Strengthening aerobic granule by salt precipitation

Bioresource Technology xxx (2016) xxx–xxx

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Short Communication

Strengthening aerobic granule by salt precipitation Yu-You Chen a, Xiangliang Pan b, Jun Li b, Duu-Jong Lee a,b,⇑ a b

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan College of Environment, Zhejiang University of Technology, Hangzhou 310014, China

h i g h l i g h t s  Structural stability of aerobic granules is enhanced by salt precipitation.  Precipitated granules have high strength with minimal loss in COD removal.  MgCO3 precipitated granules can be applied at pH 3.

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Article history: Received 30 April 2016 Received in revised form 25 June 2016 Accepted 27 June 2016 Available online xxxx Keywords: Stability Salt Strength Sequential batch reactor

a b s t r a c t Structural stability of aerobic granules is generally poor during long-term operation. This study precipitated seven salts inside aerobic granules using supersaturated solutions of (NH4)3PO4, CaCO3, CaSO4, MgCO3, Mg3(PO4)2, Ca3(PO4)2 or SiO2 to enhance their structural stability. All precipitated granules have higher interior strength at ultrasonic field and reveal minimal loss in organic matter degradation capability at 160-d sequential batch reactor tests. The strength enhancement followed: Mg3(PO4)2 = CaSO4 > SiO2 > (NH4)3PO4 > MgCO3 > CaCO3 = Ca3(PO4)2 > original. Also, the intra-granular solution environment can be buffered by the precipitate MgCO3 to make the aerobic granules capable of degradation of organic matters at pH 3. Salt precipitation is confirmed a simple and cost-effective modification method to extend the applicability of aerobic granules for wastewater treatments. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Aerobic granular sludge is a promising biotechnology for treatment of high-strength industrial wastewaters (Adav et al., 2008; Lee et al., 2010). Since the interiors of aerobic granules are compact and dense, the granules generally reveal sufficient settleability in wastewater treatment and are resistant to toxicity of industrial wastewaters (Chen and Lee, 2015; Zhang et al., 2016). The main drawback for the aerobic granular sludge process practice is its poor structural stability during long-term operation (Lee et al., 2010). Aerobic granules that can remain structurally stable over long-term, continuous-flow operation are highly desired. Ren et al. (2008) noted that calcium carbonate was accumulated at core of granules, with which the granules could have rigid structure and high strength. Lin et al. (2013) noted accumulation of CaPO4 minerals in their anammox granules and commented that the presence of minerals enhances strength of the granules. Winkler et al. (2013) measured the density of their aerobic gran⇑ Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. E-mail address: [email protected] (D.-J. Lee).

ules and noted that increase in 1–5% v/v of precipitates in granular interior strongly increase the granule density and thereby the settling velocity. Angela et al. (2011) noted mineral clusters concentrating calcium and considerable amount of phosphorus at the granules’ core. These authors claimed that the accumulation of phosphorus by granules could reach 45% of the overall P removal from the tested wastewaters. Metal precipitates commonly noted in mature granules were reported to be able to enhance granule strength and stability in long-term operation (Lee et al., 2010). Wan et al. (2015) claimed that calcium precipitate with living cells can trigger aerobic granulation. Ye et al. (2016) noted that Ca2+ ions can affect settleability and even metabolic reactivity of activated sludge. The biologically induced mineral precipitation has been extensive studied (Achal et al., 2013; Kumari et al., 2014). Liu et al. (2016) demonstrated that CaCO3 precipitates would be formed and accumulated in developed aerobic granules with quantity increasing with granule size. Juang et al. (2010) applied enriched denitrifying bacteria as seed sludge and high phosphate wastewater to stimulate mineral precipitation inside granules. Since the granule core can be deficient in oxygen content so denitrification reaction can occur locally with production of alkalinity, which facilitates calcium and iron precipitation. The

http://dx.doi.org/10.1016/j.biortech.2016.06.111 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Chen, Y.-Y., et al. Strengthening aerobic granule by salt precipitation. Bioresour. Technol. (2016), http://dx.doi.org/ 10.1016/j.biortech.2016.06.111

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so-formed granules were noted to be sable in 216-d operation in continuous-flow reactors without deterioration of structural integrity. Biologically induced mineral precipitation is a slow process. Lee and Chen (2015) proposed the use of supersaturated magnesium carbonate to lead to precipitate inside aerobic granules for enhancing their stability. These strengthened granules were noted to be able to exist stably in continuous-flow reactors. We examined in this study as the continuation work of Lee and Chen (2015) the use of precipitation of seven salts to enhance the stability of mature aerobic granules. The modified granules were tested on their strength and the bioactivity on wastewater treatment. 2. Materials and methods 2.1. Cultivation of aerobic granules The aerobic granules were cultivated in a 6 cm  180 cm sequential batch reactor with operational cycle: 1.6 L of synthetic wastewater was fed in 3 min with compositions (57.3 g/L propionate, 6.9 g/L ethanol, 6 g/L NH4Cl, 40 g/L K2HPO4, 20 g/L KH2PO4, 1.2 g/L CaCl2, 0.75 g/L MgSO4
7H2O, 0.6 g/L FeSO4
5H2O, 0.4 g/L NaHCO3, 6 g/L peptone, 3.75 g/L meat extract, pH 7.2 ± 0.1), aeration-settling for 227 min, decanting for 5 min, idle for 5 min. The seed sludge was the recycled sludge at mixed liquor suspended solids (MLSS) of 6000 mg/L. The aeration rate was 5 L/min. 2.2. Granule strengthening The cultivated granules were subjected to chemical precipitation modification. 10 g of salts was added to 100 ml distilled water which was vigorously stirred and was heated to 90 °C for 1 h. The suspension was then filtered to remove solid phase and was then slowly cooled down to 60 °C. The salts applied were (NH4)3PO4 (solubility 26.1 g/100 ml at 20 °C), CaCO3 (solubility 7.75  10 4 g/100 ml at 20 °C), CaSO4 (solubility 0.255 g/100 ml at 20 °C), Mg3(PO4)2 (solubility 2.59  10 4 g/100 ml at 20 °C), MgCO3 (solubility 0.039 g/100 ml at 20 °C), Ca3(PO4)2 (solubility 2  10 3 g/100 ml at 20 °C), SiO2 (solubility 0.012 g/100 mL at 20 °C). Two coating protocols were tested. The mature granules were first collected from the reactor with surface moisture gently removed by tissue papers. Then half of the hydrated granules was put at 4 °C freezer for 5 min to cool down the surface temperature. The cooled granules then placed into the 60 °C salt solution for 30 s, which were then collected and placed at room temperature for natural drying. Then the slightly dried granules were place in 60 °C salt bath again and the process repeated itself for up to four times. Apparently precipitates were noted on the granules’ surface after coating. These granules were named the type I granules. Another half of the collected granules were placed in the 60 °C salt bath with gentle stirring for 30 min for allowing ion penetration into granule interior. Then the granules were collected and immediately dipped into iced water for 5 s. This process was repeated for up to three times to allow mineral precipitation occurred inside the granules. These granules were named the type II granules. 2.3. Characterization and analyses The strength of aerobic granules was evaluated using ultrasound method (Wan et al., 2013): the granules were exposed to 20 25 kHz, 65 W ultrasound batch at 2.5 s (on)-3 s (off) cycles. The turbidity for the ultrasound treated suspension at 290 nm measured.

The enlarged images of tested beads were determined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) (Nova NanoSEM 230, FEI Company, Hillsboro, Oregon, USA). Size distributions of the granules were measured by particle sizer (Mastersizer 2000, Malvern UK) equipped with He-Ne laser as light source and 44-element solid-state detector as light collectors between 0.01° and 32.1° (Chen et al., 1997). The capability of aerobic granules to degrade organic matters in water was demonstrated by using 300 mL sequential batch reactors (SBR) with feed at same composition of cultivation medium at cycle time of 24 h. The organic loading rate (OLR) was 4 kg COD/m3-d. Air was fed at 3 L/min flow rate (Adav et al., 2007). Fifty granules were placed each reactor, and the number of granules left at the end of 160-d testing. The concentrations of chemical oxygen demand (COD), mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) of collected samples were measured according to Standard Methods by Taiwan EPA (NIEA W517.51B, W210.58A; W210.55A) (Annadurai et al., 2002; Show et al., 2011). Settling velocities of individual granules were measured in 1-L measure cylinders. All measurements were performed in triplicate with their mean being reported. 3. Results and discussion 3.1. Precipitated (NH4)3PO4 granules The original granules have surface with compactly crowded bacteria. After both type I and type II (NH4)3PO4 treatments, clear precipitates formed with specific crystal forms on the granule surface. The wet weights of individual granules were increased over repeated precipitation of (NH4)3PO4 salts by type I or type II protocols from 1.1 mg/granule to 2.1, 2.8, 4.2 and 5.5 mg/granule after 1–4 coatings for the former, and to 6.0, 5.8, 12 mg/granule after 1–3 coatings for the latter. Restated, type I precipitates less salt with the granules than type II. The corresponding settling velocities of the precipitated granules were increased compared with original granules. The EDS results showed that the original granules comprise principally C (67.9%), O (28.7%) and N (3.1%). The (NH4)3PO4 modified granules have C (41.1%), O (45.0%), N (8.0%), P (3.6%) for type I and C (32.0%), O (52.1%), N (2.9%), P (8.6%) for type II (figure not shown for brevity). Ultrasound tests showed that the turbidity of original granules was increased to a plateau after 30 s treatment (Fig. 1). On type I (NH4)3PO4 treatment, the coating number affected the time change of turbidity under ultrasonication. After one batch coating the turbidity reached plateau value after 72 s ultrasonication. After coating two, three and four the granules were fully disintegrated at longer testing time. On type II treatment, three coatings could lead to very tough granules with slow increase in turbidity over ultrasonication time. The original or type I (NH4)3PO4 granules placed in the continuous-flow reactors were collected and measured their size after 120-h operations in continuous-flow reactor. Most original granules were deteriorated in structure after the operation; while the type I granules had a monodispersed distribution at about 900 lm (Fig. 2). The above observations suggest that the precipitated (NH4)3PO4 granules are stronger in structure than original granules under shearing environment. 3.2. Salt precipitated granules Both type I and type II (NH4)3PO4 granules can be strengthened in structural stability. Since type II protocol can be conducted at shorter processing time than type I and the former induces more

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(a)

Original Mg3 (PO 4 )2

2000

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Effluent COD (mg/L)

Turbidity (NTU)

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Time (hr) Fig. 3. COD declines for degradation tests using original or precipitated granules.

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Time (sec) Fig. 1. Suspension turbidities under ultrasonic treatment. (a) Original and type I precipitated granules. (b) Original and type II granules.

h ou wit

t co

2 g 2 2 4 3 3 4 CO atin g SiO (PO4) (PO4) CaCO CaSO 4)3PO Mg g g H tin Mg3 a3 g n n a i i n C N t t o i ( t c g coa ating c oa coa ting coatin c oa co

Fig. 4. Residual granules (in percentage) for original and precipitated granules after 120-h continuous-flow testing.

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Size (µm) Fig. 2. Size distributions of original and (NH3)3PO4 type I coating (4 times) after COD degradation tests at aeration intensity 12 L/min for 10 h.

precipitates inside granules than the latter, the comparison tests for salt precipitation with (NH4)3PO4, CaCO3, CaSO4, Mg3(PO4)2, MgCO3, Ca3(PO4)2, or SiO2 were made only using type II with three repeated coating.

In SBR tests, the original granules could degrade most fed COD in 12 h (Fig. 3). Most salt precipitations reduced COD degradation rates, with the adverse impact following: SiO2 > CaSO4 > Mg3(PO4)2 > MgCO3 = CaCO3 = (NH4)3PO4. The precipitated Ca3(PO4)2 granules even had an enhanced COD removal rate. These tests revealed that salt precipitation affected COD degradation performances of aerobic granules, but the effects were not significant in SBR operation. The percentage of intact granule at the end of 160-d testing was 40% for original granules (Fig. 4). Salt precipitation increased the percentage of granules survived at the SBR operation: CaCO3 and Ca3(PO4)2 at 56%, MgCO3 at 64%, (NH4)3PO4 at 78%, SiO2 at 82%, and Mg3(PO4)2 and CaSO4 at 92%. The precipitated salts enhanced structural stability at the tested SBR operation. Lee and Chen (2015) applied for the first time using MgCO3 precipitation to enhance structural stability of aerobic granules. The presence study demonstrated further that certain salt precipitates such as MgCO3 inside granules can function as a buffer for pH shock. A parallel test using original and MgCO3 precipitated granules in the SBR tests just with pH being adjusted to 3.0 using HCl was performed. The original granules could not degrade COD at pH 3 (Fig. 5) (after 35 h some part of granules was dissolved so

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Appendix A. Supplementary data

100 without coating coating MgCO3

COD removal rate (%)

80

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2016.06. 111.

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References 40

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Time (hr) Fig. 5. COD degradation tests in SBR for original and MgCO3 precipitated granules at pH 3.

the COD was increased rather than decreased with time, leading to negative COD removal rate). Conversely, the MgCO3 precipitated granules could degrade COD after an adaption time, reaching 42% COD removal at hr 50. The local CO23 -HCO3-CO2 equilibrium should play important role for buffering local pH to allow the constituent cells to function at pH 3 solution. 4. Conclusions The aerobic granules with intra-granular salt precipitates were synthesized and tested in their structural stability using ultrasound tests and COD degradation capability using SBR tests. These precipitates can enhance structural stability of aerobic granules (Mg3 (PO4)2 = CaSO4 > SiO2 > (NH4)3PO4 > MgCO3 > CaCO3 = Ca3(PO4)2 > original) with minimal loss of their COD degradation capabilities (Ca3(PO4)2 > original > MgCO3 = CaCO3 = (NH4)3PO4 > Mg3(PO4)2 > CaSO4 > SiO2) at SBR operations. Intra-granular solution pH can be buffered by the precipitate to make the aerobic granules capable of degrading organic matters at pH 3. Acknowledgement The authors appreciate the financial supports by National Natural Science Foundation of China (No. 51278128).

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Please cite this article in press as: Chen, Y.-Y., et al. Strengthening aerobic granule by salt precipitation. Bioresour. Technol. (2016), http://dx.doi.org/ 10.1016/j.biortech.2016.06.111