- Email: [email protected]
Phosphorus removal using ferric–calcium complex as precipitant: Parameters optimization and phosphorus-recycling potential
Chemical Engineering Journal 268 (2015) 230–235
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Phosphorus removal using ferric–calcium complex as precipitant: Parameters optimization and phosphorus-recycling potential Qiu Lin, Zheng Ping ⇑, Zhang Meng, Yu Xiaoqing, Abbas Ghulam Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China
h i g h l i g h t s A novel technology based on the complex of ferric and calcium salts was developed. The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1. The optimized parameters were: Fe/Ca = 2.4:1, A/P = 1.5:1, pH = 7, FMS = 100 rpm. The novel technology cut down the dosage by 36% compared with the traditional one. The novel technology increased the phosphorus (P2O5) content of precipitate to 30%.
a r t i c l e
i n f o
Article history: Received 7 November 2014 Received in revised form 28 December 2014 Accepted 29 December 2014 Available online 13 January 2015 Keywords: Phosphorus removal Ferric–calcium complex Parameters optimization Phosphorus recycling
a b s t r a c t Pollution control and phosphorus recycling have become one of focuses in the field of wastewater treatment. Both the ferric and calcium salts are effective chemical reagents for the phosphorus removal from wastewater. Based on the complex of these two chemical reagents, a novel technology was developed. The results showed that ferric and calcium had an antagonistic effect at Fe/Ca molar ratio of 1:10–1:1, but a synergistic effect at Fe/Ca molar ratio of 1:1–10:1. The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1. When the wastewater with concentration of 100 mg P/L was treated using the ferric–calcium complex as precipitant, the optimized parameters were: Fe/Ca = 2.4:1, ferric–calcium complex/phosphorous (A/P) = 1.5:1, pH = 7, fast mixing speed (FMS) = 100 rpm. Compared with the traditional technology using ferric salt as sole precipitant, the phosphorus removal technology using ferric–calcium complex cut down the dosage by 35.53%, leading to a low cost of $ 3.69/kg P; and it increased the phosphorus (P2O5) content of precipitate up to 29.77%, indicating a good potential for recycling. The phosphorus removal technology using ferric–calcium complex as precipitant provides a prospective alternative for the removal and recycling of phosphorus in wastewaters. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The world is facing a threat of eutrophication nowadays. Phosphorus is one of the major factors that cause eutrophication [1–3]. As reported by Carpenter et al. [4], a water body would take on eutrophication when the total phosphorus concentration exceeded 0.015 mg/L. The concentration of phosphorus in different wastewater varies from several milligrams per liter (domestic wastewater) to a few hundred milligrams per liter (livestock wastewater) and even up to tens of thousands milligrams per liter (industrial wastewater). Since a large amount of phosphorus originates from wastewaters, the removal of phosphorus from wastewaters has become an indispensable measure to control eutrophication. ⇑ Corresponding author. E-mail address: [email protected] (P. Zheng). http://dx.doi.org/10.1016/j.cej.2014.12.107 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
On the other hand, phosphorus is a finite resource on the Earth [5,6], and it may be depleted over the next 100 years [7–9]. The analysis by the authors in Fig. 1 [9], indicated 2033 as the peak year for phosphorus production. According to previous research [10], globally 21.4 Mt elemental phosphorus from rock phosphate was consumed of which 6.1 Mt can technologically be recycled from waterways and wastewater in 2009. So, it is of great importance to recycle phosphorus from wastewater. The development of efficient technology for phosphorus removal and recovery will promote both the control of water pollution and the recycling of phosphorus resource. Currently, phosphorus removal technologies mainly include chemical, biological and bio-chemical methods [11–13]. Among them, chemical technology is widely used to achieve low residual phosphorus concentrations so as to meet the discharge standard. Ferric and calcium salts are two popular precipitants for phosphorus
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
removal. Phosphorus removal using ferric salt is an efficient method, but it has some drawbacks such as high treatment cost, low effluent pH and difficult recycling of phosphorus from precipitates [14,15]. In contrast, phosphorus removal using calcium salt is a cost-effective option for phosphorus recycling from precipitates, but it leads to an unacceptably high effluent pH of over 10 [16,17]. Based on the properties of two precipitants, the phosphorus removal using ferric–calcium complex was hypothesized to be a good technology. The objective of this work is to set up a novel technology for phosphorus removal and recycling; to optimize the parameters of the technology using ferric–calcium complex and to observe the recycling potential of produced precipitates. 2. Experimental methods The operational conditions influencing the phosphorus removal process were investigated in laboratory experiments. Batch tests were carried out with synthetic wastewater. Synthetic wastewater was prepared with deionized water and stock solutions. Stock solution of dipotassium hydrogen orthophosphate (KH2PO4) was prepared from solid reagent (AR, Yonghua Chemical Technology (Jiangsu) Co., Ltd) with concentration of 10.0 g P/L. The concentration of working phosphate solution was 100 mg P/L. The concentrations of precipitants (calcium hydroxide (Ca(OH)2, AR, Sinopharm Chemical Reagent Co., Ltd.) and ferric sulfate (Fe2(SO4)3, AR, Xilong Chemical Co., Ltd.)) were 22.0 g Ca/L and 20.1 g Fe/L respectively which were directly used so as not to dilute the reaction system. All experiments were performed in 250 mL glass beakers using mechanical stirrers to homogenize the contents. With intensive stirring in the tests (examining the effects of Fe/Ca, dose, pH, fast mixing speed (FMS)), precipitants were added to 250 mL beakers containing 150 mL synthetic wastewater. The Fe/Ca ratio was set according to the reaction equation. The reactions are shown below.
Fe3þ þ 3OH ¼ FeðOHÞ3 ðsÞ Dr Gm ðT; FeðIIIÞÞ ¼ 124:92 kJ mol 5Ca2þ þ 7OH þ 3H2 PO4 ¼ Ca5 ðOHÞðPO4 Þ3
1
Dr Gm ðT; CaðIIÞÞ 1
¼ 319:06 kJ mol
The mixing intensity was tested by initial fast mixing for 1 min, then gentle mixing (at 50 r/min) for 15 min, and finally settling for 30 min. The pH value of reaction solution was regulated beforehand using
231
sodium hydroxide (NaOH, 1 mol/L, Hangzhou Xiaoshan Chemical Reagent Co., Ltd) or hydrochloric acid (HCl, 1 mol/L) and it was no longer regulated during or after reaction. All tests were run in triplicate. The supernatant was taken to determine the residual phosphorus concentration. The precipitate was centrifuged at 7000 r/min for 5 min and dried at 70 °C for 10 h. Phosphate was determined according to the ascorbic acid photometric method, with a detection limit of 10 lg P/L. The precipitate structure was observed by scanning electron microscope (SEM, Ultra 55 field emission scanning electron microscope, the German Corlzeis D company) and the elemental component was analyzed by energy dispersive X-ray spectroscopy (EDS, X-ray spectrometer EDS7429). At the sample surface, three points were chosen to determine elemental contents. The elemental contents of phosphorus were estimated using P (wt%) and P2O5 (wt%, Eq. (2.1)) where, MP2 O5 (142 g/mol) and M p (31 g/mol) are the formula weight of P2O5 and phosphorus, respectively
P2 O5 ¼ M P2 O5 =2M p P ¼ 2:29P
ð2:1Þ
3. Results and discussion 3.1. Effects of operation parameters on phosphorus removal 3.1.1. Effect of Fe/Ca molar ratio To determine the appropriate Fe/Ca molar ratio, batch tests were conducted to investigate the effect of Fe/Ca molar ratio on the phosphorus removal. Other conditions were: concentration = 100 mg P/ L, A/P = 1, pH = 7, FMS = 200 rpm. The precipitants were added to remove equivalent amount of phosphorus stoichiometrically. For instance, Fe/Ca = 4 meant that 0.8 mol ferric salt and 0.33 mol calcium salt were added to precipitate altogether 1 mol phosphorus. If the molar phosphorus removal using ferric–calcium complex as precipitant is higher than the sum using ferric and calcium salts, it is defined as synergistic effect. On the contrary, if the molar phosphorus removal using ferric–calcium complex as precipitant is less than the sum using ferric and calcium salts, it is defined as antagonistic effect. Using ferric and calcium salts respectively, the molar phosphorus removal was 60.88% and 54.65% in the experiments. So, their average 57.76% was taken as the critical value to judge synergistic or antagonistic effect. According to the data shown in Fig. 2, the effect of ferric–calcium complex on the phosphorus removal was antagonistic when the proportion of ferric salt in the ferric–cal-
Fig. 1. Peak phosphorus curve indicating a peak in production by 2033, derived from US Geological Survey and industry data. Source: [9].
232
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
cium complex was below 50%, while the effect was synergistic when the proportion of ferric salt was over 50%. For instance, the phosphorus removal was 39.54% at Fe/Ca ratio of 2:8, far lower than that of 60.88% and 54.65% when ferric and calcium salts were dosed respectively; while the phosphorus removal was up to 77% at Fe/Ca ratio of 7:3–8:2, far higher than that of 60.88% and 54.65%. Ferric and calcium salts showed the antagonistic effect on the phosphorus removal when ferric salt was used as a minor ingredient in the ferric–calcium complex (the proportion of removed phosphorus using ferric salt was below 50%). As the fraction of removed phosphorus using ferric salt increased from 0 to 0.2, the antagonistic effect was observed to enhance. However, the antagonistic effect weakened sharply as the molar fraction of removed phosphorus using ferric salt continued to increase from 0.2 to 0.5. Ferric and calcium salts displayed the synergistic effect on the phosphorus removal if ferric salt was used as a major ingredient in the ferric–calcium complex (the proportion of removed phosphorus using ferric salt was above 50%). As the fraction of removed phosphorus using ferric salt increased from 0.5 to 0.8 in the ferric– calcium complex, the synergistic effect strengthened with the phosphorus removal and it rose from 58.63% to 77.67%. With further increase of fraction of removed phosphorus using ferric salt to 1.0, the synergistic effect became weaker and phosphorus removal dropped from 77.67% to 60.88%. It was concluded that the phosphorus removal using ferric–calcium complex as precipitant was superior to that using ferric or calcium salt as precipitant when the Fe/Ca ratio was higher than 1:1. For Fe/Ca ratio of 4:1, the phosphorus removal reached the peak value. The optimal Fe/Ca molar ratio of 2.4:1 was recommended after both the removal efficiency and treatment cost was taken into account. 3.1.2. Effect of A/P molar ratio In the case of phosphorus removal, it has been reported that the practical dose of ferric or calcium salt was larger than the predicted values (stoichiometric requirements) [18–21]. Although the ferric– calcium complex was superior to ferric or calcium salt, the appropriate molar ratio of complex-to-phosphorus (A/P) need to be determined. Thus, batch tests were carried out to set up the relationship between the ferric–calcium complex dose and the phosphorus removal. Other conditions were: concentration = 100 mg P/L, Fe/Ca = 3, pH = 7, FMS = 200 rpm. The results were shown in Fig. 2, and they followed DoseResp equation (Eq. (3.1))
ð3:1Þ
As demonstrated in Fig. 3, the phosphorus removal increased quickly from 19.31% to 90.38% when the A/P molar ratio rose from 0.25 to 1.25. At molar ratios of 1.5 and above, a good phosphorus removal (>95%) was observed by the addition of ferric–calcium complex to wastewater. As the molar ratio rose further, however, the phosphorus removal leveled out. The A/P molar ratio of 2.17 was recommended to keep the residual phosphorus concentration lower than 0.5 mg/L (The national discharge standard in China). It was reported that the required Fe/P molar ratio was 2.57 using ferric salt as precipitant to achieve the residual phosphorus concentration lower than 0.5 mg/L [22]. If the case using ferric–calcium complex at A/P molar ratio of 2.17 was compared with that using ferric salt at Fe/P molar ratio of 2.57, the novel technology could cut down the ferric salt dose by 0.4 mol Fe/mol P. The Fe/ Ca molar ratio in the ferric–calcium complex was 2.4:1, the technology using ferric–calcium complex could further cut down the ferric salt dose by 0.44 mol Fe/mol P through the substitution of ferric salt by calcium salt. Altogether, the novel technology could cut down the ferric salt dosage by 31.53%. The cost of traditional phosphorus removal using ferric salt was $ 5.33/kg P (removed), while that of phosphorus removal using ferric–calcium complex was only $ 3.69/kg P (removed), thus saving the precipitant cost by 30.7%. 3.1.3. Effect of pH pH has a significant effect on phosphorus removal by changing the form of precipitant, which led to variations in precipitant for phosphorus removal [18]. Hence, the phosphorus removal at different pH values (from 3.0 to 10.0) were tested to explore the effect of pH on phosphorus removal using ferric–calcium complex. The other conditions were: concentration = 100 mg P/L, Fe/Ca = 3, A/ P = 1.5, FMS = 200 r/min. As shown in Fig. 4., when the pH rose from 3.0 to 9.0, the phosphorus removal decreased slightly from 96.73% to 93.41%, with a small drop of 3.32%. As pH value reached 10, the phosphorus removal further decreased to 86.08%, with a drop of 10.65%. So, high efficiency of phosphorus removal could be obtained in a wide pH range (from 3.0 to 9.0) using ferric–calcium complex as precipitant. It has been reported in the earlier studies [14,20,23–25] that the phosphorus removal using ferric or calcium salts as precipitants was significantly affected by pH and the optimal pH were 5 and 10–10.5, respectively. Since the phosphorus removal using ferric–calcium complex was almost independent of pH change, the
80
110
75
100
70
90
65 60
Phosphorus Removal,%
Phosphorus Removal,%
y ¼ 0:0773 þ ð0:9976 0:0773Þ=f1 þ 10½ð0:7044xÞ1:7328 g
R2 ¼ 0:9969:
57.76%
55 50 45 40 35 30 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Ca
x(Fe)
Fig. 2. Effect of Fe/Ca molar ratio on phosphorus removal.
Fe
80
discharge standard: 0.5 mg/L dosage:2.17
70 60 50 40
Fe/Ca = 3 pH = 7 FMS = 200rpm
30 20 10
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25
dosage ratio Fig. 3. Effect of A/P on phosphorus removal.
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
233
100 95 Phosphorus Removal,%
90 85 80 75
Fe/Ca = 3 A/P = 1.5 FMS = 200 rpm
70 65 60 55 50
2
3
4
5
6
7
8
9
10
11
350
400
pH Fig. 4. Effect of pH on phosphorus removal.
100 90 Phosphorus Removal,%
80 70 60 Fe/Ca = 3 A/P = 1.5 pH = 7
50 40 30 20 10 0
50
100
150
200
250
300
FMS, rpm Fig. 5. Effect of FMS on phosphorus removal.
novel technology could cut down the consumption of reagents for pH adjustment. 3.1.4. Effect of fast mixing speed Mixing intensity is a common parameter for chemical precipitation [18]. Fast mixing speed within a certain range can promote the phosphorus removal because it breaks the precipitant into small clusters of molecules and offers the larger surface area for phosphorus sorption. In order to determine the optimum value for fast mixing speed, phosphorus removal was tested using ferric–calcium complex at fast mixing speed from 50 to 300 rpm. Other conditions were: concentration = 100 mg P/L, Fe/Ca = 3, A/P = 1.5, pH = 7. As shown in Fig. 5, phosphorus removal ranged from 81.65% to 88.09% with the increase of fast mixing speed. The phosphorus removal efficiency was high at optimum fast mixing speed of 100 rpm. 3.2. Characteristics of precipitates using ferric–calcium complex as precipitant 3.2.1. Morphological characteristics of precipitates As displayed in Fig. 6 (a, b and c), the morphological characteristics of precipitates were observed at Fe/Ca molar ratios of 3:1, 4:1 and 1:4, respectively. It can be clearly seen from Fig. 6(a and b) that the particles were almost spherical and they agglomerated. In
Fig. 6. Effect of parameter optimization on precipitate morphology. (a) A/P = 1.5:1, pH = 7, FMS = 200 rpm, Fe/Ca = 3:1; (b) A/P = 1.5:1, pH = 7, FMS = 200 rpm, Fe/ Ca = 4:1; (c) A/P = 1.5:1, pH = 7, FMS = 200 rpm, Fe/Ca = 1:4.
contrast, the particles at Fe/Ca molar ratio of 3:1 were more spherical and larger than those at Fe/Ca molar ratio of 4:1, which are more suitable as a seed crystal and easier to recycle. On the other hand, the particles at Fe/Ca molar ratio of 1:4 were circular interweaving with each other like a honeycomb. In other words, the particles were spherical and they agglomerated when calcium salt served as minor ingredient in the precipitant. On the contrary, flat and elongated particles were formed when calcium salt served as major ingredient in the precipitant. It can be clearly seen from
234
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
Fig. 7. Result of EDS analysis (A/P = 1:1, Fe/Ca = 4:1).
Fig. 8. Elemental analysis of precipitate. (a) Novel technology; (b) traditional technology.
Fig. 6(a–c) that with the increase of ferric salts proportion, the spherical particles became smaller and they adopted threadlike structure which may cause poor settleability and inconvenience of recycling.
3.2.2. Components of precipitates The phosphorus content in the precipitate was determined using EDS to estimate the potential of phosphorus recycling from the precipitate. As shown in Figs. 7 and 8, the dominant elements in the precipitate were iron (42.81 wt%) and phosphorus (13.00 wt%), with traces of calcium (2.56 wt%) (Figs. 7 and 8(a). At Fe/Ca molar ratio of 4 and A/P molar ratio of 1.5, phosphorus accounted for 14.8 wt%, indicating a phosphorus recovery of 87.8% from the precipitate. In contrast, the phosphorus content was about 7.13 wt% in the precipitate using ferric salt as precipitant [26], leading to a phosphorus recovery of 62.4% (Fig. 8(b)). With respect to the phosphorus content and recovery in traditional precipitate using ferric salt, phosphorus content in precipitates using ferric–calcium complex were 6 wt% and 25.4%, respectively. The phosphorus content in the novel precipitates was 29.77% P2O5, which was close to the phosphorus content of high-grade phosphorite (30% P2O5) [27] and it was higher than the average content of phosphorite (16.95% P2O5). It has been reported [28] that the Fe–P precipitates using ferric salt were amorphous and their recovery was difficult. On the contrary, the Ca–P precipitates using calcium salt often form crystals and their recovery is convenient [29]. The Fe–Ca–P precipitates using ferric–calcium complex displayed the advantages of Fe–P and Ca–P precipitates, so the
technology shows great promise for phosphorus removal and recycling simultaneously.
4. Conclusions A novel technology for phosphorus removal using ferric–calcium complex as precipitant was developed and its phosphorus recycling potential was investigated. (1) The phosphorus removal using ferric–calcium complex was characterized. The effect of ferric and calcium salts was antagonistic at Fe/Ca molar ratio of 1:10–1:1 and it was synergistic at Fe/Ca molar ratio of 1:1–10:0. The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1. (2) The parameters for phosphorus removal using ferric–calcium complex were optimized as follows: Concentration of 100 mg P/L, Fe/Ca = 2.4:1, A/P = 1.5:1, pH = 7, FMS = 100 rpm. (3) The phosphorus content in the precipitate using ferric–calcium complex was increased. The phosphorus content in the Fe–Ca–P precipitate was about 13 wt% (29.77 wt% P2O5), which is 6% higher than that in the traditional Fe–P precipitate using ferric salt and is close to that in the highgrade phosphorite. 5. Future issues Although the technology for phosphorus removal using ferric– calcium complex as precipitant has great prospects, several issues need to be solved before its practical applications.
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
(1) The composition and distribution of the precipitates need to be researched so as to provide guidance on their reuse in agriculture, industry and so on. (2) There is usually a high organic fraction in the real wastewaters, the effect of organic on phosphorus removal as well as the recovery of desired form of P needs to be explored in future research works.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (51278457), National Key Technology R&D Program of China (2013BAD21B04) and Key Science and Technology Innovation Team Grant of Zhejiang (2013TDXX). References [1] J. Abell, D. Özkundakci, D. Hamilton, Nitrogen and phosphorus limitation of phytoplankton growth in New Zealand lakes: implications for eutrophication control, Ecosystems 13 (2010) 966–977. [2] M. Paterson, D. Schindler, R. Hecky, D. Findlay, K. Rondeau, Comment: Lake 227 shows clearly that controlling inputs of nitrogen will not reduce or prevent eutrophication of lakes, Limnol. Oceanogr. 56 (2011) 1545–1547. [3] D.W. Schindler, The dilemma of controlling cultural eutrophication of lakes, Proc. R. Soc. B: Biol. Sci. 279 (2012) 4322–4333. [4] S.R. Carpenter, E.M. Bennett, Reconsideration of the planetary boundary for phosphorus, Environ. Res. Lett. 6 (2011) 014009. [5] D.L. Childers, J. Corman, M. Edwards, J.J. Elser, Sustainability challenges of phosphorus and food: solutions from closing the human phosphorus cycle, BioScience 61 (2011) 117–124. [6] A.E. Ulrich, E. Frossard, On the history of a reoccurring concept: phosphorus scarcity, Sci. Total Environ. 490 (2014) 694–707. [7] E.H. Oelkers, E. Valsami-Jones, Phosphate mineral reactivity and global sustainability, Elements 4 (2008) 83–87. [8] K. Ashley, D. Cordell, D. Mavinic, A brief history of phosphorus: from the philosopher’s stone to nutrient recovery and reuse, Chemosphere 84 (2011) 737–746. [9] D. Cordell, J.-O. Drangert, S. White, The story of phosphorus: global food security and food for thought, Global Environ. Change 19 (2009) 292–305. [10] R. Koppelaar, H. Weikard, Assessing phosphate rock depletion and phosphorus recycling options, Global Environ. Change 23 (2013) 1454–1466. [11] 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.
235
[12] S.K. Ramasahayam, L. Guzman, G. Gunawan, T. Viswanathan, A comprehensive review of phosphorus removal technologies and processes, J. Macromol. Sci. Part A 51 (2014) 538–545. [13] A. Oehmen, P.C. Lemos, G. Carvalho, Z. Yuan, J. Keller, L.L. Blackall, M.A. Reis, Advances in enhanced biological phosphorus removal: from micro to macro scale, Water Res. 41 (2007) 2271–2300. [14] J. Thistleton, T.-A. Berry, P. Pearce, S. Parsons, Mechanisms of chemical phosphorus removal II: iron (III) salts, Process Saf. Environ. Protection 80 (2002) 265–269. [15] T. Jianguo, L. Jiemei, Calculation of chemical P removal, Water Wastewater Eng. 9 (2000) 006. [16] G. Morse, S. Brett, J. Guy, J. Lester, Review: phosphorus removal and recovery technologies, Sci. Total Environ. 212 (1998) 69–81. [17] G. Schön, N. Jardin, Biological and Chemical Phosphorus Elimination, 2nd ed., Biotechnology Set, 2001 (pp. 285–319). [18] S.A. Parsons, J.A. Smith, Phosphorus removal and recovery from municipal wastewaters, Elements 4 (2008) 109–112. [19] A. Szabó, I. Takács, S. Murthy, G. Daigger, I. Licskó, S. Smith, Significance of design and operational variables in chemical phosphorus removal, Water Environ. Res. 80 (2008) 407–416. [20] 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. [21] M. Zhang, K. Zheng, J. Jin, X. Yu, L. Qiu, S. Ding, H. Lu, J. Cai, P. Zheng, Effects of Fe(II)/P ratio and pH on phosphorus removal by ferrous salt and approach to mechanisms, Sep. Purif. Technol. 118 (2013) 801–805. [22] M. Zhang, X.-Q. Yu, L. Qiu, H.-F. Lu, P. Zheng, Process optimization for phosphorus removal with ferric salt by response surface methodology [J], J. Zhejiang Univ. (Agric. Life Sci.) 5 (2012) 015. [23] J. Thistleton, T. Clark, P. Pearce, S. Parsons, Mechanisms of chemical phosphorus removal: 1 – iron (II) salts, Process Saf. Environ. Protection 79 (2001) 339–344. [24] I. Takcs, S. Murthy, P. Fairlamb, Chemical phosphorus removal model based on equilibrium chemistry, Water Sci. Technol. 52 (2005) 549–555. [25] E. Valsami-Jones, Phosphorus in Environmental Technologies: Principles and Applications, IWA Publishing, 2004. [26] M. Zhang, P. Zheng, G. Abbas, X. Chen, Partitionable-space enhanced coagulation (PEC) reactor and its working mechanism: a new prospective chemical technology for phosphorus pollution control, Water Res. 49 (2014) 426–433. [27] S.J. Van Kauwenbergh, M. Stewart, R. Mikkelsen, World reserves of phosphate rock. . . a dynamic and unfolding story, Better Crops With Plant Food 97 (2013) 18-20 [28] E. Pierri, E. Dalas, The precipitation of ferric phosphate on porous polymer, Colloids Surf. A 139 (1998) 335–340. [29] T. Zhang, L. Ding, H. Ren, Z. Guo, J. Tan, Thermodynamic modeling of ferric phosphate precipitation for phosphorus removal and recovery from wastewater, J. Hazard. Mater. 176 (2010) 444–450.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Phosphorus removal using ferric–calcium complex as precipitant: Parameters optimization and phosphorus-recycling potential Qiu Lin, Zheng Ping ⇑, Zhang Meng, Yu Xiaoqing, Abbas Ghulam Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China
h i g h l i g h t s A novel technology based on the complex of ferric and calcium salts was developed. The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1. The optimized parameters were: Fe/Ca = 2.4:1, A/P = 1.5:1, pH = 7, FMS = 100 rpm. The novel technology cut down the dosage by 36% compared with the traditional one. The novel technology increased the phosphorus (P2O5) content of precipitate to 30%.
a r t i c l e
i n f o
Article history: Received 7 November 2014 Received in revised form 28 December 2014 Accepted 29 December 2014 Available online 13 January 2015 Keywords: Phosphorus removal Ferric–calcium complex Parameters optimization Phosphorus recycling
a b s t r a c t Pollution control and phosphorus recycling have become one of focuses in the field of wastewater treatment. Both the ferric and calcium salts are effective chemical reagents for the phosphorus removal from wastewater. Based on the complex of these two chemical reagents, a novel technology was developed. The results showed that ferric and calcium had an antagonistic effect at Fe/Ca molar ratio of 1:10–1:1, but a synergistic effect at Fe/Ca molar ratio of 1:1–10:1. The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1. When the wastewater with concentration of 100 mg P/L was treated using the ferric–calcium complex as precipitant, the optimized parameters were: Fe/Ca = 2.4:1, ferric–calcium complex/phosphorous (A/P) = 1.5:1, pH = 7, fast mixing speed (FMS) = 100 rpm. Compared with the traditional technology using ferric salt as sole precipitant, the phosphorus removal technology using ferric–calcium complex cut down the dosage by 35.53%, leading to a low cost of $ 3.69/kg P; and it increased the phosphorus (P2O5) content of precipitate up to 29.77%, indicating a good potential for recycling. The phosphorus removal technology using ferric–calcium complex as precipitant provides a prospective alternative for the removal and recycling of phosphorus in wastewaters. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The world is facing a threat of eutrophication nowadays. Phosphorus is one of the major factors that cause eutrophication [1–3]. As reported by Carpenter et al. [4], a water body would take on eutrophication when the total phosphorus concentration exceeded 0.015 mg/L. The concentration of phosphorus in different wastewater varies from several milligrams per liter (domestic wastewater) to a few hundred milligrams per liter (livestock wastewater) and even up to tens of thousands milligrams per liter (industrial wastewater). Since a large amount of phosphorus originates from wastewaters, the removal of phosphorus from wastewaters has become an indispensable measure to control eutrophication. ⇑ Corresponding author. E-mail address: [email protected] (P. Zheng). http://dx.doi.org/10.1016/j.cej.2014.12.107 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
On the other hand, phosphorus is a finite resource on the Earth [5,6], and it may be depleted over the next 100 years [7–9]. The analysis by the authors in Fig. 1 [9], indicated 2033 as the peak year for phosphorus production. According to previous research [10], globally 21.4 Mt elemental phosphorus from rock phosphate was consumed of which 6.1 Mt can technologically be recycled from waterways and wastewater in 2009. So, it is of great importance to recycle phosphorus from wastewater. The development of efficient technology for phosphorus removal and recovery will promote both the control of water pollution and the recycling of phosphorus resource. Currently, phosphorus removal technologies mainly include chemical, biological and bio-chemical methods [11–13]. Among them, chemical technology is widely used to achieve low residual phosphorus concentrations so as to meet the discharge standard. Ferric and calcium salts are two popular precipitants for phosphorus
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
removal. Phosphorus removal using ferric salt is an efficient method, but it has some drawbacks such as high treatment cost, low effluent pH and difficult recycling of phosphorus from precipitates [14,15]. In contrast, phosphorus removal using calcium salt is a cost-effective option for phosphorus recycling from precipitates, but it leads to an unacceptably high effluent pH of over 10 [16,17]. Based on the properties of two precipitants, the phosphorus removal using ferric–calcium complex was hypothesized to be a good technology. The objective of this work is to set up a novel technology for phosphorus removal and recycling; to optimize the parameters of the technology using ferric–calcium complex and to observe the recycling potential of produced precipitates. 2. Experimental methods The operational conditions influencing the phosphorus removal process were investigated in laboratory experiments. Batch tests were carried out with synthetic wastewater. Synthetic wastewater was prepared with deionized water and stock solutions. Stock solution of dipotassium hydrogen orthophosphate (KH2PO4) was prepared from solid reagent (AR, Yonghua Chemical Technology (Jiangsu) Co., Ltd) with concentration of 10.0 g P/L. The concentration of working phosphate solution was 100 mg P/L. The concentrations of precipitants (calcium hydroxide (Ca(OH)2, AR, Sinopharm Chemical Reagent Co., Ltd.) and ferric sulfate (Fe2(SO4)3, AR, Xilong Chemical Co., Ltd.)) were 22.0 g Ca/L and 20.1 g Fe/L respectively which were directly used so as not to dilute the reaction system. All experiments were performed in 250 mL glass beakers using mechanical stirrers to homogenize the contents. With intensive stirring in the tests (examining the effects of Fe/Ca, dose, pH, fast mixing speed (FMS)), precipitants were added to 250 mL beakers containing 150 mL synthetic wastewater. The Fe/Ca ratio was set according to the reaction equation. The reactions are shown below.
Fe3þ þ 3OH ¼ FeðOHÞ3 ðsÞ Dr Gm ðT; FeðIIIÞÞ ¼ 124:92 kJ mol 5Ca2þ þ 7OH þ 3H2 PO4 ¼ Ca5 ðOHÞðPO4 Þ3
1
Dr Gm ðT; CaðIIÞÞ 1
¼ 319:06 kJ mol
The mixing intensity was tested by initial fast mixing for 1 min, then gentle mixing (at 50 r/min) for 15 min, and finally settling for 30 min. The pH value of reaction solution was regulated beforehand using
231
sodium hydroxide (NaOH, 1 mol/L, Hangzhou Xiaoshan Chemical Reagent Co., Ltd) or hydrochloric acid (HCl, 1 mol/L) and it was no longer regulated during or after reaction. All tests were run in triplicate. The supernatant was taken to determine the residual phosphorus concentration. The precipitate was centrifuged at 7000 r/min for 5 min and dried at 70 °C for 10 h. Phosphate was determined according to the ascorbic acid photometric method, with a detection limit of 10 lg P/L. The precipitate structure was observed by scanning electron microscope (SEM, Ultra 55 field emission scanning electron microscope, the German Corlzeis D company) and the elemental component was analyzed by energy dispersive X-ray spectroscopy (EDS, X-ray spectrometer EDS7429). At the sample surface, three points were chosen to determine elemental contents. The elemental contents of phosphorus were estimated using P (wt%) and P2O5 (wt%, Eq. (2.1)) where, MP2 O5 (142 g/mol) and M p (31 g/mol) are the formula weight of P2O5 and phosphorus, respectively
P2 O5 ¼ M P2 O5 =2M p P ¼ 2:29P
ð2:1Þ
3. Results and discussion 3.1. Effects of operation parameters on phosphorus removal 3.1.1. Effect of Fe/Ca molar ratio To determine the appropriate Fe/Ca molar ratio, batch tests were conducted to investigate the effect of Fe/Ca molar ratio on the phosphorus removal. Other conditions were: concentration = 100 mg P/ L, A/P = 1, pH = 7, FMS = 200 rpm. The precipitants were added to remove equivalent amount of phosphorus stoichiometrically. For instance, Fe/Ca = 4 meant that 0.8 mol ferric salt and 0.33 mol calcium salt were added to precipitate altogether 1 mol phosphorus. If the molar phosphorus removal using ferric–calcium complex as precipitant is higher than the sum using ferric and calcium salts, it is defined as synergistic effect. On the contrary, if the molar phosphorus removal using ferric–calcium complex as precipitant is less than the sum using ferric and calcium salts, it is defined as antagonistic effect. Using ferric and calcium salts respectively, the molar phosphorus removal was 60.88% and 54.65% in the experiments. So, their average 57.76% was taken as the critical value to judge synergistic or antagonistic effect. According to the data shown in Fig. 2, the effect of ferric–calcium complex on the phosphorus removal was antagonistic when the proportion of ferric salt in the ferric–cal-
Fig. 1. Peak phosphorus curve indicating a peak in production by 2033, derived from US Geological Survey and industry data. Source: [9].
232
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
cium complex was below 50%, while the effect was synergistic when the proportion of ferric salt was over 50%. For instance, the phosphorus removal was 39.54% at Fe/Ca ratio of 2:8, far lower than that of 60.88% and 54.65% when ferric and calcium salts were dosed respectively; while the phosphorus removal was up to 77% at Fe/Ca ratio of 7:3–8:2, far higher than that of 60.88% and 54.65%. Ferric and calcium salts showed the antagonistic effect on the phosphorus removal when ferric salt was used as a minor ingredient in the ferric–calcium complex (the proportion of removed phosphorus using ferric salt was below 50%). As the fraction of removed phosphorus using ferric salt increased from 0 to 0.2, the antagonistic effect was observed to enhance. However, the antagonistic effect weakened sharply as the molar fraction of removed phosphorus using ferric salt continued to increase from 0.2 to 0.5. Ferric and calcium salts displayed the synergistic effect on the phosphorus removal if ferric salt was used as a major ingredient in the ferric–calcium complex (the proportion of removed phosphorus using ferric salt was above 50%). As the fraction of removed phosphorus using ferric salt increased from 0.5 to 0.8 in the ferric– calcium complex, the synergistic effect strengthened with the phosphorus removal and it rose from 58.63% to 77.67%. With further increase of fraction of removed phosphorus using ferric salt to 1.0, the synergistic effect became weaker and phosphorus removal dropped from 77.67% to 60.88%. It was concluded that the phosphorus removal using ferric–calcium complex as precipitant was superior to that using ferric or calcium salt as precipitant when the Fe/Ca ratio was higher than 1:1. For Fe/Ca ratio of 4:1, the phosphorus removal reached the peak value. The optimal Fe/Ca molar ratio of 2.4:1 was recommended after both the removal efficiency and treatment cost was taken into account. 3.1.2. Effect of A/P molar ratio In the case of phosphorus removal, it has been reported that the practical dose of ferric or calcium salt was larger than the predicted values (stoichiometric requirements) [18–21]. Although the ferric– calcium complex was superior to ferric or calcium salt, the appropriate molar ratio of complex-to-phosphorus (A/P) need to be determined. Thus, batch tests were carried out to set up the relationship between the ferric–calcium complex dose and the phosphorus removal. Other conditions were: concentration = 100 mg P/L, Fe/Ca = 3, pH = 7, FMS = 200 rpm. The results were shown in Fig. 2, and they followed DoseResp equation (Eq. (3.1))
ð3:1Þ
As demonstrated in Fig. 3, the phosphorus removal increased quickly from 19.31% to 90.38% when the A/P molar ratio rose from 0.25 to 1.25. At molar ratios of 1.5 and above, a good phosphorus removal (>95%) was observed by the addition of ferric–calcium complex to wastewater. As the molar ratio rose further, however, the phosphorus removal leveled out. The A/P molar ratio of 2.17 was recommended to keep the residual phosphorus concentration lower than 0.5 mg/L (The national discharge standard in China). It was reported that the required Fe/P molar ratio was 2.57 using ferric salt as precipitant to achieve the residual phosphorus concentration lower than 0.5 mg/L [22]. If the case using ferric–calcium complex at A/P molar ratio of 2.17 was compared with that using ferric salt at Fe/P molar ratio of 2.57, the novel technology could cut down the ferric salt dose by 0.4 mol Fe/mol P. The Fe/ Ca molar ratio in the ferric–calcium complex was 2.4:1, the technology using ferric–calcium complex could further cut down the ferric salt dose by 0.44 mol Fe/mol P through the substitution of ferric salt by calcium salt. Altogether, the novel technology could cut down the ferric salt dosage by 31.53%. The cost of traditional phosphorus removal using ferric salt was $ 5.33/kg P (removed), while that of phosphorus removal using ferric–calcium complex was only $ 3.69/kg P (removed), thus saving the precipitant cost by 30.7%. 3.1.3. Effect of pH pH has a significant effect on phosphorus removal by changing the form of precipitant, which led to variations in precipitant for phosphorus removal [18]. Hence, the phosphorus removal at different pH values (from 3.0 to 10.0) were tested to explore the effect of pH on phosphorus removal using ferric–calcium complex. The other conditions were: concentration = 100 mg P/L, Fe/Ca = 3, A/ P = 1.5, FMS = 200 r/min. As shown in Fig. 4., when the pH rose from 3.0 to 9.0, the phosphorus removal decreased slightly from 96.73% to 93.41%, with a small drop of 3.32%. As pH value reached 10, the phosphorus removal further decreased to 86.08%, with a drop of 10.65%. So, high efficiency of phosphorus removal could be obtained in a wide pH range (from 3.0 to 9.0) using ferric–calcium complex as precipitant. It has been reported in the earlier studies [14,20,23–25] that the phosphorus removal using ferric or calcium salts as precipitants was significantly affected by pH and the optimal pH were 5 and 10–10.5, respectively. Since the phosphorus removal using ferric–calcium complex was almost independent of pH change, the
80
110
75
100
70
90
65 60
Phosphorus Removal,%
Phosphorus Removal,%
y ¼ 0:0773 þ ð0:9976 0:0773Þ=f1 þ 10½ð0:7044xÞ1:7328 g
R2 ¼ 0:9969:
57.76%
55 50 45 40 35 30 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Ca
x(Fe)
Fig. 2. Effect of Fe/Ca molar ratio on phosphorus removal.
Fe
80
discharge standard: 0.5 mg/L dosage:2.17
70 60 50 40
Fe/Ca = 3 pH = 7 FMS = 200rpm
30 20 10
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25
dosage ratio Fig. 3. Effect of A/P on phosphorus removal.
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
233
100 95 Phosphorus Removal,%
90 85 80 75
Fe/Ca = 3 A/P = 1.5 FMS = 200 rpm
70 65 60 55 50
2
3
4
5
6
7
8
9
10
11
350
400
pH Fig. 4. Effect of pH on phosphorus removal.
100 90 Phosphorus Removal,%
80 70 60 Fe/Ca = 3 A/P = 1.5 pH = 7
50 40 30 20 10 0
50
100
150
200
250
300
FMS, rpm Fig. 5. Effect of FMS on phosphorus removal.
novel technology could cut down the consumption of reagents for pH adjustment. 3.1.4. Effect of fast mixing speed Mixing intensity is a common parameter for chemical precipitation [18]. Fast mixing speed within a certain range can promote the phosphorus removal because it breaks the precipitant into small clusters of molecules and offers the larger surface area for phosphorus sorption. In order to determine the optimum value for fast mixing speed, phosphorus removal was tested using ferric–calcium complex at fast mixing speed from 50 to 300 rpm. Other conditions were: concentration = 100 mg P/L, Fe/Ca = 3, A/P = 1.5, pH = 7. As shown in Fig. 5, phosphorus removal ranged from 81.65% to 88.09% with the increase of fast mixing speed. The phosphorus removal efficiency was high at optimum fast mixing speed of 100 rpm. 3.2. Characteristics of precipitates using ferric–calcium complex as precipitant 3.2.1. Morphological characteristics of precipitates As displayed in Fig. 6 (a, b and c), the morphological characteristics of precipitates were observed at Fe/Ca molar ratios of 3:1, 4:1 and 1:4, respectively. It can be clearly seen from Fig. 6(a and b) that the particles were almost spherical and they agglomerated. In
Fig. 6. Effect of parameter optimization on precipitate morphology. (a) A/P = 1.5:1, pH = 7, FMS = 200 rpm, Fe/Ca = 3:1; (b) A/P = 1.5:1, pH = 7, FMS = 200 rpm, Fe/ Ca = 4:1; (c) A/P = 1.5:1, pH = 7, FMS = 200 rpm, Fe/Ca = 1:4.
contrast, the particles at Fe/Ca molar ratio of 3:1 were more spherical and larger than those at Fe/Ca molar ratio of 4:1, which are more suitable as a seed crystal and easier to recycle. On the other hand, the particles at Fe/Ca molar ratio of 1:4 were circular interweaving with each other like a honeycomb. In other words, the particles were spherical and they agglomerated when calcium salt served as minor ingredient in the precipitant. On the contrary, flat and elongated particles were formed when calcium salt served as major ingredient in the precipitant. It can be clearly seen from
234
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
Fig. 7. Result of EDS analysis (A/P = 1:1, Fe/Ca = 4:1).
Fig. 8. Elemental analysis of precipitate. (a) Novel technology; (b) traditional technology.
Fig. 6(a–c) that with the increase of ferric salts proportion, the spherical particles became smaller and they adopted threadlike structure which may cause poor settleability and inconvenience of recycling.
3.2.2. Components of precipitates The phosphorus content in the precipitate was determined using EDS to estimate the potential of phosphorus recycling from the precipitate. As shown in Figs. 7 and 8, the dominant elements in the precipitate were iron (42.81 wt%) and phosphorus (13.00 wt%), with traces of calcium (2.56 wt%) (Figs. 7 and 8(a). At Fe/Ca molar ratio of 4 and A/P molar ratio of 1.5, phosphorus accounted for 14.8 wt%, indicating a phosphorus recovery of 87.8% from the precipitate. In contrast, the phosphorus content was about 7.13 wt% in the precipitate using ferric salt as precipitant [26], leading to a phosphorus recovery of 62.4% (Fig. 8(b)). With respect to the phosphorus content and recovery in traditional precipitate using ferric salt, phosphorus content in precipitates using ferric–calcium complex were 6 wt% and 25.4%, respectively. The phosphorus content in the novel precipitates was 29.77% P2O5, which was close to the phosphorus content of high-grade phosphorite (30% P2O5) [27] and it was higher than the average content of phosphorite (16.95% P2O5). It has been reported [28] that the Fe–P precipitates using ferric salt were amorphous and their recovery was difficult. On the contrary, the Ca–P precipitates using calcium salt often form crystals and their recovery is convenient [29]. The Fe–Ca–P precipitates using ferric–calcium complex displayed the advantages of Fe–P and Ca–P precipitates, so the
technology shows great promise for phosphorus removal and recycling simultaneously.
4. Conclusions A novel technology for phosphorus removal using ferric–calcium complex as precipitant was developed and its phosphorus recycling potential was investigated. (1) The phosphorus removal using ferric–calcium complex was characterized. The effect of ferric and calcium salts was antagonistic at Fe/Ca molar ratio of 1:10–1:1 and it was synergistic at Fe/Ca molar ratio of 1:1–10:0. The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1. (2) The parameters for phosphorus removal using ferric–calcium complex were optimized as follows: Concentration of 100 mg P/L, Fe/Ca = 2.4:1, A/P = 1.5:1, pH = 7, FMS = 100 rpm. (3) The phosphorus content in the precipitate using ferric–calcium complex was increased. The phosphorus content in the Fe–Ca–P precipitate was about 13 wt% (29.77 wt% P2O5), which is 6% higher than that in the traditional Fe–P precipitate using ferric salt and is close to that in the highgrade phosphorite. 5. Future issues Although the technology for phosphorus removal using ferric– calcium complex as precipitant has great prospects, several issues need to be solved before its practical applications.
L. Qiu et al. / Chemical Engineering Journal 268 (2015) 230–235
(1) The composition and distribution of the precipitates need to be researched so as to provide guidance on their reuse in agriculture, industry and so on. (2) There is usually a high organic fraction in the real wastewaters, the effect of organic on phosphorus removal as well as the recovery of desired form of P needs to be explored in future research works.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (51278457), National Key Technology R&D Program of China (2013BAD21B04) and Key Science and Technology Innovation Team Grant of Zhejiang (2013TDXX). References [1] J. Abell, D. Özkundakci, D. Hamilton, Nitrogen and phosphorus limitation of phytoplankton growth in New Zealand lakes: implications for eutrophication control, Ecosystems 13 (2010) 966–977. [2] M. Paterson, D. Schindler, R. Hecky, D. Findlay, K. Rondeau, Comment: Lake 227 shows clearly that controlling inputs of nitrogen will not reduce or prevent eutrophication of lakes, Limnol. Oceanogr. 56 (2011) 1545–1547. [3] D.W. Schindler, The dilemma of controlling cultural eutrophication of lakes, Proc. R. Soc. B: Biol. Sci. 279 (2012) 4322–4333. [4] S.R. Carpenter, E.M. Bennett, Reconsideration of the planetary boundary for phosphorus, Environ. Res. Lett. 6 (2011) 014009. [5] D.L. Childers, J. Corman, M. Edwards, J.J. Elser, Sustainability challenges of phosphorus and food: solutions from closing the human phosphorus cycle, BioScience 61 (2011) 117–124. [6] A.E. Ulrich, E. Frossard, On the history of a reoccurring concept: phosphorus scarcity, Sci. Total Environ. 490 (2014) 694–707. [7] E.H. Oelkers, E. Valsami-Jones, Phosphate mineral reactivity and global sustainability, Elements 4 (2008) 83–87. [8] K. Ashley, D. Cordell, D. Mavinic, A brief history of phosphorus: from the philosopher’s stone to nutrient recovery and reuse, Chemosphere 84 (2011) 737–746. [9] D. Cordell, J.-O. Drangert, S. White, The story of phosphorus: global food security and food for thought, Global Environ. Change 19 (2009) 292–305. [10] R. Koppelaar, H. Weikard, Assessing phosphate rock depletion and phosphorus recycling options, Global Environ. Change 23 (2013) 1454–1466. [11] 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.
235
[12] S.K. Ramasahayam, L. Guzman, G. Gunawan, T. Viswanathan, A comprehensive review of phosphorus removal technologies and processes, J. Macromol. Sci. Part A 51 (2014) 538–545. [13] A. Oehmen, P.C. Lemos, G. Carvalho, Z. Yuan, J. Keller, L.L. Blackall, M.A. Reis, Advances in enhanced biological phosphorus removal: from micro to macro scale, Water Res. 41 (2007) 2271–2300. [14] J. Thistleton, T.-A. Berry, P. Pearce, S. Parsons, Mechanisms of chemical phosphorus removal II: iron (III) salts, Process Saf. Environ. Protection 80 (2002) 265–269. [15] T. Jianguo, L. Jiemei, Calculation of chemical P removal, Water Wastewater Eng. 9 (2000) 006. [16] G. Morse, S. Brett, J. Guy, J. Lester, Review: phosphorus removal and recovery technologies, Sci. Total Environ. 212 (1998) 69–81. [17] G. Schön, N. Jardin, Biological and Chemical Phosphorus Elimination, 2nd ed., Biotechnology Set, 2001 (pp. 285–319). [18] S.A. Parsons, J.A. Smith, Phosphorus removal and recovery from municipal wastewaters, Elements 4 (2008) 109–112. [19] A. Szabó, I. Takács, S. Murthy, G. Daigger, I. Licskó, S. Smith, Significance of design and operational variables in chemical phosphorus removal, Water Environ. Res. 80 (2008) 407–416. [20] 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. [21] M. Zhang, K. Zheng, J. Jin, X. Yu, L. Qiu, S. Ding, H. Lu, J. Cai, P. Zheng, Effects of Fe(II)/P ratio and pH on phosphorus removal by ferrous salt and approach to mechanisms, Sep. Purif. Technol. 118 (2013) 801–805. [22] M. Zhang, X.-Q. Yu, L. Qiu, H.-F. Lu, P. Zheng, Process optimization for phosphorus removal with ferric salt by response surface methodology [J], J. Zhejiang Univ. (Agric. Life Sci.) 5 (2012) 015. [23] J. Thistleton, T. Clark, P. Pearce, S. Parsons, Mechanisms of chemical phosphorus removal: 1 – iron (II) salts, Process Saf. Environ. Protection 79 (2001) 339–344. [24] I. Takcs, S. Murthy, P. Fairlamb, Chemical phosphorus removal model based on equilibrium chemistry, Water Sci. Technol. 52 (2005) 549–555. [25] E. Valsami-Jones, Phosphorus in Environmental Technologies: Principles and Applications, IWA Publishing, 2004. [26] M. Zhang, P. Zheng, G. Abbas, X. Chen, Partitionable-space enhanced coagulation (PEC) reactor and its working mechanism: a new prospective chemical technology for phosphorus pollution control, Water Res. 49 (2014) 426–433. [27] S.J. Van Kauwenbergh, M. Stewart, R. Mikkelsen, World reserves of phosphate rock. . . a dynamic and unfolding story, Better Crops With Plant Food 97 (2013) 18-20 [28] E. Pierri, E. Dalas, The precipitation of ferric phosphate on porous polymer, Colloids Surf. A 139 (1998) 335–340. [29] T. Zhang, L. Ding, H. Ren, Z. Guo, J. Tan, Thermodynamic modeling of ferric phosphate precipitation for phosphorus removal and recovery from wastewater, J. Hazard. Mater. 176 (2010) 444–450.