stale human urine

Accepted Manuscript Struvite precipitation from anaerobic sludge supernatant and mixed fresh/stale human urine Fanzhe Zeng, Qingliang Zhao, Wenbiao Jin, Yuxi Liu, Kun Wang, Duu-Jong Lee PII: DOI: Reference:

S1385-8947(18)30446-7 https://doi.org/10.1016/j.cej.2018.03.088 CEJ 18695

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

25 January 2018 15 March 2018 16 March 2018

Please cite this article as: F. Zeng, Q. Zhao, W. Jin, Y. Liu, K. Wang, D-J. Lee, Struvite precipitation from anaerobic sludge supernatant and mixed fresh/stale human urine, Chemical Engineering Journal (2018), doi: https://doi.org/ 10.1016/j.cej.2018.03.088

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Struvite precipitation from anaerobic sludge supernatant and mixed fresh/stale human urine

Fanzhe Zengb, Qingliang Zhaoa,b*, Wenbiao Jinc, Yuxi Liuc, Kun Wanga,b, Duu-Jong Leed,e

a

State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), Harbin Institute of Technology,

Harbin 150090, China b

School of Environment, Harbin Institute of Technology, Harbin 150090, China

c

School of Civil & Environmental Engineering, Harbin Institute of Technology, Shenzhen 518055, China

d

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607,

Taiwan e



Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

Corresponding author. Tel: +86-45186283017; Fax: +86 451 8628 3017. E-mail address:

[email protected]

1

Abstract: Phosphorus recovery attracts increasing attentions because phosphorus is a non-renewable and exhausted resource. This study for the first time remarkably improves the efficiency of phosphorus recovery in the form of struvite using the mixture of two kinds of phosphorus-rich wastewater (i.e., anaerobic sludge supernatant and human urine). An optimal volumetric ratio (VRs/f) of 1:9 of fresh and 10-d stale human urine could yield phosphorus recovery of 90.8% at [Mg2+]:[NH4+-N]:[PO43--P] =1.2:1.05:1, pH 10, and reacting time 15 min. The SEM and XRD analyses confirmed the precipitates were struvite. Struvite precipitation was determined by relative supersaturation in both nucleation and crystal growth phases, with homogenous nucleation predominated in the latter phase. One liter of mixed urine could recover the PO43--P content in 41.9 liters of anaerobic sludge supernatant to form 182.3 g of struvite.

Keywords: anaerobic sludge supernatant; human urine; phosphorus recovery; struvite precipitation; magnesium ammonium phosphate (MAP).

2

1. Introduction Phosphorus (P) is an essential ingredient for agricultural fertilizers but it is non-renewable and potentially exhausted because the phosphorus rock might be depleted in 50 years [1]. As phosphorus in the environment mainly exists in human and industrial wastes, eutrophication and algal bloom problems are likely to be induced and even even cause severe pollution of receiving waters [2]. Hence, an effective phosphorus recovery method is important to meet the needs for sufficient phosphorus supply and mitigate the receiving water pollution [3]. Phosphorus can be recovered as the precipitates in three crystal forms: struvite (MgNH4PO4.6H2O), hydroxyapatite (Ca10(PO4)6(OH)2), and calcium phosphate (Ca3(PO4)2) [4,5]. Struvite precipitates at 1:1:1 molar ratio of Mg, P and N following the general equation with n=0, 1, or 2, as shown in Eq. (1) [6, 7]:

Mg 2  Hn PO34n  NH4  6H2O  MgNH4 PO4  6H2O  nH

(1)

The struvite precipitates (MAP) can be used as agricultural fertilizer for crop productiveity enhancenment, which has been applied in Canada [8] and Italy [9].

Struvite recovery from farm wastes, industrial wastewater and municipal waste has been widely studied in previous researches and the limitations of these methods are summarized as follows: farm wastes, such as swine manure, poultry manure and cattle manure [10,11] are rich in phosphorus, but the phosphorus mainly exists in the form of particulates [12], which is difficult for struvite precipitation (phosphorus recovery efficiency is only about 70%). Industrial wastewater usually contains heavy metals and the orthophosphate concentration in some industrial wastewater is relatively low [13]. The leachate produced during municipal 3

waste landfilling is adopted for struvite recovery on the premise that the landfill leachate contains sufficient magnesium ions and phosphate sources. But extra-addition of magnesium ions and phosphate as well as the adjustment of pH may result in high salinization of leachate, which will reduce the microbial activity if the biological-treatment is used further [14]. By contrast, waste activated sludge (WAS) from wastewater treatment plants (WWTPs) is considered as a potential resource for phosphorus recovery and recycle [16], which contains the second greatest amounts of phosphorus [15]. Conventional phosphorus recovery processes from WAS are based on phosphorus fixation using biological or chemical methods. Compared with chemical methods, the biological method is verified

as

an efficient and

environment-friendly process using minimal quantity of chemicals [17], which can avoid the high costs and also skilled operators for the chemical precipitation. The biological method could effectively convert organic phosphorus in sludge phase to inorganic orthophosphate in supernatant phase [18]. Under anaerobic conditions, the polyphosphate accumulating organisms (PAOs) existing in sludge and extracellular polymeric substances (EPS) would make an obvious convertion from polyphosphate (poly-P) to orthophosphate forms [19, 20].

Human urine with little Ca2+ ions contains approximately 70% of the excreted nitrogen and 25%-60% of the excreted phosphorus [21, 22], which would place a big burden on nitrogen and phosphorus removal in WWTPs if not collected and pretreated. Therefore, human urine is regarded as an inexhaustible and easily accessible natural resource, which has been proposed as a promising feedstock for struvite crystallization [23, 24] prior to discharge. This also thanks to the NoMix technology for urine source separation to improve nutrient recovery efficiency in Sweden [25, 26]. The stale urine contains more available ammonia nitrogen than that in sludge 4

supernatant, as shown in Eq. (2), while the fresh urine contains more phosphorus for the simultaneous phosphorus recovery. There exists the need to determine the optimal dosage of anaerobic sludge supernatant and fresh/stale human urine.

NH2 (CO)NH2  2H2O  NH3  NH4  HCO3

(2)

Therefore, two phosphorus-rich waste streams (anaerobic sludge supernatant and human urine) might be mixed with added chemical (containing Mg+ ions) to realize the struvite crystallization process; while Ca2+ ions are not welcome owing to the competition to form calcium phosphate salts other than MAP [27, 28]. The phosphorous recovery process without extra alkaline chemicals added fits the merits for sustainable development with low carbon footprint and environmental impact. Thus, the use of both phosphorus-rich liquids (anaerobic sludge supernatant and human urine) is proposed for MAP precipitation of orthophosphate.

Although the stale urine has more nitrogen than fresh urine, two critical issues are needed to be faced with. First, struvite crystallization is too fast to react completely because of the high concentration of urea in the fresh human urine. Second, urea is unstable and hence difficult to monitor and control at a required dosage. Therefore, the mixed fresh/stale urine is essential for phosphorus recovery as MAP precipitation with minimal chemical dosage. According to Eq. (1), alkaline condition favors struvite formation, otherwise, phosphorus recovery from anaerobic sludge would be precipitated with the formation of Mg3(PO4)2 or Ca3(PO4)2 [29]. Due to urea hydrolysis, stale urine contains a high pH value, which is beneficial for the formation of struvite and saving alkaline chemical dosage [30]. When the mixed urine is used to recover the available phosphorus in the sludge supernatant, pH affects not only struvite crystallization process itself, but also the phosphorus species in anaerobic sludge supernatant [31] and the urea hydrolysis process [32]. Besides, struvite is nucleated under the specific supersaturation condition and the following crystal growth rate phase depends on the applied 5

mixing strength [33, 34]. However, there are few studies about the struvite nucleation and crystal growth processes using anaerobic sludge supernatant and mixed urine samples.

The scope of this work are: (1) to examine the available phosphorus in the anaerobic sludge supernatant and nitrogen/phosphorus in the fresh and stale urine; (2) to identify the simultaneous recovery efficiencies of phosphorus and nitrogen from the sludge supernatant and urine under various factors (molar ratios of [NH4+-N]/[PO43--P], molar ratios of [Mg2+]/[PO43--P], pH and reacting time); (3) to characterize the struvite nucleation and crystallization processes.

2. Materials and Methods

2.1 Phosphorus-rich sludge supernatant and ammonium-nitrogen-rich urine

The excess sludge used in this study was obtained from the secondary sedimentation tank of a WWTP with an anaerobic/anoxic/aerobic (A2/O) process located in Shenzhen, China. The anaerobic experiments were carried out in 500 mL sealed beakers with 300 mL sludge sample (97% moisture content after excess sludge thickening) at 25-30oC without pH control. The anaerobic sludge samples after 10-h reaction were immediately centrifuged at 3000 rpm for 10 min with the total nitrogen (TN) and ammonia nitrogen (NH4+-N) contents in the supernatants being measured directly, the total phosphorus (TP) and PO43--P contents in supernatant being measured after filtering through a 0.45 μm membrane.

6

The fresh urine samples (Table 1) were collected from female student restrooms on campus with 1M sodium hypochlorite at room temperature. The solution pH would increase caused by the urine hydrolysis, so it was used as the indicator for the completion of hydrolysis reaction. The stale urine samples were completely hydrolyzing after 7 d. The urine sample was placed in 500 mL sealed beakers and the supernatant was collected to measure NH4+-N concentration after 3000 rpm stirring for 10 min. The concentrations of magnesium and calcium ions in the stale urine samples were very low since they had already been precipitated as Mg3(PO4)2 or Ca3(PO4)2 spontaneously.

Table 1

2.2 MAP precipitation tests

The MAP precipitation tests were conducted in 500 mL beakers by mixing the phosphorus-rich supernatant from anaerobic sewage sludge, ammonium-nitrogen and phosphorus-rich urine (Sec. 2.1), and 1M MgCl2 at 130 rpm stirring at ambient temperature. The conditions for the studies included different volumetric ratios of stale to fresh urine sample (VRs/f: 0, 5%, 10%, 15%, 20%, 100%), molar ratios of [NH4+-N]/[PO43--P] (1.0:1, 1.05:1, 1.1:1, 1.2:1) and [Mg2+]/[PO43--P] (1.0:1, 1.1:1, 1.2:1. 1.3:1), and pH 8-11. The pH was controlled by adding HCl (0.05M) or NaOH (0.05M). Each variable was studied by holding other factors constant. Each experiment was carried out three times in parallel and the average values were recorded. The pH, molar ratios ([NH4+-N]/[PO43--P] and [Mg2+]/[PO43--P]) and reacting time 7

were analyzed by L9 (34) orthogonal table (Table S1) to determine the most important influential factors among the above four factors. The yielded precipitates were filtered through 0.45µm filters and dried at 40oC prior to analysis.

2.3 Analytical methods

The concentrations of PO43--P and TP were measured using spectrophotometer (T6, Beijing Purkinje General Instrument, Beijing, China) according to the molybdenum blue method in Standard Methods. The analysis of NH4+-N concentration was also conducted in accordance with Standard Methods and the determination of urea concentration was also performed by p-dimethylaminobenzaldehyde colorimetric method [35]. The suspension pH was measured using PHS-3C type pH meter (Shanghai Hongyi Co. Ltd., Shanghai, China). Struvite crystals were analyzed by X-ray diffraction (XRD, D/max-Rb, Rigaku Japan) and scanning electron microscopy (SEM, S-3400N, HITACHI, Tokyo, Japan).

2.4 Calculations

The efficiency of phosphorus recovery defined in Eq. (3) was determined by the difference of initial and final PO43--P concentrations in the experiments. The theoretical yield of phosphorus (W, g/L) was calculated by Eq. (4). Similarly, the efficiency of nitrogen recovery shown in Eq. (5) was defined based on the initial and finial ammonium nitrogen concentrations in the experiments. (3) 8

(4) (5) where, [PO43--P]initial and [PO43--P]final are the initial and final phosphate concentrations, mg/L; [NH4+-N]initial and [NH4+-N]final are the initial and final ammonium nitrogen concentrations, respectively, mg/L.

Induction time (tin) was defined as the time between the initial pH setting and the pH changed by 0.05. The tin could be defined in Eq. (6) [36, 37]: (6) where, A and B are empirical constants (-); Sa is the supersaturation ratio (-) used as an indicator of struvite recovery potential (Sa=σ+1); σ is the relative supersaturation for struvite (dimensionless) defined as Eq. (7). (7) where, Ksp is the thermodynamic solubility product, Ksp=5.49×10-14 (pKsp=13.26), and σa is the average value of σ during the crystal growth.

3. Results and Discussion

3.1 Available phosphorus in anaerobic sludge supernatant and nitrogen/phosphorus in stale urine

During the anaerobic treatment of sewage sludge, the TP concentration in supernatant gradually increased along with time while a reverse trend was observed in the sludge phase (Fig. S1(a)). 9

The PO43--P/TP ratio continually ascended from 38.4% to 74.9% (Fig. S1(b)), indicating additional phosphorus was released from the sludge phase during anaerobic condition. After sludge was anaerobically treated for 90 min, the PO43--P (62.5 mg/L) in the supernatant (Table 1) peaked in PO43--P/TP ratio, which was defined as the "available phosphorus" for subsequent struvite recovery.

The urea in urine samples was gradually hydrolyzed into ammonia , as noted by the increased ammonia nitrogen contents in its supernatant (Fig. S2). The pH changed from 6.7 (fresh) to 8.7 (10d) or 9.5 (15d) over the stale time (Table 1), as the neutral pH of fresh urine was not conductive for struvite precipitation.

The phosphorus (PO43--P) in the urine samples was gradually decreased along with stale time because of the formation of Mg3(PO4)2 or Ca3(PO4)2, which was not conductive to phosphorus recovery. Similar results were also reported in [6]. Moreover, the presence of urease promoted urine hydrolysis and more urea would transfer into NH4+-N accordingly. Lengthening anaerobic time would lead to the decrease of PO43--P concentration and therefore the MAP yield. Also, it induced the serious increase of ammonia concentration in the stale urine (10d/15d) so that the molar ratio of 1:1:1 failed to be met for struvite. Since the fresh urine contained high PO43--P concentration (167.8 mg/L) and excess NH4+-N, the stale urine (10 d) was selected for the following struvite precipitation experimental tests.

10

3.2 Optimum volumetric ratio of stale to fresh urine

As mentioned above, owing to hydrolysis and precipitation reactions, the stale urine contained less phosphorus and much more ammonia than the fresh urine. We herein tested the optimal volumetric ratio of 10-d stale to fresh urine (VRs/f) for maximum recovery of phosphorus in the anaerobic sludge supernatant (Fig. 1).

In the initial urine hydrolysis (0-4 min), the concentration of PO43--P in solution decreased quickly for all urine mixtures (VRs/f=0, 5%, 10%, 15%, 20%, 100%) (Fig. 1(a)). A small amount of precipitates was observed because of the occurrence of "spontaneous precipitation" (precipitate formed without addition of any artificial reagents), during which precipitate formation was minimal by low available Mg2+/Ca2+ ions as Mg3(PO4)2/Ca3(PO4)2. This phenomenon was correlated with the results by Udert et al. [38]. The PO43--P concentration of the mixed urine (VRs/f=10%, 15%, 20%) reached minimum in 20 min and then remained constant; while that in 0 or 5% samples continually decreased. Hence, the mixed urine with VRs/f of 10%, 15% and 20% fit better on phosphorus recovery than the 0 or 5% samples. The differences of PO43--P concentration among these three samples were slight despite of the small differences of the initial PO43--P concentrations in mixed urine sample.

The NH4+-N concentrations of the mixed urine (except for VRs/f =100%) increased rapidly in the first 30 min testing and then remained unchanged (Fig. 1(b)). The higher the VRs/f, the more urea was hydrolyzed into NH4+-N, likely since more urease was generated for urine hydrolysis to fulfill the need of N/P molar ratio for subsequent struvite formation. The corresponding 11

suspension pH was greater than 9 after 20-min hydrolysis with the exception of the fresh urine sample (Fig. 1(c)). The higher the VRs/f, the shorter the hydrolysis time needed to reach pH 9. As alkaline condition favored the formation of NH4+-N for struvite precipitation, pH was considered as another important factor.

The hydrolyzed urine has been verified as a valuable source, but the number of operational taxonomic units (OTUs) in stale urine significantly decreased along with the storage time [39]. As stated above, at VRs/f=10%, the mixed urine still contained a high PO43--P concentration (finally about 115 mg/L), sufficient ammonium nitrogen, and pH>9 for favorable struvite formation if the Mg2+ ion was supplied. Thus, mixed urine sample (10% volume ratio of 10-d stale to fresh urine) was selected for the phosphorus recovery tests.

Fig. 1

3.3 Recovery of phosphorus and nitrogen from sludge supernatant and urine

3.3.1 Determination of molar ratio of [NH4+-N]/[PO43--P]

The stoichiometry for formation of one mole of MAP requires the molar ratio of [Mg2+]:[NH4+-N]:[PO43--P]=1:1:1 (Eq. (1)). The 10% mixed urine contained much more nitrogen than the sum of phosphorus in urine and sludge supernatant if both streams were equally dosed. Therefore, the volumetric ratio of the mixed urine sample to sludge supernatant (VRmu/ss) should be determined reasonably to meet the demand of [NH4+-N]/[PO43--P]>1:1 (at 12

[Mg2+]/[PO43--P]=1:1, the initial pH=7.6±0.1, without pH control during experiments).

The PO43--P concentrations at different [NH4+-N]/[PO43--P] (Fig. 2(a)) declined rapidly within 5 min and remained almost unchanged after 15 min reaction. The PO43--P concentration was the lowest at [NH4+-N]/[PO43--P] of 1.05 (corresponding to VRmu/ss=2.43%) among the four [NH4+-N]/[PO43--P] conditions (1.0:1, 1.05:1, 1.1:1 and 1.2:1, or VRmu/ss=2.31%, 2.43%, 2.56% and 2.81%), indicating more available PO43--P was precipitated within the first 15 min. The recovery efficiencies of PO43--P and NH4+-N at different [NH4+-N]/[PO43--P] after 15 min reaction were compared, as shown in Fig. 2(b). The highest recovery efficiencies of PO43--P (77.2%) and NH4+-N (86.9%) were obtained at [NH4+-N]/[PO43--P] of 1.05 (VRmu/ss=2.43%), close to the stoichiometry of 1:1. The ammonium released from the stale urine significantly enhanced the phosphorous recovery in anaerobic sludge supernatant.

Fig. 2 The morphology of the precipitates was demonstrated to have the similar typical prismatic pattern of the struvite crystals by the microscopic image analysis (Fig. S3).

3.3.2 Determination of molar ratio of [Mg2+]/[PO43--P]

Mg2+ ion is the essential element for phosphorus recovery to struvite. The residual PO43--P concentrations at different [Mg2+]/[PO43--P] ([NH4+-N]/[PO43--P]=1.05, initial pH 7.6±0.1, without pH control) decreased quickly within 20 min reaction and then declined slowly (Fig. 13

3(a)). The least residual PO43--P concentration occurred at [Mg2+]/[PO43--P] >1.2 among the four [Mg2+]/[PO43--P] conditions (1.0:1, 1.1:1, 1.2:1 and 1.3:1). The recovery efficiencies of PO43--P and NH4+-N at different [Mg2+]/[PO43--P] after 20-min reaction were compared in Fig. 3(b). The higher [Mg2+]/[PO43--P] would result in a higher phosphorus recovery efficiency, which were 78.8% ([Mg2+]/[PO43--P]=1.2:1) and 79.9% ([Mg2+]/[PO43--P]=1.3:1), respectively. When [Mg2+]/[PO43--P] was increased to 1.3:1, the recovery efficiency of PO43--P remained almost constant at 78% caused by the occurrence of other side reactions (e.g. formation of magnesium phosphate complexes) . The results are similar to Zeng et al. [40] who recovered phosphorus from digested cattle manure. Wilsenach et al. also reported the potassium magnesium phosphate (KMgPO4.6H2O) was always rather richer than struvite from synthetic urine liquid with enough magnesium addition [41], and high ionic strength (Mg2+, PO43--P) of solution could hamper struvite recovery [42]. But the ion activity in mixed urine was different and the excess ammonium concentration would drive struvite precipitation, which would be beneficial for the recovery of MAP. Thus, controlling the magnesium concentration at [Mg2+]/[PO43--P] of 1.2:1 was the optimal ratio for phosphorus recovery at reduced Mg2+ dosage. The precipitate formed at [Mg2+]/[PO43--P] of 1.2 had struvite crystal morphology (Fig. S4).

Fig. 3 Fig. 4

Phosphorus reduction at different [Mg2+]/[PO43--P] ratios could be described as a function of 14

reacting time. A linear relationship between ln(C-Ce) and reacting time t are presented in Eqs. (8-9). -dC/dt =k(C-Ce)

(8)

ln(C-Ce) =-kt+ln(C0-Ce)

(9)

where, C is the reduction of phosphorus concentration (mg/L); t is the reacting time (min); k is the rate constant (1/min); C0, Ce are the initial and reference concentration (mg/L). Based on such relationship, the kinetic plots can be calculated, as shown in Fig. 4, giving k (1/min) =0.101, 0.120, 0.0947 and 0.139 at [Mg2+]/[PO43--P] ratio of 1.0:1, 1.1:1, 1.2:1 and 1.3:1, respectively. The experimental data was almost consistent with the model. According to the model, when the [Mg2+]/[PO43--P] was 1.2:1, k was the least, indicating the optimal condition of [Mg2+]/[PO43--P] for P recovery.

3.3.3 Optimum pH

The residual PO43--P and NH4+-N concentrations at different initial pH were shown in Fig. 5(a) ([NH4+-N]/[PO43--P] =1.05, [Mg2+]/[PO43--P] =1.2, reacting time=15 min). As pH increased from 8 to 10, the residual PO43--P and NH4+-N were decreased after struvite crystallization. Further increase of pH from 10 to 11 led to slight increases in residual PO43--P and NH4+-N concentrations. The lowest residual PO43--P (8.5 mg/L) and NH4+-N (412.4 mg/L) were obtained at pH 10, which was identified as the optimal initial pH condition. In addition, the final pH of mixed solution was 8.5-9.5 regardless of the initial pH (Fig. 5(b)). This phenomenon was also confirmed by other researchers [43, 44]. Compared with other 15

researches/methods for struvite precipitation, the initial pH of mixed human urine was near to the optimal pH condition and hence much less alkaline chemicals was required, which could decrease the cost and avoid the adverse effects on the environment. And as struvite solubility was a function of pH, struvite solubility decreased with the increase of pH. Generally, the mixed urine samples, with a higher pH than the anaerobic sludge supernatant, were more suitable for struvite formation.

Fig. 5

3.3.4 Maximum phosphorus recovery efficiency

Based on the orthogonal tests L9 (34) (Table S1), both [NH4+-N]/[PO43--P] and [Mg2+]/[PO43--P] were more important than pH and reacting time for struvite precipitation. The least residual phosphorus concentration occurred at the [Mg2+]:[NH4+-N]:[PO43--P] =1.2:1.05:1, pH 10 and reacting time of 15 min, yielding the maximum phosphorus recovery efficiency of 90.8%. In addition, the SEM image and XRD analysis confirmed that the precipitate under the optimal reaction condition was almost pure struvite (MgNH4PO4.6H2O) (Fig. S5).

Overall, the results showed that 10 L of the 1:9 stale/fresh urine (having 0.168 g-P and 1.818 g-P, respectively) at [Mg2+]:[NH4+-N]:[PO43--P] =1.2:1.05:1 could recover the PO43--P content in 419 L of anaerobic sludge supernatant (having 62.5 g-P) to form 182.3 g of MAP. By contrast, in the condition of digested swine wastewater as the feedstock, only 285 g struvite could be 16

recovered from one cubic meter of centrate [45]. With sewage sludge ash as feedstock, struvite was precipitated with the [Mg2+]:[NH4+-N]:[PO43--P] of 1.6:1.6:1 at pH 10.0 [46], indicating more magnesium chemicals were needed for the same amount of phosphorus recovery with a significant increase of the cost for struvite precipitation. Compared with other methods for the production of struvite, phosphorus recovery with anaerobic sludge supernatant and nitrogen/phosphorus in urine could gain much more amount of struvite and fit the merits for environmental impacts.

3.4 Struvite nucleation and crystal growth

3.4.1 Struvite nucleation

Induction time was an important parameter during struvite nucleation. Struvite crystallization was an unstable thermodynamic process, and the supersaturation was very important for struvite crystallization. The kinetic equation between the induction time and the supersaturation was correlated based on the precipitation experiments and an inversely proportional relation was presented. The lgtin data was linearly correlated with 1/(lgSa)2 (Fig. 6(a)), with the best-fit slope (A=0.766) and the intercept (B=0.859). The slope of identified model indicated the surface energy between struvite crystals and the mixed solution. Other research reported the nucleation process contained two zones, i.e., homogeneous and heterogeneous nucleation [47]. But in the present system, the linear correlation between lgtin and 1/(lgSa)2 suggested only homogenous nucleation occurred. The supersaturation ratio in this study was more than 2.11, which verified that the struvite precipitation using anaerobic 17

sewage sludge supernatant and fresh or stale urine was a crystallization process within a high supersaturation range. Homogenous nucleation process was related to the high molecular interaction, which was influenced by induction time, and the supersaturation was a primary influence on the induction time of struvite precipitation.

Fig. 6

3.4.2 Struvite crystal growth

The struvite crystal growth can be described by deposition rate (dR/dt), which could be calculated by the average decrease of substrate concentrations (Mg2+, NH4+-N and PO43--P). Over the 120-min crystal growth time, the consumptions of Mg2+, NH4+-N and PO43--P were 6.53 mmol/L, 7.17 mmol/L and 6.23 mmol/L, respectively (Table 2). Correspondingly, the consumed molar ratio of [Mg2+]:[NH4+-N]:[PO43--P] was 1.05:1.15:1.0 (higher than the stoichiometry of MAP (1:1:1)), suggesting excess Mg2+ and NH4+-N ions were needed for struvite crystal growth in the present system. The concentrations of magnesium, ammonium and phosphate ions exceeded the solubility product, and the available phosphorus could be completely precipitated as struvite within 120 min reaction. The deposition rate (dR/dt) showed a positive relationship with the average relative supersaturation (σ) (Fig. 6(b)). The exponential

dependence

gave

an

accelerating

struvite

precipitation

at

increased

supersaturation, and the high supersaturation was favorable for the fast growth of struvite crystals. During the crystal growth process, crystals in small size aggregated and formed into 18

a bigger size. The change of ionic concentrations could be predicted according to Fig. 6 and the kinetics of crystallization of struvite could determine the optimal condition for a more efficient process. Thus, the struvite crystallization process was determined by the ionic concentrations and relative supersaturation.

Table 2

4. Conclusions The phosphorus recovery as the form of struvite from anaerobic sludge supernatant and mixed human urine was feasible. The optimum volumetric ratio of fresh and stale (for 10 d) urine for recovering the available phosphorus was 10%, while the optimum volumetric ratio of mixed urine and anaerobic sludge supernatant was 2.43%. The maximum efficiency of phosphorus recovery could reach 90.8% under the condition of [Mg2+]:[NH4+-N]:[PO43--P] =1.2:1.05:1, pH 10 and reacting time=15min. Moreover, only homogenous nucleation occurred in the struvite crystallization process within a high supersaturation range. Based on the SEM image and XRD analysis, the precipitates were almost pure MAP crystals.

E-supplementary data of this work can be found in online version of the paper.

19

Acknowledgements The authors gratefully acknowledge funding from Project 51121062 (National Creative Research Groups) supported by National Nature Science Foundation of China.

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[21] E.J. Daniel Helstrom, Kerstin Grennberg, Storage of human urine acidification as a method to inhibit decomposition of urea, Ecological Engineering 12 (1999) 17. [22] M.S. Rahaman, D.S. Mavinic, N. Ellis, Effects of various process parameters on struvite precipitation kinetics and subsequent determination of rate constants, Water Science & Technology 57 (2008) 647. [23] J.R. Mihelcic, L.M. Fry, R. Shaw, Global potential of phosphorus recovery from human urine and feces, Chemosphere 84 (2011) 832-839. [24] T. Karak, P. Bhattacharyya, Human urine as a source of alternative natural fertilizer in agriculture: A flight of fancy or an achievable reality, Resources, Conservation and Recycling 55 (2011) 400-408. [25] J. Coppens, R. Lindeboom, M. Muys, W. Coessens, A. Alloul, K. Meerbergen, B. Lievens, P. Clauwaert, N. Boon, S.E. Vlaeminck, Nitrification and microalgae cultivation for two-stage biological nutrient valorization from source separated urine, Bioresource Technology 211 (2016) 41-50. [26] F. Li, R. M. Guijt, M. C. Breadmore, Nanoporous membranes for microfluidic concentration prior to electrophoretic separation of proteins in urine, Analytical chemistry, 88 (2016) 8257-8263. [27] B. Etter, E. Tilley, R. Khadka, K.M. Udert, Low-cost struvite production using source-separated urine in Nepal, Water Research 45 (2011) 852-862. [28] H. Huang, J. Liu, P. Zhang, D. Zhang, F. Gao, Investigation on the simultaneous removal of fluoride, ammonia nitrogen and phosphate from semiconductor wastewater using chemical precipitation, Chemical Engineering Journal 307 (2017) 696-706. 23

[29] N. Marti, A. Bouzas, A. Seco, J. Ferrer, Struvite precipitation assessment in anaerobic digestion processes, Chemical Engineering Journal 141 (2008) 67-74. [30] I. Kabdaşlı, O. Tünay, Ç. İşlek, E. Erdinc, S. Hüskalar, M. Tatlı, Nitrogen recovery by urea hydrolysis and struvite precipitation from anthropogenic urine, Water Science and Technology 53 (2006) 305-312. [31] X.-D. Hao, C.-C. Wang, L. Lan, M. Van Loosdrecht, Struvite formation, analytical methods and effects of pH and Ca2+, Water Science and Technology 58 (2008) 1687-1692. [32] Z. Liu, Q. Zhao, K. Wang, D. Lee, W. Qiu, J. Wang, Urea hydrolysis and recovery of nitrogen and phosphorous as MAP from stale human urine, Journal of Environmental Sciences 20 (2008) 1018-1024. [33] P. Battistoni, P. Pavan, M. Prisciandaro, F. Cecchi, Struvite crystallization: a feasible and reliable way to fix phosphorus in anaerobic supernatants, Water Research 34 (2000) 3033-3041. [34] A. Johnston, I. Richards, Effectiveness of different precipitated phosphates as phosphorus sources for plants, Soil use and management 19 (2003) 45-49. [35] M.T. Knorst, R. Neubert, W. Wohlrab, Analytical methods for measuring urea in pharmaceutical formulations, Journal of pharmaceutical and biomedical analysis 15 (1997) 1627-1632. [36] C.M. Mehta, D.J. Batstone, Nucleation and growth kinetics of struvite crystallization, Water research 47 (2013) 2890-2900. [37] M. Burns, L. N. Marin, P. A.Schneider, Investigations of a continuous Poiseuille flow struvite seed crystallizer–Mixer performance and aggregate disruption by sonication, 24

Chemical Engineering Journal 295 (2016) 552-562. [38] K.M. Udert, T.A. Larsen, M. Biebow, W. Gujer, Urea hydrolysis and precipitation dynamics in a urine-collecting system, Water Research 37 (2003) 2571-2582. [39] R.H. Lahr, H.E. Goetsch, S.J. Haig, A. Noe-Hays, N.G. Love, D.S. Aga, C.B. Bott, B. Foxman, J. Jimenez, T. Luo, Urine Bacterial Community Convergence through Fertilizer Production: Storage, Pasteurization, and Struvite Precipitation, Environmental science & technology 50 (2016) 11619-11626. [40] L. Zeng and X. Li, Nutrient removal from anaerobically digested cattle manure by struvite precipitation, Journal of Environmental Engineering and Science 5 (2006) 285-294. [41] J.A. Wilsenach, C.A.H. Schuurbiers, M.C.M. van Loosdrecht, Phosphate and potassium recovery from source separated urine through struvite precipitation, Water Research 41(2007) 458-466. [42] M.P. Huchzermeier, W. Tao, Overcoming challenges to struvite recovery from anaerobically digested dairy manure, Water Environment Research 84 (2012) 34-41. [43] K.P. Fattah, F.L. Chowdhury, Early detection of struvite formation in wastewater treatment plants, Journal of Environmental Engineering and Science 10 (2015) 19-25. [44] K.S. Le Corre, E. Valsami-Jones, P. Hobbs, S.A. Parsons, Impact of calcium on struvite crystal size, shape and purity, Journal of Crystal Growth 283 (2005) 514-522. [45] Z. Wu, S. Zou, B. Zhang, L. Wang, Z. He, Forward osmosis promoted in-situ formation of struvite with simultaneous water recovery from digested swine wastewater, Chemical Engineering Journal 342 (2018) 274-280. 25

[46] H. Xu, P. He, W. Gu, G. Wang, L. Shao, Recovery of phosphorus as struvite from sewage sludge ash, Journal of Environemntal Sciences 24 (2012) 1533-1538. M. [47] Hanhoun, L. Montastruc, C. Azzaro-Pantel, B. Biscans, M. Frèche, L. Pibouleau, Simultaneous determination of nucleation and crystal growth kinetics of struvite using a thermodynamic modeling approach, Chemical Engineering Journal 215-216 (2013) 903-912.

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Table and Figure captions Table 1 Characteristics of anaerobic sludge supernatant and fresh/stale urine Table 2 The ionic concentrations, average molar deposition rate and relative supersaturation at different time during struvite crystal growth

Fig. 1 Concentrations of NH4+-N, PO43--P and pH with hydrolysis time at different VRs/f Fig. 2 Phosphate and nitrogen recovery at different molar ratios of [NH4+-N]/[PO43--P] ([Mg2+]/[PO43--P] =1:1, initial pH=7.6±0.1, without pH control) Fig. 3 Residual PO43--P and NH4+-N concentrations at different molar ratios of [Mg2+]/[PO43--P] and residual PO43--P versus time ([NH4+-N]/[PO43--P] =1.05, initial pH=7.6±0.1, without pH control) Fig. 4 The first order kinetic of phosphorus recovery at different molar ratios of [Mg2+]/[PO43--P] ([NH4+-N]/[PO43--P] =1.05, initial pH=7.6±0.1, pH without control) Fig. 5 Residual PO43--P and NH4+-N at different pH and the pH changes during reaction at different initial pH values ([NH4+-N]/[PO43--P] =1.05, [Mg2+]/[PO43--P] =1.2) Fig. 6 Correlation between induction time, deposition rate and supersaturation

27

Table 1 Characteristics of anaerobic sludge supernatant and fresh/stale urine PO43--P (mg/L)

NH4+-N (mg/L)

pH

Mg (mg/L)

Urea (mg/L)

Anaerobic sludge supernatant

62.5±1.3

Urine-fresh

202.3±14

724.5±62

6.7±0.2

1.3±0.4

11346±21

Urine-stale 10d

167.8±6

6623.2±24

8.7±0.1

0

5458±14

Urine-stale 15d

123.2±6

7452.6±35

9.5±0.3

0

4235±8

134.6±46.7 6.8±0.2 13.6±0.3

28

Appearance description

Transparent, amber, low odor Muddy, smelly, faint yellow Muddy, smelly, tawny

Table 2 The ionic concentrations, average molar deposition rate and relative supersaturation at different time during struvite crystal growth Time (min) 0 0.3 1.5 4.0 7.0 12.0 30.0 60.0 120.0

2+

Mg 16.7 13.6 12.1 11.2 11.0 10.9 10.6 10.3 10.1

Concentration (mmol/L) PO43--P NH4+-N 6.41 93.9 3.26 89.9 1.98 88.7 1.03 87.5 0.86 87.1 0.57 86.9 0.32 86.8 0.24 86.9 0.18 86.7

29

dR/dt (mmol/min.L) 11.3 1.11 0.391 0.090 0.047 0.012 0.002 0.002



a

11.4 8.10 6.38 4.77 4.39 3.67 2.82 2.44 2.11

9.75 7.24 5.58 4.58 4.03 3.25 2.63 2.28

220 VRs/f=0 VRs/f=5% VRs/f=10% VRs/f=15% VRs/f=20% VRs/f=100%

200

PO43--P (mg/L)

180 160 140 120 100

80 60 0

10

20

30

40

50

60

70

80

90

Time (min)

(a) PO43--P concentration

7000

NH4+-N (mg/L)

6000

5000 4000 VRs/f=0 VRs/f=5% VRs/f=10% VRs/f=15% VRs/f=20% VRs/f=100%

3000 2000 1000

0 0

10

20

30

40

50

60

70

80

90

Time (min)

(b) NH4+-N concentration 10.0

pH

9.0

VRs/f=0 VRs/f=5% VRs/f=10% VRs/f=15% VRs/f=20% VRs/f=100%

8.0

7.0

6.0 0

10

20

30

40

50

60

70

80

90

Time (min)

(c) pH Fig. 1. Concentrations of NH4+-N, PO43--P and pH with hydrolysis time at different VRs/f

30

(a) Profiles of phosphate concentration 100% P recovery efficiency

Recovery efficiency

80%

N recovery efficiency

60%

40%

20%

0% 1.0:1

1.05:1 1.1:1 N/P molar ratio

1.2:1

(b) Recovery efficiency of P and N (reacting time=15min) Fig. 2. Phosphate and nitrogen recovery at different molar ratios of [NH4+-N]/[PO43--P] ([Mg2+]/[PO43--P] =1:1, initial pH=7.6±0.1, without pH control)

31

(a) Residual phosphate versus time 100% P recovery efficiency

Recovery efficiency

80%

N recovery efficiency

60%

40%

20%

0% 1.0:1

1.1:1

1.2:1

1.3:1

Mg/P molar ratio

(b) Recovery efficiency of P and N (reacting time=20min) Fig. 3. Residual PO43--P and NH4+-N concentrations at different molar ratios of [Mg2+]/[PO43--P] and residual PO43--P versus time ([NH4+-N]/[PO43--P] =1.05, initial pH=7.6±0.1, pH without control)

32

Fig. 4. The first order kinetic of phosphorus recovery at different molar ratios of [Mg2+]/[PO43--P] ([NH4+-N]/[PO43--P] =1.05, initial pH=7.6±0.1, pH without control)

33

600

30

NH4+-N

550

25

PO43--P

20

525

500

15

475

10

PO43--P (mg/L)

NH4+-N (mg/L)

575

450 5

425 400

0 8

8.5

9

9.5

10

10.5

11

pH

(a) Residual phosphate and nitrogen at different pH (reacting time=15min) 12

11

pH=8.0

pH=9.0

pH=10.0

pH=11.0

without pH control

pH

10

9

8

7 0

20

40

60

80

100

120

140

160

Time (min)

(b) pH changes during reaction Fig. 5. Residual PO43--P and NH4+-N at different pH and pH changes during reaction at different initial pH values ([Mg2+]/[PO43--P] =1:1, [NH4+-N]/[PO43--P] =1.05)

34

2.5

logt(sec)

2

y = 0.7659x - 0.8591 R² = 0.9135

1.5

1

0.5

0 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

log(Sa)-2

(a) struvite induction time vs supersaturation 12

dR/dt (mmol/min. L)

10 8

y = 9E-06x 6.0944 R² = 0.9911

6

4 2 0

0

2

4

6

8

10

12

σ

(b) deposition rate vs average relative supersaturation Fig. 6. Correlations between induction time, deposition rate and supersaturation

35

Graphical Abstract

36

Highlights ► Phosphorus recovery from sludge supernatant and mixed human urine was feasible. ► Optimum VR s/f and VR mu/ss for phosphorus recovery were 10% and 2.43%, respectively. +

3-

2+

3-

► Molar ratios of [NH4 -N]/[PO4 -P] and [Mg ]/[PO4 -P] were more important factors. ► Maximum phosphorus recovery of 90.8% could be obtained. ► The precipitates were confirmed almost pure MAP crystals.

37