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Effects of biochar on nitrogen transformation and heavy metals in sludge composting
Accepted Manuscript Effects of biochar on nitrogen transformation and heavy metals in sludge composting Wei Liu, Rong Huo, Junxiang Xu, Shuxuan Liang, Jijin Li, Tongke Zhao, Shutao Wang PII: DOI: Reference:
S0960-8524(17)30322-X http://dx.doi.org/10.1016/j.biortech.2017.03.052 BITE 17756
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
20 November 2016 6 March 2017 7 March 2017
Please cite this article as: Liu, W., Huo, R., Xu, J., Liang, S., Li, J., Zhao, T., Wang, S., Effects of biochar on nitrogen transformation and heavy metals in sludge composting, Bioresource Technology (2017), doi: http://dx.doi.org/ 10.1016/j.biortech.2017.03.052
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1
Effects of biochar on nitrogen transformation and heavy metals in sludge
2
composting
3
Short title: Application of biochar in sludge composting
4
Wei Liu1,†, Rong Huo1, Junxiang Xu 2, Shuxuan Liang1, Jijin Li2, Tongke Zhao 2, Shutao
5
Wang3, †,*
6 7
1
8
China;
9
2
College of Chemistry and Environmental Science, Hebei University, Baoding, 071002,
Institute of Plant Nutrition and Resources, Beijing Academy of Agriculture and
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Forestry Science, Beijing, 100097, China;
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3
Agriculture University of Hebei, Baoding, 071002, China
13
†
These authors contribute equally to this work.
14
Abbreviations
15
BC, bamboo charcoal; C/N, carbon-to-nitrogen; PCA, principal component analysis.
12
16 17
*Corresponding author:
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Shutao Wang
19
289 Lingyu Temple Street,
20
Baoding, 071002,
21
China;
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Tel: +86-0312-7521283;
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Email: [email protected]
24
1
25
Abstract
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Composting is regarded as an effective treatment to suppress pathogenic organisms and
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stabilize the organic material in sewage sludge. This study investigated the use of
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biochar as an amendment to improve the composting effectiveness and reduce the
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bioavailability of heavy metals and loss of nitrogen during composting. Biochar of 0%,
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1%, 3%, 5% and 7% were added into a mixture of sludge and straw, respectively. The
31
use of biochar, even in small amounts, altered the composting process and the properties
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of the end products. Biochar addition resulted in a higher pile temperature (66 °C) and
33
could reduce nitrogen loss by transforming ammonium into nitrite. In the 5% biochar
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group, the final product from sludge composting, ammonia nitrogen, decreased by
35
22.4% compared to the control, and nitrate nitrogen increased by 310.6%. Considering
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temperature and N transformation, the treatment with 5% biochar is suggested for
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sludge composting.
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Key words: biochar; nitrogen; heavy metal; composting
39 40 41 42 43 44 45 46 47 48
2
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1. Introduction
50 51
Because of the widespread practice of urban sewage treatment, sludge is produced
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as a by-product in large quantities and stockpiled in open environments. If not disposed
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of effectively, sludge easily causes secondary pollution in groundwater, soil and
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atmospheric environments. Composting is one of the more acceptable and economically
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feasible practices for recycling sludge, especially in developing countries, and enables
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the bioconversion of organic waste into a well-stabilized, value-added product.
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Composting also plays a positive role in stabilizing organic matter and reducing the
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environmental damage of heavy metals in the sludge (Dong and Huang, 2013).
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Furthermore, this practice can alleviate the pressure of sludge stacked in large quantities
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at composting sites and landfills. The addition of microbial adjustment agents and
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physical conditioners can greatly improve the efficiency of composting and nutrient
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storage. VT microbes (mainly composed of actinomycetes, yeasts, lactic acid bacteria,
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and acetobacter species and used for composting organic waste; Beijing Voto Biotech
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Co., Ltd, China) in compost fermentation can rapidly increase the temperature, increase
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the number of functional microorganisms, and promote the degradation of organic
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matter and nitrogen retention (Hu et al., 2006). Biochar agents, because of their high
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porosity and high specific surface area, provide a more suitable habitat for abundant
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functional groups than activated carbon, which not only enhances the degree of compost
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humification but also reduces nitrogen loss (N loss during composting is mainly caused
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by gaseous emissions in the form of NH3 as well as N2O (Dias et al., 2010; Jiang et al.,
71
2016; Awasthi et al., 2017) from compost and enhances compost quality (Keiji et al.,
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2012), and these agents have therefore received widespread attention from researchers.
3
73
To date, numerous studies have investigated the application of biochar in sludge
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compost with promising outcomes. Dias et al. (2010) evaluated the use of biochar as a
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bulking agent for composting poultry manure and found that a composting mixture
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prepared with biochar effectively reduces nitrogen loss compared with coffee husk and
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sawdust treatment. Jindo et al. (2012) found that biochar addition in a composting
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experiment with poultry manure and organic waste significantly impacts the
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biochemical characteristics of the compost and enhances the humification of the organic
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materials. Their study revealed a higher diversity of fungi in biochar-amended compost,
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suggesting a change in microbial composition compared to the unamended compost. Li
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et al. (2009) used bamboo charcoal (BC) for composting, showing that incorporation of
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BC into sludge composting materials significantly reduces nitrogen loss. The mobility
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of heavy metals in sludge composting material can also be reduced by the addition of
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BC. Several scholars have also suggested that biochar amendment could be a novel
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greenhouse gas mitigation strategy during composting (Wang et al., 2013; Awasthi et al.,
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2016a; Awasthi et al., 2016b). Although facilitation by biochar as a compost amendment
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has been reported, the proportion of biochar used in anaerobic digestion sludge
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composting and its effect on the speciation of heavy metals in sludge remain
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unexplored.
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Therefore, this study used anaerobic digestion sludge and straw as composting
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materials. While adding the same amount of microbial agents, different amounts of
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biochar were added as amendments, and the temperature, pH, and nitrogen content were
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measured over the course of composting. Additionally, the speciation of heavy metals
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before and after composting was compared to investigate the effect of different biochar
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amendments on composting and provide a technical reference for sludge recycling
4
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processes.
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2. Materials and methods
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2.1. Experimental materials
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The dewatered fresh sewage sludge used in this experiment was taken from an urban
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sewage treatment plant in Tianjin, China. This plant uses traditional secondary treatment
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to activate sludge, with a daily sludge volume of 8834 m3/d (moisture capacity, 97%),
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volatile solids concentration of 52.9%, and ash content of dehydrated sludge of 40.5%.
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The straw, with organic carbon and inorganic ash contents of 37.6% and 4.3%,
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respectively, was taken from a farm in Beijing. The biochar used in this experiment was
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straw biochar. The basic physical and chemical properties of the compost materials are
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shown in Table 1.
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The biochar used for the pot cultivation experiments was from wheat straw that
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had undergone pyrolysis at 350–550 °C in a commercial pyrolysis reactor at the Sanli
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New Energy Company, China. The main characteristics of the biochar were as follows:
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pH (H2O) of 8.23, methylene blue (MB) absorption capacity of 8.3 mg/g, and iodine
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adsorption capacity of 100 mg/g (Liu et al., 2016).
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2.2. Experimental design
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The composting microbial agents were complex microbial agents obtained from the
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Agricultural Culture Collection of China, including yeast (preservation no.: 20006; for
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hydrolysate fermentation), Bacillus sp. (preservation no.: 19373; for cellulose
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decomposition) and Pseudomonas sp. (preservation no.: 01021; possessing an
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antogenetic nitrogen fixation capacity), with the following composition: yeast,
5
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0.15-0.18 parts by weight; Aspergillus niger, 0.12-0.14 parts by weight; Bacillus subtilis,
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0.08-0.1 parts by weight; and Pseudomonas sp., 0.06-0.08 parts by weight. These
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microbial agents have been shown to increase the composting speed and degradation
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rate in previous experiments (Wang et al., 2012; Liu et al., 2014). To obtain
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reproducible data, the straw and sludge were thoroughly mixed at a ratio of 6:5 and
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divided into twelve parts. According to the literature (Wang et al., 2012; Liu et al.,
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2014), the design of the current experiment was as follows: Treatment 1 (hereafter
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referred to as T1), sludge + straw + 0.4% microbial agents (V/W); Treatment 2 (T2),
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sludge + straw + 0.4% microbial agents + 1% biochar (W/W); Treatment 3 (T3), sludge
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+ straw + 0.4% microbial agents + 3% biochar; Treatment 4 (T4), sludge + straw +
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0.4% microbial agents + 5% biochar; and Treatment 5 (T5), sludge + straw + 0.4%
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microbial agents + 7% biochar.
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2.3. Composting method
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This experiment adopted a fermentation tank as a simulated composting device. The
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straw was pulverized and mixed evenly with sludge. The moisture content of the
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composting mixture was determined daily with an SK-100 moisture meter (Tokyo,
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Japan). This content was adjusted to approximately 60%, and the microbial agents and
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biochar were added according to their corresponding amounts. The carbon-to-nitrogen
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ratio (C/N) was adjusted to 25:1, as determined using an elemental analyzer. The pile
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was turned every three days, and the total amount of the composting pile for each
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treatment was 2.5 kg. The maturity time was 31 d (Liu et al., 2014).
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2.4. Index measurements
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Temperature is an important factor affecting microbial activity and composting
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processes and is the second most important parameter affecting ammonia emissions
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from the composting pile after pH (Krystyna et al., 2014). During the composting
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process, a thermometer was inserted into the center of the pile, and the pile temperature
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was measured at 10:00 am and 4:00 pm every day. At other times, the temperature of the
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surrounding environment was measured.
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The five-point method, which refers to the four corners and sampling center of the
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compost pile at a depth of 30 cm, was used for sampling during the 0, 3, 6, 9, 14, 24,
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and 31 d of composting. The sample weight was approximately 200 g, divided in half.
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One half was the fresh sample, used to determine the moisture content of the pile. The
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other half was subjected to an air-drying treatment, in which the sample was mashed,
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evenly mixed, ground, sieved through a 0.25 mm mesh, sealed and stored for pH,
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nitrogen and heavy metal speciation determinations.
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pH is an important factor affecting the ammonia volatilization from the compost. A
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reasonable pH level can increase the effectiveness of the functional microorganisms in
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the compost and retain the effective nitrogen content in the compost (Jia et al., 2008).
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The pH was measured using a pH meter (Mettler Toledo, FE-20, Switzerland) according
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to Li et al. (2012). Total nitrogen was measured on a Vario EL III CHNSO elemental
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analyzer (Sigma, St. Louis, MO, USA). Measurement of nitrate nitrogen followed the
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method for determining nitrate nitrogen in fertilizers by ultraviolet spectrophotometry
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(NY/T1116-2006). Measurement of ammonia nitrogen referred to the colorimetric
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method (Bao, 2013). A modified BCR (Community Bureau of Reference) method was
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used to determine the heavy metal speciation, and the extraction solution of each species
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was analyzed by ICP-MS (inductively coupled plasma mass spectrometry) (Zhang et al.,
7
169
2012).
170 171
3. Results and discussion
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3.1. Temperature variation during composting
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The temperature variation in the different treatment groups during the composting
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process is shown in Figure 1. The temperature on the first day of composting increased
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rapidly, directly entering the high-temperature phase, but the period of the temperature
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increase was not obvious. This increase was due to the addition of microbial agents,
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which sped up the composting reactions and consequently rapidly increased the
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temperature of the pile (Karadag et al., 2013). According to China’s “Hygienic
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Requirements for Harmless Disposal of Night Soil (GB7959-2012)”, the composting
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temperature must remain above 50-55 °C for 5-7 d or above 55 °C for 3 d for standard
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sterilization. The T1 temperature was maintained above 60 °C for 4 d. The T5
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temperature was maintained above 55 °C for 3 d and then dropped gradually to room
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temperature. Neither of these two treatments achieved the required results described in
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the High-Temperature Compost Evaluation Standards (GB7959-87; the temperature of
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the compost should be at 50-55 °C for 5-7 d). The T2 temperature was maintained
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above 50 °C for on the first five d and then remained above 30 °C on the 13th-21st day,
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when secondary fermentation occurred, which was more conducive to the maturity of
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the compost. The temperature of the T3 treatment reached 70 °C on the first two days,
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quickly killing the harmful microorganisms. At the same time, the temperature of the
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compost exceeded 45 °C on the 9th-11th day, when a transient secondary fermentation
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phenomenon occurred. In T4, a temperature above 5 °C lasted for 5 d, after which the
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pile temperature gradually decreased with composting and no secondary fermentation
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occurred.
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3.2. pH variation during composting
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The pH variation during composting is shown in Figure 2. The pH in each treatment
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was highest on the first day of composting, reached a trough on the 6 th day, and then
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stabilized. During the whole composting process, the pH range in T5, T4, and T3 was
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6.6~7.5, 6.2~7.8, and 6.97~7.76, respectively, and the pH in T2 was always acidic and
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fluctuated over a range of 5.73~6.68. Microorganisms in the early stage of composting
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clearly caused the simple carbohydrates to decompose rapidly and transform into
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small-molecule organic matter. The formation of low-molecular-weight fatty acids and
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CO2 greatly contributed to the decrease in the pH of the compost pile. As composting
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proceeded, ammonia oxidation promoted the production of ammonium ions and
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increased the pH of the pile. In the later stage of composting, nitrification was enhanced
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due to the weakening of ammonia volatilization. At the same time, the macromolecular
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organic compounds decomposed to produce organic acids and phenolic compounds
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under the action of hyperthermophilic bacteria, which caused the pH of the pile to
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decline slowly and stabilize (Sun et al., 2016).
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3.3. Nitrogen variation during composting
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3.3.1. Total nitrogen variation during composting
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The variation in total nitrogen during the composting process is depicted in Figure 3.
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The total nitrogen contents in T1 to T5 first decreased due to intense NH3 emissions,
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then increased and stabilized. Chan et al. (2016) reported that the composting mass is
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reduced faster than nitrogen and that the total nitrogen increases mainly due to a
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concentration effect. After composting, the total nitrogen contents in T1, T2, T3, T4 and
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T5 increased by 6.99%, 0.48%, 11.92%, 14.25% and 11.24%, respectively. Compared
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with the other treatments, the total nitrogen content increased the most in T4, which was
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amended with 5% biochar. Over the whole composting period, the total nitrogen content
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in T3 was the lowest, followed by T1 and T5. The total nitrogen content in T4 was
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always the highest, indicating that the degradation of organic matter was the most rapid
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or the nitrogen retention was the greatest. A possible reason for these results is the
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porosity of the biochar, making the air and moisture available to the compost material,
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which is favorable for the fermentation of the pile, speeding up the degradation rate of
226
the organic matter in the pile as a result (Sánchez-García et al., 2015). On the other hand,
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the high specific surface area of the biochar was more favorable for microbial
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attachment, which accelerated the decomposition of organic matter and increased the
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total nitrogen content in the pile (Zhang et al., 2016). The ratio of biochar addition in T4
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was 5%, which was probably more suitable for the balance between the sludge and
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biochar in the composting system. The rate of addition of biochar in T2, T3 and T5 did
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not have an obvious effect compared to the control group, i.e., no biochar amendment.
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Combined with other influencing factors such as temperature, the total nitrogen content
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in the control group increased more than in T2.
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3.3.2. Variation in ammonia nitrogen during composting
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The change in ammonia nitrogen mainly depends on high temperature and pH and the
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activity of ammoniated bacteria in the compost material. Figure 4 shows that the
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contents of ammonia nitrogen in the five treatments all decreased first and then
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increased. In the early composting stage (0-6 d), the content of ammonia nitrogen
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decreased rapidly probably due to the rapid rise in temperature under the action of
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microbial agents. High temperature promoted the rapid propagation of microorganisms,
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and the ammonia nitrogen produced by organic matter mineralization was quickly
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immobilized by microorganisms as a result, which might have been due to utilization by
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ammonia oxidizers or conversion into NH3. At the end of the high-temperature period
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(after 6 d), the contents of ammonia nitrogen in the five treatments increased rapidly,
247
with that in T4 increasing the fastest. After composting, the loss rates of ammonia
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nitrogen, which is the final product of sludge composting, with biochar treatment (T2,
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T3, T4 and T5) were 35.6%, 27.8, 22.4% and 22.9% compared to the control (T1),
250
respectively. The content of ammonia nitrogen in T1 decreased the most after
251
composting probably due to the absence of biochar, leading to the resultant ammonia
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gas not being adsorbed by the biochar and instead being volatilized into the air in large
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quantities. The content of ammonia nitrogen in T3 was always lower than that in the
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other three treatments but did not undergo great fluctuations, showing a steady increase
255
with time. This result was due to the addition of biochar, which adsorbed urea, uric acid,
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and the resultant ammonia gas in the pile and thereby had a significant effect on
257
reducing nitrogen loss during composting (Meng et al., 2016). Overall, our results are
258
consistent with those of Steiner (Steiner et al., 2008).
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3.3.3. Variation in nitrate nitrogen during composting
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The changes in nitrate nitrogen during composting are shown in Figure 5. The contents
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of nitrate nitrogen in the five treatments increased first, then decreased, and gradually
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stabilized. The high-temperature environment of the compost on the first day strongly
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inhibited the activity of nitrifying bacteria as a result of rapid warming of the compost
11
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caused by the addition of microbial agents. Inorganic nitrogen mainly existed as
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ammonia nitrogen in the compost. As the composting progressed, the decomposable
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material decomposed rapidly, and the temperature of the compost pile decreased slowly.
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Furthermore, under the stimulation of biochar with ammonia-oxidizing (AOB;
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oxidizing NH4+-N to form NO2--N) and nitrite-oxidizing (NOB; oxidizing NO2--N to
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form NO3--N) bacteria, the nitrifying bacteria proliferated rapidly, and a large amount of
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ammonia nitrogen was transformed into nitrate nitrogen. Therefore, the nitrate nitrogen
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content in the initial stage of composting increased rapidly. After composting, the nitrate
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nitrogen contents in the five treatments changed quite differently, increasing by 62.4%,
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140.2%, 138.2%, 310.6%, and 280.5%, respectively. Compared with the other
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treatments, the nitrate nitrogen content in T4 increased the most, and the nitrate nitrogen
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content in T1 increased the least. Such a difference was possibly due to the addition of
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biochar, which was beneficial to the accumulation of nitrate nitrogen in the compost pile.
278
The high porosity of the biochar allowed microorganisms to better adsorb onto the
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biochar surface or pores and would also increase the looseness of the compost pile,
280
increase the oxygen flux and promote nitrification, leading to the accumulation of
281
nitrate nitrogen in the compost pile (Khan et al., 2014).
282 283
3.4. Variation in heavy metals during composting
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During sewage treatment, more than 50% to 80% of the heavy metals will be
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concentrated in the generated sludge. Heavy metals in sludge are one of the main
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restrictions on the promotion of sludge use. The content of heavy metals directly affects
287
the possibility of sludge land application. Biochar itself contains many characteristics
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conducive to the passivation of heavy metals. For example, the high contents of
12
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nitrogen and extractable inorganic nutrients such as phosphorus, potassium, calcium,
290
and magnesium in biochar, which are mostly alkaline (Jia et al., 2008), will increase the
291
pH and inhibit heavy metal activation. Both the high CEC of biochar and amount of
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oxygen-containing functional groups on its surface are conducive to the passivation of
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heavy metals (Jiang et al., 2012; Wang, 2013). The migration and phytotoxicity of
294
heavy metals are mainly determined by the heavy metal bioavailability (Lu et al., 2014).
295
In this experiment, the speciation of seven heavy metal elements, Pb, Ni, Cu, Zn,
296
As, Cr and Cd, were measured to explore the effects of different biochar additions on
297
the passivation of heavy metals. Table 2 shows the reduction in these elements in the
298
different treatments after composting compared to the total amount before composting.
299
T2 had the best effect on heavy metal passivation, followed by T5> T3> T4> T1. The
300
addition of biochar was beneficial to the passivation of heavy metal elements. However,
301
greater amounts of amended biochar do not necessarily lead to improved heavy metal
302
passivation. For example, Sun (2013) added 3%, 6% and 9% BC as an agent in compost
303
and found that the addition of 6% BC had the best passivation effect on Cu. The
304
variation in the seven heavy metal elements was not the same. After composting, the
305
content of available Pb decreased the most, followed by As, Cu, Cr and Ni, whereas the
306
contents of available Zn and Cd increased after composting. Regarding morphological
307
changes in heavy metals, different researchers have arrived at different conclusions.
308
After composting sludge amended with sawdust, Sun (2007) found that the content of
309
available Cd increased but the bioavailability of Cu, Ni, Pb, and Cr decreased. After pig
310
manure composting, Liu (2008) found that the concentrations of available Zn, Cr, Cu,
311
and Cd and Pb significantly decreased but the concentration of available Hg did not
312
change significantly.
13
313
To clearly understand the effect of biochar amendments on the bioavailability of
314
heavy metals in the composting process, the content of available heavy metals was
315
further analyzed by principal component analysis (PCA) before and after composting.
316
The contributions of PC1, PC2 and PC3 were 47.01%, 24.75% and 20.57%, respectively,
317
and the contribution of the accumulated variance of the first three principal components
318
reached 92.32%. According to the general principle of PCA, the first three
319
representative principal components were selected. In Figure 6a, rather large differences
320
can be observed in the directions of PC1, PC2 and PC3 in T1 before and after
321
composting. However, the bioavailability of heavy metals only showed obvious
322
differences in the direction of PC3 in T2 before and after composting. A large difference
323
can be observed in the PC1 direction in T3, T4 and T5 before and after composting.
324
Therefore, PC1, with 47.1%, was a main factor influencing the effect of biochar
325
compost on the bioavailability of heavy metals. Furthermore, a factor loading was
326
drawn on the three principal components of the seven heavy metals to further analyze
327
the contributions of the seven heavy metals to the three principal components. As
328
shown in Figure 6b, Pb contributed the most to PC1, Cr contributed the most to PC2,
329
and As contributed the most to PC3. These PCA results revealed that the passivation
330
effect of different treatments on heavy metals was different, i.e., the passivation effect
331
on Pb was the best, followed by Cr and As. These results are essentially consistent with
332
the results in Table 2 in that the composting treatment had the best passivation effect on
333
Pb and As in the original sludge.
334 335
4. Conclusions
336
This study confirmed the importance of biochar and microorganism amendment in
14
337
reducing not only N losses during composting but also the availability of heavy metals
338
in the compost pile, particularly Pb and As, thereby improving the maturity properties
339
and agronomic value of the end products. However, considering temperature and N
340
transformation, the treatment with 5% biochar is suggested for sludge composting to
341
enhance the fermentation of the compost and the compost product quality. Further
342
experiments are needed to verify the effects of biochar- and microorganism-amended
343
compost on plant growth to render this technology viable.
344 345
Supplementary material
346
Supplementary Figure 1 shows a photograph of biochar.
347 348
Conflict of interest
349
The authors declare no conflict of interest.
350 351
Acknowledgments
352
None.
353 354
Funding
355
This work was supported by the public-service scientific research program (agriculture)
356
“Treatment of anaerobic digestion sludge and its application in farmland utilization and
357
security evaluation [201303101-06]” and the Natural Science Foundation of Hebei
358
Province [grant number C2012201021].
359 360
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T., Matsumoto, K., Sonoki, T., Bastida, F., 2012. Biochar influences the microbial
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for culturing microbial communities in composting. Waste Manag. 54, 93-100.
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Relationship between N2O Emission and Denitrifying Community. Environ. Sci.
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swine manure-derived biochar. Thesis, Zhejiang University.
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of different microorganism inoculant on chicken manure compost. Chinese Journal of
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Soil Science 43(3), 637-643.
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456
hastens organic matter degradation and produces nutrient-rich compost. Bioresour.
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460
reference material. Ecology Evironmen Sci. 21, 1881-1884.
461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480
20
481
Figure captions
482 483
Figure 1. Temperature variation in the five treatments: T1, sludge + straw + 0.4%
484
microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar
485
(W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw +
486
0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
487
biochar.
488 489
Figure 2. pH variations in the five treatments: T1, sludge + straw + 0.4% microbial
490
agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar (W/W); T3,
491
sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw + 0.4%
492
microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
493
biochar.
494 495
Figure 3. Variation in total nitrogen in the five treatments: T1, sludge + straw + 0.4%
496
microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar
497
(W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw +
498
0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
499
biochar.
500 501
Figure 4. Variation in ammonia nitrogen in the five treatments: T1, sludge + straw +
502
0.4% microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1%
503
biochar (W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge +
504
straw + 0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial
21
505
agents + 7% biochar.
506 507
Figure 5. Variation in nitrate nitrogen in the five treatments: T1, sludge + straw + 0.4%
508
microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar
509
(W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw +
510
0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
511
biochar.
512 513
Figure 6. PCA of the bioavailability of heavy metals during composting. (a) PCA for
514
the different treatments. (b) Eigenvector coefficients for the contributions of the seven
515
heavy metals to the three factors.
516 517
22
518 519
23
520 521
24
522 523
25
524 525
26
526 527
27
528 529
28
530
Table 1. Basic physico-chemical properties of the compost materials Organi
Moistu
Volati Inorga
Total
Total Materi
c nitrog
als
re
Carbon-to-nitr
le
nic ash weig
conten
ogen ratio
solids
(%)
pH
carbon/
ht
en % Sludge
34
t 3.39
7.5
81
(%) 10
7.1 Straw
37.6
0.89
7.9
42.2
8 531 532 533 534
29
(kg)
52.9
40.5
0.41
93.1
4.3
0.49
535
Table 2. Change in available heavy metal contents in the four treatments after
536
composting
537
Treatment
Pb
Ni
Cu
Zn
As
Cr
Cd
T1
12.52%
-
-
-
2.73%
-
-
T2
0.46%
1.84%
7.25%
-
56.31%
32.09%
-
T3
51.9%
-
59.54%
-
-
-
-
T4
27.45%
-
-
-
21.69%
-
-
T5
34.91%
-
5.55%
-
32.62%
-
-
Note: “-” indicates no reduction
538 539 540
30
541
Highlights
542
1. This study explored the effect of different biochar additions on sewage sludge.
543
2. Biochar amendment was beneficial to passivation of heavy metals in the compost pile.
544
3. The treatment with 5% biochar is suggested for sludge composting to enhance quality.
545
31
S0960-8524(17)30322-X http://dx.doi.org/10.1016/j.biortech.2017.03.052 BITE 17756
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
20 November 2016 6 March 2017 7 March 2017
Please cite this article as: Liu, W., Huo, R., Xu, J., Liang, S., Li, J., Zhao, T., Wang, S., Effects of biochar on nitrogen transformation and heavy metals in sludge composting, Bioresource Technology (2017), doi: http://dx.doi.org/ 10.1016/j.biortech.2017.03.052
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1
Effects of biochar on nitrogen transformation and heavy metals in sludge
2
composting
3
Short title: Application of biochar in sludge composting
4
Wei Liu1,†, Rong Huo1, Junxiang Xu 2, Shuxuan Liang1, Jijin Li2, Tongke Zhao 2, Shutao
5
Wang3, †,*
6 7
1
8
China;
9
2
College of Chemistry and Environmental Science, Hebei University, Baoding, 071002,
Institute of Plant Nutrition and Resources, Beijing Academy of Agriculture and
10
Forestry Science, Beijing, 100097, China;
11
3
Agriculture University of Hebei, Baoding, 071002, China
13
†
These authors contribute equally to this work.
14
Abbreviations
15
BC, bamboo charcoal; C/N, carbon-to-nitrogen; PCA, principal component analysis.
12
16 17
*Corresponding author:
18
Shutao Wang
19
289 Lingyu Temple Street,
20
Baoding, 071002,
21
China;
22
Tel: +86-0312-7521283;
23
Email: [email protected]
24
1
25
Abstract
26
Composting is regarded as an effective treatment to suppress pathogenic organisms and
27
stabilize the organic material in sewage sludge. This study investigated the use of
28
biochar as an amendment to improve the composting effectiveness and reduce the
29
bioavailability of heavy metals and loss of nitrogen during composting. Biochar of 0%,
30
1%, 3%, 5% and 7% were added into a mixture of sludge and straw, respectively. The
31
use of biochar, even in small amounts, altered the composting process and the properties
32
of the end products. Biochar addition resulted in a higher pile temperature (66 °C) and
33
could reduce nitrogen loss by transforming ammonium into nitrite. In the 5% biochar
34
group, the final product from sludge composting, ammonia nitrogen, decreased by
35
22.4% compared to the control, and nitrate nitrogen increased by 310.6%. Considering
36
temperature and N transformation, the treatment with 5% biochar is suggested for
37
sludge composting.
38
Key words: biochar; nitrogen; heavy metal; composting
39 40 41 42 43 44 45 46 47 48
2
49
1. Introduction
50 51
Because of the widespread practice of urban sewage treatment, sludge is produced
52
as a by-product in large quantities and stockpiled in open environments. If not disposed
53
of effectively, sludge easily causes secondary pollution in groundwater, soil and
54
atmospheric environments. Composting is one of the more acceptable and economically
55
feasible practices for recycling sludge, especially in developing countries, and enables
56
the bioconversion of organic waste into a well-stabilized, value-added product.
57
Composting also plays a positive role in stabilizing organic matter and reducing the
58
environmental damage of heavy metals in the sludge (Dong and Huang, 2013).
59
Furthermore, this practice can alleviate the pressure of sludge stacked in large quantities
60
at composting sites and landfills. The addition of microbial adjustment agents and
61
physical conditioners can greatly improve the efficiency of composting and nutrient
62
storage. VT microbes (mainly composed of actinomycetes, yeasts, lactic acid bacteria,
63
and acetobacter species and used for composting organic waste; Beijing Voto Biotech
64
Co., Ltd, China) in compost fermentation can rapidly increase the temperature, increase
65
the number of functional microorganisms, and promote the degradation of organic
66
matter and nitrogen retention (Hu et al., 2006). Biochar agents, because of their high
67
porosity and high specific surface area, provide a more suitable habitat for abundant
68
functional groups than activated carbon, which not only enhances the degree of compost
69
humification but also reduces nitrogen loss (N loss during composting is mainly caused
70
by gaseous emissions in the form of NH3 as well as N2O (Dias et al., 2010; Jiang et al.,
71
2016; Awasthi et al., 2017) from compost and enhances compost quality (Keiji et al.,
72
2012), and these agents have therefore received widespread attention from researchers.
3
73
To date, numerous studies have investigated the application of biochar in sludge
74
compost with promising outcomes. Dias et al. (2010) evaluated the use of biochar as a
75
bulking agent for composting poultry manure and found that a composting mixture
76
prepared with biochar effectively reduces nitrogen loss compared with coffee husk and
77
sawdust treatment. Jindo et al. (2012) found that biochar addition in a composting
78
experiment with poultry manure and organic waste significantly impacts the
79
biochemical characteristics of the compost and enhances the humification of the organic
80
materials. Their study revealed a higher diversity of fungi in biochar-amended compost,
81
suggesting a change in microbial composition compared to the unamended compost. Li
82
et al. (2009) used bamboo charcoal (BC) for composting, showing that incorporation of
83
BC into sludge composting materials significantly reduces nitrogen loss. The mobility
84
of heavy metals in sludge composting material can also be reduced by the addition of
85
BC. Several scholars have also suggested that biochar amendment could be a novel
86
greenhouse gas mitigation strategy during composting (Wang et al., 2013; Awasthi et al.,
87
2016a; Awasthi et al., 2016b). Although facilitation by biochar as a compost amendment
88
has been reported, the proportion of biochar used in anaerobic digestion sludge
89
composting and its effect on the speciation of heavy metals in sludge remain
90
unexplored.
91
Therefore, this study used anaerobic digestion sludge and straw as composting
92
materials. While adding the same amount of microbial agents, different amounts of
93
biochar were added as amendments, and the temperature, pH, and nitrogen content were
94
measured over the course of composting. Additionally, the speciation of heavy metals
95
before and after composting was compared to investigate the effect of different biochar
96
amendments on composting and provide a technical reference for sludge recycling
4
97
processes.
98 99
2. Materials and methods
100
2.1. Experimental materials
101
The dewatered fresh sewage sludge used in this experiment was taken from an urban
102
sewage treatment plant in Tianjin, China. This plant uses traditional secondary treatment
103
to activate sludge, with a daily sludge volume of 8834 m3/d (moisture capacity, 97%),
104
volatile solids concentration of 52.9%, and ash content of dehydrated sludge of 40.5%.
105
The straw, with organic carbon and inorganic ash contents of 37.6% and 4.3%,
106
respectively, was taken from a farm in Beijing. The biochar used in this experiment was
107
straw biochar. The basic physical and chemical properties of the compost materials are
108
shown in Table 1.
109
The biochar used for the pot cultivation experiments was from wheat straw that
110
had undergone pyrolysis at 350–550 °C in a commercial pyrolysis reactor at the Sanli
111
New Energy Company, China. The main characteristics of the biochar were as follows:
112
pH (H2O) of 8.23, methylene blue (MB) absorption capacity of 8.3 mg/g, and iodine
113
adsorption capacity of 100 mg/g (Liu et al., 2016).
114 115
2.2. Experimental design
116
The composting microbial agents were complex microbial agents obtained from the
117
Agricultural Culture Collection of China, including yeast (preservation no.: 20006; for
118
hydrolysate fermentation), Bacillus sp. (preservation no.: 19373; for cellulose
119
decomposition) and Pseudomonas sp. (preservation no.: 01021; possessing an
120
antogenetic nitrogen fixation capacity), with the following composition: yeast,
5
121
0.15-0.18 parts by weight; Aspergillus niger, 0.12-0.14 parts by weight; Bacillus subtilis,
122
0.08-0.1 parts by weight; and Pseudomonas sp., 0.06-0.08 parts by weight. These
123
microbial agents have been shown to increase the composting speed and degradation
124
rate in previous experiments (Wang et al., 2012; Liu et al., 2014). To obtain
125
reproducible data, the straw and sludge were thoroughly mixed at a ratio of 6:5 and
126
divided into twelve parts. According to the literature (Wang et al., 2012; Liu et al.,
127
2014), the design of the current experiment was as follows: Treatment 1 (hereafter
128
referred to as T1), sludge + straw + 0.4% microbial agents (V/W); Treatment 2 (T2),
129
sludge + straw + 0.4% microbial agents + 1% biochar (W/W); Treatment 3 (T3), sludge
130
+ straw + 0.4% microbial agents + 3% biochar; Treatment 4 (T4), sludge + straw +
131
0.4% microbial agents + 5% biochar; and Treatment 5 (T5), sludge + straw + 0.4%
132
microbial agents + 7% biochar.
133 134
2.3. Composting method
135
This experiment adopted a fermentation tank as a simulated composting device. The
136
straw was pulverized and mixed evenly with sludge. The moisture content of the
137
composting mixture was determined daily with an SK-100 moisture meter (Tokyo,
138
Japan). This content was adjusted to approximately 60%, and the microbial agents and
139
biochar were added according to their corresponding amounts. The carbon-to-nitrogen
140
ratio (C/N) was adjusted to 25:1, as determined using an elemental analyzer. The pile
141
was turned every three days, and the total amount of the composting pile for each
142
treatment was 2.5 kg. The maturity time was 31 d (Liu et al., 2014).
143 144
2.4. Index measurements
6
145
Temperature is an important factor affecting microbial activity and composting
146
processes and is the second most important parameter affecting ammonia emissions
147
from the composting pile after pH (Krystyna et al., 2014). During the composting
148
process, a thermometer was inserted into the center of the pile, and the pile temperature
149
was measured at 10:00 am and 4:00 pm every day. At other times, the temperature of the
150
surrounding environment was measured.
151
The five-point method, which refers to the four corners and sampling center of the
152
compost pile at a depth of 30 cm, was used for sampling during the 0, 3, 6, 9, 14, 24,
153
and 31 d of composting. The sample weight was approximately 200 g, divided in half.
154
One half was the fresh sample, used to determine the moisture content of the pile. The
155
other half was subjected to an air-drying treatment, in which the sample was mashed,
156
evenly mixed, ground, sieved through a 0.25 mm mesh, sealed and stored for pH,
157
nitrogen and heavy metal speciation determinations.
158
pH is an important factor affecting the ammonia volatilization from the compost. A
159
reasonable pH level can increase the effectiveness of the functional microorganisms in
160
the compost and retain the effective nitrogen content in the compost (Jia et al., 2008).
161
The pH was measured using a pH meter (Mettler Toledo, FE-20, Switzerland) according
162
to Li et al. (2012). Total nitrogen was measured on a Vario EL III CHNSO elemental
163
analyzer (Sigma, St. Louis, MO, USA). Measurement of nitrate nitrogen followed the
164
method for determining nitrate nitrogen in fertilizers by ultraviolet spectrophotometry
165
(NY/T1116-2006). Measurement of ammonia nitrogen referred to the colorimetric
166
method (Bao, 2013). A modified BCR (Community Bureau of Reference) method was
167
used to determine the heavy metal speciation, and the extraction solution of each species
168
was analyzed by ICP-MS (inductively coupled plasma mass spectrometry) (Zhang et al.,
7
169
2012).
170 171
3. Results and discussion
172
3.1. Temperature variation during composting
173
The temperature variation in the different treatment groups during the composting
174
process is shown in Figure 1. The temperature on the first day of composting increased
175
rapidly, directly entering the high-temperature phase, but the period of the temperature
176
increase was not obvious. This increase was due to the addition of microbial agents,
177
which sped up the composting reactions and consequently rapidly increased the
178
temperature of the pile (Karadag et al., 2013). According to China’s “Hygienic
179
Requirements for Harmless Disposal of Night Soil (GB7959-2012)”, the composting
180
temperature must remain above 50-55 °C for 5-7 d or above 55 °C for 3 d for standard
181
sterilization. The T1 temperature was maintained above 60 °C for 4 d. The T5
182
temperature was maintained above 55 °C for 3 d and then dropped gradually to room
183
temperature. Neither of these two treatments achieved the required results described in
184
the High-Temperature Compost Evaluation Standards (GB7959-87; the temperature of
185
the compost should be at 50-55 °C for 5-7 d). The T2 temperature was maintained
186
above 50 °C for on the first five d and then remained above 30 °C on the 13th-21st day,
187
when secondary fermentation occurred, which was more conducive to the maturity of
188
the compost. The temperature of the T3 treatment reached 70 °C on the first two days,
189
quickly killing the harmful microorganisms. At the same time, the temperature of the
190
compost exceeded 45 °C on the 9th-11th day, when a transient secondary fermentation
191
phenomenon occurred. In T4, a temperature above 5 °C lasted for 5 d, after which the
192
pile temperature gradually decreased with composting and no secondary fermentation
8
193
occurred.
194 195
3.2. pH variation during composting
196
The pH variation during composting is shown in Figure 2. The pH in each treatment
197
was highest on the first day of composting, reached a trough on the 6 th day, and then
198
stabilized. During the whole composting process, the pH range in T5, T4, and T3 was
199
6.6~7.5, 6.2~7.8, and 6.97~7.76, respectively, and the pH in T2 was always acidic and
200
fluctuated over a range of 5.73~6.68. Microorganisms in the early stage of composting
201
clearly caused the simple carbohydrates to decompose rapidly and transform into
202
small-molecule organic matter. The formation of low-molecular-weight fatty acids and
203
CO2 greatly contributed to the decrease in the pH of the compost pile. As composting
204
proceeded, ammonia oxidation promoted the production of ammonium ions and
205
increased the pH of the pile. In the later stage of composting, nitrification was enhanced
206
due to the weakening of ammonia volatilization. At the same time, the macromolecular
207
organic compounds decomposed to produce organic acids and phenolic compounds
208
under the action of hyperthermophilic bacteria, which caused the pH of the pile to
209
decline slowly and stabilize (Sun et al., 2016).
210 211
3.3. Nitrogen variation during composting
212
3.3.1. Total nitrogen variation during composting
213
The variation in total nitrogen during the composting process is depicted in Figure 3.
214
The total nitrogen contents in T1 to T5 first decreased due to intense NH3 emissions,
215
then increased and stabilized. Chan et al. (2016) reported that the composting mass is
216
reduced faster than nitrogen and that the total nitrogen increases mainly due to a
9
217
concentration effect. After composting, the total nitrogen contents in T1, T2, T3, T4 and
218
T5 increased by 6.99%, 0.48%, 11.92%, 14.25% and 11.24%, respectively. Compared
219
with the other treatments, the total nitrogen content increased the most in T4, which was
220
amended with 5% biochar. Over the whole composting period, the total nitrogen content
221
in T3 was the lowest, followed by T1 and T5. The total nitrogen content in T4 was
222
always the highest, indicating that the degradation of organic matter was the most rapid
223
or the nitrogen retention was the greatest. A possible reason for these results is the
224
porosity of the biochar, making the air and moisture available to the compost material,
225
which is favorable for the fermentation of the pile, speeding up the degradation rate of
226
the organic matter in the pile as a result (Sánchez-García et al., 2015). On the other hand,
227
the high specific surface area of the biochar was more favorable for microbial
228
attachment, which accelerated the decomposition of organic matter and increased the
229
total nitrogen content in the pile (Zhang et al., 2016). The ratio of biochar addition in T4
230
was 5%, which was probably more suitable for the balance between the sludge and
231
biochar in the composting system. The rate of addition of biochar in T2, T3 and T5 did
232
not have an obvious effect compared to the control group, i.e., no biochar amendment.
233
Combined with other influencing factors such as temperature, the total nitrogen content
234
in the control group increased more than in T2.
235 236
3.3.2. Variation in ammonia nitrogen during composting
237
The change in ammonia nitrogen mainly depends on high temperature and pH and the
238
activity of ammoniated bacteria in the compost material. Figure 4 shows that the
239
contents of ammonia nitrogen in the five treatments all decreased first and then
240
increased. In the early composting stage (0-6 d), the content of ammonia nitrogen
10
241
decreased rapidly probably due to the rapid rise in temperature under the action of
242
microbial agents. High temperature promoted the rapid propagation of microorganisms,
243
and the ammonia nitrogen produced by organic matter mineralization was quickly
244
immobilized by microorganisms as a result, which might have been due to utilization by
245
ammonia oxidizers or conversion into NH3. At the end of the high-temperature period
246
(after 6 d), the contents of ammonia nitrogen in the five treatments increased rapidly,
247
with that in T4 increasing the fastest. After composting, the loss rates of ammonia
248
nitrogen, which is the final product of sludge composting, with biochar treatment (T2,
249
T3, T4 and T5) were 35.6%, 27.8, 22.4% and 22.9% compared to the control (T1),
250
respectively. The content of ammonia nitrogen in T1 decreased the most after
251
composting probably due to the absence of biochar, leading to the resultant ammonia
252
gas not being adsorbed by the biochar and instead being volatilized into the air in large
253
quantities. The content of ammonia nitrogen in T3 was always lower than that in the
254
other three treatments but did not undergo great fluctuations, showing a steady increase
255
with time. This result was due to the addition of biochar, which adsorbed urea, uric acid,
256
and the resultant ammonia gas in the pile and thereby had a significant effect on
257
reducing nitrogen loss during composting (Meng et al., 2016). Overall, our results are
258
consistent with those of Steiner (Steiner et al., 2008).
259 260
3.3.3. Variation in nitrate nitrogen during composting
261
The changes in nitrate nitrogen during composting are shown in Figure 5. The contents
262
of nitrate nitrogen in the five treatments increased first, then decreased, and gradually
263
stabilized. The high-temperature environment of the compost on the first day strongly
264
inhibited the activity of nitrifying bacteria as a result of rapid warming of the compost
11
265
caused by the addition of microbial agents. Inorganic nitrogen mainly existed as
266
ammonia nitrogen in the compost. As the composting progressed, the decomposable
267
material decomposed rapidly, and the temperature of the compost pile decreased slowly.
268
Furthermore, under the stimulation of biochar with ammonia-oxidizing (AOB;
269
oxidizing NH4+-N to form NO2--N) and nitrite-oxidizing (NOB; oxidizing NO2--N to
270
form NO3--N) bacteria, the nitrifying bacteria proliferated rapidly, and a large amount of
271
ammonia nitrogen was transformed into nitrate nitrogen. Therefore, the nitrate nitrogen
272
content in the initial stage of composting increased rapidly. After composting, the nitrate
273
nitrogen contents in the five treatments changed quite differently, increasing by 62.4%,
274
140.2%, 138.2%, 310.6%, and 280.5%, respectively. Compared with the other
275
treatments, the nitrate nitrogen content in T4 increased the most, and the nitrate nitrogen
276
content in T1 increased the least. Such a difference was possibly due to the addition of
277
biochar, which was beneficial to the accumulation of nitrate nitrogen in the compost pile.
278
The high porosity of the biochar allowed microorganisms to better adsorb onto the
279
biochar surface or pores and would also increase the looseness of the compost pile,
280
increase the oxygen flux and promote nitrification, leading to the accumulation of
281
nitrate nitrogen in the compost pile (Khan et al., 2014).
282 283
3.4. Variation in heavy metals during composting
284
During sewage treatment, more than 50% to 80% of the heavy metals will be
285
concentrated in the generated sludge. Heavy metals in sludge are one of the main
286
restrictions on the promotion of sludge use. The content of heavy metals directly affects
287
the possibility of sludge land application. Biochar itself contains many characteristics
288
conducive to the passivation of heavy metals. For example, the high contents of
12
289
nitrogen and extractable inorganic nutrients such as phosphorus, potassium, calcium,
290
and magnesium in biochar, which are mostly alkaline (Jia et al., 2008), will increase the
291
pH and inhibit heavy metal activation. Both the high CEC of biochar and amount of
292
oxygen-containing functional groups on its surface are conducive to the passivation of
293
heavy metals (Jiang et al., 2012; Wang, 2013). The migration and phytotoxicity of
294
heavy metals are mainly determined by the heavy metal bioavailability (Lu et al., 2014).
295
In this experiment, the speciation of seven heavy metal elements, Pb, Ni, Cu, Zn,
296
As, Cr and Cd, were measured to explore the effects of different biochar additions on
297
the passivation of heavy metals. Table 2 shows the reduction in these elements in the
298
different treatments after composting compared to the total amount before composting.
299
T2 had the best effect on heavy metal passivation, followed by T5> T3> T4> T1. The
300
addition of biochar was beneficial to the passivation of heavy metal elements. However,
301
greater amounts of amended biochar do not necessarily lead to improved heavy metal
302
passivation. For example, Sun (2013) added 3%, 6% and 9% BC as an agent in compost
303
and found that the addition of 6% BC had the best passivation effect on Cu. The
304
variation in the seven heavy metal elements was not the same. After composting, the
305
content of available Pb decreased the most, followed by As, Cu, Cr and Ni, whereas the
306
contents of available Zn and Cd increased after composting. Regarding morphological
307
changes in heavy metals, different researchers have arrived at different conclusions.
308
After composting sludge amended with sawdust, Sun (2007) found that the content of
309
available Cd increased but the bioavailability of Cu, Ni, Pb, and Cr decreased. After pig
310
manure composting, Liu (2008) found that the concentrations of available Zn, Cr, Cu,
311
and Cd and Pb significantly decreased but the concentration of available Hg did not
312
change significantly.
13
313
To clearly understand the effect of biochar amendments on the bioavailability of
314
heavy metals in the composting process, the content of available heavy metals was
315
further analyzed by principal component analysis (PCA) before and after composting.
316
The contributions of PC1, PC2 and PC3 were 47.01%, 24.75% and 20.57%, respectively,
317
and the contribution of the accumulated variance of the first three principal components
318
reached 92.32%. According to the general principle of PCA, the first three
319
representative principal components were selected. In Figure 6a, rather large differences
320
can be observed in the directions of PC1, PC2 and PC3 in T1 before and after
321
composting. However, the bioavailability of heavy metals only showed obvious
322
differences in the direction of PC3 in T2 before and after composting. A large difference
323
can be observed in the PC1 direction in T3, T4 and T5 before and after composting.
324
Therefore, PC1, with 47.1%, was a main factor influencing the effect of biochar
325
compost on the bioavailability of heavy metals. Furthermore, a factor loading was
326
drawn on the three principal components of the seven heavy metals to further analyze
327
the contributions of the seven heavy metals to the three principal components. As
328
shown in Figure 6b, Pb contributed the most to PC1, Cr contributed the most to PC2,
329
and As contributed the most to PC3. These PCA results revealed that the passivation
330
effect of different treatments on heavy metals was different, i.e., the passivation effect
331
on Pb was the best, followed by Cr and As. These results are essentially consistent with
332
the results in Table 2 in that the composting treatment had the best passivation effect on
333
Pb and As in the original sludge.
334 335
4. Conclusions
336
This study confirmed the importance of biochar and microorganism amendment in
14
337
reducing not only N losses during composting but also the availability of heavy metals
338
in the compost pile, particularly Pb and As, thereby improving the maturity properties
339
and agronomic value of the end products. However, considering temperature and N
340
transformation, the treatment with 5% biochar is suggested for sludge composting to
341
enhance the fermentation of the compost and the compost product quality. Further
342
experiments are needed to verify the effects of biochar- and microorganism-amended
343
compost on plant growth to render this technology viable.
344 345
Supplementary material
346
Supplementary Figure 1 shows a photograph of biochar.
347 348
Conflict of interest
349
The authors declare no conflict of interest.
350 351
Acknowledgments
352
None.
353 354
Funding
355
This work was supported by the public-service scientific research program (agriculture)
356
“Treatment of anaerobic digestion sludge and its application in farmland utilization and
357
security evaluation [201303101-06]” and the Natural Science Foundation of Hebei
358
Province [grant number C2012201021].
359 360
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Figure captions
482 483
Figure 1. Temperature variation in the five treatments: T1, sludge + straw + 0.4%
484
microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar
485
(W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw +
486
0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
487
biochar.
488 489
Figure 2. pH variations in the five treatments: T1, sludge + straw + 0.4% microbial
490
agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar (W/W); T3,
491
sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw + 0.4%
492
microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
493
biochar.
494 495
Figure 3. Variation in total nitrogen in the five treatments: T1, sludge + straw + 0.4%
496
microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar
497
(W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw +
498
0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
499
biochar.
500 501
Figure 4. Variation in ammonia nitrogen in the five treatments: T1, sludge + straw +
502
0.4% microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1%
503
biochar (W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge +
504
straw + 0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial
21
505
agents + 7% biochar.
506 507
Figure 5. Variation in nitrate nitrogen in the five treatments: T1, sludge + straw + 0.4%
508
microbial agents (V/W); T2, sludge + straw + 0.4% microbial agents + 1% biochar
509
(W/W); T3, sludge + straw + 0.4% microbial agents + 3% biochar; T4, sludge + straw +
510
0.4% microbial agents + 5% biochar; T5, sludge + straw + 0.4% microbial agents + 7%
511
biochar.
512 513
Figure 6. PCA of the bioavailability of heavy metals during composting. (a) PCA for
514
the different treatments. (b) Eigenvector coefficients for the contributions of the seven
515
heavy metals to the three factors.
516 517
22
518 519
23
520 521
24
522 523
25
524 525
26
526 527
27
528 529
28
530
Table 1. Basic physico-chemical properties of the compost materials Organi
Moistu
Volati Inorga
Total
Total Materi
c nitrog
als
re
Carbon-to-nitr
le
nic ash weig
conten
ogen ratio
solids
(%)
pH
carbon/
ht
en % Sludge
34
t 3.39
7.5
81
(%) 10
7.1 Straw
37.6
0.89
7.9
42.2
8 531 532 533 534
29
(kg)
52.9
40.5
0.41
93.1
4.3
0.49
535
Table 2. Change in available heavy metal contents in the four treatments after
536
composting
537
Treatment
Pb
Ni
Cu
Zn
As
Cr
Cd
T1
12.52%
-
-
-
2.73%
-
-
T2
0.46%
1.84%
7.25%
-
56.31%
32.09%
-
T3
51.9%
-
59.54%
-
-
-
-
T4
27.45%
-
-
-
21.69%
-
-
T5
34.91%
-
5.55%
-
32.62%
-
-
Note: “-” indicates no reduction
538 539 540
30
541
Highlights
542
1. This study explored the effect of different biochar additions on sewage sludge.
543
2. Biochar amendment was beneficial to passivation of heavy metals in the compost pile.
544
3. The treatment with 5% biochar is suggested for sludge composting to enhance quality.
545
31