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Adsorption of ammonium by aerobic granules under high ammonium levels
Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 202–206
Contents lists available at SciVerse ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Short communication
Adsorption of ammonium by aerobic granules under high ammonium levels Xiaonan Yu a, Chunli Wan a, Zhongfang Lei a,b, Xiang Liu a, Yi Zhang a, Duu-Jong Lee a,c,*, Joo-Hwa Tay a a
Department of Environmental Science and Engineering, Fudan University, 220 Handan Road, Shanghai 200433, China Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan c Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 November 2012 Received in revised form 20 April 2013 Accepted 22 April 2013 Available online 29 May 2013
High-strength ammonium (NH4+) wastewaters are difficult to be treated biologically. Aerobic granules are regarded as a promising technology for NH4+ removal from wastewaters. Adsorption using aerobic granules in 50–4000 mg NH4+-N/L wastewaters was studied at 30 8C. Adsorption capacity was increased with 50–1000 mg NH4+-N/L and reached plateau of 24.5 mg NH4+-N/g-volatile suspended solids at >2000 mg NH4+-N/L. Ion exchange mechanism controls adsorption at <300 mg NH4+-N/L and physisorption mechanism dominates the process at >1000 mg NH4+-N/L. The biomass of aerobic granules presents a recyclable adsorbent for NH4+ from high-strength wastewaters. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Aerobic granules Ammonium adsorption High concentration
1. Introduction High-strength ammonium (NH4+) wastewaters are generated by petroleum refineries, landfills, livestock farms and other sources [1,2]. The excessive NH4+ in the wastewater is toxic to aquatic species and can lead to distortion of natural nutrient cycles and eutrophication of surface waters [3,4]. Adsorption is a unit operation suitable to remove toxic substances from wastewaters [5–8]. Aerobic granulation is drawing increasing global interest in a quest for an efficient and innovative technology in wastewater treatment. Developed less than two decades ago, extensive research work on aerobic granulation has been reported [9,10]. Aerobic granules were claimed for a promising biological technology for nitrogen removal of NH4+-containing wastewaters [11,12]. The aerobic granules were tested for treating NH4+containing wastewaters with 360–1400 mg NH4+-N/L. Tsuneda et al. [13] achieved >80% NH4+ removal using aerobic granules for wastewater of 500 mg NH4+-N/L. At NH4+ level of 1000 mg NH4+-N/ L, Wei et al. [14] achieved around 50% NH4+ removal using aerobic granules. Restated, the NH4+ removal rate was decreased with increasing NH4+ concentration. Additionally, the aerobic granules were noted to be able to adsorb 10–25% of wastewaters of 30 mg NH4+-N/L and the nitrification efficiency by aerobic granules would
* Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. Tel.: +886 22362 5632; fax: +886 22362 3040. E-mail addresses: [email protected], [email protected] (D.-J. Lee).
be overestimated if the adsorption contribution was neglected [15,16]. Certain wastewaters can have very high concentrations of NH4+ [17]. At low concentration regimes the quantities of NH4+ adsorbed were noted to increase with increasing influent concentration of NH4+ [15]. No data are present on the adsorption capacity of aerobic granules with high-strength NH4+ wastewaters. The objective of present study is to investigate ammonium adsorption process onto aerobic granules at NH4-N levels up to 4000 mg/L. Mechanisms of adsorption of NH4+ on aerobic granule surface were discussed since comprehensive understanding of the mechanisms of adsorption can help accurate prediction of system performance under different wastewater compositions and operational parameters. 2. Materials and methods 2.1. Cultivation of aerobic granules Synthetic wastewater used had sodium acetate and sodium propionate as carbon source. Other nutrients contained in the synthetic wastewater were: peptone 400 mg/L, yeast exact 250 mg/L, NH4Cl 3.74 mM, KH2PO4 4.85 mM, CaCl2 0.27 mM, MgSO4 0.21 mM, FeSO4 0.13 mM, NaHCO3 0.15 mM. Aerobic granules were cultivated in column-type sequencing batch reactors (SBR) with internal diameter 6 cm and working volume of 2.2 L. The SBR was operated in a time sequence of feeding (5 min), aeration (195 min), settling (gradually decreasing from 30 min to 3 min), effluent withdrawal (5 min) and idle time (5 min). Air bubbles were supplied through an air sparger at the
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.04.017
X. Yu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 202–206
203
Fig. 1. Adsorption tests under different initial ammonium concentrations (biomass = 11.9 g/L, 30 8C, 160 rpm).
bottom of the reactors by an air pump with a superficial air velocity of 2.95 cm/s. The volume exchange ratio (volume of liquid withdrawn to the total working volume) is 0.75. The reactor was performed at 30 1 8C and at varying organic loading rates (OLR) of 0.7–3.5 kg COD/(m3 d). The dissolved oxygen and pH of liquid phase were around 7.5 mg/L and 7.1, respectively. The seeding sludge was collected from the reflux sludge stream in a municipal wastewater treatment plant in Shanghai.
(ICP-AES, P-4010, Hitachi Limited, Japan). The amounts of NH4+ absorbed from the aqueous solution were expressed as adsorption per unit mass of the granule sludge (q) as follows: q¼
ðC 0 C e ÞV m
(1)
where C0 is the initial NH4+-N concentration in solution (mg/L), Ce is the equilibrium NH4+-N concentration in solution (mg/L), V is the solution volume (L) and m is the granule biomass (gVSS).
2.2. Batch adsorption experiments 3. Results and discussion Adsorption experiments were carried out in 250 mL flasks filled with solutions of NH4Cl solution and 0.1 M Tris–HCl buffer (pH 7). The flasks were shaken in a shaker at 160 rpm for 4 h at 30 8C. 2.2.1. Effects of initial ammonium concentration Adsorption tests with solutions of NH4+-N of 50 mg/L, 100 mg/L, 200 mg/L, 300 mg/L, 400 mg/L, 600 mg/L, 1000 mg/L, 2000 mg/L and 4000 mg/L were conducted with 11.9 g/L of volatile suspended solids (VSS) aerobic granule biomass. 2.2.2. Effects of biomass concentration Adsorption tests with solutions of NH4+-N of 300 mg/L were performed at 3.4 g/L, 6.8 g/L, 11.9 g/L, 17 g/L and 25.5 g/L of VSS of granule biomass. 2.2.3. Contributions of biological role on NH4+ adsorption Mature granules were collected from the SBR and were pretreated according to protocol by [15,18,19] to inactivate their biological activities. In brief, the aerobic granules were placed in oven for drying at 105 8C for 2 h, in 75% w/w ethanol for 2 h, or in 3% w/w H2O2 for 2 h. After pretreatment, granules released <5 mg NH4+-N/L to deionized water in 5 h, assuring that the ammonium later detected from the aerobic granules were from adsorption tests. After adsorption experiments the granules or sludge samples were centrifuged at 10,000 rpm for 2 min. Then the concentrations of NH4+-N, Ca2+, Mg2+, Na+ and K+ in the supernatants were determined.
3.1. Batch adsorption tests Fig. 1a shows the time course of concentrations of NH4+ in bulk solutions and the ultimate adsorption quantities at different initial NH4+ concentrations. Adsorption equilibrium was reached in 30 min of testing from the present tests (50–4000 mg/L), much faster than the longer time needed (60 min) as reported by Bassin et al. [15] at lower initial NH4+ concentrations (5–70 mg/L). Adsorption quantity was increased with initial NH4+ concentrations from 50 to 1000 mg/L (Fig. 1b). Above 1000 mg-N/L of NH4+ the adsorption quantity reached a plateau value with 24.5 mg/g VSS. Effects of biomass concentration on NH4+ adsorption were shown in Fig. 2. With 300 mg-N/L of NH4+, equilibrium was reached in 30 min of testing (Fig. 2a). The adsorption quantity was increased with increasing VSS of biomass, while the specific adsorption capacities were respectively 2.07, 2.21, 1.62, 1.77 and 1.68 mg NH4+-N/gVSS at 3.4, 6.8, 11.9, 17 and 25.5 g/L of VSS of granule biomass (Fig. 2b). One-way analysis of variance indicated that there is no significance difference between these specific adsorption capacities so the present granule yielded an average adsorption capacity of 1.87 mg/g VSS with initial 300 mg-N/L of NH4+. This value is close to the value reported by Bassin et al. [15]. The above results suggest that specific equilibrium adsorption capacities are depending not only on the initial NH4+ concentrations, but also on the biomass dosed in the tests. 3.2. Adsorption isotherm and kinetics
2.3. Chemical analysis Ammonium concentration was analyzed according to Standard Methods [20]. Concentration of Ca2+, Mg2+, Na+ and K+ was analyzed by Inductively Coupled Plasma-Atomic Emission Spectroscopy
In the present study the Freundlich isotherm was used for data fitting since the Langmuir model failed to correlate the experimental data. The Freundlich isotherm (Eq. (2)) assumes a heterogeneous surface with a non-uniform distribution of heat
X. Yu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 202–206
204
Fig. 2. Adsorption tests under different initial ammonium concentrations (initial NH4+ concentration = 300 mg/L, 30 8C, 160 rpm).
of adsorption over the surface and binding sites are not equivalent and/or independent [21]. 1=n
q ¼ K F Ce
(2)
where KF and n are parameters accounting for the adsorption capacity and adsorption intensity of the adsorbent, respectively [22]. The Freundlich model correlates well the adsorption data with r2 = 0.91 (Fig. 3), giving 1/n < 2. Restated, the present granules can be regarded as relatively effective adsorbent. The adsorption of NH4+ onto granule surface may be achieved by multi-layer physical adsorption mechanism, particularly at high NH4+ levels. Pseudo-second-order model was used to investigate the adsorption process [23]. The pseudo-second-order model is determined by the following Eq. (3). dqt ¼ ks ðqe qt Þ2 dt
(3)
where ks is the pseudo-second-order rate constant (g/(mgmin)), qt is ammonium adsorption at time (mg/g), and qe is the equilibrium adsorption capacity (mg/g). Pseudo-second-order kinetics fit the adsorption kinetic data at r2 > 0.97 (Fig. 4). The calculated qe values (data not shown) were consistent with the experimental results in Fig. 1.
Fig. 3. Freundlich isotherm model fitting with experimental data (initial ammonia concentration 100–1000 mg/L, biomass = 11.9 gVSS/L, 30 8C, 160 rpm).
3.3. Adsorption mechanisms The granules inactivated using 75% ethanol or 3% hydrogen peroxide had similar adsorption capacities (1.5 mg/g or 1.8 mg/g) with the original granules (1.6 mg/g). However, the pre-dried granules had negligible adsorption ability (Fig. 5). Hence, aerobic granules adsorb NH4+ based on non-biological mechanisms. Adsorption process onto the granular and porous adsorbent is a sequential progression including bulk diffusion, surface diffusion, pore diffusion and adsorption on the solid surface [24]. The pseudo-second-order model correlates all testing data inferring that the chemical adsorption was the rate-limiting step [25]. Thus we can conjecture that the capability of aerobic granules to adsorb NH4+ is by surface reactions between active sites and the ions. The dried granules shrank in size to only 20% of their original size, leading to the very low surface area available for adsorption, correlating with the almost zero NH4+ adsorption by the pre-dried granules. Ning et al. [26] also claimed that physical–chemical process dominates biosorption process on 2,4-DCP. Ion exchange was proposed to be the dominating adsorption mechanisms for NH4+ onto natural zeolites [21] or on biomass [27]. Bassin et al. [15] also concluded that ion exchange process
Fig. 4. Ammonium adsorption onto inactivated and original granules (initial ammonium concentration = 300 mg/L, biomass concentration = 6.8 gVSS/L, 30 8C, 160 rpm).
X. Yu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 202–206
205
Fig. 5. Pseudo-second order kinetic fitting. (Left) Biomass = 11.9 g/L; (right) initial NH4+ concentration = 300 mg/L, 30 8C, 160 rpm.
principally contributed to NH4+ on their granules at low NH4+ concentrations (<80 mg/L). The contents of Ca2+, Mg2+, Na+ and K+ in the present adsorbed suspension were measured and the ion equivalences were calculated (Fig. 6). The extents of ion exchange accounted for 64% and 79% of adsorbed NH4+ at initial ammonium of 200 mg/L and 300 mg/L, respectively. At 1000 mg-N/L of NH4+, extent of ion exchange accounted for only 23% of the total adsorption. At initial ammonium of 2000 mg/L and 4000 mg/L, extent of ion exchange fell to only 9% of the whole adsorption. Hence, ion exchange controls the adsorption process at <300 mg/L regime; however, physical adsorption contributed most of the quantity of adsorbed NH4+ at higher concentrations. The present experimental finding suggests that the aerobic granules can be applied as adsorbents for enhanced treatment of high-levels ammonium-containing wastewaters. Particularly when treating wastewaters of >1000 mg-N/L of NH4+ the multilayer, physical adsorption is the predominant mechanism for NH4+ removal. This observation suggests the need to reexamine whether the high efficiencies of nitrification/denitrification reactions noted by aerobic granules were overestimated. Additionally, the conventional aerobic granules reactor with internal recycling to NH4+-desorption unit may be a promising technology to handle wastewaters of both high levels of NH4+ and chemical oxygen demands (COD).
4. Conclusions Aerobic granules were tested on their adsorption capacities with initial NH4+ concentrations from 50 to 4000 mg-N/L. The equilibrium adsorption capacity was increased with increasing NH4+ concentration, and reached 24.5 mg/g VSS at >1000 mg-N/L of NH4+. The Freundlich isotherm fit the equilibrium adsorption data and the pseudo-second order model correlated the kinetic data. The aerobic granules are an effective adsorbent to NH4+ while the surface reactions controlled the adsorption kinetic. The surface reactions were in non-biological origin. At <300 mg-N/L of NH4+ regime ion exchange controls the adsorption process. At >1000 mg-N/L of NH4+ the dominating mechanism is multilayer, physical adsorption. The count in nitrification/denitrification by aerobic granules may be overestimated by ignoring the contribution of NH4+ adsorption. Physical adsorption mechanism provides the potential to use aerobic granules as a recyclable adsorbent for NH4+ from high-strength wastewaters.
Acknowledgement This work is partially supported by project NSFC No. 51278128.
References
Fig. 6. The equivalent ion amounts released under different initial ammonium concentrations (biomass = 11.9 g/L, 30 8C, 160 rpm).
[1] Filali A, Manas A, Mercade M, Bessiere Y, Biscans B, Sperandio M. Stability and performance of two GSBR operated in alternative anoxic/aerobic or anaerobic/ aerobic conditions for nutrient removal. Biochem Eng J 2012;67:10–9. [2] Tamrat M, Costa C, Marquez MC. Biological treatment of leachate from solid wastes: kinetic study and simulation. Biochem Eng J 2012;67:46–51. [3] Castens DJ, Rozich AF. Analysis of batch nitrification using substrate inhibition kinetics. Biotechnol Bioeng 1986;28:461–5. [4] Rozic M, Cerjan-Stefanovic S, Kurajica S, Vancina V, Hodzic E. Ammoniacal nitrogen removal from water by treatment with clays and zeolites. Water Res 2000;34:3675–81. [5] Teng HS. Porous materials of better properties and functions: preparation and application. J Taiwan Inst Chem Engrs 2012;43:491–2. [6] Gupta N, Kushwaha AK, Chattopadhyaya MC. Adsorption studies of cationic dyes onto Ashoka (Saraca asoca) leaf powder. J Taiwan Inst Chem Engrs 2012;43:604–13. [7] Fan H, Yang JS, Gao TG, Yuan HL. Removal of a low-molecular basic dye (Azure Blue) from aqueous solutions by a native biomass of a newly isolated Cladosporium sp.: kinetics, equilibrium and biosorption simulation. J Taiwan Inst Chem Engrs 2012;43:386–92. [8] Arslan A, Veli S. Zeolite 13X for adsorption of ammonium ions from aqueous solutions and hen slaughterhouse wastewaters. J Taiwan Inst Chem Engrs 2012;43:393–8. [9] Adav SS, Lee DJ, Show KY, Tay JH. Aerobic granular sludge: recent advances. Biotechnol Adv 2008;26:411–23.
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[10] Lee DJ, Chen YY, Show KY, Whiteley CG, Tay JH. Advances in aerobic granule formation and granule stability in the course of storage and reactor operation. Biotechnol Adv 2012;28:919–34. [11] Liu YQ, Moy MYP, Tay JH. COD removal and nitrification of low-strength domestic wastewater in aerobic granular sludge sequencing batch reactors. Enzyme Microb Technol 2007;42:23–8. [12] Bassin JP, Kleerebezem R, Dezotti M, van Loosdrecht MCM. Simultaneous nitrogen and phosphate removal in aerobic granular sludge operated at different temperatures. Water Res 2012;46:3805–16. [13] Tsuneda S, Oglwara M, Ejiri Y, Hirata A. High-rate nitrification using aerobic granular sludge. Water Sci Technol 2006;53:147–54. [14] Wei YJ, Ji M, Li RY, Qin FF. Organic and nitrogen removal from landfill leachate in aerobic granular sludge sequencing batch reactors. Waste Manage 2012;32:448–55. [15] Bassin JP, Pronk M, Kraan R, Kleerebezem R, van Loosdrecht MCM. Ammonium adsorption in aerobic granular sludge, activated sludge and anammox granules. Water Res 2011;45:5257–65. [16] Nielsen PH. Adsorption of ammonium to activated sludge. Water Res 1996;30:762–4. [17] Gabarro J, Ganigue R, Gich F, Ruscalleda M, Balaguer MD, Colprim J. Effect of temperature on AOB activity of a partial nitritation SBR treating landfill leachate with extremely high nitrogen concentration. Bioresour Technol 2012;126:283–9.
[18] Madigan MT, Martinko JM, Dunlap PV, Clark DP. Brock biology of microorganisms, vol. 2. San Francisco, USA: Pearson/Benjamin Cummings; 2009. [19] Prescott LM, Harley J, Klein D. Microbiology. 5th ed. New York, NY: McGrawHill Science/Engineering/Math; 2002. [20] APHA. Standard methods for the examination of water and wastewater. 20th ed. Washington, DC, USA: American Public Health Association; 1998. [21] Widiastuti N, Wu H, Ang HM, Zhang D. Removal of ammonium from greywater using natural zeolite. Desalination 2011;277:15–23. [22] Sheng GP, Zhang ML, Yu HQ. Characterization of adsorption properties of extracellular polymeric substances (EPS) extracted from sludge. Colloids Surf B 2008;62:83–90. [23] Ho Y, McKay G. Pseudo-second order model for sorption processes. Process Biochem 1999;34:451–65. [24] Badruzzaman M. Mass transport scaling and the role of silica on arsenic adsorption onto porous iron oxide (hydroxide). Arizona, USA: Arizona State University; 2005. [25] Sun F, Sun WL, Sun HM, Ni JR. Biosorption behavior and mechanism of beryllium from aqueous solution by aerobic granule. Chem Eng J 2011;172:783–91. [26] Ning Z, Kennedy K, Fernandes L. Biosorption of 2, 4-dichlorophenol by live and chemically inactivated anaerobic granules. Water Res 1996;30:2039–44. [27] Lin Y, Bassin JJ, van Loosdrecht MCM. The contribution of exopolysaccharides induced struvites accumulation to ammonium adsorption in aerobic granular sludge. Water Res 2012;46:986–92.
Contents lists available at SciVerse ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Short communication
Adsorption of ammonium by aerobic granules under high ammonium levels Xiaonan Yu a, Chunli Wan a, Zhongfang Lei a,b, Xiang Liu a, Yi Zhang a, Duu-Jong Lee a,c,*, Joo-Hwa Tay a a
Department of Environmental Science and Engineering, Fudan University, 220 Handan Road, Shanghai 200433, China Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan c Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 November 2012 Received in revised form 20 April 2013 Accepted 22 April 2013 Available online 29 May 2013
High-strength ammonium (NH4+) wastewaters are difficult to be treated biologically. Aerobic granules are regarded as a promising technology for NH4+ removal from wastewaters. Adsorption using aerobic granules in 50–4000 mg NH4+-N/L wastewaters was studied at 30 8C. Adsorption capacity was increased with 50–1000 mg NH4+-N/L and reached plateau of 24.5 mg NH4+-N/g-volatile suspended solids at >2000 mg NH4+-N/L. Ion exchange mechanism controls adsorption at <300 mg NH4+-N/L and physisorption mechanism dominates the process at >1000 mg NH4+-N/L. The biomass of aerobic granules presents a recyclable adsorbent for NH4+ from high-strength wastewaters. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Aerobic granules Ammonium adsorption High concentration
1. Introduction High-strength ammonium (NH4+) wastewaters are generated by petroleum refineries, landfills, livestock farms and other sources [1,2]. The excessive NH4+ in the wastewater is toxic to aquatic species and can lead to distortion of natural nutrient cycles and eutrophication of surface waters [3,4]. Adsorption is a unit operation suitable to remove toxic substances from wastewaters [5–8]. Aerobic granulation is drawing increasing global interest in a quest for an efficient and innovative technology in wastewater treatment. Developed less than two decades ago, extensive research work on aerobic granulation has been reported [9,10]. Aerobic granules were claimed for a promising biological technology for nitrogen removal of NH4+-containing wastewaters [11,12]. The aerobic granules were tested for treating NH4+containing wastewaters with 360–1400 mg NH4+-N/L. Tsuneda et al. [13] achieved >80% NH4+ removal using aerobic granules for wastewater of 500 mg NH4+-N/L. At NH4+ level of 1000 mg NH4+-N/ L, Wei et al. [14] achieved around 50% NH4+ removal using aerobic granules. Restated, the NH4+ removal rate was decreased with increasing NH4+ concentration. Additionally, the aerobic granules were noted to be able to adsorb 10–25% of wastewaters of 30 mg NH4+-N/L and the nitrification efficiency by aerobic granules would
* Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. Tel.: +886 22362 5632; fax: +886 22362 3040. E-mail addresses: [email protected], [email protected] (D.-J. Lee).
be overestimated if the adsorption contribution was neglected [15,16]. Certain wastewaters can have very high concentrations of NH4+ [17]. At low concentration regimes the quantities of NH4+ adsorbed were noted to increase with increasing influent concentration of NH4+ [15]. No data are present on the adsorption capacity of aerobic granules with high-strength NH4+ wastewaters. The objective of present study is to investigate ammonium adsorption process onto aerobic granules at NH4-N levels up to 4000 mg/L. Mechanisms of adsorption of NH4+ on aerobic granule surface were discussed since comprehensive understanding of the mechanisms of adsorption can help accurate prediction of system performance under different wastewater compositions and operational parameters. 2. Materials and methods 2.1. Cultivation of aerobic granules Synthetic wastewater used had sodium acetate and sodium propionate as carbon source. Other nutrients contained in the synthetic wastewater were: peptone 400 mg/L, yeast exact 250 mg/L, NH4Cl 3.74 mM, KH2PO4 4.85 mM, CaCl2 0.27 mM, MgSO4 0.21 mM, FeSO4 0.13 mM, NaHCO3 0.15 mM. Aerobic granules were cultivated in column-type sequencing batch reactors (SBR) with internal diameter 6 cm and working volume of 2.2 L. The SBR was operated in a time sequence of feeding (5 min), aeration (195 min), settling (gradually decreasing from 30 min to 3 min), effluent withdrawal (5 min) and idle time (5 min). Air bubbles were supplied through an air sparger at the
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.04.017
X. Yu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 202–206
203
Fig. 1. Adsorption tests under different initial ammonium concentrations (biomass = 11.9 g/L, 30 8C, 160 rpm).
bottom of the reactors by an air pump with a superficial air velocity of 2.95 cm/s. The volume exchange ratio (volume of liquid withdrawn to the total working volume) is 0.75. The reactor was performed at 30 1 8C and at varying organic loading rates (OLR) of 0.7–3.5 kg COD/(m3 d). The dissolved oxygen and pH of liquid phase were around 7.5 mg/L and 7.1, respectively. The seeding sludge was collected from the reflux sludge stream in a municipal wastewater treatment plant in Shanghai.
(ICP-AES, P-4010, Hitachi Limited, Japan). The amounts of NH4+ absorbed from the aqueous solution were expressed as adsorption per unit mass of the granule sludge (q) as follows: q¼
ðC 0 C e ÞV m
(1)
where C0 is the initial NH4+-N concentration in solution (mg/L), Ce is the equilibrium NH4+-N concentration in solution (mg/L), V is the solution volume (L) and m is the granule biomass (gVSS).
2.2. Batch adsorption experiments 3. Results and discussion Adsorption experiments were carried out in 250 mL flasks filled with solutions of NH4Cl solution and 0.1 M Tris–HCl buffer (pH 7). The flasks were shaken in a shaker at 160 rpm for 4 h at 30 8C. 2.2.1. Effects of initial ammonium concentration Adsorption tests with solutions of NH4+-N of 50 mg/L, 100 mg/L, 200 mg/L, 300 mg/L, 400 mg/L, 600 mg/L, 1000 mg/L, 2000 mg/L and 4000 mg/L were conducted with 11.9 g/L of volatile suspended solids (VSS) aerobic granule biomass. 2.2.2. Effects of biomass concentration Adsorption tests with solutions of NH4+-N of 300 mg/L were performed at 3.4 g/L, 6.8 g/L, 11.9 g/L, 17 g/L and 25.5 g/L of VSS of granule biomass. 2.2.3. Contributions of biological role on NH4+ adsorption Mature granules were collected from the SBR and were pretreated according to protocol by [15,18,19] to inactivate their biological activities. In brief, the aerobic granules were placed in oven for drying at 105 8C for 2 h, in 75% w/w ethanol for 2 h, or in 3% w/w H2O2 for 2 h. After pretreatment, granules released <5 mg NH4+-N/L to deionized water in 5 h, assuring that the ammonium later detected from the aerobic granules were from adsorption tests. After adsorption experiments the granules or sludge samples were centrifuged at 10,000 rpm for 2 min. Then the concentrations of NH4+-N, Ca2+, Mg2+, Na+ and K+ in the supernatants were determined.
3.1. Batch adsorption tests Fig. 1a shows the time course of concentrations of NH4+ in bulk solutions and the ultimate adsorption quantities at different initial NH4+ concentrations. Adsorption equilibrium was reached in 30 min of testing from the present tests (50–4000 mg/L), much faster than the longer time needed (60 min) as reported by Bassin et al. [15] at lower initial NH4+ concentrations (5–70 mg/L). Adsorption quantity was increased with initial NH4+ concentrations from 50 to 1000 mg/L (Fig. 1b). Above 1000 mg-N/L of NH4+ the adsorption quantity reached a plateau value with 24.5 mg/g VSS. Effects of biomass concentration on NH4+ adsorption were shown in Fig. 2. With 300 mg-N/L of NH4+, equilibrium was reached in 30 min of testing (Fig. 2a). The adsorption quantity was increased with increasing VSS of biomass, while the specific adsorption capacities were respectively 2.07, 2.21, 1.62, 1.77 and 1.68 mg NH4+-N/gVSS at 3.4, 6.8, 11.9, 17 and 25.5 g/L of VSS of granule biomass (Fig. 2b). One-way analysis of variance indicated that there is no significance difference between these specific adsorption capacities so the present granule yielded an average adsorption capacity of 1.87 mg/g VSS with initial 300 mg-N/L of NH4+. This value is close to the value reported by Bassin et al. [15]. The above results suggest that specific equilibrium adsorption capacities are depending not only on the initial NH4+ concentrations, but also on the biomass dosed in the tests. 3.2. Adsorption isotherm and kinetics
2.3. Chemical analysis Ammonium concentration was analyzed according to Standard Methods [20]. Concentration of Ca2+, Mg2+, Na+ and K+ was analyzed by Inductively Coupled Plasma-Atomic Emission Spectroscopy
In the present study the Freundlich isotherm was used for data fitting since the Langmuir model failed to correlate the experimental data. The Freundlich isotherm (Eq. (2)) assumes a heterogeneous surface with a non-uniform distribution of heat
X. Yu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 202–206
204
Fig. 2. Adsorption tests under different initial ammonium concentrations (initial NH4+ concentration = 300 mg/L, 30 8C, 160 rpm).
of adsorption over the surface and binding sites are not equivalent and/or independent [21]. 1=n
q ¼ K F Ce
(2)
where KF and n are parameters accounting for the adsorption capacity and adsorption intensity of the adsorbent, respectively [22]. The Freundlich model correlates well the adsorption data with r2 = 0.91 (Fig. 3), giving 1/n < 2. Restated, the present granules can be regarded as relatively effective adsorbent. The adsorption of NH4+ onto granule surface may be achieved by multi-layer physical adsorption mechanism, particularly at high NH4+ levels. Pseudo-second-order model was used to investigate the adsorption process [23]. The pseudo-second-order model is determined by the following Eq. (3). dqt ¼ ks ðqe qt Þ2 dt
(3)
where ks is the pseudo-second-order rate constant (g/(mgmin)), qt is ammonium adsorption at time (mg/g), and qe is the equilibrium adsorption capacity (mg/g). Pseudo-second-order kinetics fit the adsorption kinetic data at r2 > 0.97 (Fig. 4). The calculated qe values (data not shown) were consistent with the experimental results in Fig. 1.
Fig. 3. Freundlich isotherm model fitting with experimental data (initial ammonia concentration 100–1000 mg/L, biomass = 11.9 gVSS/L, 30 8C, 160 rpm).
3.3. Adsorption mechanisms The granules inactivated using 75% ethanol or 3% hydrogen peroxide had similar adsorption capacities (1.5 mg/g or 1.8 mg/g) with the original granules (1.6 mg/g). However, the pre-dried granules had negligible adsorption ability (Fig. 5). Hence, aerobic granules adsorb NH4+ based on non-biological mechanisms. Adsorption process onto the granular and porous adsorbent is a sequential progression including bulk diffusion, surface diffusion, pore diffusion and adsorption on the solid surface [24]. The pseudo-second-order model correlates all testing data inferring that the chemical adsorption was the rate-limiting step [25]. Thus we can conjecture that the capability of aerobic granules to adsorb NH4+ is by surface reactions between active sites and the ions. The dried granules shrank in size to only 20% of their original size, leading to the very low surface area available for adsorption, correlating with the almost zero NH4+ adsorption by the pre-dried granules. Ning et al. [26] also claimed that physical–chemical process dominates biosorption process on 2,4-DCP. Ion exchange was proposed to be the dominating adsorption mechanisms for NH4+ onto natural zeolites [21] or on biomass [27]. Bassin et al. [15] also concluded that ion exchange process
Fig. 4. Ammonium adsorption onto inactivated and original granules (initial ammonium concentration = 300 mg/L, biomass concentration = 6.8 gVSS/L, 30 8C, 160 rpm).
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Fig. 5. Pseudo-second order kinetic fitting. (Left) Biomass = 11.9 g/L; (right) initial NH4+ concentration = 300 mg/L, 30 8C, 160 rpm.
principally contributed to NH4+ on their granules at low NH4+ concentrations (<80 mg/L). The contents of Ca2+, Mg2+, Na+ and K+ in the present adsorbed suspension were measured and the ion equivalences were calculated (Fig. 6). The extents of ion exchange accounted for 64% and 79% of adsorbed NH4+ at initial ammonium of 200 mg/L and 300 mg/L, respectively. At 1000 mg-N/L of NH4+, extent of ion exchange accounted for only 23% of the total adsorption. At initial ammonium of 2000 mg/L and 4000 mg/L, extent of ion exchange fell to only 9% of the whole adsorption. Hence, ion exchange controls the adsorption process at <300 mg/L regime; however, physical adsorption contributed most of the quantity of adsorbed NH4+ at higher concentrations. The present experimental finding suggests that the aerobic granules can be applied as adsorbents for enhanced treatment of high-levels ammonium-containing wastewaters. Particularly when treating wastewaters of >1000 mg-N/L of NH4+ the multilayer, physical adsorption is the predominant mechanism for NH4+ removal. This observation suggests the need to reexamine whether the high efficiencies of nitrification/denitrification reactions noted by aerobic granules were overestimated. Additionally, the conventional aerobic granules reactor with internal recycling to NH4+-desorption unit may be a promising technology to handle wastewaters of both high levels of NH4+ and chemical oxygen demands (COD).
4. Conclusions Aerobic granules were tested on their adsorption capacities with initial NH4+ concentrations from 50 to 4000 mg-N/L. The equilibrium adsorption capacity was increased with increasing NH4+ concentration, and reached 24.5 mg/g VSS at >1000 mg-N/L of NH4+. The Freundlich isotherm fit the equilibrium adsorption data and the pseudo-second order model correlated the kinetic data. The aerobic granules are an effective adsorbent to NH4+ while the surface reactions controlled the adsorption kinetic. The surface reactions were in non-biological origin. At <300 mg-N/L of NH4+ regime ion exchange controls the adsorption process. At >1000 mg-N/L of NH4+ the dominating mechanism is multilayer, physical adsorption. The count in nitrification/denitrification by aerobic granules may be overestimated by ignoring the contribution of NH4+ adsorption. Physical adsorption mechanism provides the potential to use aerobic granules as a recyclable adsorbent for NH4+ from high-strength wastewaters.
Acknowledgement This work is partially supported by project NSFC No. 51278128.
References
Fig. 6. The equivalent ion amounts released under different initial ammonium concentrations (biomass = 11.9 g/L, 30 8C, 160 rpm).
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