The use of biochar-amended composting to improve the humification and degradation of sewage sludge

Bioresource Technology xxx (2014) xxx–xxx

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The use of biochar-amended composting to improve the humification and degradation of sewage sludge Jining Zhang a,b, Fan Lü a,b,⇑, Liming Shao b,c, Pinjing He b,c a

State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China Institute of Waste Treatment and Reclamation, Tongji University, Shanghai 200092, China c Centre for the Technology Research and Training on Household Waste in Small Towns & Rural Area, Ministry of Housing and Urban-Rural Development of PR China (MOHURD), China b

h i g h l i g h t s  Biochar accelerated the humification progress of sludge organics.  Biochar increased oxygen uptake rates of sewage sludge during aerobic degradation.  SEM showed the porosity of the sludge surface increased with 12–18% biochar amended.

a r t i c l e

i n f o

Article history: Received 16 December 2013 Received in revised form 20 February 2014 Accepted 22 February 2014 Available online xxxx Keywords: Water-extractable organic matter Fluorescence excitation and emission matrix FT-IR SEM Humus substances

a b s t r a c t Wood biochar (6%, 12% and 18% of fresh sludge weight) adding to a sludge-and-straw composting system was investigated to assess the potential of biochar as a composting amendment. Organic degradation efficiency, temporal humification profile of the water-extractable organic fraction and solid organic matter, through spectroscopic, microscopic and elementary analysis were monitored. Fluorescent excitation and emission matrix indicated that concentrations of aqueous fulvic-acid-like and humic-acid-like compounds were, respectively, 13–26% and 15–30% higher in the biochar-amended treatments, than those in the control without biochar-amended. On the first day of sludge aerobic incubation, the presence of biochar resulted in increased oxygen uptake rates of 21–37% due to its higher nano-porosity and surface area. SEM indicated that, in the biochar-amended sludge, the dense microstructure on the sludge surface disintegrated into fragments with organic fraction degraded and water lost. Results indicated that 12–18% w/w addition of wood biochar to sludge composting was recommended. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Due to its putrescible characteristics and the continuous and increasing quantities of its generation, sewage sludge has become a major cause for environmental concern. Composting, a traditional process used for sludge treatment, involves the recycling of nutrients and their re-use, as a biofertilizer for land application. Biochar is comprised of carbonaceous matter, formed during biomass pyrolysis (Luque et al., 2012). It contains abundant quantities of recalcitrant aromatic ring structures (Zhang et al., 2014) that have a long half-life in the soil (Fischer and Glaser, 2012). Biochar is widely used to facilitate soil amendment, thus improving soil properties (Schmidt and Noack, 2000; Kramer et al., 2004; Liang et al., 2010; Shrestha et al., 2010; Lehmann et al., 2011; Deal ⇑ Corresponding author at: Institute of Waste Treatment and Reclamation, Tongji University, Shanghai 200092, China. Tel./fax: +86 21 65981383. E-mail addresses: [email protected], [email protected] (F. Lü).

et al., 2012; Schulz and Glaser, 2012). This amendment process has also been applied to the composting of organic substances (Hua et al., 2009; Dias et al., 2010; Steiner et al., 2010; Jindo et al., 2012a). A number of additional benefits are also associated with the use of biochar for the amendment for organics composting. The use of biochar as a bulking agent can result in decreasing bulk density and increasing aeration conditions (Steiner et al., 2010), improving microbial growth and microbial respiration rates (Jindo et al., 2012a), as well as enhancing the absorption of gaseous NH3 and water-soluble NHþ 4 (Hua et al., 2009; Steiner et al., 2011). Dias et al. (2010) achieved an organic-matter degradation of 73.2% of the initial content when poultry manure was mixed with wood biochar in a proportion of 1:1 (fresh weight: w/w). When sawdust and coffee husk were amended with poultry manure, organic matter degradation levels of 65.0% and 84.2% were, achieved, respectively. A review of the above literatures indicated that the amount of biochar added to organics composting ranged from 2%

http://dx.doi.org/10.1016/j.biortech.2014.02.080 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Zhang, J., et al. The use of biochar-amended composting to improve the humification and degradation of sewage sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.080

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J. Zhang et al. / Bioresource Technology xxx (2014) xxx–xxx

to 50% of the total organics weight. However, knowledge of the effects of biochar on compost humification is limited. Jindo et al. (2012b), who assessed the effects of a 2% (v/v) wood biochar addition to poultry manure compost, found a 10% increase in carbon content in water extracted humic-like substances and a 30% decrease in the water-soluble carbon content. It should however be noted that this study was confined to the chemical and biochemical characteristics of the final composting product. The objective of the present study was to develop an efficient humification process with the assistance of wood biochar. For this purpose two specific aspects were focused: investigating the temporal humification profile of sewage sludge composting amended with wood biochar; and the effect of supplying different levels of biochar to the system. During the composting process, the following parameters: pH, electrical conductivity (EC), dissolved organic carbon (DOC), the C/N ratio, specific ultraviolet absorbance (SUV254/DOC), and three-dimensional fluorescence excitation– emission matrix spectroscopy (EEM) were determined in the water extracted fraction during the aqueous phase. Elementary analysis involving Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) morphology was also undertaken, to investigate the development of structural changes to organic matter during the composting process. 2. Methods 2.1. Properties of sewage sludge, wood biochar and rice straw Dewatered sewage sludge was obtained from a local municipal wastewater treatment plant in Shanghai, China where 75,000 m3 of wastewater (93% domestic and 7% industrial sewage) was treated per day, using an anaerobic–anoxic–oxic (A2O) process. Water was removed from the sludge by centrifugation, with the addition of organic flocculating agents. Chopped paddy straw (2–5 cm) was used as a bulking agent. Commercial wood biochar (Qunfang Gardening Co., China) was pyrolyzed at 500–600 °C in kilns. Dried wood biochar was crushed and sieved to 2–5 mm and then used as an amendment during composting. The characteristics of the raw materials are listed in Table 1. The wood biochar used was basic, and had advantageous hydrophobic, sorptive, aromatic, as well as it contained more water extractable fulvic-acid-like compounds (Enders et al., 2012; Zhang et al., 2014). 2.2. Composting system The composting process was performed in foam rectangle reactors, each with a working volume of 12.5 L. The outer wall of each reactor was 4 cm thick. A whirlpool pump and a gas-flow meter were used for aeration in each reactor. A time-based aeration control system was adopted for intermittent O2 supply, with an air Table 1 The characteristics of the raw materials.

Moisture content (%) VS (%, dry basis) pH value EC value (ms cm1) DOC (mg L1) DN (mg L1) 1 NHþ 4 AN (mg L ) C (%, dry basis) N (%, dry basis) H (%, dry basis) C/N (w/w)

Sludge

Wood biochar

Straw

81.6 ± 0.5 62.2 ± 3.2 6.8 ± 0.2 1.9 ± 0.0 6680.0 ± 335 3076.4 ± 16 2329.5 ± 275 33.1 ± 0.1 5.3 ± 0.3 4.8 ± 0.1 6.2 ± 0.8

0 40.0 ± 2.4 8.4 ± 0.2 0.6 ± 0.0 662.8 ± 55 134.1 ± 26 0 49.7 ± 1. 7 0.8 ± 0.0 2.4 ± 0.1 62.2 ± 7.3

10.0 ± 0.3 95.2 ± 1.2 7.2 ± 0.1 5.7 ± 0.3 23320.5 ± 64 3148.4 ± 24 444.0 ± 20 42.2 ± 0.3 1.1 ± 0.1 5.8 ± 0.0 39.4 ± 4.9

flow rate of 0.03 m3 h1 kg1 (wet basis). The layout of the reactor used for the experiment is presented schematically in Fig. S1 (Supplemental information: Fig. S1). The initial amount of sludge was 1.5 kg. Wood biochar and paddy straw were amended with sludge. First, the paddy straw at 20% of sludge wet weight as recommended by Zhao et al. (2011), and then the wood biochar at 0%, 6%, 12% and 18% of sludge wet weight were homogeneous added to the sludge-and-straw composting system. These tests are hereinafter referred to as Trial WB0, Trial WB06, Trial WB12 and Trial WB18, respectively. The mixing ratio (%) on a dry weight basis to each treatment was sludge: straw: biochar = 9:1:0, 9:1:3, 9:1:6 and 9:1:9, respectively. Each trial was conducted in duplicate. The water content of the sludge, wood biochar and straw were listed in Table 1, and the blended water content was adjusted to 70–75% and kept constant throughout all trials. The composting materials were turned every seventh day, to homogenize the materials. At the same time, fresh samples were collected for chemical analysis. The aeration process was terminated on the 21st day and the active composting trials terminated on the 42nd day. 2.3. Analytical methods The aerobic degradability for sludge, the sludge–straw complex, and the sludge–straw-biochar complex was indicated by the oxygen uptake rate and investigated separately, by means of incubation at constant temperature. Erlenmeyer flasks with 1.5 L volume were used, and the proportion of raw materials was maintained at the same level during this test and during subsequent composting trials. About 10.0 g sludge (fresh wet weight) was spread over the bottom of the first Erlenmeyer flask, to ensure enough space for aerobic respiration of the materials and for the generation of gases. In addition, 10.0 g sludge and 2.0 g straw were placed into the second Erlenmeyer flask. To represent the composition of the initial material in WB06, WB12 and WB18, the 0.6, 1.2 and 1.8 g wood biochar were separately placed into three additional Erlenmeyer flasks filled with a complex of 10.0 g sludge with 2.0 g straw. The mixtures in the flasks were uniformly mixed and maintained the same water-content conditions as the first Erlenmeyer flask. All flasks were placed in hermetically-sealed incubators, maintained at a constant temperature of 35 °C. Prior to sealing, the flasks were aerated with fresh air. During the incubation, about 50 ml of gas was extracted from the flasks on a daily (24 h) basis and the O2 content was measured using a detector (CYS-1, Xuelian Co., China). After sampling, the flasks were aerated prior to sealing. The total incubation time was 10 days and all tests were duplicated. Oxygen uptake rate was determined by measuring the oxygen concentration in the outlet air flow, which had passed through the raw material of the flask. The oxygen uptake rate (mg O2 g1 DM h1) was then calculated (Supplemental information: Equation). Fresh composting samples were collected on the start of the experiment (Day 0) and thereafter on the following days: 7th, 14th, 21st, 28th, 35th and 42nd days, based on quartering. Samples were collected after the compost materials had been turned and uniformly mixed. Aqueous extracts were obtained by shaking the samples with deionized water (1:10, w/v) at 200 r min1 for 4 h in a horizontal shaker kept at room temperature. Thereafter, measurements were taken of the pH (AWWA-4500, Clesceri et al., 1998) of the supernatant, using a pH electrode (pHS-2F, Jingke Co., China), and the electrical conductivity (EC, AWWA-2520, Clesceri et al., 1998) (EC meter; DDS-307A, Jingke Co., China). The filtrate of each sample was passed through a 0.45-lm polytetrafluoroethylene filter, after which the dissolved organic matter (DOM) was determined. A total organic carbon analyzer (TOC-VCPH, Shimadzu, Japan) was used to measure dissolved organic carbon (DOC) and total soluble nitrogen (DN); NHþ 4 AN concentration

Please cite this article in press as: Zhang, J., et al. The use of biochar-amended composting to improve the humification and degradation of sewage sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.080

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was analyzed by a Kjeltec 8400 analyzer (Foss, Sweden); organic nitrogen represented the difference between DN and NHþ 4 AN & NO 3 AN. The DOC concentration and organic nitrogen concentration were used to determine the ratio of C/N in the aqueous phase; a UV spectrophotometer (UV-1800, Shimadzu, Japan) was used to determine UV light absorption at 254 nm wavelength (SUV254) and the ratio of SUV254/DOC was determined by dividing the value of SUV254 by the DOC concentration. For the purpose of fluorescence excitation–emission matrix (EEM) analysis, the filtrate was adjusted to pH 7.0 with HCl solution and then diluted to a DOC of <10 mg L1. Fluorescence EEM spectroscopy was subsequently conducted using a fluorescence spectrophotometer (Cary Eclipse, Varian Inc., USA) in scan mode. Interpolation in affected areas was used to minimize first- and second-order Rayleigh and Raman scatter, and a fluorescence regional integration (FRI) technique was adopted for analysis (Chen et al., 2003; Lü et al., 2009). Samples to be used for SEM analysis were initially freeze-dried, crushed and ground, and then passed through a 75 lm sieve. A scanning electron microscope (S-3400 N, Hitachi, Japan) was then used to examine the phase development and microstructure of the dry composting materials. The SEM was operated at 15 kV acceleration voltage; samples were gilded with Au and kept in a vacuum desiccator until further analysis. The percentage content of the elements, including carbon, hydrogen, and nitrogen of the composting samples, were measured using an Elementary Analyzer (Vario EL III, Germany). Ground dry-compost materials were mixed with KBr wafers to prepare them for diffusereflectance FT-IR measurement (Nicolet 5700, USA). The spectra for all samples were obtained by subtracting the value obtained from a blank sample.

highest values, of 4.5, 6.0, 7.0, and 7.1 mg O2 g1 DM h1 in the WB0, WB06, WB12 and WB18 trials, respectively. In the case of pure sludge incubation, the oxygen uptake rate increased to its highest value (2.1 mg O2 g1 DM h1) on the third day, after which the value decreased and remained at levels of 1.3–1.4 mg O2 g1 DM h1 until the end of the test period. The lowest values recorded in the pure sludge incubation indicated that the pure sludge incubation process cannot last long, because an optimal C/N ratio, suitable for a rapid increase in microbial activity, was required. It was not possible to obtain such a ratio without the presence of straw in the sample (Iranzo et al., 2004). After one day, the biodegradability levels attained by the sludge–straw-biochar system were 21.1–37.0% higher than those obtained in samples incubated without the presence of biochar. The addition of biochar also resulted in a proportional increase in the oxygen uptake rate. It is likely that this was due to the higher nano-particle size 2505 nm and larger surface area 6.40 m2 g1 containing in the sample biochar. This would enhance aeration (Steiner et al., 2010) and provide a suitable habitat for microorganisms, promoting metabolic activity and affecting various microbial processes associated with organic matter decomposition (Thies and Rillig, 2009; Jindo et al., 2012a), which would lead to increased rates of oxygen uptake. As the treatment experiment progressed, a decrease in metabolic rates was observed in all incubation treatments. However, the oxygen uptake rates remained higher in WB18 than in WB12, WB06 and WB0 after incubation for 1–2 days. These results suggest that the assistance of wood biochar resulted in increasing oxygen uptake rates, as the levels of microbial degradation of organic matter increased. The biochar played a role as an oxygen carrier that, due to its porosity, enhanced composting. This enhanced the activity of microorganisms coming from sludge and straw, which were adsorptive in pure biochar or in the composting materials.

3. Results and discussion 3.1. Oxygen uptake rate of the sludge–straw-biochar complex

3.2. Variation of pH and EC values during the composting process

During the composting process, oxygen is consumed, and CO2 produced, due to the aerobic degradation of organic matter by aerobic microorganisms. The oxygen uptake rate is therefore an indication of a decrease in biological activity, related to the aerobic stabilization processes of organic matter during the composting (Cossu and Raga, 2008). Oxygen uptake rates measured during various 10-day incubation trials are presented in Fig. 1. The results indicate that, for the incubation in the sludge–straw system and the sludge–straw-biochar system, all the oxygen uptake rates increased rapidly and, after the first day of bioreaction, reached their

Variation in the pH value during the composting process is shown in Fig. 2. The initial pH values were 6.9 in all the trials. Their pH values could be attributed to the nature of the pure sludge (pH6.8). By the end of the 7th day, the pH values had increased to 8.6, 8.6, 8.4 and 8.3 for trials WB0, WB06, WB12 and WB18, respectively. The increasing peak values for all the trials were due to ammonia volatilization, mineralization of organic nitrogen and the basic pH8.4 of biochar during the early stage of composting. Thereafter, the pH values of biochar-amended trials decreased rapidly, to reach levels of about 8.0 at the end of 14th day. The

9.0

8.0

pH

4.0

6.0

7.5

2.0

4.0

7.0 0.0

8.0

6.5

-1

-1

6.0

WB0-pH WB06-pH WB12-pH WB18-pH WB0-EC WB06-EC WB12-EC WB18-EC

8.5

EC (ms cm )

Sludge WB0 WB06 WB12 WB18

-1

Oxygen uptake rate (mg O2 g DM h )

8.0

2.0

6.0 0

1

2

3

4

5

6

7

8

9

10

Test duration (day) Fig. 1. Changes in oxygen uptake rates during the aerobic incubation process (error bars indicate the data range of two duplicates).

0

7

14

21

28

35

42

Time (day) Fig. 2. Variation in pH and EC values during the process of composting (error bars indicate the data range of two duplicates).

Please cite this article in press as: Zhang, J., et al. The use of biochar-amended composting to improve the humification and degradation of sewage sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.080

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J. Zhang et al. / Bioresource Technology xxx (2014) xxx–xxx

value 8.6 in trial WB0 remained constant for 2 weeks and then began to decline, until the 14th day when it reach a pH value of 7.4, which then remained unchanged until the end of the experiment. The highest levels of ammonia volatilization usually occurred during the first two weeks. During this time microbial activity was low and the release of NH3 declined during the later stages of composting. In the biochar-amended trials, the pH values were lower than those measured in the control tests (without biochar). The results noted in the biochar-amended trials may have been caused by decreased availability of N, since biochar can efficiently adsorb and retain ammonia gas and ammonium as well as nitrate ions (Liang et al., 2006; Hua et al., 2009; Taghizadeh-Toosi et al., 2012). A rapid composting process, as well as lower pH levels in the biochar-amended trials, was thus observed in this study. In all cases the final pH values were 7.3. Electricity conductivity (EC) is a measure of total ion concentration and describes the variation in levels of organic ions and inor 2 ganic ions, such as Cl, Na+, K+, NHþ in the different 4 , NO3 , SO4 samples, during the composting process. As shown in Fig. 2, the initial EC values in all samples were in the range of 2.0– 2.5 ms cm1. The EC values remained constant during the first 14 days, after which they gradually increased with increased composting times. By the end of the compositing period they had increased to 4.0, 3.3, 2.7 and 2.7 ms cm1. The slight changes in EC values were due to ammonia volatilization during the first 14 days. The increase in EC may have been due to the release of mineral salts, as a result of the degradation of organic matter (Campbell et al., 1997; Rasapoor et al., 2009). In this context it should be noted that Sánchez-Monedero et al. (2001) maintained þ that the production of NO 3 and NH4 could probably explain the increase of the EC. The EC values of biochar-amended trials were 20.7–35.6% lower than those of the control, in which biochar was

not present in the final composting mixtures. This is important from an agricultural point of view, since an increase in EC would result in phyto-inhibitory effects in the soils and in the plants. 3.3. Fluorescence excitation–emission matrix (EEM) spectra of DOM Biochemical transformation of organic compounds took place during the water-soluble phase. It was therefore important to investigate the characterization of water soluble organic compounds after composting, as this was critically important for understanding the composting process. Three-dimensional fluorescence EEM was used to study aqueous humus-like compounds, or aromatic compounds, generated during the composting process. The location and intensity of fluorescence was analyzed using the methods described by Chen et al. (2003), Senesi and Plaza (2007) and Lü et al. (2009). More information was presented in supplementary information: EEM. Three-dimensional fluorescence EEM spectra, normalized to DOC concentration for primary sludge, wood biochar and straw – as well as the four trials on the 21st and 42nd days during the process of composting – were recorded (Fig. 3). It was found that, in the contour of primary sludge itself, organic compounds were composed of aromatic protein (Region I + Region II) and soluble microbial by-product-like material (Region IV). The volumes of fulvic-acid-like material and humic-acid-like material were low. After 21-day and 42-day composting, the fluorescence intensity of Regions I, II and IV had declined, while the fluorescence intensity and areas of fulvic-acid-like material and humic-acid-like material had increased in each trial. Easilydegradable organic matter components, such as aliphatic chains, polysaccharides, alcohols and protein, had decomposed and the humification level had increased. The mature compost therefore

500

Ex/nm

450

(b)

(a)

(c)

0 20 60 100 140

400 350

V

IV

0 10 20 30 40 50 60

300 250 200 500 450

I

II

III

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

for (a)-(c)

for (d)-(k)

Ex/nm

400 350 300 250 200 500 450

Ex/nm

400 350

Part A

300

Part B

250 200

250 300 350 400 450 500 550250 300 350 400 450 500 550 250 300 350 400 450 500 550 250 300 350 400 450 500 550

Em/nm

Em/nm

Em/nm

Em/nm

Fig. 3. EEM spectra of primary sludge, wood biochar, straw, and compost materials at the 21st and 42nd days. Region I: tyrosine-like organic compounds; Region II: tryptophan-like organic compounds; Region III: fulvic-acid-like materials; Region IV: soluble microbial byproduct-like materials; Region V: humic acid-like materials. (a) Primary sludge; (b) wood biochar; (c) straw; (d) WB0 at 21st day; (e) WB06 at 21st day; (f) WB12 at 21st day; (g) WB18 at 21st day; (h) WB0 at 42nd day; (i) WB06 at 42nd day; (j) WB12 at 42nd day; (k) WB18 at 42nd day.

Please cite this article in press as: Zhang, J., et al. The use of biochar-amended composting to improve the humification and degradation of sewage sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.080

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Distribution of Regions (%)

100

0

21 42

0

21 42

0

21 42

0

Region V Region IV Region III Region II Region I

21 42

80

biochar was about 1.5 times higher than that in bamboo biochar (Zhang et al., 2014). Thus, the levels of decomposition and humification increased with the addition of wood biochar. 3.4. The UV/Vis spectroscopy

60

40

20

0

WB 0

WB06

WB12

WB18

Fig. 4. Distribution DOM (assessed using the FRI technique) for trials WB0, WB06, WB12 and WB18 during the process of composting. 0 = the initial day; 21 = the 21st day; 42 = the 42nd day, during the composting process.

contained more fulvic-acid-like substances (in Region III) and humic-acid-like substances (in Region V), as confirmed in EEM spectra. The contours of fluorescence regions were different in the four trials, particularly in the humic-acid-like region (Region V). Within this region, a division between two fluorescence regions was noted. These were named after part A (Ex/Em: 300–370/ >380 nm) and part B (Ex/Em: 250–300/>380 nm), respectively. Contours were evident in Part A and these were similar in all the trials. The size and intensity of the fluorescence area of part B was, however, the largest, as indicated in trial WB18. This decreased from trial WB12 to trial WB06 to trial WB0. These results suggest that the transformation of fluorescence from the fulvic-acid-like region towards the humic-acid-like region was continuous and flowing. Fluorescence regional integration (FRI), a quantitative technique that integrates the volume beneath an EEM, was developed to analyze the EEM spectra. The distribution of volumetric fluorescence among the five regions for different trials during the process of composting is presented in Fig. 4. According to this calculation, the distribution of the fulvic-acid-like region and humic-acid-like region increased with the addition of biochar. The quantities of fulvic-acid-like compounds in the biochar-amended trials were respectively 13.1–42.3% and 12.5–25.8% higher than that in the control (without biochar) on the 21st day and the 42nd day of composting. Meanwhile, the quantities of humic-acid-like compounds in the biochar-amended trials were respectively 21.8–42.3% and 15.0–29.6% higher than that in the control (without biochar) on the 21st day and the 42nd day of composting. These results may be due to the wood biochar being comprised of higher levels of fulvic-acid-like material (as shown in Fig. 4) in the water-extractable fraction, which played a role in the composting process. It should also be noted that biochar presents functional groups on the edges of the poly-aromatic rings, and these functional groups were mainly comprised of carboxyl groups, lactones, and phenols (Fischer and Glaser, 2012). All of them could enhance the capacity of biochar to chemisorb nutrients, minerals and dissolved organic matter, in association with the surface oxidation capacity of the biochar (Haug, 1993; Song and Guo, 2012; Joseph et al., 2013). The quantity of functional groups in wood

The ratio of SUV254/DOC is related to the degree of condensation of the aromatic carbon network, with a high ratio being indicative of a relatively high degree of condensation of aromatic humic constituents. Ratios of SUV254/DOC (measured as L mg1 m1) are listed in Table 2. The initial ratio was the lowest, being almost 0 L mg1 m1 but, after 7-day composting, the ratio exceeded 1 L mg1 m1 and continued to increase during the process of composting, with the values increasing progressively, irrespective of the composting stage, from the trial WB0, WB06, and WB12 until WB18. The ratios became stable after 28 days of composting, in the case of the biochar-amended trials. However, the ratios remained unchanged until the period following 35 days of composting. Ratios of trials WB12 and WB18 increased by 15.0–20.0% and were higher than those of the control (that had not been biocharamended). These data indicated that the humification degree and aromatization degree had improved and that the sample containing 12–18% of wood biochar had a favorable effect on sludge composting. The results of SUV254/DOC were agreement with EEM interpretation. The final compost products present favorable quality and stability due to high aromatization and humification. Such compost would produce the best possible for recycling solid waste and improving soil properties. Wood biochar contained a higher level of surface oxygen functional groups, which increased the capacity of biochar to chemisorb nutrients, moisture, minerals and dissolved organic matter. The increased amount, and retention time, of the absorbed compounds enhanced chemical and biochemical reactions associated with humification (Song and Guo, 2012; Fischer and Glaser, 2012). Furthermore, pure wood biochar was also found contained higher levels of aqueous fulvic-acid-like and humic-acid-like compounds in the water-extractable fraction (Zhang et al., 2014). The presence of these large amounts of organic compounds may facilitate their incorporation of humus-like substances and subsequent conversion into humified end-products. 3.5. SEM on solid samples The microscopic structure of solid samples was visualized, to assess the microstructure and porosity of wood biochar, primary sludge and dried composting mixtures. The SEM images indicate a large degree of disordered clumpy structures, with open porosity, covering the surface of wood biochar (Supplemental information: Fig. S2). As seen in Fig. S2(b), the plate-like layer construction and the poor structure of the raw sludge were smoothly compacted (Fig. S2(b)). The initial composting materials were similar in all the trials and exhibited a low-porosity, dense, and homogeneous microstructure (Fig. S2(c, d)). With time it was noted that the dense and tightly-packed microstructure had disintegrated, gradually converting into fragments (Fig. S2(e–l)). The SEM images also revealed an additional 12–18% w/w of wood biochar, which resulted in increasing quantities of plate-like fragments and

Table 2 Variation in SUV254/DOC (L mg1 m1) during the process of composting.

WB0 WB06 WB12 WB18

0-day

7-day

14-day

21-day

28-day

35-day

42-day

0.20 0.11 0.11 0.10

1.10 ± 0.02 1.15 ± 0.03 1.37 ± 0.02 1.40 ± 0.03

1.45 ± 0.03 1.50 ± 0.04 1.77 ± 0.06 1.89 ± 0.01

1.81 ± 0.05 2.01 ± 0.04 2.13 ± 0.05 2.28 ± 0.06

2.03 ± 0.08 2.15 ± 0.10 2.28 ± 0.08 2.55 ± 0.08

2.12 ± 0.07 2.20 ± 0.06 2.31 ± 0.01 2.60 ± 0.07

2.15 ± 0.05 2.20 ± 0.08 2.33 ± 0.06 2.58 ± 0.04

Please cite this article in press as: Zhang, J., et al. The use of biochar-amended composting to improve the humification and degradation of sewage sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.080

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decreasing fragment volumes (Fig. S2(k, l)). The resultant irregular and discontinuous surface, with many channels or voids, thus covered the coating of sludge (Fig. S2(k, l)). This may favor the disintegration of sludge as well as oxygen fill-into enhance aerobic biodegradation. The improved organics degradation and water loss could promote the sludge disintegration. Comparatively, the amendment of 6% w/w biochar did not impose significant changes on the sludge microstructure. 3.6. Elementary analysis of solid organics and the soluble C/N of aqueous phase An elemental analysis was performed to determine the percentage content of carbon, hydrogen and nitrogen in the solid composting materials at each stage of the trials. The elementary composition, C/N values, and atomic ratios of H/C values are listed in the Table 3. In all cases, the solid C/N values decreased during the composting process, with initial values (ranging from 9.4 to 14.0) falling to a range of 7.7–12.2 after maturation. The percentage content of nitrogen in compost is one of the basic criteria that should be taken into consideration when determining the usefulness of an organic material for agricultural purposes. In all cases this was found to increase after composting. A higher content of nitrogen was obtained for the control (without biochar). This was probably due to the higher nitrogen content in primary sludge. The nitrogen content in the final compost product increased by 14.5–15.0%, compared with the initial nitrogen content measured in trials WB12 and WB18. This, however, increased by only 4.3– 5.1%, compared to an increase of 0–6% in the biochar-amended trials. The use of 12–18% biochar thus had a positive effect on nitrogen enrichment during the composting process. Moreover, the values of H/C atomic ratio, listed in Table 3, decreased in all the cases. A large decrease in the H/C atomic ratio, of over 20%, was noted in trial WB18. In the case of the control, however, the H/C atomic ratio amounted to only 7.6%. This ratio provides information on the progress of the condensation and

aromatization processes. Lower H/C atomic ratios indicate a higher degree of aromatization. Changes in the C/N ratio of aqueous phase reflected the degree of organic matter decomposition and stabilization that has been achieved during composting. As shown in Table 3, the water soluble C/N ratio decreased during composting in all cases, declining from a wide range (of 10.1–15.3) in the initial samples to a very narrow range (of 6.6–7.3) on day 21, and then to a range of 6.2– 7.4 in the final composting products. This indicated that, in cases where the water soluble C/N ratio was in the range of 5–6, the composting product of trials WB12 and WB18 had almost reached maturation (Chanyasak and Kubota, 1981). 3.7. FT-IR spectroscopy on solid organics FT-IR spectroscopy was employed to achieve a better understanding of the structural changes in organic matter during composting. This information is useful for monitoring and evaluating efficiency and compost maturity. The FT-IR spectra (Supplemental information: Fig. S3) of primary pure sludge and composting materials of sludge, straw and biochar on the 21st and 42nd days are shown. The sharp peak in the FT-IR spectra of primary sludge in the region 1030 cm1 was assigned to CAO stretching of polysaccharides or polysaccharide-like substances. After composting, the sharp peak decreased with time and eventually appeared in the spectrum as a shoulder in the final composting product. Meanwhile, a peak of 1080 cm1, which appeared in a similar position in the broad region, was assigned to SiAO, indicating silicate impurities and clay minerals, possibly in a complex with humic acids. Peaks at 1540 and 1650 cm1 were assigned to the amide II band and amide I bands of protein origin. The amide I band was noted to disappear in the composted sludge spectra. The 2925 and 2855 cm1 peaks were attributed to aliphatic, methylene groups and assigned to fats and lipids. The intensity of peaks at 1540, 1650 and 2950 cm1 decreased with time in all trials. The 1384 cm1 peak represented a nitrate band, which appeared in

Table 3 Elementary compositions of solid organics and water soluble C/N in different trials during the composting process. Trials

Time

C%

H%

H/C atomic ratio

Water soluble C/N

WB0

0 day 7 day 14 day 21 day 28 day 35 day 42 day

33.3 ± 0.1 31.1 ± 0.2 28.1 ± 0.3 29.6 ± 0.5 31.5 ± 0.0 29.0 ± 0.6 28.4 ± 0.1

5.4 ± 0.0 4.9 ± 0.1 4.3 ± 0.0 4.4 ± 0.1 4.4 ± 0.0 4.4 ± 0.1 4.4 ± 0.1

N% 3.5 ± 0.2 3.3 ± 0.1 3.3 ± 0.1 3.4 ± 0.1 3.4 ± 0.2 3.5 ± 0.1 3.7 ± 0.1

C/N 9.4 ± 0.2 9.3 ± 0.2 8.6 ± 0.2 8.8 ± 0.3 9.3 ± 0.3 8.2 ± 0.3 7.7 ± 0.4

2.0 ± 0.0 1.9 ± 0.0 1.8 ± 0.0 1.8 ± 0.0 1.7 ± 0.1 1.8 ± 0.0 1.8 ± 0.0

15.3 ± 0.2 10.0 ± 0.3 7.7 ± 0.4 8.0 ± 0.1 7.5 ± 0.1 7.5 ± 0.1 7.4 ± 0.2

WB06

0 day 7 day 14 day 21 day 28 day 35 day 42 day

34.0 ± 0.2 33.0 ± 0.2 31.3 ± 0.4 32.5 ± 0.5 31.7 ± 0.2 32.3 ± 0.0 31.3 ± 0.3

4.9 ± 0.0 4.5 ± 0.1 4.0 ± 0.0 4.1 ± 0.1 4.0 ± 0.0 4.1 ± 0.0 4.1 ± 0.1

3.2 ± 0.1 3.0 ± 0.0 2.8 ± 0.1 3.0 ± 0.0 3.2 ± 0.0 3.2 ± 0.0 3.3 ± 0.1

10.8 ± 0.2 10.9 ± 0.2 11.1 ± 0.2 10.6 ± 0.3 9.9 ± 0.4 10.2 ± 0.2 9.4 ± 0.2

1.7 ± 0.0 1.6 ± 0.0 1.5 ± 0.0 1.5 ± 0.0 1.5 ± 0.0 1.5 ± 0.0 1.6 ± 0.0

14.8 ± 0.2 8.4 ± 0.3 7.2 ± 0.1 6.7 ± 0.1 6.9 ± 0.1 7.2 ± 0.1 7.0 ± 0.4

WB12

0 day 7 day 14 day 21 day 28 day 35 day 42 day

33.1 ± 0.1 33.5 ± 0.1 33.8 ± 0.3 33.9 ± 0.1 33.6 ± 0.1 33.7 ± 0.1 33.6 ± 0.3

4.4 ± 0.0 4.0 ± 0.0 3.8 ± 0.0 3.7 ± 0.0 3.7 ± 0.1 3.6 ± 0.0 3.8 ± 0.0

2.6 ± 0.0 2.7 ± 0.0 2.8 ± 0.1 2.8 ± 0.0 3.01 ± 0.1 3.0 ± 0.1 3.0 ± 0.1

12.8 ± 0.1 12.4 ± 0.1 11.9 ± 0.2 11.9 ± 0.4 11.2 ± 0.4 11.2 ± 0.4 11.1 ± 0.0

1.6 ± 0.0 1.4 ± 0.0 1.3 ± 0.0 1.3 ± 0.0 1.3 ± 0.0 1.3 ± 0.0 1.4 ± 0.0

11.9 ± 0.2 7.3 ± 0.2 6.9 ± 0.2 6.6 ± 0.1 6.8 ± 0.1 6.8 ± 0.3 6.3 ± 0.1

WB18

0 day 7 day 14 day 21 day 28 day 35 day 42 day

32.7 ± 0.1 33.1 ± 0.2 33.8 ± 0.2 37.1 ± 0.4 34.6 ± 0.8 35.0 ± 0.2 32.4 ± 0.4

4.1 ± 0.0 3.9 ± 0.1 3.3 ± 0.1 3.4 ± 0.0 3.4 ± 0.0 3.4 ± 0.1 3.3 ± 0.0

2.3 ± 0.0 2.4 ± 0.1 2.4 ± 0.1 2.6 ± 0.0 2.7 ± 0.1 2.7 ± 0.0 2.7 ± 0.0

14.1 ± 0.1 14.0 ± 0.2 13.8 ± 0.2 14.4 ± 0.3 13.0 ± 0.2 12.8 ± 0.4 12.2 ± 0.0

1.5 ± 0.0 1.4 ± 0.0 1.2 ± 0.0 1.1 ± 0.0 1.2 ± 0.1 1.2 ± 0.0 1.2 ± 0.0

10.1 ± 0.1 7.1 ± 0.3 6.6 ± 0.1 6.6 ± 0.2 6.7 ± 0.3 6.6 ± 0.1 6.2 ± 0.2

Please cite this article in press as: Zhang, J., et al. The use of biochar-amended composting to improve the humification and degradation of sewage sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.080

J. Zhang et al. / Bioresource Technology xxx (2014) xxx–xxx

the spectra and increased in intensity from the 21st day to the 42nd day of composting. The occurrence of this band indicated that nitrogen from decomposed components had been oxidized. It was detected exclusively during the later composting phase, when the material was well composted (Grube et al., 2006). These results indicated that easily-degradable organic matter components, such as aliphatic chains, polysaccharides, fats and protein, had been decomposed. An increase in the peak value 1384/2925 and decrease of the peak values 1925/1034 and 1034/1384 indicated the progress of composting and consequent higher degree of decomposition (Grube et al., 2006). With respect to the composting materials under the present study, the peak ratios of 1384/2925 were 0.75, 0.83, 1.41, and 1.47 for trial WB0, WB06, WB12 and WB18, respectively. These results indicated that the relative increase in aromatic components and compost maturity was associated with increasing levels of wood biochar addition. However, the band at 1030 cm1 appeared in the spectra as a shoulder, and its intensity was indistinct. The ratios of 2925/1034 and 1034/1384 had thus become unusable as a means for evaluating maturity. 4. Conclusions This work has demonstrated that the use of wood biochar, at 12–18% w/w, had a positive effect on the properties of sewage sludge composting materials. Literature accounts indicate that pure biochar can enhance soil fertility by improving soil structure, water retention capacity, and nutrient availability to crops. Soil amendments based on biochar-blended composts will therefore combine the positive effects of pure biochar and the well-known effects of composting materials, for promoting sustainable agriculture. Further investigation is needed to evaluate the effect of the biochar-compost complex on the development of crops and its effect on soil properties. Acknowledgements The authors express appreciation to the following: the National Basic Research Program (973) of China (No. 2012CB719801), the Key Special Program on the Science & Technology for the Pollution Control and Treatment of Water Bodies (No. 2011ZX07303-004-03), the National High Technology Research and Development Program (863) of China (No. 2012AA063504) and the Fok Ying-Tong Education Foundation (No. 132012). The authors also appreciated the comments and suggestions from the anonymous reviewers and editors. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech. 2014.02.080. References Campbell, A.G., Folk, R.L., Tripepi, R.R., 1997. Wood ash as an amendment in municipal sludge and yard waste composting processes. Compost Sci. Util. 5 (1), 62–73. Chanyasak, V., Kubota, H., 1981. Carbon/organic nitrogen ratio in water extract as measure of compost degradation. J. Ferment. Technol. 59, 215–219. Chen, W., Westerhoff, P., Leenheer, J.A., Booksh, K., 2003. Fluorescence excitationemission matrix regional integration to quantify spectra for dissolved organic matter. Environ. Sci. Technol. 37 (24), 5701–5710. Clesceri, L.S., Greenberg, A.E., Eaton, A.D. (Eds.), 1998. Standard Methods for the Examination of Water and Wastewater, twentieth ed. Published by the American Public Health Association, American Water Works Association, Water Environment Federation, Washington, DC. Cossu, R., Raga, R., 2008. Test methods for assessing the biological stability of biodegradable waste. Waste Manage. 28, 381–388.

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Please cite this article in press as: Zhang, J., et al. The use of biochar-amended composting to improve the humification and degradation of sewage sludge. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.080