Enhanced simultaneous nitrogen and phosphorus removal

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Enhanced simultaneous nitrogen and phosphorus removal performance by anammox-HAP symbiotic granules in the attached film expanded bed reactor Yanlong Zhang, Haiyuan Ma, Lan Lin, Wenzhi Cao, Tong Ouyang, and Yu-You Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02414 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced simultaneous nitrogen and phosphorus removal performance by anammox-HAP symbiotic granules in the attached film expanded bed reactor Yanlong Zhang †, Haiyuan Ma ‡, Lan Lin †, Wenzhi Cao † *, Tong Ouyang †, Yu-You Li ‡* † College

of the Environment & Ecology, Xiamen University, South Xiang’an Road, Xiang’an

District, Xiamen, Fujian 361102, China ‡

Department of Civil and Environmental Engineering, Graduate School of Engineering, Tohoku

University, 6-6-06 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

Corresponding authors： Yu-You Li E-mail: [email protected]; Phone: +81 22 795 7464; Fax: +81 22 795 7465 Wenzhi Cao E-mail: [email protected]; Phone: +86 592-2185877; Fax: +86 592-2185877

1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Dissolved nitrogen and phosphorus compounds are responsible for eutrophication and usually be treated by separate processes in wastewater treatment plants. In this study, the simultaneous, long-term, stable removal of phosphorus and nitrogen was achieved in a high-loading anammox attached film expanded bed (AAFEB) reactor. As an energy-saving and high efficiency biological process, the anammox reaction performed a stable and high nitrogen removal. In addition, the chemical environment and hydrodynamic conditions of the AAFEB system, as well as the microenvironment of the anammox granules, induced the crystallization of hydroxyapatite (HAP). As an inorganic carrier of anammox biofilms, HAP enhanced the settling velocity and shear strength of the anammox granules and thus promoted the operational stability of the AAFEB reactor. In the coupled anammox-HAP system, the nitrogen removal rate (NRR) reached 44.8 gN/L/d, with a stable phosphorus removal rate of 71.61 ± 6.82%. The results indicated that the coupled anammox-HAP system enhanced the nitrogen removal performance and provided a possible phosphorus recovery pathway through biomineralization processes.

Keywords: anammox-HAP symbiotic granules; phosphorus removal; biomineralization; granule properties


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Introduction The heavy use of fertilizers in modern agriculture has enhanced world crop yields and made it possible for the global population to exceed 7.4 billion 1. However, the nitrogen and phosphorus consumed in food typically exceed nutritional requirements, and the remainder leaches into waterbodies, resulting in eutrophication and other concomitant environmental problems

1, 2

. In

addition, the demand for phosphorus is increasing, while the available phosphorus rock is shrinking 3

. An obvious way to reduce risks to water quality and resource exhaustion is to remove phosphorus

and nitrogen from wastewater and recover phosphorus in the form of valuable products. In a wastewater treatment plant, biological processes are usually applied as economic pathways for meeting the effluent quality standards of COD, nitrogen and phosphorus. To meet increasingly stricter environmental protection requirements, new processes with high efficiency and low carbon footprint are required. Due to the low electricity consumption and low total cost (including construction and operational cost), the anaerobic ammonia oxidation (anammox) process has attracted a great deal of attention for the treatment of ammonium-rich wastewater 4. The anammox process is usually applied for sidestream treatment of municipal wastewater, accounting for 75% of the total number of full-scale anammox-based installations 5. The phosphorus concentration in reject water can vary at high levels of 60-100 mg P/L and needs to be further treated via other biological or chemical processes 6. The recovery of phosphorus from reject water as solid phosphorus



compounds decreases the risk of blockage in pipes and pumps and extends the life of the facility; moreover, the recovered phosphorus provides a local source of fertilizers or eco-materials 1, 7. Chemical precipitation process is a typical process for the recovery of high-strength phosphoruscontaining wastewater. As a typical recycled product, struvite precipitation is induced under the conditions of high pH, CO2 stripping and magnesium addition 8, 9. The recovery cost is thus high due to the input of energy and chemicals. Induced crystallization in the form of hydroxyapatite (HAP) is promising for the effective recovery of phosphorus; in addition, the recovered product is of high quality and can be used as an adsorbent for treating wastewater containing heavy metals 1012

. The mechanism of HAP crystallization follows the sequence of supersaturation, nucleation and

crystal growth, and thus, the concentration of amorphous calcium precursors (ACP) and the operational pH are key factors for HAP crystallization 13, 14. Biomineralization induced by gradients in pH and substrate concentration inside or between the cells provides a possible pathway toward the recovery of phosphorus in a microbial system 15, 16. Moreover, recent studies indicate that HAP accumulates in the core of anaerobic/aerobic granules 17, 18. It can be assumed that the participation of microorganisms may decrease the cost and enhance the efficiency of phosphorus recovery; meanwhile, the generated crystalline phosphate may affect the biological and physical performance of the anaerobic/aerobic granules. The anammox process consumes H+ and results in an alkaline environment, which provides the


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essential conditions for phosphate crystallization 17. In addition, the endogenous organic matter produced in the biological metabolism induces phosphate biomineralization Ruscalleda and Colprim

16

19

. Johansson,

reported a two-layer structure of anammox granules, in which the

anammox biofilm was attached to an inorganic core, in that study, Energy Dispersive X-ray coupled with Scanning Electron Microscopy (SEM-EDX) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) analysis confirmed that the inorganic core formed in the partial nitritation-anammox

reactor

was

HAP

(Ca5(PO4)3(OH),

with

minor

apatite

((Ca4Na0.01Mg0.02)(Ca6Na0.13Mg0.03)(PO4)6), chloroapatite (Ca5(PO4)3Cl) and calcium sulfide phosphate (Ca10(PO4)3S). Johansson, Ruscalleda and Colprim 16 proved that the generated HAP complied with the proposed EU fertilizer standard. In addition, Lin, Lotti, Sharma and van Loosdrecht 17 indicated that the presence of calcium phosphate was important for the mechanical strength of the granules. Overall, the anammox process induced the HAP crystallization through biomineralization process in an economic way, the produced HAP in anammox reactor effects on the granule characteristics and provides a possible pathway for phosphorus recovery. The combination of the anammox process and phosphorus recovery is possible for constructing a renewable and sustainable process for treating wastewater containing high concentrations of nitrogen and phosphorus. In this study, long-term experiments were carried out in an anammox attached film expanded bed (AAFEB) reactor, in which the nitrogen and phosphorus removal



performance and granule characteristics were evaluated. Furthermore, the formation mechanism of coupled anammox-HAP granules induced by anammox metabolism was investigated.

Materials and methods Reactor setup and biomass An AAFEB reactor with a 5 L working volume was used in this study. The operational temperature was controlled at 35 ℃. Effluent recirculation was applied to dilute the substrate concentration and to maintain fluidization condition in the AAFEB reactor. With appropriate recirculation rate, the concentrations of free ammonia (FA) and free nitrous acid (FNA) can be diluted lower than the toxic threshold concentrations, and thus enhancing the stability of the AAFEB reactor. In addition, the fluidization condition enhances the mass transfer and improves the efficiency of the AAFEB reactor 20, 21. Furthermore, higher up-flow velocity enhances the shear strength that benefit for the formation of granules 22. Diluted sulfuric acid (2-5%) was continuously fed into the recirculation water to adjust the operational pH to a proper condition for both the anammox process and HAP crystallization. The anammox-HAP symbiotic granules enriched from the mixed anaerobic digestion sludge, denitrifying activated sludge and methanogenic granules were produced in the same reactor and operated for over four years. Synthetic wastewater was continuously fed from the bottom of the AAFEB reactor. The nitrogen sources were prepared from 135-473 mgN/L (NH4)2SO4-N and 178-567 mgN/L NaNO2-N.


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The phosphorus source was prepared from 11.4 mg P/L KH2PO4-P. The calcium chloride (CaCl2·2H2O) was supplied as the configurational ion source, with a constant Ca/P molar ratio of 5.5. The other components of the mineral medium and trace element were generally consistent with the pervious study 20. Reactor operation As shown in Table 1, during the long-term experiment, the nitrogen concentration gradually increased, while the HRT decreased. As a result, the nitrogen loading rate (NLR) and phosphorus loading rate (PLR) were increased to 50.0 gN/L/d and 0.55 gP/L/d, respectively. To avoid the risk of inhibition by the high ammonium and nitrite concentrations, the recirculation rate was increased from 2.5 to 3.5 to sufficiently dilute the influent FA and FNA below the toxic thresholds of 15 mg/L and 10 μg/L, respectively 20. Table 1 Operational conditions of the AAFEB reactor Period (day) 1-30 31-51 52-72 73-103 103-132 133-158 159-180

NH4+-N (mgN/L) 135 313 284 284 284 377 473

NO2--N (mgN/L) 178 313 341 341 341 453 567

HRT (h) 1.50 1.50 1.00 0.75 0.50 0.50 0.50

P (mg/L) 11.4 11.4 11.4 11.4 11.4 11.4 11.4

Ca (mg/L) 81.6 81.6 81.6 81.6 81.6 81.6 81.6

NLR (gN/L/d) 5.0 10.0 15.0 20.0 30.0 40.0 50

PLR (gP/L/d) 0.18 0.18 0.27 0.36 0.55 0.55 0.55

Recirculation rate 2.5 2.5 2.5 2.5 2.5 3.0 3.5

Granule analysis Analysis of physical properties The physical properties (size distribution, settling velocity and shear strength) of the produced



anammox-HAP granules were evaluated at the end of each operational period. Photographic analysis using ImageJ (1.48v, USA) was applied to evaluate the particle distribution of the granules, which could be divided by particle size into five groups: < 1.0 mm, 1.0 ~ 2.0 mm, 2.0 ~ 3.0 mm, 3.0 ~ 4.0 mm and > 4.0 mm. Each group was separated by sifters with different mesh apertures for further tests. The settling velocity of the granules (10-15 in number) was measured in a series of settlement experiments 23. Ni, et al. 24 reported the procedures for measuring the integrity coefficient of the granules, which can be used to quantify the shear strength of the coupled anammox-HAP granules and is expressed as follows: 𝑆𝑆

Integrity coefficient = (1 − 𝑆𝑆𝑡 ) × 100% 0

Eq. 1

where SS0 is the total number of coupled anammox-HAP granules used in the shaking test, g, and SSt is the amount of sludge in the supernatant after 20 min of shaking (200 rpm) in a water bath shaker, g. Analysis of the morphology and chemical components The morphology and chemical components of the coupled anammox-HAP granules were evaluated through various methods during the experiment. SEM-EDX was applied to determine the granule structure and elemental distribution at different points in the particle cross section. For analyzing the components of organic matters in the granules, the carbon, hydrogen, nitrogen and sulfur were measured by an Elemental Analyzer (Vario Micro Cube, Elementar), while the oxygen


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content was calculated as the weight of dry granules subtracting the ash after burning under 600℃ for 2h and CHNS content of the samples. For analyzing the inorganic component, 0.2g dry granules were mixed with 8 mL of nitric acid and then digested in a microwave digester (Milestone, ETHOS UP) for 12 h, the calcium, iron, potassium, magnesium, cobalt, molybdenum, sodium, nickel, phosphorus and zinc contents were analyzed by an Agilent 720 ICP-OES system (Agilent Technologies, Wilmington, USA). As the metabolic product is an important parameter for assaying the status of a biological reaction and the shear strength of the granules

25, 26

, the extracellular

polymeric substance (EPS) was extracted by the cation exchange resin (CER) method (Dowes Marathon C, 20-50 mesh, sodium form), and the EPS content was calculated as the sum of the protein (PN) and polysaccharide (PC) content. PN was measured using the Lowry method with bovine serum albumin as the standard 27, and the PC content was measured using the phenol-sulfuric acid method with glucose as the standard 28. The inorganic species were evaluated by X-ray Diffraction (XRD), where the diffraction patterns were recorded in Bragg-Brentano geometry using a Bruker D5005 diffractometer equipped with a Huber incident-beam monochromator and a Braun PSD detector. Analysis of the specific anammox activity (SAA) The SAAs of the coupled anammox-HAP granules, the anammox biofilm and the inner cores of the granules, which are regarded as the biofilm carriers, were tested separately at day 70. Since



granules and micro-crystals possibly coexist in the sludge and influence the SAA calculation of individual samples, granules with diameters larger than 1 mm were selected from the sludge and used in the following tests to avoid the influence of micro-crystals. The anammox biofilm was peeled from the carriers by gently rubbing on the surface of a sieve. The procedures for the SAA tests were basically consistent with those in a previous study 20. The coupled anammox-HAP granules, the separated biofilm and the carriers were put into different serum flasks and incubated in a thermostatic water bath shaker (110 rpm) at 35 ℃. The nitrogen source was supplemented with a mixture of (NH4)2SO4 and NaNO2 (1:1.32 molar ratio), with a TN concentration of 200 mg/L. The produced gas volume was recorded for the subsequent SAA calculation and simulated by the modified Gompertz equation 20. Water quality analysis Samples were collected from the influent and effluent every 2 days and filtered through a 0.45 μm syringe filter. The nitrogenous compounds were analyzed by capillary electrophoresis (Agilent 7100). Phosphorus was analyzed by an Agilent 720 ICP-OES system (Agilent Technologies, Wilmington, USA). Analyses of the suspended solids (SS) and volatile suspended solids (VSS) were carried out according to the standard method 29.


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Results and discussion

40 30 20 10 0 0

pH

A

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100 Duration (days)

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C

100 90 80 70 60 50 40 30 20 10 0

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80 100 120 Duration (days)

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Ca removal efficiency P removal efficiency

0

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B

Ca loading rate P loading rate

3.5

Ca/P (in mole)=5.5

3.0 2.5 2.0 1.5 1.0 0.5 0.0

200

9.4 9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8 7.6 0

4.0

0 10

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Effluent P concentration

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Effluent Ca concentration

D

8

70 60 50

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30 20

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10 0 0

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0 200

Effulent Ca concentration (mg/L)

N loading rate N removal rate

50

Ca and P loading rate (m/L/d)

60

Effluent P concentration (mg/L)

N loading and removal rate (gN/L/d)

Long-term N and P removal performance

Ca and P removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


E


140

160

180

200

Fig. 1 Long-term operation and performance of the AAFEB reactor: nitrogen loading rate and nitrogen removal rate (A); Ca and P loading rate (B); pH variation in the reactor (C); effluent Ca and P concentration (D); Ca and P removal rate (E).

The AAFEB reactor maintained a stable nitrogen removal performance as the NLR was increased from 5.0 to 50.0 gN/L/d. The maximum nitrogen removal rate (NRR) reached 44.8 gN/L/d (Fig. 1A), which is higher than that in most previous studies 21. A higher NRR of 76.7 ±4.5 gN/L/d was reported by Tang, et al. 30, the TN removal efficiency in that study was only 50.9-61.7%, which is much lower than the value obtained in this study (89.8 ±0.7%). Chen, et al. 31 also reported



a higher NRR of 52.6 gN/L/d in an UASB reactor, however, granule floatation and floccule floatation occurred during the operational period and resulted in the unstable reactor performance. The results indicated that the AAFEB reactor was highly efficient and stable for nitrogen removal. When the HRT was shortened from 1.5 h to 0.5 h, the phosphorus loading rate increased from 0.18 to 0.55 gP/L/d, and the effluent P concentration was basically staying lower than 4.0 mg/L with the stable P removal efficiency of 71.61 ±6.82% (Fig. 1B, D, E). The Ca/P ratio is a key factor that determines the effect of crystallization 13, 32. Since a higher Ca/P ratio is beneficial for the formation of HAP, micro-crystals formed in a supersaturated solution adversely influence the recovery of phosphorus 33. In this study, the influent Ca/P molar ratio was controlled at a constant value of 5.5, and the Ca loading rate was increased from 1.31 to 3.92 g/L/d, with a stable Ca removal rate of 31.16 ±2.46%. Though various calcium phosphates may be generated based on the operational conditions, all the precipitates are likely to be converted into the thermally stable product, HAP 34. During the long-term experiment, the consumed Ca/P molar ratio of 2.08 ±0.40 was higher than the ratio of HAP (1.67) and higher than that of all other HAP precursors: amorphous calcium phosphate (ACP, 1.5), dicalcium phosphate dehydrate (DCPD, 1.0), octacalcium phosphate (OCP, 1.33), hydroxy dicalcium phosphate (HDP, 2.0) and tricalcium phosphate (TCP, 1.5). The results indicated that not only did a precipitation reaction occur between Ca and P but that other calcium salt precipitation may occur in the AAFEB reactor, thus resulting in a higher Ca/P


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consumption ratio. The precipitate analysis is discussed in the following sections.

The operational pH is an important factor in both the anammox process and crystallization. For the anammox process, an excessively high or low pH may result in increased FA or FNA concentrations and thus adversely affect the anammox performance 35. In addition, precipitation may not occur at excessively low pH due to the low saturation of configurational ions. Although a higher pH is beneficial for crystallization, the formed micro-crystals are hard to recover due to their supersaturation 13. In the anammox reaction, an increase in nitrogen concentration may result in heavy acid consumption and increase the pH to 9.05 36, which is not suitable for the anammox reaction. Furthermore, Zhang, Ma, Chen, Niu and Li 21 indicated that more acid will be consumed to convert the same equivalent of nitrogen under higher NLR conditions. The effects of pH on phosphorus removal by Ca-P precipitation was reported in a previous study 15, in which 25% of the phosphorus could precipitate at a pH of 7, whereas approximately 80% of the phosphorus could precipitate at a pH of 9. In this study, the operational pH may increase to an excessively high level with an increase in nitrogen concentration and NLR. To avoid the risk from high pH to the anammox process and recovery of phosphorus, sulfuric acid (2-5‰) was added to the recirculation water to control the operational pH in the optimum range of 8.0 to 8.5 (Fig. 1C). As a result, the P and Ca removal rates were stably maintained at 71.61 ±6.82% and 31.16 ±2.46%, respectively, while nitrogen removal was stably maintained at high efficiency (Fig. 1A).



Characteristics of the granules Distribution of particle size and settling velocity

200

20 15

100

10

40 30 20 15 10

NLR= 20gN/L/d N/P=123 (in mole)

400

30

300

25 20

200

15 10

100

1.0-2.0 2.0-3.0 Granule diameter (mm)

3.0-4.0

45 40

0

500 Proportion settling velocity

NLR= 30 gN/L/d N/P= 121

400

35 30

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25 20

200

15 10

100

5

5 1.0-2.0 2.0-3.0 Granule diameter (mm)

3.0-4.0

50 Proportion settling velocity

300

30 25

200

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10 0

0

Enhanced simultaneous nitrogen and phosphorus removal

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