Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting

Bioresource Technology xxx (2016) xxx–xxx

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Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting Tao Jiang a,b, Xuguang Ma a, Qiong Tang a, Juan Yang a, Guoxue Li b,⇑, Frank Schuchardt c a

College of Chemistry, Leshan Normal University, Leshan 614004, China College of Resources and Environment Sciences, China Agricultural University, Beijing 100193, China c Johann Heinrich von Thuenen-Institute, Institute of Agricultural Technology and Biosystems Engineering, Bundesallee 50, 38116 Braunschweig, Germany b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


DCD reduces the N2O emission by

CK: control; C0: 15% Mg and P salts; C1: 15% Mg and P salts + 2.5% DCD; C2: 15% Mg and P salts + 5.0% DCD; C3: 15% Mg and P salts + 7.5% DCD; C4: 15% Mg and P salts + 10.0% DCD. GHG: greenhouse gas. Global warming potential calculation: 1 mol NH3 = 3.86 mol CO2-eq, 1 mol N2O = 298 mol CO2-eq. GHG emissionl equivalent (kg CO2-eq t-1)

76.1–77.6%.
Struvite crystal process reduces the NH3 loss by 45–53%.
DCD is decomposed faster in thermophilic phase.
DCD added by 2.5% of TN is enough to inhibit nitrification at maturing stage.

100 GHG emission equivalent from nitrogen gas 80 Ammonia

60

Nitrous oxide 40

20

0 CK

a r t i c l e

i n f o

Article history: Received 2 December 2015 Received in revised form 14 January 2016 Accepted 17 January 2016 Available online xxxx Keywords: Composting Dicyandiamide Struvite Ammonia Nitrous oxide

C0

C1

C2

C3

C4

a b s t r a c t Struvite crystallization (SCP) is combined with a nitrification inhibitor (dicyandiamide, DCD) to mitigate the NH3 and N2O emission during composting. The MgO and H3PO4 were added at a rate of 15% (mole/mole) of initial nitrogen, and the DCD was added at rates of 0%, 2.5%, 5.0%, 7.5% and 10% (w/w) of initial nitrogen respectively. Results showed that the combination use of SCP and DCD was phytotoxin free. The SCP could significantly reduce NH3 losses by 45–53%, but not the DCD. The DCD significantly inhibits nitrification when the content was higher than 50 mg kg1, and that could reduce the N2O emission by 76.1–77.6%. The DCD degraded fast during the thermophilic phase, as the nitrification will be inhibited by the high temperature and high free ammonia content in this stage, the DCD was suggested to be applied in the maturing periods by 2.5% of initial nitrogen. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Composting of animal manure is a widely used and effective technology, however harmful gasses, such as ammonia (NH3),

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

and nitrous oxide (N2O), are emitted during the process as secondary pollution. N2O for example is a significant greenhouse gas (GHG) with global warming potential 298 times higher than that of carbon dioxide (CO2) (IPCC, 2007) and is considered to be an important factor in ozone depletion (Ravishankara et al., 2009). Ammonia has been shown to be a significant, and increasing, component of airborne fine particulate matter (PM2.5) in

E-mail address: [email protected] (G. Li). http://dx.doi.org/10.1016/j.biortech.2016.01.089 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Jiang, T., et al. Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting. Bioresour. Technol. (2016), http://dx.doi.org/10.1016/j.biortech.2016.01.089

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T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx

northern China (Li et al., 2013), from 2003 to 2012 its proportion has increased from 7.5% to 12%. Unlike nitrate and sulphate pollution, ammonium in the atmosphere is largely generated from agricultural activities (Shen et al., 2011). During composting, N2O can be produced through both nitrification and de-nitrification processes. One formation mechanism is through incomplete nitrification of NH3 which is oxidized to hydroxylamine (NH2OH) by ammonia mono-oxygenase (AMO), which can be further oxidized to nitroxyl (NOH) by hydroxylamine oxidoreductase which produces N2O after polymerization and dehydration (Canfield et al., 2010). During denitrification, nitrite is reduced to NO by nitrite reductase, which is further reduced by nitric oxide reductase to N2O (Moenne-Loccoz and Fee, 2010). As a result of these processes, 0.4–9.9% of total nitrogen (TN) is emitted as N2O during the composting of animal manure (Tsutsui et al., 2015). The N2O emission rate is affected by the feedstocks C/N ratio, aeration conditions, and turning frequency (Jiang et al., 2011). When compost is well operated, the global warming potential of N2O can be reduced to about 11–18 CO2-eq t1, but this still accounts for 35–74% of total GHG emission (Jiang et al., 2013). NH3 emission accounts for 9–32% of initial total nitrogen (Fukumoto et al., 2011; Jiang et al., 2011). The emission rate can be affected by C/N ratio, aeration condition, moisture content, porosity, pile density, and so on (El Kader et al., 2007; Jiang et al., 2011). Stuvite crystallization (SCP) is one of the most effective methods of mitigating NH3 emission in waste water treatment and in recent years has been used in composting to reduce NH3 loss and to increase the compost quality (Fukumoto et al., 2011; Wang et al., 2013; Chan et al., 2016). When the application ratio of phosphate and magnesium salts is 1:1, the first reaction is:

MgðOHÞ2 þ H3 PO4 ! MgHPO4 þ 2H2 O Subsequently, the following reaction occurs under alkaline conditions and the struvite was formatted

MgHPO4 þ NHþ4 þ OH þ 5H2 O ! MgNH4 PO4  6H2 O The best H3PO4 and MgO application rate is about 10–20% of total nitrogen (mole/mole) (Jeong and Hwang, 2005), and with appropriate application rate, SCP can decrease NH3 emission by 40–84% (Zhang and Lau, 2007; Ren et al., 2010). Dicyandiamide (DCD, C2H4N4) is a well-known nitrification inhibitor that has been studied for over 90 years (Kelliher et al., 2008). DCD works by reducing the amoA gene in ammonia oxidizing bacteria (AOB) especially at high nitrogen application rates (Dai et al., 2013). Slowing nitrification results in decreased N2O production and emission rates, and reduced nitrate concentrations in soil decreases the potential for N2O production from denitrification (Kelliher et al., 2008). DCD operates in a bacteriostatic mode and does not kill soil bacteria but rather inhibits or reduces their activity. O’Callaghan et al. (2010) reported that AOB are significantly affected by DCD in which reduces population size and activity, while having little impact on the overall soil bacterial activity. DCD has been widely used in agriculture due to its low cost, minimal volatility, and solubility in water (Tian et al., 2015). It has been documented that DCD is effective at decreasing N2O emissions from fields treated with mineral fertilizer or urine. Depending on the crop system and climate, the N2O emission rate was reduced by 17–90% (Kelliher et al., 2008; Dai et al., 2013; Cahalan et al., 2015; Wang et al., 2015). While literature relating to DCD is extensive, only few published studies examine the use of DCD during composting, especially in combination with the SCP process. The purpose of the present study is to evaluate this combination of nitrification inhibitor and stuvite crystallization on NH3 and N2O emission

during composting and to determine the most effective application rate and application time. 2. Methods 2.1. Raw materials and composting installation Pig feces and corn stalk were used as raw materials in this research. Pig feces were taken from a pig fattening farm located in Beijing. Corn stalk was obtained from Shangzhuang research station of China Agricultural University. To achieve the appropriate moisture content and C/N ratio, pig feces and corn stalks were mixed at a ratio of 7:1 (wet weight). Compositions of the raw materials and the mixture are shown in Table 1. In order to simulate the forced aeration system, trials were carried out in a series of 60 L composting vessels (Fig. 1). Aeration of the vessels was controlled by a program, which also recorded the temperature automatically. 2.2. Experiment design and sample collection Six treatments were conducted in triplicate to evaluate the combination effects of the nitrification inhibitor and stuvite crystallization and to determine the best DCD application rate (Table 2). The CK without any chemical additives was used as a control treatment. For C0–C4, H3PO4 + MgO were added to induce struvite crystallization. The application rates were all supplemented on a molar basis equivalent to 15% of the initial nitrogen. For C1–C4 treatments, DCD was added as a nitrification inhibitor. Application rates were set as 2.5%, 5%, 7.5% and 10.0% of the initial nitrogen (w/w) (Table 2). The DCD application times are also shown in Table 2. Continuous aeration was employed in all experiments and the aeration rate of all treatments was 0.25 L kg DM1 min1. Materials were composted for 4 weeks and turned at days 4, 10, and 17 in order to homogenize the materials and improve the porosity. During each turning, about 100 g samples were removed for analysis. Samples were separated into 2 parts: one part was airdried, ground, passed through a 0.1 mm sieve and stored as a dry sample while the other part was immediately frozen as a fresh sample. 2.3. Analytical methods and calculations Gaseous (N2O, NH3, O2) concentrations were measured daily during the first 2 weeks, and then 3–4 times per week thereafter. Cumulative emissions for the whole composting period were calculated from the daily flux. Data for non-measured days were obtained by averaging the closest measured days. N2O and O2 were analyzed by gas chromatograph equipped with electron capture detectors (Agilent 7890A, USA) and thermal conductivity detector (Beifen 3420A, China) respectively. NH3 was absorbed by a washing bottle with boric acid (2%) and then titrated using 0.05 M H2SO4. Total nitrogen content (TN) and total organic carbon content (TOC) were measured using an Element analyzer (Elementar vario MACRO cube, Germany). To determine moisture content, fresh samples were dried at 105 °C in an oven until they reached a constant weight. Inorganic nitrogen (NH+4–N, NO 3 –N) was extracted with 2 M KCl (1:20) and analyzed using a segmented flow analyzer (Technicon Autoanalyser II System, Germany). Fresh samples were mixed with deionized water at a ratio of 10:1 (w/w) and shaken for 30 min, filtered and the supernatant was used for the measure of germination index (GI) and pH value (Guo et al., 2012). The GI was determined in triplicate using cucumber seeds. Supernatant (8 ml) was pipetted into petri dishes packed with a

Please cite this article in press as: Jiang, T., et al. Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting. Bioresour. Technol. (2016), http://dx.doi.org/10.1016/j.biortech.2016.01.089

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T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx Table 1 The compositions of raw materials and mixture. TOC (g kg Pig feces Corn stalk Mixture

NH+4–N

TN 1

NO 3 –N 1

DM)

(mg kg

408.6 ± 12.1 419.0 ± 15.8 412.7 ± 10.6

30.4 ± 0.3 9.9 ± 0.1 22.3 ± 0.3

6.1 ± 0.5 – 3.7 ± 0.1

Moisture content DM)

92.1 ± 10.1 – 55.7 ± 5.1

C/N

(%) 80.2 ± 2.8 9.3 ± 0.1 52.2 ± 1.8

13.4 ± 0.5 42.3 ± 1.5 24.8 ± 0.7

TOC: total organic carbon; TN: total nitrogen content; DM: dry matter.

Fig. 1. Sketch map of the composting vessel.

2.4. Statistical analysis

Table 2 Design of experiment and the chemical additives application methods.

a b

No.

Abb.

H3PO4 + MgO

DCD

0th (%)

0th (%)

10th (%)

16th (%)

Total (%)

1 2 3 4 5 6

CK C0 C1 C2 C3 C4

– 15.0a 15.0 15.0 15.0 15.0

– – 2.5b 2.5 2.5 5.0

– – – 2.5 2.5 2.5

– – – – 2.5 2.5

– – 2.5 5.0 7.5 10.0

Based on the mole ratio of initial total nitrogen. Based on the weight of initial nitrogen.

piece of filter paper. Ten seeds were evenly scattered on the filter paper and incubated at 20 ± 1 °C for 48 h in the dark. Deionized water was used as a control. The composting control program measured the air temperature and the temperature at the center of vessel automatically every 2 h. The DCD concentration was measured using high-performance liquid chromatography–mass spectrometry (SHIMADZU LCMS8030, Japan). The samples were extracted with water, centrifuged, precipitated with acetonitrile, and separated on a Thermo syncronis column using a mixture of acetonitrile (containing 0.2% formic acid) and 0.1% formic acid (containing 10 mmol L1 ammonium formate) (95:5) as the mobile phase and a flow rate of 0.3 mL min1.

All data were analyzed using One-way Analysis of Variance (ANOVA); LSD-t was used to test for significant differences (Jiang et al., 2015). SPSS 17 for windows was used for all statistical analyses. 3. Results and discussion 3.1. Temperature and oxygen concentration evolution The temperature of all treatments increases sharply at the beginning of composting and exceeded 65 °C after 3 days (Fig. 2A). The thermophilic phase lasted around 2 weeks which is long enough to satisfy the Chinese national standard GB 7989-87 for sanitation. After the exhaustion of easily degradable carbon, the thermophilic phase ends and the temperatures decrease gradually reaching atmospheric temperature after the third week. When compare to the windrow system the thermophilic phase in this trial is significantly shorter (Jiang et al., 2013), possibly as a result of the forced aeration accelerating the degradation process. In contrast, due to intensive degradation, the O2 content in the outlet of all treatments decreased to 5–10% by the second day and maintained a low level for 10 days (Fig. 2B). After the second turning, the O2 content began to increase, and reached the air concentration after the third turning. Data analysis showed that differences in temperature (P = 0.806) and O2 (P = 0.898) content

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T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx

80 CK C0 C4 Air Turn

60 40 20

Temperature ( C)

Temperature ( C)

80

C1 C2 C3 Air Turn

60 40 20

A

0

0 0

7

14

21

0

28

7

21

28

20

O2 concentration (%)

20

O2 concentration (%)

14

Composting time (d)

Composting time (d)

15 10 5

15 10 5

B 0

0 0

7

14 21 Composting time (d)

28

0

7

14 21 Composting time (d)

28

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

between all treatments were not significant. This indicates that the chemical additives do not significantly affect the degradation process. 3.2. Chemical characteristics of compost The NH+4 content of the initial raw materials is 3.7 g kg1 DM, indicating that the initial mixture is unstable (Fig. 3A). Initially the NH+4 content in the compost increased in the experiment due to the conversion of organic nitrogen to NH+4 via ammonification (Luo et al., 2013). The maximum NH+4 content for each treatment occurred during the first turning. Afterwards, the NH+4 content declined due to intensive NH3 losses. After each trial, the NH+4 contents of treatments C0–C4 were higher than that of CK (Fig. 3A). Addition of P and Mg salts results in an increase in NH+4 content in the end product which is consistent with previous studies (Jeong and Hwang, 2005; Ren et al., 2010; Fukumoto et al., 2011). Jeong and Hwang (2005) have attributed increased NH+4 content to crystallization of struvite. In this research the pH range of the compost was 8.2–8.7, which is favorable for struvite crystallization (Fig. 4A). The solubility product (Ksp) of struvite is 2.5  1013, meaning that it is insoluble in neutral/alkaline conditions and therefore difficult to dissolve in KCl solution as in Bremner’s method. Fukumoto et al. (2011) reported that NH+4 nitrogen extracted by HCl solution (3.7–17.5 g) is significantly higher than that extracted by KCl solution (2.7– 4.6 g). Therefore the difference in NH+4 extracted with 2.0 mol L1 KCl and with 0.5 mol L1 HCl corresponds to the struvite nitrogen. In this research, 2.0 mol L1 KCl solution was used as the NH+4 nitrogen extracting agent, which means that the NH+4 content is all free ammonia. The final NH+4 contents (1.1–2.0 g kg1 DM) of the C0–C4 treatments were lower than previous studies (6–14 g kg1 DM), in which P and Mg were added at similar rates (Jeong and Hwang, 2005; Zhang and Lau, 2007; Ren et al., 2010). In these studies, the NH+4 content increased gradually throughout the composting process and peaked at the end of the experiment. Different

extraction solvents may be responsible for these differences. When using water as the extraction solvent, the NH+4 content (1.4–3.0 g kg1 DM) was equivalent to the present study (Lee et al., 2009). At the end of the experiment the NH+4 concentration of the C4 treatment was significantly higher than that of C0 (P = 0.039). During the maturing period, the DCD inhibits the nitrification of NH+4 to + form NO 3 , which contributes to the accumulation of NH4. This accumulation was also observed when DCD was applied to grasslands in the UK (Barneze et al., 2015). Due to denitrification, the NO 3 content of all treatments was also observed to decrease quickly below the detection limit after the start of the trial (Fig. 3B). Afterwards, the higher temperatures and higher free ammonia content inhibited the activity of nitrobacteria, which maintained the NO 3 content at a low level throughout the thermophilic phase (Sánchez-Monedero et al., 2010). After two weeks, nitrification coupled with a decrease in temperature caused the NO 3 content to increase in the CK and C0 treatments. However, in the cases of C2–C4, the DCD inhibited the nitrifiers and kept the 1 NO ) until the end of the 3 content at a low level (<50 mg kg experiment. During the C1 treatment, the nitrification inhibitor did not work. DCD is susceptible to biodegradation via guanylic urea, guanidine and urea to yield CO2 and NH+4, with a regression of t1=2 ¼ 168  e0:084T in soils (Kelliher et al., 2008). During the thermophilic phase, the DCD degraded rapidly, especially at temperatures exceeding 60 °C (Fig. 4C). After the tenth day, the DCD content of C1 was lower than 10 g kg1, which is too low to inhibit nitrification. Therefore, the degradation of DCD is responsible for the high NO 3 concentration observed in C1. After the third turning, the temperature reached the air temperature (about 20 °C) and the degradation of DCD slowed, allowing the C2–C4 treatments to maintain their DCD content until the end of the trial. During the thermophilic phase, the high temperatures and high NH+4 content may inhibit nitrification. Based on this, DCD was suggested to be added during the maturing period to avoid degradation. After the second turning, the DCD content of C2 was lower than 150 g kg1 and close to the initial DCD content of C1. If DCD

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T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx

C0

C4

C1 NH4+-N concentration (g·kg-1DM)

NH4+-N concentration (g kg-1DM)

CK 6

4

2

0 0

7

14 21 Composting time (d)

4

2

0 0

7

14 21 Composting time (d)

28

600 NO3--N content (mg kg-1DM)

NO3--N content (mg kg-1DM)

C3

A

28

600

C2

6

400

200

0

B 400

200

0 0

7

14 21 Composting time (d)

28

0

45

7

14 21 Composting time (d)

28

45 TN content (g kg-1 DM)

TN content (g kg-1 DM)

C 35

25

15

35

25

15 0

7

14 21 Composting time (d)

28

0

7

14 21 Composting time (d)

28

Fig. 3. Ammonium (A), nitrate (B) and total nitrogen (C) content evolution during the composting. Diamonds indicate the turning days.

was added during the maturing period when the nitrifiers are most active, 2.5% of the initial nitrogen content is enough to inhibit nitrification. Data analysis shows that the differences in NO 3 content between all treatments are statistically significantly (P = 0.000). Changes in the TN during these reactions are shown in Fig. 3C. During the first week, the TN of CK treatment decreased slightly due to intensive NH3 emission. Although emissions of NH3 were significant after this point, the TN increased gradually. The SCP not only reserved the nitrogen in the form of struvite, but also could increase the NH+4 content in the end compost production (Jeong and Hwang, 2005; Ren et al., 2010). Chan et al. (2016) reported that the composting mass is reduced faster than the nitrogen, the total Kjelhdal nitrogen increased mainly as a concentration effect. In the present work, the TN of the CK treatment also significantly increases, which indicates that the intensive degradation of organic carbonaceous compounds may also be responsible for the increase in TN content. After the experiment, the TN values of the C0–C4 treatments were higher than that of CK, indicating that struvite crystallization may be responsible for these differences. Germination index (GI) is a good indicator of maturity and phytotoxicity and can be used to ascertain whether the end

product is thoroughly decomposed (Guo et al., 2012). During the first week, the GI of all treatments decreased slightly (Fig. 4B). This drop is most likely due to the production of low molecular weight short chain volatile fatty acids (mainly acetic acid) and high NH+4 concentrations (Guo et al., 2012). After the first turning, the GI of all treatments increased gradually as the NH+4 content declined until the end of the experiment. The GI of C0 treatment at the end of the experiment was 107%. Since there is also no significant difference from the CK treatment (P = 0.832), indicating that addition of H3PO4 + MgO at 15% of initial total nitrogen (mole/mole) was phytotoxin free. Lee et al. (2009) reported the inhibition of composting process started at a molar rate 6% and ceased completely when the application rate was 10% or higher. In this study, KH2PO4 and MgCl2 were used as P and Mg salts, respectively. MgCl2, MgSO4 and MgO can all be used as Mg sources in the composting process for struvite formation. However, if the deterioration of compost quality with residual Cl or SO 4 is a concern then MgO rather than MgCl2 and MgSO4 should be used (Lee et al., 2009; Ren et al., 2010). The GI values of the C2–C4 treatments were lower than that of the CK treatment, mainly due to the concentration of NH+4. A

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T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx

C0

C4

C1 9.0

8.7

8.7

pH

pH

CK 9.0

8.4

8.1

A

8.4

7.8 0

7

14 21 Composting time (d)

28

0

7

14 21 Composting time (d)

80

40

0

28

B

120 Germination index (%)

120 Germination index (%)

C3

8.1

7.8

80

40

0 0

7

14 21 Composting time (d)

28

0

7

14 21 Composting time (d)

300

200

100

0

28

C

400 DCD content (mg·kg-1)

400 DCD content (mg·kg-1)

C2

300

200

100

0 0

7

14 21 Composting time (d)

28

0

7

14 21 Composting time (d)

28

Fig. 4. pH value (A), germination index (B) and DCD (C) content evolution during the composting. Diamonds indicate the turning days. Square indicate the DCD adding days (purple for C1, orange for C2, black for C3 and blue for C4). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

previous study reported that the NH+4 content and the GI were significantly and negatively correlated (R = 0.556, P = 0.037) (Guo et al., 2012).

3.3. Ammonia emission The NH3 emission pattern during composting is shown in Fig. 5A. NH3 emissions of all the treatments increased sharply after the start of the trial, with maximum emission occurring after the first turning. The NH3 emission pattern is similar to results from previous studies, which used similar aeration systems (Fukumoto et al., 2011). However, the NH3 emission rate was significantly accelerated compared to the self-aeration window system (Jiang et al., 2013). Forced aeration may expedite the degradation process, accelerate the increase in temperature, and therefore speed up the NH3 emission rate. While intensive NH3 emission last for about 10 days, afterwards the degradable materials were exhausted and the degradation rate slow down, resulting in a decline in NH3 emissions. After 2 weeks, the NH3 emissions were almost undetectable in all treatments.

After each turning, a notable NH3 emission was observed. Similar NH3 emission patterns were also observed in previous studies (Szanto et al., 2007; Jiang et al., 2015), which are attributed to the degradation of the partially decomposed materials. High NH3 emissions generally coincide with high temperatures and high O2 uptake rates (Jiang et al., 2011). In the CK treatment, the NH3 emission and O2 concentration in the outlet were negatively correlated (R = 0.861, P = 0.013). The NH3 emission rates of the C0–C4 treatments were significantly lower than that of the CK treatment. The SCP decreased the available NH+4 content during the thermophilic phase that reduced the NH3 emission. During the composting of food waste the addition of MgO and KH2PO4 even promote the NH3 losses, the low pH value (pH < 6) and low degradation rate of control treatment should responsible for the unusual results (Wang et al., 2013; Chan et al., 2016). Literature studies show that the effect of DCD on NH3 emissions is inconsistent, ranging from a 9–56% increase (Zaman et al., 2009) to a 50.9% decrease (Tian et al., 2015). In this study, the differences between the C0 and C4 treatments is not significant (P = 0.819), indicating that the addition of DCD does not affect the emission of NH3.

Please cite this article in press as: Jiang, T., et al. Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting. Bioresour. Technol. (2016), http://dx.doi.org/10.1016/j.biortech.2016.01.089

7

12 9

CK C0

6

C4 Turn

3

NH3 emission rate (g·d-1)

NH3 emission rate (g·d-1)

T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx

0

A

12 9

C1 C2

6

C3 Turn

3 0

0

7

14

21

28

0

7

Composting time (d)

N2O emission rate (g·d-1)

0.8 N2O mission rate (g·d-1)

14

21

28

Composting time (d)

0.6 0.4 0.2 0.0

B

0.8 0.6 0.4 0.2 0.0

0

7

14

21

28

Composting time (d)

0

7

14

21

28

Composting time (d)

Fig. 5. NH3 (A) and N2O (B) emission patterns during the composting. Diamonds indicate the turning days.

3.4. Nitrous oxide emission The pattern of N2O emissions are shown in Fig. 5B. At the beginning of the experiment, significant N2O emissions were immediately observed in every treatment. This emission pattern is also observed in the literatures (Hao et al., 2004; El Kader et al., 2007; Jiang et al., 2011). Hao et al. (2004) and Jiang et al. (2011) suggested that the presence of NO 3 in the pig feces led to the production of N2O during early composting. Hao et al. (2004) observed significant correlations between the rate of N2O emission and the average concentration of NO 3 in the compost (R = 0.080, n = 16). El Kader et al. (2007) suggested that the production of N2O in turkey manure occurred before the compost pile was started. In this case, the N2O emissions were already high at the start of the experiment and decreased immediately after the pile was made. In our study, the temperature during the first day was suitable for the nitrosomonas, and DCD was able to inhibit the nitrification activities in the C1–C4 treatments. Therefore, the N2O emissions during the first day occurred through the denitrification of nitrate in the raw materials. After the second day, the initial NO 3 concentrations decreased along with the N2O emissions to a low level and were maintained for about one week. High temperatures and high concentration of free ammonia may have inhibited the nitrifiers. In addition, the lack of available nitrate sources may have rendered the heterotrophic denitrifiers inactive. A notable N2O emission was observed in CK and C0 treatment since day 7 and reached the peak emission after the second turning. The NO 3 maintain at a very low level (20–90 mg kg1) during the thermophilic phase (Fig. 3B), cannot support such a significant N2O production by denitrification, so the N2O can only be produced by nitrification during this stage. Although the high temperatures and high NH+4 concentrations could inhibit the nitrifiers, the methanotrophs are capable of oxidizing ammonium under thermophilic conditions (Szanto et al., 2007). During this period, the N2O emission rate in the C4 treatment was undetectable, which indicates that the DCD not only inhibits the nitrifiers but can also slow the activities of the

methane oxidizers. Cahalan et al. (2015) also reported that the use of DCD could potentially increase CH4 emissions, since the enzyme ammonia monooxygenase, which DCD is thought to inhibit, can oxidize CH4 as well as NH3. After the third turning, significant N2O emissions were observed in the CK, C0 and C1 treatments. This phenomenon has also been reported in previous literature (El Kader et al., 2007; Jiang et al., 2011). It is well documented that the observed N2O emission is produced by the denitrification of NO 3 , which is transported from the aerobic area to the anaerobic regions by the turning. The N2O emissions of the C2–C4 treatments were significantly lower than that of the CK treatment, especially during the maturation period. DCD inhibited the nitrification activities and maintained the nitrate concentration at a low level (Fig. 4B), making denitrification difficult. Data analysis shows that the differences between the C2 and C4 treatments are not significant (P = 0.682) and that a 5% DCD addition rate is adequate for inhibiting nitrification. As previously discussed, the DCD was degraded fast during the thermophilic phase, resulting in poor nitrification inhibition effect (Fig. 4C). Since the high temperatures and high free ammonia content inhibit nitrification during thermophilic period, the addition of DCD should occur during the maturing period. 3.5. Nitrogen balance After the experiment, the total nitrogen losses ranged from 19.3% to 43.9% (Table 3). Most nitrogen is lost in the form of NH3 (79–88% of total losses), but a considerable part is also lost as N2O. In this study, NH3 emissions account for 15.8–37.8% of the initial nitrogen content. Struvite crystallization can reduce the NH3 losses by 52.4–59.1%. This mitigation rate was lower than what was observed by Ren et al. (2010), where similar materials were used. In their study, MgO was added at a concentration equal to 20% of the initial nitrogen, while H3PO4 was added at 1–3 times this concentration. Excess H3PO4 reduced the pH of the compost which reduces the volatility of ammonia, but also increases the cost and reduces the quality of the compost.

Please cite this article in press as: Jiang, T., et al. Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting. Bioresour. Technol. (2016), http://dx.doi.org/10.1016/j.biortech.2016.01.089

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T. Jiang et al. / Bioresource Technology xxx (2016) xxx–xxx

Table 3 Nitrogen balance. Nitrogen in compost (%)

CK C0 C1 C2 C3 C4 a b c

Initial Final Initial Final Initial Final Initial Final Initial Final Initial Final

Nitrogen losses (%)

NH+4

NO 3

Org-N

Crystal-N

NH3 loss

N2O loss

Sample loss

Other lossesc

Total N losses

16.5 ± 1.1 1.0 ± 0.0 16.6 ± 0.9 2.3 ± 0.2 16.4 ± 0.7 2.7 ± 0.0 16.2 ± 1.2 4.3 ± 0.1 15.9 ± 0.8 4.1 ± 0.2 15.6 ± 13.3 4.2 ± 0.1

0.2 ± 0.01 0.9 ± 0.02 0.3 ± 0.00 1.1 ± 0.03 0.2 ± 0.00 0.8 ± 0.03 0.3 ± 0.02 0.3 ± 0.01 0.2 ± 0.01 0.2 ± 0.00 0.2 ± 0.01 0.2 ± 0.00

83.2 ± 5.9 54.3 ± 1.5 83.2 ± 6.2 59.3 ± 3.3 83.4 ± 9.7 61.4 ± 5.3 83.5 ± 8.1 58.2 ± 3.2 83.9 ± 6.6 61.4 ± 3.8 84.2 ± 9.1 59.3 ± 2.9

– 0.0 – 15.0 – 15.0 – 15.0 – 15.0 – 15.0

– 38.7 ± 3.2 – 17.9 ± 1.7 – 16.1 ± 2.1 – 18.4 ± 1.3 – 15.8 ± 1.9 – 17.2 ± 2.8

– 1.5 ± 0.02 – 1.4 ± 0.01 – 1.0 ± 0.03 – 0.3 ± 0.02 – 0.3 ± 0.00 – 0.3 ± 0.02

– 1.3 ± 0.01 – 1.6 ± 0.00 – 1.5 ± 0.01 – 1.6 ± 0.01 – 1.5 ± 0.00 – 1.5 ± 0.00

– 2.5 ± 0.02 – 1.4 ± 0.01 – 1.7 ± 0.03 – 1.8 ± 0.02 – 1.6 ± 0.01 – 2.5 ± 0.01

– 43.9 ± 2.3 – 22.3 ± 2.1 – 20.2 ± 1.6 – 22.1 ± 1.9 – 19.3 ± 2.5 – 21.4 ± 1.2

a

b

The organic N = TN  NH+4–N  NO 3 N  crystal N. Calculate number, seems that the chemical additives were all crystal in the form of struvite. Calculate number, other losses = total N losses  NH3 loss  N2O loss  sample loss.

Mass balance showed that the addition of Mg and P salts not only conserved the nitrogen as struvite, but also increased the proportion of NH+4 and organics in the end product. The SCP process reduces NH3 emission during the thermophilic phase resulting in NH+4, some of which is converted to organic nitrogen by microbes during the maturing period. Fukumoto et al. (2011) reported that the application of Mg and P salts decrease available nitrogen for nitrifiers, resulting in low NO 3 content in the final product. But in our research, the NH+4 concentration of C0 was close to the CK, and the end NO 3 content of the 2 treatments were similar (Fig. 3B). Total N2O emissions of CK and C0 accounted for 1.3–1.6% of the initial nitrogen. This is comparable to the results obtained by Fukumoto et al. (2011), but significantly higher than those of Hao et al. (2004) who used similar raw materials. Composting size and aeration conditions may be responsible for the changes in N2O emissions. Literature studies have shown that low emission rates are often coupled with large pile size and self-aeration (Hao et al., 2004; Jiang et al., 2015). Application of DCD can significantly reduce N2O losses (P = 0.000). When the DCD application rate is higher than 5%, it can reduce the total N2O emission by 76.1–77.6%. This mitigation rate is similar or higher than the application rate in croplands (Cahalan et al., 2015; Tian et al., 2015). Dai et al. (2013) reported that DCD has the largest reduction of AOB amoA gene copy numbers under the highest nitrogen application rates. Therefore the high nitrogen concentration of composting materials is suitable for the work of DCD. Luo et al. (2013) used DCD (0.1–0.2% of dry weight) and superphosphate (5.0–10.0% of dry weight) jointly and achieved only a 35.5% mitigation rate after the trial concluded. In their research, DCD and superphosphate were mixed into the compost at the beginning of the experiment, so as discussed above, DCD may have been degraded during the thermophilic phase.

4. Conclusion Results of this study suggested that the SCP and DCD could be combined used during the composting without any phytotoxin. Combination use could reduce the NH3 and N2O by 45–53% and 76–78%, respectively. The DCD degraded fast during the thermophilic phase, as the high temperature and high free ammonia content could inhibit the nitrifiers, the DCD is suggested to be applied in the maturing stage (<45 °C). Nitrification inhibition works well when the DCD content is higher than 50 mg kg1 that means 2.5% application rate was enough if the DCD was employed during the maturing periods.

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Please cite this article in press as: Jiang, T., et al. Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting. Bioresour. Technol. (2016), http://dx.doi.org/10.1016/j.biortech.2016.01.089