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Continuous anaerobic digestion of food waste and design of digester with lipid removal Dong Li
a b
a
a
a
a
, Yongming Sun , Yanfeng Guo , Zhenhong Yuan , Yao Wang & Feng Zhen
a
a
Chinese Academy of Sciences , Guangzhou Institute of Energy Conversion , Guangzhou , People's Republic of China b
Chinese Academy of Sciences , Key Laboratory of Renewable Energy and Gas Hydrate , Guangzhou , People's Republic of China Accepted author version posted online: 03 Jun 2013.Published online: 21 Jun 2013.
To cite this article: Dong Li , Yongming Sun , Yanfeng Guo , Zhenhong Yuan , Yao Wang & Feng Zhen (2013) Continuous anaerobic digestion of food waste and design of digester with lipid removal, Environmental Technology, 34:13-14, 2135-2143, DOI: 10.1080/09593330.2013.808237 To link to this article: http://dx.doi.org/10.1080/09593330.2013.808237
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2135–2143, http://dx.doi.org/10.1080/09593330.2013.808237
Continuous anaerobic digestion of food waste and design of digester with lipid removal Dong Lia,b , Yongming Suna , Yanfeng Guoa , Zhenhong Yuana∗ , Yao Wanga and Feng Zhena a Chinese b Chinese
Academy of Sciences, Guangzhou Institute of Energy Conversion, Guangzhou, People’s Republic of China; Academy of Sciences, Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou, People’s Republic of China
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(Received 8 November 2012; final version received 15 May 2013 ) Separation of municipal solid waste has been implemented in many cities in China. As a major component of municipal solid waste, food waste can be treated by anaerobic digestion (AD) for energy production. To provide reference data for disposing of food waste through engineering applications, continuous AD was carried out under various organic loading rates (OLRs) at 27 ± 2◦ C in the laboratory. The anaerobic reactor was stable with pH 7.0–7.1, total volatile fatty acid (VFA) concentrations of 206–746 mg/L, and NH+ 4 − N concentrations of 525–1293 mg/L when the OLR was 1.118–5.588 kg volatile solids (VS)/m3 ·d. The maximum volumetric biogas production rate was 4.41 L/L·d when the OLR was increased to 5.588 kg VS/m3 ·d with a hydraulic retention time of 30 d. When the OLR was increased to 6.706 and 8.382 kg VS/m3 ·d, biogas production was seriously inhibited by VFAs, with maximum total VFA and propionate concentrations of 8738 mg/L and 2864 mg/L, respectively. Due to the incomplete degradation of lipids, the specific methane production rate of 353– 488 L/kg VS accounted for 55.2–76.3% of the theoretical methane potential calculated based on the component composition. A retrofitted anaerobic digester with lipid removal was designed to improve the efficiency. Keywords: food waste; anaerobic digestion; organic loading rate; biogas production rate; lipid removal
Introduction Benefiting from the introduction of source separation at collection and mechanical separation plants, anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) has substantially increased over the past 15 years in Europe. A wide range of bench, pilot, and commercial scale AD systems have been applied to treat food waste, fruit and vegetable waste, household waste, sourcesorted OFMSW, mechanically sorted OFMSW, municipal sewage sludge, and mixtures of the above.[1–8] In China, due to the lack of developed waste separation technology or source separation collection systems, there is not yet a fullscale application of AD for OFMSW. As a major component of OFMSW, food waste is partially disposed of with other municipal solid waste in a landfill or by incineration; other food waste is collected from restaurants and used for pig feed or cooking oil refining, eventually returning to dining tables. The biochemical composition of food waste is mainly soluble carbohydrates, starches, dietary fibre, proteins, lipids, and salt. Landfill disposal of this perishable waste may give rise to greenhouse gas emissions, leachate production, and further air, groundwater, and soil pollution. When incinerated, its high water content results in low heat value and greater auxiliary fuel requirements. On the other hand, its characteristically high organic matter and water ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
contents make food waste suitable for AD. AD is considered a commercially feasible option for both waste treatment and energy/nutrition recovery, if the digestion residue is used as fertilizer. Recently, certain cities in highly developed regions of China have initiated source separation programmes. In Guangzhou, for example, the Guangzhou Municipal Solid Waste Management Interim Provisions for Classification were implemented in 2011. Domestic waste is divided into four categories, including recyclables, food waste, hazardous waste, and other garbage. Advances in waste classification systems will promote food waste disposal through anaerobic fermentation for biogas production. To provide reference data for engineering applications of AD of food waste, continuous AD pilot tests were run with various organic loading rates (OLRs). Based on the results, a new digester was designed to remove lipids and thereby improve efficiency. Materials and methods Food waste and inoculum Food waste was collected from a refectory at South China Agricultural University. At breakfast, lunch, and dinner, 10, 20, and 20 kg food wastes, respectively, were collected. After removing large bones, the 50 kg of food waste was
2136 Table 1.
D. Li et al. Characteristics of food waste and inoculum.
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Parameter Particle size (mm) Total solids (TS; g/kg) Heat value (MJ/kg TS) VS (% of TS) Ash (% of TS) Carbohydrates (% of TS) Crude fibre (% of TS) Proteins (% of TS) Total Kjeldahl N (% of TS) Lipids (% of TS) Carbon (% of TS) Hydrogen (% of TS) Oxygen (% of TS) Nitrogen (% of TS) Sulphur (% of TS) Phosphorus (% of TS) C:N ratio pH VFAs (mg/L) NH+ 4 − N(mg/L) TMP (L/kg VS)a TMP (L/kg VS)b
Food waste
Inoculum
≤4 241.80 25.14 92.44 7.56 38.6 3.30 17.30 2.77 34.90 50.12 7.81 32.64 1.79 0.06 0.02 28.38 4.25 137.00 72.00 607.00 639.00
≤2 24.00 – 58.30 41.70 – – – – – – – – – – – – 7.32 253.00 510.00 – –
a TMP
based on elemental composition (C, H, O, and N). based on component composition (carbohydrates, proteins, and lipids).
b TMP
pulverized, completely mixed, and refrigerated at 4◦ C. As source of inoculum, the effluent from a mesophilic AD in laboratory fed with pig waste, was used as inoculum after removing large particles. Characteristics of the food waste and inoculum are given in Table 1.
Experimental setup and procedures Continuous AD was conducted in a bench-scale reactor with a total volume of 40 L (Figure 1). Five separate ports on each reactor were designed to perform different functions: online pH monitoring, mechanical agitation, temperature control, and gas, and liquid sampling. Because the average temperature of Guangzhou is about 27◦ C, AD was carried out at 27 ± 2◦ C. Mixing was conducted six times per day at 20 rpm for 10 min. To avoid pipe blockage and consequent expansion of digesting material, the loading rate of the reactor was set at ∼75%. After a 1-week idle period, the first batch of food waste was fed into the reactor when negligible biogas production was observed. The food waste was added once a day without dilution. A 200-mL liquid sample was discharged from the reactor before adding new material. The daily amount of food waste addition is given in Table 2. The OLR ranged from 1.118 to 8.832 kg VS/m3 ·d, and the corresponding hydraulic retention time (HRT) ranged from 150 to 20 d. Two parallel experiments were conducted and the data are presented as mean values.
Analytical methods Total solids (TS) and volatile solids (VS'jwere determined using standard methods.[9] Elemental analysis was performed using a Vario EL elemental analyzer (Elementar Analysensysteme GmbH, Germany). The compositional analysis (carbohydrates, crude fibre, lipids, proteins, and total Kjeldahl nitrogen) was conducted based on the Chinese Standard GB/T 5009-2003. Heat values were measured using a WGR-1 heat analyzer (Changsha Bente Instrument Corporation, China), assuming complete combustion of fuel and that the water was heated in a completely insulated container, then using the measured temperature rise of the liquid to calculate the heat. Ammonia nitrogen (NH+ 4 − N) was determined using an FC-100 ammonia analyzer (Shanghai Super Info. Tech. Co. Ltd., China) with an electrochemical sensor. The pH was determined with a pHS-3C pH metre (Shanghai Precision & Scientific Instrument Co., Ltd, China). Biogas production was measured with an LML-1 wet gas metre (Changchun Automobile Filter Co., Ltd, China). The H2 , CH4 , and CO2 contents of the biogas were determined using a gas chromatograph (6890; Agilent, Santa Clara, CA, USA) equipped with a thermal conductivity detector and a 2-m stainless column packed with Porapak Q (50/80 mesh). The operational temperatures of the injection port, column oven, and detector were 100, 70, and 150◦ C, respectively. Argon was used as the carrier gas at a flow rate of 30 mL/min. Liquid samples were centrifuged at 5000 rpm at 0–4◦ C and then filtered through 0.22- μm cellulose acetate membranes. The concentration of total volatile fatty acids (VFAs, including acetate, propionate, butyrate, isobutyrate, valerate, and isovalerate) was determined using high performance liquid chromatography (Alliance 2695; Waters, Milford, MA, USA) equipped with a 241 refractive index detector (Waters) and a 300 × 8-mm S-DVB gel column (RSpak KC-811; New York, NY, USA). The mobile phase was 0.1% (m/m) phosphoric acid solution with flow rate of 0.7 mL/min. The temperature of the column oven was 40◦ C and the sample size was 10 μL. Calculations Free ammonia and free VFAs were calculated using the following formulas: CT [H + ] , KA + [H + ] KB CTotal [NH3 ] = , KB + [H + ] [HA] =
(1) (2)
where [HA] and [NH3 ] are the concentrations (mol/L) of free VFAs and free NH3 , respectively, CT and CTotal are the concentrations (mol/L) of total VFAs and total NH3 , respectively, [H+ ] is the hydrogen ion concentration
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Figure 1. Experimental apparatus for 40-L AD: (1) hot water tank, (2) circulating pump, (3) liquid sampling port, (4) digester, (5) mixer, (6) thermocouple probe, (7) hot water interlayer, (8) gas sampling port, (9) rotametre, (10) wet gas metre, (11) biogas outlet, (12) temperature controller, (13) timer, (14) pH metre, (15) rpm governor, and (16) pH probe. Table 2.
Experimental design.
Period (d) Food waste addition (kg/d, wet basis) Food waste addition (kg VS/d) OLR (kg VS/m3 ·d) HRT (d)
1st stage
2nd stage
3rd stage
4th stage
5th stage
6th stage
7th stage
8th stage
9th stage
1–7 0 0 0 –
8–14 0.2 0.045 1.118 150.0
15–21 0.3 0.067 1.676 100.0
22–28 0.4 0.089 2.235 75.0
29–35 0.6 0.134 3.353 50.0
36–42 0.8 0.179 4.470 37.5
43–49 1.0 0.224 5.588 30.0
50–57 1.2 0.268 6.706 25.0
58–61 1.5 0.335 8.382 20.0
(mol/L), and KA and KB are the dissociation equilibrium constants for VFAs and NH3 , respectively. The proportion of free VFAs depends on pH rather than temperature, because KA is negligible over the range 0 − −60◦ C. The proportion of free NH3 depends on both pH and temperature, as temperature has a significant effect on KB , with a higher temperature resulting in a higher proportion of free NH3 . pKA and pKB are 4.8 and 9.1, respectively, at 27◦ C. Results and discussion Trends in pH, VFAs, and NH+ 4 −N Figure 2 illustrates the changes in pH, VFA concentration, and NH+ 4 − N concentration over time. The pH in the initial idle period (first stage) was 7.0–7.1. The pH was maintained at 7.0–7.2 during the stable operational period (second to seventh stages). During the stable period, the total VFA and NH+ 4 − N concentrations increased from
206 to 746 mg/L and from 525 to 1293 mg/L, respectively. When the OLR increased to 6.706 and 8.382 kg VS/m3 ·d in the eighth and ninth stages, the total VFA concentration increased from 972 to 8738 mg/L and the propionate concentration increased from 357 to 2864 mg/L. Hanaki et al. pointed out that oxidation of propionate to acetate was more difficult than that of butyrate and valerate to acetate, and the tolerated propionate concentration for methanogens was < 1000 mg/L.[10] During these last two stages, the volumetric biogas production rate (VBPR) began to decrease. Therefore, the eighth and ninth stages represent an overloaded period. VFAs and NH3 are intermediate metabolites in methanogenesis as well as inhibitors when excessive accumulation occurs. The concentrations of VFAs and NH3 during AD are determined by their production and consumption rates. Ammonium ion (NH+ 4 ) and free NH3 are the two principal forms of inorganic ammonia nitrogen in aqueous solution. VFA ions (such as CH3 COO− ) and free VFAs (such
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Concentration of NH4 - N (mg/L)
(c)
Evaluation of (a) pH, (b) total VFAs and propionate, and (c) NH+ 4 − N.
3000 2500 2000 1500 1000 500 0
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Concentration of total VFAs(mg/L)
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20000 10000 5000
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Free NH3 concentration (mg/L)
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Figure 3. Inhibitory conditions for methane production by anaerobic fermentation: (A: Methane production; B: Inhibited steady state; C: Inhibition by VFAs; D: Inhibition by ammonia).
as CH3 COOH) are the two forms of VFAs in aqueous solution. Free NH3 and free VFAs have been suggested as the main cause of inhibition, since they are freely membranepermeable.[11] The hydrophobic free NH3 and free VFA molecules may diffuse passively into cells, causing proton imbalances and/or potassium deficiency.[12] The inhibition of methanogens by free VFAs is stronger than that of acidogens; the concentration at which the inhibition of acidogens occurs is 2400–3000 mg/L, while for methanogens, it is 30–60 mg/L.[11,13] Free NH3 mainly inhibits methanogens, particularly aceticlastic methanogens. It does not have a significant effect on hydrogenotrophic methanogens or acidogens. Such inhibition is reversible and adaptive; it can be relieved by diluting the digest liquid or acclimating the methanogens. Koster et al. reported that the concentration of free NH3 inhibiting unacclimated methanogens was 50 mg/L.[14] Based on Equations (1) and (2) and the information presented above, a diagram of inhibition by VFAs and NH3 on anaerobic methane production at 27◦ C is shown in Figure 3. Normally, methane production occurs under neutral conditions (region A). Acidification and alkalization result from inhibition by VFAs and NH3 , represented by regions C and D, respectively. However, there may be little or no methane production even when the pH is neutral. Digestion of easily degradable and nitrogen-rich substrates produces a large amount of VFAs and NH3 during acidogenesis. According to Equations (1) and (2), free VFA and free ammonia concentrations can be higher than inhibitory levels even when the pH is close to 7. Interactions between VFAs, NH3 , and pH lead to this ‘inhibited steady state’ (region B).[15,16] In this study, biogas production was successful in region A during the initial and stable periods. However, the overloaded period was characterized by region C, during which hydrolysis and acidogenesis rates were higher than that of methanogenesis. Free VFAs, rather than free
NH3 , inhibited methane production. Accumulated VFAs lowered the activity of aceticlastic methanogens and the pH decreased. The lower pH increased free VFAs, further inhibiting methanogens. Among the VFAs, inhibition by propionate was strongest. Degradation of propionate was much slower than that of acetate, primarily because acetate can be directly broken down to CH4 and CO2 (Equation (3)), while propionate must be degraded to acetate before methane production (Equation (4)). Moreover, degradation of propionate is a thermodynamically unfavourable reaction unless the hydrogen partial pressure is maintained at a very low level: CH3 COOH → CH4 + CO2
G0 = −31.0 kJ/mol, (3)
CH3 CH2 COOH + 4H2 O → 2CH3 COOH + 3H2 + 2CO2 G0 = +76.1 kJ/mol.
(4)
Biogas production The specific biogas and methane production rates (SBPR and SMPR), VBPR, and CH4 and H2 contents were used to describe the results of the biogas production process (Figure 4). The SMPR and SBPR profiles were similar. During the stable period, the OLR and VBPR were positively and linearly correlated (Figures 4 and 5). The maximum VBPR was 4.41 L/L·d when the OLR was 5.588 kg VS/m3 ·d with an HRT of 30 d. The corresponding volumetric methane production rate was ∼2.42 L/L·d, similar to the results of Cho et al.[17] In that study, dry AD of food waste was conducted under mesophilic conditions. Stable dry AD was achieved by controlling the HRT without the addition of alkali agents. The average methane production rate, methane content, and VS reduction were 2.51 L/L·d, 66%, and 65.8%, respectively, at an HRT of 40 d with an OLR of 5.0 kg VS/m3 ·d.
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(32,365) OLR SMPR Fitted SMPR
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Time (day)
Time (day)
Figure 4.
Evaluation of (a) the VBPR, (b) SBPR, (c) methane concentration, and (d) SMPR.
4.5 4.0
Experimental data Linear fit of experimental data
3.5 3.0 2.5 2.0 1.5 1.0
y = 0.7025x 2 R = 0.9874
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Initial period
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SBPR (L/kgVS)
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2 3 4 3 OLR (kgVS/ (m ·day))
5
6
Figure 5. Relationship between OLR and VBPR during the stable period.
The methane concentration initially increased and then decreased. The maximum methane concentration of 66% was observed on the 35th day with an OLR of 3.353 kg VS/m3 ·d. No H2 was detected during the stable period.
Based on the best fit of the SMPR data, a higher OLR does not necessarily correspond to a higher SMPR. The optimal SMPR of 353–488 L/kg VS was observed during the fifth stage with an OLR of 3.353 kg VS/m3 ·d and an HRT of 50 d, similar to results for a study of food waste in San Francisco, CA.[18] In that study, anaerobic methane yields from food waste were evaluated using batch tests performed at 50◦ C. The methane yield was determined to be 348 and 435 L/kg VS after 10 and 28 days of digestion, respectively. The average VS reduction was 81% at the end of the 28-day digestion test. When the OLR was increased to 6.706 and 8.382 kg VS/m3 ·d, the VBPR, SBPR, and SMPR first slowly and then sharply decreased until little biogas was produced. When the reactor was run in an overloaded state, methane concentrations were 100μm floats on the water as a contiguous oil film in the absence of mixing. It can easily be removed by gravity separation. Dispersed oil with particle sizes of 25–100 μm is suspended in the liquid phase as small droplets. It can also be easily removed when the droplets gather to form oil beads and float on the surface after a period of standing. Emulsified oil has particle sizes of 0.1–25 μm when there are surfactants in the liquid phase and is difficult to remove. Dissolved oil of nanometre size disperses in the water phase to form ultrafine droplets in molecular or chemical forms. It is difficult to remove because the oil and water form a very stable homogenous phase system. However, the solubility of oil in water is very low (5–15 mg/L). Solid inner fat is closely associated with solid particles from which it cannot be directly separated. Lipids in food waste mainly exist in the form of solid inner fat and floating oil, which account for more than 80% and 10–15% of the total lipids on a mass basis, respectively.[28,29] The key to lipid removal from food waste is to release the solid inner fat from the solid phase to the liquid phase. The Chinese patent application No. 200710132239.4 and [29] state that wet hot treatment with high-temperature steaming can release solid inner fat from the solid phase of food waste to the liquid phase. The Chinese patent application No. 200710306010.8 describes a method for converting dispersed and emulsified oil in wastewater into floating oil through a combination of coarse-graining and microfloating. However, wet hot treatment is a high energy consumption process, as the target temperature is ∼150◦ C. Microfloating requires energy to drive the microbubble generator. Coarse-graining not only requires a filter, which increases the equipment cost, but also requires finely controlled process conditions to ensure the efficiency of oil recovery. We propose a simpler and more cost-effective method and associated device for removing lipids from food waste. Figure 6 presents an integrated anaerobic digester for food waste with lipid removal. In the anaerobic digester, solid organic particles are broken down and released to the water phase through physical decomposition and extracellular enzymatic hydrolysis. The solid inner fat is also released to the water phase, then forms dispersed and emulsified oil. Afterwards, the dispersed and emulsified oil adhere to micro biogas bubbles as a result of surface tension. They then rise along with the ascending bubbles and become floating oil. At the level of the floating oil layer, two oil scraper plates are installed on the shaft and an oil discharge port and baffle are placed on the wall of the digester. A rubber contact pad is fixed at the end of the oil scraper plate. When the stirrer is operating, the oil scraper plates and rubber contact pads rotate clockwise. Once the floating oil is gathered to the oil discharge port, it is obstructed by the baffle and discharged.
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(b)
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(a)
(c)
Figure 6. Designed integrated anaerobic digester for food waste with lipid removal: (a) cutaway view, (b) oblique cutaway view, and (c) top view; (1) inlet, (2) outlet, (3) slag discharge port, (4) oil discharge port, (5) oil discharge pipe, (6) motor, (7) shaft, (8) oil scraper plate, (9) rubber contact pad, (10) baffle, (11) biogas outlet, (12) heating band, and (13) valve.
The outside wall of the digester at the level of the floating oil is encircled with a heating band to melt the solidified oil adhered onto the inside wall of the digester. This design takes advantage of physical decomposition and extracellular enzymatic hydrolysis of solid inner fat to form dispersed and emulsified oil, makes full use of methanogenesis microorganisms as ‘microbubble generators’ for converting dispersed and emulsified oil to floating oil, and introduces a set of simple additional parts to remove floating oil from the digester. This retrofitted anaerobic digester does not require either separate lipid removal equipment or substantial energy consumption for steaming food waste or generating microbubbles. However, the current design is a technical proposal that requires additional work to evaluate its feasibility and efficiency. Conclusions AD of food waste performed stably with OLRs of 1.118–5.588 kg VS/m3 ·d and HRTs of 150–30 d. Biogas production was seriously inhibited by VFAs when the OLR was increased to 6.706 and 8.382 kg VS/m3 ·d. The recommended OLR for AD of food waste at
27◦ C in practical engineering applications is about 4.47 kg VS/m3 ·d, at which the VBPR, SMPR, and methane content were 2.80–3.24 L/L·d, 300–420 L/kg VS, and 57–64%, respectively. To address challenges associated with the presence of lipids, a method and supporting device for removing lipids from food waste is proposed. It takes full advantage of physical decomposition, extracellular enzymatic hydrolysis, and methanogenesis microorganisms as ‘microbubble generators’, and introduces a set of simple added parts to remove floating oil from the anaerobic digester. These results provide important reference data for treatment of food waste by AD in China. Acknowledgements This research was financially supported by the National Key Technologies R&D Programme of China (No. 2011BAD15B02) and the Science and Technology Planning Project of Guangdong Province (No. 2011A030600005).
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fraction of household municipal solid waste: start-up procedure for continuously stirred tank reactor. Water Res. 2006;40:2621–2628. Bolzonella D, Innocenti L, Pavan P, Traverso P, Cecchi F. Semi-dry thermophilic anaerobic digestion of the organic fraction of municipal solid waste: focusing on the start-up phase. Bioresour Technol. 2003;86:123–129. Bolzonella D, Pavan P, Mace S, Cecchi F. Dry anaerobic digestion of differently sorted organic municipal solid waste: a full-scale experience. Water Sci Technol. 2006; 53:23–32. Bolzonella D, Battistoni P, Susini C, Cecchi F. Anaerobic codigestion of waste activated sludge and OFMSW: the experiences of viareggio and treviso plants (Italy). Water Sci Technol. 2006;53:203–211. Bouallagui H, Touhami Y, Cheikh RB, Hamdi M. Bioreactor performance in anaerobic digestion of fruit and vegetable wastes. Process Biochem. 2005;40:989–995. Charles W, Walker L, Cord-Ruwisch R. Effect of pre-aeration and inoculum on the start-up of batch thermophilic anaerobic digestion of municipal solid waste. Bioresour Technol. 2009;100:2329–2335. Dinsdale RM, Premier GC, Hawkes FR, Hawkes DL. Twostage anaerobic co-digestion of waste activated sludge and fruit/vegetable waste using inclined tubular digesters. Bioresour Technol. 2000;72:159–168. Forster-Carneiro T, Perez M, Romero LI. Thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste. Bioresour Technol. 2008;99: 6763–6770. APHA. Standard method for the examination of water and wastewater. New York: American Public Health Association; 1995. Hanaki K, Hirunmasuwan S, Matsuo T. Protection of methanogenic bacteria from low pH and toxic materials by immobilization using polyvinyl alcohol. Water Res. 1994;28:877–885. Kroeker EJ, Schulte DD, Sparling AB, Lapp HM. Anaerobic treatment process stability. J Water Pollut Control Fed. 1979;51:718–727. Chen Y, Cheng JJ, Creamer KS. Inhibition of anaerobic digestion process: a review. Bioresour Technol. 2008; 99:4044–4064. Wang G, Wang DIC. Elucidation of growth-inhibition and acetic-acid production by clostridium-thermoaceticum. Appl Environ Microbiol. 1984;47:294–298. Koster IW, Lettinga G. Ammonium Toxicity in Anaerobic Digestion. Proceeding of Anaerobic Wastewater Treatment Symposium, The Netherland, Hague, 1983, p. 553.
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[15] Angelidaki I, Ahring BK. Thermophilic anaerobic-digestion of livestock waste – the effect of ammonia. Appl MicrobiolBiotechnol. 1993;38:560–564. [16] Li D, Yuan ZH, Zhang Y, Sun YM, Kong XY, Li LH. Anaerobic biochemical methane potential of organic fraction of municipal solid waste. Acta Scientiae Circumstantiae. 2008;28:2284–2290. [17] Cho SK, Im WT, Kim DH, Kim MH, Shin HS, Oh SE. Dry anaerobic digestion of food waste under mesophilic conditions: performance and methanogenic community analysis. Bioresour Technol. 2013;131:210–217. [18] Zhang RH, El-Mashad HM, Hartman K, Wang FY, Liu GQ, Choate C, Gamble P. Characterization of food waste as feedstock for anaerobic digestion. Bioresour Technol. 2007;98:929–935. [19] Buswell AM, Mueller HF. Mechanism of methane fermentation. Ind Eng Chem. 1952;44:550–552. [20] Li D, Sun YM, Yuan ZH, Kong XY, Zhang Y. Kinetic study of the mesophilic anaerobic digestion of organic waste components. Acta Energiae Solaris Sinica. 2010;31:385–390. [21] Hwu CS, Donlon B, Lettinga G. Comparative toxicity of long-chain fatty acid to anaerobic sludges from various origins. Water Sci Technol. 1996;34:351–358. [22] Hwu CS, Molenaar G, Garthoff J, vanLier JB, Lettinga G. Thermophilic high-rate anaerobic treatment of wastewater containing long-chain fatty acids: impact of reactor hydrodynamics. Biotechnol Lett. 1997;19:447–451. [23] Hwu CS, van Lier JB, Lettinga G. Physicochemical and biological performance of expanded granular sludge bed reactors treating long-chain fatty acids. Process Biochem. 1998;33:75–81. [24] Chipasa KB, Medrzycka K. Behavior of lipids in biological wastewater treatment processes. J Ind Microbiol Biotechnol. 2006;33:635–645. [25] Cirne DG, Paloumet X, Björnsson L, Alves MM, Mattiasson B. Anaerobic digestion of lipid-rich waste – effects of lipid concentration. Renewable Energy. 2007;32:965–975. [26] Rinzema A, Boone M, van Knippenberg K, Lettinga G. Bactericidal effect of long chain fatty acids in anaerobic digestion. Water Environ Res. 1994;66:40–49. [27] Rinzema A, Alphenaar A, Lettinga G. The effect of lauric acid shock loads on the biological and physical performance of granular sludge in Uasb reactors digesting acetate. J Chem Technol Biotechnol. 1989;46:257–266. [28] Ren LH, Nie YF, Liu JG, Jin YY. Impact of hydrothermal process on effluent of restaurant garbage. China Water Wastewater. 2006;22:73–76. [29] Ren LH, Jin YY, Liu JG, Nie YF. Factors influencing the liquefaction, separation, and recovery of solid grease from restaurant garbage. J Tsinghua Univ. 2009;49:387–390.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Continuous anaerobic digestion of food waste and design of digester with lipid removal Dong Li
a b
a
a
a
a
, Yongming Sun , Yanfeng Guo , Zhenhong Yuan , Yao Wang & Feng Zhen
a
a
Chinese Academy of Sciences , Guangzhou Institute of Energy Conversion , Guangzhou , People's Republic of China b
Chinese Academy of Sciences , Key Laboratory of Renewable Energy and Gas Hydrate , Guangzhou , People's Republic of China Accepted author version posted online: 03 Jun 2013.Published online: 21 Jun 2013.
To cite this article: Dong Li , Yongming Sun , Yanfeng Guo , Zhenhong Yuan , Yao Wang & Feng Zhen (2013) Continuous anaerobic digestion of food waste and design of digester with lipid removal, Environmental Technology, 34:13-14, 2135-2143, DOI: 10.1080/09593330.2013.808237 To link to this article: http://dx.doi.org/10.1080/09593330.2013.808237
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2135–2143, http://dx.doi.org/10.1080/09593330.2013.808237
Continuous anaerobic digestion of food waste and design of digester with lipid removal Dong Lia,b , Yongming Suna , Yanfeng Guoa , Zhenhong Yuana∗ , Yao Wanga and Feng Zhena a Chinese b Chinese
Academy of Sciences, Guangzhou Institute of Energy Conversion, Guangzhou, People’s Republic of China; Academy of Sciences, Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou, People’s Republic of China
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(Received 8 November 2012; final version received 15 May 2013 ) Separation of municipal solid waste has been implemented in many cities in China. As a major component of municipal solid waste, food waste can be treated by anaerobic digestion (AD) for energy production. To provide reference data for disposing of food waste through engineering applications, continuous AD was carried out under various organic loading rates (OLRs) at 27 ± 2◦ C in the laboratory. The anaerobic reactor was stable with pH 7.0–7.1, total volatile fatty acid (VFA) concentrations of 206–746 mg/L, and NH+ 4 − N concentrations of 525–1293 mg/L when the OLR was 1.118–5.588 kg volatile solids (VS)/m3 ·d. The maximum volumetric biogas production rate was 4.41 L/L·d when the OLR was increased to 5.588 kg VS/m3 ·d with a hydraulic retention time of 30 d. When the OLR was increased to 6.706 and 8.382 kg VS/m3 ·d, biogas production was seriously inhibited by VFAs, with maximum total VFA and propionate concentrations of 8738 mg/L and 2864 mg/L, respectively. Due to the incomplete degradation of lipids, the specific methane production rate of 353– 488 L/kg VS accounted for 55.2–76.3% of the theoretical methane potential calculated based on the component composition. A retrofitted anaerobic digester with lipid removal was designed to improve the efficiency. Keywords: food waste; anaerobic digestion; organic loading rate; biogas production rate; lipid removal
Introduction Benefiting from the introduction of source separation at collection and mechanical separation plants, anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) has substantially increased over the past 15 years in Europe. A wide range of bench, pilot, and commercial scale AD systems have been applied to treat food waste, fruit and vegetable waste, household waste, sourcesorted OFMSW, mechanically sorted OFMSW, municipal sewage sludge, and mixtures of the above.[1–8] In China, due to the lack of developed waste separation technology or source separation collection systems, there is not yet a fullscale application of AD for OFMSW. As a major component of OFMSW, food waste is partially disposed of with other municipal solid waste in a landfill or by incineration; other food waste is collected from restaurants and used for pig feed or cooking oil refining, eventually returning to dining tables. The biochemical composition of food waste is mainly soluble carbohydrates, starches, dietary fibre, proteins, lipids, and salt. Landfill disposal of this perishable waste may give rise to greenhouse gas emissions, leachate production, and further air, groundwater, and soil pollution. When incinerated, its high water content results in low heat value and greater auxiliary fuel requirements. On the other hand, its characteristically high organic matter and water ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
contents make food waste suitable for AD. AD is considered a commercially feasible option for both waste treatment and energy/nutrition recovery, if the digestion residue is used as fertilizer. Recently, certain cities in highly developed regions of China have initiated source separation programmes. In Guangzhou, for example, the Guangzhou Municipal Solid Waste Management Interim Provisions for Classification were implemented in 2011. Domestic waste is divided into four categories, including recyclables, food waste, hazardous waste, and other garbage. Advances in waste classification systems will promote food waste disposal through anaerobic fermentation for biogas production. To provide reference data for engineering applications of AD of food waste, continuous AD pilot tests were run with various organic loading rates (OLRs). Based on the results, a new digester was designed to remove lipids and thereby improve efficiency. Materials and methods Food waste and inoculum Food waste was collected from a refectory at South China Agricultural University. At breakfast, lunch, and dinner, 10, 20, and 20 kg food wastes, respectively, were collected. After removing large bones, the 50 kg of food waste was
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D. Li et al. Characteristics of food waste and inoculum.
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Parameter Particle size (mm) Total solids (TS; g/kg) Heat value (MJ/kg TS) VS (% of TS) Ash (% of TS) Carbohydrates (% of TS) Crude fibre (% of TS) Proteins (% of TS) Total Kjeldahl N (% of TS) Lipids (% of TS) Carbon (% of TS) Hydrogen (% of TS) Oxygen (% of TS) Nitrogen (% of TS) Sulphur (% of TS) Phosphorus (% of TS) C:N ratio pH VFAs (mg/L) NH+ 4 − N(mg/L) TMP (L/kg VS)a TMP (L/kg VS)b
Food waste
Inoculum
≤4 241.80 25.14 92.44 7.56 38.6 3.30 17.30 2.77 34.90 50.12 7.81 32.64 1.79 0.06 0.02 28.38 4.25 137.00 72.00 607.00 639.00
≤2 24.00 – 58.30 41.70 – – – – – – – – – – – – 7.32 253.00 510.00 – –
a TMP
based on elemental composition (C, H, O, and N). based on component composition (carbohydrates, proteins, and lipids).
b TMP
pulverized, completely mixed, and refrigerated at 4◦ C. As source of inoculum, the effluent from a mesophilic AD in laboratory fed with pig waste, was used as inoculum after removing large particles. Characteristics of the food waste and inoculum are given in Table 1.
Experimental setup and procedures Continuous AD was conducted in a bench-scale reactor with a total volume of 40 L (Figure 1). Five separate ports on each reactor were designed to perform different functions: online pH monitoring, mechanical agitation, temperature control, and gas, and liquid sampling. Because the average temperature of Guangzhou is about 27◦ C, AD was carried out at 27 ± 2◦ C. Mixing was conducted six times per day at 20 rpm for 10 min. To avoid pipe blockage and consequent expansion of digesting material, the loading rate of the reactor was set at ∼75%. After a 1-week idle period, the first batch of food waste was fed into the reactor when negligible biogas production was observed. The food waste was added once a day without dilution. A 200-mL liquid sample was discharged from the reactor before adding new material. The daily amount of food waste addition is given in Table 2. The OLR ranged from 1.118 to 8.832 kg VS/m3 ·d, and the corresponding hydraulic retention time (HRT) ranged from 150 to 20 d. Two parallel experiments were conducted and the data are presented as mean values.
Analytical methods Total solids (TS) and volatile solids (VS'jwere determined using standard methods.[9] Elemental analysis was performed using a Vario EL elemental analyzer (Elementar Analysensysteme GmbH, Germany). The compositional analysis (carbohydrates, crude fibre, lipids, proteins, and total Kjeldahl nitrogen) was conducted based on the Chinese Standard GB/T 5009-2003. Heat values were measured using a WGR-1 heat analyzer (Changsha Bente Instrument Corporation, China), assuming complete combustion of fuel and that the water was heated in a completely insulated container, then using the measured temperature rise of the liquid to calculate the heat. Ammonia nitrogen (NH+ 4 − N) was determined using an FC-100 ammonia analyzer (Shanghai Super Info. Tech. Co. Ltd., China) with an electrochemical sensor. The pH was determined with a pHS-3C pH metre (Shanghai Precision & Scientific Instrument Co., Ltd, China). Biogas production was measured with an LML-1 wet gas metre (Changchun Automobile Filter Co., Ltd, China). The H2 , CH4 , and CO2 contents of the biogas were determined using a gas chromatograph (6890; Agilent, Santa Clara, CA, USA) equipped with a thermal conductivity detector and a 2-m stainless column packed with Porapak Q (50/80 mesh). The operational temperatures of the injection port, column oven, and detector were 100, 70, and 150◦ C, respectively. Argon was used as the carrier gas at a flow rate of 30 mL/min. Liquid samples were centrifuged at 5000 rpm at 0–4◦ C and then filtered through 0.22- μm cellulose acetate membranes. The concentration of total volatile fatty acids (VFAs, including acetate, propionate, butyrate, isobutyrate, valerate, and isovalerate) was determined using high performance liquid chromatography (Alliance 2695; Waters, Milford, MA, USA) equipped with a 241 refractive index detector (Waters) and a 300 × 8-mm S-DVB gel column (RSpak KC-811; New York, NY, USA). The mobile phase was 0.1% (m/m) phosphoric acid solution with flow rate of 0.7 mL/min. The temperature of the column oven was 40◦ C and the sample size was 10 μL. Calculations Free ammonia and free VFAs were calculated using the following formulas: CT [H + ] , KA + [H + ] KB CTotal [NH3 ] = , KB + [H + ] [HA] =
(1) (2)
where [HA] and [NH3 ] are the concentrations (mol/L) of free VFAs and free NH3 , respectively, CT and CTotal are the concentrations (mol/L) of total VFAs and total NH3 , respectively, [H+ ] is the hydrogen ion concentration
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Figure 1. Experimental apparatus for 40-L AD: (1) hot water tank, (2) circulating pump, (3) liquid sampling port, (4) digester, (5) mixer, (6) thermocouple probe, (7) hot water interlayer, (8) gas sampling port, (9) rotametre, (10) wet gas metre, (11) biogas outlet, (12) temperature controller, (13) timer, (14) pH metre, (15) rpm governor, and (16) pH probe. Table 2.
Experimental design.
Period (d) Food waste addition (kg/d, wet basis) Food waste addition (kg VS/d) OLR (kg VS/m3 ·d) HRT (d)
1st stage
2nd stage
3rd stage
4th stage
5th stage
6th stage
7th stage
8th stage
9th stage
1–7 0 0 0 –
8–14 0.2 0.045 1.118 150.0
15–21 0.3 0.067 1.676 100.0
22–28 0.4 0.089 2.235 75.0
29–35 0.6 0.134 3.353 50.0
36–42 0.8 0.179 4.470 37.5
43–49 1.0 0.224 5.588 30.0
50–57 1.2 0.268 6.706 25.0
58–61 1.5 0.335 8.382 20.0
(mol/L), and KA and KB are the dissociation equilibrium constants for VFAs and NH3 , respectively. The proportion of free VFAs depends on pH rather than temperature, because KA is negligible over the range 0 − −60◦ C. The proportion of free NH3 depends on both pH and temperature, as temperature has a significant effect on KB , with a higher temperature resulting in a higher proportion of free NH3 . pKA and pKB are 4.8 and 9.1, respectively, at 27◦ C. Results and discussion Trends in pH, VFAs, and NH+ 4 −N Figure 2 illustrates the changes in pH, VFA concentration, and NH+ 4 − N concentration over time. The pH in the initial idle period (first stage) was 7.0–7.1. The pH was maintained at 7.0–7.2 during the stable operational period (second to seventh stages). During the stable period, the total VFA and NH+ 4 − N concentrations increased from
206 to 746 mg/L and from 525 to 1293 mg/L, respectively. When the OLR increased to 6.706 and 8.382 kg VS/m3 ·d in the eighth and ninth stages, the total VFA concentration increased from 972 to 8738 mg/L and the propionate concentration increased from 357 to 2864 mg/L. Hanaki et al. pointed out that oxidation of propionate to acetate was more difficult than that of butyrate and valerate to acetate, and the tolerated propionate concentration for methanogens was < 1000 mg/L.[10] During these last two stages, the volumetric biogas production rate (VBPR) began to decrease. Therefore, the eighth and ninth stages represent an overloaded period. VFAs and NH3 are intermediate metabolites in methanogenesis as well as inhibitors when excessive accumulation occurs. The concentrations of VFAs and NH3 during AD are determined by their production and consumption rates. Ammonium ion (NH+ 4 ) and free NH3 are the two principal forms of inorganic ammonia nitrogen in aqueous solution. VFA ions (such as CH3 COO− ) and free VFAs (such
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Figure 3. Inhibitory conditions for methane production by anaerobic fermentation: (A: Methane production; B: Inhibited steady state; C: Inhibition by VFAs; D: Inhibition by ammonia).
as CH3 COOH) are the two forms of VFAs in aqueous solution. Free NH3 and free VFAs have been suggested as the main cause of inhibition, since they are freely membranepermeable.[11] The hydrophobic free NH3 and free VFA molecules may diffuse passively into cells, causing proton imbalances and/or potassium deficiency.[12] The inhibition of methanogens by free VFAs is stronger than that of acidogens; the concentration at which the inhibition of acidogens occurs is 2400–3000 mg/L, while for methanogens, it is 30–60 mg/L.[11,13] Free NH3 mainly inhibits methanogens, particularly aceticlastic methanogens. It does not have a significant effect on hydrogenotrophic methanogens or acidogens. Such inhibition is reversible and adaptive; it can be relieved by diluting the digest liquid or acclimating the methanogens. Koster et al. reported that the concentration of free NH3 inhibiting unacclimated methanogens was 50 mg/L.[14] Based on Equations (1) and (2) and the information presented above, a diagram of inhibition by VFAs and NH3 on anaerobic methane production at 27◦ C is shown in Figure 3. Normally, methane production occurs under neutral conditions (region A). Acidification and alkalization result from inhibition by VFAs and NH3 , represented by regions C and D, respectively. However, there may be little or no methane production even when the pH is neutral. Digestion of easily degradable and nitrogen-rich substrates produces a large amount of VFAs and NH3 during acidogenesis. According to Equations (1) and (2), free VFA and free ammonia concentrations can be higher than inhibitory levels even when the pH is close to 7. Interactions between VFAs, NH3 , and pH lead to this ‘inhibited steady state’ (region B).[15,16] In this study, biogas production was successful in region A during the initial and stable periods. However, the overloaded period was characterized by region C, during which hydrolysis and acidogenesis rates were higher than that of methanogenesis. Free VFAs, rather than free
NH3 , inhibited methane production. Accumulated VFAs lowered the activity of aceticlastic methanogens and the pH decreased. The lower pH increased free VFAs, further inhibiting methanogens. Among the VFAs, inhibition by propionate was strongest. Degradation of propionate was much slower than that of acetate, primarily because acetate can be directly broken down to CH4 and CO2 (Equation (3)), while propionate must be degraded to acetate before methane production (Equation (4)). Moreover, degradation of propionate is a thermodynamically unfavourable reaction unless the hydrogen partial pressure is maintained at a very low level: CH3 COOH → CH4 + CO2
G0 = −31.0 kJ/mol, (3)
CH3 CH2 COOH + 4H2 O → 2CH3 COOH + 3H2 + 2CO2 G0 = +76.1 kJ/mol.
(4)
Biogas production The specific biogas and methane production rates (SBPR and SMPR), VBPR, and CH4 and H2 contents were used to describe the results of the biogas production process (Figure 4). The SMPR and SBPR profiles were similar. During the stable period, the OLR and VBPR were positively and linearly correlated (Figures 4 and 5). The maximum VBPR was 4.41 L/L·d when the OLR was 5.588 kg VS/m3 ·d with an HRT of 30 d. The corresponding volumetric methane production rate was ∼2.42 L/L·d, similar to the results of Cho et al.[17] In that study, dry AD of food waste was conducted under mesophilic conditions. Stable dry AD was achieved by controlling the HRT without the addition of alkali agents. The average methane production rate, methane content, and VS reduction were 2.51 L/L·d, 66%, and 65.8%, respectively, at an HRT of 40 d with an OLR of 5.0 kg VS/m3 ·d.
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Figure 5. Relationship between OLR and VBPR during the stable period.
The methane concentration initially increased and then decreased. The maximum methane concentration of 66% was observed on the 35th day with an OLR of 3.353 kg VS/m3 ·d. No H2 was detected during the stable period.
Based on the best fit of the SMPR data, a higher OLR does not necessarily correspond to a higher SMPR. The optimal SMPR of 353–488 L/kg VS was observed during the fifth stage with an OLR of 3.353 kg VS/m3 ·d and an HRT of 50 d, similar to results for a study of food waste in San Francisco, CA.[18] In that study, anaerobic methane yields from food waste were evaluated using batch tests performed at 50◦ C. The methane yield was determined to be 348 and 435 L/kg VS after 10 and 28 days of digestion, respectively. The average VS reduction was 81% at the end of the 28-day digestion test. When the OLR was increased to 6.706 and 8.382 kg VS/m3 ·d, the VBPR, SBPR, and SMPR first slowly and then sharply decreased until little biogas was produced. When the reactor was run in an overloaded state, methane concentrations were 100μm floats on the water as a contiguous oil film in the absence of mixing. It can easily be removed by gravity separation. Dispersed oil with particle sizes of 25–100 μm is suspended in the liquid phase as small droplets. It can also be easily removed when the droplets gather to form oil beads and float on the surface after a period of standing. Emulsified oil has particle sizes of 0.1–25 μm when there are surfactants in the liquid phase and is difficult to remove. Dissolved oil of nanometre size disperses in the water phase to form ultrafine droplets in molecular or chemical forms. It is difficult to remove because the oil and water form a very stable homogenous phase system. However, the solubility of oil in water is very low (5–15 mg/L). Solid inner fat is closely associated with solid particles from which it cannot be directly separated. Lipids in food waste mainly exist in the form of solid inner fat and floating oil, which account for more than 80% and 10–15% of the total lipids on a mass basis, respectively.[28,29] The key to lipid removal from food waste is to release the solid inner fat from the solid phase to the liquid phase. The Chinese patent application No. 200710132239.4 and [29] state that wet hot treatment with high-temperature steaming can release solid inner fat from the solid phase of food waste to the liquid phase. The Chinese patent application No. 200710306010.8 describes a method for converting dispersed and emulsified oil in wastewater into floating oil through a combination of coarse-graining and microfloating. However, wet hot treatment is a high energy consumption process, as the target temperature is ∼150◦ C. Microfloating requires energy to drive the microbubble generator. Coarse-graining not only requires a filter, which increases the equipment cost, but also requires finely controlled process conditions to ensure the efficiency of oil recovery. We propose a simpler and more cost-effective method and associated device for removing lipids from food waste. Figure 6 presents an integrated anaerobic digester for food waste with lipid removal. In the anaerobic digester, solid organic particles are broken down and released to the water phase through physical decomposition and extracellular enzymatic hydrolysis. The solid inner fat is also released to the water phase, then forms dispersed and emulsified oil. Afterwards, the dispersed and emulsified oil adhere to micro biogas bubbles as a result of surface tension. They then rise along with the ascending bubbles and become floating oil. At the level of the floating oil layer, two oil scraper plates are installed on the shaft and an oil discharge port and baffle are placed on the wall of the digester. A rubber contact pad is fixed at the end of the oil scraper plate. When the stirrer is operating, the oil scraper plates and rubber contact pads rotate clockwise. Once the floating oil is gathered to the oil discharge port, it is obstructed by the baffle and discharged.
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Figure 6. Designed integrated anaerobic digester for food waste with lipid removal: (a) cutaway view, (b) oblique cutaway view, and (c) top view; (1) inlet, (2) outlet, (3) slag discharge port, (4) oil discharge port, (5) oil discharge pipe, (6) motor, (7) shaft, (8) oil scraper plate, (9) rubber contact pad, (10) baffle, (11) biogas outlet, (12) heating band, and (13) valve.
The outside wall of the digester at the level of the floating oil is encircled with a heating band to melt the solidified oil adhered onto the inside wall of the digester. This design takes advantage of physical decomposition and extracellular enzymatic hydrolysis of solid inner fat to form dispersed and emulsified oil, makes full use of methanogenesis microorganisms as ‘microbubble generators’ for converting dispersed and emulsified oil to floating oil, and introduces a set of simple additional parts to remove floating oil from the digester. This retrofitted anaerobic digester does not require either separate lipid removal equipment or substantial energy consumption for steaming food waste or generating microbubbles. However, the current design is a technical proposal that requires additional work to evaluate its feasibility and efficiency. Conclusions AD of food waste performed stably with OLRs of 1.118–5.588 kg VS/m3 ·d and HRTs of 150–30 d. Biogas production was seriously inhibited by VFAs when the OLR was increased to 6.706 and 8.382 kg VS/m3 ·d. The recommended OLR for AD of food waste at
27◦ C in practical engineering applications is about 4.47 kg VS/m3 ·d, at which the VBPR, SMPR, and methane content were 2.80–3.24 L/L·d, 300–420 L/kg VS, and 57–64%, respectively. To address challenges associated with the presence of lipids, a method and supporting device for removing lipids from food waste is proposed. It takes full advantage of physical decomposition, extracellular enzymatic hydrolysis, and methanogenesis microorganisms as ‘microbubble generators’, and introduces a set of simple added parts to remove floating oil from the anaerobic digester. These results provide important reference data for treatment of food waste by AD in China. Acknowledgements This research was financially supported by the National Key Technologies R&D Programme of China (No. 2011BAD15B02) and the Science and Technology Planning Project of Guangdong Province (No. 2011A030600005).
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