15 53LED™ HACH (USA) DO analyzer and pH was measured with a PHB-4 pH meter (INESA, Shanghai, China). Furthermore, hydraulic loading rate (HLR, m/d) and nitrogen volumetric removal rate (NVRR, g N/(m3·d)) were estimated as follows:
HLR =
2. Materials and methods
0.001Q πd2
Q×(CNH+4 − Nin + CNO−2 − Nin + CNO−3 − Nin − CNH+4 − Nef − CNO−2 − Nef
2.1. Experiment setup
− CNO−3 − Nef )
The fixed-bed bioreactor (Æ5 cm and 55 cm height) made of plexiglas contained a perforated plate as supporting layer 5 cm above the bottom and the outlet was placed 5 cm below the top (Fig. 1). The working volume was 0.88 L and the PHBV/PLA blends were packed to a height of 25 cm (Fig. 1). PHBV/PLA blends were cylindrical in shape with weight ratio PHBV: PLA of 1 : 1 (Tian'an Biopolymer co. LTD, Zhejiang province, China).
NVRR =
2.2. Experimental procedure
2.4. Statistical analysis
The synthetic wastewater was injected into the column at the bottom at a flow rate of 0.44 L/h and aeration rate of 3–5 mL/min. The column was percolated for 134 days at an indoor air temperature average of 25.9 ± 1.6 °C. The empty bed hydraulic retention time (HRT) was 2 h corresponding to the hydraulic loading rate (HLR) of 5.4 m/d. The influent nitrogen concentration of 5 mg/L NH4+-N and 10 mg/L NO3−-N was in accordance with the Class I (A) of the Wastewater Discharge Standard (GB 18918-2002, China) (Sun et al., 2019). The synthetic wastewater was prepared by mixing 19.1 mg/L NH4Cl and 60.7 mg/L NaNO3 with tap water. Reactors were inoculated with 0.28 L high nutrient solution (25 mg/ L NH4+-N and 50 mg/L NO3−-N, respectively) mixed with biomass from a secondary settling tank of an activated sludge wastewater plant (v: v = 1:1). During one week, the reactor was subjected to cycles of intermittent aeration at 3–5 mL/min for 12 h followed by 12 h
Statistical analyses were performed using PASW Statistics 18.0 (IBM Corporation, Route 100, Somers, NY 10589, USA). Student's t-test was used to determine the difference of the sample data under different conditions. F-test was used to provide volatility test of the effluent water quality between different periods. p < 0.05 were taken to indicate statistical significance at a 95.0% confidence level.
v
where,Q(L/d)itsheinflowrate;d(m)itsheinnerdiameter;CNH4+ -Nin, − + − CNO2− -Nin and CNO3-Nin are the influent concentration of NH4 -N, NO2 − + − − N and NO3 -N, all mg/L and CNH4-Nout, CNO2-Nout and CNO3-Nout are the effluent concentration of NH4+-N, NO2−-N and NO3−-N all mg/L; v (L) is the working volume of the system.
3. Results and discussion 3.1. Nitrogen removal performance of the fixed-bed system Fig. 2 showed the nitrogen removal performance in the present system. Effluent NH4+-N concentration dropped sharply to 3.6 mg/L on the first day, indicating that the rapid start-up of nitrification was obtained in the present system (Fig .2a). The nitrifying bacteria were 2
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Fig. 2. Removal performance of NH4+-N (a), NO3−-N (b), TN (c) and the variation of effluent NO2−-N (b) during the operation period in the experimental system.
inner layer in the attached biofilms (Nerenberg, 2016). In the present system, soluble organics were supplied from the location of the enzymatic attack on the PHBV/PLA blends that have a dual role as electron donor and biofilm carrier. Therefore, since DO was possibly consumed by the aerobic bacteria in the outer biofilm layer (like nitrification bacteria), denitrification bacteria might experience high soluble organic and low DO concentration simultaneously in the inner layer. This kind of “counter-diffusional” biofilms probably further enhanced the SND in the present system. As a result, satisfactory TN removal performance was obtained during the stable period, with average removal efficiency of 89.3 ± 6.3% and average effluent concentration of 1.6 ± 0.9 mg/L without significant accumulation of NO2−-N (Fig. 2 b and c) (p < 0.05). During the initial period, the contaminant transformation fluxes were low due to the thinner biofilms. As the biofilm thickness increased, the fluxes increased with the higher mass transfer resistance in the biofilm. Finally, the biofilms were stable when the decay and detachment was balanced. This dynamic equilibrium process took time to establish. This may be a factor leading to the fluctuation of effluent concentration of NH4+-N, NO3−-N and TN before 90 d (p < 0.05).
present in the inoculum. Furthermore, DO was easily distributed throughout the thin biofilm in the initial period (Modin et al., 2007). Whereafter, NH4+-N removal performance was variable from 5 to 90 d, and subsequently stabilized in the period from 91 to 134 d (p < 0.05). The average effluent NH4+-N concentration was 0.1 ± 0.1 mg/L, with average removal efficiency of 98.1 ± 2.9% during the stable period. These results indicated that complete nitrification could occur in SPD systems without being inhibited. As can be seen in Fig. 2b, NO3−-N removal could be divided into three phases: start-up period for days 1–26 (phase Ⅰ), fluctuation period for days 27–90 (phase Ⅱ) and stable period for days 91–134 (phase Ⅲ). Average removal efficiencies in phase Ⅰ to Ⅲ were 5.5 ± 11.3, 65.3 ± 14.0 and 87.2 ± 6.8%, with average effluent NO3−-N concentration of 10.0 ± 1.1, 4.0 ± 1.5 and 1.3 ± 0.7 mg/L, respectively. Unlike the rapid start-up of nitrification, a lag-time of NO3−-N removal presented during the start-up period. Xu and Chai (2017) reported that the longest lag-time was merely 13 days in the SPD system with different weight ratios of PHBV/PLA, which was shorter than that in the present study. It was observed that volatile fatty acids (VFAs), such as acetic acid and n-butyric acid, were the main available soluble carbon sources released by PHBV (Xu et al., 2019). The inoculum was offered sodium acetate as external carbon source during the denitrification. The denitrification bacteria needed to take some time to adapt the soluble carbon sources released by the PHBV/PLA blends. Furthermore, aeration was supplied in the present system for nitrification. In the initial stage, DO was easily distributed throughout the biomass (Modin et al., 2007). The denitrification relied on the inner anoxic area, which could be provided after the formation of the thicker biofilms. This probably led to the extension of the lag-time of the denitrification. Previously reported biological removals of NO3−-N in PHBV/PLA supported SPD systems optimized for denitrification were all higher than 90% at proper operated parameters (Qiu et al., 2017; Wu et al., 2012; Xu et al., 2018a, 2018b). These removal efficiencies were higher than the removal efficiency observed during the stable period in this study. NO3−-N would be produced as the terminal product of nitrification. Therefore, the removed NO3−-N in the system included two parts: NO3−-N in the influent and that produced in the reactor by NH4+-N oxidation. However, the calculation of NO3−-N removal efficiency was merely based on the influent NO3−-N concentration. The calculated removal efficiency of NO3−-N was certainly lower than the real removal efficiency. On account of this, denitrification performance in this aerated system still maintained comparable level with that in other studies. Virdis et al. (2011) reported that biofilms in the SND systems were divided into two layers due to the oxygen gradient with the outer layers occupied by nitrifying bacteria and the inner layers occupied by denitrifying bacteria. In most attached-growth systems, the biofilms are established on inert substrates, such as zeolite, plastic foams, or ceramsite. The electron donors (soluble organics) and electron acceptors (NO3−-N, DO) diffuse in the same direction from the outer layer to the
3.2. Variation of DOC, pH and DO As depicted in Fig. 3a, DOC increased rapidly in the first 70 d up to the maximum concentration of 36.4 mg/L. A certain degree of organic loading was produced by PHBV/PLA blends considering that there was no DOC in the influent. During this period, biofilms were not fully mature and the consumption of the soluble organic matter was less than the production, which led to the increase of effluent DOC (Wang and Chu, 2016). During 70–90 d, DOC decreased corresponding with the decline of effluent NO3−-N (Figs. 2b and 3a). On the one hand, more available soluble organic matter was utilized by the attached increased biomass with the demand for assimilation or respiration. On the other hand, the less degradable regions of PHBV were exposed as operation time progressed (Timmins et al., 1997). With the maturation of biofilms and the stability of hydrolysis rate of PHBV, the system achieved the approximate balance between the production of DOC and the sum of the consumption of DOC and effluent DOC after 90 d. Effluent DOC maintained stability during the stable period with the average of 9.0 ± 3.4 mg/L, which was in accordance with the stable nitrogen removal during this period. Xu et al. (2018a) reported that the effluent DOC was 7.0–55.6 mg/L under different operation conditions in PHBV/ PLA blends supported SPD system. Chu and Wang (2016) compared the denitrification performance in the SPD systems filled with three kinds of biopolymers, PHBV, PHBV/starch and PHBV/bamboo powder (BP) blends. Effluent DOC was 10.5 mg/L observed in PHBV/BP system, which was considered to minimize the negative impact of external carbons. The present system further reduced the risk of DOC secondary pollution produced by PHBV/PLA during the stable period, which indicated that DOC leaching of the PHBV/PLA blends showed 3
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Fig. 3. Variation of DOC (a), pH and DO (b) during the operation period in the experimental system.
Fig. 4. Nitrogen transformation (a) and DO concentration (b) along the flow direction. Table 1 Cost evaluation of different carbon sources for nitrate removal. Substrate
Price of substrate ($/kg)
Consumption of substrate (kg/kg NO3−-N)
Cost of denitrification ($/kg NO3−-N)
References
PHBV/PLA PHBV PCL Methanol Ethanol Sodium acetate
3.05 4.37 3.93 0.33 0.84 1.03
1.4 1.5–1.7 1.6–3.7 2.0–3.2 2.0–3.3 3.5–4.3
4.27 6.56–7.43 6.29–14.54 0.66–1.06 1.68–2.77 3.61–4.43
This study Chu & Wang (2016) Chu & Wang (2013) (Kim et al., 2017; Mohseni-Bandpi et al., 2013) (Kim et al., 2017; Mohseni-Bandpi et al., 2013) (Kim et al., 2017; Mohseni-Bandpi et al., 2013)
the influent (7.5 ± 0.1, Fig. 3b) (p < 0.05). This is different from most previous studies on SPD where effluent pH generally decreased compared to influent pH (Shen et al., 2013; Xie et al., 2017; Xu and Chai, 2017; Xu et al., 2018a, 2018b). Soluble organic acids could be released through the degradation of solid carbon sources, and these acidic substances could provide enough acidity to offset the alkalinity produced by denitrification (Wang and Chu, 2016). Therefore, DOC concentration in the SPD system is a major determinant of pH decrease or increase. In the present system, effluent DOC concentration was at relative low level compared with the previous studies, so slight increase of pH was observed in the effluent. Average DO was 4.9 ± 0.7 mg/Lin the effluent and 3.2 ± 0.9 mg/ L in the effluent. These DO levels were similar to those recorded in our previous study in which satisfactory nitrogen removal performance was achieved by SND in a constructed wetland filled with PHBV (Sun et al., 2018). As mentioned above, denitrification bacteria could accumulate in the inner zones of the attached biofilms, and the oxygen might be consumed by aerobic or facultative bacteria in the outer layer. Therefore, from the point of view of nitrogen removal performance, the
Table 2 Comparison of NVRRs by using various carbon sources for nitrogen removal. Carbon sources
Types of system
Optimal NVRRs
References
PHBV/PLA Methanol
Fixed bed PFRa SBRb MBRc PFR SBR MBR PFR SBR MBR
162.2 83.8 147.5 107.0 85.3 175.5 56.4 203.8 183.2 70.0–412.5
This study Nyberg et al. (1996) Peng et al. (2007) Ivanovic & Leiknes (2011) Nyberg et al. (1996) Puig et al. (2007) Ylinen (2013) Yoo et al. (1999) Lim et al. (2008) Kumar et al. (2012)
Ethanol
Sodium acetate
a b c
Plug flow reactor. Sequencing batch reactor. Membrane bioreactor.
controllable and sustainable characteristics without adverse effect during SND. The average effluent pH (7.6 ± 0.1) was slightly higher than that in 4
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The other point that should not be ignored is that soluble carbon such as sodium acetate are not suitable for advanced nitrogen removal, e.g., nitrogen removal in sensitive water environment or sewage plant effluent, due to the risk of overdosing. We conclude that the PHBV/PLA supported system is a competitive technology for nitrogen removal in large-scale applications, especially in the field of advanced nitrogen removal.
observed DO level was in a reasonable range favoring SND in the present system. Furthermore, the oxygen-rich environment further strengthened the control of effluent DOC for that some soluble organics could be utilized by the aerobic bacteria for respiration in the presence of adequate DO (Gutierrez-Wing et al., 2012). 3.3. Nitrogen removal profile in the column
4. Conclusion
In order to further understand SND dynamics in the reactor, nitrogen removal performance along the flow direction was evaluated. NH4+-N was nearly completely removed in the first 5 cm height of the substrate (Fig. 4a). In contrast, NO3−-N concentration increased at 5 cm height, and then gradually decreased along the flow direction. DO dropped from 5.3 ± 0.5 to 3.4 ± 0.2 mg/L in the first 5 cm, and then varied in the range of 3.0–3.5 mg/L in the remainder of the column (Fig. 4b). As the terminal product of nitrification, the net accumulation of NO3−-N at 5 cm depth was attributed to the oxidation of NH4+-N, which could also explain the DO decline from 0 to 5 cm. TN was removed along the flow direction (Fig. 4a). In the first 5 cm, the TN concentration declined from 15.2 ± 0.3 to 12.3 ± 0.6 mg/L, even though the NO3−-N concentration increased at 5 cm depth (Fig. 4a). This also indicated that SND occurred in this region. Denitrification accounted for most of the TN removal in the remaining sections of the column (Fig. 4a). The zero-order model was applied to describe the TN removal after the conversion of height to HRT, which fitted well in the present study (Fig. 4a). The zero-order constant was 10.9 mg N/(L·h) (Fig. 4a), which was acceptable compared with previous studies (Wang and Chu, 2016).
The SND fixed-bed system packed with PHBV/PLA blends as carbon source and biofilm carrier was verified as an effective technology for nitrogen removal from sewage plant effluent. Average TN removal efficiency of 89.3 ± 6.3% could be achieved with controllable effluent DOC concentration of 9.0 ± 3.4 mg/L during the stable period. The oxygen-rich environment is beneficial to decrease the risk of DOC excess to the extreme extent in the present study. SND dominated in the first 5 cm and denitrification occurred in the remainder of the column. Furthermore, the potential large-scale application of the present system was evaluated from the perspective of cost and volumetric removal rate. The cost of nitrate removal using PHBV/PLA was estimated at 4.27 $/kg NO3−-N, while the NVRR was 162.2 g N/(m3·d). These two data indicated competitive performance to those of conventional systems with soluble carbon sources. Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
3.4. Evaluation of the potential large-scale application Acknowledgement Cost and the volumetric removal rate are two important indicators to evaluate the system for potential large-scale application (Zhang et al., 2016). The two indicators of the present system were calculated and then compared with those of conventional systems (Tables 1 and 2). Table 1 presented the denitrification costs by using PHBV/PLA and other carbon sources. The cost of the carbon source for nitrate removal was calculated according to the consumption of the substrate and the unit price of the substrate. The results showed that the PHBV/PLA (4.27 $/kg NO3−-N) was more economical than PHBV and PCL, indicating that the blends improved the practical applicability of solid carbon sources. When compared with soluble carbon source, the cost of using PHBV/PLA was 4.0–6.5 and 1.5–2.5 times higher than that using methanol and ethanol, respectively, and at the same level with sodium acetate. However, in addition to the cost of feedstock, additional expenses are incurred due to the need of auxiliary facilities such as control systems and dosing pumps (Wang and Chu, 2016). Therefore, the economic performance of the PHBV/PLA for denitrification is comparable to that of conventional carbon sources. Table 2 summarized the NVRRs in the present system and other traditional systems. It should be noted that, all systems were capable of simultaneous removal of NH4+-N and NO3−-N. Furthermore, some NVRRs were obtained after calculation according to the results reported by the referenced articles. As can be seen in Table 2, PHBV/PLA provided higher NVRR (162.2 g N/(m3·d)) than that in most systems with methanol and ethanol as carbon sources. Compared with methanol and ethanol, sodium acetate could be easily metabolized and directly utilized by bacteria (Fowdar et al., 2015). It is mainly for this reason that NVRR of the present system was slightly lower than that of the activated sludge systems with sodium acetate as carbon source. However, in such suspended-growth systems, nitrification and denitrification need to be partitioned based on space or time sequence due to the uniformly distributed DO concentration in the aerobic region. This will certainly bring higher construction costs or more operational troubles.
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