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Phosphorus recovery as vivianite from waste activated sludge via optimizing iron source and pH value during anaerobic fermentation
Bioresource Technology 293 (2019) 122088
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Phosphorus recovery as vivianite from waste activated sludge via optimizing iron source and pH value during anaerobic fermentation
T
Jiashun Cao, Yang Wu, Jianan Zhao, Shuo Jin, Muhammad Aleem, Qin Zhang, Fang Fang, ⁎ Zhaoxia Xue, Jingyang Luo Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, College of Environment, Hohai University, Nanjing 210098, China College of Environment, Hohai University, Nanjing 210098, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphorus (P) recovery Vivianite Waste activated sludge (WAS) Anaerobic fermentation (AF) Clostridiaceae
This study presented an innovative method for phosphorus (P) recovery as vivianite from waste activated sludge (WAS) via optimizing iron dosing and pH value during anaerobic fermentation (AF). The optimal conditions for vivianite formation were in the pH range of 6.0–9.0 with initial PO43− > 5 mg/L and Fe/P molar ratio of 1.5. Notably, FeCl3 showed advantages over ZVI for the simultaneous release of Fe2+ and PO43− during WAS fermentation, especially in acidic conditions. The FeCl3 dosing at pH 3.0 could contribute to 78.81% Fe2+ release and 85.69% of total PO43− release from WAS. They were ultimately recovered in the form of high-purity vivianite (93.67%). Clostridiaceae (40.25%) was the predominant bacteria in FeCl3-pH3 reactors, which played key roles in inducing dissimilatory iron reduction for Fe2+ formation. Therefore, P recovery as vivianite from WAS fermentation might be a promising and highly valuable approach to relieve the P crisis.
1. Introduction Phosphorus (P) is one of the vital elements for living organisms in aspect of biological growth as well as energy supply and transfer, especially for the formation of DNA and RNA (Jalali and Jalali, 2016; Selbig, 2016). However, as a limited and non-renewable resource, P is ⁎
predicted to be exhausted in the next century (Cordell et al., 2009). The efficient recovery of P is thereby quite urgent and significant. Wastewater treatment plants (WWTPs) is an important source for P recovery as approximate 1.3 million tons of P are annually entered via the wastewater discharge (Li and Li, 2017), which are able to meet 15–20% of the global demand for P (Carpenter, 2008; Yuan et al.,
Corresponding author. E-mail address: [email protected] (J. Luo).
https://doi.org/10.1016/j.biortech.2019.122088 Received 31 July 2019; Received in revised form 27 August 2019; Accepted 28 August 2019 Available online 30 August 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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oxygen demand was 19923 mg/L. Also, the concentration of total Fe (TFe) and total phosphorus (TP) were 184.09 mg/L and 318.64 mg/L separately. Besides, the initial pH of WAS was 7.4 ± 0.1. Iron(II) chloride tetrahydrate (FeCl2·4H2O, XiLong Scientific, China, AR grade) and potassium dihydrogen phosphate (KH2PO4, XiLong Scientific, China, AR grade) were taken as iron and P source for jar tests respectively, while Iron(III) chloride hexahydrate (FeCI3·6H2O, Kermel, China, AR grade) and zero valent iron (ZVI, Kermel, China, AR grade) were selected as external Fe source for anaerobic fermentation experiments.
2012). Notably, the removed P in WWTPs is basically transferred and concentrated in waste activated sludge (WAS) through the biological and chemical processes (Zhang et al., 2019). Anaerobic fermentation (AF) is currently considered as one of the major methods for WAS disposal, which could achieve the resource recovery (i.e. volatile fatty acids as carbon source) and reduce environmental risks simultaneously (Luo et al., 2018). However, a large amount of PO43− from both enhanced biological phosphorus removal (EBPR) and Fe-enhanced P removal sludge would be released into the supernatant during AF. The high concentration of PO43− would reduce the utilization value of VFA as carbon source due to the extra P loading for nutrient removal. Given the phenomenon, PO43− should be also removed from the fermented supernatant to avoid its negative influence on subsequent application. Compared with the traditional struvite precipitation from WAS, the P recovery as vivianite (Fe3(PO4)2·8H2O) has currently attracted substantial attention due to its natural ubiquity, foreseeable economic value and extensive utilization (Wu et al., 2019). For example, vivianite is a fundamental source for lithium iron phosphate (LiFePO4) manufacturing, which is increasingly exploited as a precursor when fabricating Li-ion secondary batteries (Priambodo et al., 2017). Moreover, the market price of vivianite (approximate 10,000 €/ton) is far more expensive than the struvite (approximate 500 €/ton) (Wu et al., 2019). Generally, the efficient formation of vivianite in WAS requires adequate PO43− and Fe2+ concentration. In view of obtaining P, several studies have indicated that P could be released into the supernatant during the fermentation process with proper pH control (Latif et al., 2017; Latif et al., 2015; Zou et al., 2018). For example, Latif et al observed that the acidic pH (< 5.7) could increase 3.6 times of P release compared with that in neutral pH during AF (Latif et al., 2015). As to Fe2+ acquisition, iron salts are ubiquitous in WWTPs, as they are generally fed into wastewater as flocculants in large amounts to remove P (Fan et al., 2018). Under the anaerobic environment, Fe3+ could be reduced as Fe2+ by dissimilatory metal-reducing bacteria (DMRB) (O'Loughlin et al., 2013; Rothe et al., 2016), which could provide the necessary raw materials for vivianite formation. Although several studies have found that the vivianite was the major Fe-P fraction in the anaerobic sludge, it is difficult to separate and purify from the sludge directly due to its effective combination with organic matters and other metals (Wilfert et al., 2018; Wilfert et al., 2016). Consequently, the strategy of releasing P and Fe from sludge phase into liquid phase and then re-precipitating after separation seemed to be more feasible and effective to obtain high-purity vivianite. Nevertheless, the researches on the synchronously efficient PO43− and Fe2+ release during WAS fermentation process have been seldom reported and required additional exploitation. Besides, the main influencing factors on efficient vivianite formation have not been systematically investigated, which will limit its application in practice. Given the above facts, the main aims of this study are to (1) explore the optimal conditions of vivianite formation; (2) evaluate Fe2+ and PO43− release during AF under different iron source and pH conditions, and disclose the underlying mechanisms (3) investigate the efficiency of P recovery as vivianite from fermented supernatant. This study would give a new insight of promoting P recovery as vivianite from WAS.
2.2. Jar tests In order to explore the potential influencing factors on efficient vivianite formation (e.g. pH, the concentration of phosphate, Fe/P ratio, temperature and etc.), jar tests were conducted in constantly stirring conical flask reactor which was firstly filled with 400 mL deionized water, and then fed with certain amount of FeCl2 and KH2PO4 solution by peristaltic pumps. In addition, the reactor was purged with high purity N2 during the whole experiments to maintain the oxygen-free environments and stirred at 250 revolutions per minutes (rpm) to mix the solution homogenously. The pH value was controlled manually by the coordination of pH meter, the HCl/NaOH flask and the peristaltic pump. The temperature was adjusted automatically by the sensor and heating rod. The main parameters for the jar tests were set as below: pH (3.0–12.0), P concentration (1, 5, 20, 100, 500 mg/L), Fe/P molar ratio (1.0, 1.5, 2.0), and temperatures (15, 25, 35, 45, 55 °C). After 200 min agitation, the concentration of aqueous Fe2+ and PO43− were detected immediately to prevent Fe2+ oxidation. 2.3. WAS fermentation for the release and recovery of Fe and P Based on the results of jar tests, external iron was required for efficient vivianite formation considering the high concentration of P but relatively low level of Fe salts. As FeCl3 was the one of the main flocculants used in WWTPs for P removal, and ZVI exhibited positive effects on WAS fermentation (Wei et al., 2018), they were chosen as the external iron source for the experiments. The WAS fermentation for the release and recovery of Fe and P under different iron source and pH conditions were occurred in 500 mL glass bottles. The FeCl3 and ZVI were added into 300 mL WAS to ensure that Fe/P (TP in the sludge) molar ratio is 1.5, which was theoretical value for vivianite formation. The mixtures under five pH levels (3.0, 5.0, 10.0, 12.0, not control (NC)) were examined for the AF experiment under different iron source. The reactors were purged with pure N2 gas and sealed with butyl rubber to keep anaerobic environment. Not dosing (ND) samples were set as control groups without external iron addition. Then they were operated at 35 °C with at an agitation rate of 180 rpm. During the whole process, multiple indicators in liquid phase were tested, including levels of pH, TFe, Fe2+, TP and PO43−. After the WAS fermentation, the fermented supernatant was extracted from the fermenter. After adjusting the pH of supernatant to 7.0, the precipitates were firstly collected and then freeze-drying for 24 h to obtain the recovered precipitates.
2. Materials and methods 2.4. Analytic methods 2.1. Waste activated sludge and chemicals 2.4.1. Chemical analysis The pH was determined by a pH meter (PHB-4, INESA, China). The sludge mixtures were sampled from the reactor and centrifuged at 10000 rpm for 5 min to separate the solids and supernatant. The centrifuged supernatant was filtered by 0.45 μm acetate fiber membrane for analysis. The concentration of TSS, VSS, TFe, Fe2+, TP and PO43− were determined according to the standards methods (APHA, 2005). For aqueous Fe2+ analysis, the centrifuged supernatant was immediately acidified by the 1 mol/L HCl and determined within 1 h to
Waste activated sludge (WAS) used in this study was obtained from the secondary sedimentation tank of a municipal WWTP (Nanjing, China). Anaerobic-anoxic-aerobic (A2/O) process is conducted as the major biological treatment technology, and PAFC is used as flocculant for enhancing phosphorus removal. The withdrawn WAS was firstly thickened by gravity at 4 °C for 24 h and then filtered by a stainless steel mesh (2.0 mm) for later use. The MLSS and MLVSS of the thickened WAS were 18.16 g/L and 8.17 g/L respectively while the total chemical 2
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However, the increase of PO43− concentration could visibly widen the effective pH region. For instance, the effective regions were 6.0–10.0 and 4.0–11.0 when the PO43− concentration was controlled at 5 mg/L and 500 mg/L, respectively, indicating that a higher initial PO43− concentration could gather a stronger ionic strength to compete with OH− ions and meet the solubility product constant (Ksp) of vivianite formation (März et al., 2018). Therefore, a high concentration of PO43− is preferable, for the purposes of reducing and breaking narrow pH limitation. It should be noticed that the Fe/P molar ratio also exhibited evident influence on Fe2+/PO43− combination. At the low Fe/P molar ratio (1.0), only about 66.7% of PO43− was effectively recovered in all tests. However, nearly all the PO43− (> 99%) could be recovered when the Fe/P molar ratio increased to 1.5. The PO43− recovery was also quite desirable at the Fe/P molar ratio of 2.0. But it showed insignificant difference compared with that of 1.5. The overdose of Fe is thereby unnecessary. Moreover, the excess Fe2+ tended to combine with OH− to form the insoluble Fe(OH)2 in high pH values, which would reduce the purity of recovered vivianite. Thus, in light of economic evaluation and vivianite’s purity, the optimal molar ratio of Fe/P should be controlled at 1.5. However, the effects of temperature on the vivianite formation between the effective pH regions were limited. Several studies have also reported that Ksp of vivianite formation showed insignificant distinctions at various temperatures (from 5 °C to 90 °C) (Al-Borno and Tomson, 1994; Madsen and Hansen, 2014). These results demonstrated that the temperature variation during vivianite formation could be neglected. According to the jar test, the optimal conditions for vivianite formation were at Fe/P molar ratio of 1.5 and pH range of 6.0–9.0 with an initial PO43− > 5 mg/L.
prevent the oxidation. In order to determine the variation of phosphorus fraction in sludge, the Standards, Measurements and Testing (SMT) protocol (Medeiros et al., 2005) was been adopted to identify the total phosphorus (TP), inorganic phosphorus (IP), organic phosphorus (OP), non-apatite inorganic phosphorus (NAIP) and apatite phosphorus (AP). AP includes various inorganic phosphorus bound with Ca while NAIP is combined with Fe, Mn, Al oxides and their hydroxides. 2.4.2. Characterization of the recovered precipitates The scanning electron microscopy (SEM) coupled with energy dispersive spectrometer (EDS) (JSM-5900, JEOL, Japan) was used to analyze the micromorphology and elemental composition of recovered precipitates, while X-ray diffraction (XRD) (D max/RB, RIGAKU, Japan) was employed to determine the crystallographic structure of recovered precipitates (Liu et al., 2018). 2.4.3. Microbial population analysis The Illumina MiSeq technique was applied for clear understanding of microbial community evolution in WAS fermentation system, CTAB/ SDS method was utilized to extract the total genomic DNA amplified by specific primers (Bacteria: 341F-806R and Archaea: Arch519FArch915R) to target V3-V4 region of 16S rRNA. Then DNA samples were quantified and the PCR products were checked on 2% agarose gels. The bright main strip between 400 and 450 bp were chosen to further analyze. The Illumina HiSeq 2500 platform (Illumina, USA) were applied for purification, combination and sequence of PCR products. The detailed methodology and related analysis were referred to the study (Luo et al., 2018). 2.5. Data analysis
3.2. WAS anaerobic fermentation for Fe2+ and P release under different conditions
All tests were conducted in triplicate. An analysis of variance was used to evaluate the significance of the results, and p < 0.05 was considered to be statistically significant.
Since the WAS contain large amounts of P, the efficient P recovery as vivianite from WAS is an innovative method to solve the crisis of P deficiency and bring external benefits. However, due to its difficulty of directly separating and purifying from WAS, the release and re-precipitation of P could be a more feasible way to recover P as vivianite (Wu et al., 2019). Several studies have presented that the pH adjustment during AF could efficiently release P into the supernatant (Latif et al., 2017; Latif et al., 2015; Zou et al., 2018). The pH values of 3.0, 5.0, 10.0 and 12.0 were chosen as the controlling factors within the process to avoid direct vivianite formation in WAS but effectively release the P into supernatant (Based on the results obtained from jar tests). Also, to explore the effect of different iron source on Fe2+ and P release, the ZVI and FeCl3 were selected as the external iron source with 1.5 Fe/P molar ratio.
3. Results and discussion 3.1. Jar tests of phosphate recovery as vivianite The influences of pH, Fe/P molar ratio and PO43− concentration on vivianite formation (identified in Section 3.3) were presented in Fig. 1. Generally, pH is the key factors for efficient P recovery as vivianite, and three distinct regions are formed at different pH values. Specifically, when the pH value was below 5.0, almost all the PO43− (> 96.81%) and Fe2+ (> 97.04%) were remaining in the liquid phase at the concentration of 1 mg/L PO43− with the Fe/P ratio of 1.0 (shown in Fig. 1(A)). The reason could be attributed to the high solubility of Fe2+P compound at a lower pH (Wu et al., 2019). Nevertheless, the efficient pH regions for vivianite formation were in the range of 7.0–9.0. The aqueous PO43− and Fe2+ were effectively combined and precipitated as vivianite (Eq. (1)) in all reactors, and the residual concentration of soluble PO43− and Fe2+ were at an extremely low level (< 0.1 mg/L). This conclusion is similar to Liu’s (Liu et al., 2018), reporting that the optimal pH values for vivianite crystallization were 7.0–9.0. Nevertheless, as pH continued to rise, the aqueous P was observed to be noticeably increased. The reason might be contributed to formation of the insoluble Fe(OH)2, which could compete with Fe2+ ions to hinder the formation of vivianite and release PO43− into the solution (Eq. (2)) (Priambodo et al., 2017). For example, in Fig. 1(B), when pH raised from 10.0 to 11.0, the aqueous PO43− was increased from 0.14 to 4.97 mg/l, while the aqueous Fe2+ were still lower than 0.1 mg/L.
3F e2 + + 2PO43 − + 8H2 O → F e3 (PO4 )2 ·8H2 O
(1)
F e3 (PO4 )2 ·8H2 O + 6OH− → 3F e(OH )2 + 2PO43 − + 8H2 O
(2)
3.2.1. Fe release during WAS fermentation The Fe2+ release during AF at 1.5 Fe/P molar ratio was displayed in Fig. 2. Generally, the dominant fraction of aqueous TFe in all samples was Fe2+ (> 95%) at low pH values, which was beneficial for the subsequent P recovery as vivianite. However, the concentration of Fe2+ and TFe were negligible (< 1 mg/L) under alkaline environments. This consequence is consistent with jar tests in which ferric and ferrous hydroxide would be generated under alkaline condition because of its low solubility (Zou et al., 2017). The iron elements would thereby remain in the sludge. Noticeably, the dosing of ZVI and FeCl3 in acidic environment could markedly improve the Fe2+ concentration in supernatant, especially in the FeCl3 dosing reactors. Moreover, the lower pH value contributed more to the increase of Fe2+ in WAS fermentation reactors. The highest aqueous Fe2+ contents for ZVI- and FeCl3-dosing sludge at pH 3 were respectively 552.23 and 825.57 mg/L, accounting for 52.71% and 78.81% of TFe in each sludge, both of which were 3
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Fig. 1. Correlations of pH, Fe/P and PO43− concentration with P recovery as vivianite by jar tests at temperature 25 °C.
fermentation.
greater than that of ND sludge (maximum 74.85 mg/L making up 40.27%). The increase of aqueous Fe2+ in ZVI sludge might induced by ZVI dissolution in acidic environment (Eq. (3)) (Liu and Wang, 2019). Nonetheless, ZVI dissolution rate was relatively slow in weak acidic environment, leading to the relatively low aqueous Fe2+ concentration consequently.
F e0 + 2H+ → Fe 2 + + H2
F e3 + + 3H2 O → Fe (OH )3(s) + 3H+
Fe (OH )3(s) +
H+
+
e−
(4)
→F e(OH )2(S ) + H2 O
(5) 2+
reIn order to further disclose the underlying mechanisms of Fe lease with the addition of FeCl3 in WAS fermentation reactors, the microbial population in sludge samples was analyzed by high throughput sequencing and the results are shown in Table 1. Additionally, differing from raw and ND-NC sludge, the sludge with FeCl3 addition had much lower Chao and Shannon index value, indicating the decrease of richness and diversity of microbial population (Li et al., 2018; Zhang and Zhao, 2014). The dominant microbial populations in the different sludge samples are exhibited in Fig. 3(A). Obviously, the microbial structure was decreased significantly. The predominant bacteria in FeCl3-pH3 sludge was the Clostridiaceae (40.25%), belonging to the order Clostridiales (42.33%), However, it was only 14.79% and 19.67%, in FeCl3-NC and FeCl3-pH5 sludges, respectively, and was negligible in Raw and ND-NC sludge. The Clostridiaceae mainly consists of typical iron-reducing bacteria, i.e. Clostridium beijerinckii and Alkaliphilus metalliredigenes (Weber et al., 2006). During WAS fermentation, these functional bacteria could
(3)
The increase aqueous Fe2+ in FeCl3 sludge in acidic environment was primarily contributed to the biological Fe3+ reduction. The FeCl3 in water is mainly hydrolyzed to form hydrous iron oxide (HFO) precipitates as shown in Eq. (4) (Chen et al., 2019). Under anaerobic reduction conditions, Fe3+ is reduced to Fe2+ by (DMRB) as shown in Eq. (5) (Vuillemin et al., 2013). Thus, the Fe2+ from insoluble Fe(OH)2 was tend to release into supernatant due to its high solubility in acidic conditions (Zou et al., 2017). Nevertheless, the maximum concentration of aqueous Fe2+ in FeCl3 sludge was 1.51 times higher than it in ZVI sludge. Also, it took only 6 d in FeCl3-dosing sludge to reached the maximal Fe2+ concentration while it was 12 d in ZVI sludge, which indicated that biological Fe3+ reduction might be a better way to obtain sufficient aqueous Fe2+ compared with ZVI dissolution during WAS 4
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Fig. 2. Concentration of Fe2+ and TFe in supernatant during the AF under different pH values and iron amendments (A) aqueous Fe2+ in ND sludge, (B) aqueous TFe in ND sludge, (C) aqueous Fe2+ in ZVI sludge, (D) aqueous TFe in ZVI sludge, (E) aqueous Fe2+ in FeCl3 sludge, (F) aqueous TFe in FeCl3 sludge (As the concentration of Fe2+ release for control, pH 10 and pH 12 were extremely low (< 0.1 mg/L), the symbols and lines were overlapped).
efficiently promote the reduction and release of iron in the form of Fe2+, which is favorable for the subsequent reaction of P recovery as vivianite from the fermented supernatant.
Table 1 Alpha biodiversity analysis of sludge samples with different treatment. Sample
Raw ND-NC FeCl3-NC FeCl3-pH 3 FeCl3-pH 5
α = 0.03 Sequences
OTUs
Chaoa
Coverage
Shannonb
Simpson
45647 43505 42370 49816 49,007
1089 1025 1034 376 925
1141 1082 1165 586 1117
0.998357 0.997678 0.995492 0.996728 0.995572
5.83 5.58 4.16 2.45 4.27
0.0077 0.0101 0.1131 0.1948 0.0511
3.2.2. P release during WAS fermentation The P release during WAS fermentation with different Fe source and pH values were showed in Fig. 4. The majority of aqueous TP in all samples was PO43− (> 95%), which was also beneficial for the subsequent P recovery as vivianite. Contrary to the Fe2+ release, the PO43− was found to be dramatically released in the supernatant at alkaline environments. The highest aqueous P content in ND sludge at acidic (pH = 3.0) and alkaline (pH = 12.0) environment were respectively 100.34 mg/L and 208.26 mg/L, taking up 30.00% and 65.36% of TP in each sludge sample. Several studies had proved that alkaline fermentation could energetically improve the release of P, from WAS by enhancing the hydrolysis of P-containing organics (Chen et al., 2019; Wang et al., 2017; Zaman et al., 2019). Also, this phenomenon was consistent with the jar test where OH− replaced the site of PO43− in FeP compounds at alkaline condition, and caused the PO43− releasing into the liquid phase largely. However, it was worth mentioning that the dosage of ZVI or FeCl3 had little effects on P release under alkaline conditions but it apparently facilitated the release of P under acidic conditions, especially the FeCl3. In ZVI-pH3 sludge, the P releasing efficiency could reach 48.61%,
a Chao richness estimator: the total number of OTUs estimated by infinite sampling. A higher number indicates higher richness. b Shannon diversity index: an index to characterize species diversity. A higher value represents more diversity.
utilize the organic substrates as carbon sources and electron donors for Fe3+ reduction. Therefore, the large occupation of Clostridiaceae in FeCl3-pH3 reactor had a great positive impact on the acceleration of Fe3+ reduction under acid condition, which resulted in the high level of Fe2+. To sum up, ZVI may not be the preferable iron source for P recovery as vivianite due to the relatively slow rate of iron dissolution during AF. The dosage of FeCl3 and acidic environments during AF could 5
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Fig. 3. (A) Variations of microbial community in reactors with different treatment (B) Redundancy analysis for environmental variables and Fe2+ and PO43− release during WAS fermentation (family level).
release, probably by enhancing the hydrolysis of P-containing organic matters (Lin and Li, 2018). On the contrary, the concentration of soluble PO43− (SP) increased remarkably with pH control, which might attribute to the decrease of OP, NAIP and AP. In ND-pH3 sludge, the SP increased from 0.92 mg/L in the raw sludge to 100.34 mg/L, whereas OP, NAIP and AP were all decreased and contributed 37.39, 50.69 and 11.92% to the SP rise, respectively. Due to the relatively low solubility of Fe3+-P compounds and Al-P compounds, the majority of P in fermentation reactors would remain in the solid phase. The ZVI and FeCl3 addition could further promote the increase of SP, especially the FeCl3. The concentration of SP was respectively 154.92 and 273.04 mg/L in ZVI-pH3 and FeCl3-pH3 sludge samples, which accounted for 48.61% and 85.69% of the TP, respectively. The proportions of NAIP were respectively contributed to 61.94 and 65.61%. It seemed that the drop of NAIP might be the major cause for the rising SP. In retrospect with Fe release efficiency raised above, this result verified that the addition of ZVI and FeCl3 could enhance the reduction of Fe3+ through chemical and biological methods respectively. For instance, in the FeCl3-pH 3 reactor, the
which was higher than that in ND-pH3 (30.00%). Interestingly, approximate 85.69% of TP was released into fermentation liquid within 12 d in FeCl3-pH3 fermentation reactors, which was even far greater than that in alkaline environment (pH = 12.0). Thus, it can be concluded that the dosage of FeCl3 under acidic fermentation might be more conducive to the release of P. In order to explore the specific reasons for PO43− releasing efficiency in different reactors, the SMT method was conducted to determine the specific components of phosphorus in raw and fermented sludge. As shown in Fig. 5, the TP was 318.64 mg/L in raw sludge among which the OP and IP were occupied 23.96% and 76.04%, respectively. Specifically, NAIP occupied 76.73% of the IP, which indicated that P in the sludge was mainly in the form of Fe-P and Al-P. However, the main P components were changed greatly during WAS fermentation under different conditions. The pH adjustment obviously reduced the level of OP. For example, there are respectively 80.09 and 69.97% decrease of OP in FeCl3-pH3 and FeCl3-pH12 sludge samples, whereas it was merely 22.12% drop in the reactors without pH control, revealing acidic and alkaline treatment could contribute to the OP 6
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Fig. 4. Concentration of PO43− and TP in supernatant during the AF under different pH values and iron amendments (A) ND, (B) ZVI, (C) FeCl3.
form the insoluble Ca3(PO43−)2 in sludge, generating 44.85% increase of AP. However, the Fe2+ was basically remained in sludge phase as hydroxide precipitate under alkaline environment after AF, which was disadvantageous to the later vivianite recovery from fermentation supernatants (Lin et al., 2017). Overall, the FeCl3 with lower pH was more conducive to the release of Fe2+ and PO43− during WAS fermentation. Compared with the NDNC, the FeCl3-dosing reactors showed closer correlations with the aqueous Fe2+ and PO43−, which were the raw materials for vivianite formation reactors (shown in Fig. 3(B)). The fermentation pH is an influencing factor in determining the Fe2+ and PO43− release. The RDA analysis found that the angles between the pH arrows and AP, OP, NAIP
predominant bacteria was the Clostridiaceae consisted of typical ironreducing bacteria. It had great positive impacts on the acceleration of Fe3+ reduction under acid condition. Consequently, Fe3+-P compounds in sludge were reduced to Fe2+-P compounds which have high solubility under acidic environment. The P was thereby released into the liquid phase. Besides, the SP is also evidently increased in FeCl3-pH12 sludge (60.09% of the TP) with large drop of NAIP (decreased by 89.34%). It primarily attributed to the replacement of OH− for PO43− in the Fe-P and Al-P due to the much lower solubility of hydroxide precipitate. Desorption and release of PO43− derived from Fe-P and AlP would thereby desorb and release into the aqueous solution (Kim et al., 2003). The released P then re-combined with the Ca2+ to newly
Fig. 5. Proportions of different phosphorus forms in WAS fermentation reactors under different pH and iron source amendments. 7
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arrows were very small, indicating the positive correlation of fermentation pH with AP, OP and NAIP. Thus, the low pH would result in the decrease of AP, OP and NAIP, but with the increase of SP concentration. Moreover, the abundance of Clostridiaceae, which was responsible for Fe3+ reduction, showed negative correlations with pH strongly. It was disadvantageous to the final Fe2+ formation. However, the aqueous Fe2+ and PO43− as well as Clostridiaceae were clustered firmly in the FeCl3-pH3 reactors, which is consistent with results of maximal release of Fe2+ and PO43− in fermentation supernatant.
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Improving anaerobic fermentation of waste activated sludge using iron activated persulfate treatment. Bioresour. Technol. 268, 68–76. März, C., Riedinger, N., Sena, C., Kasten, S., 2018. Phosphorus dynamics around the sulphate-methane transition in continental margin sediments: authigenic apatite and Fe(II) phosphates. Mar. Geol. 404, 84–96. Madsen, H.E.L., Hansen, H.C.B., 2014. Kinetics of crystal growth of vivianite, Fe 3 (PO 4) 2 ·8H 2 O, from solution at 25, 35 and 45°C. J. Cryst. Growth 401, 82–86. Medeiros, J.J.G., Cid, B.P., Gomez, E.F., 2005. Analytical phosphorus fractionation in sewage sludge and sediment samples. Anal. Bioanal. Chem. 381 (4), 873–878. O'Loughlin, E.J., Boyanov, M.I., Flynn, T.M., Gorski, C.A., Hofmann, S.M., Mccormick, M.L., Scherer, M.M., Kemner, K.M., 2013. Effects of bound phosphate on the bioreduction of lepidocrocite (γ-FeOOH) and maghemite (γ-Fe2O3) and formation of secondary minerals. Environ. Sci. Technol. 47 (16), 9157–9166. Priambodo, R., Shih, Y.-J., Huang, Y.-H., 2017. Phosphorus recovery as ferrous phosphate (vivianite) from wastewater produced in manufacture of thin film transistor-liquid crystal displays (TFT-LCD) by a fluidized bed crystallizer (FBC). RSC Adv. 7 (65), 40819–40828. Rothe, M., Kleeberg, A., Hupfer, M., 2016. The occurrence, identification and environmental relevance of vivianite in waterlogged soils and aquatic sediments. Earth Sci. Rev. 158, 51–64. Selbig, W.R., 2016. Evaluation of leaf removal as a means to reduce nutrient concentrations and loads in urban stormwater. Sci. Total Environ. 571, 124–133. Vuillemin, A., Ariztegui, D., Coninck, A.S.D., Lücke, A., Mayr, C., Schubert, C.J., Team, T.P.S., 2013. Origin and significance of diagenetic concretions in sediments of Laguna Potrok Aike, southern Argentina. J. Paleolimnol. 50 (3), 275–291. 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. Weber, K.A., Achenbach, L.A., Coates, J.D., 2006. Microorganisms pumping iron: anaerobic microbial iron oxidation and reduction. Nat. Rev. Microbiol. 4, 752. Wei, J., Hao, X.D., van Loosdrecht, M.C.M., Li, J., 2018. Feasibility analysis of anaerobic digestion of excess sludge enhanced by iron: a review. Renew. Sustain. Energy Rev. 89, 16–26. Wilfert, P., Dugulan, A.I., Goubitz, K., Korving, L., Witkamp, G.J., Van Loosdrecht, M.C.M., 2018. Vivianite as the main phosphate mineral in digested sewage sludge and its role for phosphate recovery. Water Res. 144, 312–321. Wilfert, P., Mandalidis, A., Dugulan, A.I., Goubitz, K., Korving, L., Temmink, H., Witkamp, G.J., Van Loosdrecht, M.C.M., 2016. Vivianite as an important iron phosphate precipitate in sewage treatment plants. Water Res. 104, 449–460. Wu, Y., Luo, J., Zhang, Q., Aleem, M., Fang, F., Xue, Z., Cao, J., 2019. Potentials and challenges of phosphorus recovery as vivianite from wastewater: a review. Chemosphere 226, 246–258. Yuan, Z.G., Pratt, S., Batstone, D.J., 2012. Phosphorus recovery from wastewater through microbial processes. Curr. Opin. Biotechnol. 23 (6), 878–883.
3.3. Phosphorus recovery as vivianite from WAS fermentation supernatant As mentioned above, around 78.81% of Fe2+ and 85.69% of PO43− was released into the supernatant from FeCl3-dosing sludge at pH 3.0 via AF. After the supernatant was extracted from the fermenter, Fe2+ and PO43− were then re-precipitated and recovered by adjusting pH to 7.0. Also the XRD analysis indicated that the recovered precipitates showed distinctively strong and sharp diffraction peaks at 11.16°, 13.16°, 18.06° and 27.80°, which was the same to the structure of vivianite (PDF-79–1928#). Besides, EDS results presented that the Fe:P:O molar ratios were respectively 1.54:1:8.13 (jar tests) and 1.58:1:8.14 (fermentation), which were consistent with the theoretical value of vivianite (1.5:1:8). This results clearly indicated that the main fraction of the recovered precipitates both in jar tests (97.83%) and WAS fermentation (93.67%) were vivianite. In summary, over 82.60% of P from the sludge could be recovered as vivianite via FeCl3-dosing and acidic fermentation, which is much higher than that of traditional P recovery as struvite (30–40%) (Egle et al., 2015). Moreover, the vivianite is far more valuable than the struvite considering its wide application and high market price (Jowett et al., 2018; Priambodo et al., 2017; Wu et al., 2019). Hence, it is a novel and promising alternative for P recovery from WAS. 4. Conclusion The efficient vivianite formation was synchronously influenced by operating pH, PO43− concentration and Fe/P molar ratio. The P in WAS could be effectively recovered as vivianite during AF with optimized Fe dosing and pH control. The FeCl3 dosing with lower pH was more conducive to the release of Fe2+ and PO43− during WAS fermentation. Also, the Clostridiaceae with the ability of dissimilatory iron reduction was highly enriched in FeCl3 dosing reactors. Approximate 82.60% of the TP in WAS was finally recovered in the form of high-purity vivianite (93.67%). Acknowledgements The work is financially supported by the “National Natural Science Foundation of China (No. 51708171 and 51878243)”, “China Postdoctoral Science Foundation (No. 2018M630508 and No. 2019T120390)”, “Key Special Program for the Pollution Control (2014ZX07305-002)” and “Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China”. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122088. References Al-Borno, A., Tomson, M.B., 1994. The temperature dependence of the solubility product constant of vivianite. Geochim. Cosmochim. Acta 58 (24), 5373–5378. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21 ed. American Public Health Association, Washington, DC. Carpenter, S.R., 2008. Phosphorus control is critical to mitigating eutrophication. PNAS 105 (32), 11039–11040.
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Treat. 52 (37–39), 6868–6877. Zou, J., Pan, J., He, H., Wu, S., Xiao, N., Ni, Y., Li, J., 2018. Nitrifying aerobic granular sludge fermentation for releases of carbon source and phosphorus: the role of fermentation pH. Bioresour. Technol. 260, 30–37. 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.
Zaman, M., Kim, M., Nakhla, G., Singh, A., Yang, F., 2019. Enhanced biological phosphorus removal using thermal alkaline hydrolyzed municipal wastewater biosolids. J. Environ. Sci. 86, 164–174. Zhang, B., Wang, L., Li, Y., 2019. Fractionation and identification of iron-phosphorus compounds in sewage sludge. Chemosphere 223, 250–256. Zhang, Y., Zhao, X., 2014. The effects of powdered activated carbon or ferric chloride on sludge characteristics and microorganisms in a membrane bioreactor. Desalin. Water
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Phosphorus recovery as vivianite from waste activated sludge via optimizing iron source and pH value during anaerobic fermentation
T
Jiashun Cao, Yang Wu, Jianan Zhao, Shuo Jin, Muhammad Aleem, Qin Zhang, Fang Fang, ⁎ Zhaoxia Xue, Jingyang Luo Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, College of Environment, Hohai University, Nanjing 210098, China College of Environment, Hohai University, Nanjing 210098, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphorus (P) recovery Vivianite Waste activated sludge (WAS) Anaerobic fermentation (AF) Clostridiaceae
This study presented an innovative method for phosphorus (P) recovery as vivianite from waste activated sludge (WAS) via optimizing iron dosing and pH value during anaerobic fermentation (AF). The optimal conditions for vivianite formation were in the pH range of 6.0–9.0 with initial PO43− > 5 mg/L and Fe/P molar ratio of 1.5. Notably, FeCl3 showed advantages over ZVI for the simultaneous release of Fe2+ and PO43− during WAS fermentation, especially in acidic conditions. The FeCl3 dosing at pH 3.0 could contribute to 78.81% Fe2+ release and 85.69% of total PO43− release from WAS. They were ultimately recovered in the form of high-purity vivianite (93.67%). Clostridiaceae (40.25%) was the predominant bacteria in FeCl3-pH3 reactors, which played key roles in inducing dissimilatory iron reduction for Fe2+ formation. Therefore, P recovery as vivianite from WAS fermentation might be a promising and highly valuable approach to relieve the P crisis.
1. Introduction Phosphorus (P) is one of the vital elements for living organisms in aspect of biological growth as well as energy supply and transfer, especially for the formation of DNA and RNA (Jalali and Jalali, 2016; Selbig, 2016). However, as a limited and non-renewable resource, P is ⁎
predicted to be exhausted in the next century (Cordell et al., 2009). The efficient recovery of P is thereby quite urgent and significant. Wastewater treatment plants (WWTPs) is an important source for P recovery as approximate 1.3 million tons of P are annually entered via the wastewater discharge (Li and Li, 2017), which are able to meet 15–20% of the global demand for P (Carpenter, 2008; Yuan et al.,
Corresponding author. E-mail address: [email protected] (J. Luo).
https://doi.org/10.1016/j.biortech.2019.122088 Received 31 July 2019; Received in revised form 27 August 2019; Accepted 28 August 2019 Available online 30 August 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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oxygen demand was 19923 mg/L. Also, the concentration of total Fe (TFe) and total phosphorus (TP) were 184.09 mg/L and 318.64 mg/L separately. Besides, the initial pH of WAS was 7.4 ± 0.1. Iron(II) chloride tetrahydrate (FeCl2·4H2O, XiLong Scientific, China, AR grade) and potassium dihydrogen phosphate (KH2PO4, XiLong Scientific, China, AR grade) were taken as iron and P source for jar tests respectively, while Iron(III) chloride hexahydrate (FeCI3·6H2O, Kermel, China, AR grade) and zero valent iron (ZVI, Kermel, China, AR grade) were selected as external Fe source for anaerobic fermentation experiments.
2012). Notably, the removed P in WWTPs is basically transferred and concentrated in waste activated sludge (WAS) through the biological and chemical processes (Zhang et al., 2019). Anaerobic fermentation (AF) is currently considered as one of the major methods for WAS disposal, which could achieve the resource recovery (i.e. volatile fatty acids as carbon source) and reduce environmental risks simultaneously (Luo et al., 2018). However, a large amount of PO43− from both enhanced biological phosphorus removal (EBPR) and Fe-enhanced P removal sludge would be released into the supernatant during AF. The high concentration of PO43− would reduce the utilization value of VFA as carbon source due to the extra P loading for nutrient removal. Given the phenomenon, PO43− should be also removed from the fermented supernatant to avoid its negative influence on subsequent application. Compared with the traditional struvite precipitation from WAS, the P recovery as vivianite (Fe3(PO4)2·8H2O) has currently attracted substantial attention due to its natural ubiquity, foreseeable economic value and extensive utilization (Wu et al., 2019). For example, vivianite is a fundamental source for lithium iron phosphate (LiFePO4) manufacturing, which is increasingly exploited as a precursor when fabricating Li-ion secondary batteries (Priambodo et al., 2017). Moreover, the market price of vivianite (approximate 10,000 €/ton) is far more expensive than the struvite (approximate 500 €/ton) (Wu et al., 2019). Generally, the efficient formation of vivianite in WAS requires adequate PO43− and Fe2+ concentration. In view of obtaining P, several studies have indicated that P could be released into the supernatant during the fermentation process with proper pH control (Latif et al., 2017; Latif et al., 2015; Zou et al., 2018). For example, Latif et al observed that the acidic pH (< 5.7) could increase 3.6 times of P release compared with that in neutral pH during AF (Latif et al., 2015). As to Fe2+ acquisition, iron salts are ubiquitous in WWTPs, as they are generally fed into wastewater as flocculants in large amounts to remove P (Fan et al., 2018). Under the anaerobic environment, Fe3+ could be reduced as Fe2+ by dissimilatory metal-reducing bacteria (DMRB) (O'Loughlin et al., 2013; Rothe et al., 2016), which could provide the necessary raw materials for vivianite formation. Although several studies have found that the vivianite was the major Fe-P fraction in the anaerobic sludge, it is difficult to separate and purify from the sludge directly due to its effective combination with organic matters and other metals (Wilfert et al., 2018; Wilfert et al., 2016). Consequently, the strategy of releasing P and Fe from sludge phase into liquid phase and then re-precipitating after separation seemed to be more feasible and effective to obtain high-purity vivianite. Nevertheless, the researches on the synchronously efficient PO43− and Fe2+ release during WAS fermentation process have been seldom reported and required additional exploitation. Besides, the main influencing factors on efficient vivianite formation have not been systematically investigated, which will limit its application in practice. Given the above facts, the main aims of this study are to (1) explore the optimal conditions of vivianite formation; (2) evaluate Fe2+ and PO43− release during AF under different iron source and pH conditions, and disclose the underlying mechanisms (3) investigate the efficiency of P recovery as vivianite from fermented supernatant. This study would give a new insight of promoting P recovery as vivianite from WAS.
2.2. Jar tests In order to explore the potential influencing factors on efficient vivianite formation (e.g. pH, the concentration of phosphate, Fe/P ratio, temperature and etc.), jar tests were conducted in constantly stirring conical flask reactor which was firstly filled with 400 mL deionized water, and then fed with certain amount of FeCl2 and KH2PO4 solution by peristaltic pumps. In addition, the reactor was purged with high purity N2 during the whole experiments to maintain the oxygen-free environments and stirred at 250 revolutions per minutes (rpm) to mix the solution homogenously. The pH value was controlled manually by the coordination of pH meter, the HCl/NaOH flask and the peristaltic pump. The temperature was adjusted automatically by the sensor and heating rod. The main parameters for the jar tests were set as below: pH (3.0–12.0), P concentration (1, 5, 20, 100, 500 mg/L), Fe/P molar ratio (1.0, 1.5, 2.0), and temperatures (15, 25, 35, 45, 55 °C). After 200 min agitation, the concentration of aqueous Fe2+ and PO43− were detected immediately to prevent Fe2+ oxidation. 2.3. WAS fermentation for the release and recovery of Fe and P Based on the results of jar tests, external iron was required for efficient vivianite formation considering the high concentration of P but relatively low level of Fe salts. As FeCl3 was the one of the main flocculants used in WWTPs for P removal, and ZVI exhibited positive effects on WAS fermentation (Wei et al., 2018), they were chosen as the external iron source for the experiments. The WAS fermentation for the release and recovery of Fe and P under different iron source and pH conditions were occurred in 500 mL glass bottles. The FeCl3 and ZVI were added into 300 mL WAS to ensure that Fe/P (TP in the sludge) molar ratio is 1.5, which was theoretical value for vivianite formation. The mixtures under five pH levels (3.0, 5.0, 10.0, 12.0, not control (NC)) were examined for the AF experiment under different iron source. The reactors were purged with pure N2 gas and sealed with butyl rubber to keep anaerobic environment. Not dosing (ND) samples were set as control groups without external iron addition. Then they were operated at 35 °C with at an agitation rate of 180 rpm. During the whole process, multiple indicators in liquid phase were tested, including levels of pH, TFe, Fe2+, TP and PO43−. After the WAS fermentation, the fermented supernatant was extracted from the fermenter. After adjusting the pH of supernatant to 7.0, the precipitates were firstly collected and then freeze-drying for 24 h to obtain the recovered precipitates.
2. Materials and methods 2.4. Analytic methods 2.1. Waste activated sludge and chemicals 2.4.1. Chemical analysis The pH was determined by a pH meter (PHB-4, INESA, China). The sludge mixtures were sampled from the reactor and centrifuged at 10000 rpm for 5 min to separate the solids and supernatant. The centrifuged supernatant was filtered by 0.45 μm acetate fiber membrane for analysis. The concentration of TSS, VSS, TFe, Fe2+, TP and PO43− were determined according to the standards methods (APHA, 2005). For aqueous Fe2+ analysis, the centrifuged supernatant was immediately acidified by the 1 mol/L HCl and determined within 1 h to
Waste activated sludge (WAS) used in this study was obtained from the secondary sedimentation tank of a municipal WWTP (Nanjing, China). Anaerobic-anoxic-aerobic (A2/O) process is conducted as the major biological treatment technology, and PAFC is used as flocculant for enhancing phosphorus removal. The withdrawn WAS was firstly thickened by gravity at 4 °C for 24 h and then filtered by a stainless steel mesh (2.0 mm) for later use. The MLSS and MLVSS of the thickened WAS were 18.16 g/L and 8.17 g/L respectively while the total chemical 2
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However, the increase of PO43− concentration could visibly widen the effective pH region. For instance, the effective regions were 6.0–10.0 and 4.0–11.0 when the PO43− concentration was controlled at 5 mg/L and 500 mg/L, respectively, indicating that a higher initial PO43− concentration could gather a stronger ionic strength to compete with OH− ions and meet the solubility product constant (Ksp) of vivianite formation (März et al., 2018). Therefore, a high concentration of PO43− is preferable, for the purposes of reducing and breaking narrow pH limitation. It should be noticed that the Fe/P molar ratio also exhibited evident influence on Fe2+/PO43− combination. At the low Fe/P molar ratio (1.0), only about 66.7% of PO43− was effectively recovered in all tests. However, nearly all the PO43− (> 99%) could be recovered when the Fe/P molar ratio increased to 1.5. The PO43− recovery was also quite desirable at the Fe/P molar ratio of 2.0. But it showed insignificant difference compared with that of 1.5. The overdose of Fe is thereby unnecessary. Moreover, the excess Fe2+ tended to combine with OH− to form the insoluble Fe(OH)2 in high pH values, which would reduce the purity of recovered vivianite. Thus, in light of economic evaluation and vivianite’s purity, the optimal molar ratio of Fe/P should be controlled at 1.5. However, the effects of temperature on the vivianite formation between the effective pH regions were limited. Several studies have also reported that Ksp of vivianite formation showed insignificant distinctions at various temperatures (from 5 °C to 90 °C) (Al-Borno and Tomson, 1994; Madsen and Hansen, 2014). These results demonstrated that the temperature variation during vivianite formation could be neglected. According to the jar test, the optimal conditions for vivianite formation were at Fe/P molar ratio of 1.5 and pH range of 6.0–9.0 with an initial PO43− > 5 mg/L.
prevent the oxidation. In order to determine the variation of phosphorus fraction in sludge, the Standards, Measurements and Testing (SMT) protocol (Medeiros et al., 2005) was been adopted to identify the total phosphorus (TP), inorganic phosphorus (IP), organic phosphorus (OP), non-apatite inorganic phosphorus (NAIP) and apatite phosphorus (AP). AP includes various inorganic phosphorus bound with Ca while NAIP is combined with Fe, Mn, Al oxides and their hydroxides. 2.4.2. Characterization of the recovered precipitates The scanning electron microscopy (SEM) coupled with energy dispersive spectrometer (EDS) (JSM-5900, JEOL, Japan) was used to analyze the micromorphology and elemental composition of recovered precipitates, while X-ray diffraction (XRD) (D max/RB, RIGAKU, Japan) was employed to determine the crystallographic structure of recovered precipitates (Liu et al., 2018). 2.4.3. Microbial population analysis The Illumina MiSeq technique was applied for clear understanding of microbial community evolution in WAS fermentation system, CTAB/ SDS method was utilized to extract the total genomic DNA amplified by specific primers (Bacteria: 341F-806R and Archaea: Arch519FArch915R) to target V3-V4 region of 16S rRNA. Then DNA samples were quantified and the PCR products were checked on 2% agarose gels. The bright main strip between 400 and 450 bp were chosen to further analyze. The Illumina HiSeq 2500 platform (Illumina, USA) were applied for purification, combination and sequence of PCR products. The detailed methodology and related analysis were referred to the study (Luo et al., 2018). 2.5. Data analysis
3.2. WAS anaerobic fermentation for Fe2+ and P release under different conditions
All tests were conducted in triplicate. An analysis of variance was used to evaluate the significance of the results, and p < 0.05 was considered to be statistically significant.
Since the WAS contain large amounts of P, the efficient P recovery as vivianite from WAS is an innovative method to solve the crisis of P deficiency and bring external benefits. However, due to its difficulty of directly separating and purifying from WAS, the release and re-precipitation of P could be a more feasible way to recover P as vivianite (Wu et al., 2019). Several studies have presented that the pH adjustment during AF could efficiently release P into the supernatant (Latif et al., 2017; Latif et al., 2015; Zou et al., 2018). The pH values of 3.0, 5.0, 10.0 and 12.0 were chosen as the controlling factors within the process to avoid direct vivianite formation in WAS but effectively release the P into supernatant (Based on the results obtained from jar tests). Also, to explore the effect of different iron source on Fe2+ and P release, the ZVI and FeCl3 were selected as the external iron source with 1.5 Fe/P molar ratio.
3. Results and discussion 3.1. Jar tests of phosphate recovery as vivianite The influences of pH, Fe/P molar ratio and PO43− concentration on vivianite formation (identified in Section 3.3) were presented in Fig. 1. Generally, pH is the key factors for efficient P recovery as vivianite, and three distinct regions are formed at different pH values. Specifically, when the pH value was below 5.0, almost all the PO43− (> 96.81%) and Fe2+ (> 97.04%) were remaining in the liquid phase at the concentration of 1 mg/L PO43− with the Fe/P ratio of 1.0 (shown in Fig. 1(A)). The reason could be attributed to the high solubility of Fe2+P compound at a lower pH (Wu et al., 2019). Nevertheless, the efficient pH regions for vivianite formation were in the range of 7.0–9.0. The aqueous PO43− and Fe2+ were effectively combined and precipitated as vivianite (Eq. (1)) in all reactors, and the residual concentration of soluble PO43− and Fe2+ were at an extremely low level (< 0.1 mg/L). This conclusion is similar to Liu’s (Liu et al., 2018), reporting that the optimal pH values for vivianite crystallization were 7.0–9.0. Nevertheless, as pH continued to rise, the aqueous P was observed to be noticeably increased. The reason might be contributed to formation of the insoluble Fe(OH)2, which could compete with Fe2+ ions to hinder the formation of vivianite and release PO43− into the solution (Eq. (2)) (Priambodo et al., 2017). For example, in Fig. 1(B), when pH raised from 10.0 to 11.0, the aqueous PO43− was increased from 0.14 to 4.97 mg/l, while the aqueous Fe2+ were still lower than 0.1 mg/L.
3F e2 + + 2PO43 − + 8H2 O → F e3 (PO4 )2 ·8H2 O
(1)
F e3 (PO4 )2 ·8H2 O + 6OH− → 3F e(OH )2 + 2PO43 − + 8H2 O
(2)
3.2.1. Fe release during WAS fermentation The Fe2+ release during AF at 1.5 Fe/P molar ratio was displayed in Fig. 2. Generally, the dominant fraction of aqueous TFe in all samples was Fe2+ (> 95%) at low pH values, which was beneficial for the subsequent P recovery as vivianite. However, the concentration of Fe2+ and TFe were negligible (< 1 mg/L) under alkaline environments. This consequence is consistent with jar tests in which ferric and ferrous hydroxide would be generated under alkaline condition because of its low solubility (Zou et al., 2017). The iron elements would thereby remain in the sludge. Noticeably, the dosing of ZVI and FeCl3 in acidic environment could markedly improve the Fe2+ concentration in supernatant, especially in the FeCl3 dosing reactors. Moreover, the lower pH value contributed more to the increase of Fe2+ in WAS fermentation reactors. The highest aqueous Fe2+ contents for ZVI- and FeCl3-dosing sludge at pH 3 were respectively 552.23 and 825.57 mg/L, accounting for 52.71% and 78.81% of TFe in each sludge, both of which were 3
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Fig. 1. Correlations of pH, Fe/P and PO43− concentration with P recovery as vivianite by jar tests at temperature 25 °C.
fermentation.
greater than that of ND sludge (maximum 74.85 mg/L making up 40.27%). The increase of aqueous Fe2+ in ZVI sludge might induced by ZVI dissolution in acidic environment (Eq. (3)) (Liu and Wang, 2019). Nonetheless, ZVI dissolution rate was relatively slow in weak acidic environment, leading to the relatively low aqueous Fe2+ concentration consequently.
F e0 + 2H+ → Fe 2 + + H2
F e3 + + 3H2 O → Fe (OH )3(s) + 3H+
Fe (OH )3(s) +
H+
+
e−
(4)
→F e(OH )2(S ) + H2 O
(5) 2+
reIn order to further disclose the underlying mechanisms of Fe lease with the addition of FeCl3 in WAS fermentation reactors, the microbial population in sludge samples was analyzed by high throughput sequencing and the results are shown in Table 1. Additionally, differing from raw and ND-NC sludge, the sludge with FeCl3 addition had much lower Chao and Shannon index value, indicating the decrease of richness and diversity of microbial population (Li et al., 2018; Zhang and Zhao, 2014). The dominant microbial populations in the different sludge samples are exhibited in Fig. 3(A). Obviously, the microbial structure was decreased significantly. The predominant bacteria in FeCl3-pH3 sludge was the Clostridiaceae (40.25%), belonging to the order Clostridiales (42.33%), However, it was only 14.79% and 19.67%, in FeCl3-NC and FeCl3-pH5 sludges, respectively, and was negligible in Raw and ND-NC sludge. The Clostridiaceae mainly consists of typical iron-reducing bacteria, i.e. Clostridium beijerinckii and Alkaliphilus metalliredigenes (Weber et al., 2006). During WAS fermentation, these functional bacteria could
(3)
The increase aqueous Fe2+ in FeCl3 sludge in acidic environment was primarily contributed to the biological Fe3+ reduction. The FeCl3 in water is mainly hydrolyzed to form hydrous iron oxide (HFO) precipitates as shown in Eq. (4) (Chen et al., 2019). Under anaerobic reduction conditions, Fe3+ is reduced to Fe2+ by (DMRB) as shown in Eq. (5) (Vuillemin et al., 2013). Thus, the Fe2+ from insoluble Fe(OH)2 was tend to release into supernatant due to its high solubility in acidic conditions (Zou et al., 2017). Nevertheless, the maximum concentration of aqueous Fe2+ in FeCl3 sludge was 1.51 times higher than it in ZVI sludge. Also, it took only 6 d in FeCl3-dosing sludge to reached the maximal Fe2+ concentration while it was 12 d in ZVI sludge, which indicated that biological Fe3+ reduction might be a better way to obtain sufficient aqueous Fe2+ compared with ZVI dissolution during WAS 4
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Fig. 2. Concentration of Fe2+ and TFe in supernatant during the AF under different pH values and iron amendments (A) aqueous Fe2+ in ND sludge, (B) aqueous TFe in ND sludge, (C) aqueous Fe2+ in ZVI sludge, (D) aqueous TFe in ZVI sludge, (E) aqueous Fe2+ in FeCl3 sludge, (F) aqueous TFe in FeCl3 sludge (As the concentration of Fe2+ release for control, pH 10 and pH 12 were extremely low (< 0.1 mg/L), the symbols and lines were overlapped).
efficiently promote the reduction and release of iron in the form of Fe2+, which is favorable for the subsequent reaction of P recovery as vivianite from the fermented supernatant.
Table 1 Alpha biodiversity analysis of sludge samples with different treatment. Sample
Raw ND-NC FeCl3-NC FeCl3-pH 3 FeCl3-pH 5
α = 0.03 Sequences
OTUs
Chaoa
Coverage
Shannonb
Simpson
45647 43505 42370 49816 49,007
1089 1025 1034 376 925
1141 1082 1165 586 1117
0.998357 0.997678 0.995492 0.996728 0.995572
5.83 5.58 4.16 2.45 4.27
0.0077 0.0101 0.1131 0.1948 0.0511
3.2.2. P release during WAS fermentation The P release during WAS fermentation with different Fe source and pH values were showed in Fig. 4. The majority of aqueous TP in all samples was PO43− (> 95%), which was also beneficial for the subsequent P recovery as vivianite. Contrary to the Fe2+ release, the PO43− was found to be dramatically released in the supernatant at alkaline environments. The highest aqueous P content in ND sludge at acidic (pH = 3.0) and alkaline (pH = 12.0) environment were respectively 100.34 mg/L and 208.26 mg/L, taking up 30.00% and 65.36% of TP in each sludge sample. Several studies had proved that alkaline fermentation could energetically improve the release of P, from WAS by enhancing the hydrolysis of P-containing organics (Chen et al., 2019; Wang et al., 2017; Zaman et al., 2019). Also, this phenomenon was consistent with the jar test where OH− replaced the site of PO43− in FeP compounds at alkaline condition, and caused the PO43− releasing into the liquid phase largely. However, it was worth mentioning that the dosage of ZVI or FeCl3 had little effects on P release under alkaline conditions but it apparently facilitated the release of P under acidic conditions, especially the FeCl3. In ZVI-pH3 sludge, the P releasing efficiency could reach 48.61%,
a Chao richness estimator: the total number of OTUs estimated by infinite sampling. A higher number indicates higher richness. b Shannon diversity index: an index to characterize species diversity. A higher value represents more diversity.
utilize the organic substrates as carbon sources and electron donors for Fe3+ reduction. Therefore, the large occupation of Clostridiaceae in FeCl3-pH3 reactor had a great positive impact on the acceleration of Fe3+ reduction under acid condition, which resulted in the high level of Fe2+. To sum up, ZVI may not be the preferable iron source for P recovery as vivianite due to the relatively slow rate of iron dissolution during AF. The dosage of FeCl3 and acidic environments during AF could 5
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Fig. 3. (A) Variations of microbial community in reactors with different treatment (B) Redundancy analysis for environmental variables and Fe2+ and PO43− release during WAS fermentation (family level).
release, probably by enhancing the hydrolysis of P-containing organic matters (Lin and Li, 2018). On the contrary, the concentration of soluble PO43− (SP) increased remarkably with pH control, which might attribute to the decrease of OP, NAIP and AP. In ND-pH3 sludge, the SP increased from 0.92 mg/L in the raw sludge to 100.34 mg/L, whereas OP, NAIP and AP were all decreased and contributed 37.39, 50.69 and 11.92% to the SP rise, respectively. Due to the relatively low solubility of Fe3+-P compounds and Al-P compounds, the majority of P in fermentation reactors would remain in the solid phase. The ZVI and FeCl3 addition could further promote the increase of SP, especially the FeCl3. The concentration of SP was respectively 154.92 and 273.04 mg/L in ZVI-pH3 and FeCl3-pH3 sludge samples, which accounted for 48.61% and 85.69% of the TP, respectively. The proportions of NAIP were respectively contributed to 61.94 and 65.61%. It seemed that the drop of NAIP might be the major cause for the rising SP. In retrospect with Fe release efficiency raised above, this result verified that the addition of ZVI and FeCl3 could enhance the reduction of Fe3+ through chemical and biological methods respectively. For instance, in the FeCl3-pH 3 reactor, the
which was higher than that in ND-pH3 (30.00%). Interestingly, approximate 85.69% of TP was released into fermentation liquid within 12 d in FeCl3-pH3 fermentation reactors, which was even far greater than that in alkaline environment (pH = 12.0). Thus, it can be concluded that the dosage of FeCl3 under acidic fermentation might be more conducive to the release of P. In order to explore the specific reasons for PO43− releasing efficiency in different reactors, the SMT method was conducted to determine the specific components of phosphorus in raw and fermented sludge. As shown in Fig. 5, the TP was 318.64 mg/L in raw sludge among which the OP and IP were occupied 23.96% and 76.04%, respectively. Specifically, NAIP occupied 76.73% of the IP, which indicated that P in the sludge was mainly in the form of Fe-P and Al-P. However, the main P components were changed greatly during WAS fermentation under different conditions. The pH adjustment obviously reduced the level of OP. For example, there are respectively 80.09 and 69.97% decrease of OP in FeCl3-pH3 and FeCl3-pH12 sludge samples, whereas it was merely 22.12% drop in the reactors without pH control, revealing acidic and alkaline treatment could contribute to the OP 6
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Fig. 4. Concentration of PO43− and TP in supernatant during the AF under different pH values and iron amendments (A) ND, (B) ZVI, (C) FeCl3.
form the insoluble Ca3(PO43−)2 in sludge, generating 44.85% increase of AP. However, the Fe2+ was basically remained in sludge phase as hydroxide precipitate under alkaline environment after AF, which was disadvantageous to the later vivianite recovery from fermentation supernatants (Lin et al., 2017). Overall, the FeCl3 with lower pH was more conducive to the release of Fe2+ and PO43− during WAS fermentation. Compared with the NDNC, the FeCl3-dosing reactors showed closer correlations with the aqueous Fe2+ and PO43−, which were the raw materials for vivianite formation reactors (shown in Fig. 3(B)). The fermentation pH is an influencing factor in determining the Fe2+ and PO43− release. The RDA analysis found that the angles between the pH arrows and AP, OP, NAIP
predominant bacteria was the Clostridiaceae consisted of typical ironreducing bacteria. It had great positive impacts on the acceleration of Fe3+ reduction under acid condition. Consequently, Fe3+-P compounds in sludge were reduced to Fe2+-P compounds which have high solubility under acidic environment. The P was thereby released into the liquid phase. Besides, the SP is also evidently increased in FeCl3-pH12 sludge (60.09% of the TP) with large drop of NAIP (decreased by 89.34%). It primarily attributed to the replacement of OH− for PO43− in the Fe-P and Al-P due to the much lower solubility of hydroxide precipitate. Desorption and release of PO43− derived from Fe-P and AlP would thereby desorb and release into the aqueous solution (Kim et al., 2003). The released P then re-combined with the Ca2+ to newly
Fig. 5. Proportions of different phosphorus forms in WAS fermentation reactors under different pH and iron source amendments. 7
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arrows were very small, indicating the positive correlation of fermentation pH with AP, OP and NAIP. Thus, the low pH would result in the decrease of AP, OP and NAIP, but with the increase of SP concentration. Moreover, the abundance of Clostridiaceae, which was responsible for Fe3+ reduction, showed negative correlations with pH strongly. It was disadvantageous to the final Fe2+ formation. However, the aqueous Fe2+ and PO43− as well as Clostridiaceae were clustered firmly in the FeCl3-pH3 reactors, which is consistent with results of maximal release of Fe2+ and PO43− in fermentation supernatant.
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3.3. Phosphorus recovery as vivianite from WAS fermentation supernatant As mentioned above, around 78.81% of Fe2+ and 85.69% of PO43− was released into the supernatant from FeCl3-dosing sludge at pH 3.0 via AF. After the supernatant was extracted from the fermenter, Fe2+ and PO43− were then re-precipitated and recovered by adjusting pH to 7.0. Also the XRD analysis indicated that the recovered precipitates showed distinctively strong and sharp diffraction peaks at 11.16°, 13.16°, 18.06° and 27.80°, which was the same to the structure of vivianite (PDF-79–1928#). Besides, EDS results presented that the Fe:P:O molar ratios were respectively 1.54:1:8.13 (jar tests) and 1.58:1:8.14 (fermentation), which were consistent with the theoretical value of vivianite (1.5:1:8). This results clearly indicated that the main fraction of the recovered precipitates both in jar tests (97.83%) and WAS fermentation (93.67%) were vivianite. In summary, over 82.60% of P from the sludge could be recovered as vivianite via FeCl3-dosing and acidic fermentation, which is much higher than that of traditional P recovery as struvite (30–40%) (Egle et al., 2015). Moreover, the vivianite is far more valuable than the struvite considering its wide application and high market price (Jowett et al., 2018; Priambodo et al., 2017; Wu et al., 2019). Hence, it is a novel and promising alternative for P recovery from WAS. 4. Conclusion The efficient vivianite formation was synchronously influenced by operating pH, PO43− concentration and Fe/P molar ratio. The P in WAS could be effectively recovered as vivianite during AF with optimized Fe dosing and pH control. The FeCl3 dosing with lower pH was more conducive to the release of Fe2+ and PO43− during WAS fermentation. Also, the Clostridiaceae with the ability of dissimilatory iron reduction was highly enriched in FeCl3 dosing reactors. Approximate 82.60% of the TP in WAS was finally recovered in the form of high-purity vivianite (93.67%). Acknowledgements The work is financially supported by the “National Natural Science Foundation of China (No. 51708171 and 51878243)”, “China Postdoctoral Science Foundation (No. 2018M630508 and No. 2019T120390)”, “Key Special Program for the Pollution Control (2014ZX07305-002)” and “Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China”. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122088. References Al-Borno, A., Tomson, M.B., 1994. The temperature dependence of the solubility product constant of vivianite. Geochim. Cosmochim. Acta 58 (24), 5373–5378. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21 ed. American Public Health Association, Washington, DC. Carpenter, S.R., 2008. Phosphorus control is critical to mitigating eutrophication. PNAS 105 (32), 11039–11040.
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Zaman, M., Kim, M., Nakhla, G., Singh, A., Yang, F., 2019. Enhanced biological phosphorus removal using thermal alkaline hydrolyzed municipal wastewater biosolids. J. Environ. Sci. 86, 164–174. Zhang, B., Wang, L., Li, Y., 2019. Fractionation and identification of iron-phosphorus compounds in sewage sludge. Chemosphere 223, 250–256. Zhang, Y., Zhao, X., 2014. The effects of powdered activated carbon or ferric chloride on sludge characteristics and microorganisms in a membrane bioreactor. Desalin. Water
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