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Influence of thermal pretreatment on physical and chemical properties of kitchen waste and the efficiency of anaerobic digestion
Journal of Environmental Management 180 (2016) 291e300
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Influence of thermal pretreatment on physical and chemical properties of kitchen waste and the efficiency of anaerobic digestion Yiying Jin, Yangyang Li*, Jinhui Li School of Environment, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 1 June 2016
The effects of thermal pretreatment at moderate temperatures (70, 80 and 90 C) and high temperatures (120, 140 and 160 C) over heating durations of 10e120 min on the physical and chemical properties of kitchen waste and on anaerobic digestion were investigated. The results show that thermal pretreatment significantly enhances the solubilisation of organic compounds (chemical oxygen demand, crude proteins, crude fats and volatile fatty acids) and their biodegradability during subsequent anaerobic digestion. High temperature and long heating duration are beneficial for the release and reduction of organic compounds, and the efficiency of subsequent anaerobic digestion is improved markedly under these conditions. Moreover, both the methane production rate and methane yield were observed to increase significantly at moderate treatment temperatures when the anaerobic digestion time was longer than 50 h. © 2016 Published by Elsevier Ltd.
Keywords: Thermal pretreatment Anaerobic digestion Kitchen waste Biodegradability
1. Introduction Approximately 60 million tons of kitchen waste (KW) is produced annually (China Statistical Yearbook, 2010). Due to its characteristic high organic content, high moisture content, and susceptibility to rotting and breeding flies in the short term, normal municipal solid waste (MSW) disposal technologies cannot directly be applied to KW, and improper disposal of KW can easily cause secondary pollution, such as odour pollution and mosquito breeding. To comply with EHS (Environmental, Health, and Safety) regulations and policies, the final disposal of KW requires thorough sterilization and effective resource recycling (Adhikari and Barrington, 2006; Khalid et al., 2011). Alternative processing technologies, such as anaerobic digestion (AD), offer some potential for the recovery of valuable resources from organic wastes by producing biogas (Stabnikova et al., 2008; Carvalheiro et al., 2008) and have been widely used for the treatment of KW in recent years. However, KW is rich in carbohydrates, which can relatively easily be converted to volatile fatty acids, accumulating lactic acid at an early stage of the digestion process; this conversion can result in a dramatic pH drop if the anaerobic digestion system lacks sufficient buffering capacity (Veeken et al.,
* Corresponding author. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jenvman.2016.05.047 0301-4797/© 2016 Published by Elsevier Ltd.
2000). In addition, KW contains higher fat and protein contents than does MSW, which can lead to inhibitory levels of ammonia, sulphide and long-chain fatty acids (Braun et al., 2003; Amaral et al., 2004). Hence, the digestion and stability of the digestion process could be easily diminished and impeded. Numerous pretreatment methods, including chemical, biological and thermal processes, have been suggested to improve the properties of organic waste by promoting the solubilisation of particulate organics and biogas production and enhancing the digestion's rate-limiting step, i.e., the hydrolysis of organic matter. Among these methods, a number of thermal pretreatments, which disintegrate sludge cells under the application of various temperatures, ranging from 60 to 270 C for numerous durations, lasting from 15 min to several hours, have been studied (Liu et al., 2012; Climent et al., 2007). Thermal pretreatment has been proposed to improve organic waste properties and promote biogas production, which also has the effect of sterilization and sanitation. However, previous studies mostly have focused on the thermal hydrolysis of sewage sludge, and very few reports are available on the varying properties of KW (Chowdhury et al., 2007; Aragon et al., 2009). In addition, previous studies have mainly concentrated on direct anaerobic digestion and combined digestion with other biomass waste, such as sludge, by adjusting pH, mixing ratio, retention time and organic loading (Wilson and Novak, 2009; Appels et al., 2008; Liu et al., 2011). Furthermore, the high oil content of KW, which is a result of Chinese dietary customs, directly restricts the
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fermentation speed and operating cost when KW is directly used for anaerobic fermentation or composting. Hence, it is particularly important to investigate the effects of thermal pretreatment on the physical and chemical properties of KW, as well as the effects on subsequent anaerobic digestion. The objective of the current work was to evaluate the influence of thermal pretreatment on the physical and chemical properties of KW and determine the relationship between thermal hydrolysis and anaerobic biodegradability. To determine the influence of treatment on physical properties, pH, density and the characteristics of the three-phase boundary were discussed. The chemical properties include volatile solids, soluble chemical oxygen demand and nutritional components including crude fats, crude proteins and volatile fatty acids, which reflect solubilisation effects of organic compounds in KW. The influence of physical and chemical properties variations on subsequent anaerobic digestion of KW were determined by effects of organic biodegradation and production of methane. Thus, the effect of thermal pretreatment on the efficiency of subsequent anaerobic digestion was analysed and discussed, and more specific parameters of thermal pretreatment can be recommended. 2. Materials and methods 2.1. KW characteristics KW was collected on a weekly basis from a canteen at Tsinghua University. The major components were carbohydrates derived from bread, cooked noodles and rice; proteins and fat from different types of meat and fish; and various vegetables and fruits. The received KW was mixed in a kitchen blender and stored at 4 C in a refrigerator after being crushed into particles with an average size of 1e2 mm. Table 1 shows the basic characteristics of the KW. 2.2. Thermal pretreatment Thermal pretreatment was performed in a 1.5 L stainless steel hydrolysis reactor, which was constructed as a pressure vessel with a heating shell. The temperature was kept constant by controlling
Degree of reductionð%Þ ¼
2.3. Anaerobic digestion tests The digestion experiments were conducted in 1 L glass bottles at 35 C to determine the biodegradability of the raw and thermally treated materials. The seed sludge was obtained from a steadily operating digester at the Gaobeidian waste water treatment plant with 2 d of gravity sedimentation prior to inoculation. Each bottle was fed with a mixture of 80 g kW and 800 g seed sludge so that the effects of thermal pretreatment could be more pronounced. The digestion experiments were run for 30 days. In each experimental run, three control digesters were operated with thermally treated sludge. 2.4. Analytical methods The following components were analysed before and after thermal treatment: pH, density (r), total solids (TS), volatile solids (VS), soluble chemical oxidation demand (SCOD), carbohydrate concentration, protein concentration, crude fat concentration, total alkalinity (TALK) and concentration of VFAs. The TS and VS analyses were based on the Standard Analytical Methods developed by the National Environmental Protection Agency of China (1989). SCOD was determined according to the standard methods of the American Public Health Association (APHA, 2005). TALK (endpoint pH titration) and VFAs (distillation method) were measured according to procedures described in Clesceri et al. (1998). Characteristics of the three-phase boundary (including solid phase, liquid phase and floating oil) after thermal pretreatment were also analysed. The analyses were performed on KW after thermal pretreatment; this index could also be used to evaluate the effect of thermal hydrolysis. Three phases appeared from top to bottom after centrifugation at 5000 r/min for 5 min. Then, the mass of each phase was weighed. The concentrations of crude protein and crude fat were determined according to the Kjeldahl method, using a Soxhlet device (Methodenbuch, 1993). The reduction in the concentration of the organic components after anaerobic digestion was calculated by the following equation:
Concentration before AD Concentration after AD 100 Concentration before AD
the temperature of the thermal fluid circulating through the outer heating shell of the reactor. During the pretreatment, approximately 1 kg kW was transferred into the vessel, which had been preheated to a predetermined temperature. After hydrolysation at this temperature for a selected period, the heating process was stopped, and samples were then chilled by circulating 10 C water until the vessel cooled to room temperature. After pretreatment, the samples were stored at 4 C in a refrigerator to minimise the volatilisation of organic compounds while waiting for the next step of the analysis.
The statistical and correlation analysis (Pearson correlation) between treatment temperatures and physical and chemical properties were performed using the statistical software IBM SPSS version 20 for windows, and a p-value of <0.05 was considered statistically significant. 3. Results and discussion 3.1. Effect of thermal pretreatment on physical properties Thermal treatment was performed on KW at six different temperatures and over six different treatment durations. An overview of the results is shown in Fig. 1.
Table 1 Characteristic of the KW. Sample
pH
TS
C
H
N
S
O
KW
6.47
18.66%
46.11%
6.89%
3.19%
0.29%
37.80%
3.1.1. pH The measured pH values are presented in Fig. 1(a). With a pH value of 6.47, the KW was mildly acidic. After thermal pretreatment,
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Fig. 1. Variation in pH (a), phase proportions (b, c, d) and density (e) during thermal pretreatment at different temperatures and for different treatment durations.
the pH of the KW decreased significantly, and it decreased further by increasing the pretreatment temperature and the duration. Higher temperatures and longer durations increased the pH reduction rate because organic acids were continuously released into the liquid phase from the solid phase in the KW during treatment. Therefore, to some extent, the decrease in the pH reflected the extent of the thermal hydrolysis reaction. At 55e90 C, the decrease in pH was limited compared to that at higher temperatures. Even over a treatment time of 50 min, the pH was essentially stable in the range of 4.77e5.33. As the temperature and duration were further increased, the pH reduction rate increased. For example, at 120 C, the pH decreased to 4.71 when the thermal treatment lasted 50 min, an approximately 27.20% decrease in the original pH of the KW. When the treatment
temperature climbed to 160 C, the decrease in pH became more apparent. These results also suggested that both the rate and amount of organic acid release increased, resulting in a lower pH value.
3.1.2. Characteristics of three-phase boundary The organic matter in the KW was effectively hydrolysed into small molecules and inorganic matter after thermal pretreatment, which provided high temperature and high pressure. Additionally, bound water was released, and oil dissolved out of the KW. Therefore, after thermal pretreatment, the KW could be divided into three distinct layers from top to bottom, an oil, a liquid and a solid phase, after centrifugal treatment.
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3.1.2.1. Floating oil. The animal fat content of KW is high, and mainly in five states, floating oil, dispersed oil, emulsified oil, dissolved oil and oily solids, and oil removal is difficult to perform directly without any pretreatment. The content of floating oil served as an indicator of the KW de-oiling performance. The deoiling performance of oil-water mixtures is usually measured by the content of the floating oil. After thermal pretreatment and sequenced centrifugation, the removed oil that floats on the surface can be recycled, and the spare liquor is clear and transparent. Fig. 1(b) shows the increase in the proportion of floating oil with respect to different thermal pretreatment parameters. Apparently, raising the thermal treatment temperature and heating time was advantageous to the dissolution of floating oil. Untreated KW contained approximately 8.35% floating oil, and this proportion increased rapidly with a rise in temperature and treatment duration. The thermal pretreatment time and temperature affected the proportion of floating oil very little at 55e70 C and 50 min, with the proportion remaining similar at 9% and 9.4%, respectively. A continuous increase in the temperature of the steel hydrolysis reactor would have led to a continuous rise in the floating oil proportion. To achieve the same 10% scale, the heating time could be decreased by nearly 60 min at 120 C compared with the 120 min required at 90 C. The percentage of floating oil could be increased more quickly and noticeably when the heating temperature was enhanced. Above 120 C, the growth rate rose distinctly. However, the proportion of floating oil did not change greatly for a heating time greater than 50 min, remaining at 11% and 12%. Therefore, thermal hydrolysis can improve the oil content of KW, affecting the efficiency of waste oil removal. 3.1.2.2. Solid phase. The solid phase of KW mainly contains food residues from rice and flour, vegetables, meats, fats, etc., existing in the chemical forms of amylum, cellulose, proteins, lipids and inorganic salts, which are useful for subsequent utilisation, such as a for aerobic composting. However, these ingredients were gradually exposed to the dissolution process, and small-molecule organic matter was dissolved into the liquid phase and, in the case of some fats, into the floating oil, which triggered a reduction in the solid phase. During the thermal treatment process, the following reactions occur: first, fats from adipose tissue are broken down into glycerol and free fatty acids. Second, carbohydrates are broken down into three types of sugar, monosaccharides, disaccharides and polysaccharides. Third, most proteins are degraded to small peptides and amino acids. All of these hydrolysis processes will effectively promote the reduction in solid-phase quality. Fig. 1(c) presents the variations in the solid phase mass ratio; it was apparent that increasing the heating temperature and heating time enhanced the dissolution of the solid phase, resulting in a decrease in the solid-phase mass ratio. The higher the temperature and the longer the period for thermal treatment were, the faster the solids mass ratio decreased. The solid proportion was only decreased by 3.29% after treating at 55 C for 120 min, in contrast with reductions of 15.55% and 20.68% observed for treatment times of 30 min and 90 min, respectively, at 160 C. Thus, increasing the thermal treatment temperature and duration is advantageous to the rapid hydrolysis of large particulate matter and organic matter in the solid phase.
treatment process. The effects of heating temperature and time on the quality of the liquid phase in KW were determined, and the results are shown in Fig. 1(d). The proportion of the liquid phase was evidently improved after thermal pretreatment, and the rate of increase grew with the heating temperature and heating time due to the enhancement in the thermal hydrolysis rate of the solid phase in KW. Hence, the proportion of the liquid phase changed slightly at low temperature, e.g., the liquid phase increased by 2.5% and 4.5% at 55 C and 90 C, respectively, for a treatment duration of 120 min. When the heating temperature increased above 120 C, the liquid proportion increased significantly and stabilised at approximately 24% and 30% at 120 and 140 C, respectively, for a treatment duration of 50 min. When the temperature climbed further to 160 C, the liquid proportion continued to increase, and approximately 20 min of reaction time could be eliminated, while still achieving the same percentage as that at 120 C and a duration of 50 min. In general, the moisture distribution in KW could be effectively altered, thus improving the mechanical dewatering properties of KW. Combined with variations in the floating oil and solid phase, from the perspective of material balance analysis, the large reduction in the solid phase could be attributed to two sinks, a relatively small part of the floating oil and the main part of the liquid phase. In general, thermal treatment is beneficial for waste oil recycling from KW, especially at higher temperatures, such as 140 C and 160 C, thus enhancing oil separation from water in KW and promoting floating oil recycling, which could be used as raw materials for biodiesel. These optimal conditions can lead to a 29e40% increase. The dehydration performance and viscosity, which are useful parameters for designing and monitoring biological processes and determining the mode of agitation and energy consumption of bioreactors, could be effectively improved due to the higher efficiency of waste oil removal and liquefaction, making KW a very suitable substrate for subsequent anaerobic digestion. 3.1.3. Density Fats in KW are readily dissolved and are normally collected for recycling after thermal treatment. Due to their low density compared with that of the liquid phase and that of solid phase, after the oil is removed, the density of the remaining solid-liquid mixture would be altered. The densities of the tested materials after oil removal were analysed, and the results are shown in Fig. 1(e). Due to the presence of oils, the density of the raw KW was approximately 0.92 mg/L, slightly lower than that of water. When the KW was heated to relatively low temperatures (55e90 C), the density rose slightly due to the increased removal proportion of floating oil, but was still lower than that of water. When the heating temperature was higher than 120 C, the effects of lipid dissolution became more apparent, which led to a rapid rise in the floating oil content. At the same time, after degreasing, the density of the remaining material increased. After being heated for 50e70 min by thermal conditioning, the density tended to stabilise after degreasing to approximately 1.20 mg/L. Hence, raising the thermal treatment temperature and prolonging the heating time facilitated the dissolution of floating oil and subsequent oil collection and removal, which would be beneficial to achieving higher density. 3.2. Effects of thermal pretreatment on solubilisation
3.1.2.3. Liquid phase. Due to its high moisture content and migration-flow characteristics, KW is particularly difficult to treat, and more environmental pollution would be produced without timely and effective supervision and disposal, compared with ordinary household waste. Hence, dehydration is crucial to the KW
The effects of thermal pretreatment at different treatment temperatures and for different durations on the solubilisation of organic compounds were investigated, and the results are shown in Fig. 2.
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Fig. 2. Solubilisation of organic compounds during thermal pretreatment at different temperatures and for different treatment durations. (a) SCOD; (b) SS; (c) VSS; (d) Crude protein in solid phase; (e) Crude fat in solid phase; (f) VFA.
3.2.1. SCOD SCOD can be used to characterise the concentration of soluble organic matter, which consists mainly of dissolved organic matter. Hence, SCOD changes, to a certain extent, can indicate digestion performance. The release of organics is due to the disruption of chemical bonds by thermal treatment, including degradation and thermal hydrolysis of polysaccharides, proteins, and volatile fatty acids (VFAs) from KW. It was apparent that the SCOD content varied greatly when increasing the heating temperature and duration in all tests. The higher the temperature was, the faster the rate of SCOD formation rate became. The SCOD results are presented in Fig. 2(a). It could be concluded that at a lower temperature and shorter duration under thermal treatment, the macromolecular
organic matter in KW was not effectively disintegrated, nor could it be transformed into small molecules that would be released into the liquid phase and result in an increase in SCOD. At 55e90 C, the increase in SCOD was not distinct, and the value tended to stabilise at a heating time of 30 min; the maximum degrees of hydrolysis observed were 8.01%, 12.60% and 22.70% at 55, 70 and 90 C respectively. The degrees of liquefaction and hydrolysis of organics were relatively weak. When the heating temperature rose to 120e160 C and the reaction time was extended, both the values of SCOD and the rate of increase were enhanced significantly compared to the results obtained below 100 C, and they tended to stabilise when the heating time was longer than 50 min. The highest hydrolysis effect was achieved at 160 C and 120 min; the SCOD value was approximately
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188,970 mg/L, and the degree of hydrolysis increased by nearly 56% compared with that of the original sample. The increase in SCOD demonstrated that thermal pretreatment was beneficial for improving the digestion performance of KW, whether it may be subsequently applied as feed or fertiliser. Additionally, the SCOD increase suggests that some organic nutrients were transformed from the solid phase into the liquid phase, which would increase the possibility of producing liquid organic fertiliser from the KW. However, excessively high temperature and prolonged extension of thermal treatment would decrease the amount of soluble organic matter and change the trophic structure in the solid phase after processing; thus, fewer nutrients would remain in the product after dehydration and drying. 3.2.2. SS & VSS After thermal conditioning and subsequent centrifugation, the liquid phase and floating oil in the KW were poured out, and the remaining solid phase was used to detect SS and VSS. The effects of thermal treatment on SS and VSS are shown in Fig. 2(b) and (c). It is apparent from the three-phase interface and change in SCOD that the proportion of SS in the solid phase decreased with increasing reaction temperature and reaction time. The proportion of SS varied slightly at 55 C with different thermal treatment durations, and less than 2% was added when the heating temperature rose to 90 C. Once KW was heated to above 100 C, the SS ratio decreased rapidly, and the lowest SS was achieved at 160 C and 90 min, with a reduction of nearly 38% compared with the SS of raw KW. Three main causes resulted in the loss of SS during thermal processing. First, some organic matter was converted to dissolved compounds, accompanied by the reactions of hydrolysis and liquefaction, thus transforming from the solid phase into the liquid phase. Second, the evaporation of volatile organic compounds in the solid phase tended to cause this phenomenon. Third, carbon dioxide and ammonia gas were generated and released from the hydrolysis of partial organic matter. Among these three causes, the first process served as the most important factor for the reduction of SS in the solid phase, which could also be concluded from the changing trends of SCOD (Table 2). The VSS content in the solid phase after thermal pretreatment decreased with the heating temperature and duration in all tests. The SS changes could serve as the explanation for the observed trends of VSS, which were analysed descriptively and comparatively. The thermal instability of solid components would lead to their gradual decomposition with increasing temperature and heating time, causing a reduction in VSS content. The VSS content was altered slightly at 55e90 C when the heating time reached 50 min; less than a 0.5% increase was obtained, and the rate increased rapidly as the heating temperature and duration rose further. At 160 C, particularly when the heating time was 90 min, VSS was reduced to 3.92%. 3.2.3. Protein solubilisation Proteins were solubilised due to thermal treatment. Fig. 2(d) shows the protein concentration in the solid phase. The results indicate that the proportion of crude protein in the solid phase was reduced by 1e2.41% after thermal processing, compared with 24.62% for the untreated samples. It could also be concluded that at temperatures of 55e120 C, increasing the processing temperature would lead to protein denaturation and coagulation, especially in the form of animal protein, reducing the proportion of protein in the solid phase. Thermal treatment can further affect the interaction and hydration of water molecules and the side chains of amino acids and peptide bonds, first by forming bound and adjacent water and then by forming a multilayer of water. With rising temperature
and heating duration, crude protein would experience further hydration and gradually became a colloidal solution due to waterswelling. The opposite trend was observed when increasing the heating duration at temperatures of 140e160 C, which could be explained by the exposure of hydrophobic groups in proteins at high pressure and even the irreversible encapsulation of hydrophilic proteins by the gel formed by the proteins when the pressure continued to increase (Campbell et al., 2005; Chapleau and De LamballerieAnton, 2003). Additionally, non-protein sulphydryl (NPSH) was mainly affected by non-covalent bonds between protein molecules, which were significantly affected by high pressure. As the thermal treatment temperature and processing duration increased, the content of NPSH decreased remarkably, thus resulting in the augmentation of molecular polymerisation due to hydrophobic interaction and once again promoting the formation of disulphide bonds (Messens et al., 1997). In addition, after the thermal treatment of KW at 140 C and 160 C for 50 min and 30 min, respectively, the colour of the material became black and the Maillard reaction occurred. These changes resulted in a loss of nutrients due to the combination of amino acids and sugars, the product of which could not easily be digested and absorbed fully, even leading to the production of recalcitrant soluble organics or toxic/inhibitory intermediates and hence reducing the biodegradability (Wilson and Novak, 2009). 3.2.4. Crude fat solubilisation Normally, the lipid content in KW is high, and KW is readily subject to deterioration and rancidity because of its unsaturated fatty acids, which cause numerous adverse effects, such as inhibiting the activity of digestion bacteria and methanogens; such effects may even lead to anaerobic digestion failure by preventing substance transfer caused by high levels of long chain fatty acids (LCFA) adsorbed on the membrane surface and the wrapping of microorganisms by undegraded oil after condensation, when conducting anaerobic digestion. With thermal pretreatment, the dissolution of lipids is accelerated and lipid reactions are more complex, mainly involving hydrolysis and oxidation reactions, producing monoglycerides and fatty acids, thereby the digestibility is improved, which also reduces the stability of oil, thus accelerating rancidity. The crude fat content in the solid phase at different temperatures over given heating times are shown in Fig. 2 (e). The results showed that the crude fat content in the solid phase first increased and then decreased with an increase in the heating time but was still less than that of raw KW. The higher the heating temperature became, the earlier the decline in crude fat appeared. Taking 55 C for example, the crude fat content began to drop after rising upon reaching a heating duration of 90 min, nearly 20 min longer than at 90 C. When the temperature rose above 100 C to 120, 140 and 160 C, the turning points were all approximately 50 min. Overall, a relatively lower crude fat content was achieved at 90 C, and higher contents were observed at 140 C compared with those recorded at other temperatures and heating times. During the early stages of thermal treatment, at low heating temperature, some of the free fatty acids were solidified, and the coagulation was separated from the liquid phase and entered the solid phase, resulting in an increase in the fat content of the solid phase. Upon further raising the heating temperature and prolonging the heating time, partial solid fats began to dissolve and decompose into the liquid phase, resulting in fat loss in the solid phase; moreover some of the volatile lipid compounds volatised into the gas phase and drained, leading to lower fat content in the solid phase of the
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Table 2 Pearson correlation analysis between treatment temperatures and physical and chemical properties. (* Correlation is significant at the 0.05 level (2-tailed). ** Correlation is significant at the 0.01 level (2-tailed).)
T ¼ 55 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 70 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 90 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 120 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 140 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 160 C T pH Floating oil Soild phase Liquid phase Density
T
pH
Floating oil
Soild phase
Liquid phase
Density
SCOD
SS
VSS
Crude protein
Crude fat
VFA
1 0.784* 0.627 0.548 0.395 0.843* 0.684 0.949** 0.902** 0.699 0.857* 0.857*
1 0.665 0.458 0.596 0.812* 0.471 0.921** 0.801* 0.588 0.750 0.750
1 0.928** 0.010 0.937** 0.545 0.755* 0.807* 0.370 0.899** 0.899**
1 0.218 0.845* 0.354 0.626 0.741 0.110 0.768* 0.768*
1 0.217 0.128 0.463 0.369 0.388 0.207 0.207
1 0.606 0.928** 0.953** 0.557 0.955** 0.955**
1 0.604 0.510 0.831* 0.808* 0.808*
1 0.952** 0.663 0.890** 0.890**
1 0.565 0.883** 0.883**
1 0.684 0.684
1 1.000**
1
1 0.685 0.553 0.929** 0.953** 0.316 0.978** 0.338 0.901** 0.987** 0.738 0.912**
1 0.747 0.841* 0.789* 0.307 0.569 0.666 0.899** 0.729 0.607 0.813*
1 0.676 0.545 0.306 0.538 0.764* 0.812* 0.623 0.884** 0.657
1 0.953** 0.538 0.854* 0.619 0.965** 0.907** 0.706 0.988**
1 0.369 0.872* 0.454 0.922** 0.937** 0.629 0.950**
1 0.232 0.567 0.382 0.211 0.250 0.608
1 0.247 0.839* 0.971** 0.779* 0.827*
1 0.657 0.336 0.450 0.600
1 0.920** 0.808* 0.941**
1 0.795* 0.882**
1 0.701
1
1 0.847* 0.893** 0.940** 0.883** 0.810* 0.948** 0.553 0.851* 0.849* 0.660 0.889**
1 0.807* 0.941** 0.948** 0.593 0.882** 0.689 0.994** 0.928** 0.914** 0.955**
1 0.875** 0.805* 0.739 0.862* 0.750 0.835* 0.882** 0.722 0.817*
1 0.978** 0.662 0.944** 0.727 0.948** 0.961** 0.793* 0.988**
1 0.618 0.870* 0.665 0.939** 0.926** 0.788* 0.972**
1 0.613 0.195 0.563 0.515 0.300 0.563
1 0.702 0.908** 0.919** 0.797* 0.934**
1 0.754 0.882** 0.770* 0.760*
1 0.957** 0.933** 0.965**
1 0.873* 0.971**
1 0.838*
1
1 0.560 0.943** 0.695 0.533 0.629 0.837* 0.896** 0.797* 0.507 0.757* 0.932**
1 0.712 0.926** 0.926** 0.992** 0.892** 0.769* 0.913** 0.936** 0.945** 0.787*
1 0.742 0.595 0.778* 0.878** 0.887** 0.908** 0.616 0.860* 0.952**
1 0.973** 0.932** 0.957** 0.885** 0.911** 0.879** 0.958** 0.858*
1 0.908** 0.874* 0.767* 0.831* 0.868* 0.900** 0.730
1 0.927** 0.805* 0.940** 0.917** 0.964** 0.844*
1 0.937** 0.941** 0.850* 0.959** 0.968**
1 0.925** 0.785* 0.915** 0.936**
1 0.845* 0.990** 0.911**
1 0.876** 0.740
1 0.900**
1
1 0.603 0.840* 0.888** 0.874* 0.566 0.431 0.910** 0.568 0.594 0.677 0.963**
1 0.807* 0.869* 0.876** 0.990** 0.924** 0.844* 0.941** 0.924** 0.955** 0.758*
1 0.968** 0.958** 0.766* 0.797* 0.926** 0.856* 0.902** 0.902** 0.878**
1 0.997** 0.833* 0.774* 0.983** 0.872* 0.877** 0.929** 0.949**
1 0.834* 0.776* 0.981** 0.882** 0.874* 0.927** 0.939**
1 0.908** 0.817* 0.910** 0.911** 0.949** 0.739
1 0.690 0.960** 0.939** 0.917** 0.571
1 0.800* 0.837* 0.896** 0.980**
1 0.923** 0.960** 0.697
1 0.958** 0.731
1 0.818*
1
1 0.733 0.737 0.901** 0.913** 0.554
1 0.987** 0.940** 0.918** 0.965**
1 0.923** 0.899** 0.928**
1 0.997** 0.849*
1 0.820*
1 (continued on next page)
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Table 2 (continued )
SCOD SS VSS Crude protein Crude fat VFA
T
pH
Floating oil
Soild phase
Liquid phase
Density
SCOD
SS
VSS
Crude protein
Crude fat
VFA
0.255 0.897** 0.504 0.523 0.660 0.901**
0.378 0.874* 0.926** 0.947** 0.980** 0.954**
0.405 0.882** 0.912** 0.902** 0.962** 0.945**
0.127 0.952** 0.811* 0.834* 0.901** 0.987**
0.091 0.949** 0.783* 0.805* 0.878** 0.976**
0.510 0.755* 0.954** 0.996** 0.969** 0.856*
1 0.128 0.630 0.532 0.424 0.111
1 0.796* 0.746 0.857* 0.945**
1 0.962** 0.959** 0.805*
1 0.960** 0.831*
1 0.913**
1
product. In addition, the hydrolysis of some lipid compounds at higher temperature resulted in the volatilisation of small amounts of volatile fatty acids in the liquid, promoting the reduction in fat content (Table 2). 3.2.5. VFAs Volatile fatty acids (VFAs) are fatty acids with a carbon chain composed of six carbons or fewer; these fatty acids are usually referred to as short-chain fatty acids (SCFA) and include acetic acid (C2), propionic acid (C3), butyric acid (C4) and valeric acid (C5). The degree of hydrolysis of organic matter in KW and the biodegradability of the products after thermal treatment could be correlated with variations in VFAs, and VFA concentrations under different thermal treatment conditions were determined to investigate the effect of hydrolysis and calculate the biochemical degradability (Fig. 2(f)). The hydrolysis of organic matter in KW directly leads to changes in VFA content; with the increase in thermal treatment temperature and heating time, the concentration of total organic acids also rises, similarly to SCOD and VSS, indicating an improvement in the degree of thermal hydrolysis. In addition, changes in VFA content were consistent with those in pH obtained in previous experiments after thermal treatment at increased temperature and heating time (Table 2). At a low pretreatment temperature (55e90 C), the VFA content gradually stabilised to a constant value when a heating time of 50 min was reached. With a further increase in temperature, the largest proportional increase in VFAs ranged from 2.5% to 9.5%; among these, the highest rate of increase in content was obtained at 160 C, nearly 140% higher than the original VFA content in KW when the thermal treatment temperature and time were 160 C and 120 min, respectively (Table 2). However, a higher total VFA content in the anaerobic digestion device was not necessarily representative of better biochemical quality because a higher amount of VFAs would easily result in a decrease in pH and gradually threaten the normal progression of the subsequent anaerobic reaction. According to the analysis of organics solubilisation effect during thermal treatment of KW at 120 C and 140 C, the intracellular and cell wall polymers (including polysaccharides, proteins, lipids, and other macromolecules) in KW are effectively released into the surrounding medium, becoming more available to microorganisms. Thus, inhibitory levels of ammonia, sulphide and long chain fatty acids, which are caused by high protein and fat content during anaerobic digestion, could be effectively alleviated. These optimal conditions can lead to 20e40% hydrolysis ratio of volatile solid (VS). 3.3. Effect of thermal treatment on anaerobic digestion 3.3.1. Anaerobic biodegradability The methane production potential under mesophilic conditions was assessed using BMP batch tests throughout a 30-day assay. Fig. 3 shows the evolution of the net accumulated methane production for KW without thermal pretreatment and for samples with thermal pretreatment at moderate temperature (55e90 C)
Fig. 3. Methane production (in mL/g-VSS) after 30 days of KW treatment for different treatment temperatures and for different treatment durations.
and high temperature (120e160 C), respectively, for durations of 70 min and 50 min. Both the methane production rate and the cumulative methane production of raw KW were the lowest compared with those of the pretreated KW, and the highest methane production of KW was 707.84 mL CH4/g-VSS, 5.44e12.90% and 3.74e7.67% less than the results obtained for the two above mentioned temperature ranges. The anaerobic digestion of KW without thermal treatment required a relatively longer time to complete than that of pretreated samples because large molecules and refractory organic matter were destroyed and the particle size decreased in advance at high temperature and pressure; thus, the proportion of biodegradable ingredients increased and the digestion process could be completed in less time. For example, if KW without pretreatment was directly inoculated according to the experimental method described in section 2.3, approximately 532 h would be required to produce 600 mL of methane, whereas this time could be reduced by 112e172 h and 39e60 h at moderate temperatures and high temperatures, respectively. Two distinct phenomena are indicated by the BMP tests of KW with thermal pretreatment. First, over the two temperature ranges mentioned above, as the thermal pretreatment temperature increased, a relatively higher methane production rate and methane yield could be obtained, whereas the total methane yield of KW with thermal pretreatment at moderate temperature over a period of 70 min was 12e38 mL greater than that of samples treated at high temperature for 50 min. Second, 50 h was identified as the turning point in methane production rate and methane yield for samples pretreated at 160 C þ 50 min before subsequent anaerobic digestion; the trend gradually became less distinct and the turning point then became 55e90 C þ 70 min. 3.3.2. Degradation of organic components after anaerobic digestion The effects of the degradation of various organic components
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299
Fig. 4. Organic component reduction after 30 days of anaerobic digestion of KW at different temperatures and durations of thermal pretreatment. (a) SOCD reduction; (b) VSS reduction; (c) Crude protein reduction; (d) Crude fat reduction.
(including SCOD, VSS, crude protein and crude fat) after 30 days of anaerobic digestion of KW at different thermal pretreatment temperatures were examined, and the results are shown in Fig. 4. 3.3.2.1. SCOD reduction. Higher temperatures and longer durations yielded better SCOD removal rates than lower temperatures and durations, which demonstrated that thermal pretreatment improves the biodegradability of particulate organics and that more volatile dissolved solids were degraded during anaerobic digestion. However, when the heating duration was longer than 50 min and 70 min at 140 and 160 C, respectively, the removal efficiency began to decline and decreased rapidly as the heating time increased further, on the other hand, the removal rates at other temperatures continued to increase with heating time (Fig. 4(a)). In particular, for a heating time of 120 min at 160 C, the removal rate was relatively close to the rates observed for samples without thermal pretreatment, whereas the SCOD reduction remained near 56% at 140 C when the heating duration ranged from 70 min to 120 min. Hence, the occurrence of the Maillard reaction to a certain extent hindered the further anaerobic biodegradation that appeared at 140 C and 160 C after 50 min and 30 min, respectively. 3.3.2.2. VSS degradation. The variation in VSS with the thermal pretreatment temperature and heating time was similar to that observed for SCOD (Fig. 4(b)), and a higher degradation rate and degree of degradation could be obtained at higher temperature and longer treatment duration. The removal rate of VSS was greatly
enhanced at thermal pretreatment temperatures in the range of 55e90 C when the treatment time was shorter than 50 min, compared with the removal rate observed for a treatment of 30 min when the heating temperature rose above 120 C; on the other hand, a smooth increase of approximately 1e5% was obtained when the heating durations at both temperatures were further prolonged and even reduced for temperatures of 120e160 C. 3.3.2.3. Crude fat degradation. As shown in Fig. 4(c), although the removal rate of crude fat was greater at a high thermal pretreatment temperature, it did not fluctuate significantly after a heating duration longer than 15 min; the removal rate increased by only 2e5%. The proportion of crude fat in samples after thermal pretreatment remained steady when the heating duration reached 15 min, and slight changes occurred when the heating time was further increased; thus, a better anaerobic degradation effect was obtained at lower fat content due to the inhibiting effect caused by the adsorption of crude fat on the membrane surface, preventing the substance transfer of bacteria by LCFA and wrapping microorganisms in undegraded oil after condensation, as indicated in Fig. 4(c). 3.3.2.4. Crude protein degradation. The reduction in crude protein after anaerobic biodegradation was also investigated, and the results are shown in Fig. 4(d). Compared with the 14.61% reduction observed in raw materials without thermal pretreatment, the reduction was effectively improved when numerous thermal processing approaches were introduced, and the maximum value was obtained at 160 C after a treatment duration of 30 min, which was
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twice that observed for the raw materials. The variations in crude protein were relatively smooth at lower temperature and could even be considered to be constant at a treatment temperature of 55 C. Thus, a low thermal treatment temperature is not conducive to accelerating the further degradation of crude protein during anaerobic digestion. The reduction in protein was strongly enhanced at temperatures above 90 C but only after a treatment time of 15 min. Hence, adequate solubilisation of both crude protein and crude fat in KW during thermal pretreatment could enhance the degradation rate of subsequent anaerobic digestion. However, for the sake of ensuring economic efficiency and to avoid loss of nutrition, the applied thermal pretreatment parameters should not exceed 50 min at 140 C or 30 min at 160 C, at which points the reaction of soluble carbohydrates with themselves or with soluble proteins (forming, e.g., Amadori compounds or melanoidins) begins to emerge; furthermore, the supernatant turns brown to a large extent after treatment. Cumulative methane production and methane yield rates were obtained for KW with thermal pretreatment were higher, especially at higher temperatures, such as 120 Ce160 C due to the intensification of liquefaction and organics hydrolysis in KW. However, thermal pretreatment with higher treatment temperatures has already achieved more organic reductions compared with lower treatment temperatures and KW without treatment, thus less nutrients were left to microorganisms for methane production. Therefore, KW with lower treatment temperature showed higher cumulative methane production whereas lower organic reduction after anaerobic digestion. These optimal conditions can lead to an 8%e13% methane production increase for KW. 4. Conclusions This study compared and analysed the physical and chemical properties and their effects on anaerobic biodegradability at moderate temperatures (70, 80 and 90 C) and high temperatures (120, 140 and 160 C) when thermal hydrolysis was applied as a pretreatment prior to anaerobic digestion of KW. Thermal pretreatment could not only markedly disintegrate organic and inorganic compounds efficiently but could also enhance the subsequent anaerobic digestion, especially for KW pretreated at a higher temperature and for a longer heating duration. Additionally, the methane production rate at moderate temperatures increased significantly and even exceeded that at high temperatures when the anaerobic digestion time surpassed 50 h. Acknowledgements This work was financially supported by China Twelfth Five-Year
plan of Scientific and Technical program (2009BAC64B06). References Adhikari, B.K., Barrington, S., 2006. Predicted growth of world urban food waste and methane production. Waste Manag. Res. 24 (5), 421e433. Amaral, A.L., Pereira, M.A., DaMotta, M., Pons, M.N., Mota, M., Ferreira, E.C., Alves, M.M., 2004. Development of image analysis techniques as a tool to detect and quantify morphological changes in anaerobic sludge: II application to a granule deterioration process triggered by contact with oleic acid. Biotechnol. Bioeng. 87 (2), 194e199. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21th ed. American Public Health Association, American Water Works Association. Water Environmental Federation, Washington, DC, New York, USA. ve, J., Dewil, R., 2008. Principles and potential of the Appels, L., Baeyens, J., Degre anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 34, 755e781. Aragon, C., Quiroga, J.M., Coello, M.D., 2009. Comparison of four chemical uncouplers for excess sludge reduction. Environ. Technol. 30, 707e714. Braun, R., Bracht, E., Grasmug, M., 2003. Codigestion of proteinaceous industrial waste. Appl. Biochem. Biotechnol. 109 (1e3), 139e153. Campbell, L., Raikos, V., Euston, S.R., 2005. Heat stability and emulsifying ability of whole egg and egg yolk as related to heat treatment. Food Hydrocoll. 19 (3), 533e539. Carvalheiro, F., Duarte, L.C., Gírio, F.M., 2008. Hemicellulose biorefineries: a review on biomass pretreatments. J. Sci. Ind. Res. 67, 849e864. Chapleau, N., De Lamballerie-Anton, M., 2003. Improvement of emulsifying properties of lupin proteins by high-pressure induced aggregation. Food Hydrocoll. 17, 273e280. China Statistical Yearbook, 2010. National Bureau of Statistics of China. Chowdhury, N., Lalman, J.A., Seth, R., Ndegwa, P., 2007. Biohydrogen production by mesophilic anaerobic fermentation of glucose in the presence of linoleic acid. J. Environ. Eng. e ASCE 133, 1145e1152. Clesceri, L.S., Greenberg, A.E., Eaton, A.D., 1998. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C. Climent, M., Ferrer, I., Baeza, M.D., Artola, A., Vazquez, F., Font, X., 2007. Effects of thermal and mechanical pre-treatments of secondary sludge on biogas production under thermophilic conditions. Chem. Eng. J. 133, 335e342. Khalid, A., Arshad, M., Anjum, M., Mahmood, T., Dawson, L., 2011. The anaerobic digestion of solid organic waste. Waste Manag. Oxf. 31, 1737e1744. Liu, G.Q., Liu, X.Y., Li, Y.Q., He, Y.F., Zhang, R.H., 2011. Influence of pH adjustment and inoculum on anaerobic digestion of kitchen waste for biogas producing. J. Biobaded. Mater. Bioenergy 5 (3), 390. Liu, X., Wang, W., Gao, X.B., Zhou, Y.J., Shen, R.J., 2012. Effect of thermal pretreatment on the physical and chemical properties of municipal biomass waste. Waste Manag. 32, 249e255. Messens, W., Van Camo, J., Huyghebaert, A., 1997. The use of high-pressure to modify the functionality of food proteins. Trends Food Sci. Technol. 8, 107e112. Methodenbuch III, Band, 1993. Die chemische Untersuchung von Futtermitteln. €nzungslieferung. VDLUFA, Verlag. Darmstadt/Verband Deutscher LandErga wirtschaftlicher Untersuchungs- und Forschungsanstalten, Methodenbuch Band III-Die Chemische Untersuchung Von Futtermitteln, VDLUFA, Darmstadt, Germany, 1997. Stabnikova, O., Liu, X.Y., Wang, J.Y., 2008. Digestion of frozen/thawed food waste in the hybrid anaerobic solideliquid system. Waste Manag. 28 (9), 1654e1659. Veeken, A., Kalyuzhnyi, S., Scharff, H., Hamelers, B., 2000. Effect of pH and VFA hydrolysis of organic waste. J. Environ. Eng. e ASCE 126 (12), 1076e1081. Wilson, C.A., Novak, J.T., 2009. Hydrolysis of macromolecular components of primary and secondary wastewater sludge by thermal hydrolytic pretreatment. Water Res. 43, 4489e4498.
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Influence of thermal pretreatment on physical and chemical properties of kitchen waste and the efficiency of anaerobic digestion Yiying Jin, Yangyang Li*, Jinhui Li School of Environment, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 1 June 2016
The effects of thermal pretreatment at moderate temperatures (70, 80 and 90 C) and high temperatures (120, 140 and 160 C) over heating durations of 10e120 min on the physical and chemical properties of kitchen waste and on anaerobic digestion were investigated. The results show that thermal pretreatment significantly enhances the solubilisation of organic compounds (chemical oxygen demand, crude proteins, crude fats and volatile fatty acids) and their biodegradability during subsequent anaerobic digestion. High temperature and long heating duration are beneficial for the release and reduction of organic compounds, and the efficiency of subsequent anaerobic digestion is improved markedly under these conditions. Moreover, both the methane production rate and methane yield were observed to increase significantly at moderate treatment temperatures when the anaerobic digestion time was longer than 50 h. © 2016 Published by Elsevier Ltd.
Keywords: Thermal pretreatment Anaerobic digestion Kitchen waste Biodegradability
1. Introduction Approximately 60 million tons of kitchen waste (KW) is produced annually (China Statistical Yearbook, 2010). Due to its characteristic high organic content, high moisture content, and susceptibility to rotting and breeding flies in the short term, normal municipal solid waste (MSW) disposal technologies cannot directly be applied to KW, and improper disposal of KW can easily cause secondary pollution, such as odour pollution and mosquito breeding. To comply with EHS (Environmental, Health, and Safety) regulations and policies, the final disposal of KW requires thorough sterilization and effective resource recycling (Adhikari and Barrington, 2006; Khalid et al., 2011). Alternative processing technologies, such as anaerobic digestion (AD), offer some potential for the recovery of valuable resources from organic wastes by producing biogas (Stabnikova et al., 2008; Carvalheiro et al., 2008) and have been widely used for the treatment of KW in recent years. However, KW is rich in carbohydrates, which can relatively easily be converted to volatile fatty acids, accumulating lactic acid at an early stage of the digestion process; this conversion can result in a dramatic pH drop if the anaerobic digestion system lacks sufficient buffering capacity (Veeken et al.,
* Corresponding author. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jenvman.2016.05.047 0301-4797/© 2016 Published by Elsevier Ltd.
2000). In addition, KW contains higher fat and protein contents than does MSW, which can lead to inhibitory levels of ammonia, sulphide and long-chain fatty acids (Braun et al., 2003; Amaral et al., 2004). Hence, the digestion and stability of the digestion process could be easily diminished and impeded. Numerous pretreatment methods, including chemical, biological and thermal processes, have been suggested to improve the properties of organic waste by promoting the solubilisation of particulate organics and biogas production and enhancing the digestion's rate-limiting step, i.e., the hydrolysis of organic matter. Among these methods, a number of thermal pretreatments, which disintegrate sludge cells under the application of various temperatures, ranging from 60 to 270 C for numerous durations, lasting from 15 min to several hours, have been studied (Liu et al., 2012; Climent et al., 2007). Thermal pretreatment has been proposed to improve organic waste properties and promote biogas production, which also has the effect of sterilization and sanitation. However, previous studies mostly have focused on the thermal hydrolysis of sewage sludge, and very few reports are available on the varying properties of KW (Chowdhury et al., 2007; Aragon et al., 2009). In addition, previous studies have mainly concentrated on direct anaerobic digestion and combined digestion with other biomass waste, such as sludge, by adjusting pH, mixing ratio, retention time and organic loading (Wilson and Novak, 2009; Appels et al., 2008; Liu et al., 2011). Furthermore, the high oil content of KW, which is a result of Chinese dietary customs, directly restricts the
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fermentation speed and operating cost when KW is directly used for anaerobic fermentation or composting. Hence, it is particularly important to investigate the effects of thermal pretreatment on the physical and chemical properties of KW, as well as the effects on subsequent anaerobic digestion. The objective of the current work was to evaluate the influence of thermal pretreatment on the physical and chemical properties of KW and determine the relationship between thermal hydrolysis and anaerobic biodegradability. To determine the influence of treatment on physical properties, pH, density and the characteristics of the three-phase boundary were discussed. The chemical properties include volatile solids, soluble chemical oxygen demand and nutritional components including crude fats, crude proteins and volatile fatty acids, which reflect solubilisation effects of organic compounds in KW. The influence of physical and chemical properties variations on subsequent anaerobic digestion of KW were determined by effects of organic biodegradation and production of methane. Thus, the effect of thermal pretreatment on the efficiency of subsequent anaerobic digestion was analysed and discussed, and more specific parameters of thermal pretreatment can be recommended. 2. Materials and methods 2.1. KW characteristics KW was collected on a weekly basis from a canteen at Tsinghua University. The major components were carbohydrates derived from bread, cooked noodles and rice; proteins and fat from different types of meat and fish; and various vegetables and fruits. The received KW was mixed in a kitchen blender and stored at 4 C in a refrigerator after being crushed into particles with an average size of 1e2 mm. Table 1 shows the basic characteristics of the KW. 2.2. Thermal pretreatment Thermal pretreatment was performed in a 1.5 L stainless steel hydrolysis reactor, which was constructed as a pressure vessel with a heating shell. The temperature was kept constant by controlling
Degree of reductionð%Þ ¼
2.3. Anaerobic digestion tests The digestion experiments were conducted in 1 L glass bottles at 35 C to determine the biodegradability of the raw and thermally treated materials. The seed sludge was obtained from a steadily operating digester at the Gaobeidian waste water treatment plant with 2 d of gravity sedimentation prior to inoculation. Each bottle was fed with a mixture of 80 g kW and 800 g seed sludge so that the effects of thermal pretreatment could be more pronounced. The digestion experiments were run for 30 days. In each experimental run, three control digesters were operated with thermally treated sludge. 2.4. Analytical methods The following components were analysed before and after thermal treatment: pH, density (r), total solids (TS), volatile solids (VS), soluble chemical oxidation demand (SCOD), carbohydrate concentration, protein concentration, crude fat concentration, total alkalinity (TALK) and concentration of VFAs. The TS and VS analyses were based on the Standard Analytical Methods developed by the National Environmental Protection Agency of China (1989). SCOD was determined according to the standard methods of the American Public Health Association (APHA, 2005). TALK (endpoint pH titration) and VFAs (distillation method) were measured according to procedures described in Clesceri et al. (1998). Characteristics of the three-phase boundary (including solid phase, liquid phase and floating oil) after thermal pretreatment were also analysed. The analyses were performed on KW after thermal pretreatment; this index could also be used to evaluate the effect of thermal hydrolysis. Three phases appeared from top to bottom after centrifugation at 5000 r/min for 5 min. Then, the mass of each phase was weighed. The concentrations of crude protein and crude fat were determined according to the Kjeldahl method, using a Soxhlet device (Methodenbuch, 1993). The reduction in the concentration of the organic components after anaerobic digestion was calculated by the following equation:
Concentration before AD Concentration after AD 100 Concentration before AD
the temperature of the thermal fluid circulating through the outer heating shell of the reactor. During the pretreatment, approximately 1 kg kW was transferred into the vessel, which had been preheated to a predetermined temperature. After hydrolysation at this temperature for a selected period, the heating process was stopped, and samples were then chilled by circulating 10 C water until the vessel cooled to room temperature. After pretreatment, the samples were stored at 4 C in a refrigerator to minimise the volatilisation of organic compounds while waiting for the next step of the analysis.
The statistical and correlation analysis (Pearson correlation) between treatment temperatures and physical and chemical properties were performed using the statistical software IBM SPSS version 20 for windows, and a p-value of <0.05 was considered statistically significant. 3. Results and discussion 3.1. Effect of thermal pretreatment on physical properties Thermal treatment was performed on KW at six different temperatures and over six different treatment durations. An overview of the results is shown in Fig. 1.
Table 1 Characteristic of the KW. Sample
pH
TS
C
H
N
S
O
KW
6.47
18.66%
46.11%
6.89%
3.19%
0.29%
37.80%
3.1.1. pH The measured pH values are presented in Fig. 1(a). With a pH value of 6.47, the KW was mildly acidic. After thermal pretreatment,
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Fig. 1. Variation in pH (a), phase proportions (b, c, d) and density (e) during thermal pretreatment at different temperatures and for different treatment durations.
the pH of the KW decreased significantly, and it decreased further by increasing the pretreatment temperature and the duration. Higher temperatures and longer durations increased the pH reduction rate because organic acids were continuously released into the liquid phase from the solid phase in the KW during treatment. Therefore, to some extent, the decrease in the pH reflected the extent of the thermal hydrolysis reaction. At 55e90 C, the decrease in pH was limited compared to that at higher temperatures. Even over a treatment time of 50 min, the pH was essentially stable in the range of 4.77e5.33. As the temperature and duration were further increased, the pH reduction rate increased. For example, at 120 C, the pH decreased to 4.71 when the thermal treatment lasted 50 min, an approximately 27.20% decrease in the original pH of the KW. When the treatment
temperature climbed to 160 C, the decrease in pH became more apparent. These results also suggested that both the rate and amount of organic acid release increased, resulting in a lower pH value.
3.1.2. Characteristics of three-phase boundary The organic matter in the KW was effectively hydrolysed into small molecules and inorganic matter after thermal pretreatment, which provided high temperature and high pressure. Additionally, bound water was released, and oil dissolved out of the KW. Therefore, after thermal pretreatment, the KW could be divided into three distinct layers from top to bottom, an oil, a liquid and a solid phase, after centrifugal treatment.
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3.1.2.1. Floating oil. The animal fat content of KW is high, and mainly in five states, floating oil, dispersed oil, emulsified oil, dissolved oil and oily solids, and oil removal is difficult to perform directly without any pretreatment. The content of floating oil served as an indicator of the KW de-oiling performance. The deoiling performance of oil-water mixtures is usually measured by the content of the floating oil. After thermal pretreatment and sequenced centrifugation, the removed oil that floats on the surface can be recycled, and the spare liquor is clear and transparent. Fig. 1(b) shows the increase in the proportion of floating oil with respect to different thermal pretreatment parameters. Apparently, raising the thermal treatment temperature and heating time was advantageous to the dissolution of floating oil. Untreated KW contained approximately 8.35% floating oil, and this proportion increased rapidly with a rise in temperature and treatment duration. The thermal pretreatment time and temperature affected the proportion of floating oil very little at 55e70 C and 50 min, with the proportion remaining similar at 9% and 9.4%, respectively. A continuous increase in the temperature of the steel hydrolysis reactor would have led to a continuous rise in the floating oil proportion. To achieve the same 10% scale, the heating time could be decreased by nearly 60 min at 120 C compared with the 120 min required at 90 C. The percentage of floating oil could be increased more quickly and noticeably when the heating temperature was enhanced. Above 120 C, the growth rate rose distinctly. However, the proportion of floating oil did not change greatly for a heating time greater than 50 min, remaining at 11% and 12%. Therefore, thermal hydrolysis can improve the oil content of KW, affecting the efficiency of waste oil removal. 3.1.2.2. Solid phase. The solid phase of KW mainly contains food residues from rice and flour, vegetables, meats, fats, etc., existing in the chemical forms of amylum, cellulose, proteins, lipids and inorganic salts, which are useful for subsequent utilisation, such as a for aerobic composting. However, these ingredients were gradually exposed to the dissolution process, and small-molecule organic matter was dissolved into the liquid phase and, in the case of some fats, into the floating oil, which triggered a reduction in the solid phase. During the thermal treatment process, the following reactions occur: first, fats from adipose tissue are broken down into glycerol and free fatty acids. Second, carbohydrates are broken down into three types of sugar, monosaccharides, disaccharides and polysaccharides. Third, most proteins are degraded to small peptides and amino acids. All of these hydrolysis processes will effectively promote the reduction in solid-phase quality. Fig. 1(c) presents the variations in the solid phase mass ratio; it was apparent that increasing the heating temperature and heating time enhanced the dissolution of the solid phase, resulting in a decrease in the solid-phase mass ratio. The higher the temperature and the longer the period for thermal treatment were, the faster the solids mass ratio decreased. The solid proportion was only decreased by 3.29% after treating at 55 C for 120 min, in contrast with reductions of 15.55% and 20.68% observed for treatment times of 30 min and 90 min, respectively, at 160 C. Thus, increasing the thermal treatment temperature and duration is advantageous to the rapid hydrolysis of large particulate matter and organic matter in the solid phase.
treatment process. The effects of heating temperature and time on the quality of the liquid phase in KW were determined, and the results are shown in Fig. 1(d). The proportion of the liquid phase was evidently improved after thermal pretreatment, and the rate of increase grew with the heating temperature and heating time due to the enhancement in the thermal hydrolysis rate of the solid phase in KW. Hence, the proportion of the liquid phase changed slightly at low temperature, e.g., the liquid phase increased by 2.5% and 4.5% at 55 C and 90 C, respectively, for a treatment duration of 120 min. When the heating temperature increased above 120 C, the liquid proportion increased significantly and stabilised at approximately 24% and 30% at 120 and 140 C, respectively, for a treatment duration of 50 min. When the temperature climbed further to 160 C, the liquid proportion continued to increase, and approximately 20 min of reaction time could be eliminated, while still achieving the same percentage as that at 120 C and a duration of 50 min. In general, the moisture distribution in KW could be effectively altered, thus improving the mechanical dewatering properties of KW. Combined with variations in the floating oil and solid phase, from the perspective of material balance analysis, the large reduction in the solid phase could be attributed to two sinks, a relatively small part of the floating oil and the main part of the liquid phase. In general, thermal treatment is beneficial for waste oil recycling from KW, especially at higher temperatures, such as 140 C and 160 C, thus enhancing oil separation from water in KW and promoting floating oil recycling, which could be used as raw materials for biodiesel. These optimal conditions can lead to a 29e40% increase. The dehydration performance and viscosity, which are useful parameters for designing and monitoring biological processes and determining the mode of agitation and energy consumption of bioreactors, could be effectively improved due to the higher efficiency of waste oil removal and liquefaction, making KW a very suitable substrate for subsequent anaerobic digestion. 3.1.3. Density Fats in KW are readily dissolved and are normally collected for recycling after thermal treatment. Due to their low density compared with that of the liquid phase and that of solid phase, after the oil is removed, the density of the remaining solid-liquid mixture would be altered. The densities of the tested materials after oil removal were analysed, and the results are shown in Fig. 1(e). Due to the presence of oils, the density of the raw KW was approximately 0.92 mg/L, slightly lower than that of water. When the KW was heated to relatively low temperatures (55e90 C), the density rose slightly due to the increased removal proportion of floating oil, but was still lower than that of water. When the heating temperature was higher than 120 C, the effects of lipid dissolution became more apparent, which led to a rapid rise in the floating oil content. At the same time, after degreasing, the density of the remaining material increased. After being heated for 50e70 min by thermal conditioning, the density tended to stabilise after degreasing to approximately 1.20 mg/L. Hence, raising the thermal treatment temperature and prolonging the heating time facilitated the dissolution of floating oil and subsequent oil collection and removal, which would be beneficial to achieving higher density. 3.2. Effects of thermal pretreatment on solubilisation
3.1.2.3. Liquid phase. Due to its high moisture content and migration-flow characteristics, KW is particularly difficult to treat, and more environmental pollution would be produced without timely and effective supervision and disposal, compared with ordinary household waste. Hence, dehydration is crucial to the KW
The effects of thermal pretreatment at different treatment temperatures and for different durations on the solubilisation of organic compounds were investigated, and the results are shown in Fig. 2.
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Fig. 2. Solubilisation of organic compounds during thermal pretreatment at different temperatures and for different treatment durations. (a) SCOD; (b) SS; (c) VSS; (d) Crude protein in solid phase; (e) Crude fat in solid phase; (f) VFA.
3.2.1. SCOD SCOD can be used to characterise the concentration of soluble organic matter, which consists mainly of dissolved organic matter. Hence, SCOD changes, to a certain extent, can indicate digestion performance. The release of organics is due to the disruption of chemical bonds by thermal treatment, including degradation and thermal hydrolysis of polysaccharides, proteins, and volatile fatty acids (VFAs) from KW. It was apparent that the SCOD content varied greatly when increasing the heating temperature and duration in all tests. The higher the temperature was, the faster the rate of SCOD formation rate became. The SCOD results are presented in Fig. 2(a). It could be concluded that at a lower temperature and shorter duration under thermal treatment, the macromolecular
organic matter in KW was not effectively disintegrated, nor could it be transformed into small molecules that would be released into the liquid phase and result in an increase in SCOD. At 55e90 C, the increase in SCOD was not distinct, and the value tended to stabilise at a heating time of 30 min; the maximum degrees of hydrolysis observed were 8.01%, 12.60% and 22.70% at 55, 70 and 90 C respectively. The degrees of liquefaction and hydrolysis of organics were relatively weak. When the heating temperature rose to 120e160 C and the reaction time was extended, both the values of SCOD and the rate of increase were enhanced significantly compared to the results obtained below 100 C, and they tended to stabilise when the heating time was longer than 50 min. The highest hydrolysis effect was achieved at 160 C and 120 min; the SCOD value was approximately
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188,970 mg/L, and the degree of hydrolysis increased by nearly 56% compared with that of the original sample. The increase in SCOD demonstrated that thermal pretreatment was beneficial for improving the digestion performance of KW, whether it may be subsequently applied as feed or fertiliser. Additionally, the SCOD increase suggests that some organic nutrients were transformed from the solid phase into the liquid phase, which would increase the possibility of producing liquid organic fertiliser from the KW. However, excessively high temperature and prolonged extension of thermal treatment would decrease the amount of soluble organic matter and change the trophic structure in the solid phase after processing; thus, fewer nutrients would remain in the product after dehydration and drying. 3.2.2. SS & VSS After thermal conditioning and subsequent centrifugation, the liquid phase and floating oil in the KW were poured out, and the remaining solid phase was used to detect SS and VSS. The effects of thermal treatment on SS and VSS are shown in Fig. 2(b) and (c). It is apparent from the three-phase interface and change in SCOD that the proportion of SS in the solid phase decreased with increasing reaction temperature and reaction time. The proportion of SS varied slightly at 55 C with different thermal treatment durations, and less than 2% was added when the heating temperature rose to 90 C. Once KW was heated to above 100 C, the SS ratio decreased rapidly, and the lowest SS was achieved at 160 C and 90 min, with a reduction of nearly 38% compared with the SS of raw KW. Three main causes resulted in the loss of SS during thermal processing. First, some organic matter was converted to dissolved compounds, accompanied by the reactions of hydrolysis and liquefaction, thus transforming from the solid phase into the liquid phase. Second, the evaporation of volatile organic compounds in the solid phase tended to cause this phenomenon. Third, carbon dioxide and ammonia gas were generated and released from the hydrolysis of partial organic matter. Among these three causes, the first process served as the most important factor for the reduction of SS in the solid phase, which could also be concluded from the changing trends of SCOD (Table 2). The VSS content in the solid phase after thermal pretreatment decreased with the heating temperature and duration in all tests. The SS changes could serve as the explanation for the observed trends of VSS, which were analysed descriptively and comparatively. The thermal instability of solid components would lead to their gradual decomposition with increasing temperature and heating time, causing a reduction in VSS content. The VSS content was altered slightly at 55e90 C when the heating time reached 50 min; less than a 0.5% increase was obtained, and the rate increased rapidly as the heating temperature and duration rose further. At 160 C, particularly when the heating time was 90 min, VSS was reduced to 3.92%. 3.2.3. Protein solubilisation Proteins were solubilised due to thermal treatment. Fig. 2(d) shows the protein concentration in the solid phase. The results indicate that the proportion of crude protein in the solid phase was reduced by 1e2.41% after thermal processing, compared with 24.62% for the untreated samples. It could also be concluded that at temperatures of 55e120 C, increasing the processing temperature would lead to protein denaturation and coagulation, especially in the form of animal protein, reducing the proportion of protein in the solid phase. Thermal treatment can further affect the interaction and hydration of water molecules and the side chains of amino acids and peptide bonds, first by forming bound and adjacent water and then by forming a multilayer of water. With rising temperature
and heating duration, crude protein would experience further hydration and gradually became a colloidal solution due to waterswelling. The opposite trend was observed when increasing the heating duration at temperatures of 140e160 C, which could be explained by the exposure of hydrophobic groups in proteins at high pressure and even the irreversible encapsulation of hydrophilic proteins by the gel formed by the proteins when the pressure continued to increase (Campbell et al., 2005; Chapleau and De LamballerieAnton, 2003). Additionally, non-protein sulphydryl (NPSH) was mainly affected by non-covalent bonds between protein molecules, which were significantly affected by high pressure. As the thermal treatment temperature and processing duration increased, the content of NPSH decreased remarkably, thus resulting in the augmentation of molecular polymerisation due to hydrophobic interaction and once again promoting the formation of disulphide bonds (Messens et al., 1997). In addition, after the thermal treatment of KW at 140 C and 160 C for 50 min and 30 min, respectively, the colour of the material became black and the Maillard reaction occurred. These changes resulted in a loss of nutrients due to the combination of amino acids and sugars, the product of which could not easily be digested and absorbed fully, even leading to the production of recalcitrant soluble organics or toxic/inhibitory intermediates and hence reducing the biodegradability (Wilson and Novak, 2009). 3.2.4. Crude fat solubilisation Normally, the lipid content in KW is high, and KW is readily subject to deterioration and rancidity because of its unsaturated fatty acids, which cause numerous adverse effects, such as inhibiting the activity of digestion bacteria and methanogens; such effects may even lead to anaerobic digestion failure by preventing substance transfer caused by high levels of long chain fatty acids (LCFA) adsorbed on the membrane surface and the wrapping of microorganisms by undegraded oil after condensation, when conducting anaerobic digestion. With thermal pretreatment, the dissolution of lipids is accelerated and lipid reactions are more complex, mainly involving hydrolysis and oxidation reactions, producing monoglycerides and fatty acids, thereby the digestibility is improved, which also reduces the stability of oil, thus accelerating rancidity. The crude fat content in the solid phase at different temperatures over given heating times are shown in Fig. 2 (e). The results showed that the crude fat content in the solid phase first increased and then decreased with an increase in the heating time but was still less than that of raw KW. The higher the heating temperature became, the earlier the decline in crude fat appeared. Taking 55 C for example, the crude fat content began to drop after rising upon reaching a heating duration of 90 min, nearly 20 min longer than at 90 C. When the temperature rose above 100 C to 120, 140 and 160 C, the turning points were all approximately 50 min. Overall, a relatively lower crude fat content was achieved at 90 C, and higher contents were observed at 140 C compared with those recorded at other temperatures and heating times. During the early stages of thermal treatment, at low heating temperature, some of the free fatty acids were solidified, and the coagulation was separated from the liquid phase and entered the solid phase, resulting in an increase in the fat content of the solid phase. Upon further raising the heating temperature and prolonging the heating time, partial solid fats began to dissolve and decompose into the liquid phase, resulting in fat loss in the solid phase; moreover some of the volatile lipid compounds volatised into the gas phase and drained, leading to lower fat content in the solid phase of the
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297
Table 2 Pearson correlation analysis between treatment temperatures and physical and chemical properties. (* Correlation is significant at the 0.05 level (2-tailed). ** Correlation is significant at the 0.01 level (2-tailed).)
T ¼ 55 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 70 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 90 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 120 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 140 C T pH Floating oil Soild phase Liquid phase Density SCOD SS VSS Crude protein Crude fat VFA T ¼ 160 C T pH Floating oil Soild phase Liquid phase Density
T
pH
Floating oil
Soild phase
Liquid phase
Density
SCOD
SS
VSS
Crude protein
Crude fat
VFA
1 0.784* 0.627 0.548 0.395 0.843* 0.684 0.949** 0.902** 0.699 0.857* 0.857*
1 0.665 0.458 0.596 0.812* 0.471 0.921** 0.801* 0.588 0.750 0.750
1 0.928** 0.010 0.937** 0.545 0.755* 0.807* 0.370 0.899** 0.899**
1 0.218 0.845* 0.354 0.626 0.741 0.110 0.768* 0.768*
1 0.217 0.128 0.463 0.369 0.388 0.207 0.207
1 0.606 0.928** 0.953** 0.557 0.955** 0.955**
1 0.604 0.510 0.831* 0.808* 0.808*
1 0.952** 0.663 0.890** 0.890**
1 0.565 0.883** 0.883**
1 0.684 0.684
1 1.000**
1
1 0.685 0.553 0.929** 0.953** 0.316 0.978** 0.338 0.901** 0.987** 0.738 0.912**
1 0.747 0.841* 0.789* 0.307 0.569 0.666 0.899** 0.729 0.607 0.813*
1 0.676 0.545 0.306 0.538 0.764* 0.812* 0.623 0.884** 0.657
1 0.953** 0.538 0.854* 0.619 0.965** 0.907** 0.706 0.988**
1 0.369 0.872* 0.454 0.922** 0.937** 0.629 0.950**
1 0.232 0.567 0.382 0.211 0.250 0.608
1 0.247 0.839* 0.971** 0.779* 0.827*
1 0.657 0.336 0.450 0.600
1 0.920** 0.808* 0.941**
1 0.795* 0.882**
1 0.701
1
1 0.847* 0.893** 0.940** 0.883** 0.810* 0.948** 0.553 0.851* 0.849* 0.660 0.889**
1 0.807* 0.941** 0.948** 0.593 0.882** 0.689 0.994** 0.928** 0.914** 0.955**
1 0.875** 0.805* 0.739 0.862* 0.750 0.835* 0.882** 0.722 0.817*
1 0.978** 0.662 0.944** 0.727 0.948** 0.961** 0.793* 0.988**
1 0.618 0.870* 0.665 0.939** 0.926** 0.788* 0.972**
1 0.613 0.195 0.563 0.515 0.300 0.563
1 0.702 0.908** 0.919** 0.797* 0.934**
1 0.754 0.882** 0.770* 0.760*
1 0.957** 0.933** 0.965**
1 0.873* 0.971**
1 0.838*
1
1 0.560 0.943** 0.695 0.533 0.629 0.837* 0.896** 0.797* 0.507 0.757* 0.932**
1 0.712 0.926** 0.926** 0.992** 0.892** 0.769* 0.913** 0.936** 0.945** 0.787*
1 0.742 0.595 0.778* 0.878** 0.887** 0.908** 0.616 0.860* 0.952**
1 0.973** 0.932** 0.957** 0.885** 0.911** 0.879** 0.958** 0.858*
1 0.908** 0.874* 0.767* 0.831* 0.868* 0.900** 0.730
1 0.927** 0.805* 0.940** 0.917** 0.964** 0.844*
1 0.937** 0.941** 0.850* 0.959** 0.968**
1 0.925** 0.785* 0.915** 0.936**
1 0.845* 0.990** 0.911**
1 0.876** 0.740
1 0.900**
1
1 0.603 0.840* 0.888** 0.874* 0.566 0.431 0.910** 0.568 0.594 0.677 0.963**
1 0.807* 0.869* 0.876** 0.990** 0.924** 0.844* 0.941** 0.924** 0.955** 0.758*
1 0.968** 0.958** 0.766* 0.797* 0.926** 0.856* 0.902** 0.902** 0.878**
1 0.997** 0.833* 0.774* 0.983** 0.872* 0.877** 0.929** 0.949**
1 0.834* 0.776* 0.981** 0.882** 0.874* 0.927** 0.939**
1 0.908** 0.817* 0.910** 0.911** 0.949** 0.739
1 0.690 0.960** 0.939** 0.917** 0.571
1 0.800* 0.837* 0.896** 0.980**
1 0.923** 0.960** 0.697
1 0.958** 0.731
1 0.818*
1
1 0.733 0.737 0.901** 0.913** 0.554
1 0.987** 0.940** 0.918** 0.965**
1 0.923** 0.899** 0.928**
1 0.997** 0.849*
1 0.820*
1 (continued on next page)
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Table 2 (continued )
SCOD SS VSS Crude protein Crude fat VFA
T
pH
Floating oil
Soild phase
Liquid phase
Density
SCOD
SS
VSS
Crude protein
Crude fat
VFA
0.255 0.897** 0.504 0.523 0.660 0.901**
0.378 0.874* 0.926** 0.947** 0.980** 0.954**
0.405 0.882** 0.912** 0.902** 0.962** 0.945**
0.127 0.952** 0.811* 0.834* 0.901** 0.987**
0.091 0.949** 0.783* 0.805* 0.878** 0.976**
0.510 0.755* 0.954** 0.996** 0.969** 0.856*
1 0.128 0.630 0.532 0.424 0.111
1 0.796* 0.746 0.857* 0.945**
1 0.962** 0.959** 0.805*
1 0.960** 0.831*
1 0.913**
1
product. In addition, the hydrolysis of some lipid compounds at higher temperature resulted in the volatilisation of small amounts of volatile fatty acids in the liquid, promoting the reduction in fat content (Table 2). 3.2.5. VFAs Volatile fatty acids (VFAs) are fatty acids with a carbon chain composed of six carbons or fewer; these fatty acids are usually referred to as short-chain fatty acids (SCFA) and include acetic acid (C2), propionic acid (C3), butyric acid (C4) and valeric acid (C5). The degree of hydrolysis of organic matter in KW and the biodegradability of the products after thermal treatment could be correlated with variations in VFAs, and VFA concentrations under different thermal treatment conditions were determined to investigate the effect of hydrolysis and calculate the biochemical degradability (Fig. 2(f)). The hydrolysis of organic matter in KW directly leads to changes in VFA content; with the increase in thermal treatment temperature and heating time, the concentration of total organic acids also rises, similarly to SCOD and VSS, indicating an improvement in the degree of thermal hydrolysis. In addition, changes in VFA content were consistent with those in pH obtained in previous experiments after thermal treatment at increased temperature and heating time (Table 2). At a low pretreatment temperature (55e90 C), the VFA content gradually stabilised to a constant value when a heating time of 50 min was reached. With a further increase in temperature, the largest proportional increase in VFAs ranged from 2.5% to 9.5%; among these, the highest rate of increase in content was obtained at 160 C, nearly 140% higher than the original VFA content in KW when the thermal treatment temperature and time were 160 C and 120 min, respectively (Table 2). However, a higher total VFA content in the anaerobic digestion device was not necessarily representative of better biochemical quality because a higher amount of VFAs would easily result in a decrease in pH and gradually threaten the normal progression of the subsequent anaerobic reaction. According to the analysis of organics solubilisation effect during thermal treatment of KW at 120 C and 140 C, the intracellular and cell wall polymers (including polysaccharides, proteins, lipids, and other macromolecules) in KW are effectively released into the surrounding medium, becoming more available to microorganisms. Thus, inhibitory levels of ammonia, sulphide and long chain fatty acids, which are caused by high protein and fat content during anaerobic digestion, could be effectively alleviated. These optimal conditions can lead to 20e40% hydrolysis ratio of volatile solid (VS). 3.3. Effect of thermal treatment on anaerobic digestion 3.3.1. Anaerobic biodegradability The methane production potential under mesophilic conditions was assessed using BMP batch tests throughout a 30-day assay. Fig. 3 shows the evolution of the net accumulated methane production for KW without thermal pretreatment and for samples with thermal pretreatment at moderate temperature (55e90 C)
Fig. 3. Methane production (in mL/g-VSS) after 30 days of KW treatment for different treatment temperatures and for different treatment durations.
and high temperature (120e160 C), respectively, for durations of 70 min and 50 min. Both the methane production rate and the cumulative methane production of raw KW were the lowest compared with those of the pretreated KW, and the highest methane production of KW was 707.84 mL CH4/g-VSS, 5.44e12.90% and 3.74e7.67% less than the results obtained for the two above mentioned temperature ranges. The anaerobic digestion of KW without thermal treatment required a relatively longer time to complete than that of pretreated samples because large molecules and refractory organic matter were destroyed and the particle size decreased in advance at high temperature and pressure; thus, the proportion of biodegradable ingredients increased and the digestion process could be completed in less time. For example, if KW without pretreatment was directly inoculated according to the experimental method described in section 2.3, approximately 532 h would be required to produce 600 mL of methane, whereas this time could be reduced by 112e172 h and 39e60 h at moderate temperatures and high temperatures, respectively. Two distinct phenomena are indicated by the BMP tests of KW with thermal pretreatment. First, over the two temperature ranges mentioned above, as the thermal pretreatment temperature increased, a relatively higher methane production rate and methane yield could be obtained, whereas the total methane yield of KW with thermal pretreatment at moderate temperature over a period of 70 min was 12e38 mL greater than that of samples treated at high temperature for 50 min. Second, 50 h was identified as the turning point in methane production rate and methane yield for samples pretreated at 160 C þ 50 min before subsequent anaerobic digestion; the trend gradually became less distinct and the turning point then became 55e90 C þ 70 min. 3.3.2. Degradation of organic components after anaerobic digestion The effects of the degradation of various organic components
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299
Fig. 4. Organic component reduction after 30 days of anaerobic digestion of KW at different temperatures and durations of thermal pretreatment. (a) SOCD reduction; (b) VSS reduction; (c) Crude protein reduction; (d) Crude fat reduction.
(including SCOD, VSS, crude protein and crude fat) after 30 days of anaerobic digestion of KW at different thermal pretreatment temperatures were examined, and the results are shown in Fig. 4. 3.3.2.1. SCOD reduction. Higher temperatures and longer durations yielded better SCOD removal rates than lower temperatures and durations, which demonstrated that thermal pretreatment improves the biodegradability of particulate organics and that more volatile dissolved solids were degraded during anaerobic digestion. However, when the heating duration was longer than 50 min and 70 min at 140 and 160 C, respectively, the removal efficiency began to decline and decreased rapidly as the heating time increased further, on the other hand, the removal rates at other temperatures continued to increase with heating time (Fig. 4(a)). In particular, for a heating time of 120 min at 160 C, the removal rate was relatively close to the rates observed for samples without thermal pretreatment, whereas the SCOD reduction remained near 56% at 140 C when the heating duration ranged from 70 min to 120 min. Hence, the occurrence of the Maillard reaction to a certain extent hindered the further anaerobic biodegradation that appeared at 140 C and 160 C after 50 min and 30 min, respectively. 3.3.2.2. VSS degradation. The variation in VSS with the thermal pretreatment temperature and heating time was similar to that observed for SCOD (Fig. 4(b)), and a higher degradation rate and degree of degradation could be obtained at higher temperature and longer treatment duration. The removal rate of VSS was greatly
enhanced at thermal pretreatment temperatures in the range of 55e90 C when the treatment time was shorter than 50 min, compared with the removal rate observed for a treatment of 30 min when the heating temperature rose above 120 C; on the other hand, a smooth increase of approximately 1e5% was obtained when the heating durations at both temperatures were further prolonged and even reduced for temperatures of 120e160 C. 3.3.2.3. Crude fat degradation. As shown in Fig. 4(c), although the removal rate of crude fat was greater at a high thermal pretreatment temperature, it did not fluctuate significantly after a heating duration longer than 15 min; the removal rate increased by only 2e5%. The proportion of crude fat in samples after thermal pretreatment remained steady when the heating duration reached 15 min, and slight changes occurred when the heating time was further increased; thus, a better anaerobic degradation effect was obtained at lower fat content due to the inhibiting effect caused by the adsorption of crude fat on the membrane surface, preventing the substance transfer of bacteria by LCFA and wrapping microorganisms in undegraded oil after condensation, as indicated in Fig. 4(c). 3.3.2.4. Crude protein degradation. The reduction in crude protein after anaerobic biodegradation was also investigated, and the results are shown in Fig. 4(d). Compared with the 14.61% reduction observed in raw materials without thermal pretreatment, the reduction was effectively improved when numerous thermal processing approaches were introduced, and the maximum value was obtained at 160 C after a treatment duration of 30 min, which was
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twice that observed for the raw materials. The variations in crude protein were relatively smooth at lower temperature and could even be considered to be constant at a treatment temperature of 55 C. Thus, a low thermal treatment temperature is not conducive to accelerating the further degradation of crude protein during anaerobic digestion. The reduction in protein was strongly enhanced at temperatures above 90 C but only after a treatment time of 15 min. Hence, adequate solubilisation of both crude protein and crude fat in KW during thermal pretreatment could enhance the degradation rate of subsequent anaerobic digestion. However, for the sake of ensuring economic efficiency and to avoid loss of nutrition, the applied thermal pretreatment parameters should not exceed 50 min at 140 C or 30 min at 160 C, at which points the reaction of soluble carbohydrates with themselves or with soluble proteins (forming, e.g., Amadori compounds or melanoidins) begins to emerge; furthermore, the supernatant turns brown to a large extent after treatment. Cumulative methane production and methane yield rates were obtained for KW with thermal pretreatment were higher, especially at higher temperatures, such as 120 Ce160 C due to the intensification of liquefaction and organics hydrolysis in KW. However, thermal pretreatment with higher treatment temperatures has already achieved more organic reductions compared with lower treatment temperatures and KW without treatment, thus less nutrients were left to microorganisms for methane production. Therefore, KW with lower treatment temperature showed higher cumulative methane production whereas lower organic reduction after anaerobic digestion. These optimal conditions can lead to an 8%e13% methane production increase for KW. 4. Conclusions This study compared and analysed the physical and chemical properties and their effects on anaerobic biodegradability at moderate temperatures (70, 80 and 90 C) and high temperatures (120, 140 and 160 C) when thermal hydrolysis was applied as a pretreatment prior to anaerobic digestion of KW. Thermal pretreatment could not only markedly disintegrate organic and inorganic compounds efficiently but could also enhance the subsequent anaerobic digestion, especially for KW pretreated at a higher temperature and for a longer heating duration. Additionally, the methane production rate at moderate temperatures increased significantly and even exceeded that at high temperatures when the anaerobic digestion time surpassed 50 h. Acknowledgements This work was financially supported by China Twelfth Five-Year
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