wastewater by internal recycling of struvite chlorination product

, Li Dool of En

Nitrogen (N) and phosphorus (P) are the indispensableelements for all living organisms. They play an irreplaceable rolein the breeding, growth, and development of organisms. Nitrogenis abundant in the nature, and its resource is sufcient and hencenot a cause for concern. However, the resource amount of P israther limited; it has been estimated that the phosphate rock

form of7 10 metric tons (MT) (Shu et al., 2006). Phosphate roctite) is mainly used as a raw material in fertilizer manufacWith the increasing world population, the demand for P fehas been estimated to increase from 34.3 106 MT in 2000 to47.6 106 in 2020 and to 83.7 106 in 2050 (Tilman et al.,2001). Given the current rate of consumption, it can be predictedthat the P resource in the world would last for a maximum of200 years more. Therefore, it is of great importance to recoverthe used P from the society for sustainable development of Presource.

Corresponding author. Tel.: +86 335 8387 741; fax: +86 335 8061 569.E-mail address: huanghaiming52hu@163.com (H. Huang).

Bioresource Technology 172 (2014) 253259

Contents lists availab

T

els1. Introduction reserves in the world that can be mined in the9http://dx.doi.org/10.1016/j.biortech.2014.09.0240960-8524/ 2014 Elsevier Ltd. All rights reserved.P2O5 isk (apa-turing.rtilizerKeywords:AmmonianitrogenPhosphateStruviteChlorinationRecycling

Mg(OH)2 (pH > 9) were conrmed to be responsible for the decrease in the purity of struvite. The decom-position of recovered struvite by sodium hypochlorite (NaClO) was feasible. The TAN concentration of theswine wastewater was decreased to 63 mg/L by internal recycling of the chlorination decompositionproduct for seven cycles. An economic evaluation showed that 37% of the treatment cost of the proposedprocess could be saved as compared with struvite precipitation using pure chemicals.

2014 Elsevier Ltd. All rights reserved.completely oxidized to N2 gas byNaClO.

High NH4-N removal was achievedusing the chlorination decompositionproduct.

84.5% of NH4-N can be removed fromthe swine wastewater by theproposed process.

a r t i c l e i n f o

Article history:Received 19 July 2014Received in revised form 31 August 2014Accepted 4 September 2014Available online 16 September 2014a b s t r a c t

The recovery of the total orthophosphate (PT) and removal of the total ammonianitrogen (TAN) fromswine wastewater were investigated through a combined technology of using bittern as the magnesiumsource in struvite precipitation along with internal recycling of the chlorination product of the recoveredstruvite. Results revealed that the PT recovery efciency and the struvite purity was mainly depended onthe wastewater pH and the Mg:PT molar ratio. Co-precipitations of Mg3(PO4)2, MgKPO4, Ca3(PO4)2, and Nitrogen in struvite could beHaiming Huang , Yang JiangHebei Key Laboratory of Applied Chemistry, Sch

h i g h l i g h t s

Recovery of phosphate wassignicantly inuenced by thesolution pH.

Purity of struvite was mainlydetermined by the solution pH andMg:P molar ratio.ingvironmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China

g r a p h i c a l a b s t r a c tRecovery and removal of ammonianitrogen and phosphate from swineBioresource

journal homepage: www.le at ScienceDirect

echnology

evier .com/locate /bior tech

included the recovery of PT and the reuse of recovered struvitewas proposed. In this process, bittern as the external Mg sourcein struvite precipitation was added to the swine wastewater forthe recovery of PT. The recovered struvite was decomposed bysodium hypochlorite (NaClO), and its product was continuouslyreused for the removal of TAN from the supernatant after PT recov-ery. The effects of pH and Mg dose on the recovery efciency of PTand the purity of struvite were rst explored. Subsequently, theinuences of pH and Cl/N weight ratio on the chlorination decom-position efciency of struvite were investigated. Finally, multiplerecycling of the chlorinated product was performed and an eco-nomic evaluation of the proposed process was performed.

Technology 172 (2014) 253259Swine wastewater is a wastewater type that contains abundantN and P, which are major pollutant source to water bodies if dis-charged without adequate treatment. Pollution problems relatedto the N and P discharge of swine wastewater commonly includeeutrophication and dissolved oxygen depletion in water bodies aswell as toxicity to the aquatic life and increase in the chloride dosefor drink water disinfection due to the presence of ammonia (Dongand Reddy, 2012; Huang et al., 2010). In order to avoid the appear-ance of these problems, more stringent water quality standardshave been established to decrease the levels of P and N enteringsurface water resources (Adnan et al., 2003). Therefore, the recov-ery of P from swine wastewater is not only an available means torealize sustainable development of P resource but also an essentialmeasure for maintaining the water environment quality. For Precovery, various physicalchemical and biological methods suchas metal ion (e.g., Al, Fe, and Ca) precipitation (Xu et al., 2014),adsorption (Alshameri et al., 2014), biological nutrient removalprocess (Bassin et al., 2012), and struvite (magnesium ammoniumphosphate, MAP) crystallization (Huang et al., 2014) have beenreported. Among these processes, struvite crystallization is apromising and feasible method to recover nutrients from swinewastewater as the total ammonianitrogen (TAN) and the totalorthophosphate (PT) content can rapidly react with magnesium(Mg) to form white struvite crystal, the solubility of which is only0.023 mg per 100 g water (Li et al., 1999).

Swine wastewater generally contains several hundreds milli-gram per liter of TAN, less than 200 mg/L of PT, and small amountof Mg (Liu et al., 2011). To obtain a high recovery efciency of PT,addition of external Mg source is required in the process of struvitecrystallization. Pure Mg salts such as magnesium chloride (MgCl2)and magnesium sulfate (MgSO4) are often selected as the Mgsource of struvite crystallization, but owing to their high cost price,their use tends to increase the recovery cost of PT. Therefore, usinglow-grade Mg-containing materials such as brucite mineral(Huang et al., 2012), magnesite pyrolysate (Chen et al., 2009), mag-nesite mineral (Gunay et al., 2008), and seawater (Liu et al., 2013)as alternatives of Mg is a good strategy to decrease the recoverycost. Bittern is the remaining mother liquid in the process of sea-water salt manufacturing and contains mostly MgCl2, MgSO4,sodium chloride, and small amounts of other inorganic compounds(Lee et al., 2003). With an increase in the degree of seawaterenrichment, the maximum Mg2+ content in bittern is near60,000 mg/L. In a previous study (Lee et al., 2003), bittern hasproved to be an effective Mg source for struvite crystallization.

Although, in the process of PT recovery by struvite crystalliza-tion, some proportion of the TAN in swine wastewater can beremoved, but the removed amount is small as the amount ofTAN in swine wastewater is much more than that of PT. Highcontents of TAN remaining in the swine wastewater may inhibitthe activity of microorganisms in the biological treatment system,thereby lowering the treatment efciency of wastewater (Vadiveluet al., 2007). Therefore, there is a need to decrease the TAN concen-tration of swine wastewater before biological processing. Gener-ally, under this condition, additional phosphate has to be addedto the wastewater if high removal of TAN is desired. Nevertheless,this operation would signicantly increase the treatment cost. Toovercome this issue, recycling of struvite pyrogenation product isoften considered to be an alternative process (Sugiyama et al.,2005; Trker and elen, 2007). However, with an increase in therecycling time, the removal ratio of TAN by this process decreasedprogressively due to the accumulation of byproducts such asMg2P2O7 and Mg3(PO4)2 during struvite pyrogenation and the lossof phosphate and Mg in during pyrolysate reuse (Huang et al.,

254 H. Huang et al. / Bioresource2011; Trker and elen, 2007).In this study, in order to simultaneously recover PT and remove

TAN from swine wastewater, a novel treatment process that2. Methods

2.1. Materials

Swine wastewater used during this study was collected from apig farm located in Beijing. Before use, the swine wastewater wasltered through lter paper to remove the suspended solids andthen stored in a 30-L plastic bucket at 5 1 C. The chemicalcharacteristics of the ltered wastewater are given in Table 1.The bittern used in the experiments was collected from a solar salteld in Tianjin, and its main components are listed in Table 1.

2.2. Experimental methods

The ow chart of the proposed process is shown in Fig. 1. Asseen in Fig. 1, the ow chart was divided into three stages: phos-phate recovery, struvite decomposition, and product multiple recy-cling. (a) Swine wastewater was poured into the phosphaterecovery reactor (PRR), and the phosphate in the wastewater wasrecovered with bittern as Mg source of struvite crystallization,followed by the addition of the recovered struvite and thesupernatant into the struvite decomposition reactor (SDR) andmultiple-recycle reactor (MRT), respectively. (b) The recoveredstruvite was decomposed using NaClO as an oxidant, and the result-ing decomposition product was added to the MRT for the removalof the remaining TAN in the supernatant. (c) The struvite formed inMRT was returned to SDR and step (b) was repeated. (d) Afterseveral repetitions of steps (b) and (c), the supernatant aftersolidliquid separation was discharged to the next treatmentprocess and the formed struvite was recovered.

Here, batch experiments were conducted to optimize the oper-ating conditions of each stage. The specic experiment procedureswere as follows:

1. PT recovery: For the rst-stage recovery of phosphate, 1000 mLof swine wastewater was added into the PRR (a 1500-mL jarwith an airtight lid). Subsequently, bittern was added to theswine wastewater at different Mg/PT molar ratios, and the

Table 1Compositions of ltered swine wastewater and bittern used in this study.

Swine wastewater Bittern

Parameter Value and S.D. Parameter Value and S.D.

pH 7.5 0.2 Ca2+ (mg/L) 90 20Alkalinity (as CaCO3) (mg/L) 2939 193 Mg2+ (mg/L) 44,000 2100COD (mg/L) 3298 276 K+ (mg/L) 12,300 600TAN (mg/L) 406 28 Na+ (mg/L) 58,000 1800PT (mg/L) 128 13 Cl (mg/L) 202,000 13,000K+ (mg/L) 338 21 SO42 (mg/L) 60,000 3000Ca2+ (mg/L) 58 9 Br (mg/L) 5300 400Mg2+ (mg/L) 28 5Fe3+ (mg/L) 1.2 0.3

Zn2+ (mg/L) 0.6 0.2Al3+ (mg/L) 0.5 0.1

uvit

Techmixture was agitated by a magnetic stirrer at pH of 810.5.After 30 min of the reaction, the mixture was allowed to settlefor another 30 min. The supernatant was ltered through a0.45-lm membrane lter for component analysis. The recov-ered struvite was washed with deionized water thrice and thendried in an oven at 35 C for 48 h. The dried precipitates weredissolved in 0.5% nitric acid solution and analyzed for the purityof struvite and the contents of various elements.

2. Struvite decomposition: The precipitates obtained under the

Fig. 1. The ow chart of the designed str

H. Huang et al. / Bioresourceabove-mentioned optimal conditions were added to the SDR(a 50-mL jar) to decompose struvite via chlorination reaction.The subsequent experiment procedures were as follows: Firstly,10 mL of deionized water was added to the SDR. Then, NaClOsolution was added to the solidliquid system at the designedchlorine/nitrogen (Cl/N) weight ratio and the mixture was stir-red at 200 rpm. The pH was adjusted with 1.5 M NaOH or 1.5 MHCl as required (59). The reaction lasted for 10 min and then1 mL of 1 M sodium hyposulte solution was added into thereaction mixture to stop the reaction. Finally, the solution pHafter reaction was adjusted to 3 with 0.5% nitric acid to com-pletely dissolve the solid matter, and 5 mL of the resulting solu-tion was assessed by component analysis. In addition, toinvestigate the TAN removal performance of the decompositionproduct obtained under the optimal conditions, it was reused toremove TAN from synthetic wastewater, which was preparedby dissolving NH4Cl of analytical grade into pure water (TANconcentration, 406 mg/L) at the TAN:PT ratio of 1:1 and at agiven experimental pH of 810.5. The experimental procedureswere similar to those detailed in step (1).

3. Product multiple recycling: Based on the results of the steps (1)and (2), the entire decomposition product of the precipitatesobtained from 1000 mL swine wastewater was added to thesupernatant (approximately 1000 mL). Then, the recoveredstruvite precipitate was recycled, [i.e., steps (2) and (3) wererepeated]. After treatment in MRT, the nal struvite precipitatewas recovered and the treated swine wastewater was dis-charged into the subsequent treatment vessel. The efuentand the recovered struvite were analyzed according to theabove-mentioned methods.ter; China), respectively. Metal ions such as K+, Ca2+, Na+, and

Mg2+ were quantied by using an atomic adsorption photometerand Br were quantied by using the Dionex Series 4500i Ion Chro-

(AA-6800; Shimadzu, Japan). Anions in bittern such as Cl, SO42,

matograph. The collected solids in the experiments was washedwith deionized water thrice and then dried in an oven at 35 Cfor 48 h. The dried solids were characterized under a scanning elec-tron microscope (SEM; SUPRA 55 SAPPHIRE; Germany) and X-ray2.3. Analytical methods

In this study, water samples were analyzed according to thestandard methods detailed elsewhere (APHA, 1998). The pH ofsolution was measured using a pHmeter (pHS-3C; China). The con-centrations of the TAN and PT in the reaction solution were deter-mined using the Nesslers reagent spectrophotometric method andMoSb anti-spectrophotometric method (752 N-spectrophotome-

e recovery and multiple-recycle process.nology 172 (2014) 253259 255diffraction analyzer (XRD; DMAX-RB; Rigaku, Japan). All tests wereperformed in triplicate, and their average data was reported.

3. Results and discussion

3.1. Recovery of phosphate

The crystallization of struvite occurs only when the ionic activ-ity products of Mg2+, NH4+, and PO43 exceeds the thermodynamicsolubility product of struvite (Wang et al., 2006). However, the spe-cies of the three constitutional ions were signicantly inuencedby the solution pH. Therefore, to effectively recover the TAN andPT from swine wastewater by struvite crystallization, control ofthe solution pH is the key factor. In addition, increasing the ionicactivity product through increase in the Mg2+ concentration is alsoan effective method. The changes in the removal efciencies of TANand PT as well as the purity of struvite with the solution pH andMg:PT molar ratio are shown in Fig. 2.

As shown in Fig. 2a, at the pH range of 89.5, the removal ef-ciency of PT signicantly increased with an increase in the pH andMg:PT molar ratio. Nevertheless, at pH > 9.5, further increases ofpH and Mg:PT molar ratio incurred a slight decrease in the removal

pH

Techefciency of PT. It was observed from Fig. 2b that an increase in theamount of added Mg2+ has a negligible effect on the removal ofTAN. At a given Mg:PT ratio, the TAN removal efciency rstincreased at a pH 89 and then decreased at pH 9.510.5. At pH9 and Mg:PT molar ratio of 1.2:1, the removal efciencies of theTAN and PT were 12.9% and 98%, respectively. The changes in thepurity of struvite in the obtained precipitates with pH and Mg:PTratio are shown in Fig. 2c. From this gure, it can be observed thatthe purity of struvite slightly decreased in the pH range of 89, butsharply decreased in the pH range of 910.5. Furthermore, atpH > 9, an increase in the Mg:PT molar ratio further decreased

Fig. 2. The changes in the removal efciencies of the TAN and PT and the purity ofstruvite with pH and Mg:PT molar ratio.8.0 8.5 9.0 9.5 10.0 10.520

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256 H. Huang et al. / Bioresourcethe purity of struvite.Although struvite is constituted by Mg2+, NH4+, and PO43 in

equal molar ratio, its formation reaction proceeds by the Eq. (1)given below (Wu and Zhou, 2012).

Mg2 NH4 HPO24 6H2O ! MgNH4PO4 6H2O # H 1At solution pH < 8, although the majority of the TAN in solution

was presented in the form of NH4+ species, high concentration of H+

may inhibit the formation of struvite [according to Eq. (1)], result-ing in low removal efciencies of the TAN and PT. When the pHincreased from 8 to 9, although the ratio of NH4+ species in theTAN rapidly decreased from >90% to about 55% (Li et al., 2012),the concentration of HPO42 rapidly increased along with adecrease in the H+ concentration. This event could promote theproceeding of the precipitation reaction, resulting in an increasein the removal efciencies of the TAN and PT. However, atpH > 9, the conversion from NH4+ to NH3 may accelerate, and atpH 10.5, 95% of NH4+ can be converted to NH3 (Li et al., 2012),which cannot be precipitated by the formation of struvite. Inaddition, some Mg2+ in the solution may react with PO43 andOH to form amorphous Mg3(PO4)2 and Mg(OH)2 (Wu and Zhou,2012). Consequently, the ionic activity product of the struvite con-stitutional ions decreased, leading to a decrease in the removalefciency of the TAN. Nevertheless, the PT removal efciencymaintained a stable value owing to the formation of other amor-phous solids such as Mg3(PO4)2 and Ca3(PO4)2. This event may beresponsible for the rapid decrease in the purity of struvite at thepH of 910.5.In order to further determine the relationship between purity ofstruvite and pH, the components of the precipitates obtained atdifferent pHs and at the Mg:P molar ratio of 1.2:1 were analyzed.The contents of K, Ca, N, Mg, P, and TOC are given in Fig. 3. Asobserved in the gure, with an increase in the pH, the molar con-centration of N in the precipitates progressively decreased,whereas those of Mg and P gradually increased. Furthermore, itseemed that the molar concentration of Mg obviously exceededthat of P at pH > 9. The stoichiometric ratio of Mg2+, NH4+, andPO43 in struvite is 1:1:1. The Mg:N (or P:N) molar ratio exceedingthis value in the obtained precipitates indicated formation of someimpurity during precipitation. In this work, it can be clearlyobserved that the Mg:N (or P:N) molar ratio increased withincreasing pH from 8 to 10.5, which suggested that some Mg-containing compounds were formed and their amount increasedwith an increase in the pH. These Mg-containing compoundsmay include Mg3(PO4)2, MgKPO46H2O, MgHPO43H2O, andMg(OH)2. Since the formation of MgHPO43H2O occurs signicantlyonly at pH around 6 and at high concentrations of Mg and phos-phate (Musvoto et al., 2000), it was not formed in this study. TheK+ present in the swine wastewater could compete with NH4+ toform MgKPO46H2O (struvite-K), which is one of the struvite ana-logs (Wilsenach et al., 2007). The data shown in Fig. 3 revealed that

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Fig. 3. The analysis of the elements of the precipitates obtained at different pHs andits TOC value (Mg:PT = 1.2:1).

nology 172 (2014) 253259MgKPO46H2O formed at all the tested pH and that its amountincreased slightly with an increase of pH. Mg(OH)2 generallybegins to form at pH > 9, and its amount increases with an increasein the pH (Wu and Zhou, 2012). This may explain why the Mg:Pmolar ratio in the precipitates gradually increased at pH > 9.Certainly, this may also be caused by the increase in the amountof Mg3(PO4)2. Hence, it was conrmed that, at the pH range of89, the possible byproducts containing Mg may includeMg3(PO4)2 and MgKPO46H2O, and the other byproducts furtherincluded Mg(OH)2 at pH > 9. Presence of high concentration ofCa2+ in swine wastewater could react with PO43 to form Ca3(PO4)2.As observed from Fig. 3, at the tested pHs, all of the obtainedprecipitates contained Ca, and its content progressively increasedwith an increase in the pH. In addition, the TOC values of the pre-cipitates were measured. The results indicated that, at pH > 9.5, theTOC value obviously increased, suggesting an increase in theamount of organic matter in the precipitates, probably caused bythe adsorption of these amorphous compounds. Nonetheless, itcan be speculated that the organic matter content in the precipi-tates have a weak effect on the chlorination decomposition ofstruvite.

Solution pH can inuence the ionic equilibrium system in swinewastewater, which resulted in the changes of composition and

morphology of the precipitates. In order to further corroborate therelation between the purity of struvite and pH, the precipitatesobtained at Mg:PT 1.2:1 and at pH 8.5, 9.5, and 10.5, respectively,were characterized by SEM and XRD. The SEM image showed thatsome regular prismatic crystal with a diameter of 2050 lm andsmooth surfaces were obtained at pH 8.5. When pH increased to9.5, the size of the crystals decreased (1030 lm) and their sur-faces became coarse. At pH 10.5, no obvious crystal was observed,and amorphous compounds amounted for the majority of the pre-cipitates. XRD patterns demonstrated that the intensity of charac-teristic peaks of struvite in the precipitates decreased with anincrease in the pH of 8.510.5, suggesting that the content of stru-vite in the precipitates reduced with an increase in the pH.

3.2. Chlorination decomposition of the recovered struvite

The chlorination decomposition of the struvite obtained at

the reaction in Eq. (1).In the previous literatures, Zhang et al. (2009) reported that

approximately 83% of the TAN could be removed from the cokingwastewater when the product of struvite pyrolyzed with NaOHwas recycled at pH 9.5. He et al. (2007) recycled the NaOH decom-position product of struvite at pH 9 for the removal of TAN fromlandll leachate to obtain a high degree of TAN removal. Yu et al.(2013) found that the optimum pH for the recycling of the struviteNaOH pyrolysate was 9.5, and the TAN removal ratio could bemaintained at >80%. Huang et al. (2011) reported that the maxi-mum TAN removal ratio (82.5%) was achieved at the pH range of88.5 when the struvite pyrolysate was recycled for the treatmentof swine wastewater. Comparing with the results reported by theseliteratures, it was found that the TAN removal efciency for recy-cling the chlorination decomposition product clearly exceeded thatof using the struvite NaOH pyrolysate.

3.3. Mutiple-recycle of decomposition products

H. Huang et al. / Bioresource TechMg:PT 1.2:1 and pH 9 was performed at the pH range of 59 andCl/N ratio of 6.5:1 to 9.5:1. The results shown in Fig. 4 indicate thatthe decomposition efciencies of nitrogen in struvite were inu-enced by the pH and Cl/N ratio. Under the same pH values, the Ndecomposition efciencies signicantly increased with an increasein the Cl/N ratio. For example, at pH 6.0, the N decomposition ef-ciencies increased from 80% to 99% as the Cl/N ratio increased from6.5 to 8.5; however, it remained unchanged when the Cl/N ratioincreased to 9.5. Under the same Cl/N conditions, the N decompo-sition efciencies increased at pH 56, reached a peak at pH 6, andthen slightly decreased at pH 79.

It is well known that the TAN in swine wastewater can be oxi-dized to nitrogen gas by breakpoint chlorination. In this study,with the addition of NaClO to the struvite solidliquid system, itcan be converted to hypochlorous acid through adjustment of thepH. In the chlorination process, the overall reaction occurring inthe solution system can be expressed as follows:

ClO H ! HOCl 2

MgNH4PO4 H $ Mg2 NH4 HPO24 3

2NH4 3HOCl ! N2 3H2O 5H 3Cl 4As the chlorination reaction proceeded, the concentrations of

Mg2+ and HPO42 in solution system rapidly increased. Under thispH condition, supersaturated Mg2+ and HPO42 in the solutionmay form insoluble MgHPO43H2O and Mg3(PO4)2 (Mijangoset al., 2004; Wu and Zhou, 2012). During the experiments,

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Cl/N=6.5:1 Cl/N=8.5:1 Cl/N=7.5:1 Cl/N=9.5:1Fig. 4. The decomposition ratio of nitrogen in struvite at different pH and Cl/Nratios.abundant white aggregations with large particles were formed atthe original solidliquid system. Furthermore, the precipitatesobtained at pH 6 and Cl/N ratio 8.5 was collected and characterizedby SEM and XRD. SEM image showed that the morphology of theprecipitates was rectangular block with a diameter of 520 lm.XRD pattern indicated that the main composition of the solid prod-uct was MgHPO43H2O. Although the formation of MgHPO43H2OandMg3(PO4)2 occurred in the chlorination reaction, large amountsof Mg2+ and phosphate anions still remained in the solution. Hence,the total solid and liquid products after chlorination decomposi-tion should be added into the wastewater for reuse.

Since the chlorination decomposition product contained a cer-tain amount of Mg3(PO4)2, which has a weak effect on the removalof TAN (Yu et al., 2012), the solution pH of the product had to beadjusted to reduce the effect of Mg3(PO4)2. In this study, the pHof the chlorination product solution was adjusted to 4 before recy-cling. The TAN removal efciencies of recycling the nal productsolution at the pH of 810.5 were determined (Fig. 5). As seen inFig. 5, it was observed that the TAN removal efciencies rapidlyincreased from 73% to 88% when the pH of the wastewater solutionincreased to 89 and then slightly decreased to 85% at pH 9.5. AtpH 10.5, the TAN removal efciency rapidly declined to 74%. Theresults were similar to those of using pure Mg and phosphate salts(Li and Zhao, 2003; Yetilmezsoy and Sapci-Zengin, 2009). Here, therecycling of the decomposition product was mainly achieved by

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Fig. 5. TAN removal ratio for recycling the struvite chlorinated product at differentpHs.

nology 172 (2014) 253259 257Under the above-mentioned optimal conditions, the mutiple-recycle of struvite decomposition product was performed (Fig. 6).It can be noted from Fig. 6 that, with an increase in the number

Techof times of recycling, the TAN concentration of swine wastewaterdecreased gradually and the remaining PT concentration in thesolution remained constant at the range of 25 mg/L. Whenthe decomposition product was internally recycled seven times,the TAN concentration reduced to 63 mg/L. From calculation, itwas derived that 84.5% of the TAN could be removed by theproposed process. The amount of the TAN removed at differentrecycling times was maintained at approximately 43 mg/L waste-water. This suggested that the TAN removal performance of thestruvite decomposition product did not reduce with an increasein the number of recycle times. This may be due to the fact thatthe amounts of active Mg and phosphate kept the same duringthe mutiple-recycle. Trker and elen (2007) reported that whenthe struvite pyrolysate was repeatedly reused ve times, the TANremoval efciency progressively decreased from 92% in the rstcycle to 77% in the fth cycle. Huang et al. (2011) found that theTAN removal efciency was initially 80% and rapidly reduced to67% in the fth cycle recycling of the struvite pyrolysate as theMg and phosphate sources in struvite precipitation. He et al.(2007) recycled the pyrolysate of struvite and achieved the TANremoval of >90% in the rst cycle; however, it rapidly decreased

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258 H. Huang et al. / Bioresourceto

and the Free Research Foundation for Young Teachers of YanshanUniversity (13LGB023).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.biortech.2014.09.024.

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Recovery and removal of ammonianitrogen and phosphate from swine wastewater by internal recycling of struvite chlorination product1 Introduction2 Methods2.1 Materials2.2 Experimental methods2.3 Analytical methods

3 Results and discussion3.1 Recovery of phosphate3.2 Chlorination decomposition of the recovered struvite3.3 Mutiple-recycle of decomposition products3.4 Economic evaluation

4 ConclusionsAcknowledgementsAppendix A Supplementary dataReferences