Effect of different struvite crystallization methods on gaseous emission and the comprehensive comparison during the composting

Bioresource Technology xxx (2016) xxx–xxx

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Effect of different struvite crystallization methods on gaseous emission and the comprehensive comparison during the composting Tao Jiang a,b, Xuguang Ma a, Juan Yang a, Qiong Tang a, Zhigang Yi a, Maoxia Chen a, Guoxue Li b,⇑ a b

College of Chemistry, Leshan Normal University, Leshan 614004, China College of Resources and Environment Sciences, China Agricultural University, Beijing 100193, China

h i g h l i g h t s
Struvite crystal formation reduces NH3 loss by 51–82%. 2


SO4

reduces CH4 emission by 62–70%.


Mg(OH)2 + H3PO4 treatment yields the highest struvite content in the end product.
MgSO4 + H3PO4 treatment yields the lowest struvite content due to low pH.
Ca(H2PO4)2 + MgSO4 treatment is most suitable for practical use.

a r t i c l e

i n f o

Article history: Received 25 December 2015 Received in revised form 10 February 2016 Accepted 11 February 2016 Available online xxxx Keywords: Composting Struvite Ammonia Nitrous oxide Methane

a b s t r a c t This study compared 4 different struvite crystallization process (SCP) during the composting of pig feces. Four combinations of magnesium and phosphate salts (H3PO4 + MgO (PMO), KH2PO4 + MgSO4 (KPM), Ca (H2PO4)2 + MgSO4 (CaPM), H3PO4 + MgSO4 (PMS)) were assessed and were also compared to a control group (CK) without additives. The magnesium and phosphate salts were all supplemented at a level equivalent to 15% of the initial nitrogen content on a molar basis. The SCP significantly reduced NH3 emission by 50.7–81.8%, but not the N2O. Although PMS group had the lowest NH3 emission rate, the PMO treatment had the highest struvite content in the end product. The addition of sulphate decreased CH4 emission by 60.8–74.6%. The CaPM treatment significantly decreased NH3 (59.2%) and CH4 (64.9%) emission and yielded compost that was completely matured. Due to its effective performance and low cost, the CaPM was suggested to be used in practice. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Composting is a widely-used and effective technology for the utilization of organic waste. However, the process emits harmful gasses, such as ammonia (NH3), as secondary pollution. During composting of animal waste about 9.6–46% of the initial nitrogen is lost in the form of NH3, which accounts for 79–94% of the total nitrogen loss (Jiang et al., 2011; Fukumoto et al., 2011; Yang et al., 2015). NH3 emission causes not only a decline of compost quality, but also serious environmental problems such as odor and haze (Jiang et al., 2015). In northern China, ammonium has been shown to be a significant and increasing component of airborne fine particles (PM2.5): from 2003 to 2012, its proportion has increased from 7.5% to 12% (Li et al., 2013).

⇑ Corresponding author. Tel.: +86 01062733498; fax: +86 01062731016.

Several approaches have been taken to reduce the volatilization of NH3 during composting, including optimization of the composition of raw materials, use of different bulking agents (Nakhshiniev et al., 2014; Chowdhury et al., 2014), improvement of composting process conditions (El Kader et al., 2007), use of chemical additives (Liu et al., 2015; Yang et al., 2015), and use of microbial amendments (DeLaune et al., 2004). Struvite crystallization process (SCP) which was first used in the treatment of waste water, has recently been used in composting (Jeong and Kim, 2001; Lee et al., 2009; Wang et al., 2013).The SCP can decrease NH3 emission by generate struvite (MgNH4PO46H2O), a high-quality, slowrelease fertilizer. In previous work, different magnesium (Mg) salts (e.g., Mg (OH)2, MgCl2, MgSO4) and phosphorus (P) salts (e.g., H3PO4, KH2PO4, NaH2PO4, Na2HPO4) were used for struvite formation, but no published study has specifically evaluated the effect of different Mg and P salts on struvite crystal formation and gaseous emission.

E-mail address: [email protected] (G. Li). http://dx.doi.org/10.1016/j.biortech.2016.02.046 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Jiang, T., et al. Effect of different struvite crystallization methods on gaseous emission and the comprehensive comparison during the composting. Bioresour. Technol. (2016), http://dx.doi.org/10.1016/j.biortech.2016.02.046

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The purpose of the present study was to identify the most valuable and practical SCP method for composting by evaluating the differences of 4 struvite precipitation methods on N conservation, greenhouse gas emission, struvite crystal formation, and the cost balance. 2. Methods 2.1. Raw materials and composting installation Pig feces and cornstalk were mixed at a ratio of 7:1 (w/w) as raw materials. Pig feces were taken from a pig fattening farm located in Beijing. Air-dried corn stalk was obtained from the Shangzhuang research station of China Agricultural University. The compositions of the raw materials and the mixture are shown in Table 1. Although the C/N ratio is lower than the traditionally suggested level, it is similar to ratios seen in current practice in Chinese compost plants. Experiment was carried out in a laboratory-scale composting system (Fig. 1). Each compost vessel in this system had a capacity of 60 L and was controlled by a composting program. Under this program, the aeration was controlled automatically. The temperatures in the vessels were also recorded by the program at 02:00 and 14:00 every day. 2.2. Experimental design and sample collection Five treatments were conducted in triplicate to evaluate the effects of the different MAP methods (Table 2). According to the results of previous researches (Jeong and Hwang, 2005; Ren et al., 2010), the P and Mg salts were all supplemented at a level equivalent to 15% of the initial nitrogen content on a molar basis. All the P and Mg salts were gotten form agriculture market as chemical fertilizer, not the analytical reagents. In the CaPM treatment, triple superphosphate (with P2O5 content of 46%) was used instead of pure Ca(H2PO4)2. Continuous aeration at a rate of 0.3 L kg1 DM min1 was employed in all treatments. The materials were composted for 5 weeks and turned at days 4, 10, 18, and 26 in order to homogenize the materials and improve porosity. Samples of approximately 100 g were taken for analysis at the beginning of the experiment, during each turning, and at the end of the experiment. Samples were divided into two parts: one part was air-dried and ground to pass through a 0.1 mm sieve as a dry sample; the other part was immediately frozen as a fresh sample. 2.3. Analytical methods and calculations Total nitrogen (TN) and total organic carbon (TOC) content were measured by an elemental analyzer (Elementar vario MACRO cube, Germany). Mineral compositions were analyzed by a powder X-ray diffraction analyzer (Rigaku Dmax 12 kW, Japan). Fresh samples were mixed with deionized water at a ratio of 10:1 (w/w) and shaken for 30 min, and then filtered. The supernatant was then collected for measurement of germination index (GI) (Guo et al., 2012). NH+4 and NO 3 were extracted with 2 M KCl (1:20) and were

analyzed by a segmented flow analyzer (Technicon Autoanalyzer II system, Germany). CH4 and N2O content were analyzed by gas chromatographs (Agilent 7890A, USA) equipped with electron capture and flame ionization detectors, respectively (Jiang et al., 2011). The O2 were analyzed by gas chromatograph equipped with thermal conductivity detector (Beifen 3420A, China). The NH3 was absorbed by a washing bottle with boric acid (2%) and then titrated using 0.05 M H2SO4. Gas emission rates (N2O, CH4, NH3) were measured daily during the first 2 weeks, and then 3–4 times per week thereafter. Based on previous experience, N2O increases rapidly after turning the compost. Thus, after each turning, the N2O was measured hourly until the emission rate declined to a constant level. Cumulative emissions for the whole composting period were calculated from the daily flux. Data for non-measured days were obtained by averaging that of the closest measured days. All data were analyzed using One-way Analysis of Variance (ANOVA). LSD-t was used to test for significant differences. SPSS 17 for Windows was used for all statistical analyses.

3. Results and discussion 3.1. Evolution of temperature and oxygen concentration Once the experiment began, the temperature of all treatments increased quickly (Fig. 2A). For all except the PMS group, the temperature exceeded 65 °C after 3 days. The duration of the thermophilic phase of these treatments was approximately 2 weeks, which is a sufficient length of time to achieve a sanitation effect. With the exhaustion of easily degradable carbon, the temperatures decreased gradually and approached air temperature after 4 weeks. The temperature of the PMS treatment group was significantly lower than that of CK (P = 0.033), and its thermophilic phase lasted only five days. The addition of H3PO4 + MgSO4 (PMS treatment) inhibited the degradation process significantly. Lee et al. (2009) reported that both Mg (MgCl2) and P (KH2PO4) salts inhibit organic matter decomposition during composting when present in molar ratios of greater than 0.05 with regard to TN. In this study, significant inhibition was not observed in the CaPM (Ca(H2PO4)2 + MgSO4) or KPM (KH2PO4 + MgSO4) treatment, even when the salts were added at a concentration of 15% (mole/mole) of the initial nitrogen content. Jeong and Hwang (2005) reported that the optimal dose of Mg and P salts should be approximately 20% (mole/mole) of the initial nitrogen content to ensure both the complete mineralization of organic materials and increased conservation of nitrogen. In our research, the addition of H3PO4 + MgSO4 significantly decreased the pH value (Fig. 3), which is likely responsible for the low degradation rate. The low pH not only inhibits the composting process directly, but also increases the accumulation of VFAs and NH+4 in the compost that further enhances the inhibition on degradation of organic matter (Wong et al., 2009; Wang et al., 2013). Due to intensive degradation, the O2 content in the outlet for all treatments decreased to 8–13% by the second day and maintained this low level for approximately 1 week (Fig. 2B). After that, the O2 content began to increase until it gradually reached the air

Table 1 Compositions of raw materials and mixture.

Pig feces Corn stalk Mixture

TOC (g kg1 DM)

TN (g kg1 DM)

NH+4-N (g kg1 DM)

1 NO DM) 3 -N (mg kg

MC (%)

C/N

349 ± 22.1 419 ± 8.2 374.8 ± 11.8

26.5 ± 2.1 9.9 ± 0.7 20.4 ± 2.9

6.1 ± 1.2 – 3.8 ± 0.9

116.2 ± 15.1 – 73.8 ± 10.2

77.8 ± 2.1 9.3 ± 0.2 69.2 ± 3.8

13.2 ± 0.8 42.3 ± 0.7 18.4 ± 0.5

DM, dry matter; TOC, total organic carbon; TN, total nitrogen; MC, moisture content; – , undetected.

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Fig. 1. Sketch of the composting vessel. PI, pressure indicator; FI, flow indicator; QI, quantity indicator; TIR, temperature information recorder; GC, gas chromatograph.

CK CaPM Turn

No.

Chemical additives

Abbreviation

1 2 3 4 5

– H3PO4 + Mg(OH)2 KH2PO4 + MgSO4 Ca(H2PO4)2 + MgSO4 H3PO4 + MgSO4

CK PMO KPM CaPM PMS

concentration. Due to the significant inhibition caused by H3PO4 + MgSO4, the O2 content of the PMS treatment was significant higher than that of the CK treatment (P = 0.002). After each turning, a significant decrease in O2 content was observed. When partially decomposed materials are turned, they are moved to areas with abundant O2, thus facilitating intensive degradation (Jiang et al., 2013).

Temperature (eC)

Table 2 Design of experiment and chemical additives.

KPM Air

70

A 55

40

25 0

7

14

21

28

35

Composting time (d)

3.2. Chemical characteristics

20 O2 concentration (%)

The pH value of all treatments increased after the experiment was started (Fig. 3A). Intensive degradation of substances with high nitrogen content (e.g., urea) caused an increase in the NH+4 content of the compost (Fig. 3E), resulting in the pH change. During the thermophilic phase, the high NH3 emission (Fig. 4A) induced a gradual decrease in pH value. Three weeks later, as the compost transitioned to the maturing period and the degradation process slowed down, the pH remained at a constant level until the end of the experiment. The pH of the PMS treatment was clearly lower than that of the other groups because the starting materials were weakly acidic due to the chemical additives (H3PO4 + MgSO4). The initial low pH increased the accumulation of VFAs that further decreased the pH of compost. The TN content of the CK group decreased slightly in the first week due to intensive NH3 emission (Fig. 3B). Subsequently, the TN content gradually increased, despite significant NH3 emission. This increase is consistent with several previous reports (Luo

PMO PMS

15

B 10

5 0

7

14 21 28 Composting time (d)

35

Fig. 2. Temperature (A) and O2 concentration (B) evolution during composting. Diamonds indicate the turning days.

et al., 2013; Jiang et al., 2015; Nasini et al., 2016) and is thought to be due to a higher degradation rate of organic carbonaceous

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CK

KPM

PMO

A

7.5

B

27

22

17

6.5 0

7

14 21 28 Composting time (d)

0

35

7

35

14 21 28 Composting time (d)

35

14 21 28 Composting time (d)

35

D Germination index (%)

C TOC content (g kg-1DM)

14 21 28 Composting time (d)

120

400

350

300

250

80

40

0 0

7

14 21 28 Composting time (d)

0

35

7

400

E 6

4

2

0

NO3--N content (mg kg-1)

8

NH4+-N content (g kg-1)

PMS

32

TN content (g kg-1DM)

pH

8.5

CaPM

F 300

200

100

0 0

7

14 21 28 Composting time (d)

35

0

7

Fig. 3. Evolution of chemical characteristics (A, pH value; B, total nitrogen content; C, total organic carbon; D, germination index; E, ammonium content; F, nitrate) during composting.

compounds compared to the emission rate of NH3. The initial decrease in TN was not observed in all SCP treatments, as the addition of P and Mg salts conserved NH+4 in the form of struvite in the compost (Jeong and Hwang, 2005). The TOC of the CK group remained at a constant level after the third turning, whereas that of the SCP treatments exhibited a notable decrease (Fig. 3C). The addition of chemical additives not only reduced the degradation rate, but also prolonged the degradation process. The sustained degradation of carbonaceous compounds resulted in a continuous rise in TN content until the end of the experiment. After the experiment, the TN of the PMO, CaPS, and KPS treatments were higher than that of the CK group, indicating that struvite crystallization may be responsible for these differences. The low degradation rate of the PMS group resulted in a final TN content that was significantly lower than that of the PMO group (P = 0.001). The GI of all treatments decreased slightly between the start of the experiment and the first turning (Fig. 3D). A previous study

attributed this drop to the production of low molecular weight short-chain volatile fatty acids (primarily acetic acid) and high NH+4 concentrations (Guo et al., 2012). After the first turning, the GI of all treatments increased gradually until the end of the experiment. The final GI of all treatments except PMS exceeded 80%, which indicated that when supplemented at a molar percentage of 15% of TN, P and Mg salts were phytotoxin-free. In a similar study using KH2PO4 and MgCl2 as P and Mg salts, respectively, Lee et al. (2009) reported that the inhibition of decomposition started at a molar percentage of 6% and ceased completely at a percentage of 10% or higher. MgCl2, MgSO4, and MgO can all be used as sources of Mg for struvite formation in the composting process. However, if the toxicity of Cl to the seeds is a concern then MgO and MgSO4 rather than MgCl2 should be used (Lee et al., 2009; Ren et al., 2010). The GI of the PMS treatment was significantly lower than that of the PMO treatment (P = 0.000). In the PMS treatment, the low pH caused more nitrogen to be preserved in the form of NH+4, which

Please cite this article in press as: Jiang, T., et al. Effect of different struvite crystallization methods on gaseous emission and the comprehensive comparison during the composting. Bioresour. Technol. (2016), http://dx.doi.org/10.1016/j.biortech.2016.02.046

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CK CaPM

NH3 emission rate (g d-1)

12

PMO PMS

KPM Turn

A 9 6 3 0 0

7

14

21

28

35

N2O emission rate (g d-1)

Composting time (d)

B

0.8 0.6 0.4 0.2

process, causing earlier maturation of the compost (Jiang et al., 2013). The SCP treatments had a similar NH3 emission pattern but lower emission rate compared to that of the CK group. Mg and P salt supplementation can preserve NH+4 in the form of struvite crystal (Jeong and Kim, 2001). After the second turning, a decrease in NH+4 content was observed in all treatments except the PMS group, which maintain NH+4 at a high level until the end of the composting process. This observed difference may be mainly due to the low pH of the PMS group. In addition, the low pH condition would not allow further transformation of inorganic nitrogen by nitrifiers (Wang et al., 2013; Chan et al., 2016). At the end of the experiment, the NH+4 contents of all SCP treatments were higher than that of the CK group, which is consistent with previous studies (Jeong and Hwang, 2005; Ren et al., 2010; Fukumoto et al., 2011). Jeong and Hwang (2005) attributed an increase in NH+4 content to the crystallization of struvite. In our work, 2.0 mol L1 KCl solution was used as the NH+4-N extracting agent which could not extract the NH+4 in the struvite crystal (Fukumoto et al., 2011). Therefore, the NH+4 that was quantified in our study is free ammonium. The NH+4 content of the PMS treatment was significantly higher than that of the PMO group (P = 0.007). The lower pH of the PMS group reduced NH3 emission and preserved more nitrogen in the form of NH+4 (Ren et al., 2010). 3.4. Nitrate content evolution and nitrous oxide emission

0.0 0

7

14

21

28

35

Composting time (d)

10 CH4 emission rate (g d-1)

5

C

8 6 4 2 0 0

7

14

21

28

35

Composting time (d) Fig. 4. NH3 (A), N2O (B), and CH4 (C) emission patterns during composting. Diamonds indicate the turning days.

decreased the final GI (Ren et al., 2010). Guo et al. (2012) reported that NH+4 content and GI were significantly and negatively correlated (R = 0.556, P = 0.037). 3.3. Ammonium content evolution and NH3 emission pattern NH3 emissions of all groups increased sharply after the start of the experiment, with maximum emission occurring after the first turning (Fig. 4A). This high emission was caused by the intensive degradation of easily degradable organic matter and consequent high temperature, high pH, and high NH+4 content (Fig. 3E). Peak NH+4 content always coincides with active degradation of organic matter during the thermophilic phase (Chan et al., 2016). After intensive NH3 emission was observed for approximately 2 weeks, the degradation rate slowed, causing the NH+4 content and temperature to decline. NH3 emission then decreased as well. After the third week, NH3 emissions were almost undetectable in all treatments. This NH3 emission pattern is consistent with that observed in previous studies that used similar aeration systems (Fukumoto et al., 2011; Chan et al., 2016). Compared with selfaerated compost, forced aeration can accelerate the degradation

Significant N2O emission was observed immediately after the start of the experiment (Fig. 4B), consistent with other reports in the literatures (Hao et al., 2004; El Kader et al., 2007; Jiang et al., 2011). El Kader et al. (2007) suggested that the production of N2O in turkey manure occurred before composting began. Maeda et al. (2010) found that denitrification occurred rapidly, especially after turning the compost. Hao et al. (2004) reported that denitrification of NO 3 in pig feces led to the production of N2O during early composting. They observed a significant correlation between the N2O emission rate and the average NO 3 concentration in the compost (R = 0.080, n = 16). In our work, N2O emissions were already high at the start of the experiment and decreased immediately after the compost pile was made. The N2O emission coincided with the decline of NO 3 content (Fig. 3F), indicating that the denitrification of NO 3 is likely responsible for the initial N2O emission. After the initial emission, NO 3 -N was depleted, and N2O emissions decreased and maintained a low level throughout the thermophilic phase. During this stage, the high temperature and high content of free ammonia may have inhibited the nitrifiers. After the second turning, slight N2O emissions were detected in all treatments and lasted for 4–6 days. Because the molecular structures of methane (CH4) and NH3 are similar, methanotrophs are capable of oxidizing both NH3 and CH4 under thermophilic conditions (Szanto et al., 2007). Cahalan et al. (2015) also reported that the enzyme ammonia monooxygenase can oxidize both CH4 and NH3. After the third and fourth turnings, significant N2O emissions were observed in all treatments, as has been reported previously in the literature (El Kader et al., 2007; Maeda et al., 2010; Jiang et al., 2013). During turning, NO 3 is transported from aerobic regions to anaerobic regions, and the denitrification of NO 3 produces N2O. Data analysis showed that the differences in N2O emissions between all treatments were not significant (P = 0.823), indicating that the SCP did not significantly affect N2O emission. 3.5. Methane emission After composting began, CH4 emission from all treatments increased rapidly (Fig. 4C). Peak emission occurred over a period

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of 4–5 days, after which the emission levels decreased due to the exhaustion of easily degradable carbon. CH4 emission during the thermophilic phase is inevitable. CH4 fermentation is a complex biological process that can be divided into 4 phases: hydrolysis, acidogenesis, acetogenesis, and methanation (Sawatdeenarunat et al., 2015). In the methanogenic phase, methane formation occurs under strict anaerobic conditions (redox potential <250 mV) (Chandra et al., 2012). In our work, the O2 content in the outlet of the composting vessel ranged from 8 to 13% (Fig. 2B), which is unsuitable for the production of CH4. Jiang et al. (2015) reported that even under a high aeration rate of 0.54 L kg1 DM min1, CH4 emission was still clearly observed, especially in the treatments that had more pig feces in the raw materials and, consequently, a lower C/N ratio. The authors hypothesized that CH4 was produced under conditions of abundant O2 due to the copious amounts of large feces particles. Under natural conditions, O2 can only infiltrate 1–3 mm of the pig feces, resulting in serous anaerobic conditions within the bulk feces mass. After the first turning, CH4 emission began to decrease, and after the third turning, CH4 was undetectable. Jiang et al. (2013) reported that by turning the compost, CH4 emission was decreased by 83–93% compared with static compost. Turning not only homogenized the material and increased the porosity, but also broke apart the large particles of the pig feces. In our study, the CK and PMO groups exhibited notable CH4 emission during the maturing period. The high degradation rate of these 2 treatments produced a significant amount of water, which accumulated in the composting vessel and generated significant anaerobic conditions in some areas (Fig. 4F). The CH4 losses in the PMS, KPM, and CaPM groups were clearly lower than that of the CK. There are many reports in the literature indicating that the presence of SO2 4 can decrease CH4 production through several mechanisms (Hao et al., 2005; Luo et al., 2013; Yang et al., 2015). First, under anaerobic conditions, both sulfatereducing bacteria and methanogens compete for the same organic carbon and energy sources, and the sulfate-reducing bacteria may out-compete the methanogens (Hao et al., 2005; Dan et al., 2010). 2 Second, sulfur compounds (SO2 , SO2 4 , S 3 ) have toxic effects on methanogens during anaerobic digestion (Hao et al., 2005). Third, reduction of SO2 4 during composting may enhance the oxidation rate of CH4 (Yang et al., 2015). Furthermore, the additives in these treatments caused a decrease in degradation rate and moisture content, thus decreasing the anaerobic conditions compared to the CK group.

CaPM treatment was significantly lower than that of the PMO group (P = 0.001). Under alkaline conditions, the presence of Ca2+ can promote the precipitation of Ca3(PO4)2, Mg3(PO4)2, and even CaHPO4 (Yigit and Mazlum, 2007). Compared with H3PO4 and KH2PO4, Ca(H2PO4)2 has a lower solubility and is therefore less suitable for struvite formation. Struvite formation in the PMS treatment was significantly lower than that of the PMO group (P = 0.000). The low pH value of PMS treatment (initial pH = 6.85) could be responsible for the low struvite content observed at the end of composting. When the application ratio of Mg:P is 1:1, the first reaction is:

MgðOHÞ2 þ H3 PO4 ! MgHPO4 þ 2H2 O Subsequently, the following reaction occurs under alkaline conditions:

MgHPO4 þ NHþ4 þ OH þ 5H2 O ! MgNH4 PO4  6H2 O Struvite formation may be significantly affected by pH value, and the best pH range for struvite precipitation is 7.5–9.0 (Wang et al., 2010). Under acidic conditions, Mg(H2PO4)2, rather than struvite, is the main product. At pH levels greater than 9, Mg3(PO4)2 precipitation is favored. 3.7. Mass balance and cost calculation At the end of the experiment, the total NH3 losses ranged from 5.9% to 32.5% (Table 4). The SCP treatments reduced NH3 emission by 55.5–81.8%. This mitigation rate was similar to that reported by Zhang and Lau (2007) in a study employing KH2PO4 and MgCl2 as additives. Lee et al. (2009) also used KH2PO4 and MgCl2 as P and Mg salts, respectively, but they observed significant inhibition when the molar percentage of the salts was higher than 5% of the initial N. In our study, the NH3 loss rate of the PMS treatment was significantly lower than that of the PMO group (P = 0.000). As discussed above, this decreased NH3 emission may result from the presence of H3PO4 + MgSO4, which lowers the pH and inhibits the degradation process. Although the PMS treatment had the lowest N loss rate, its low GI indicated that it cannot be used directly in the composting of animal wastes (Fig. 3F). Total N2O emissions accounted for 1.0–1.5% of the initial nitrogen. Although SCP slightly reduced N2O emission, the difference in N2O emission between all treatments was not statistically signifi-

Table 4 Mass balance.

3.6. Mineral composition analysis

No.

Struvite is the main crystal in all SCP treatments and accounts for 36.7–78.3% of the total crystal content (Table 3). The highest struvite content was found in the PMO treatment indicating that Mg(OH)2 + H3PO4 is the most effective additive for struvite formation during the composting of pig feces. The struvite content of the

CK PMO KPM CaPM PMS

C balance (% initial C)

N balance (% initial N)

CH4 loss

Total C losses

N2O loss

NH3 loss

Total N losses

1.7 ± 0.01 1.8 ± 0.01 0.7 ± 0.01 0.6 ± 0.00 0.5 ± 0.00

64.9 ± 2.9 63.1 ± 5.3 60.6 ± 3.8 58.5 ± 4.6 42.9 ± 2.1

1.5 ± 0.01 1.4 ± 0.00 1.2 ± 0.01 1.3 ± 0.02 1.0 ± 0.00

32.5 ± 1.8 14.5 ± 1.1 16.0 ± 1.0 13.3 ± 0.6 5.9 ± 0.4

37.5 ± 2.1 18.8 ± 1.2 20.8 ± 1.3 17.6 ± 0.8 9.8 ± 0.8

Table 3 Mineral composition of the final compost product. Mineral

Molecular formula

CK

PMO

KPM

CaPM

PMS

Struvite (%) Calcite (%) Quartz (%) Albite (%) Talc (%) Gypsum (%) Montmorillonite (%) Dittmarite (%) Brushite (%)

NH4MgPO46H2O CaCO3 SiO2 (Na, Ca)Al(Si, Al)3O8 Mg3Si4O10(OH)2 CaSO42H2O Na0.3(Al, Mg)2Si4O10(OH)24H2O NH4MgPO4H2O CaPO3(OH) 2H2O

5.7 ± 0.6 9.3 ± 0.6 48.0 ± 3.9 26.7 ± 1.2 4.0 ± 1.0 1.3 ± 0.6 3.3 ± 0.6 – 2.0 ± 0.6

78.3 ± 5.3 3.0 ± 0.0 10.7 ± 0.7 7.3 ± 1.5 – – – 1.0 ± 0.0 –

61.3 ± 3.6 2.0 ± 1.0 21.3 ± 1.4 12.7 ± 1.2 – – 2.0 ± 0.0 1.7 ± 0.6 –

52.3 ± 5.1 4.3 ± 1.1 18.7 ± 1.1 7.7 ± 0.6 1.3 ± 0.6 1.7 ± 0.6 – 5.7 ± 1.5 8.3 ± 1.5

36.7 ± 4.0 6.3 ± 1.5 30.3 ± 3.5 18.7 ± 2.1 2.3 ± 0.6 1.0 ± 0.0 2.0 ± 0.0 2.7 ± 1.1 –

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T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx Table 5 Cost balance. No.

Additive costa (dollar t1)b Mg

CK PMO KPM CaPM PMS

c

0.0 7.3 7.5 3.2 7.3

Nutrient conservation profitsd (dollar t1)

Net cost (dollar t1)

P

Total costs

N

P

Mg

Total profits

0.0 0.5 2.0 2.0 2.0

0.0 7.9 9.5 5.3 9.4

0.0 0.9 0.8 1.0 1.3

0.0 3.3 3.3 3.3 3.3

0.0 2.0 2.0 2.0 2.0

0.0 6.2 6.1 6.3 6.7

0.0 1.6 3.4 1.0 2.7

a

The cost was calculated by per ton raw materials. 1 US dollar = 6.4537 CNY (China Yuan). The market prices of MgO and MgCl26H2O are 125 and 100 dollar t1, respectively. The market prices of H3PO4, KH2PO4, and triple superphosphate are 730, 620, and 240 dollar t1, respectively. The market prices of MgO and MgCl26H2O are 125 and 100 dollar t1, respectively. The market price of urea is 360 dollar t1. All market prices were obtained from Alibaba wholesale (http://www.1688.com/). d The value of the nutrients conserved in the compost is equated to the price of commonly use chemical fertilizers (which contain the same amount of nutrients). Nitrogen is equated to urea, P is equated to superphosphate, and Mg is equated to MgSO4. b

c

cant (P = 0.287). In a study by Fukumoto et al. (2011), MgCl26H2O and H3PO4 were used for MAP treatment during the composting of swine manure. Struvite formation significantly reduced NH3 and other nitrogenous emissions, but not N2O. The MgSO4 application reduced CH4 emission by 61.5–70.4% compared with the CK group. In a study by Luo et al. addition of 5–10% (w/w) phosphogypsum to compost resulted in a 56.3– 62.5% mitigation rate. Yang et al. (2015) reported that during the composting of kitchen waste, CH4 emission was decreased by 85.5% and 80.5% with the addition of phosphogypsum and superphosphate, respectively (10% of dry weight). Here, addition of MgSO4 only accounted for 3.3% of the dry weight. Due to its high solubility, MgSO4 is more effective than CaSO4 in the inhibition of methanogens. The total cost of Mg and P salts ranged from 5.3 to 9.5 US dollars (Table 5). CaPM was the least expensive treatment due to the low price of triple superphosphate. In the Chinese market, the cost of triple superphosphate (P2O5 > 46%) is significant lower than that of H3PO4 and KH2PO4 (Table 5). If we factor in the nutrients conserved by the additives, the net cost ranged from 1.0 to 3.4 dollar t1. The CaPM treatment had a net profit of 1.0 dollar t1. Although Ca(H2PO4)2 is not favorable for the formation of struvite, it reduced NH3 emission more effectively than PMO (Table 4). Lower degradation rates may be responsible for the inconsistency of these findings. Additionally, the final GI of CaPM was 100.2%, indicating that the compost was completely matured. After a comprehensive comparison of the 4 SCP treatments, we suggest that the CaPM treatment (Ca(H2PO4)2 + MgSO4) should be used for composting in practice.

4. Conclusion The SCP significantly decreased NH3 loss by 50.6–81.8%, but not the N2O. Addition of SO2 reduced CH4 emission by 61.5–70.4%. 4 PMO and KPS treatments were most suitable for struvite formation. Although the PMS group had the lowest NH3 emission rate, its low pH was not advantageous for struvite formation, and inhibited the degradation process. CaPM significantly decreased NH3 (59.2%) and CH4 (64.9%) emission and yielded compost that was completely matured. Due to its effective performance and low cost, we suggest that CaPM be used for composting.

Acknowledgements Financial support for this study was provided by the National Natural Science Foundation of China (No. 41201282). In addition, this research is part of the Chinese National Science and Technology

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