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Ammonia emissions and biodegradation of organic carbon during sewage sludge composting with different extra carbon sources

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Page 1: Ammonia emissions and biodegradation of organic carbon during sewage sludge composting with different extra carbon sources

at SciVerse ScienceDirect

International Biodeterioration & Biodegradation 85 (2013) 624e630

Contents lists available

International Biodeterioration & Biodegradation

journal homepage: www.elsevier .com/locate/ ibiod

Ammonia emissions and biodegradation of organic carbon duringsewage sludge composting with different extra carbon sources

Yunbei Li a, Weiguang Li a,b,c,*, Baiyin Liu d, Ke Wang a,b, Chengyuan Su a, Chuandong Wu a

a School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, Chinab State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, Heilongjiang 150090, ChinacNational Engineering Research Center of Urban Water Resources, Harbin 150090, ChinadChinese Research Academy of Environmental Science, Beijing 100012, China

a r t i c l e i n f o

Article history:Received 24 November 2012Received in revised form25 April 2013Accepted 25 April 2013Available online 20 May 2013

Keywords:Sludge compostingNitrogen lossAmmonia emissionsBioavailabilityCarbon source

* Corresponding author. State Key Laboratory ofEnvironment, Harbin Institute of Technology, Harbin,Tel./fax: þ86 0451 86283003.

E-mail addresses: [email protected], [email protected] (K. Wang).

0964-8305/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.ibiod.2013.04.013

a b s t r a c t

Ammonia emissions during composting result in the reduced value of agronomic production and canalso pollute the air. To evaluate the influence of various carbon sources on ammonia emissions, sixcomposting experiments were carried out with different amendments of carbon sources (glucose, su-crose and straw powder). The cumulative ammonia volatilizations were reduced from 3.11 g/kg (R6) to2.46 g/kg (R1), 2.17 g/kg (R2), 2.23 g/kg (R4) and 1.93 g/kg (R5). Compared to the control, no significantdifference of ammonia emissions and carbon degradation was observed for the mixture of R3 (3.15 g/kg),which was amended with straw powder alone. The co-addition of sucrose and straw powder led to thelowest ammonia emissions. According to these results, a higher C/N ratio did not necessarily indicate aneffective solution for reducing ammonia emissions, and not all readily available carbon compounds werehelpful in reducing ammonia emissions. The addition of sucrose promoted the decomposition of organiccarbon during the intensive stage of ammonia emissions, and the combination of straw and sucroseprolonged this promotion. Thus, the co-addition of sucrose and straw powder made it possible to reduceammonia emissions drastically by nitrogen immobilization.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Sewage sludge is an inevitable by-product of the wastewatertreatment process. Recently, the production of sewage sludgeincreased rapidly with the improvement of municipal wastewatertreatment capacity in China (Wang, 1997). Composting is one of themost widely employed approaches for disposing of sewage sludgebecause of its cost reduction, easy application and low pollution(Wang et al., 2011). During the aerobic thermophilic compostingprocess, complex and unstable organic constituents convert into arelatively stable humus-likematerial, which can be handled, stored,and used as a soil conditioner and fertilizer (Haug, 1993).

Nitrogen is easily lost through ammonia volatilization, whichreduces the fertilizing ability of the manure and causes environ-mental odor pollution problems (Ogunwande et al., 2008). It is wellknown that an excess amount of nitrogen is released when theinitial C/N ratio is small (Jiang et al., 2011); therefore, to reduce

Urban Water Resource andHeilongjiang 150090, China.

hotmail.com (W. Li), hitwk@

All rights reserved.

ammonia emissions, several researchers have sought to increasethe C/N ratio by using carbon amendments. Various carbon-richcompounds such as sawdust, straw and paper have been used ascarbon amendments in several studies (Kirchmann and Witter,1989; Nakasaki et al., 2001; Torkashvand, 2009). However, whendifferent materials have been added, their effectiveness inreducing ammonia emissions has not been evident. For example,Mahimairaja et al. (1994) demonstrated that ammonia loss wasreduced by 33% in manure amended with wheat straw. Increasingthe straw additions to manure reduced ammonia volatilizationduring aerobic composting from 44% to 9% (Kirchmann and Witter,1989). In contrast, Gilhespy et al. (2009) indicated that theincreasing the concentration of straw amendments increased thetemperature of the composting pile and hence promoted ammoniaemissions. A few studies have reported that molasses, a readilyavailable carbon, appeared to effectively reduce ammonia emis-sions (Subair, 1995; Liang et al., 2006; Torkashvand, 2009).Furthermore, because of its high C/N ratio, sawdust was alsopopularly used to regulate such ratios in composts. However, Subair(1995) found that the effectiveness of sawdust on reducingammonia emissions may not be due to nitrogen bio-immobilizationbut to physical adsorption.

Page 2: Ammonia emissions and biodegradation of organic carbon during sewage sludge composting with different extra carbon sources

Table 1The characteristics of the raw composting materials.

Parameters Moisturecontent (%)

pH Kjeldahl N (%) TOC (%) C/N

Sewage Sludge 86.6 � 0.6 6.9 � 0.4 3.13 � 0.12 23.1 � 1.7 7.38Sawdust 9.1 � 0.05 7.1 � 0.5 0.12 � 0.004 48.8 � 1.2 406.7Straw powder 6.7 � 0.03 7.2 � 0.04 0.24 � 0.005 44.3 � 2.3 184.6Glucose nd nd nd 39.96 ndSucrose nd nd nd 42.07 ndR1 63.4 � 1.5 5.45 � 0.04 2.09 � 0.04 26.8 � 0.7 12.8R2 62.6 � 2.1 5.23 � 0.17 2.06 � 0.17 26.7 � 1.12 12.9R3 63.4 � 1.8 7.52 � 0.21 2.03 � 0.21 26.2 � 0.87 12.9R4 61.8 � 1.5 5.31 � 0.23 2.02 � 0.23 27.9 � 0.54 13.8R5 62.4 � 1.5 6.14 � 0.06 1.95 � 0.06 27.6 � 0.87 14.15R6 62.1 � 1.5 7.63 � 0.04 2.10 � 0.04 24.3 � 0.97 11.5

nd, not detected; Each value is the mean of triplicates (dry weight basis).

Y. Li et al. / International Biodeterioration & Biodegradation 85 (2013) 624e630 625

It is difficult to evaluate the direct effects of the C/N ratio onammonia emissions. Therefore, this factor is not a suitable andexclusive indictor for reducing nitrogen loss. Other factors, such asthe types of carbon sources, chemical form and the particle size ofthe carbon source may also influence ammonia emissions. Despitethe fact that several researchers have observed significant effects ofcarbon-rich compounds on nitrogen loss, there is little informationavailable in literature regarding the interaction between carbondegradation rates and ammonia emissions using different carbonsamendments (Matsumura et al., 2010). It is difficult to explain whydifferent types of carbon sources show different effects on reducingammonia emissions; thus, it is necessary to examine factors otherthan the C/N ratio that can affect ammonia emissions.

Thus, in our study, different carbon-rich amendments withdifferent degradation rates, such as glucose, sucrose, straw powderand mixtures of straw powder and glucose or sucrose were addedat the beginning of composting. The objectives of the current studyare 1) to assess the effects of these carbon compounds on com-posting performance and ammonia emissions during sewagesludge composting and 2) to discuss the interaction betweendegradation rate of carbon compounds and ammonia emissions.

2. Materials and methods

2.1. Composting materials and experimental design

Dewatered sewage sludge (SS) was collected from the Wen-chang (China) urban sewage treatment plant. Because of theexcessive humidity of SS, sawdust was used as a bulking agent toregulate moisture content and provide optimum free air space.Before mixing with the SS, the sawdust, which was considered aninner material, was manually cut to a width of 2.5 mm and a lengthof 16 mm. The soluble organic matter from the sawdust could beignored because of its insignificant effects compared to SS or othercarbon-rich amendments.

To obtain reproducible data, the same composting material wasused throughout the experiment. Approximately 1460 g of SS and280 g of sawdust were well mixed and divided into six parts. Eachpart had the same composition (243.3 g SS and 46.6 g sawdust).Various carbon-rich amendments d glucose, sucrose, straw pow-der and mixtures of glucose or sucrose and straw powder d wereadded. Six laboratory-scale composting experiments with differentamendments were set up as following: R1 (5 g glucose, 243.3 g SSand 46.6 g sawdust), R2 (5 g sucrose, 243.3 g SS and 46.6 gsawdust), R3 (5 g straw powder, 243.3 g SS and 46.6 g sawdust), R4(5 g glucose, 5 g straw powder, 243.3 g SS and 46.6 g sawdust), R5(5 g sucrose, 5 g straw powder, 243.3 g SS and 46.6 g sawdust), R6(No amendment except for 243.3 g SS and 46.6 g sawdust). Thecharacteristics of the raw materials are presented in Table 1.

The composting experiments were conducted in column re-actors (100 mm in diameter, 300 mm in depth) made of glass. Thisreactor was equipped with a silicone rubber stopper at the top.Three holes were drilled into this silicone rubber stopper, the firstone used for aeration, the second for detecting temperature, andthe last one for collecting exhaust gas. The aeration rate wasmonitored by a flowmeter (LZB, China) and maintained at0.135 L h�1, which was considered sufficient for maintaining aer-obic conditions based on our preliminary experiment. The exhaustgas from composting reactor passed through a 250 mL Erlenmeyerflask containing 50 mL 2% boric acid to capture the ammonia. Tominimize heat loss and simulate a self-heating reactor, all of thecomposting reactors were placed in a water bath with a tempera-ture sensor in it to measure the environmental temperature. Thetemperature of the water bath was maintained below that insidethe reactor (approximately 3e5 �C). This temperature-controlling

procedure has been reported by Lashermes et al. (2012) andMason and Milke (2005).

2.2. Chemical analysis

A thermometer was inserted into the composting pile to recordthe temperature every 24 h at the same point inside the reactor.Ammonia in the exhaust gas was trapped by a 2% H3BO4 solutionand measured by titration (Al-Kanani et al., 1992). The concentra-tions of ammonia were measured every 24 h until the temperaturedropped below 40 �C. After the initial measurements, the exhaustgas was analyzed every 48 h due to a reduction in emissions, andthe absorbent solution was replenished after each titration.

The solid composting samples were collected at specific intervalsbased on temperature evolution: day 0, the starting period; day 3,the thermophilic phase; day 6, the end of thermophilic phase; andday 22, the end of the composting process. To reduce the mea-surement error, all samples were taken from the center of thecomposter after stirring the composted material, and then mixedtogether to form a homogeneous samplematerial. Moisture content,pH and NH4

þ-N were determined by analyzing fresh compostingsamples. Total organic carbon, organic matter and Kjeldahl nitrogenwere determined using samples after drying. Moisture content wasmeasured by weight loss of the compost sample after drying at105 �C for 24 h (Nakasaki et al., 1998). After measuring the moisturecontent, the organic matter or ash of the dried sample was deter-mined by burning it at 550 �C for 4 h (Witter and Lopez-Real, 1988).The pHwas determined using the pHmeter (PH-S) by dissolving 1 gcompost sample (<1 mm) in 10 mL distilled water (Rihani et al.,2010). NH4

þ-N was extracted with 50 mL 2 mol/L KCl (1 g solidsample with a 50 mL KCl solution) on a shaker at 150 rpm andenvironmental temperature for 1 h, measured by the IndophenolBlue method and followed by colorimetric (Li et al., 2012). The totalorganic carbon was determined following the standardized methodof WalkleyeBlack wet combustion method (Walkley and Black,1934). The Kjeldahl nitrogen of the composting sample was deter-mined by using the Kjeldahlmethod (Bremner andMulvaney,1982).

All presented results were average values of triplicated mea-surements, and the maximum difference among the triplicate re-sults was 5%. During composting, significant differences among thevalues of each studied parameter were calculated by the least sig-nificant difference (LSD) test at p < 0.05 by SPSS 17.0.

3. Results and discussion

3.1. Temperature

Temperature is an important indicator both for both decompo-sition rates and microbial activity in composting (Woodford, 2009).

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Fig. 2. Changes in pH in the different treatments during composting.

Y. Li et al. / International Biodeterioration & Biodegradation 85 (2013) 624e630626

In our study, the temperature measurements were taken from thecore of the composting reactor because of the small volume of thereactor. Fig. 1 shows the change in temperature over time for thedifferent treatments. No significant difference was observed in anyof the runs until the peak values were achieved at 72 h, after whichtime the temperatures declined. The maximum temperaturereached during composting was 62 �C in R4 and R5. Although hightemperature conditions (50e60 �C) are needed to ensure thedestruction of pathogenic organisms (Shammas and Wang, 2008),the thermophilic phase (T > 55 �C) was maintained for only 60 h,which was shorter than the other study (Kulikowska and Klimiuk,2011; Lashermes et al., 2012). This phenomenon could beexplained by the smaller size of the laboratory-scale compostingreactor because the higher temperature condition could be easilyachieved with full-scale composting (Pagans et al., 2006). After-ward, the temperatures began to decrease and the compostingprocess entered into a mesophilic phase at 192 h. Finally, contin-uous decreases in temperature were observed before the temper-ature stabilized at a lower ambient level in all of the runs. Thesetemperature results indicated that the small amount of carbonamendment added to the compost was not sufficient enough toinfluence the pile temperature because the heat generated fromcarbon amendment was insignificant compared to that of thesewage sludge.

3.2. pH

As shown in Fig. 2, the addition of glucose and sucrosedecreased the initial pH in R1, R2, R4 and R5, but the straw powderhad no effect. An explanation for these results is that the glucoseand sucrose could be easily degraded bymicrobes which causes therelease of acid intermediates (Nakasaki et al., 2000; Torkashvand,2009). Moreover, the pH values of 5% glucose and sucrose solu-tions were 6.02 and 6.28, respectively, which are slight acidic; thus,the initial addition of glucose or sucrose could decrease the pH insome degree. However, when the temperature began to reach thethermophilic phase on the third day, this initial difference in pHdisappeared and alkaline ranges (8.0e8.5) were observed in all ofthe runs. This increase in pH was the result of protein degradationthe consumption of the organic acids, indicating that the concen-tration of acid intermediates, which came from the degradation ofglucose and sucrose, was not enough to completely neutralize thealkaline materials. Afterward, the pH values began to decrease to

Fig. 1. Changes of temperature in different treatments during composting.

approximately 7.0 during the final days of composting because ofthe ammonia volatilization and ammonium oxidation by nitro-bacteria (Rihani et al., 2010). The final neutral pH implied that thecomposting production stabilized, supporting by Hachicha et al.(2009) who reported that the decrease in pH promoted the for-mation of humic substances and was an indicator of organic matterstability. Moreover, the composting production with a near neutralpH value is beneficial for plant growth and soil improvement(Lasaridi et al., 2006).

3.3. Ammonium (NH4þ-N)

The evolution of NH4þ-N is shown in Fig. 3. During the first 3 days

of composting, NH4þ-N concentrations increased rapidly and

reached maximum values in all treatments. These increases inNH4

þ-Nweremost likely due to the intensive degradation of organicnitrogen compounds (Sánchez-Monedero et al., 2001). In R6,without the addition of carbon compounds, the NH4

þ-N concen-tration (6.52 g/kg) was significantly higher than those of othertreatments. This higher content of NH4

þ-N is related to increasedammonia volatilization, especially during the thermophilic phasebecause of higher temperatures and pH values (Pagans et al., 2006).In R4 and R5, the concentrations of NH4

þ-Nwere 5.56 g/kg and 5.3 g/

Fig. 3. Changes in NH4þ-N in different treatments during composting.

Page 4: Ammonia emissions and biodegradation of organic carbon during sewage sludge composting with different extra carbon sources

Fig. 4. Changes in TOC in different treatments during composting.

Table 2Degradation rates of TOC during different phase of composting.

0e3 d (%) 3e6 d (%) 6e22 d (%) 0e22 d (%)

R1 23.7 1.80 2.23 27.7R2 18.7 6.72 5.17 30.5R3 20.5 2.03 1.52 24.0R4 25.4 4.73 3.91 35.1R5 20.9 11.5 5.28 37.7R6 16.9 1.93 0.66 19.5

d, day.

Y. Li et al. / International Biodeterioration & Biodegradation 85 (2013) 624e630 627

kg, respectively, which were significantly lower than the R6 con-centration. This difference could be explained by the suppression ofthe ammonification caused by the lower initial pH value in R6. Huet al. (2007) and Yu and Huang (2009) indicated that lower initialpH values (<7.0) caused negative effects on microbial activity andcould even kill the microbes.

After this initial increase, the NH4þ-N contents began to decrease

until the end of the composting process. According to previousstudies, there are three reasons for this decrease: (1) An increase inammonia emissions, (2) immobilization bymicroorganisms, and (3)nitrification (SánchezeMonedero et al., 1999; Huang et al., 2004).Considering the reactions in this study, the emission of ammoniawas themost likely cause for the NH4

þ-N decrease. Liang et al. (2006)also indicated that the ammonia emissions were higher in ther-mophilic stage and accounted for 78% of the nitrogen losses. At thefinal stage of composting, the NH4

þ-N contents in R1, R3, R5 and R6were 3.02 g/kg, 2.22 g/kg, 3.44 g/kg and 2.12 g/kg, respectively,which were significantly lower than in R2 and R5 (p < 0.05), thusindicating that large amounts of nitrogenwere lost in R1, R3, R5 andR6. A possible explanation for this observation was that there wasnot enough available carbon compounds for microorganisms toutilize for nitrogen immobilization in R1, R3, R5 and R6; thus, theNH4

þ-N was easily lost through ammonia emissions. During com-posting, ammonification and nitrogen immobilization occurredsimultaneously, but the dominant reaction was variable accordingto the substrate compositilon and environmental condition. Nitro-gen loss always occurred when ammonification was the dominantreaction, while nitrogen was conserved when nitrogen immobili-zation was the dominant reaction. In our study, the final NH4

þ-Nconcentrations for all of the runs were high compared to concen-trations measured by other research studies (Rihani et al., 2010).This disparity was attributed to the different initial nitrogen contentof the composting materials applied in this study. Moreover, thelower ammonia emissions and shorter composting time could alsoexplain this phenomenon. The present study only observed the firststage of composting, but a second stage allowing for further reac-tion was needed to achieve stability and maturity.

3.4. Carbon degradation and decomposition rate

To quantitatively analyze the degradation of organic matter, weinvestigated the changes in the total organic carbon during thecomposting process. The initial contents of organic carbon in R1eR5 were higher than that of R6 due to the addition of carbonamendments (Fig. 4). However, as the composting process devel-oped, the carbon degradation trends showed significant differencesbased on the composition of composting substrate. Table 2 showsthe performance of organic carbon degradation during differentstages of composting in detail. The degradation of organic carbon inall runs occurred principally during the initial 3 days, indicatingthat most of the readily degradable carbon sources were utilized bymicroorganisms for production of heat during this stage. Themaximum degradation rates of organic carbon during initial 3 dayswere observed in R1 and R4, the treatments in which glucose wasused as carbon amendment. In contrast, during the thermophilicstage (days 3e6), the treatments of R2 and R5 had the maximumorganic carbon degradation rate. Moreover, the same tendency wasobserved from day 6 to the end of composting. This implied that acontinuous intensive degradation of organic carbon was observedin R2 and R5 throughout the entire thermophilic phase (3e6 days).A similar phenomenon of carbon degradation has been reported byFernández et al. (2008). This tendency was attributed to the highmicrobial activity in R2 and R5 caused by the addition of sucrose.The total degradation rates of organic carbon were 27.7%, 30.5%,24.0%, 35.1%, 37.7% and 19.5% for the treatments of R1 through R6,

respectively. The poor degradation capacity of organic carbon in R3and R6 seemed to be due to the lower microbial availability ofcomposting substrates such as straw powder and sawdust. R4 andR5 had the highest total degradation rate, which could be explainedby the co-addition of large amounts of easily biodegradable carbonsources and straw powder. Straw has a relative complicatedstructure with cellulose and lignin-type compounds, which aredifficult for microbes to decompose (Rihani et al., 2010). However, ifglucose or sucrose were added, the microbial activity could beenhanced and some resistant biodegradable material from thestraw powder could possibly be available for microorganisms. Janget al. (2002) found that addition of glucose increased the number ofboth bacteria and actinomycetes by 10e100 times. Other authorsalso reported that glucose could enhance the mineralization of theindigenous carbon matter of the compost (Bellia et al., 2000;Kuzyakov et al., 2000). It is well known that sucrose and glucose,which have very similar compositions, are readily degradable bymicrobes. Therefore, it is not hard to deduce that sucrose hassimilar effects on carbonmineralization. From these results, it couldbe concluded that the carbon mineralization of the compostingprocess was enhanced by the addition of glucose or sucrose but notstraw powder. Although the addition of carbon amendmentsincreased the C/N ratio of composting materials, the structure ofthe added carbon compounds was more important than the abso-lute amount of organic carbon for promoting microbial activity.

3.5. Instantaneous and cumulative ammonia emissions duringcomposting

The instantaneous and cumulative ammonia emissions overtime during the composting process are shown in Fig. 5. With the

Page 5: Ammonia emissions and biodegradation of organic carbon during sewage sludge composting with different extra carbon sources

Fig. 5. Changes of instantaneous and cumulative ammonia emissions in differenttreatments during composting: (1) instantaneous ammonia emissions, (2) cumulativeammonia emissions.

Y. Li et al. / International Biodeterioration & Biodegradation 85 (2013) 624e630628

increase of temperature, ammonia emissions reached peak valueson the fourth day in R1, R3, R4 and R6. In contrast, the peak valuesof ammonia emissions for R2 and R5 occurred on the fifth day,which was later than the other treatments. The peak value ofammonia emissions in R6 (with no carbon amendment) was largerthan other treatments, indicating that the addition of a carbonsource to the composting material would reduce ammonia emis-sions during the thermophilic phase (Fig. 5-1). After reaching thepeak value, the ammonia emissions began to decrease rapidly in allruns. Second peaks in ammonia emissions were observed in R1, R2,R4 and R5, inwhich the readily degradable carbon sources (glucoseand sucrose) were used as carbon amendments. There are twopossible explanations for this phenomenon: 1) the decompositionof organic nitrogen compounds that were formed by the nitrogenimmobilization, or 2) the addition of easily degradable carboncompounds decreased the initial pH and thus inhibited theammonification in the initial 3 days, leading to the partial organicnitrogen decomposition in the subsequent 3 days. According to thecumulative ammonia emission results, there was no significantdifference in the total emission of ammonia between R3 and R6(Fig. 5-2). This implied that the addition of straw powder in R3 hadno effect in reducing ammonia losses. This phenomenon could beexplained by the poor degradability and complex composition of

straw powder, which contains a large amount of cellulose andhemicelluloses that are difficult for microbes to utilize. Otherstudies, however, found contrary results; Kirchmann and Witter(1989), for example, indicated that the addition of straw resultedin the decrease of ammonia losses during 200 days of poultrymanure composting. This contrary conclusion could have resultedfrom the different incubation period. In our study, the compostingphase was only 22 days, and approximately 70% of the ammoniaemissions occurred in the initial 8 days; but the indigenous mi-croorganisms could not decompose straw in such a short time. Incontrast, the additions of sucrose and the mixture of straw powderand sucrose reduced the total ammonia emissions by up to 30.2%and 37.9%, respectively. Several authors have reported that readilyavailable compounds, such as molasses, glucose and sugar, showedpositive effects on reducing nitrogen losses (Subair, 1995; Lianget al., 2006; Torkashvand, 2009). However, to our knowledge,there is little information available in literature about the effects ofsucrose on ammonia emissions.

According to the results of this study, the readily degradablecarbon sources had better success at reducing ammonia emissionsthan did resistant materials. Comparing different readily degrad-able carbon sources, the effects of sucrose were more significantthan those of glucose, and the co-addition of straw powder hadmore significant effects than when only one amendment was used.These results indicated that the effectiveness of carbon amend-ments in reducing ammonia volatilization was strongly dependenton the composition of the carbon amendments. These findings arediscussed in detail in the following section.

3.6. Interaction between ammonia emissions and decompositionrate of organic carbon

It is well known that the amount of ammonia emissions stronglydepend on the initial C/N ratio of the composting materials (Huanget al., 2004). Lower C/N ratios cause an excess amount of nitrogen,which is lost through ammonia volatilization. However, a few au-thors found that the C/N ratio was not always a reliable indicatorof nitrogen loss (Komilis and Ham, 2006; Liang et al., 2006;Torkashvand, 2009). The results of the current study presented inSections 3.4 and 3.5 showed that although the addition of glucose,sucrose and straw powder all increase the C/N ratio, there was anobvious difference in the observed ammonia emissions for treat-ments with different carbon amendments. In this section, based oncomprehensive consideration of ammonia emissions and carbondegradation, themechanism of carbon source in reducing ammoniaemissions will be interpreted.

The results seem to indicate that the carbon sources with amorereadily degradable composition are more effective at reducingammonia emissions. However, although both glucose and sucrosecan enhance microbial activity, glucose proved less effective thansucrose. Compared with sucrose, glucose is easier to decomposeand its decomposition rate is faster. As shown in Table 2, theaddition of sucrose in R2 and R5 enhanced the degradation oforganic carbon from day 3 to day 6. In contrast, in the glucosetreatments of R1 and R4, these enhancements were observedduring the initial 3 days of composting. Although various carbonamendments could promote organic carbon degradation, theirtimespan was different.

As shown in Fig. 5, ammonia emissions primary occurred afterthe third day of composting and begun to decrease after the eighthday, indicating that this period was crucial for controlling ammoniaemissions. Therefore, comprehensive consideration of the results ofammonia emissions and organic carbon degradation demonstratedthat the carbon amendment that could promote the organic carbondegradation during the intensive phase of ammonia emissions (3e

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Y. Li et al. / International Biodeterioration & Biodegradation 85 (2013) 624e630 629

8 days) was effective at reducing ammonia emissions. To explainthis finding, one must focus on the relationship between carbonsource and ammonia emissions.

According to previous studies, increasing the C/N ratio of thecomposting material promoted the nitrogen immobilization, thusleading to the reduction of ammonia emissions. As early as 1970,Tempest et al. (1970) found that microorganisms could assimilateammonia into glutamate with essential enzymes, with theammonia converted into organic nitrogen being used as a bacterialnitrogen compound. The carbon source plays an important role innitrogen immobilization as it can provide the energy source andcarbon framework for microorganisms. Thus, the ability of micro-organism to decompose a carbon source during the intensive phaseof ammonia emissions is related to the nitrogen immobilization.Moreover, nitrogen immobilization depends not only on the con-tent of available carbon compounds but also on the presence ofNH4

þ-N. The NH4þ-N obtained from the ammonification provides key

substrate for nitrogen immobilization. Therefore, although theglucose addition in R1 and R4 promoted the microbial activity andorganic carbon degradation, the accumulation of NH4

þ-N mainlyappeared after this promotion of glucose (Fig. 3). In other words,although the glucose addition promoted themicrobial activity in R1and R4, the higher decomposition rates of organic carbon were nothelpful in reducing ammonia emissions. While both of the aboveconditions were satisfied, inorganic nitrogen could be converted toorganic nitrogen by microorganisms.

Thus, based on the above analysis, the superior effectiveness ofsucrose could be related to the positive influence on nitrogenimmobilization. The ability of nitrogen immobilization stronglydepended on the composition of carbon source, and the C/N ratiowas not sufficient to explain the immobilization of nitrogen.Additionally, another observed phenomenon was that the com-bination of straw powder with sucrose reduced ammonia emis-sions more efficiently than sucrose alone. As mentioned before,the readily available carbon in sucrose promoted the microbialactivity, which caused the nitrogen immobilization. However, thisimmobilization may end when the carbon compounds aredepleted. In contrast, the addition of resistant material such asstraw powder could provide more carbon sources for nitrogenimmobilization, leading to more inorganic nitrogen immobiliza-tion by microorganisms.

The findings obtained in the present study are limited in thelaboratories. When the carbon sources are used in large-scalecomposting, it is necessary to discuss the economic effects of theintroduction of external carbon sources. In China, home price isabout 9000e12,000 RMB yuan/t for industrial sucrose, and about200e300 RMB yuan/t for straw. Thus, the cost of adding carbonsources is approximately 184e246 RMB yuan/t sludge (80% mois-ture content, wet basis). The co-addition of sucrose and strawpowder has a cost advantage compared to other methods such asbiological enhancement and physicalechemical absorption (Witterand Lopez-Real, 1988; Mahimairaja et al., 1994; DeLaune et al.,2004). In addition, to further decrease the cost, other substituteamendments for sucrose such as sugar industry waste residue willbe investigated in the future.

4. Conclusions

To reduce ammonia emissions during sewage sludge compost-ing, various carbon-rich compounds were added to increase the C/N ratio; this study investigated the composting performance andammonia emissions of these various experiments.

It was found that the readily available carbon sources (glucoseand sucrose) were more effective than the resistant materials(straw) in promoting organic carbon degradation and reducing

ammonia emissions. In R5, in which the mixture of sucrose andstraw powder were added, the degradation of organic carbon wasenhanced during the intensive stage of ammonia emissions (days3e8), and the ammonia emissions significantly decreased by asmuch as 37.9% compared to the control. However, not all readilyavailable carbon compounds were helpful in reducing ammoniaemissions, and the occurrence time of carbon amendment pro-motion influenced ammonia emissions reduction. This findingcould be explained by the mechanism of nitrogen immobilizationinwhich the availability of the carbon source and presence of NH4

þ-N played an important role.

Acknowledgements

This research was financially supported by the National NaturalScience Foundation of China (No. 51278146). The authors wouldlike to thank all of the technical staff from State Key Laboratory ofUrban Water Resource and Environment in Harbin for their help inthe chemical analysis of the composting samples.

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