Bioresource Technology 163 (2014) 112–122
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effects of rhamnolipid and initial compost particle size on the two-stage composting of green waste Lu Zhang, Xiangyang Sun ⇑ College of Forestry, Beijing Forestry University, Beijing 100083, PR China
h i g h l i g h t s Effects of rhamnolipid (RL) and particle size (IPS) on composting were studied. RL at 0.15% and IPS of 15 mm reduced the two-stage composting time to 24 days. Physico-chemical and biological characteristics explain the rapid decomposition. Microbial communities, nutrient contents, and cellulose degradation are optimized.
a r t i c l e
i n f o
Article history: Received 10 March 2014 Received in revised form 8 April 2014 Accepted 11 April 2014 Available online 19 April 2014 Keywords: Green waste Compost Rhamnolipid Initial compost particle size Two-stage composting
a b s t r a c t Composting is a potential alternative to green waste incineration or deposition in landfills. The effects of the biosurfactant rhamnolipid (RL) (at 0.0%, 0.15%, and 0.30%) and initial compost particle size (IPS) (10, 15, and 25 mm) on a new, two-stage method for composting green waste was investigated. A combination of RL addition and IPS adjustment improved the quality of the finished compost in terms of its physical characteristics, pH, C/N ratio, nutrient content, cellulose and hemicellulose contents, water-soluble carbon (WSC) content, xylanase and CMCase activities, numbers of culturable microorganisms (bacteria, actinomycetes, and fungi), and toxicity to germinating seeds. The production of a stable and mature compost required only 24 days with the optimized two-stage composting method described here rather than the 90–270 days required with traditional composting. The best quality compost was obtained with 0.15% RL and an IPS of 15 mm. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Organic solid waste, which includes animal manure, agricultural and forestry residues, sewage sludge, and green waste, is an inexpensive, renewable, and abundant resource. The quantity of green waste, i.e., park and garden litter and trimmings, generated in cities has increased dramatically with the rapid development of urban green space in China; as a consequence, disposal of green waste is a major problem affecting the environment and sustainable development of cities (Zhang et al., 2013). For obvious reasons, recycling of green waste by composting is preferred to its deposition in landfills or its incineration. In composting, thermophilic, aerobic microorganisms transform organic materials into a hygienic, biostable product that can be used as a soil amendment, an organic fertilizer, or an alternative to peat in soilless culture ⇑ Corresponding author. Address: College of Forestry, Beijing Forestry University, P.O. Box 111, Beijing 100083, PR China. Tel./fax: +86 01062338103. E-mail addresses: [email protected] (L. Zhang), [email protected] (X. Sun). http://dx.doi.org/10.1016/j.biortech.2014.04.041 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
(Gabhane et al., 2012). However, there are still some problems associated with traditional composting that must be overcome, i.e., traditional compositing is labor intensive and time consuming and produces an odorous gas and an unstable compost product. The time required is especially long for green waste because green waste generally contains up to 75% cellulose and hemicellulose (Liu et al., 2006; Shi et al., 2006). We previously demonstrated that composting can be improved by using an innovative two-stage composting technology, which includes a primary fermentation (PF) and a secondary fermentation (SF) (Zhang et al., 2013). With this new method, the highest fermentation temperature (55–60 °C or even higher) is attained twice, and thus the thermophilic period can last for a long time. As a consequence, the production of a mature compost requires only 30 days rather than the 90–270 days required for traditional composting (Zhang et al., 2013). Because the composting process occurs in a very thin layer of liquid on the surface of the organic particles, the physical and chemical conditions in the gaps that separate the particles greatly affect the process (Xi et al., 2005). Researchers have attempted to
113
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
improve the composting microenvironment by adding surfactants (Shi et al., 2006). Surfactants, which include ionic and non-ionic forms, are molecules with a polar head group and a nonpolar hydrocarbon tail and are therefore amphiphilic and have the ability to reduce the surface tension between a liquid and a solid (Xi et al., 2005). Increased rates of decomposition after the addition of a surfactant have been reported for wood, bagasse, and corn stover; by modifying the surface properties of cellulose and minimizing irreversible binding, the surfactant enhanced the enzymatic hydrolysis of cellulose and promoted the production of compost (Eriksson et al., 2002). Biosurfactants are anionic compounds that are produced as cometabolites by particular microorganisms (Fu et al., 2007). The use of biosurfactants has recently increased because they are highly effective and, unlike synthetic surfactants, they are non-toxic to microorganisms and are biodegradable (Mulligan, 2005; Xi et al., 2005). The addition of the biosurfactant rhamnolipid (RL) enhanced the activities of microorganisms and their enzymes and thereby promoted the conversion of organic waste containing high percentages of cellulose and hemicellulose into compost (Fu et al., 2007). When RL is added to an organic waste during composting, the hydrophobic group of RL binds to the surface of the organic material while the hydrophilic group dissolves in the water, which accelerates the degradation of materials and reduces the time required to produce a mature compost (Gabhane et al., 2012). The effects of RL on the composting of green waste, however, have not been determined. A number of factors should be considered in designing a composting process, and these include: the biodegradability of the waste; physical pretreatment to alter waste particle size and porosity; the nature of the aeration system (static, passive, or dynamic); and the frequency of turning (Barrington et al., 2002). Numerous studies have demonstrated that the biodegradability of organic waste can be increased through chemical or biochemical measures, but few have investigated the effects of initial compost particle size (IPS). Adjusting the IPS could enhance microbial activity throughout the composting period and increase the rate at which macromolecules are degraded (Manpreet et al., 2005). Adjusting the IPS could also enhance water penetration and gas
exchange and prevent compaction (Zhou et al., 2014). If the IPS is too small, on the one hand, the transfer of oxygen and carbon dioxide will be inhibited, the compost process will become anaerobic, and microbial activity will be reduced. If the IPS is too large, on the other hand, the compositing materials may collapse and water-holding capacity may decline during the composting process. A large IPS can also create voids, resulting in excessive ventilation and insufficient self-heating. Adjusting the IPS and other physical parameters of the waste is important for the composting of sludge waste (Diaz, 2007). In similar studies, wood chips with IPS ranging from 5 to 25 cm were found to be optimal for composting (Barrington et al., 2003). Little information is available, however, on the effects of IPS on green waste composting. The overall goal of the present study was to optimize the twostage composting of green waste. The specific objective was to measure the effects of RL and IPS on the two-stage composting of green waste in terms of the time required to produce compost and the quality of the compost. The changes in the physical, chemical, biochemical, and microbial properties of the compost were measured and analyzed during the two-stage composting process.
2. Methods 2.1. Preparation of compost materials The green waste that was used as the raw material for composting consisted mainly of fallen leaves and branch cuttings produced by urban landscape maintenance in Beijing in the spring of 2012. Tables 1 and 2 list the main physico-chemical characteristics of the initial material. RL in the form of a brown, water-soluble paste was purchased from Zijin Biological Technology Co. (Huzhou, China) and the critical micelle concentration of RL used in the current study was 0.10%. Bamboo vinegar, which can reduce nitrogen volatilization during composting, was purchased from the Beijing Kaiyin Organic Fertilizer Production Co. (China). The microbial inoculum, which was a mixture of Trichoderma spp. inoculum (60%, v/v) and Phanerochaete chrysosporium Burdsall inoculum (40%, v/v), was prepared as described by Wei et al. (2007).
Table 1 Physico-chemical characteristics of initial material (IM) and finished composts (means ± SD; n = 3). Treatments T1–T9 are described in Table 3. Treatment
BD (g/cm3)
WHC (%)
TPS (%)
AP (%)
WHP (%)
IM T1 T2 T3 T4 T5 T6 T7 T8 T9
0.8759(0.023) 0.3156(0.039)f 0.3290(0.017)e 0.3022(0.024)g 0.3493(0.056)d 0.4021(0.019)b 0.4217(0.047)a 0.3954(0.038)b 0.3735(0.040)c 0.3828(0.016)c
– 1.19(0.10)g 1.26(0.07)f 1.02(0.02)h 1.57(0.04)e 1.94(0.12)a 1.66(0.09)d 1.81(0.03)b 1.63(0.06)d 1.75(0.08)c
– 47.24(0.34)f 45.17(0.22)g 42.35(0.47)h 52.67(0.89)e 64.02(0.31)a 55.06(0.40)d 59.49(0.63)b 56.31(0.26)cd 57.44(0.55)c
– 14.92(0.11)f 13.11(0.23)g 11.05(0.19)h 16.86(0.35)e 20.69(0.16)a 18.41(0.30)c 19.37(0.27)b 17.56(0.48)d 18.14(0.21)c
–
pH
EC (mS/cm)
HA (%)
TOC (%)
C/N ratio
7.33(0.18) 7.00(0.21)d 6.99(0.75)d 6.86(0.33)e 7.42(0.54)c 7.61(0.77)a 7.43(0.63)c 7.54(0.39)b 7.40(0.40)c 7.52(0.82)b
1.71(0.06) 0.77(0.03)ab 0.74(0.02)b 0.80(0.05)a 0.67(0.01)cd 0.49(0.07)g 0.58(0.04)f 0.62(0.02)e 0.65(0.05)de 0.69(0.01)c
14.59(0.64) 11.98(0.78)de 11.62(0.43)ef 11.39(0.65)f 12.70(0.81)c 14.06(0.52)a 13.04(0.97)bc 13.22(0.59)b 12.82(0.20)bc 12.91(0.36)bc
40.41(1.55) 31.70(1.09)a 30.20(1.23)b 32.49(1.87)a 27.58(1.20)c 22.06(1.58)f 25.26(1.11)e 24.31(1.64)e 26.04(1.32)cd 24.87(1.30)e
IM T1 T2 T3 T4 T5 T6 T7 T8 T9
32.32(0.49)e 32.06(0.52)ef 31.30(0.33)f 35.81(0.50)d 43.33(0.23)a 36.65(0.37)d 40.12(0.15)b 38.75(0.26)c 39.30(0.12)bc
43.45(0.86) 10.27(0.54)ab 9.93(0.29)b 10.44(0.62)a 8.41(0.23)c 6.16(0.43)f 8.08(0.51)c 7.58(0.37)de 8.03(0.16)cd 7.39(0.68)e
BD = bulk density; WHC = water-holding capacity; TPS = total porosity; AP = aeration porosity; WHP = water-holding porosity; EC = electrical conductivity (25 °C); HA = humic acid; TOC = total organic carbon. Means in a column followed by the same letter are not significantly different at p 6 0.05 by LSD. All percentages are based on air-dry weight.
114
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
Table 2 Contents of macro-nutrients (TN, TP, TK, Ca, Mg, and S) and micro-nutrients (Fe, Cu, Mn, Zn, and B) in the initial material (IM) and in the finished composts (means ± SD; n = 3). Treatments T1–T9 are described in Table 3. Treatment
IM T1 T2 T3 T4 T5 T6 T7 T8 T9
Macro-nutrients TN (%)A
TP (%)A
TK (%)B
Ca (%)B
Mg (%)B
S (%)B
0.93(0.01) 3.09(0.07)ef 3.04(0.02)f 3.11(0.05)e 3.27(0.04)c 3.58(0.01)a 3.13(0.06)e 3.21(0.03)d 3.24(0.08)cd 3.36(0.03)b
0.13(0.02) 0.24(0.01)f 0.29(0.02)e 0.19(0.02)g 0.34(0.03)cd 0.48(0.01)a 0.36(0.03)cd 0.40(0.02)b 0.33(0.04)d 0.37(0.01)bc
0.42(0.01) 0.71(0.01)g 0.76(0.03)f 0.63(0.01)h 0.84(0.02)c 0.92(0.01)a 0.81(0.04)de 0.87(0.03)b 0.79(0.02)e 0.82(0.01)cd
0.63(0.03) 0.78(0.02)h 0.95(0.04)g 0.70(0.05)i 1.23(0.03)f 1.59(0.01)a 1.41(0.04)de 1.49(0.05)b 1.38(0.03)e 1.45(0.06)cd
0.69(0.02) 0.77(0.01)g 0.80(0.02)f 0.73(0.03)h 0.84(0.01)de 0.99(0.02)a 0.86(0.04)cd 0.92(0.01)b 0.83(0.03)e 0.90(0.01)b
6.01(0.05) 7.46(0.04)f 7.90(0.06)f 6.35(0.03)g 8.57(0.05)e 12.03(0.02)a 9.52(0.01)cd 10.09(0.07)b 9.13(0.03)d 9.90(0.01)bc
Micro-nutrients Fe (10 IM T1 T2 T3 T4 T5 T6 T7 T8 T9
3
%)B
9.14(0.11) 12.78(0.23)f 15.39(0.42)e 10.07(0.61)g 18.55(0.48)cd 25.66(0.29)a 20.01(0.50)c 22.34(0.37)b 17.26(0.26)d 22.61(0.45)b
Cu (10
3
%)B
Mn (10
0.23(0.03) 0.48(0.01)f 0.56(0.04)e 0.40(0.05)g 0.67(0.03)cd 0.88(0.02)a 0.64(0.09)d 0.75(0.06)b 0.62(0.03)d 0.71(0.01)bc
3
%)B
7.13(0.07) 10.46(0.10)e 12.29(0.09)d 10.03(0.06)e 13.55(0.12)c 15.06(0.09)a 12.63(0.11)d 14.04(0.13)bc 13.72(0.04)bc 14.18(0.08)b
Zn (10
3
%)B
0.92(0.04) 1.36(0.03)g 1.49(0.07)f 1.28(0.03)h 1.62(0.02)e 1.83(0.03)a 1.69(0.05)cd 1.74(0.01)b 1.66(0.02)de 1.71(0.02)bc
%)B
–
1.26(0.09) 1.74(0.04)f 1.80(0.10)f 1.59(0.03)g 2.21(0.02)e 2.84(0.04)a 2.39(0.01)d 2.72(0.06)b 2.58(0.05)c 2.63(0.02)bc
– – – – – – – – – –
B (10
3
TN = total nitrogen; TP = total phosphorus; TK = total potassium. Means in a column followed by the same letter are not significantly different at p 6 0.05 by LSD. A Percentages are based on air-dry weight. B Percentages are based on oven-dry weight.
2.2. Composting experiment Before the beginning of the composting experiment, the green waste was processed with a grinder until the required IPSs were obtained according to Table 3. The moisture content of initial materials was adjusted to 60% (w/w), and the C/N ratio was adjusted to 25–30 by application of urea. During the whole composting process, the moisture content was checked once every day, and water was added to maintain the moisture content at 60%. The different dosages of RL (Table 3) were dissolved in 1 L of water to facilitate the mixing of the RL with the green waste. Microbial inoculum was also added to the materials (5 ml kg 1 dry green waste) before the start of the experiment. After the moisture content and the C/N ratio of initial materials were adjusted, the RL solution and microbial inoculum were sprayed on the materials and mixed evenly. As indicated in the Section 1, the compost mixtures were subjected to a PF followed by an SF. In the PF, the mixtures were added on day 0 to each of 27 digester cells, which were non-covered cement containers. Each digester cell was 6 m long, 2 m wide, and 1.5 m high, and had an automatic compost-turning and Table 3 Orthogonal design L9(34) of the experiment. Treatment
RL (% in each compost, dry weight)a
IPS (mm)
T1 T2 T3 T4 T5 T6 T7 T8 T9
0 0 0 0.15 0.15 0.15 0.30 0.30 0.30
10 15 25 10 15 25 10 15 25
RL = rhamnolipid; IPS = initial compost particle size. a RL was added with water.
watering system. The mixture added to the digester cells was assigned to one of nine treatments, which are described in Table 3. Each treatment was represented by three replicate digester cells. An automatic system turned the mixture in each digester cell every day during the PF. The temperature in the center of the mixture in each digester cell was determined daily using a self-made temperature sensor with a temperature dial and 1-m-long rod. When the temperature of the mixture increased to 60–70 °C during the PF, 2 ml of bamboo vinegar (diluted in 2 L of water) was added per 100 kg of waste (dry weight equivalent). The vinegar solution was sprinkled onto the mixtures as they were being turned and watered. When the temperature dropped to 45–55 °C, the PF was considered complete. The temperature in all treatments decreased to 45–55 °C by day 6. At this time, the mixtures were once again treated with the vinegar solution and were also treated with the RL solution. In addition, the mixture was removed from each cell and formed into three windrows (three windrows per cell). The SF of all treatments was begun on day 6 by creating three windrows from the mixture in each digester cell. Each windrow had a trapezoidal cross-section and was 2 m long, 1.5 m wide, and 1 m high. A mini-excavator was used to turn the windrows every 3 days in order to aerate and homogenize the materials and to stimulate microbial activity. RL solution and diluted bamboo vinegar were added every 6 days during the SF; these liquids were added as the windrows were being turned and watered. The temperature in the center of each windrow was measured daily. When the temperature of a windrow decreased to the ambient temperature, the compost was considered mature. During the whole composting process, the temperatures of treatments T1– T3 increased to 60–70 °C on day 3 in the PF and on day 18 in the SF. In contrast, the temperatures of treatments T4–T9 increased to 60–70 °C on day 2 in the PF and on day 12 in the SF. Treatments T1–T3 required 30 days to mature while treatments T4–T9 required only 24 days to mature.
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
2.3. Sample collection Compost samples were collected while the compost was being turned on day 0, 2, 3, 6, 9, 12, 15, 18, 24, and 30. On each of these days, three subsamples (200 g per subsample) were collected from the top, middle, and bottom of each cell or windrow and were combined to form one composite sample per digester cell or windrow. Each composite sample was mixed and then divided into three parts. The first part was air-dried (3–5% moisture content), and the second was oven-dried at 65 °C. When dry, the samples were crushed in a small grinder, passed through soil sieves (0.25 and 0.1 mm), sealed in plastic containers, and stored at 4 °C. The third part served as a fresh sample and was kept in a plastic container at 4 °C. Air-dried samples were used for determination of physical characteristics, pH, electrical conductivity (EC), and the contents of humic acid (HA), total organic carbon (TOC), total Kjeldahl nitrogen (TN), total phosphorus (TP), cellulose, hemicellulose, and ash. Oven-dried samples were used for determination of contents of total potassium (TK) and macro- and micro-nutrients. Fresh samples were used to quantify bacteria, actinomycetes, fungi, xylanase and CMCase activities, and water-soluble carbon (WSC) content; fresh samples were also used for a seed germination test.
115
2.6.2. Enzyme assays Xylanase and CMCase activities were assayed in a reaction mixture containing 1% (w/v) oat spelt xylan and 1% (w/v) carboxymethyl cellulose, respectively, 1.0 g of compost sample, and 80 ml of 50 mM acetate buffer (pH 5.0) (Saha et al., 2005). After 24 h at 50 °C, the reducing sugar liberated in the reaction mixture was measured by the dinitrosalicylic acid method. The activities of these enzymes were measured in international units (IUs), in which one unit (IU) of enzyme activity was defined as the amount of enzyme required to release 1 lmol of glucose or xylose per minute under the given assay conditions (Liu et al., 2006). Activities are expressed per g dry weight of compost sample (IU/g dry wt). 2.7. Crude fiber analysis The cellulose and hemicellulose contents of the initial material and the mature compost were determined according to the method of Goering and Van Soest (1970). Cellulose content was estimated as the difference between acid-detergent fiber and acid-detergent lignin contents. Hemicellulose content was estimated as the difference between neutral-detergent fiber and acid-detergent fiber contents. The initial material and finished compost were ashed at 550 °C for 6 h in a muffle furnace.
2.4. Physical analysis 2.4.1. Bulk density, water-holding capacity, and porosity Bulk density (BD), water-holding capacity (WHC), total porosity (TPS), aeration porosity (AP), and water-holding porosity (WHP) of the initial material and finished composts were determined by the ring knife method described by Zhang et al. (2013). 2.4.2. Particle-size determination The particle-size distribution of the finished compost was determined by the sieve method of Gabhane et al. (2012), i.e., air-dried compost samples were passed through sieves (mesh sizes of 0.1, 0.25, 0.5, 1, 2, and 12 mm), and the material retained on each sieve was weighed. 2.5. Chemical analysis pH and EC were measured in a 1:10 distilled-water soluble extract (w/v) with an MP521 pH/EC meter (Shuangxu Electronics Co., Ltd., Shanghai, China). Sodium pyrophosphate as the immersible reagent was used to extract dissoluble HA to determine HA content (Brittain et al., 2012). TOC was determined with a ‘Liqui TOC’ total organic carbon analyzer (S&M International Inc., America). WSC was extracted using the method of Shi et al. (2006). TN was measured by the modified micro-Kjeldahl procedure with a KDY-9830 automatic Kjeldahl apparatus (Ruibang Technology Co., Ltd., Beijing, China). TP was estimated by the Anti-Mo-Sb spectrophotometry method using a 721 Spectrophotometer (Precision and Scientific Instrument Co., Ltd., Shanghai, China). TK was determined by flame photometry using a 425 Flame Photometer (Spring Instrument Equipment Co., Ltd., Shanghai, China). Macro- and micro-nutrients were digested with sulfuric acid, and the digested liquid was analyzed by inductively coupled plasma mass spectrometry (Prodigy; Leeman Labs Inc., America). 2.6. Microbiological analysis 2.6.1. Estimation of microbial counts Throughout the composting process, the abundances of culturable bacteria, actinomycetes, and fungi in the compost samples were determined by the method of Shi et al. (2006) and Sen and Chandra (2009).
2.8. Germination assay The negative effects of finished compost on plant growth were measured based on seed germination rate (SGR) and a germination index (GI) using seeds and seedlings of pakchoi (Brassica rapa L., Chinensis group) and the method of Zhang et al. (2013). Controls were treated in the same way but with distilled water rather than compost filtrate as the culture solution. SGR (%) = mean number of germinated seeds per dish 100%/mean number of seeds per dish. GI (%) = (mean number of germinated seeds per dish mean root length per dish 100%)/(mean number of germinated seeds in the control average root length of the control). Each treatment was represented by three replicate dishes. 2.9. Statistical analysis One-way analyses of variance (ANOVAs) were used to determine whether the physical, chemical, biochemical, and microbial characteristics of the mature composts were affected by the treatments. When ANOVAs were significant, means were separated with an LSD test. As noted earlier, the samples collected at the same time from digester cells and from individual windrows were treated as replicates. All statistical analyses were performed with SPSS16.0 software. 3. Results and discussion 3.1. Effects of RL and IPS on bulk density and water-holding capacity The main physical characteristics of the initial material and the finished composts are summarized in Table 1. The BD dropped from 0.88 g/cm3 for the initial material to 0.30–0.42 g/cm3 for the finished composts. The BD for all treatments, and especially for treatments T5 and T7, were near 0.40 g/cm3, which is the optimum according to Zhang et al. (2013). Because a drop in BD will result in increased aeration, water penetration, water drainage, and available surface area, the results indicate that green waste composting could be improved by a combination of IPS adjustment and RL addition; the optimum combination was treatment T5 (0.15% RL and 15 mm IPS).
116
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
WHC was significantly (p < 0.05) different for all of the finished composts (Table 1). WHC was greatest in treatment T5 (0.15% RL and 15 mm IPS), and was smallest in treatment T3. RL, which was added in treatments T4–T9, could increase the penetration of water into the pores of green waste particles and thus increase water absorption and could also reduce evaporation from the surface of waste particles (Bridgeman et al., 2007; Zeng et al., 2006). As noted earlier, an IPS that is too small may contribute to a high WHC but result in poor air permeability, while an IPS that is too large will have the opposite effect. These results indicate that an inappropriate IPS negatively affects physical conditions during composting and may reduce microbial activity and the degradation rate. Similar results were obtained by Zhou et al. (2014). 3.2. Effects of RL and IPS on the particle-size distribution of the finished composts The particle-size distribution is relevant to compost maturity because it indicates the extent of degradation of complex substances (McGlinchey, 2005). Research has previously determined that the percentage of particles between 0.1 and 0.5 mm is an indicator of compost maturity (Gabhane et al., 2012). Based on Table 4, the percentage of particles between 0.1 and 0.5 mm in treatments T1, T2, T3, T4, T5, T6, T7, T8, and T9 was 13.4%, 14.3%, 11.4%, 17.5%, 33.4%, 20.8%, 25.4%, 18.4%, and 22.4%, respectively, indicating that the percentages were higher in treatments that received RL than in treatments that did not. The increase in particles of this size was greatest in treatment T5 and smallest in treatment T3. This again indicates that combination of RL and IPS, especially the most
proper treatment of 0.15% RL and 15 mm IPS, could improve the quality of the finished compost. Particle-size distribution affects pore space, BD, and the balance between water and air movement in the finished compost (Fornes et al., 2012). The positive effects of treatment T5 on particle-size distribution also help explain why WHC was highest for treatment T5. 3.3. Effects of RL and IPS on pH during composting For all treatments, the initial pH (7.33) quickly increased and then declined during the PF (Fig. 1). During the PF, the pH values peaked at day 3 for treatments T1–T3 but peaked at day 2 for treatments T4–T9. From day 6 onward, pH values fluctuated for all treatments. During the SF, the pH values were highest at day 18 for treatments T1–T3 and at day 12 for treatments T4–T9. After increasing to these high values during the SF, the pH subsequently declined in all treatments. According to previous research, the optimal pH value during green waste composting ranges from 7.5 to 8.5 (Zhang et al., 2009). These pH values are closely related to the activity of microorganisms participating in the process of compost formation (Neklyudov et al., 2006). In the current study, the pH during the composting process was closer to the optimal range for treatments T4–T9, and especially for treatment T5, than for treatments T1–T3. In addition, the pH increased sooner and to higher values in treatments T4–T9 than in treatments T1–T3, probably because the proper combination of RL addition and IPS adjustment supported microbial activity. An increase in microbial activity would increase
Table 4 Particle-size distribution of finished composts. The values indicate the percentage of each particle size (in mm) for each treatment (values in each row add to 100%; means ± SD; n = 3). Treatments T1–T9 are described in Table 3. Treatment
>12 (mm)
12–2.0
1.0–2.0
0.5–1.0
0.5–0.25
0.25–0.1
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effects of rhamnolipid and initial compost particle size on the two-stage composting of green waste Lu Zhang, Xiangyang Sun ⇑ College of Forestry, Beijing Forestry University, Beijing 100083, PR China
h i g h l i g h t s Effects of rhamnolipid (RL) and particle size (IPS) on composting were studied. RL at 0.15% and IPS of 15 mm reduced the two-stage composting time to 24 days. Physico-chemical and biological characteristics explain the rapid decomposition. Microbial communities, nutrient contents, and cellulose degradation are optimized.
a r t i c l e
i n f o
Article history: Received 10 March 2014 Received in revised form 8 April 2014 Accepted 11 April 2014 Available online 19 April 2014 Keywords: Green waste Compost Rhamnolipid Initial compost particle size Two-stage composting
a b s t r a c t Composting is a potential alternative to green waste incineration or deposition in landfills. The effects of the biosurfactant rhamnolipid (RL) (at 0.0%, 0.15%, and 0.30%) and initial compost particle size (IPS) (10, 15, and 25 mm) on a new, two-stage method for composting green waste was investigated. A combination of RL addition and IPS adjustment improved the quality of the finished compost in terms of its physical characteristics, pH, C/N ratio, nutrient content, cellulose and hemicellulose contents, water-soluble carbon (WSC) content, xylanase and CMCase activities, numbers of culturable microorganisms (bacteria, actinomycetes, and fungi), and toxicity to germinating seeds. The production of a stable and mature compost required only 24 days with the optimized two-stage composting method described here rather than the 90–270 days required with traditional composting. The best quality compost was obtained with 0.15% RL and an IPS of 15 mm. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Organic solid waste, which includes animal manure, agricultural and forestry residues, sewage sludge, and green waste, is an inexpensive, renewable, and abundant resource. The quantity of green waste, i.e., park and garden litter and trimmings, generated in cities has increased dramatically with the rapid development of urban green space in China; as a consequence, disposal of green waste is a major problem affecting the environment and sustainable development of cities (Zhang et al., 2013). For obvious reasons, recycling of green waste by composting is preferred to its deposition in landfills or its incineration. In composting, thermophilic, aerobic microorganisms transform organic materials into a hygienic, biostable product that can be used as a soil amendment, an organic fertilizer, or an alternative to peat in soilless culture ⇑ Corresponding author. Address: College of Forestry, Beijing Forestry University, P.O. Box 111, Beijing 100083, PR China. Tel./fax: +86 01062338103. E-mail addresses: [email protected] (L. Zhang), [email protected] (X. Sun). http://dx.doi.org/10.1016/j.biortech.2014.04.041 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
(Gabhane et al., 2012). However, there are still some problems associated with traditional composting that must be overcome, i.e., traditional compositing is labor intensive and time consuming and produces an odorous gas and an unstable compost product. The time required is especially long for green waste because green waste generally contains up to 75% cellulose and hemicellulose (Liu et al., 2006; Shi et al., 2006). We previously demonstrated that composting can be improved by using an innovative two-stage composting technology, which includes a primary fermentation (PF) and a secondary fermentation (SF) (Zhang et al., 2013). With this new method, the highest fermentation temperature (55–60 °C or even higher) is attained twice, and thus the thermophilic period can last for a long time. As a consequence, the production of a mature compost requires only 30 days rather than the 90–270 days required for traditional composting (Zhang et al., 2013). Because the composting process occurs in a very thin layer of liquid on the surface of the organic particles, the physical and chemical conditions in the gaps that separate the particles greatly affect the process (Xi et al., 2005). Researchers have attempted to
113
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
improve the composting microenvironment by adding surfactants (Shi et al., 2006). Surfactants, which include ionic and non-ionic forms, are molecules with a polar head group and a nonpolar hydrocarbon tail and are therefore amphiphilic and have the ability to reduce the surface tension between a liquid and a solid (Xi et al., 2005). Increased rates of decomposition after the addition of a surfactant have been reported for wood, bagasse, and corn stover; by modifying the surface properties of cellulose and minimizing irreversible binding, the surfactant enhanced the enzymatic hydrolysis of cellulose and promoted the production of compost (Eriksson et al., 2002). Biosurfactants are anionic compounds that are produced as cometabolites by particular microorganisms (Fu et al., 2007). The use of biosurfactants has recently increased because they are highly effective and, unlike synthetic surfactants, they are non-toxic to microorganisms and are biodegradable (Mulligan, 2005; Xi et al., 2005). The addition of the biosurfactant rhamnolipid (RL) enhanced the activities of microorganisms and their enzymes and thereby promoted the conversion of organic waste containing high percentages of cellulose and hemicellulose into compost (Fu et al., 2007). When RL is added to an organic waste during composting, the hydrophobic group of RL binds to the surface of the organic material while the hydrophilic group dissolves in the water, which accelerates the degradation of materials and reduces the time required to produce a mature compost (Gabhane et al., 2012). The effects of RL on the composting of green waste, however, have not been determined. A number of factors should be considered in designing a composting process, and these include: the biodegradability of the waste; physical pretreatment to alter waste particle size and porosity; the nature of the aeration system (static, passive, or dynamic); and the frequency of turning (Barrington et al., 2002). Numerous studies have demonstrated that the biodegradability of organic waste can be increased through chemical or biochemical measures, but few have investigated the effects of initial compost particle size (IPS). Adjusting the IPS could enhance microbial activity throughout the composting period and increase the rate at which macromolecules are degraded (Manpreet et al., 2005). Adjusting the IPS could also enhance water penetration and gas
exchange and prevent compaction (Zhou et al., 2014). If the IPS is too small, on the one hand, the transfer of oxygen and carbon dioxide will be inhibited, the compost process will become anaerobic, and microbial activity will be reduced. If the IPS is too large, on the other hand, the compositing materials may collapse and water-holding capacity may decline during the composting process. A large IPS can also create voids, resulting in excessive ventilation and insufficient self-heating. Adjusting the IPS and other physical parameters of the waste is important for the composting of sludge waste (Diaz, 2007). In similar studies, wood chips with IPS ranging from 5 to 25 cm were found to be optimal for composting (Barrington et al., 2003). Little information is available, however, on the effects of IPS on green waste composting. The overall goal of the present study was to optimize the twostage composting of green waste. The specific objective was to measure the effects of RL and IPS on the two-stage composting of green waste in terms of the time required to produce compost and the quality of the compost. The changes in the physical, chemical, biochemical, and microbial properties of the compost were measured and analyzed during the two-stage composting process.
2. Methods 2.1. Preparation of compost materials The green waste that was used as the raw material for composting consisted mainly of fallen leaves and branch cuttings produced by urban landscape maintenance in Beijing in the spring of 2012. Tables 1 and 2 list the main physico-chemical characteristics of the initial material. RL in the form of a brown, water-soluble paste was purchased from Zijin Biological Technology Co. (Huzhou, China) and the critical micelle concentration of RL used in the current study was 0.10%. Bamboo vinegar, which can reduce nitrogen volatilization during composting, was purchased from the Beijing Kaiyin Organic Fertilizer Production Co. (China). The microbial inoculum, which was a mixture of Trichoderma spp. inoculum (60%, v/v) and Phanerochaete chrysosporium Burdsall inoculum (40%, v/v), was prepared as described by Wei et al. (2007).
Table 1 Physico-chemical characteristics of initial material (IM) and finished composts (means ± SD; n = 3). Treatments T1–T9 are described in Table 3. Treatment
BD (g/cm3)
WHC (%)
TPS (%)
AP (%)
WHP (%)
IM T1 T2 T3 T4 T5 T6 T7 T8 T9
0.8759(0.023) 0.3156(0.039)f 0.3290(0.017)e 0.3022(0.024)g 0.3493(0.056)d 0.4021(0.019)b 0.4217(0.047)a 0.3954(0.038)b 0.3735(0.040)c 0.3828(0.016)c
– 1.19(0.10)g 1.26(0.07)f 1.02(0.02)h 1.57(0.04)e 1.94(0.12)a 1.66(0.09)d 1.81(0.03)b 1.63(0.06)d 1.75(0.08)c
– 47.24(0.34)f 45.17(0.22)g 42.35(0.47)h 52.67(0.89)e 64.02(0.31)a 55.06(0.40)d 59.49(0.63)b 56.31(0.26)cd 57.44(0.55)c
– 14.92(0.11)f 13.11(0.23)g 11.05(0.19)h 16.86(0.35)e 20.69(0.16)a 18.41(0.30)c 19.37(0.27)b 17.56(0.48)d 18.14(0.21)c
–
pH
EC (mS/cm)
HA (%)
TOC (%)
C/N ratio
7.33(0.18) 7.00(0.21)d 6.99(0.75)d 6.86(0.33)e 7.42(0.54)c 7.61(0.77)a 7.43(0.63)c 7.54(0.39)b 7.40(0.40)c 7.52(0.82)b
1.71(0.06) 0.77(0.03)ab 0.74(0.02)b 0.80(0.05)a 0.67(0.01)cd 0.49(0.07)g 0.58(0.04)f 0.62(0.02)e 0.65(0.05)de 0.69(0.01)c
14.59(0.64) 11.98(0.78)de 11.62(0.43)ef 11.39(0.65)f 12.70(0.81)c 14.06(0.52)a 13.04(0.97)bc 13.22(0.59)b 12.82(0.20)bc 12.91(0.36)bc
40.41(1.55) 31.70(1.09)a 30.20(1.23)b 32.49(1.87)a 27.58(1.20)c 22.06(1.58)f 25.26(1.11)e 24.31(1.64)e 26.04(1.32)cd 24.87(1.30)e
IM T1 T2 T3 T4 T5 T6 T7 T8 T9
32.32(0.49)e 32.06(0.52)ef 31.30(0.33)f 35.81(0.50)d 43.33(0.23)a 36.65(0.37)d 40.12(0.15)b 38.75(0.26)c 39.30(0.12)bc
43.45(0.86) 10.27(0.54)ab 9.93(0.29)b 10.44(0.62)a 8.41(0.23)c 6.16(0.43)f 8.08(0.51)c 7.58(0.37)de 8.03(0.16)cd 7.39(0.68)e
BD = bulk density; WHC = water-holding capacity; TPS = total porosity; AP = aeration porosity; WHP = water-holding porosity; EC = electrical conductivity (25 °C); HA = humic acid; TOC = total organic carbon. Means in a column followed by the same letter are not significantly different at p 6 0.05 by LSD. All percentages are based on air-dry weight.
114
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
Table 2 Contents of macro-nutrients (TN, TP, TK, Ca, Mg, and S) and micro-nutrients (Fe, Cu, Mn, Zn, and B) in the initial material (IM) and in the finished composts (means ± SD; n = 3). Treatments T1–T9 are described in Table 3. Treatment
IM T1 T2 T3 T4 T5 T6 T7 T8 T9
Macro-nutrients TN (%)A
TP (%)A
TK (%)B
Ca (%)B
Mg (%)B
S (%)B
0.93(0.01) 3.09(0.07)ef 3.04(0.02)f 3.11(0.05)e 3.27(0.04)c 3.58(0.01)a 3.13(0.06)e 3.21(0.03)d 3.24(0.08)cd 3.36(0.03)b
0.13(0.02) 0.24(0.01)f 0.29(0.02)e 0.19(0.02)g 0.34(0.03)cd 0.48(0.01)a 0.36(0.03)cd 0.40(0.02)b 0.33(0.04)d 0.37(0.01)bc
0.42(0.01) 0.71(0.01)g 0.76(0.03)f 0.63(0.01)h 0.84(0.02)c 0.92(0.01)a 0.81(0.04)de 0.87(0.03)b 0.79(0.02)e 0.82(0.01)cd
0.63(0.03) 0.78(0.02)h 0.95(0.04)g 0.70(0.05)i 1.23(0.03)f 1.59(0.01)a 1.41(0.04)de 1.49(0.05)b 1.38(0.03)e 1.45(0.06)cd
0.69(0.02) 0.77(0.01)g 0.80(0.02)f 0.73(0.03)h 0.84(0.01)de 0.99(0.02)a 0.86(0.04)cd 0.92(0.01)b 0.83(0.03)e 0.90(0.01)b
6.01(0.05) 7.46(0.04)f 7.90(0.06)f 6.35(0.03)g 8.57(0.05)e 12.03(0.02)a 9.52(0.01)cd 10.09(0.07)b 9.13(0.03)d 9.90(0.01)bc
Micro-nutrients Fe (10 IM T1 T2 T3 T4 T5 T6 T7 T8 T9
3
%)B
9.14(0.11) 12.78(0.23)f 15.39(0.42)e 10.07(0.61)g 18.55(0.48)cd 25.66(0.29)a 20.01(0.50)c 22.34(0.37)b 17.26(0.26)d 22.61(0.45)b
Cu (10
3
%)B
Mn (10
0.23(0.03) 0.48(0.01)f 0.56(0.04)e 0.40(0.05)g 0.67(0.03)cd 0.88(0.02)a 0.64(0.09)d 0.75(0.06)b 0.62(0.03)d 0.71(0.01)bc
3
%)B
7.13(0.07) 10.46(0.10)e 12.29(0.09)d 10.03(0.06)e 13.55(0.12)c 15.06(0.09)a 12.63(0.11)d 14.04(0.13)bc 13.72(0.04)bc 14.18(0.08)b
Zn (10
3
%)B
0.92(0.04) 1.36(0.03)g 1.49(0.07)f 1.28(0.03)h 1.62(0.02)e 1.83(0.03)a 1.69(0.05)cd 1.74(0.01)b 1.66(0.02)de 1.71(0.02)bc
%)B
–
1.26(0.09) 1.74(0.04)f 1.80(0.10)f 1.59(0.03)g 2.21(0.02)e 2.84(0.04)a 2.39(0.01)d 2.72(0.06)b 2.58(0.05)c 2.63(0.02)bc
– – – – – – – – – –
B (10
3
TN = total nitrogen; TP = total phosphorus; TK = total potassium. Means in a column followed by the same letter are not significantly different at p 6 0.05 by LSD. A Percentages are based on air-dry weight. B Percentages are based on oven-dry weight.
2.2. Composting experiment Before the beginning of the composting experiment, the green waste was processed with a grinder until the required IPSs were obtained according to Table 3. The moisture content of initial materials was adjusted to 60% (w/w), and the C/N ratio was adjusted to 25–30 by application of urea. During the whole composting process, the moisture content was checked once every day, and water was added to maintain the moisture content at 60%. The different dosages of RL (Table 3) were dissolved in 1 L of water to facilitate the mixing of the RL with the green waste. Microbial inoculum was also added to the materials (5 ml kg 1 dry green waste) before the start of the experiment. After the moisture content and the C/N ratio of initial materials were adjusted, the RL solution and microbial inoculum were sprayed on the materials and mixed evenly. As indicated in the Section 1, the compost mixtures were subjected to a PF followed by an SF. In the PF, the mixtures were added on day 0 to each of 27 digester cells, which were non-covered cement containers. Each digester cell was 6 m long, 2 m wide, and 1.5 m high, and had an automatic compost-turning and Table 3 Orthogonal design L9(34) of the experiment. Treatment
RL (% in each compost, dry weight)a
IPS (mm)
T1 T2 T3 T4 T5 T6 T7 T8 T9
0 0 0 0.15 0.15 0.15 0.30 0.30 0.30
10 15 25 10 15 25 10 15 25
RL = rhamnolipid; IPS = initial compost particle size. a RL was added with water.
watering system. The mixture added to the digester cells was assigned to one of nine treatments, which are described in Table 3. Each treatment was represented by three replicate digester cells. An automatic system turned the mixture in each digester cell every day during the PF. The temperature in the center of the mixture in each digester cell was determined daily using a self-made temperature sensor with a temperature dial and 1-m-long rod. When the temperature of the mixture increased to 60–70 °C during the PF, 2 ml of bamboo vinegar (diluted in 2 L of water) was added per 100 kg of waste (dry weight equivalent). The vinegar solution was sprinkled onto the mixtures as they were being turned and watered. When the temperature dropped to 45–55 °C, the PF was considered complete. The temperature in all treatments decreased to 45–55 °C by day 6. At this time, the mixtures were once again treated with the vinegar solution and were also treated with the RL solution. In addition, the mixture was removed from each cell and formed into three windrows (three windrows per cell). The SF of all treatments was begun on day 6 by creating three windrows from the mixture in each digester cell. Each windrow had a trapezoidal cross-section and was 2 m long, 1.5 m wide, and 1 m high. A mini-excavator was used to turn the windrows every 3 days in order to aerate and homogenize the materials and to stimulate microbial activity. RL solution and diluted bamboo vinegar were added every 6 days during the SF; these liquids were added as the windrows were being turned and watered. The temperature in the center of each windrow was measured daily. When the temperature of a windrow decreased to the ambient temperature, the compost was considered mature. During the whole composting process, the temperatures of treatments T1– T3 increased to 60–70 °C on day 3 in the PF and on day 18 in the SF. In contrast, the temperatures of treatments T4–T9 increased to 60–70 °C on day 2 in the PF and on day 12 in the SF. Treatments T1–T3 required 30 days to mature while treatments T4–T9 required only 24 days to mature.
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
2.3. Sample collection Compost samples were collected while the compost was being turned on day 0, 2, 3, 6, 9, 12, 15, 18, 24, and 30. On each of these days, three subsamples (200 g per subsample) were collected from the top, middle, and bottom of each cell or windrow and were combined to form one composite sample per digester cell or windrow. Each composite sample was mixed and then divided into three parts. The first part was air-dried (3–5% moisture content), and the second was oven-dried at 65 °C. When dry, the samples were crushed in a small grinder, passed through soil sieves (0.25 and 0.1 mm), sealed in plastic containers, and stored at 4 °C. The third part served as a fresh sample and was kept in a plastic container at 4 °C. Air-dried samples were used for determination of physical characteristics, pH, electrical conductivity (EC), and the contents of humic acid (HA), total organic carbon (TOC), total Kjeldahl nitrogen (TN), total phosphorus (TP), cellulose, hemicellulose, and ash. Oven-dried samples were used for determination of contents of total potassium (TK) and macro- and micro-nutrients. Fresh samples were used to quantify bacteria, actinomycetes, fungi, xylanase and CMCase activities, and water-soluble carbon (WSC) content; fresh samples were also used for a seed germination test.
115
2.6.2. Enzyme assays Xylanase and CMCase activities were assayed in a reaction mixture containing 1% (w/v) oat spelt xylan and 1% (w/v) carboxymethyl cellulose, respectively, 1.0 g of compost sample, and 80 ml of 50 mM acetate buffer (pH 5.0) (Saha et al., 2005). After 24 h at 50 °C, the reducing sugar liberated in the reaction mixture was measured by the dinitrosalicylic acid method. The activities of these enzymes were measured in international units (IUs), in which one unit (IU) of enzyme activity was defined as the amount of enzyme required to release 1 lmol of glucose or xylose per minute under the given assay conditions (Liu et al., 2006). Activities are expressed per g dry weight of compost sample (IU/g dry wt). 2.7. Crude fiber analysis The cellulose and hemicellulose contents of the initial material and the mature compost were determined according to the method of Goering and Van Soest (1970). Cellulose content was estimated as the difference between acid-detergent fiber and acid-detergent lignin contents. Hemicellulose content was estimated as the difference between neutral-detergent fiber and acid-detergent fiber contents. The initial material and finished compost were ashed at 550 °C for 6 h in a muffle furnace.
2.4. Physical analysis 2.4.1. Bulk density, water-holding capacity, and porosity Bulk density (BD), water-holding capacity (WHC), total porosity (TPS), aeration porosity (AP), and water-holding porosity (WHP) of the initial material and finished composts were determined by the ring knife method described by Zhang et al. (2013). 2.4.2. Particle-size determination The particle-size distribution of the finished compost was determined by the sieve method of Gabhane et al. (2012), i.e., air-dried compost samples were passed through sieves (mesh sizes of 0.1, 0.25, 0.5, 1, 2, and 12 mm), and the material retained on each sieve was weighed. 2.5. Chemical analysis pH and EC were measured in a 1:10 distilled-water soluble extract (w/v) with an MP521 pH/EC meter (Shuangxu Electronics Co., Ltd., Shanghai, China). Sodium pyrophosphate as the immersible reagent was used to extract dissoluble HA to determine HA content (Brittain et al., 2012). TOC was determined with a ‘Liqui TOC’ total organic carbon analyzer (S&M International Inc., America). WSC was extracted using the method of Shi et al. (2006). TN was measured by the modified micro-Kjeldahl procedure with a KDY-9830 automatic Kjeldahl apparatus (Ruibang Technology Co., Ltd., Beijing, China). TP was estimated by the Anti-Mo-Sb spectrophotometry method using a 721 Spectrophotometer (Precision and Scientific Instrument Co., Ltd., Shanghai, China). TK was determined by flame photometry using a 425 Flame Photometer (Spring Instrument Equipment Co., Ltd., Shanghai, China). Macro- and micro-nutrients were digested with sulfuric acid, and the digested liquid was analyzed by inductively coupled plasma mass spectrometry (Prodigy; Leeman Labs Inc., America). 2.6. Microbiological analysis 2.6.1. Estimation of microbial counts Throughout the composting process, the abundances of culturable bacteria, actinomycetes, and fungi in the compost samples were determined by the method of Shi et al. (2006) and Sen and Chandra (2009).
2.8. Germination assay The negative effects of finished compost on plant growth were measured based on seed germination rate (SGR) and a germination index (GI) using seeds and seedlings of pakchoi (Brassica rapa L., Chinensis group) and the method of Zhang et al. (2013). Controls were treated in the same way but with distilled water rather than compost filtrate as the culture solution. SGR (%) = mean number of germinated seeds per dish 100%/mean number of seeds per dish. GI (%) = (mean number of germinated seeds per dish mean root length per dish 100%)/(mean number of germinated seeds in the control average root length of the control). Each treatment was represented by three replicate dishes. 2.9. Statistical analysis One-way analyses of variance (ANOVAs) were used to determine whether the physical, chemical, biochemical, and microbial characteristics of the mature composts were affected by the treatments. When ANOVAs were significant, means were separated with an LSD test. As noted earlier, the samples collected at the same time from digester cells and from individual windrows were treated as replicates. All statistical analyses were performed with SPSS16.0 software. 3. Results and discussion 3.1. Effects of RL and IPS on bulk density and water-holding capacity The main physical characteristics of the initial material and the finished composts are summarized in Table 1. The BD dropped from 0.88 g/cm3 for the initial material to 0.30–0.42 g/cm3 for the finished composts. The BD for all treatments, and especially for treatments T5 and T7, were near 0.40 g/cm3, which is the optimum according to Zhang et al. (2013). Because a drop in BD will result in increased aeration, water penetration, water drainage, and available surface area, the results indicate that green waste composting could be improved by a combination of IPS adjustment and RL addition; the optimum combination was treatment T5 (0.15% RL and 15 mm IPS).
116
L. Zhang, X. Sun / Bioresource Technology 163 (2014) 112–122
WHC was significantly (p < 0.05) different for all of the finished composts (Table 1). WHC was greatest in treatment T5 (0.15% RL and 15 mm IPS), and was smallest in treatment T3. RL, which was added in treatments T4–T9, could increase the penetration of water into the pores of green waste particles and thus increase water absorption and could also reduce evaporation from the surface of waste particles (Bridgeman et al., 2007; Zeng et al., 2006). As noted earlier, an IPS that is too small may contribute to a high WHC but result in poor air permeability, while an IPS that is too large will have the opposite effect. These results indicate that an inappropriate IPS negatively affects physical conditions during composting and may reduce microbial activity and the degradation rate. Similar results were obtained by Zhou et al. (2014). 3.2. Effects of RL and IPS on the particle-size distribution of the finished composts The particle-size distribution is relevant to compost maturity because it indicates the extent of degradation of complex substances (McGlinchey, 2005). Research has previously determined that the percentage of particles between 0.1 and 0.5 mm is an indicator of compost maturity (Gabhane et al., 2012). Based on Table 4, the percentage of particles between 0.1 and 0.5 mm in treatments T1, T2, T3, T4, T5, T6, T7, T8, and T9 was 13.4%, 14.3%, 11.4%, 17.5%, 33.4%, 20.8%, 25.4%, 18.4%, and 22.4%, respectively, indicating that the percentages were higher in treatments that received RL than in treatments that did not. The increase in particles of this size was greatest in treatment T5 and smallest in treatment T3. This again indicates that combination of RL and IPS, especially the most
proper treatment of 0.15% RL and 15 mm IPS, could improve the quality of the finished compost. Particle-size distribution affects pore space, BD, and the balance between water and air movement in the finished compost (Fornes et al., 2012). The positive effects of treatment T5 on particle-size distribution also help explain why WHC was highest for treatment T5. 3.3. Effects of RL and IPS on pH during composting For all treatments, the initial pH (7.33) quickly increased and then declined during the PF (Fig. 1). During the PF, the pH values peaked at day 3 for treatments T1–T3 but peaked at day 2 for treatments T4–T9. From day 6 onward, pH values fluctuated for all treatments. During the SF, the pH values were highest at day 18 for treatments T1–T3 and at day 12 for treatments T4–T9. After increasing to these high values during the SF, the pH subsequently declined in all treatments. According to previous research, the optimal pH value during green waste composting ranges from 7.5 to 8.5 (Zhang et al., 2009). These pH values are closely related to the activity of microorganisms participating in the process of compost formation (Neklyudov et al., 2006). In the current study, the pH during the composting process was closer to the optimal range for treatments T4–T9, and especially for treatment T5, than for treatments T1–T3. In addition, the pH increased sooner and to higher values in treatments T4–T9 than in treatments T1–T3, probably because the proper combination of RL addition and IPS adjustment supported microbial activity. An increase in microbial activity would increase
Table 4 Particle-size distribution of finished composts. The values indicate the percentage of each particle size (in mm) for each treatment (values in each row add to 100%; means ± SD; n = 3). Treatments T1–T9 are described in Table 3. Treatment
>12 (mm)
12–2.0
1.0–2.0
0.5–1.0
0.5–0.25
0.25–0.1
