- Email: [email protected]
Phosphorus release and recovery from Fe-enhanced primary sedimentation sludge via alkaline fermentation
Bioresource Technology 278 (2019) 266–271
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Phosphorus release and recovery from Fe-enhanced primary sedimentation sludge via alkaline fermentation Yun Chena,1, Hui Lina,b,1, Nan Shena, Wang Yana,b, Jieai Wanga, Guoxiang Wanga,b, a b
T
⁎
School of Environment, Nanjing Normal University, Nanjing, Jiangsu 210023, People’s Republic of China Jiangsu Engineering Lab of Water and Soil Eco-remediation, Nanjing Normal University, Nanjing, Jiangsu 210023, People’s Republic of China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Alkaline fermentation Chemically enhanced primary sedimentation (CEPS) Magnesium ammonium phosphate (MAP) Phosphorus release and recovery
Phosphorus release and recovery from Fe-based chemically enhanced primary sedimentation (CEPS) sludge via alkaline fermentation was investigated. The coagulation results showed that 78% of organic matter and 95% of phosphorus were concentrated from sewage into sludge with the optimum dosages of 25 mg/L FeCl3. The batch fermentation results revealed that 69.35% of the phosphorus in the Fe-sludge can be released and the maximum phosphorus concentration was 20.57 mg/L at pH 11. In the recovery stage, 90% of the P released in the fermented sludge supernatant was precipitated at a 2:1 ratio of magnesium to phosphorus and pH 11. The result of X-ray diffraction indicated that magnesium ammonium phosphate (MAP) was the major component of the precipitated solids. Thus, the present study provides an alternative option for phosphorus release and recovery as MAP from CEPS sludge via alkaline fermentation.
1. Introduction Excessive phosphorus (P) discharge could induce eutrophication in natural waters. Thus, removing P from wastewater to avoid P flowing into nature waters is an urgent task (Xu et al., 2018a). Until now, dosing with coagulant chemicals or the biological P removal process are
known as the most common methods for P removal. Compared to the biological P removal process, which removes P by accumulating excess phosphate in polyphosphate-accumulating organisms (PAOs), the chemical precipitation process is more popular due to the easier operation, lower cost, and higher P removal efficiency. Furthermore, in wastewater, limited chemical dosing of metal salts (e.g., FeCl3 and AlCl3) is
Corresponding author at: School of Environment, Nanjing Normal University, Nanjing, Jiangsu 210023, People’s Republic of China. E-mail address: [email protected] (G. Wang). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.biortech.2019.01.094 Received 4 December 2018; Received in revised form 19 January 2019; Accepted 21 January 2019 Available online 23 January 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
required to reduce the concentration of total P < 0.5 mg/L in the effluent, and this process is called chemically enhanced primary sedimentation (CEPS) (Li et al., 2018a). CEPS is deemed to be a cost-effective method for wastewater treatment, especially in large cities (e.g., Shanghai and Hong Kong), which could meet the stringent P regulations of wastewater effluent. For example, Wang et al. (2009) reported P and chemical oxygen demand (COD) removal efficiencies were enhanced using high performance AlCl3 in the Bailonggang Wastewater Treatment Plant in Shanghai. As reported by Lin et al. (2017a), FeCl3 (10–12 mg-Fe/L) was utilized at the Stonecutters Island Sewage Treatment Works in Hong Kong, the biggest CEPS treatment plant of the world. Nevertheless, large amount of CEPS sludge as the by-product has been generated. Currently, CEPS sludge is disposed by landfilling, which is expensive and raises environmental risks. Anaerobic digestion is recommended as an effective method to treat CEPS sludge, as it contains large amounts of valuable resources including organic matters and P. The organics can be used to produce valuable products, such as methane, hydrogen, and volatile fatty acids (VFAs) (Chen et al., 2018; Lin et al., 2018). Several studies have described that CEPS sludge can be treated to produce VFAs via acidic anaerobic fermentation. For example, Lin et al. (2017b) reported recovery of VFAs from Al-enhanced primary sedimentation sewage sludge at pH 5.7–5.8. Lin and Li (2018a) investigated the influence of acidic pH on fermentation of FeCl3-based CEPS process (Fesludge). Meanwhile, P was also released to fermented liquid due to the acidic condition, limiting its direct utilization as additional carbon source for biological nutrient removal (BNR) (Li et al., 2018b). Thus, P should be recovered firstly from fermented liquid to avoid the increasing P loading in the influent. P recovery as magnesium ammonium phosphate (MAP) has been typically applied in full-scale plants, as it can simultaneously recover P and N from fermented liquid (Bi et al., 2014; Huang et al., 2015; Mulder et al., 2018; Ye et al., 2017) and the MAP can potentially be used as a fertilizer (Tansel et al., 2018; Xu et al., 2018b). However, extra alkaline addition is necessary to format the MAP from acidic fermented liquid as low pH is unfavorable to produce MAP. In addition, other metals dissolving in the acidic fermentation liquid limits the application in practice (Fang et al., 2018). As described by several published studies (Chen et al., 2017a; Chen et al., 2018; Wang et al., 2017), alkaline fermentation was recommended to treat waste activated sludge (WAS) as it could enhance VFAs production and improve the sludge destruction. In comparison with WAS, CEPS sludge contains not only organics, but also large chemical coagulants. Until now, little is known about the performance of VFAs production via alkaline fermentation of CEPS sludge. However, several studies have indicated P can be also released under the alkaline condition. For example, Ye et al. (2017) reported that alkali leaching caused the separation of P and heavy metals in sewage sludge on account of the insolubilization of most heavy metals under alkaline condition. Fang et al. (2018) indicated that P was released, as OH− replaced PO43− in Fe-P under an alkaline condition. Xu et al. (2018a) reported that alkali cannot be used to extract heavy metals and Ca-P but can be used to dissolve Al-P and Fe-P. Thus, the performance of P release from CEPS sludge during alkaline fermentation is necessary to be investigated. More importantly, if the proper alkaline pH works, P can be in-situ recovered as MAP from fermented liquid. To date, little study was investigated P release and recovery from CEPS sludge during alkaline fermentation. Thus, the aims of this study were to (i) evaluate P and COD removal from wastewater under different FeCl3 dosing concentrations; (ii) investigate P release and VFAs production via CEPS sludge fermentation under different alkaline pHs; (iii) analyze P recovery as MAP from the alkaline fermented supernatant.
2. Methods and materials 2.1. Feedstock preparation and coagulation experiment Raw wastewater was obtained weekly from the Xianlin Sewage Treatment Works in Nanjing Normal University and stored at 4 °C until further use. The total chemical oxygen demand (TCOD), total phosphorus (TP), PO43−-P, total nitrogen (TN) and NH4+-N of the raw wastewater were 303.7 ± 2.25, 2.81 ± 0.06, 1.42 ± 0.01, 23.72 ± 1.12 and 17.57 ± 0.77 mg/L, respectively. FeCl3 was used as the coagulant to concentrate P and organics into the CEPS sludge in this study, as iron salts are becoming the mainstream chemical additive to remove P in wastewater treatment plants (Hu et al., 2018). A concentrated FeCl3 reagent of 1000 mg/L was prepared and used as a coagulant to remove pollutants from the wastewater during batch operations. A laboratory jar-test was conducted to determine the optimum coagulant dose. Every 500 mL raw wastewater were filled into 9 beakers without adjusting the pH. The FeCl3 concentration ranged from 0 mg/L sewage to 40 mg/L sewage with 5 mg/L increments in the beakers. Stirring velocity was 350 rpm for 1 min then changed to 150 rpm for 15 min, after that 30 min of sedimentation was carried out. The supernatant was used for the further pollutant removal analysis. 2.2. Batch alkaline fermentation of Fe-sludge 25 mg/L of FeCl3 was added into sewage for the CEPS treatment. The dose was determined according to the batch test results shown in Fig. 1. A predetermined dose of coagulant was added to 20 L of raw wastewater, and the same coagulation process was performed as described above. And then 1.6 L of sediment was obtained as the Fe-sludge and 18.4 L supernatant was taken out, which reduced the volume by 12.5 times. Subsequently, 0.2 L of inoculum collected from a laboratory anaerobic digester was added and mixed well. Glass bottles (550 mL) were used for the batch fermentation tests with a working volume of 450 mL. These fermenters were placed at room temperature with stirring velocity of 500 rpm for mixing. The fermenters were purged with nitrogen gas (99.99%) for 20 min to ensure anaerobic conditions. The pH of the four fermenters were initially controlled at 8.0, 9.0, 10.0, and 11.0 and the pH was adjusted twice every day throughout the test. The fermenters were operated for 10 days and the concentrations of the metabolites in the liquid solution were collected and monitored daily.
Fig. 1. Removal efficiencies of turbidity, chemical oxygen demand (COD), NH4+-N, and PO43−-P from raw wastewater using the chemically enhanced primary sedimentation (CEPS) process under different FeCl3 dosing concentrations. 267
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
All tests were carried out in duplicate and the results were statistically significant with the probability (P value) less than 0.05. 2.3. Phosphorus recovery from the fermented supernatant The tests of P recovery as MAP were conducted in 100 mL beakers with the fermented supernatant. Magnesium ions are generally used as precipitators to react with P and N to form MAP, in which the chemical precipitation reaction occurred under alkaline conditions with a molar ratio of Mg:P:N = 1:1:1, as shown in Eq. (1):
Mg2 + + HPO24 + NH+4 + 6H2 O = MgNH 4 PO4 6H2 O
+H+
(1)
MgCl2 was added to the fermented supernatants at pH 10 and 11, and the molar ratios of Mg:P were 1:1, 2:1, and 3:1 at room temperature, respectively. The stirring velocity was initially set at 200 rpm for 1 h and followed by sedimentation for 1 h. Finally, the precipitates were obtained by centrifugation and then washed three times with deionized water. Finally, the precipitates was dried at 40 °C for 2 days (Zhou et al., 2015).
Fig. 2. Variations in soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) under different alkaline conditions during Fe-sludge fermentation (25 mg/L FeCl3).
slightly. The result was similar to previous studies (Lin et al., 2017b; Wang et al., 2009) in which chemical coagulation did not improve N removal from wastewater. The positively charged Fe3+ and Fe (III) hydrolysis species were not neutralized or removed by the positively charged NH4+-N based on the CEPS mechanism. Hence, CEPS efficiently removed negatively charged organic matter and phosphorus, while the removal efficiency of positively charged ammonia nitrogen remained low.
2.4. Analytical methods The pH of fermenter was measured online using pH meter (Starter 3100, Ohaus Corp., Parsippany, NJ, USA). The fermented mixtures were centrifuged for 30 min at 5000 rpm then the supernatant after centrifugation was filtered for further analysis using a 0.45 μm membrane filter. Soluble chemical oxygen demand (SCOD), PO43−-P, TP, NH4+-N, and TN were measured in accordance with other researches (APHA, 2005). The VFAs concentrations were measured using a gas chromatograph (Agilent 7890A; Agilent Technologies, Palo Alto, CA, USA) with a flame ionization detector which was instrumented in Chen et al. (2016) study. The recovered P-containing product was collected and used for scanning electron microscopy (SEM; ZEISS SUPRA® 55, Zeiss, Zena, Germany) and energy-dispersive X-ray analyses (EDX; Oxford Isis, UK analysis; SIRION200, FEI, Hillsboro, OR, USA). The precipitates were also analyzed by X-ray diffraction (XRD, D/max, Japan).
3.2. Production of SCOD and VFAs during alkaline fermentation The effect of alkaline pH on SCOD release during Fe-sludge fermentation is shown in Fig. 2. The concentration of SCOD released from Fe-sludge increased with the pH increasing from 8 to 11. The maximum concentration of SCOD released from the sludge was 1488 mg/L at pH 11, whereas the SCOD concentrations were only 360, 435, and 710 mg/ L at pH of 8, 9, and 10, respectively. The alkaline pH significantly improved the hydrolysis of Fe-sludge, and sludge hydrolysis can be expressed by the changes of SCOD concentration (Lin & Li, 2018b). In this study, COD was concentrated to the Fe-sludge as the HFO absorption. When the pH increased, the electrostatic repulsion between negatively charged SCOD and HFO was increased, resulting in the SCOD release. Further, higher pH could improve organic matters hydrolysis as chemical reactions (e.g., solvation and saponification) (Zhen et al., 2017) and biological reactions (Chen et al., 2017a). Thus, these factors contributed to the SCOD release at higher pH. As shown in Fig. 2, VFAs concentration increased with the increase of pH, meanwhile acetate, and propionate comprised the main VFAs. Furthermore, the maximum VFAs concentration was 417.82 mg COD/L at pH 11, whereas VFAs concentrations at pH 8 and 9 were always < 1 mmol/L. This result was consistent with previous studies demonstrating that VFAs accumulation from WAS fermentation can be significantly improved in an high alkaline pH (Chen et al., 2017a). This observation was mainly attributed to more soluble organic matter released from sludge and the methanogens activity could be inhibited under a higher pH condition. The optimal pH for VFAs accumulation in this study (pH = 11) was inconsistent with other alkaline fermentation studies. Chen et al. (2017a) reported a maximum VFAs yield of 423.22 ± 25.49 mg COD/g VSS at pH 8.9 from WAS in a thermophilic fermenter. Yuan et al. (2015) found that the optimum pH for VFAs accumulation was 10 under mesophilic fermentation from WAS. The differences may be attributed to different substrate types, inocula and operational temperatures.
3. Results and discussion 3.1. Effect of different FeCl3 dosage on pollutants removal The performance of CEPS was affected by the FeCl3 dosage during the coagulation experiment (Fig. 1). The jar-test results showed that only 47.8% of the turbidity, 29.8% of the COD, and 2.08% of the PO43−-P were removed from the raw wastewater with 213.2 mg COD/L and 1.39 mg P/L remaining in the effluent when no coagulant (0 mg/L) was added for sedimentation (Fig. 1). The pollutants removal efficiencies clearly increased with the increase of FeCl3 dosage from 10 to 25 mg/L sewage. The maximum pollutants removal efficiency was achieved at 25 mg/L FeCl3 with 66.4 mg/L of COD and 0.06 mg/L of PO43−-P remained in the effluent. Thus, up to 78% of COD and 95% of P were concentrated from sewage into the Fe-sludge based on the results of jar-test (Fig. 1). However, adding more FeCl3 did not improve pollutant removal efficiency. 25 mg FeCl3/L sewage was determined to be the optimum dosage considering the cost and the removal efficiency of COD and PO43−-P. The high removal efficiencies of phosphorus, i.e., 95% in this study, were attributed to two reasons. Firstly, Fe3+ in water is mainly hydrolyzed to form hydrous iron oxide (HFO) precipitates. These hydrolyzed products show strong adsorption performance due to its large specific surface area, leading to mostly PO43− adsorption onto HFO. Secondly, unhydrolyzed Fe(III) can react with phosphate to form the precipitation (FePO4) (Li et al., 2018b). The removal efficiencies of NH4+-N were only about 9.1% with different FeCl3 dosage and changed 268
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
Fig. 3. Variations in total nitrogen (TN) (a), NH4+-N (b), total phosphorous (TP) (c), and PO43−-P (d) under different alkaline conditions during Fe-sludge fermentation (25 mg/L FeCl3).
leading to PO43− release. In addition, the metabolites produced by bacterial (Eq. (2)) as well as the cell walls (Eq. (3)) could play the role of organic-ligand. Both of the two could react with metal ions to form chelate which resulted in further phosphate release from the precipitates (Luo et al., 2018). Thus, PO43−-P release can be enhanced by chemical reactions, biological hydrolysis, and acidification at pH 11. Nevertheless, further study is needed to differentiate the true contribution from biological and chemical hydrolysis at high pH to promote P release.
3.3. Phosphorus and nitrogen released during alkaline fermentation The effect of different alkaline pHs on nitrogen release in Fe-sludge fermentation is shown in Fig. 3a and b. The elevated pH resulted in more TN released during anaerobic fermentation, which was attributed to hydrolysis of the nitrogenous organic matter, such as proteins and DNA in the wastewater. The concentration of ammonium, which is the key intermediate released from the breakdown of protein or other nitrogenous organic compounds by anaerobic bacteria (Chen et al., 2017b), increased with the increase of pH, and the maximum ammonium concentration was 155 mg/L. This result indicates the microbial population at pH 11 possessed the relatively higher hydrolysis and acidification activities in our study. Fig. 3c and d shows the profile of P released during Fe-sludge fermentation at different pHs. The PO43−-P concentration increased with increasing fermentation time from days 0 to 6 under all the pH conditions, and subsequently PO43−-P concentration was relatively stable. Furthermore, the PO43−-P concentration in the fermented liquid at pH 11 was much higher than the other pH conditions and its concentration reached 20.57 mg/L at the end of fermentation, which was three times of the initial value (6.98 mg/L). The trends of TP in all samples were almost similar to PO43−-P, as PO43−-P contributed the most to TP. In this study, the PO43−-P concentration of raw wastewater (1.42 ± 0.01 mg/L) was concentrated 12.5 times by FeCl3 dosing, as ferric phosphate precipitates and its concentration was 16.98 mg/L in the Fe-sludge for alkaline fermentation. More PO43−-P was released during fermentation at pH 11 (i.e., 20.57 mg/L) than other pH conditions. Ferric hydroxide would generated under alkaline conditions due to the lower solubility than ferric phosphate (Zou et al., 2017). Thus, P was released as OH− replaced PO43− at an alkaline condition. Furthermore, the Fe-sludge underwent hydrolysis and acidogenesis during alkaline anaerobic fermentation (pH 11), resulting in more PO43−-P release into the supernatant from organic P. And the electrostatic repulsion between PO43− and HFO increased with the pH increased,
Orangic(bacterial metabolites) + Fe(III) + Orangic(cell walls) + Fe(III)
P
Fe(III)
Orangic (2)
PO34 P
Fe(III)
Orangic + PO34
(3)
In this study, Fe-sludge via alkaline fermentation with different pHs were investigated. The efficiency of VFAs recovery from Fe-sludge under an alkaline condition in this study (i.e., 17%) was lower than published studies under acidic pH conditions, which may be attributed to the different wastewater types and solid CEPS concentrations. However, more PO43−-P was released (> 69.35%) during alkaline fermentation compared to acidic fermentation (Lin et al., 2017b; Lin & Li, 2018a), as alkaline condition promoted PO43−-P release from ferric phosphate precipitates and HFO and then enhanced TP hydrolysis to form PO43−-P. This process also simultaneously reduced the content of heavy metals and other toxic substances in the supernatant (Ye et al., 2017). Thus, using alkaline fermentation is a potential way to release PO43−-P from Fe-sludge and improve the purity of MAP. 3.4. Phosphorus recovery from the fermented supernatant Phosphate plays an irreplaceable role for MAP production while magnesium ions are generally used as precipitators to react with P and N to form MAP from the liquid phase and pH > 9 are favorable for MAP formation if Mg2+ ions are supplied (Zeng et al., 2018). Thus, the 269
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
Fig. 4. Recovery efficiencies of PO43−-P and NH4+-N from the fermented supernatant under different Mg/P molar ratios.
performance of MAP formation with different molar ratios of Mg:P from fermented liquid at pH 10 and 11 was investigated in Fig. 4. The results show that up to 90% of PO43−-P was recovered from the fermented supernatant at pH 11 when the molar ratios of Mg:P were 2:1 and 3:1. Considering the cost of magnesium dosing and the optimal removal efficiency of PO43−-P, the optimal molar ratio of Mg:P was determined to be 2:1 at pH 11. The struvite crystals were primarily rhombus-shaped with sizes of 0.1 mm, which was similar to a previous study (Wang et al., 2018). The result of XRD patterns of the harvested precipitates at pH 11 clearly showed a series of strong diffraction peaks of 2-theta at 15.809°, 20.850°, and 21.451° could be indexed to the pure phase of MAP (PDF-15-0762#), which the Miller indices were (0 2 0), (1 1 1) and (0 2 1), respectively. Hence, the XRD and SEM/EDS results showed that MAP was the major component of the precipitated solids, which could be added to soils as a fertilizer. The dose of magnesium in this study was clearly much larger than the theoretical value, as humic substances (Zhou et al., 2015) and other dissolved organic matter (Lei et al., 2018) from the Fe-sludge combined with magnesium ions resulted in the reduction of valid magnesium ions for the formation of MAP. However, the recovery efficiency of PO43−-P was only about 33% with all the three ratios at pH 10. This may has been due to the high alkalinity (mainly as HCO3− and CO32−) in the fermentation supernatant, which significantly inhibits the formation of MAP, limiting phosphorus recovery (Huang et al., 2015). The fractions of different carbonate species depend on the solution pH value. The bicarbonate (HCO3−) turns to the carbonate (CO32−), leading to the highest carbonate concentration at pH 11. These produced carbonates may form precipitates with other metal ions (Ca2+) (Sheng et al., 2019). Thus, the pH of the liquid should be maintained at 11–12 to reduce the negative effect of high alkalinity on MAP formation from the fermentation liquid. While, more byproducts (amorphous Mg3(PO4)2 and Mg(OH)2) were produced at pH 11–12 (Huang et al., 2015). Hence, further efforts are needed to improve MAP purity from fermented liquid. As mentioned above, > 69.35% of PO43−-P was effectively released from the Fe-sludge via alkaline fermentation. Thus, it is valuable for recovering PO43−-P from the supernatant. In this study, 90% of the P released in the fermented sludge supernatant was precipitated as MAP with a molar ratio of Mg:P = 2:1 at pH 11. No additional alkali was required to the supernatant under alkaline fermentation compared with acidification fermentation, which should reduce the cost. However, this method requires a high resource supplement to maintain the alkaline condition during the release stage and magnesium source at the recovery stage, which limits its industrial applications. Several studies have demonstrated that CO2 stripping to increase pH instead of adding NaOH solution. Meanwhile, Mg(OH)2 or MgO as a magnesium source are economical ways compared with adding MgCl2 (Luo et al., 2018; Ye et al., 2017). Furthermore, Mostly iron metal is concentrated in the
fermented sludge, which can be recycled for coagulation (Yang et al., 2018). Therefore, further research is needed to investigate the ability of these approaches to reduce costs. 4. Conclusions The present study demonstrated phosphorus release and recovery from Fe-based CEPS sludge via alkaline fermentation. The results showed that 78% of organic matter and 95% of P were concentrated from the sewage into the sludge at 25 mg FeCl3/L. 69.35% of the phosphorus in the Fe-sludge was released and the maximum phosphorus concentration (i.e., 20.57 mg/L) was produced during alkaline fermentation at pH 11. Finally, about 90% of the released P in the alkaline fermentation supernatant could be precipitated as MAP during the recovery stage. Acknowledgements This study was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (No. 2017ZX07203-003), the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (No. 18KJB610013), and the Natural Science Found of Jiangsu Province (No. BK20160475). References APHA, 2005. Standard methods for the examination of water and wastewater. American Public Health Association (APHA), Washington, DC, USA. Bi, W., Li, Y., Hu, Y., 2014. Recovery of phosphorus and nitrogen from alkaline hydrolysis supernatant of excess sludge by magnesium ammonium phosphate. Bioresour. Technol. 166, 1–8. Chen, Y., Jiang, X., Xiao, K., Shen, N., Zeng, R.J., Zhou, Y., 2017a. Enhanced volatile fatty acids (VFAs) production in a thermophilic fermenter with stepwise pH increase – Investigation on dissolved organic matter transformation and microbial community shift. Water Res. 112, 261–268. Chen, Y., Wang, T., Shen, N., Zhang, F., Zeng, R.J., 2016. High-purity propionate production from glycerol in mixed culture fermentation. Bioresour. Technol. 219, 659–667. Chen, Y., Xiao, K., Jiang, X., Shen, N., Zeng, R.J., Zhou, Y., 2017b. In-situ sludge pretreatment in a single-stage anaerobic digester. Bioresour. Technol. 238, 102–108. Chen, Y., Xiao, K., Shen, N., Zeng, R.J., Zhou, Y., 2018. Hydrogen production from a thermophilic alkaline waste activated sludge fermenter: effects of solid retention time (SRT). Chemosphere 206, 101–106. Fang, L., Li, J.-S., Donatello, S., Cheeseman, C.R., Wang, Q., Poon, C.S., Tsang, D.C.W., 2018. Recovery of phosphorus from incinerated sewage sludge ash by combined twostep extraction and selective precipitation. Chem. Eng. J. 348, 74–83. Hu, P., Liu, J., Wu, L., Zou, L., Li, Y.Y., Xu, Z.P., 2018. Simultaneous release of polyphosphate and iron-phosphate from waste activated sludge by anaerobic fermentation combined with sulfate reduction. Bioresour. Technol. 271, 182–189. Huang, H., Liu, J., Ding, L., 2015. Recovery of phosphate and ammonia nitrogen from the anaerobic digestion supernatant of activated sludge by chemical precipitation. J. Clean. Prod. 102, 437–446. Lei, Y., Song, B., Saakes, M., van der Weijden, R.D., Buisman, C.J.N., 2018. Interaction of calcium, phosphorus and natural organic matter in electrochemical recovery of phosphate. Water Res. 142, 10–17.
270
Bioresource Technology 278 (2019) 266–271
Y. Chen et al. Li, R.-H., Wang, X.-M., Li, X.-Y., 2018a. A membrane bioreactor with iron dosing and acidogenic co-fermentation for enhanced phosphorus removal and recovery in wastewater treatment. Water Res. 129, 402–412. Li, R.H., Cui, J.L., Li, X.D., Li, X.Y., 2018b. Phosphorus removal and recovery from wastewater using Fe-dosing bioreactor and cofermentation: investigation by X-ray absorption near-edge structure spectroscopy. Environ. Sci. Technol. 52 (24), 14119–14128. Lin, L., Li, R.-H., Li, X.-Y., 2018. Recovery of organic resources from sewage sludge of Alenhanced primary sedimentation by alkali pretreatment and acidogenic fermentation. J. Clean. Prod. 172, 3334–3341. Lin, L., Li, R.-H., Li, Y., Xu, J., Li, X.-Y., 2017a. Recovery of organic carbon and phosphorus from wastewater by Fe-enhanced primary sedimentation and sludge fermentation. Proc. Biochem. 54 (Supplement C), 135–139. Lin, L., Li, R.-H., Yang, Z.-Y., Li, X.-Y., 2017b. Effect of coagulant on acidogenic fermentation of sludge from enhanced primary sedimentation for resource recovery: comparison between FeCl3 and PACl. Chem. Eng. J. 325 (Supplement C), 681–689. Lin, L., Li, X.-Y., 2018a. Acidogenic fermentation of iron-enhanced primary sedimentation sludge under different pH conditions for production of volatile fatty acids. Chemosphere 194, 692–700. Lin, L., Li, X.-Y., 2018b. Effects of pH adjustment on the hydrolysis of Al-enhanced primary sedimentation sludge for volatile fatty acid production. Chem. Eng. J. 346, 50–56. Luo, Y., Li, H., Huang, Y.-R., Zhao, T.-L., Yao, Q.-Z., Fu, S.-Q., Zhou, G.-T., 2018. Bacterial mineralization of struvite using MgO as magnesium source and its potential for nutrient recovery. Chem. Eng. J. 351, 195–202. Mulder, M., Appeldoorn, K., Weij, P., van Kempen, R., 2018. Full scale optimisation of sludge dewatering and phosphate removal at Harnaschpolder wwtp (The Hague, NL). Water Pract. Technol. 13 (1), 21–29. Sheng, L., Liu, J., Zhang, C., Zou, L., Li, Y.-Y., Xu, Z.P., 2019. Pretreating anaerobic fermentation liquid with calcium addition to improve short chain fatty acids extraction via in situ synthesis of layered double hydroxides. Bioresour. Technol. 271, 190–195. Tansel, B., Lunn, G., Monje, O., 2018. Struvite formation and decomposition characteristics for ammonia and phosphorus recovery: a review of magnesium-ammoniaphosphate interactions. Chemosphere 194, 504–514. Wang, D., Liu, Y., Ngo, H.H., Zhang, C., Yang, Q., Peng, L., He, D., Zeng, G., Li, X., Ni, B.J., 2017. Approach of describing dynamic production of volatile fatty acids from sludge
alkaline fermentation. Bioresour. Technol. 238, 343–351. Wang, H., Li, F., Keller, A.A., Xu, R., 2009. Chemically enhanced primary treatment (CEPT) for removal of carbon and nutrients from municipal wastewater treatment plants: a case study of Shanghai. Water Sci. Technol. 60 (7), 1803–1809. Wang, Q., Zhang, W., Yang, Z., Xu, Q., Yang, P., Wang, D., 2018. Enhancement of anaerobic digestion sludge dewatering performance using in-situ crystallization in combination with cationic organic polymers flocculation. Water Res. 146, 19–29. Xu, D.-C., Zhong, C.-Q., Yin, K.-H., Peng, S.-H., Zhu, T.-T., Cheng, G., 2018a. Alkaline solubilization of excess mixed sludge and the recovery of released phosphorus as magnesium ammonium phosphate. Bioresour. Technol. 249, 783–790. Xu, S., Feng, S., Sun, H., Wu, S., Zhuang, G., Deng, Y., Bai, Z., Jing, C., Zhuang, X., 2018b. Linking N2O emissions from biofertilizer-amended soil of tea plantations to the abundance and structure of N2O-reducing microbial communities. Environ. Sci. Technol. 52 (19), 11338–11345. Yang, L., Wang, L., Zhang, H., Li, C., Zhang, X., Hu, Q., 2018. A novel low cost microalgal harvesting technique with coagulant recovery and recycling. Bioresour. Technol. 266, 343–348. Ye, Y., Ngo, H.H., Guo, W., Liu, Y., Li, J., Liu, Y., Zhang, X., Jia, H., 2017. Insight into chemical phosphate recovery from municipal wastewater. Sci. Total Environ. 576, 159–171. Yuan, Y., Wang, S., Liu, Y., Li, B., Wang, B., Peng, Y., 2015. Long-term effect of pH on short-chain fatty acids accumulation and microbial community in sludge fermentation systems. Bioresour. Technol. 197, 56–63. Zeng, F., Zhao, Q., Jin, W., Liu, Y., Wang, K., Lee, D.-J., 2018. Struvite precipitation from anaerobic sludge supernatant and mixed fresh/stale human urine. Chem. Eng. J. 344, 254–261. Zhen, G., Lu, X., Kato, H., Zhao, Y., Li, Y.-Y., 2017. Overview of pretreatment strategies for enhancing sewage sludge disintegration and subsequent anaerobic digestion: current advances, full-scale application and future perspectives. Renew. Sust. Energy Rev. 69, 559–577. Zhou, Z., Hu, D., Ren, W., Zhao, Y., Jiang, L.-M., Wang, L., 2015. Effect of humic substances on phosphorus removal by struvite precipitation. Chemosphere 141, 94–99. Zou, J., Zhang, L., Wang, L., Li, Y., 2017. Enhancing phosphorus release from waste activated sludge containing ferric or aluminum phosphates by EDTA addition during anaerobic fermentation process. Chemosphere 171, 601–608.
271
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Phosphorus release and recovery from Fe-enhanced primary sedimentation sludge via alkaline fermentation Yun Chena,1, Hui Lina,b,1, Nan Shena, Wang Yana,b, Jieai Wanga, Guoxiang Wanga,b, a b
T
⁎
School of Environment, Nanjing Normal University, Nanjing, Jiangsu 210023, People’s Republic of China Jiangsu Engineering Lab of Water and Soil Eco-remediation, Nanjing Normal University, Nanjing, Jiangsu 210023, People’s Republic of China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Alkaline fermentation Chemically enhanced primary sedimentation (CEPS) Magnesium ammonium phosphate (MAP) Phosphorus release and recovery
Phosphorus release and recovery from Fe-based chemically enhanced primary sedimentation (CEPS) sludge via alkaline fermentation was investigated. The coagulation results showed that 78% of organic matter and 95% of phosphorus were concentrated from sewage into sludge with the optimum dosages of 25 mg/L FeCl3. The batch fermentation results revealed that 69.35% of the phosphorus in the Fe-sludge can be released and the maximum phosphorus concentration was 20.57 mg/L at pH 11. In the recovery stage, 90% of the P released in the fermented sludge supernatant was precipitated at a 2:1 ratio of magnesium to phosphorus and pH 11. The result of X-ray diffraction indicated that magnesium ammonium phosphate (MAP) was the major component of the precipitated solids. Thus, the present study provides an alternative option for phosphorus release and recovery as MAP from CEPS sludge via alkaline fermentation.
1. Introduction Excessive phosphorus (P) discharge could induce eutrophication in natural waters. Thus, removing P from wastewater to avoid P flowing into nature waters is an urgent task (Xu et al., 2018a). Until now, dosing with coagulant chemicals or the biological P removal process are
known as the most common methods for P removal. Compared to the biological P removal process, which removes P by accumulating excess phosphate in polyphosphate-accumulating organisms (PAOs), the chemical precipitation process is more popular due to the easier operation, lower cost, and higher P removal efficiency. Furthermore, in wastewater, limited chemical dosing of metal salts (e.g., FeCl3 and AlCl3) is
Corresponding author at: School of Environment, Nanjing Normal University, Nanjing, Jiangsu 210023, People’s Republic of China. E-mail address: [email protected] (G. Wang). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.biortech.2019.01.094 Received 4 December 2018; Received in revised form 19 January 2019; Accepted 21 January 2019 Available online 23 January 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
required to reduce the concentration of total P < 0.5 mg/L in the effluent, and this process is called chemically enhanced primary sedimentation (CEPS) (Li et al., 2018a). CEPS is deemed to be a cost-effective method for wastewater treatment, especially in large cities (e.g., Shanghai and Hong Kong), which could meet the stringent P regulations of wastewater effluent. For example, Wang et al. (2009) reported P and chemical oxygen demand (COD) removal efficiencies were enhanced using high performance AlCl3 in the Bailonggang Wastewater Treatment Plant in Shanghai. As reported by Lin et al. (2017a), FeCl3 (10–12 mg-Fe/L) was utilized at the Stonecutters Island Sewage Treatment Works in Hong Kong, the biggest CEPS treatment plant of the world. Nevertheless, large amount of CEPS sludge as the by-product has been generated. Currently, CEPS sludge is disposed by landfilling, which is expensive and raises environmental risks. Anaerobic digestion is recommended as an effective method to treat CEPS sludge, as it contains large amounts of valuable resources including organic matters and P. The organics can be used to produce valuable products, such as methane, hydrogen, and volatile fatty acids (VFAs) (Chen et al., 2018; Lin et al., 2018). Several studies have described that CEPS sludge can be treated to produce VFAs via acidic anaerobic fermentation. For example, Lin et al. (2017b) reported recovery of VFAs from Al-enhanced primary sedimentation sewage sludge at pH 5.7–5.8. Lin and Li (2018a) investigated the influence of acidic pH on fermentation of FeCl3-based CEPS process (Fesludge). Meanwhile, P was also released to fermented liquid due to the acidic condition, limiting its direct utilization as additional carbon source for biological nutrient removal (BNR) (Li et al., 2018b). Thus, P should be recovered firstly from fermented liquid to avoid the increasing P loading in the influent. P recovery as magnesium ammonium phosphate (MAP) has been typically applied in full-scale plants, as it can simultaneously recover P and N from fermented liquid (Bi et al., 2014; Huang et al., 2015; Mulder et al., 2018; Ye et al., 2017) and the MAP can potentially be used as a fertilizer (Tansel et al., 2018; Xu et al., 2018b). However, extra alkaline addition is necessary to format the MAP from acidic fermented liquid as low pH is unfavorable to produce MAP. In addition, other metals dissolving in the acidic fermentation liquid limits the application in practice (Fang et al., 2018). As described by several published studies (Chen et al., 2017a; Chen et al., 2018; Wang et al., 2017), alkaline fermentation was recommended to treat waste activated sludge (WAS) as it could enhance VFAs production and improve the sludge destruction. In comparison with WAS, CEPS sludge contains not only organics, but also large chemical coagulants. Until now, little is known about the performance of VFAs production via alkaline fermentation of CEPS sludge. However, several studies have indicated P can be also released under the alkaline condition. For example, Ye et al. (2017) reported that alkali leaching caused the separation of P and heavy metals in sewage sludge on account of the insolubilization of most heavy metals under alkaline condition. Fang et al. (2018) indicated that P was released, as OH− replaced PO43− in Fe-P under an alkaline condition. Xu et al. (2018a) reported that alkali cannot be used to extract heavy metals and Ca-P but can be used to dissolve Al-P and Fe-P. Thus, the performance of P release from CEPS sludge during alkaline fermentation is necessary to be investigated. More importantly, if the proper alkaline pH works, P can be in-situ recovered as MAP from fermented liquid. To date, little study was investigated P release and recovery from CEPS sludge during alkaline fermentation. Thus, the aims of this study were to (i) evaluate P and COD removal from wastewater under different FeCl3 dosing concentrations; (ii) investigate P release and VFAs production via CEPS sludge fermentation under different alkaline pHs; (iii) analyze P recovery as MAP from the alkaline fermented supernatant.
2. Methods and materials 2.1. Feedstock preparation and coagulation experiment Raw wastewater was obtained weekly from the Xianlin Sewage Treatment Works in Nanjing Normal University and stored at 4 °C until further use. The total chemical oxygen demand (TCOD), total phosphorus (TP), PO43−-P, total nitrogen (TN) and NH4+-N of the raw wastewater were 303.7 ± 2.25, 2.81 ± 0.06, 1.42 ± 0.01, 23.72 ± 1.12 and 17.57 ± 0.77 mg/L, respectively. FeCl3 was used as the coagulant to concentrate P and organics into the CEPS sludge in this study, as iron salts are becoming the mainstream chemical additive to remove P in wastewater treatment plants (Hu et al., 2018). A concentrated FeCl3 reagent of 1000 mg/L was prepared and used as a coagulant to remove pollutants from the wastewater during batch operations. A laboratory jar-test was conducted to determine the optimum coagulant dose. Every 500 mL raw wastewater were filled into 9 beakers without adjusting the pH. The FeCl3 concentration ranged from 0 mg/L sewage to 40 mg/L sewage with 5 mg/L increments in the beakers. Stirring velocity was 350 rpm for 1 min then changed to 150 rpm for 15 min, after that 30 min of sedimentation was carried out. The supernatant was used for the further pollutant removal analysis. 2.2. Batch alkaline fermentation of Fe-sludge 25 mg/L of FeCl3 was added into sewage for the CEPS treatment. The dose was determined according to the batch test results shown in Fig. 1. A predetermined dose of coagulant was added to 20 L of raw wastewater, and the same coagulation process was performed as described above. And then 1.6 L of sediment was obtained as the Fe-sludge and 18.4 L supernatant was taken out, which reduced the volume by 12.5 times. Subsequently, 0.2 L of inoculum collected from a laboratory anaerobic digester was added and mixed well. Glass bottles (550 mL) were used for the batch fermentation tests with a working volume of 450 mL. These fermenters were placed at room temperature with stirring velocity of 500 rpm for mixing. The fermenters were purged with nitrogen gas (99.99%) for 20 min to ensure anaerobic conditions. The pH of the four fermenters were initially controlled at 8.0, 9.0, 10.0, and 11.0 and the pH was adjusted twice every day throughout the test. The fermenters were operated for 10 days and the concentrations of the metabolites in the liquid solution were collected and monitored daily.
Fig. 1. Removal efficiencies of turbidity, chemical oxygen demand (COD), NH4+-N, and PO43−-P from raw wastewater using the chemically enhanced primary sedimentation (CEPS) process under different FeCl3 dosing concentrations. 267
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
All tests were carried out in duplicate and the results were statistically significant with the probability (P value) less than 0.05. 2.3. Phosphorus recovery from the fermented supernatant The tests of P recovery as MAP were conducted in 100 mL beakers with the fermented supernatant. Magnesium ions are generally used as precipitators to react with P and N to form MAP, in which the chemical precipitation reaction occurred under alkaline conditions with a molar ratio of Mg:P:N = 1:1:1, as shown in Eq. (1):
Mg2 + + HPO24 + NH+4 + 6H2 O = MgNH 4 PO4 6H2 O
+H+
(1)
MgCl2 was added to the fermented supernatants at pH 10 and 11, and the molar ratios of Mg:P were 1:1, 2:1, and 3:1 at room temperature, respectively. The stirring velocity was initially set at 200 rpm for 1 h and followed by sedimentation for 1 h. Finally, the precipitates were obtained by centrifugation and then washed three times with deionized water. Finally, the precipitates was dried at 40 °C for 2 days (Zhou et al., 2015).
Fig. 2. Variations in soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) under different alkaline conditions during Fe-sludge fermentation (25 mg/L FeCl3).
slightly. The result was similar to previous studies (Lin et al., 2017b; Wang et al., 2009) in which chemical coagulation did not improve N removal from wastewater. The positively charged Fe3+ and Fe (III) hydrolysis species were not neutralized or removed by the positively charged NH4+-N based on the CEPS mechanism. Hence, CEPS efficiently removed negatively charged organic matter and phosphorus, while the removal efficiency of positively charged ammonia nitrogen remained low.
2.4. Analytical methods The pH of fermenter was measured online using pH meter (Starter 3100, Ohaus Corp., Parsippany, NJ, USA). The fermented mixtures were centrifuged for 30 min at 5000 rpm then the supernatant after centrifugation was filtered for further analysis using a 0.45 μm membrane filter. Soluble chemical oxygen demand (SCOD), PO43−-P, TP, NH4+-N, and TN were measured in accordance with other researches (APHA, 2005). The VFAs concentrations were measured using a gas chromatograph (Agilent 7890A; Agilent Technologies, Palo Alto, CA, USA) with a flame ionization detector which was instrumented in Chen et al. (2016) study. The recovered P-containing product was collected and used for scanning electron microscopy (SEM; ZEISS SUPRA® 55, Zeiss, Zena, Germany) and energy-dispersive X-ray analyses (EDX; Oxford Isis, UK analysis; SIRION200, FEI, Hillsboro, OR, USA). The precipitates were also analyzed by X-ray diffraction (XRD, D/max, Japan).
3.2. Production of SCOD and VFAs during alkaline fermentation The effect of alkaline pH on SCOD release during Fe-sludge fermentation is shown in Fig. 2. The concentration of SCOD released from Fe-sludge increased with the pH increasing from 8 to 11. The maximum concentration of SCOD released from the sludge was 1488 mg/L at pH 11, whereas the SCOD concentrations were only 360, 435, and 710 mg/ L at pH of 8, 9, and 10, respectively. The alkaline pH significantly improved the hydrolysis of Fe-sludge, and sludge hydrolysis can be expressed by the changes of SCOD concentration (Lin & Li, 2018b). In this study, COD was concentrated to the Fe-sludge as the HFO absorption. When the pH increased, the electrostatic repulsion between negatively charged SCOD and HFO was increased, resulting in the SCOD release. Further, higher pH could improve organic matters hydrolysis as chemical reactions (e.g., solvation and saponification) (Zhen et al., 2017) and biological reactions (Chen et al., 2017a). Thus, these factors contributed to the SCOD release at higher pH. As shown in Fig. 2, VFAs concentration increased with the increase of pH, meanwhile acetate, and propionate comprised the main VFAs. Furthermore, the maximum VFAs concentration was 417.82 mg COD/L at pH 11, whereas VFAs concentrations at pH 8 and 9 were always < 1 mmol/L. This result was consistent with previous studies demonstrating that VFAs accumulation from WAS fermentation can be significantly improved in an high alkaline pH (Chen et al., 2017a). This observation was mainly attributed to more soluble organic matter released from sludge and the methanogens activity could be inhibited under a higher pH condition. The optimal pH for VFAs accumulation in this study (pH = 11) was inconsistent with other alkaline fermentation studies. Chen et al. (2017a) reported a maximum VFAs yield of 423.22 ± 25.49 mg COD/g VSS at pH 8.9 from WAS in a thermophilic fermenter. Yuan et al. (2015) found that the optimum pH for VFAs accumulation was 10 under mesophilic fermentation from WAS. The differences may be attributed to different substrate types, inocula and operational temperatures.
3. Results and discussion 3.1. Effect of different FeCl3 dosage on pollutants removal The performance of CEPS was affected by the FeCl3 dosage during the coagulation experiment (Fig. 1). The jar-test results showed that only 47.8% of the turbidity, 29.8% of the COD, and 2.08% of the PO43−-P were removed from the raw wastewater with 213.2 mg COD/L and 1.39 mg P/L remaining in the effluent when no coagulant (0 mg/L) was added for sedimentation (Fig. 1). The pollutants removal efficiencies clearly increased with the increase of FeCl3 dosage from 10 to 25 mg/L sewage. The maximum pollutants removal efficiency was achieved at 25 mg/L FeCl3 with 66.4 mg/L of COD and 0.06 mg/L of PO43−-P remained in the effluent. Thus, up to 78% of COD and 95% of P were concentrated from sewage into the Fe-sludge based on the results of jar-test (Fig. 1). However, adding more FeCl3 did not improve pollutant removal efficiency. 25 mg FeCl3/L sewage was determined to be the optimum dosage considering the cost and the removal efficiency of COD and PO43−-P. The high removal efficiencies of phosphorus, i.e., 95% in this study, were attributed to two reasons. Firstly, Fe3+ in water is mainly hydrolyzed to form hydrous iron oxide (HFO) precipitates. These hydrolyzed products show strong adsorption performance due to its large specific surface area, leading to mostly PO43− adsorption onto HFO. Secondly, unhydrolyzed Fe(III) can react with phosphate to form the precipitation (FePO4) (Li et al., 2018b). The removal efficiencies of NH4+-N were only about 9.1% with different FeCl3 dosage and changed 268
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
Fig. 3. Variations in total nitrogen (TN) (a), NH4+-N (b), total phosphorous (TP) (c), and PO43−-P (d) under different alkaline conditions during Fe-sludge fermentation (25 mg/L FeCl3).
leading to PO43− release. In addition, the metabolites produced by bacterial (Eq. (2)) as well as the cell walls (Eq. (3)) could play the role of organic-ligand. Both of the two could react with metal ions to form chelate which resulted in further phosphate release from the precipitates (Luo et al., 2018). Thus, PO43−-P release can be enhanced by chemical reactions, biological hydrolysis, and acidification at pH 11. Nevertheless, further study is needed to differentiate the true contribution from biological and chemical hydrolysis at high pH to promote P release.
3.3. Phosphorus and nitrogen released during alkaline fermentation The effect of different alkaline pHs on nitrogen release in Fe-sludge fermentation is shown in Fig. 3a and b. The elevated pH resulted in more TN released during anaerobic fermentation, which was attributed to hydrolysis of the nitrogenous organic matter, such as proteins and DNA in the wastewater. The concentration of ammonium, which is the key intermediate released from the breakdown of protein or other nitrogenous organic compounds by anaerobic bacteria (Chen et al., 2017b), increased with the increase of pH, and the maximum ammonium concentration was 155 mg/L. This result indicates the microbial population at pH 11 possessed the relatively higher hydrolysis and acidification activities in our study. Fig. 3c and d shows the profile of P released during Fe-sludge fermentation at different pHs. The PO43−-P concentration increased with increasing fermentation time from days 0 to 6 under all the pH conditions, and subsequently PO43−-P concentration was relatively stable. Furthermore, the PO43−-P concentration in the fermented liquid at pH 11 was much higher than the other pH conditions and its concentration reached 20.57 mg/L at the end of fermentation, which was three times of the initial value (6.98 mg/L). The trends of TP in all samples were almost similar to PO43−-P, as PO43−-P contributed the most to TP. In this study, the PO43−-P concentration of raw wastewater (1.42 ± 0.01 mg/L) was concentrated 12.5 times by FeCl3 dosing, as ferric phosphate precipitates and its concentration was 16.98 mg/L in the Fe-sludge for alkaline fermentation. More PO43−-P was released during fermentation at pH 11 (i.e., 20.57 mg/L) than other pH conditions. Ferric hydroxide would generated under alkaline conditions due to the lower solubility than ferric phosphate (Zou et al., 2017). Thus, P was released as OH− replaced PO43− at an alkaline condition. Furthermore, the Fe-sludge underwent hydrolysis and acidogenesis during alkaline anaerobic fermentation (pH 11), resulting in more PO43−-P release into the supernatant from organic P. And the electrostatic repulsion between PO43− and HFO increased with the pH increased,
Orangic(bacterial metabolites) + Fe(III) + Orangic(cell walls) + Fe(III)
P
Fe(III)
Orangic (2)
PO34 P
Fe(III)
Orangic + PO34
(3)
In this study, Fe-sludge via alkaline fermentation with different pHs were investigated. The efficiency of VFAs recovery from Fe-sludge under an alkaline condition in this study (i.e., 17%) was lower than published studies under acidic pH conditions, which may be attributed to the different wastewater types and solid CEPS concentrations. However, more PO43−-P was released (> 69.35%) during alkaline fermentation compared to acidic fermentation (Lin et al., 2017b; Lin & Li, 2018a), as alkaline condition promoted PO43−-P release from ferric phosphate precipitates and HFO and then enhanced TP hydrolysis to form PO43−-P. This process also simultaneously reduced the content of heavy metals and other toxic substances in the supernatant (Ye et al., 2017). Thus, using alkaline fermentation is a potential way to release PO43−-P from Fe-sludge and improve the purity of MAP. 3.4. Phosphorus recovery from the fermented supernatant Phosphate plays an irreplaceable role for MAP production while magnesium ions are generally used as precipitators to react with P and N to form MAP from the liquid phase and pH > 9 are favorable for MAP formation if Mg2+ ions are supplied (Zeng et al., 2018). Thus, the 269
Bioresource Technology 278 (2019) 266–271
Y. Chen et al.
Fig. 4. Recovery efficiencies of PO43−-P and NH4+-N from the fermented supernatant under different Mg/P molar ratios.
performance of MAP formation with different molar ratios of Mg:P from fermented liquid at pH 10 and 11 was investigated in Fig. 4. The results show that up to 90% of PO43−-P was recovered from the fermented supernatant at pH 11 when the molar ratios of Mg:P were 2:1 and 3:1. Considering the cost of magnesium dosing and the optimal removal efficiency of PO43−-P, the optimal molar ratio of Mg:P was determined to be 2:1 at pH 11. The struvite crystals were primarily rhombus-shaped with sizes of 0.1 mm, which was similar to a previous study (Wang et al., 2018). The result of XRD patterns of the harvested precipitates at pH 11 clearly showed a series of strong diffraction peaks of 2-theta at 15.809°, 20.850°, and 21.451° could be indexed to the pure phase of MAP (PDF-15-0762#), which the Miller indices were (0 2 0), (1 1 1) and (0 2 1), respectively. Hence, the XRD and SEM/EDS results showed that MAP was the major component of the precipitated solids, which could be added to soils as a fertilizer. The dose of magnesium in this study was clearly much larger than the theoretical value, as humic substances (Zhou et al., 2015) and other dissolved organic matter (Lei et al., 2018) from the Fe-sludge combined with magnesium ions resulted in the reduction of valid magnesium ions for the formation of MAP. However, the recovery efficiency of PO43−-P was only about 33% with all the three ratios at pH 10. This may has been due to the high alkalinity (mainly as HCO3− and CO32−) in the fermentation supernatant, which significantly inhibits the formation of MAP, limiting phosphorus recovery (Huang et al., 2015). The fractions of different carbonate species depend on the solution pH value. The bicarbonate (HCO3−) turns to the carbonate (CO32−), leading to the highest carbonate concentration at pH 11. These produced carbonates may form precipitates with other metal ions (Ca2+) (Sheng et al., 2019). Thus, the pH of the liquid should be maintained at 11–12 to reduce the negative effect of high alkalinity on MAP formation from the fermentation liquid. While, more byproducts (amorphous Mg3(PO4)2 and Mg(OH)2) were produced at pH 11–12 (Huang et al., 2015). Hence, further efforts are needed to improve MAP purity from fermented liquid. As mentioned above, > 69.35% of PO43−-P was effectively released from the Fe-sludge via alkaline fermentation. Thus, it is valuable for recovering PO43−-P from the supernatant. In this study, 90% of the P released in the fermented sludge supernatant was precipitated as MAP with a molar ratio of Mg:P = 2:1 at pH 11. No additional alkali was required to the supernatant under alkaline fermentation compared with acidification fermentation, which should reduce the cost. However, this method requires a high resource supplement to maintain the alkaline condition during the release stage and magnesium source at the recovery stage, which limits its industrial applications. Several studies have demonstrated that CO2 stripping to increase pH instead of adding NaOH solution. Meanwhile, Mg(OH)2 or MgO as a magnesium source are economical ways compared with adding MgCl2 (Luo et al., 2018; Ye et al., 2017). Furthermore, Mostly iron metal is concentrated in the
fermented sludge, which can be recycled for coagulation (Yang et al., 2018). Therefore, further research is needed to investigate the ability of these approaches to reduce costs. 4. Conclusions The present study demonstrated phosphorus release and recovery from Fe-based CEPS sludge via alkaline fermentation. The results showed that 78% of organic matter and 95% of P were concentrated from the sewage into the sludge at 25 mg FeCl3/L. 69.35% of the phosphorus in the Fe-sludge was released and the maximum phosphorus concentration (i.e., 20.57 mg/L) was produced during alkaline fermentation at pH 11. Finally, about 90% of the released P in the alkaline fermentation supernatant could be precipitated as MAP during the recovery stage. Acknowledgements This study was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (No. 2017ZX07203-003), the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (No. 18KJB610013), and the Natural Science Found of Jiangsu Province (No. BK20160475). References APHA, 2005. Standard methods for the examination of water and wastewater. American Public Health Association (APHA), Washington, DC, USA. Bi, W., Li, Y., Hu, Y., 2014. Recovery of phosphorus and nitrogen from alkaline hydrolysis supernatant of excess sludge by magnesium ammonium phosphate. Bioresour. Technol. 166, 1–8. Chen, Y., Jiang, X., Xiao, K., Shen, N., Zeng, R.J., Zhou, Y., 2017a. Enhanced volatile fatty acids (VFAs) production in a thermophilic fermenter with stepwise pH increase – Investigation on dissolved organic matter transformation and microbial community shift. Water Res. 112, 261–268. Chen, Y., Wang, T., Shen, N., Zhang, F., Zeng, R.J., 2016. High-purity propionate production from glycerol in mixed culture fermentation. Bioresour. Technol. 219, 659–667. Chen, Y., Xiao, K., Jiang, X., Shen, N., Zeng, R.J., Zhou, Y., 2017b. In-situ sludge pretreatment in a single-stage anaerobic digester. Bioresour. Technol. 238, 102–108. Chen, Y., Xiao, K., Shen, N., Zeng, R.J., Zhou, Y., 2018. Hydrogen production from a thermophilic alkaline waste activated sludge fermenter: effects of solid retention time (SRT). Chemosphere 206, 101–106. Fang, L., Li, J.-S., Donatello, S., Cheeseman, C.R., Wang, Q., Poon, C.S., Tsang, D.C.W., 2018. Recovery of phosphorus from incinerated sewage sludge ash by combined twostep extraction and selective precipitation. Chem. Eng. J. 348, 74–83. Hu, P., Liu, J., Wu, L., Zou, L., Li, Y.Y., Xu, Z.P., 2018. Simultaneous release of polyphosphate and iron-phosphate from waste activated sludge by anaerobic fermentation combined with sulfate reduction. Bioresour. Technol. 271, 182–189. Huang, H., Liu, J., Ding, L., 2015. Recovery of phosphate and ammonia nitrogen from the anaerobic digestion supernatant of activated sludge by chemical precipitation. J. Clean. Prod. 102, 437–446. Lei, Y., Song, B., Saakes, M., van der Weijden, R.D., Buisman, C.J.N., 2018. Interaction of calcium, phosphorus and natural organic matter in electrochemical recovery of phosphate. Water Res. 142, 10–17.
270
Bioresource Technology 278 (2019) 266–271
Y. Chen et al. Li, R.-H., Wang, X.-M., Li, X.-Y., 2018a. A membrane bioreactor with iron dosing and acidogenic co-fermentation for enhanced phosphorus removal and recovery in wastewater treatment. Water Res. 129, 402–412. Li, R.H., Cui, J.L., Li, X.D., Li, X.Y., 2018b. Phosphorus removal and recovery from wastewater using Fe-dosing bioreactor and cofermentation: investigation by X-ray absorption near-edge structure spectroscopy. Environ. Sci. Technol. 52 (24), 14119–14128. Lin, L., Li, R.-H., Li, X.-Y., 2018. Recovery of organic resources from sewage sludge of Alenhanced primary sedimentation by alkali pretreatment and acidogenic fermentation. J. Clean. Prod. 172, 3334–3341. Lin, L., Li, R.-H., Li, Y., Xu, J., Li, X.-Y., 2017a. Recovery of organic carbon and phosphorus from wastewater by Fe-enhanced primary sedimentation and sludge fermentation. Proc. Biochem. 54 (Supplement C), 135–139. Lin, L., Li, R.-H., Yang, Z.-Y., Li, X.-Y., 2017b. Effect of coagulant on acidogenic fermentation of sludge from enhanced primary sedimentation for resource recovery: comparison between FeCl3 and PACl. Chem. Eng. J. 325 (Supplement C), 681–689. Lin, L., Li, X.-Y., 2018a. Acidogenic fermentation of iron-enhanced primary sedimentation sludge under different pH conditions for production of volatile fatty acids. Chemosphere 194, 692–700. Lin, L., Li, X.-Y., 2018b. Effects of pH adjustment on the hydrolysis of Al-enhanced primary sedimentation sludge for volatile fatty acid production. Chem. Eng. J. 346, 50–56. Luo, Y., Li, H., Huang, Y.-R., Zhao, T.-L., Yao, Q.-Z., Fu, S.-Q., Zhou, G.-T., 2018. Bacterial mineralization of struvite using MgO as magnesium source and its potential for nutrient recovery. Chem. Eng. J. 351, 195–202. Mulder, M., Appeldoorn, K., Weij, P., van Kempen, R., 2018. Full scale optimisation of sludge dewatering and phosphate removal at Harnaschpolder wwtp (The Hague, NL). Water Pract. Technol. 13 (1), 21–29. Sheng, L., Liu, J., Zhang, C., Zou, L., Li, Y.-Y., Xu, Z.P., 2019. Pretreating anaerobic fermentation liquid with calcium addition to improve short chain fatty acids extraction via in situ synthesis of layered double hydroxides. Bioresour. Technol. 271, 190–195. Tansel, B., Lunn, G., Monje, O., 2018. Struvite formation and decomposition characteristics for ammonia and phosphorus recovery: a review of magnesium-ammoniaphosphate interactions. Chemosphere 194, 504–514. Wang, D., Liu, Y., Ngo, H.H., Zhang, C., Yang, Q., Peng, L., He, D., Zeng, G., Li, X., Ni, B.J., 2017. Approach of describing dynamic production of volatile fatty acids from sludge
alkaline fermentation. Bioresour. Technol. 238, 343–351. Wang, H., Li, F., Keller, A.A., Xu, R., 2009. Chemically enhanced primary treatment (CEPT) for removal of carbon and nutrients from municipal wastewater treatment plants: a case study of Shanghai. Water Sci. Technol. 60 (7), 1803–1809. Wang, Q., Zhang, W., Yang, Z., Xu, Q., Yang, P., Wang, D., 2018. Enhancement of anaerobic digestion sludge dewatering performance using in-situ crystallization in combination with cationic organic polymers flocculation. Water Res. 146, 19–29. Xu, D.-C., Zhong, C.-Q., Yin, K.-H., Peng, S.-H., Zhu, T.-T., Cheng, G., 2018a. Alkaline solubilization of excess mixed sludge and the recovery of released phosphorus as magnesium ammonium phosphate. Bioresour. Technol. 249, 783–790. Xu, S., Feng, S., Sun, H., Wu, S., Zhuang, G., Deng, Y., Bai, Z., Jing, C., Zhuang, X., 2018b. Linking N2O emissions from biofertilizer-amended soil of tea plantations to the abundance and structure of N2O-reducing microbial communities. Environ. Sci. Technol. 52 (19), 11338–11345. Yang, L., Wang, L., Zhang, H., Li, C., Zhang, X., Hu, Q., 2018. A novel low cost microalgal harvesting technique with coagulant recovery and recycling. Bioresour. Technol. 266, 343–348. Ye, Y., Ngo, H.H., Guo, W., Liu, Y., Li, J., Liu, Y., Zhang, X., Jia, H., 2017. Insight into chemical phosphate recovery from municipal wastewater. Sci. Total Environ. 576, 159–171. Yuan, Y., Wang, S., Liu, Y., Li, B., Wang, B., Peng, Y., 2015. Long-term effect of pH on short-chain fatty acids accumulation and microbial community in sludge fermentation systems. Bioresour. Technol. 197, 56–63. Zeng, F., Zhao, Q., Jin, W., Liu, Y., Wang, K., Lee, D.-J., 2018. Struvite precipitation from anaerobic sludge supernatant and mixed fresh/stale human urine. Chem. Eng. J. 344, 254–261. Zhen, G., Lu, X., Kato, H., Zhao, Y., Li, Y.-Y., 2017. Overview of pretreatment strategies for enhancing sewage sludge disintegration and subsequent anaerobic digestion: current advances, full-scale application and future perspectives. Renew. Sust. Energy Rev. 69, 559–577. Zhou, Z., Hu, D., Ren, W., Zhao, Y., Jiang, L.-M., Wang, L., 2015. Effect of humic substances on phosphorus removal by struvite precipitation. Chemosphere 141, 94–99. Zou, J., Zhang, L., Wang, L., Li, Y., 2017. Enhancing phosphorus release from waste activated sludge containing ferric or aluminum phosphates by EDTA addition during anaerobic fermentation process. Chemosphere 171, 601–608.
271