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The removal of phosphate from wastewater using anoxic reduction of iron ore in the rotating reactor
Biochemical Engineering Journal 46 (2009) 223–226
Contents lists available at ScienceDirect
Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej
Short communication
The removal of phosphate from wastewater using anoxic reduction of iron ore in the rotating reactor Chenghong Guo a , Viktor Stabnikov b , Shengli Kuang a , Volodymyr Ivanov a,∗ a b
School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Institute of Municipal Activity, National Aviation University, Kiev, Ukraine
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 12 May 2009 Accepted 12 May 2009
Keywords: Iron ore Reject water Phosphorus removal Rotating reactor
a b s t r a c t The removal of phosphorus from reject water, which is the liquid fraction produced after dewatering of anaerobically digested activated sludge on the municipal wastewater treatment plants (MWWTPs), can significantly reduce the phosphorus load to the main stream of the MWWTPs. Ferric or ferrous reagents can be used for this removal but the significantly cheaper option could be the production of ferrous reagent using bioreduction of iron ore. The removal of phosphorus from reject water using anoxic bioreduction of iron ore was studied in the rotating reactor, which was selected to avoid the clogging of the pores between iron ore particles. The highest phosphorus removal rate from reject water in the rotating reactor, i.e. the parameter which can be used in the design of the continuous process, was 25 mg P/L day. Significant role in the iron ore bioreduction is playing the formation of the fine particles from initially loaded coarse particles of iron ore during rotation of the reactor. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The discharge of phosphorus from municipal wastewater treatment plants (MWWTPs) to aquatic systems can cause their eutrophication. In order to diminish the negative effect of phosphorus discharged to environment, the emission limits of phosphorus in effluent from MWWTPs are becoming more stringent and vary from 0.05 to 2 mg P/L [1]. One of the most effective ways to decrease phosphorus concentration in the effluent of MWWTP could be removal of phosphorus from reject water (other terms are return liquor, sewage sludge filtrate or centrifugate), which is the liquid fraction produced after dewatering of anaerobically digested activated sludge. The concentration of phosphorus in municipal wastewater is from 5 to 20 mg/L; meanwhile the concentration of phosphorus in reject water is up to 130 mg/L [2]. Although the flow of reject water is only 2% of the raw sewage flow [3], it contributes from 10 to 80% of phosphorous load on the activated sludge tank [4]. Therefore, the removal of phosphorus from reject water can significantly reduce the phosphorus load to the main stream of the MWWTPs. Major form of phosphorus in reject water is orthophosphate.
∗ Corresponding author at: Nanyang Technological University, School of Civil and Environmental Engineering, Blk N1, CEE, NTU, 50 Nanyang Avenue, Singapore. Tel.: +65 6790 6934; fax: +65 6792 1650. E-mail address: [email protected] (V. Ivanov). 1369-703X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2009.05.011
Magnesium hydroxide, Mg(OH)2 with sodium hydroxide, NaOH, are commonly used for the chemical removal of phosphate from wastewater as struvite, MgNH4 PO4 ·6H2 O [5,6] but the crystallization rate of struvite is low. Another, lower cost and faster process is precipitation of phosphate as ferric phosphate using ferric chloride, FeCl3 , or ferrous sulfate, FeSO4 [5,6]. However, the disadvantage of this chemical precipitation of phosphate is the high cost of the iron salts, about US$ 1015 per ton of Fe in FeCl3 [5,6]. Alternative method for the removal of phosphate is BioIronTech process, where ferrous ions, precipitating phosphate, are producing from cheap iron ore/iron hydroxide reduced by iron-reducing bacteria (IRB) under anaerobic conditions [1,7–10]. It was shown that the BioIronTech process can be used for improvement of sludge quality [7], treatment of fat-containing wastewater [11], removal of xenobiotics such as diphenylamine, m-cresol, 2,4-dichlorphenol and p-phenylphenol [1]. The cost of Fe of iron ore is approximately US$ 140 per ton of Fe in iron ore, which is 7.2 times cheaper than Fe of iron salt [12]. The feasibility of phosphorus removal from reject water using anoxic bioreduction of iron ore has been demonstrated [12]. However, that study was performed in the static reactors, where diffusion was the major mechanism of mass transfer for Fe2+ and HPO4 2− ions and the rate of ferrous production was associated with size of the iron ore particles and their specific surface. The application of the fixed bed reactor, fluidized bed reactor, or rotating reactor instead of static reactor a priori will increase the mass transfer rate and promote the iron ore bioreduction rate. The rotating reactor is most preferable system due to the high risk
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of clogging caused by ferrous phosphate and ferrous hydroxide precipitation in the fixed bed reactor and high energy expenses in the fluidized bed reactor due to the high density of iron ore (4045 ± 3 g/L for 2.4 mm size particles). Therefore, the aim of this research was to study the production of ferrous and the removal of phosphorus from reject water using bioreduction of iron ore in the rotating reactor. 2. Materials and methods 2.1. Composition of reject water, anaerobic sludge and iron ore The anaerobic sludge, which was used as a source of IRB [13], and reject water were collected from a local municipal wastewater treatment plant in Singapore. The content of dissolved TOC, TSS, VSS, dissolved phosphate, PO4 3− , total concentration of Fe2+ ions in reject water and anaerobic sludge, mg/L, were 525 ± 43 and 1900 ± 152; 550 ± 38 and 5240 ± 298; 470 ± 33 and 4000 ± 160; 87 ± 7 and 80 ± 8; 5.5 ± 0.6 and 167 ± 12, respectively. pH of reject water and anaerobic sludge were 7.6 and 7.8, respectively. The iron ore, with iron content 60% (w/w), was supplied from China. Major mineral was hematite (Fe2 O3 ). The porosity of the iron ore particles ranged from 45 to 55% (v/v) depending on the particle size. Three sets of iron ore particles with the sizes of 0.6 ± 0.1 mm, 2.4 ± 0.4 mm, and 7.6 ± 1.9 mm, were used in the experiments. Iron ore particles of different sizes were produced by sieving the crushed iron ore.
Fig. 1. The production of ferrous in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
to the standard method [14]. pH was measured by the EUTECH pH meter (Cyberscan PCD 6500) after calibration with buffer solutions. All analytical determinations were performed at least in triplicate and mean values ± standard deviations are shown. 3. Results and discussion 3.1. Fe(II) production
2.2. Experimental setup 35 ◦ C
The experiment was performed in batch culture at for 15 days. Iron ore, 400 g, reject water, 1.8 L, and anaerobic sludge, 200 mL (10%, v/v), were added in each 2 L reactor. The reactors were rotated on VELP® OVERHEAD MIXER rotax 6.8 (Velp Scientifica) at 5 rpm/min. Abiotic controls were used to evaluate the phosphate adsorption onto the surface of iron ore particles in the rotating reactors. For abiotic control, mixture of reject water with anaerobic sludge was autoclaved at 121 ◦ C for 15 min, while the other conditions were the same as in experiment. All experiments were performed with iron ore particles of three different sizes. Each bioreactor was purged with nitrogen gas for 5 min and sealed up to ensure anaerobic conditions. To evaluate the production of fine iron ore particles due to mechanism impulse, the iron ore particles were washed several times with distilled water. Iron ore particles of three different sizes, 160 g, were placed in the 1 L plastic bottles with 800 mL of distilled water and put in the rotating reactor (5 rpm/min) for 14 days. The wellmixed samples were taken out from the bottles every 2 days to measure the concentration of total suspended solids (TSS) using filtration of the liquid samples through 0.45 m membrane filter. The particles size of the produced fine iron ore particles were measured by the particle size analyzer (Malvern Instrument).
The ferrous production was observed during the cultivation process (Fig. 1). The highest ferrous concentration in the rotating reactor was 490 mg/L. The highest ferrous production rate of 44.5 mg/L day was in the reactor with the iron ore particle size of 7.6 mm. However, it was shown previously that the ferrous production rate in the static reactors statistically reliably and positively correlated with the specific surface area (surface-to-volume ratio) of iron ore particles and negatively with their size, so that the highest rate of ferrous production was in the static reactor with the smallest particles, 0.6 mm [12]. This contradiction between rotating and static reactors can be explained by the formation of fine iron ore powder in the rotating reactor. The formation of these fine particles of iron ore was the biggest for largest particles (Fig. 2). It was probably due to the strongest mechanical impulses and friction between largest iron ore particles. Formed fine particles of iron ore with high specific surface can be bioreduced faster than big particles of iron ore. Maximum and average fine iron ore particles production rates were 3.66 and 0.78 g/L day; 8.61 and 3.87 g/L day; 15.25 and 7.64 g/L day in the rotating reactors with iron ore particles with size 0.6, 2.4 and
2.3. Chemical analysis Total suspended solids (TSS) and volatile suspended solids (VSS) were determined in the well-mixed sample by standard methods [14]. The concentrations of total ferrous ions, dissolved phosphate, and dissolved total organic carbon (TOC) were measured in the well-mixed samples from the reactors. The concentration of total ferrous ions was measured according to a modified phenanthroline method [1]. The modified vanadomolybdophosphoric acid colorimetric method was used to determine the concentration of dissolved orthophosphate [14]. The concentration of TOC was measured by TOC analyzer C-Vcsh (Shimadzu, Kyoto, Japan) according
Fig. 2. The production of fine iron ore particles in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
C. Guo et al. / Biochemical Engineering Journal 46 (2009) 223–226
225
Fig. 3. Concentration of dissolved phosphorus in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
Fig. 4. Concentrations of dissolved TOC in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
7.6 mm, respectively. The pH of reject water, 7.6, was suitable for iron reducing bacteria activity and the pH of reject water did not change significantly during experiment. So, final pH after 15 days of cultivation was 8.0. The ferrous ions concentration in the abiotic control was not increased.
WWTP effluent, thus preventing eutrophication of aquatic systems and deterioration of water quality. Another benefit of this technology could be recovery of phosphorus as fertilizer. By our calculations, the annual supplies of phosphorus with reject water in Singapore with 4 million population is 5700 tons. Considering that world population, serving with centralized sewage treatment, is 1.5 billions, and the average domestic water consumption is the same as in Singapore, the annual recovery of phosphorus from reject water of MWWTPs in the world could be approximately 2 million tons.
3.2. Phosphorus removal The concentration of dissolved phosphorus decreased in all rotating reactors (Fig. 3). Maximum of the phosphorus removal efficiency was 97% in the reactor with mean sizes of iron ore particles of 7.6 mm. This efficiency is comparable with the conventional chemical precipitation of phosphate as struvite or ferric phosphate [5,6]. Similar efficiency of phosphate precipitation was also in the experiments where ferric hydroxide was used as source of Fe(III) for iron-reducing bacteria [1,9]. Final concentration of phosphate in the rotating reactor with mean sizes of iron ore particles of 7.6 mm was 1.7 mg/L, which is lower than permitted level for effluent of MWWTP in many countries. The process with largest iron ore particles 7.6 mm in the rotating reactors was characterized with the highest phosphorus removal rate, 25 mg P/L day. It is known that chemical removal of phosphate is accompanied with adsorption of phosphate ions and organic dissolved phosphorus on ferric hydroxide flocs [15]. Significant phosphorus removal due to adsorption of phosphorus on surface of iron ore particles in the static reactors was shown in the previous study [12]. However, removal of phosphate by adsorption in the rotating reactors was not associated with size of initially loaded iron ore particles and was 65% of total phosphorus removal in all rotating reactors. This effect also can be hypothetically explained by the formation of fine iron ore particles during mechanical mixing of iron ore in the rotating reactor. In this case, the percentage of phosphate removed by adsorption was associated with the size distribution of the formed fine particles. This distribution was approximately same in all rotating reactors and the size of these fine particles was within the range from 0.2 to 2.0 m (the data are not shown). The size of iron ore particles and anaerobic conditions are major factors affecting the ferrous production and phosphate removal by the proposed method. By our data for MWWTPs in Singapore, reject water contributed 4% of the total flow of raw sewage. However, the phosphate of reject water formed up to 40% of phosphorus load in the MWWTP. Therefore, removal of phosphate from reject water prior its recycling into aerobic tanks could significantly reduce the phosphorus load to the main stream of MWWTP and diminish concentration in
3.3. TOC and removal The changes of TOC concentration are shown in Fig. 4. The removal efficiencies of TOC were 31, 41 and 63% after 15 days of cultivation with iron ore particles of 0.6, 2.4 and 7.6 mm, respectively. The reject water contains a lot of branched volatile fatty acids, mainly isobutyric acid (C4 H8 O2 ) and isocaproic acids (C6 H12 O2 ) [1]. In case when TOC components in the reject water are only isobutyric and isocaproic acids in ratio of 1:1, empirical formula of VFA of reject water is C5 H10 O2 ; and theoretical molar ratio of oxidized TOC/reduced Fe should be 0.19: 26Fe3+ + C5 H10 O2 + 26OH− → 26Fe2+ + 5CO2 + 18H2 O
(1)
The experimental values of the molar ratio of TOC removal/Fe2+ production were 10.3, 4.0, and 2.8 for the reactors with the loaded iron ore particles 0.6, 2.4 and 7.6 mm, respectively. These values were higher than theoretical value 0.19. So, the removal of organics from reject water was due to degradation by other anaerobic bacteria. 4. Conclusions The highest phosphorus removal rate from reject water using bioreduction of iron ore was 25 mg P/L day and phosphorus removal efficiency was 97% in the rotating reactor with the iron ore particles 7.6 mm. The rates of iron ore bioreduction and phosphate precipitation in the rotating reactor were associated with the production of fine iron ore particles from initially loaded coarse iron ore particles. References [1] V. Ivanov, V. Stabnikov, W.Q. Zhuang, J.H. Tay, S.T.L. Tay, Phosphate removal from the returned liquor of municipal wastewater treatment plant using ironreducing bacteria, J. Appl. Microbiol. 98 (2005) 1152–1161.
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[2] A.R. Pitman, Management of biological nutrient removal plant sludges—change the paradigms, Water Res. 33 (1999) 1141–1146. [3] H.M. Janus, H.F. Van der Roset, Don’t reject the idea of treating reject water, Water Sci. Technol. 35 (1997) 27–34. [4] M.C.M. Van Loosdrecht, S. Salem, Biological treatment of sludge digester liquids, Water Sci. Technol. 53 (2006) 11–20. [5] L.E. De-Bashan, Y. Bashan, Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997–2003), Water Res. 38 (2004) 4222–4246. [6] G.K. Morse, S.W. Brett, J.A. Guy, J.N. Lester, Review: phosphorus removal and recovery technologies, Sci. Total Environ. 212 (1998) 69–81. [7] V. Ivanov, J.Y. Wang, V. Stabnikov, X. Zikun, J.H. Tay, Improvement of sludge quality by iron-reducing bacteria, J. Residuals Sci. Technol. 1 (2004) 165–168. [8] V. Ivanov, J.Y. Wang, O. Stabnikova, V. Krasinko, V. Stabnikov, S.T.L. Tay, J.H. Tay, Iron-mediated removal of ammonium from strong nitrogenous wastewater from food processing, Water Sci. Technol. 49 (2004) 421–431. [9] V.P. Stabnikov, S.T.L. Tay, J.H. Tay, V.N. Ivanov, Effect of iron hydroxide on phosphate removal during anaerobic digestion of activated sludge, Appl. Biochem. Microbiol. 40 (2004) 376–380.
[10] J.H. Tay, S.T.L. Tay, V. Ivanov, O. Stabnikova, J.Y. Wang, Compositions and methods for the treatment of wastewater and other waste, US Patent 7,393,452 (2008). Date of grant July 1, 2008, Date of filing April 11, 2003. [11] V. Ivanov, E.V. Stabnikova, V.P. Stabnikov, I.S. Kim, A. Zubair, Effects of iron compound on the treatment of fat-containing wastewater, Appl. Biochem. Microbiol. 38 (3) (2002) 255–258. [12] V. Ivanov, S.-L. Kuang, V. Stabnikov, C.-H. Guo, The removal of phosphorus from reject water in a municipal wastewater treatment plant using iron ore, J. Chem. Technol. Biotechnol. 84 (2009) 78–82. [13] J.L. Nielsen, S. Juretschko, M. Wagner, P.H. Nielsen, Abundance and phylogenetic affiliation of iron reducers in activated sludge as assessed by fluorescence in situ hybridization and microautoradiography, Appl. Environ. Microbiol. 68 (2002) 4629–4636. [14] Standard Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Association/Water Environment Federation, Washington, DC, 1998. [15] I. Takacs, S. Murthy, S. Smith, M. McGrath, Chemical phosphorus removal to extremely low levels: experience of two plants in the Washington, DC area, Water Sci. Technol. 53 (2006) 21–28.
Contents lists available at ScienceDirect
Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej
Short communication
The removal of phosphate from wastewater using anoxic reduction of iron ore in the rotating reactor Chenghong Guo a , Viktor Stabnikov b , Shengli Kuang a , Volodymyr Ivanov a,∗ a b
School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Institute of Municipal Activity, National Aviation University, Kiev, Ukraine
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 12 May 2009 Accepted 12 May 2009
Keywords: Iron ore Reject water Phosphorus removal Rotating reactor
a b s t r a c t The removal of phosphorus from reject water, which is the liquid fraction produced after dewatering of anaerobically digested activated sludge on the municipal wastewater treatment plants (MWWTPs), can significantly reduce the phosphorus load to the main stream of the MWWTPs. Ferric or ferrous reagents can be used for this removal but the significantly cheaper option could be the production of ferrous reagent using bioreduction of iron ore. The removal of phosphorus from reject water using anoxic bioreduction of iron ore was studied in the rotating reactor, which was selected to avoid the clogging of the pores between iron ore particles. The highest phosphorus removal rate from reject water in the rotating reactor, i.e. the parameter which can be used in the design of the continuous process, was 25 mg P/L day. Significant role in the iron ore bioreduction is playing the formation of the fine particles from initially loaded coarse particles of iron ore during rotation of the reactor. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The discharge of phosphorus from municipal wastewater treatment plants (MWWTPs) to aquatic systems can cause their eutrophication. In order to diminish the negative effect of phosphorus discharged to environment, the emission limits of phosphorus in effluent from MWWTPs are becoming more stringent and vary from 0.05 to 2 mg P/L [1]. One of the most effective ways to decrease phosphorus concentration in the effluent of MWWTP could be removal of phosphorus from reject water (other terms are return liquor, sewage sludge filtrate or centrifugate), which is the liquid fraction produced after dewatering of anaerobically digested activated sludge. The concentration of phosphorus in municipal wastewater is from 5 to 20 mg/L; meanwhile the concentration of phosphorus in reject water is up to 130 mg/L [2]. Although the flow of reject water is only 2% of the raw sewage flow [3], it contributes from 10 to 80% of phosphorous load on the activated sludge tank [4]. Therefore, the removal of phosphorus from reject water can significantly reduce the phosphorus load to the main stream of the MWWTPs. Major form of phosphorus in reject water is orthophosphate.
∗ Corresponding author at: Nanyang Technological University, School of Civil and Environmental Engineering, Blk N1, CEE, NTU, 50 Nanyang Avenue, Singapore. Tel.: +65 6790 6934; fax: +65 6792 1650. E-mail address: [email protected] (V. Ivanov). 1369-703X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2009.05.011
Magnesium hydroxide, Mg(OH)2 with sodium hydroxide, NaOH, are commonly used for the chemical removal of phosphate from wastewater as struvite, MgNH4 PO4 ·6H2 O [5,6] but the crystallization rate of struvite is low. Another, lower cost and faster process is precipitation of phosphate as ferric phosphate using ferric chloride, FeCl3 , or ferrous sulfate, FeSO4 [5,6]. However, the disadvantage of this chemical precipitation of phosphate is the high cost of the iron salts, about US$ 1015 per ton of Fe in FeCl3 [5,6]. Alternative method for the removal of phosphate is BioIronTech process, where ferrous ions, precipitating phosphate, are producing from cheap iron ore/iron hydroxide reduced by iron-reducing bacteria (IRB) under anaerobic conditions [1,7–10]. It was shown that the BioIronTech process can be used for improvement of sludge quality [7], treatment of fat-containing wastewater [11], removal of xenobiotics such as diphenylamine, m-cresol, 2,4-dichlorphenol and p-phenylphenol [1]. The cost of Fe of iron ore is approximately US$ 140 per ton of Fe in iron ore, which is 7.2 times cheaper than Fe of iron salt [12]. The feasibility of phosphorus removal from reject water using anoxic bioreduction of iron ore has been demonstrated [12]. However, that study was performed in the static reactors, where diffusion was the major mechanism of mass transfer for Fe2+ and HPO4 2− ions and the rate of ferrous production was associated with size of the iron ore particles and their specific surface. The application of the fixed bed reactor, fluidized bed reactor, or rotating reactor instead of static reactor a priori will increase the mass transfer rate and promote the iron ore bioreduction rate. The rotating reactor is most preferable system due to the high risk
224
C. Guo et al. / Biochemical Engineering Journal 46 (2009) 223–226
of clogging caused by ferrous phosphate and ferrous hydroxide precipitation in the fixed bed reactor and high energy expenses in the fluidized bed reactor due to the high density of iron ore (4045 ± 3 g/L for 2.4 mm size particles). Therefore, the aim of this research was to study the production of ferrous and the removal of phosphorus from reject water using bioreduction of iron ore in the rotating reactor. 2. Materials and methods 2.1. Composition of reject water, anaerobic sludge and iron ore The anaerobic sludge, which was used as a source of IRB [13], and reject water were collected from a local municipal wastewater treatment plant in Singapore. The content of dissolved TOC, TSS, VSS, dissolved phosphate, PO4 3− , total concentration of Fe2+ ions in reject water and anaerobic sludge, mg/L, were 525 ± 43 and 1900 ± 152; 550 ± 38 and 5240 ± 298; 470 ± 33 and 4000 ± 160; 87 ± 7 and 80 ± 8; 5.5 ± 0.6 and 167 ± 12, respectively. pH of reject water and anaerobic sludge were 7.6 and 7.8, respectively. The iron ore, with iron content 60% (w/w), was supplied from China. Major mineral was hematite (Fe2 O3 ). The porosity of the iron ore particles ranged from 45 to 55% (v/v) depending on the particle size. Three sets of iron ore particles with the sizes of 0.6 ± 0.1 mm, 2.4 ± 0.4 mm, and 7.6 ± 1.9 mm, were used in the experiments. Iron ore particles of different sizes were produced by sieving the crushed iron ore.
Fig. 1. The production of ferrous in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
to the standard method [14]. pH was measured by the EUTECH pH meter (Cyberscan PCD 6500) after calibration with buffer solutions. All analytical determinations were performed at least in triplicate and mean values ± standard deviations are shown. 3. Results and discussion 3.1. Fe(II) production
2.2. Experimental setup 35 ◦ C
The experiment was performed in batch culture at for 15 days. Iron ore, 400 g, reject water, 1.8 L, and anaerobic sludge, 200 mL (10%, v/v), were added in each 2 L reactor. The reactors were rotated on VELP® OVERHEAD MIXER rotax 6.8 (Velp Scientifica) at 5 rpm/min. Abiotic controls were used to evaluate the phosphate adsorption onto the surface of iron ore particles in the rotating reactors. For abiotic control, mixture of reject water with anaerobic sludge was autoclaved at 121 ◦ C for 15 min, while the other conditions were the same as in experiment. All experiments were performed with iron ore particles of three different sizes. Each bioreactor was purged with nitrogen gas for 5 min and sealed up to ensure anaerobic conditions. To evaluate the production of fine iron ore particles due to mechanism impulse, the iron ore particles were washed several times with distilled water. Iron ore particles of three different sizes, 160 g, were placed in the 1 L plastic bottles with 800 mL of distilled water and put in the rotating reactor (5 rpm/min) for 14 days. The wellmixed samples were taken out from the bottles every 2 days to measure the concentration of total suspended solids (TSS) using filtration of the liquid samples through 0.45 m membrane filter. The particles size of the produced fine iron ore particles were measured by the particle size analyzer (Malvern Instrument).
The ferrous production was observed during the cultivation process (Fig. 1). The highest ferrous concentration in the rotating reactor was 490 mg/L. The highest ferrous production rate of 44.5 mg/L day was in the reactor with the iron ore particle size of 7.6 mm. However, it was shown previously that the ferrous production rate in the static reactors statistically reliably and positively correlated with the specific surface area (surface-to-volume ratio) of iron ore particles and negatively with their size, so that the highest rate of ferrous production was in the static reactor with the smallest particles, 0.6 mm [12]. This contradiction between rotating and static reactors can be explained by the formation of fine iron ore powder in the rotating reactor. The formation of these fine particles of iron ore was the biggest for largest particles (Fig. 2). It was probably due to the strongest mechanical impulses and friction between largest iron ore particles. Formed fine particles of iron ore with high specific surface can be bioreduced faster than big particles of iron ore. Maximum and average fine iron ore particles production rates were 3.66 and 0.78 g/L day; 8.61 and 3.87 g/L day; 15.25 and 7.64 g/L day in the rotating reactors with iron ore particles with size 0.6, 2.4 and
2.3. Chemical analysis Total suspended solids (TSS) and volatile suspended solids (VSS) were determined in the well-mixed sample by standard methods [14]. The concentrations of total ferrous ions, dissolved phosphate, and dissolved total organic carbon (TOC) were measured in the well-mixed samples from the reactors. The concentration of total ferrous ions was measured according to a modified phenanthroline method [1]. The modified vanadomolybdophosphoric acid colorimetric method was used to determine the concentration of dissolved orthophosphate [14]. The concentration of TOC was measured by TOC analyzer C-Vcsh (Shimadzu, Kyoto, Japan) according
Fig. 2. The production of fine iron ore particles in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
C. Guo et al. / Biochemical Engineering Journal 46 (2009) 223–226
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Fig. 3. Concentration of dissolved phosphorus in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
Fig. 4. Concentrations of dissolved TOC in the rotating reactors. The sizes of iron ore particles were as follows: () 0.6 mm; () 2.4 mm; () 7.6 mm. Error bar indicates standard deviation.
7.6 mm, respectively. The pH of reject water, 7.6, was suitable for iron reducing bacteria activity and the pH of reject water did not change significantly during experiment. So, final pH after 15 days of cultivation was 8.0. The ferrous ions concentration in the abiotic control was not increased.
WWTP effluent, thus preventing eutrophication of aquatic systems and deterioration of water quality. Another benefit of this technology could be recovery of phosphorus as fertilizer. By our calculations, the annual supplies of phosphorus with reject water in Singapore with 4 million population is 5700 tons. Considering that world population, serving with centralized sewage treatment, is 1.5 billions, and the average domestic water consumption is the same as in Singapore, the annual recovery of phosphorus from reject water of MWWTPs in the world could be approximately 2 million tons.
3.2. Phosphorus removal The concentration of dissolved phosphorus decreased in all rotating reactors (Fig. 3). Maximum of the phosphorus removal efficiency was 97% in the reactor with mean sizes of iron ore particles of 7.6 mm. This efficiency is comparable with the conventional chemical precipitation of phosphate as struvite or ferric phosphate [5,6]. Similar efficiency of phosphate precipitation was also in the experiments where ferric hydroxide was used as source of Fe(III) for iron-reducing bacteria [1,9]. Final concentration of phosphate in the rotating reactor with mean sizes of iron ore particles of 7.6 mm was 1.7 mg/L, which is lower than permitted level for effluent of MWWTP in many countries. The process with largest iron ore particles 7.6 mm in the rotating reactors was characterized with the highest phosphorus removal rate, 25 mg P/L day. It is known that chemical removal of phosphate is accompanied with adsorption of phosphate ions and organic dissolved phosphorus on ferric hydroxide flocs [15]. Significant phosphorus removal due to adsorption of phosphorus on surface of iron ore particles in the static reactors was shown in the previous study [12]. However, removal of phosphate by adsorption in the rotating reactors was not associated with size of initially loaded iron ore particles and was 65% of total phosphorus removal in all rotating reactors. This effect also can be hypothetically explained by the formation of fine iron ore particles during mechanical mixing of iron ore in the rotating reactor. In this case, the percentage of phosphate removed by adsorption was associated with the size distribution of the formed fine particles. This distribution was approximately same in all rotating reactors and the size of these fine particles was within the range from 0.2 to 2.0 m (the data are not shown). The size of iron ore particles and anaerobic conditions are major factors affecting the ferrous production and phosphate removal by the proposed method. By our data for MWWTPs in Singapore, reject water contributed 4% of the total flow of raw sewage. However, the phosphate of reject water formed up to 40% of phosphorus load in the MWWTP. Therefore, removal of phosphate from reject water prior its recycling into aerobic tanks could significantly reduce the phosphorus load to the main stream of MWWTP and diminish concentration in
3.3. TOC and removal The changes of TOC concentration are shown in Fig. 4. The removal efficiencies of TOC were 31, 41 and 63% after 15 days of cultivation with iron ore particles of 0.6, 2.4 and 7.6 mm, respectively. The reject water contains a lot of branched volatile fatty acids, mainly isobutyric acid (C4 H8 O2 ) and isocaproic acids (C6 H12 O2 ) [1]. In case when TOC components in the reject water are only isobutyric and isocaproic acids in ratio of 1:1, empirical formula of VFA of reject water is C5 H10 O2 ; and theoretical molar ratio of oxidized TOC/reduced Fe should be 0.19: 26Fe3+ + C5 H10 O2 + 26OH− → 26Fe2+ + 5CO2 + 18H2 O
(1)
The experimental values of the molar ratio of TOC removal/Fe2+ production were 10.3, 4.0, and 2.8 for the reactors with the loaded iron ore particles 0.6, 2.4 and 7.6 mm, respectively. These values were higher than theoretical value 0.19. So, the removal of organics from reject water was due to degradation by other anaerobic bacteria. 4. Conclusions The highest phosphorus removal rate from reject water using bioreduction of iron ore was 25 mg P/L day and phosphorus removal efficiency was 97% in the rotating reactor with the iron ore particles 7.6 mm. The rates of iron ore bioreduction and phosphate precipitation in the rotating reactor were associated with the production of fine iron ore particles from initially loaded coarse iron ore particles. References [1] V. Ivanov, V. Stabnikov, W.Q. Zhuang, J.H. Tay, S.T.L. Tay, Phosphate removal from the returned liquor of municipal wastewater treatment plant using ironreducing bacteria, J. Appl. Microbiol. 98 (2005) 1152–1161.
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