Enhanced split-phase resource utilization of kitchen waste by thermal pre-treatment

Energy 98 (2016) 155e167

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Enhanced split-phase resource utilization of kitchen waste by thermal pre-treatment Yangyang Li, Yiying Jin*, 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: Received 16 April 2015 Received in revised form 27 October 2015 Accepted 8 January 2016 Available online 6 February 2016

Our society currently faces the twin challenges of resource reclamation from rapidly escalating KW (kitchen waste) and increasingly expensive depletion costs and restrictive disposal legislation due to environmental impacts and fast depleting global resources necessitate action. This work studied the influence of thermal hydrolysis on the utilization of KW based on the principle of split phase processing, including solid phase for pig feed, liquid phase for anaerobic digestion and floating oil for biodiesel. It shows that the solid phase of KW after thermal treatment could satisfy the nutrition content requirements as raw materials for pig feed. The efficiency of the subsequent anaerobic digestion of liquid phase increased for KW pretreated at 120
C and higher methane production and soluble chemical oxygen demand reduction were achieved after a pretreatment time of 40 min. Composition analysis of floating oil during thermal hydrolysis indicates that unsaturated fatty acid accounts for more than 61% and the main ingredients are monounsaturated fatty acid (more than 36%). All parameters important for biodiesel quality except the acid value could satisfy the biodiesel requirements according to the European standard. From overall analysis, the thermal pre-treatment was profitable with output value of $ 57.52 ton1 KW. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Thermal hydrolysis Kitchen waste Feed Biodiesel Anaerobic digestion

1. Introduction More than 30 million tons of KW (kitchen wastes) are produced in China every year. Approximately 80% of the collected KW has been directly utilized as feedstuff in pig farms in China, which is facing strict restrictions by China's Ministry of Agriculture due to concerns of foot and mouth disease, and raw materials for illegal extraction of hogwash oil, which is unsanitary and can cause serious illness. In addition, the universal concern on environmental

Abbreviations: KW, kitchen waste; AD, anaerobic digestion; CF, crude fiber; CP, crude protein; EE, ether extract; Ash, crude ash; NFE, nitrogen free extract; SCOD, soluble chemical oxygen demand; GE, gross energy; DE, digestible energy; ME, metabolizable energy; NE, net energy; AA, Amino acids; DAA, dispensable AA; IDAA, indispensable AA; Ala, alanine; Asp, Aspartic acid; Glu, Glutamic acid; Gly, Glycine; Pro, Proline; Ser, Serine; Tyr, Tyrosine; Thr, Threonine; Val, Valine; ILe, Isoleucine; Leu, Leucine; Phe, Phenylalanine; His, Histidine; Lys, Lysine; Arg, Arginine; VFA, volatile fatty acids; BMP, biochemical methane potential; FA, fatty acids; DU, degree of unsaturation; LCSF, long chain saturated factor; SFA, saturated fatty acids; UFA, unsaturated fatty acids; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids. * Corresponding author. Tel.: þ86 10 627 943 52. ** Corresponding author. Tel.: þ86 10 627 943 52. E-mail addresses: [email protected] (Y. Jin), [email protected] (J. Li). http://dx.doi.org/10.1016/j.energy.2016.01.013 0360-5442/© 2016 Elsevier Ltd. All rights reserved.

protection, resource utilization and food safety has brought increasing research on KW processing technology. Due to high moisture and salt content, there are two potential problems on the incineration of KW, including extra energy consumption, and generation and release of toxic pollutants to the environment. Such negative aspects coupled with high oil content, KW does not favor the compost process [1], thus restricts their application in fertilizer utilization. In addition, AD (anaerobic digestion) can be used to convert organics into biogas for energy recovery and achieve waste reduction [2,3]. When KW is treated anaerobically as a single substrate, common problems would appear during conventional AD because of their high oil content and the presence of macromolecular compounds, including the accumulation of lactic acid at an early stage of the digestion process resulting in a sudden pH drop and inhibitory levels of ammonia, sulphide and long-chain fatty acids due to the high protein and fat content [4]. These factors usually impede digestion stability, thus restricting the application of AD. Besides, the lack of efficient technology for disposal of biogas residues, the secondary pollutant during anaerobic digestion, also limits the application of anaerobic digestion in the recycling of KW. In the technical aspect, numerous pretreatment methods and new process techniques, for example co-digestion with substrates

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containing high levels of ammonium nitrogen and alkalinity have been proposed to improve the physical and chemical properties of organic waste to enhance the solubilisation of organic particulates and promote biogas production [5,6]. Among these methods, thermal treatment has been demonstrated to be an effective method to provide better phase separation, higher hydrolysis rate of complex particulate organic substrates and apparent sterilization effects, thus improving the subsequent disposal, such as enhancing the efficiency of anaerobic digestion process [7]. In addition, thermal pretreatment of KW could meet these requirements of guarantying the effect of disinfection sterilization, avoid the premise of exogenous pollution and improve the recovery rate of useful resource [8], such as nutrients and lipids when using as pig and other animal feed due to the enormous demand [9,10], and could also enhance the production of waste edible oils, which could not only be used as the raw oil for biodiesel production [11], but also effectively alleviate biological inhibiting reactions induced by the high concentrations of oil and grease in KW [12]. According to the policy perspective on promoting the recycling application and resource saving of KW, developing resource-saving and environment-friendly society as well as circular economy and protecting the ecological environment in both China (FAGAIHUANZI [2010] No. 1020) and other countries due to environmental impacts and fast depleting global resources necessitate action [13e15], it is required especially in China that the construction of pilot projects should be conducted overall planning and combinational optimization to enhance resource-oriented utilization and harmless treatment of three phases in KW, including oil, solid and liquid phase (FAGAIHUANZI [2010] No. 1020). Because of policy encouragement, environmental concern and economic incentives by local and central governments in China, more diverse methods after thermal pretreatment should be developed as the amount of KW production increases rapidly. Besides, basing on the increasing universal concern on safety, energy and environmental preservation, finding proper and effective disposal methods of KW for energy production and solution for KW treatment and valorization, enhancing biogas production and reducing the amount of final residue is extremely important [16,17]. In addition, basing on technological limitations and knowledge-based processing and efficient and cost-competitive ways converting KW into valuable products, the objective of this study is to estimate and analyze the impacts of the split-phase processing with thermal hydrolysis of KW. In this regard, this paper aims at a comprehensive study on the solubilisation effects of organic compounds in three phases (solid, liquid and oil phase) of KW, and the recovery rate of floating oil and nutritional components, such as protein and amino acid, during different thermal pretreatments. Furthermore, the potential of floating oil to produce biodiesel, solid phase to produce pig feed after thermal drying process, and liquid phase to produce biogas under mesophilic anaerobic digestion condition (35
C) from KW is evaluated. This study is unique in that it proposes a split-phase processing method to maximize the recycling utilization of KW and analyzes process that produces resources comprehensively from KW, while comparison of results in this study highlights the significant impact of thermal hydrolysis degrees. Results and conclusions presented are intended to contribute to processing knowledge of KW in the areas of biomethane, pig feed and biodiesel production, waste management and related policy. 2. Materials and methods 2.1. Kitchen waste The characteristics of KW (kitchen waste) are closely relevant to local living standards, eating habits, etc. In particular, according to

investigations carried out in some cities in China, it could be found that food waste and bones accounted for more than 90% in KW, while the other proportions were mainly paper, plastic, wood, metal, etc. And the characteristics of KW include high moisture content (70%e87%), high organic content (80%e93%, dry basis), and the content of oil ranged from 2% to 3% with the salt content as approximately 1%. Moreover, the volatile solid content in KW was also pretty high (75%e90%, dry basis) and some nutrient content, such as crude protein and crude fat, showed no significant difference from the typical feed, i.e. soybean, corn and etc. Table 1(a) shows the basic characteristics of KW and the characteristics of KW collected from a canteen in Tsinghua University were conformed to the current situation of basic components of KW in China. Specifically, the major components were carbohydrates derived from bread, cooked noodles and rice; proteins and fat from different types of meat and fish. The KW was mixed with a kitchen blender to ensure uniform and representative experimental materials. It was then crushed into particles with an average size of 1e2 mm and stored at 4
C in a refrigerator. 2.2. Reactors and processing method A thermal hydrolysis reactor was used to perform the pretreatment of KW; a semi-continuously operating continuouslystirred flow tank reactor was used to study the effect of thermal Table 1 (a). Characteristics of the KW. (b). Characteristics of the instruments used in this study. (a) Parameters

KW

pH Total solids (%) Volatile solids (%, wet basis) Proteins (%, wet basis) Lipids (%, wet basis) Carbon (%, dry basis) Hydrogen (%, dry basis) Oxygen (%, dry basis) Nitrogen (%, dry basis) Sulfur (%, dry basis)

6.50 ± 0.2 18.7 ± 1.2 17.5 ± 1.4 2.8 ± 0.2 3.7% ± 0.3 46.1 ± 1.6 6.9 ± 0.2 37.8 ± 1.6 3.2 ± 0.4 0.3 ± 0.01

(b) Item Thermal hydrolysis reactor

Specification

Effective volume: 20 L Highest pressure: 2.0 MPa Highest temperature: 220
C Three-phase separator Power: 220 V, 50 HZ Max RCF: 4650 g Highest speed: 5500 rpm Rotating accuracy: ±50 rpm Duration: 0e99 min Transesterification Power: 220 V, 50 Hz system Temperature range: RT e 100
C Temperature accuracy: ±1
C Thermal drying system Power: 220 V, 50 HZ Temperature range: RT e 250
C Temperature accuracy: ±1
C Anaerobic digester Power: 220 V, 50e60 Hz Temperature range: RT e 100
C Temperature accuracy: ±0.5
C Working volume: 250 mL

Parameter in this study 90e140
C

5000 rpm

Moisture removal: 105
C Reaction time, 1 h Reaction temperature: 60
C Temperature: 105
C

Temperature: 35
C Feed: Inoculum ¼ 1:5 (volume)

Y. Li et al. / Energy 98 (2016) 155e167

pretreatment on the digestion of liquid phase in KW; while a thermal drying system was utilized to dry the solid phase in KW (Fig. 1). 2.2.1. Thermal hydrolysis Thermal hydrolysis pretreatment was performed in a 20 L stainless steel hydrolysis reactor, which was constructed as a pressure vessel with a heating shell. Approximately 1 kg kW was transferred into the vessel and preheated at 90, 100, 120 and 140
C over heating durations of 10, 30, 40, 50 and 60 min respectively. 2.2.2. Centrifugation and three-phase separation After hydrolysis at a certain temperature for a selected time, KW samples were cooled down to room temperature by circulating 10
C water. Then some of the KW samples were centrifuged at 2950  G (5000 r/min) for 5 min, and as a consequence, it could be divided into three distinct layers from top to bottom, e.g., oil, liquid and solid phase. Finally, all the three phases were taken out for subsequent experiments and analysis. 2.2.3. Anaerobic digestion Batch anaerobic digestion tests were carried out in 250 mL glass bottles at 35
C to measure the biogas production of different liquid products obtained after the thermal pretreatment and centrifugal separation. The seed sludge was obtained from a steady-operation digester in a waste water treatment plant after two-day gravity sedimentation prior to inoculation. Furthermore, the feed and inoculums were put into the bottles with volume as 30 mL and 150 mL respectively. Then the upper space of each reactor was flushed with nitrogen for at least 1 min to guarantee the anaerobic conditions and then sealed quickly. Moreover, in each experimental run, three control digesters were operated. And at the same time, two blank digesters containing inoculums only were incubated to correct for the biogas yield from the inoculums. Then the volume of biogas produced during anaerobic digestion was read from the graduated cylinders filled with saturated salt water and connected with the bottles according to rubber hosepipe. Particularly, all assays were shaken by hand twice and the volume of gas produced was recorded twice per day. 2.2.4. Thermal drying system After thermal hydrolysis and subsequent three phase separation, the solid phase was transferred into the thermal drying system. Prior to the drying step, samples were sliced and heated at 110 ± 5
C for 2.5 h in the thermal drying equipment to ensure uniform temperature in each part of the system. In this experiment, materials were grinded and then analyzed for the contents of CF (crude fiber), CP (crude protein), EE (ether extract), Ash (crude ash) and NFE (nitrogen free extract) after the above drying process. 2.2.5. Experimental procedure of biodiesel Biodiesel is made from renewable biological sources such as vegetable oils and animal fats, and is biodegradable and non-toxic with low emission profiles compared to petroleum diesel. Moreover, the floating oil was applied to produce biodiesel which was obtained from KW treated at 120
C and 40 min. In addition, transesterification experiments were carried out in a 200 mL four-necked glass reactor with a reflux condenser to reduce the loss of methanol, an electric stirrer to keep materials completely mixing, and a thermometer to control reaction temperature. Moreover, two stoppers were used for the raw oil feeding, sampling and analyzing respectively. Firstly, the floating oil was preheated at 105
C to remove the moisture and then appropriate amounts of methanol with sodium hydroxide catalyst were placed into a reactor equipped with heater

157

and stirrer. Then, the reacting mixture was blended for 30 min at 60
C. Consequently, the reactor was cooled to 10
C quickly in cold water and after a certain time of gravity settling, the ester at the top layer could be separated from the glycerol at the down layer. Additionally, the ester layer was removed, and the glycerol was further purified and thus obtained as a valuable product. And the following experimental procedure was referred as described in Ref. [18]. 2.3. Analytical methods The pH values and SCOD (soluble chemical oxygen demand) were determined in well mixed samples in triplicate following standard methods [19]. The floating oil content was analyzed by weighing the mass of waste oil on the top layer of liquid after centrifugation at 5000 r/min for 5 min. Total Kjeldahl nitrogen was determined according to the Kjeldahl method [20]. Materials were dried at 105
C until the dry material content reached 87.50% and grinded before the analyses of CF, CP, EE, Ash and NFE, which were performed following methods described by the Association of Analytical Communities [21]. The concentrations of GE (gross energy), DE (digestible energy), ME (metabolizable energy) and NE (net energy) were determined according to methods proposed by Refs. [22e24]. Amino acid content was determined as described by Ref. [25]. The testing methods of biodiesel were added in Section 2.4 Analytical methods. And the analytical methods of biodiesel properties were referred as described in Ref. [18]. 3. Results and discussion 3.1. Influence of thermal hydrolysis on chemical compositions of solid phase 3.1.1. Nutritional components in solid phase Nutrition contents such as CF (crude fiber), CP (crude protein), EE (ether extract), crude ash, etc are normally considered as the prerequisite for application of raw materials in the process of feed mixing and production. When the solid phase in KW is used to produce animal feed, the content of these indicators should be analyzed. The change of these nutritional components at different thermal treatment temperatures and heating durations are shown in Fig. 2. Ether extract with high energy content could ensure energy supply when KW is utilized as raw materials of animal feed. As shown in Fig. 2 (a), thermal pretreatment can accelerate the dissolution of lipids, the process of which mainly includes hydrolysis and oxidation reactions, producing monoglycerides and fatty acids. The results showed that EE content in the solid phase of raw KW was 17.39%, which decreased with increasing heating time. Increasing the treatment temperature could promote fast initial decline of EE content. Taking 140
C for example, the EE content dropped rapidly during a heating duration of 10 min. Thermal treatment of KW could promote the hydrolysis of intracellular and cell wall polymers (including polysaccharides, proteins, lipids, and other macromolecules) in KW and release into the surrounding medium, thus, leading to the dissolution of fat in the solid phase. In addition, higher treatment temperatures at 120 and 140
C could achieve higher EE contents due to the solidification of some free fatty acids and the transfer of coagulation into solid phase from the liquid phase. Upon prolonging the heating time, partial solid fats in the solid phase began to dissolve and decompose into the liquid phase, resulting in the reduction in oil and fat content.

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(a)

Oil phase

6

Biodiesel

Solid phase

7

Pig feed

Liquid phase

8

Biogas

2

5

3

1 4

Blender Motor

T

P

Pressure relief valve

Controller Heating layer

Discharging gate

(b) Blender Motor

Water in Condenser Water out Sampling

Thermostatic water bath

(c)

Fig. 1. Experimental setup of the system with thermal pre-treatment.1 Thermal hydrolysis reactor (a); 2 Stirrer; 3 Temperature controller; 4 Pump; 5 Three-phase separator; 6 Transesterification system (b); 7 Thermal drying system; 8 Anaerobic digester (c).

Y. Li et al. / Energy 98 (2016) 155e167

Fig. 2(b) shows in general an increase of crude protein content in the solid phase after thermal processing, approximately 7e11% increase for treatment at 90e120
C and 3e5% increase for treatment at 140
C compared with 19.39% of the raw samples. At temperatures of 90e120
C, increasing the processing temperature would lead to molecular polymerization due to hydrophobic groups of proteins at high pressure, or irreversible encapsulation of hydrophilic proteins by gel formed when the pressure continued to increase [27], thus increase the proportion of protein in the solid phase. With increasing temperature and heating duration, CP would experience further hydration and gradually became a colloidal solution due to waterswelling. In addition, thermal pretreatment could reduce the proportion of EE in solid waste, thus the proportion of CP in solid waste is reduced. Fig. 2(c) showed that crude fiber content in the solid phase increased first but subsequently decreased with the increase of heating time, and was in general a little higher than that of the raw KW. The higher the heating temperature, the earlier the decline of crude fiber content appeared. Taking 90
C for example, the CF content began to drop after a heating duration of 50 min, nearly 20 min later than that at 100
C and 140
C. When the temperature is 120
C, the turning point was not obvious compared with that at other treatment temperatures. The increase of CF was caused by the change of ether extract and crude protein contents, and partly due to the thermal decomposition of cellulose, hemi-cellulose and other components of CF, such as levoglucosan [28]. Crude ash is not nutrition of the feed. Higher content of crude ash in feed means lower quality and the crude ash content index is used to prevent the addition of mineral materials with low prices but no nutrition function purposely, such as zeolite powder and bentonite. Fig. 2(d) shows the crude ash concentration in the solid phase. The crude ash concentration in the samples could be considered constant when treated at 90
C, 100
C and 120
C, ranging from 5% to 6%. This implies that organic compounds are not degraded significantly during low temperature treatment. The concentration increased apparently at 140
C, but only after a treatment time of 30 min, and a 38% increase of crude ash concentration was observed after treatment for 50 min. It suggests that the volatile solid content in the solid phase of KW decreased with the heating duration. The thermal instability of solid components would lead to their gradual decomposition with increasing heating time, causing a reduction in volatile solid content and an increase in crude ash content. NFE (Nitrogen free extract) mainly consists of soluble carbohydrates including amylum, monosaccharide, disaccharides etc, which are easy to digest and absorb. High content of NFE is beneficial for the animal feed to achieve better palatability and digestibility. Fig. 2(e) shows a decrease of NFE concentration in the solid phase after thermal treatment and drying. This phenomenon could be explained by the solubilisation of ether extract, carbohydrates and proteins in KW. The reduction of ether extract in solid phase accounts for the main reason due to lower solubilisation efficiency of carbohydrates and proteins at low treatment temperatures. Lower NFE was obtained when treated at 100
C and 120
C compared with that at 90
C, due to the large hydrolysis degree of organics at higher treatment temperatures. However, the NFE at 140
C was the highest among all treatment temperatures though it fell down to the bottom value (approximately 40%) at a treatment time of 30 min, after which, NFE began to increase again to about 42%, approximately 1% less than the raw KW due to the pronouncedly release of carbohydrates and proteins. 3.1.2. Variations of energy content Energy is generated from the oxidation of organic compounds, such as carbohydrates, lipids, and proteins in feed ingredients [29].

159

Available energy in feeds for pigs has been characterized as DE (digestible energy), ME (metabolizable energy) or NE (net energy) by considering sequential energy losses during digestion and metabolism from GE (gross energy) in feeds [30]. GE, DE, ME and NE of the solid phase (TS% ¼ 87.5%) of KW after thermal treatment were calculated to evaluate the pig feed quality (Fig. 3). The GE concentration of a feed ingredient is dependent on the proportions of carbohydrate, fat and protein present. Fig. 3(a) shows the concentration of GE with different thermal treatment temperatures and durations. It can be seen that longer heating duration would lead to the decrease of GE and this trend was alleviated after 30 min. A significant decrease of GE was observed at 90
C and 140
C for all treatment durations, which is approximately 6% less than that of untreated KW. The decrease of GE was slower at 100
C and 120
C, as clearly can be seen in Fig. 3(a), but still reached a low concentration of 5000e5100 kcal/kg after 30 min. The decrease is due to the degradation of ether extract, crude protein, and volatile solids during thermal treatment. Moreover, differences in nutrient levels under different treatment temperatures and durations were in general relatively low. Coefficients of ether extract, crude protein and crude ash played significant roles in the calculation of GE values [GE ¼ 4143 þ (56  EE) þ (15  CP)(44  Ash)]. The coefficient of ether extract is the largest compared with those of crude protein and crude ash, which are 73% and 21% less. DE values are available for most of the commonly used feeds [31]. It has been suggested that DE is preferred to be used to describe the energy requirements of pig and the energy content of pig feeds, because DE is easily and precisely determined and is additive in principle. Chemical composition of feed ingredients is a major determinant of DE, with positive effects of ether extract and negative effects of crude fiber and Ash [22]. As shown in Fig. 3(b), there is a slight increase of DE prior to a continuous decrease after 10 min treatment at 90
C, 100
C and 120
C, while a general decrease occurred at 140
C. Furthermore, fluctuation of DE values appeared after 40 min due to the combined effects of the concentration of EE, CP, CF and crude ash. The energy requirement for maintenance in pigs has typically been calculated as the ME requirement for maintenance. Fig. 3(c) shows a continuous reduction of ME concentration at temperatures of 90
C, 100
C and 140
C, and slight increase at 120
C in 10 min prior to the decrease process. Compared with untreated KW, ME treated at 120
C are the highest and a reduction of less than 1% is obtained. Due to the high concentration of protein, ME decreases because the amino acids not used for protein synthesis are catabolized and used as a source of energy, and nitrogen is excreted as urea [31]. Therefore, as the nitrogen content of the urine increases, the energy losses in urine increase and the ME of the diet decreases. In addition, the increasing use of high fiber ingredients for diets fed to pigs would increase the importance of gaseous energy losses when ME is calculated, because increased dietary fiber results in increased hindgut fermentation and increased synthesis and loss of methane. Thus, the ME of diets varies with the quantities and characteristics of dietary fiber. Net energy is the energy that the animal uses for maintenance and production. It can usually be considered as energy values of ingredients and diets that most closely describe the available energy to animals, because it takes the heat increment from digestive utilization and metabolism of feeds into account, which is considered to be the best indication of the energy available to an animal for maintenance and production [24]. As shown in Fig. 3(d), changes of net energy at different treatment temperatures show a similar trend. The ratio of NE to ME

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Y. Li et al. / Energy 98 (2016) 155e167

(a)

(c)

(b)

(d)

(e) Fig. 2. Content of EE (a), CP (b), CF (c), crude ash (d)and NFE (e)in solid phase of KWafter thermal treatment and thermal drying (TS% ¼ 87.5%).

represents the energy efficiency of ME for maintenance and production, and thus depends on the particular biochemical pathways for the purpose of utilization (e.g., maintenance vs. energy retention) in animals [32]. Theoretical values for NE/ME have been proposed for energy efficiencies of digestible nutrients for adenosine triphosphate production and lipid retention. The ratios

between NE and ME are all above 63%, varying from 63.0% to 64.8%, which are all higher than that of soybean meal (53e60%) [29]. Less than 3% reduction in NE/ME ratio was achieved after treatment at different temperatures compared with raw KW, while samples treated at 120
C show higher ratios than other treatment temperatures.

Y. Li et al. / Energy 98 (2016) 155e167

(a)

(b)

161

(c)

(d)

Fig. 3. Changes of GE (a), DE (b), ME (c) and NE (d)of the solid phase of KW after thermal treatment and thermal drying (TS% ¼ 87.5%).

3.1.3. Compositions of amino acids AA (Amino acids), including nutritionally DAA (dispensable AA) which can be synthesized by animals and nutritionally IDAA (indispensable AA) whose carbon skeletons cannot be formed by animals, are crucial for animal growth, development, reproduction, lactation and health. For mammals, DAA include alanine, arginine (except carnivores, ferrets, minks, and young animals), asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine and tyrosine, while IDAA mainly consist of histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan andvaline. The objective of this section was to study and determine composition of DAA (including Ala (alanine), Asp (Aspartic acid), Glu (Glutamic acid), Gly (Glycine), Pro (Proline), Ser (Serine) and Tyr (Tyrosine)) in feed ingredients of animal diets for comparison with IDAA content (including Thr (Threonine), Val (Valine), ILe (Isoleucine), Leu (Leucine), Phe (Phenylalanine), His (Histidine), Lys (Lysine) and Arg (Arginine)). Table 2 shows an increase in AA content appeared at three different treatment temperatures (90, 100 and 120
C); At 90
C, the increase in AA content was more apparent compared with the other two treatment temperatures, while the content of AA began to decrease after 30 min of treatment at 140
C. Over 20% increase was achieved at 90
C, 100
C and 120
C while a 1%~18% increase was obtained at 140
C compared with raw materials. Meanwhile, higher treatment temperatures and longer treatment durations gave lower content of AA. This could be explained by the

solubilisation of crude protein, which is a measure of the amino acid content in the feed, and the change tendency of the content of AA is in accordance with CP. In addition, DAA were the main components in AA, accounting for about 55%~58%. Thus, the IDAA concentrations were relatively low, which needed extra adding. The main components of both DAA and IDAA are shown as in Table 2. It can be concluded that Leu, Arg and Val are the three main ingredients in IDAA at different treatment temperatures. Their contents are 8.3e9.3%, 6.0e7.2% and 5.1e7.1% respectively; while Glu (16.5e19.7%) and Asp (9.4e10.4%) are the main components for DAA. It could also be seen that thermal treatment with reasonable temperatures and durations is beneficial for raising the content of some ingredients such as Ser and Tyr in DAA, Val and Phe in IDAA, while decreasing others, such as Arg in IDAA and Gly in DAA. Thus, thermal hydrolysis could be used as an efficient way to regulate the nutrition level. 3.1.4. Comparison of feed quality The requirements (listed in Table 3(a) and (b)) are both China and USA committees’ best estimates of minimum and maximum requirements of young weanling pigs (from 3 to 20 kg body weight), and of growing-finishing pigs (from 20 to 120 kg body weight). It is obvious that the standards performed in China are less strict than those in the United States, such as lower limitation values and more control index. Moreover, taking dietary amino acid

4.25 162.5 10.7 42.6 5.2 2.5 5.0 9.1 3.6 6.1 4.6 6.4 57.8 6.8 9.9 19.2 6.2 6.2 5.2 4.3 19.4 4.35 161.4 10.5 44.0 5.5 2.4 5.3 9.3 3.9 6.2 4.6 6.9 56.4 6.7 10.0 18.5 5.8 5.7 4.9 4.7 19.8 4.46 157.8 10.2 45.1 5.9 2.5 5.4 9.4 4.3 6.3 4.6 6.9 55.3 6.6 10.1 17.9 5.5 5.4 4.9 4.9 20.6 4.68 139.9 9.8 45.0 6.2 2.5 5.3 9.3 4.5 6.0 4.6 6.7 55.3 6.5 10.1 18.0 5.4 5.6 5.1 4.5 21.2 4.50 151.6 10.3 45.9 6.4 2.5 5.1 9.2 5.1 6.3 4.6 6.7 54.5 6.7 10.2 16.5 5.7 5.7 4.7 5.0 22.4 4.55 151.0 10.2 45.3 6.4 2.5 5.0 9.1 4.6 6.4 4.6 6.8 55.2 6.8 10.3 16.5 6.0 5.7 4.8 5.1 22.4 4.61 150.1 10.1 44.8 6.5 2.5 4.9 9.1 4.6 6.0 4.6 6.6 55.7 6.8 10.3 16.7 6.0 5.8 4.8 5.2 21.4 4.65 147.8 9.7 44.8 6.5 2.5 4.8 9.0 4.7 6.0 4.6 6.6 55.6 6.8 10.4 16.8 6.0 5.7 4.9 5.1 21.8 4.86 134.3 9.2 44.7 6.7 2.5 5.0 9.0 4.4 6.0 4.3 6.8 55.7 7.0 10.3 16.6 6.6 5.9 4.4 5.0 19.4 4.62 147.5 10.1 44.1 6.5 2.5 4.9 9.1 3.9 6.4 4.1 6.7 56.4 6.6 9.6 18.5 5.9 6.5 4.5 4.8 21.6 4.74 147.4 9.9 44.6 6.5 2.4 4.9 9.3 3.9 6.3 4.1 7.2 55.9 6.6 9.4 17.8 6.3 6.7 4.4 4.7 21.9 4.87 147.3 9.7 43.2 6.6 2.4 4.7 8.9 3.8 5.9 4.2 6.7 57.3 6.7 9.5 18.2 6.6 6.8 4.6 4.8 21.8 4.71 145.0 9.6 42.6 6.8 2.5 4.7 8.6 4.5 5.5 4.4 5.7 57.5 6.3 10.0 19.1 6.5 6.6 5.0 4.2 23.0

c

a

b

Measured on liquid phase of KW. Measured on oil phase of KW. Measured on dry weight basis of the solid phase in KW.

4.84 144.7 9.5 43.0 6.7 2.4 4.8 8.7 4.3 5.6 4.5 6.1 57.2 6.2 9.9 19.3 6.0 6.6 5.1 4.1 22.9 5.05 144.0 9.3 43.1 6.5 2.5 4.8 8.8 4.3 5.6 4.4 6.1 57.1 6.2 9.9 19.5 5.8 6.5 5.1 4.0 22.4 5.45 121.2 8.5 42.1 7.1 2.4 4.6 8.3 4.5 5.0 4.2 6.0 57.9 6.8 10.3 18.5 7.5 6.5 4.8 3.4 17.8 pH SCOD,a  103 mg/L Floating oil,b g/kg KW Indispensable AA,c % Arg,c % His,c % Ile,c % Leu,c % Lys,c % Phe,c % Thr,c % Val,c % Dispensable AA,c % Ala,c % Asp,c % Glu,c % Gly,c % Pro,c % Ser,c % Tyr,c % All AA,c %

50 min 40 min

5.12 133.5 9.0 42.6 6.5 2.4 4.6 8.7 3.8 6.0 4.2 6.4 57.9 6.7 9.5 18.1 6.8 7.0 4.8 5.0 19.9 5.15 141.3 9.2 42.7 6.5 2.5 4.7 8.8 4.2 5.6 4.5 6.1 57.4 6.1 10.0 19.6 5.9 6.6 5.1 4.0 22.2 5.24 131.3 8.9 43.5 6.7 2.5 4.9 8.7 4.5 5.6 4.3 6.3 56.6 6.2 9.9 19.2 5.9 6.6 4.8 4.1 22.2

4.93 145.4 9.5 43.2 6.5 2.4 4.6 8.8 3.8 6.3 4.1 6.6 57.3 6.7 9.6 18.2 6.6 6.7 4.6 4.9 21.2

50 min 40 min 30 min 10 min 10 min

a

60 min 30 min 10 min

30 min

40 min

50 min

60 min

10 min

30 min

40 min

50 min

60 min

T ¼ 140
C T ¼ 120
C T ¼ 100
C T ¼ 90
C Blank Parameter

Table 2 Concentration and degradation of the organic components after thermal pretreatment.

4.21 164.2 10.9 41.5 5.1 2.2 4.8 9.0 3.5 5.6 4.8 6.5 58.8 6.6 10.4 19.7 6.9 6.0 5.1 4.3 18.1

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60 min

162

requirements for example, 12 kinds of amino acids are required to be constrained in NRC (1998) [33] compared unfortunately with just 3 respectively Lysine, Methionine and Threonine in Chinese standard GB/T 5915-2008 [34] for a complete and balanced diet. Overall, when the solid phase of KW with a dry material content of 87.5% is used as the raw materials of pig feed, the four indexes above could meet the nutrition requirements in Chinese standard GB/T 5915e2008 [34]. Furthermore the contents of ether extract in solid phase of KW after thermal treatment are 4e6 times higher than the value of the lowest requirement in Chinese standard set for young weanling pigs, while 2.5e4 times higher than requirements in NIH-2004 [35]. The crude protein contents present a 10e50% increase compared with standard values in Chinese standard set for young weanling pigs. Moreover, the content of crude protein in KW treated at 90e120
C within 10e60 min is 1%~18% higher than the required values in NRC (1998) [33], while materials treated at 140
C within 10e60 min could meet nutritional requirements set for pigs with weight of more than 10 kg in NRC (1998) [33]. Meanwhile, for the content of crude fiber and crude ash, they could meet the limitation values set for growing-finishing pigs (more than 20 kg) and they are both 2% less than the maximum values set by the national standard in China. In particular, the content of crude fiber after KW treatment ranges from 2% to 4.7%, which is less than the limited value (6%) in NIH e 2004 [35]. Thus, with regard to the nutrition content, the solid phase of KW after thermal treatment could meet the relevant requirements as raw materials for pig feed. Moreover, the thermal process can effectively provide high temperature as well as pressure required for the sterilization [9] [36]. According to the nutrition requirements in Chinese standard GB/T 5915e2008 [34], the minimum concentration requirements of two kinds of DAA, i.e., Lys and Thr, are 0.6e1.4% and 0.5e0.9% respectively, which are normally deficient in raw materials of Chinese animal feed. And as shown in Table 2, the concentration of Lys ranges from 3.53% to 5.08%, with that of Thr ranging from 4.09% to 4.76%, which are both much better than the values in national standards and NIH-2004 [35]. In addition, methionine had not been detected in raw and thermal treated KW, which could be conducted by adding the methionine hydroxy analogue in feeds according to Chinese standard. Concentrations of other components in AA were all higher than the limitation values in NRC (1998) [33]. Thermal pretreatment with higher treatment temperatures is beneficial for the release of organics, for example proteins, lipids, and fibers in KW into the surrounding medium, thus becoming more available to microorganisms, such as at 120
C and 140
C. In addition, it could promote more organic reductions compared with lower treatment temperatures and KW without treatment, therefore more nutrients were released into the liquid phase and less were left in the solid phase used for providing energy as feed. Hence, KW with lower treatment temperature, such as 120
C, showed higher energy content in solid phase. 3.2. Influence of thermal hydrolysis on chemical properties of liquid phase 3.2.1. pH of liquid phase Raw KW was mildly acidic with a pH value of 5.45. After thermal pretreatment, the pH of the liquid phase of KW decreased significantly with increasing temperature and duration (Table 2). At 90
C, the decrease in pH was limited compared to that at higher temperatures. Higher temperatures and longer durations led to fast pH reduction rate because organic acids were continuously released into the liquid phase from the solid phase during treatment, which resulted in an increase of VFA (volatile fatty acids).

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163

Table 3 (a). Contents of primary nutritional compositions in feeds for starter and growing-finishing pigs (GB/T 5915-2008). (b). Dietary nutrition requirements of Growing pigs (90% dry matter). (a) Parameter

Limit value

Body weight (kg) 3e10

10e20

20e40

40e70

70e120

CP(%) EE (%) CF(%) Crude ash (%) Lysine (%) Methionine (%) Threonine (%)

Minimum Minimum Maximum Maximum Minimum Minimum Minimum

18 2.5 4.0 7.0 1.35 0.40 0.86

17 2.5 5.0 7.0 1.15 0.30 0.75

15 1.5 7.0 8.0 0.90 0.24 0.58

14 1.5 7.0 8.0 0.75 0.22 0.50

13 1.5 8.0 9.0 0.60 0.19 0.45

(b) Parameter

Limit value

Body weight (kg)

Standard

3e5

5e10

10e20

20e50

50e80

80e120

DE (kcal/kg) ME (kcal/kg) CP/% Lysine (%) Methionine (%) Threonine (%) EE/% CF/%

Minimum Minimum Minimum Minimum Minimum Minimum Minimum Maximum

3400 3265 26.0 1.34 0.36 0.84 4.0 6.0

3400 3265 23.7 1.19 0.32 0.74 4.0 6.0

3400 3265 20.9 1.01 0.27 0.63 4.0 6.0

3400 3265 18.0 0.83 0.22 0.52 4.0 6.0

3400 3265 15.5 0.66 0.18 0.43 4.0 6.0

3400 3265 13.2 0.52 0.14 0.34 4.0 6.0

Therefore, to some extent, the decrease of the pH reflected the degree of the thermal hydrolysis reaction. 3.2.2. SCOD of liquid phase Table 2 shows an increase of SCOD at all treatment temperatures: it is apparent that the SCOD content changed more significantly when increasing the treatment temperature and duration. The higher the temperature, the faster the SCOD formation rate. The increase in SCOD was not obvious at low treatment temperatures, and the values tended to stabilize after 30 min. The maximum hydrolysis degrees at 90
C, 100
C and 120
C were 19.6%, 21.6% and 25.0% respectively compared with raw KW. At 140
C, the amount of liquefaction and the hydrolysis of organics were significant, although they tended to stabilize after 40 min; the highest hydrolysis effect was achieved after 60 min, where the SCOD value was approximately 1,64,000 mg/L. The degree of hydrolysis increased by nearly 35.4% compared with the original sample. The release of organics is due to the disruption of chemical bonds in polysaccharides, proteins, and VFA from KW by thermal treatment. Higher temperatures and longer durations were beneficial for both the disintegration of the macromolecular organic matter in KW and the release of small molecules into the liquid phase, thus result in an increase of SCOD. 3.2.3. Anaerobic biodegradability The methane production potential under mesophilic conditions was assessed using BMP (biochemical methane potential) batch tests during a 30-day assay. Fig. 4 shows the evolution of the net accumulated methane (calculated at STP (standard temperature and pressure)) from the liquid phase of KW treated at 120
C after different pretreatment time, while Table 4 presents the results of cumulative methane yields, methane production rate constant and SCOD removal rate. Both the SCOD removal rates and the cumulative methane production from liquid phase were lower after 10 min treatment compared with other treatment durations. Samples treated at 40 min obtained the highest methane production (149 mL CH4/mL-liquid), which was 34.8% and 8.8% more than the amount obtained after 10 min and 30 min treatment; while 4.9%

NRC-1998

NIH-2004

and 24.0% less methane were produced when treated with 50 min and 60 min. SCOD can be used to characterize the concentration of soluble organic matter, which consists mainly of dissolved organic matter. Hence, SCOD changes can somehow indicate digestion performance. As shown in Table 4, increasing the treatment durations could give higher SCOD removal rates, although a slight decrease appeared after 40 min. This demonstrates that thermal pretreatment was beneficial for improving the digestion performance of the liquid phase in KW. More than 56% of SCOD was removed after anaerobic digestion of the liquid phase in KW when treated at 120
C after 30e40 min. The decrease in both cumulative methane production and SCOD removal rate when samples were treated for 50 and 60 min at 120
C could probably be explained by the occurrence of Maillard reaction, because the color of the liquid phase turned black. Maillard reaction would result in a loss of nutrients due to the combination of amino acids and sugars, the

Fig. 4. Cumulative methane production (at STP, in mL/mL-liquid) from the liquid phase after 30 days for different treatment durations at 120
C.

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Y. Li et al. / Energy 98 (2016) 155e167

Table 4 Methane production and SCOD removal of the liquid phase in KW treated at 120
C after 30 days BMP tests. Samples

Cumulative methane yield (mL CH4/mL)

Production rate (d1)

SCOD removal (%)

10 30 40 50 60

112 152 168 161 129

0.179 0.194 0.195 0.196 0.198

36.3 53.7 57.4 56.2 48.4

min min min min min

product of which could not easily be digested and absorbed, or even lead to the production of recalcitrant soluble organics or toxic/ inhibitory intermediates, hence reduce the biodegradability. Table 4 also shows that the methane production rate constants increased with the increase of thermal pretreatment durations at 120
C, ranging from 0.179 d1 at 10 min to 0.198 d1 at 60 min. The production rates of samples pretreated at 30, 40 and 50 min are comparable. This could be explained by the continuous solubilisation and hydrolysis of macromolecular compounds (such as proteins, grease and oil) with the increase of thermal pretreatment durations, under the condition of which, shorter time is required for microbial adaptation to the new circumstance and the organic degradation in the case of the same microbial inoculation amount, thus improve the digestion performance. Higher accumulated methane production and methane yield rates were obtained for KW with thermal pretreatment, especially at longer treatment durations due to the intensification of liquefaction and hydrolysis of organic proportions in KW [37]. When the thermal pretreatment duration was more than 40e50 min at 120
C, the occurrence of Maillard reaction could lead to low cumulative methane production and SCOD reduction after anaerobic digestion.

3.3. Influence of thermal hydrolysis on floating oil 3.3.1. Content of floating oil Oil and grease content are high in KW, existing mainly in five states, i.e., floating oil, dispersed oil, emulsified oil, dissolved oil and oily solids. Oil removal is difficult to perform directly without any pretreatment. The content of the floating oil could be used as an indicator of the de-oiling performance of KW. After thermal pretreatment and subsequent centrifugation, the removed oil that floats on the surface of liquid phase can be recycled. Table 2 shows an increase of floating oil when increasing the treatment temperature and time. Raw KW contains approximately 8.51% floating oil. To achieve the same 17.5% increase of floating oil compared with raw KW, the heating time could be decreased by almost 40 min at 140
C compared with the 50 min required at 100
C.

3.3.2. Properties of floating oil There are three main types of FA (fatty acids) present in a triglyceride: saturated (Cn: 0), monounsaturated (Cn: 1) and polyunsaturated with two or three double bonds (Cn: 2, 3). The percentage of these compounds for each vegetable oil is given in Table 5. Two parameters based on the type of fatty acids were defined: DU (degree of unsaturation) and LCSF (long chain saturated factor), which are shown in Table 5. Degree of unsaturation parameter was obtained from the empirical Eq. (1), taking into account the amount of monounsaturated and polyunsaturated fatty acids (wt. %) present in the vegetable oil.

DU ¼ (monounsaturated Cn: 1; wt. %) þ 2  (polyunsaturated Cn: 2, 3, wt. %) (1) Long Chain Saturated Factor parameter was obtained from the empirical Eq. (2), taking into account the composition of saturated fatty acids and weighing more on the composition of fatty acids with long chains. LCSF (B) ¼ 0.1  C16 (wt. %) þ 0.5  C18 (wt. %) þ 1  C20 (wt. %) þ 1.5  C22 (wt. %) þ 2  C24 (wt. %) (2) For all treatment temperatures and durations, the main fatty acids in floating oil are palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1) and linoleic acid (C18:2) (Table 5), and the contents of SFAs (saturated fatty acids) (26.11e38.78%) and UFAs (unsaturated fatty acids) (61.22e73.89%) are similar. The contents of MUFAs (monounsaturated fatty acids) and PUFAs (polyunsaturated fatty acids) are 30.22e39.14% and 24.29e43.67%, respectively. The content of most free fatty acids, such as palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1) and linoleic acid (C18:2) varied significantly, while less variance occurred to lauric acid (C12:0), myristic acid (C14:0), a-linolenic (C18:3), arachic acid (20:0), docosanoic acid (22:0) and lignoceric acid (24:0) after thermal treatment. It could be concluded that increasing thermal treatment temperature and duration would lead to the increase of SFA, UFA, PUFA and MUFA, although a continuous decrease occurred after a 10 min treatment. These results showed that the degradation of long-chain PUFA may contribute to the increase in short-chain SFAs [38]. Table 5 also shows the increase of DU and LSCF after 10 min and then continuous decrease at all treatment temperatures. The properties of a biodiesel fuel, including iodine value, oxidative stability, cetane number, cold-flow properties, viscosity and lubricity, can be directly determined by DU of its component fatty esters. Iodine value and cold-flow properties increase as the number of double bonds in the fatty acid molecular increases [39], while cetane number and the oxidative stability decrease with an increasing DU in the fatty acid chain [40]. Thus, thermal pretreatment could serve as a promising method for regulating and modifying the DU levels because only with a proper DU can biodiesel be produced from starting materials of high DU meet the specifications. The iodine value could be considered as a measure of total unsaturation of a fatty material. The more unsaturation present in the oil, the higher the iodine value [39]. The longer the carbon chains in the biodiesel, the worse their low-temperature properties [26]. Consequently, a lower iodine value and worse lowtemperature property at 120
C could be expected compared with at other treatment temperatures. Meanwhile, the correlation between the CFPP (cold filter plugging point) and the saturated methyl ester content could be simplified as a linear equation [18]. Thus, it is possible to directly predict the CFPP of biodiesels from the content of saturated methyl esters. 3.3.3. Properties of biodiesel Biodiesel is made from renewable biological sources such as vegetable oils and animal fats, which is biodegradable and nontoxic and has low emission profiles as compared to petroleum diesel [41]. Floating oil, obtained from KW treated at 120
C and 40 min, was applied to produce biodiesel according to method mentioned by Ref. [42]. Table 6 shows the results and the limit values established by the European standard of UNE-EN 14214 (2003) [43]. And it could be concluded that all the parameters except acid value could meet the limits required in the standard UNE-EN 14214. The quality of the

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165

Table 5 Compositions of FA (mg/g, dry basis) of floating oils. FA

C12:0 C14:0 C16:0 C18:0 C20:0 C22:0 C24:0 C14:1 C16:1 C18:1 C20:1 C22:1 C24:1 C18:2 C20:2 C18:3 SFA MUFA PUFA DU LCSF

KW

0.1 0.8 22.2 8.7 0.4 0.1 0.1 0.1 1.5 32.8 0.6 0.3 0.1 27.8 0.2 1.5 32.3 35.3 29.5 94.2 7.3

90
C

100
C

120
C

140
C

10 min

30 min

50 min

10 min

30 min

50 min

10 min

30 min

50 min

10 min

30 min

50 min

0.1 0.9 23.4 9.2 0.4 0.1 0.2 0.1 1.8 37.2 0.6 2.6 0.1 36.7 0.6 2.0 34.3 42.5 39.2 120.8 8.0

0.1 0.8 19.6 7.5 0.3 0.1 0.1 0.1 1.5 31.2 0.5 2.4 0.0 31.7 0.6 1.8 28.4 35.7 34.0 103.8 6.4

0.1 0.7 19.8 7.8 0.3 0.1 0.1 0.1 1.5 31.8 0.5 1.5 0.0 32.8 0.6 1.9 28.9 35.5 35.3 106.1 6.6

0.1 0.9 26.5 9.9 0.5 0.1 0.1 0.1 1.7 42.0 0.7 0.4 0.1 37.2 0.4 2.1 38.1 45.1 39.6 124.3 8.5

0.0 0.8 23.0 8.5 0.5 0.0 0.1 0.1 1.5 36.2 0.6 0.4 0.1 31.9 0.4 1.3 32.9 38.8 33.5 105.9 7.3

0.0 0.7 19.7 7.2 0.4 0.1 0.1 0.0 1.3 29.9 0.5 0.3 0.1 26.7 0.4 1.4 28.1 32.1 28.5 89.1 6.2

0.1 1.4 36.7 14.2 0.7 0.1 0.2 0.1 2.5 56.1 0.9 0.5 0.2 40.5 0.5 2.4 53.3 60.3 43.5 147.2 12.0

0.1 0.9 24.8 9.7 0.5 0.1 0.1 0.1 1.8 40.0 0.7 0.3 0.1 34.0 0.5 2.1 36.2 42.9 36.6 116.1 8.2

0.1 1.0 27.5 10.5 0.5 0.1 0.2 0.1 2.0 42.8 0.7 0.3 0.1 35.5 0.6 30.3 39.7 46.0 66.4 178.8 8.9

0.1 0.9 27.1 10.3 0.5 0.1 0.1 0.1 1.7 38.9 0.7 0.3 0.1 24.2 0.4 1.2 39.0 41.7 25.9 93.5 8.7

0.1 0.9 26.4 10.2 0.5 0.1 0.2 0.1 1.8 41.1 0.7 0.0 0.1 29.9 0.5 1.5 38.3 43.9 31.9 107.7 8.7

0.0 0.8 24.2 10.7 0.4 0.1 0.1 0.0 1.4 31.9 0.6 0.0 0.1 22.0 0.4 1.1 36.4 33.9 23.5 80.9 8.5

Table 6 Properties of biodiesel produced from floating oils obtained from KW treated at 120
C and 40 min. Property

Unit

Ester content Acid value Iodine value Density at 15
C Flash point Cetane number Kinematic viscosity, 40
C CFPP Ash contentc

wt.% mgKOH/g g I2/100 g kg/m3 o C e mm2/s o C %

a b c

Limitsa Minimum

Maximum

96.50 e e 860 101 51 3.5 e e

e 0.50 120 900 e e 5 10 0.020

Meanb

Standard deviation

97.43 1.43 99.67 875 176.33 55.7 4.2 3.1 0.009

0.709 0.709 2.082 0.005 1.528 1.528 0.040 0.153 0.000

UNE-EN 14214 standard. Measured values. ASTM D6751.

synthesized biodiesels was tested according to the European Standard UNE-EN 14214 (2003) [43] and ASTM D6751 (2008) [44] as shown in Table 6. The quality of some parameters depends on the degree of the oil refinement, transesterification process as well as purification quality [18,41,45,46]. Iodine value is a measure of total unsaturation within a mixture of fatty acid. It is expressed in grams of iodine which react with 100 g of the respective sample when formally adding iodine to the double bonds. The flash point method serves to restrict the amount of alcohol in the biodiesel fuel to a maximum of about 0.1%. The iodine value of a vegetable oil or animal fat is almost identical to that of the corresponding methyl esters. Iodine value is limited to 120 g I2/100 g in the European biodiesel standard UNEEN 14214 (2003) [43] (Table 6). Oils satisfying the limit for the iodine value could also meet the minimum limit for the ester content, density at 15
C and flash point. Acid value is expressed in mg KOH required to neutralize 1 g of fatty acid methyl esters and is set to a maximum value of 0.50 mg KOH/g in the European standard. It could be concluded from Table 6 that the acid value was nearly 3 times higher than the standard due to the hydrolysis of organic matter to free fat acids in KW. The reason for the out of specification of acidity could also be led by using the acidified water to purify the crude biodiesel, thus forming fatty acids as the reaction product [47].

Thermal treatment of KW is beneficial for floating oil recycling, especially at higher temperatures and longer treatment durations, such as 120e140
C, which could also influence FA composition in floating oil which could affect the quality of biodiesel as well. When floating oil collected from KW treatment at 120
C and 50 min was used for biodiesel production, all parameters except the acid value could meet the biodiesel requirements in UNE-EN 14214, 2003 [43]. In addition, the higher efficiency of waste oil removal and liquefaction caused by thermal pre-treatment could also alleviate the inhibitory levels caused by high protein and fat content, thus making the liquid phase of KW a very suitable substrate for subsequent anaerobic digestion.

3.4. Economic considerations In order to enhance the accuracy, reliability and comparability of the evaluation results, two KW treatment plants were chose to investigate the economic analysis: Plant A in Suzhou and Plant B in Chongqing in both east and southwest of China respectively, both of which operate steadily till now. Among them, proposed techniques in this study were adopted in the demonstration project of Plant A, while Plant B did not use thermal pretreatment prior to the same subsequent resource utilization techniques. A summary of the

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Y. Li et al. / Energy 98 (2016) 155e167

treatment plant information and economic analysis results including production costs and net profit is developed in Table 7. According to Table 7, collection and transportation cost of KW are very close for the collection and transportation of 380 ton and 1020 ton of raw materials including KW and waste cooking oil. On the contrary, greater difference between processing cost, including water, electricity and CNG (compressed natural gas) consumption and other costs, of two KW processing techniques could be concluded, and the total cost associated with KW processing (apart from collection and transportation cost) of Plant A were $15.74 per ton KW, 57.44% lower compared with that of Plant B without thermal pretreatment. In addition, biogas produced from the liquid phase of KW with thermal pretreatment in Plant A were just 30.56% less than that of both solid phase and liquid phase of KW without thermal pretreatment in Plant B. Biodiesel generation per ton KW in Plant B was 50% less than Plant A, but higher selling price per ton of biodiesel was achieved. According to the comprehensive analysis of total costs and recycling products, such phenomena could mean that thermal pretreatment could enhance the subsequent utilization efficiency and reduce the processing cost. According to Table 7, the output value associated with process in Plant A can be estimated to $ 57.52 for the production of 1 ton of product, which was about three times as much as that of Plant B. However, the net profits in both KW treatment plants, generated by selling those products and after deducting the expenses in collection-transportation section, are negative and equate to $ 19.69 per ton of KW in Plant A and -$ 34.18 in Plant B, demonstrating the economic feasibility of treatment technique in this study. Nonetheless government subsidies measures are taken to ensure safe and normal operation of KW plants in China. So, it does seem that a thermal pretreatment and split-phase utilization of KW could be more cost-effective. Besides, one phenomenon should be recognized about these profits based on the actual market price for the products which differ significantly between two districts in China and production techniques.

Table 7 Information and costs distribution of two KW treatment plants. Items

1 Treatment capacity 1.1 KW 1.2 Waste cooking oil 2 Collection-transportation costs 2.1 Vehicle maintenance 2.2 Fuel consumption 2.3 Labor 3 Processing costs 3.1 Water consumption 3.2 Electricity consumption 3.3 CNG consumption 3.4 Others 4 Biogas 4.1 Biogas yield 4.2 Electricity power from biogas 4.3 Amount of electricity for self use 4.4 Amount of biogas for self use 5 Biodiesel 5.1 Biodiesel production 5.2 Unit price of biodiesel 6 Animal feed 6.1 Animal feed production 6.2 Unit price of animal feed 7 Net profit 8 Output value

Units

Values Plant A

Plant B

350 30 24.76 1.26 3.31 20.19 15.74 0.23 2.70 3.64 9.17

1000 20 21.40 1.42 8.18 11.80 36.98 0.33 7.33 0.00 29.33

50 11.84 7.89 31.58

72 89.87 47.22 11

ton$ton1-KW $$ton1

2.3% 629.52

1.2% 944.29

ton$ton1-KW $$ton1 $$ton1 $$ton1

15% 39.35 19.69 57.52

0 0 34.18 19.64

ton$d1 ton$d1 $$ton1 $$ton1 $$ton1 $$ton1 $$ton1 $$ton1 $$ton1 $$ton1 $$ton1 m3$ton1-KW kWh$ton1-KW kWh$ton1-KW m3$ton1-KW

4. Conclusions This work was to study the effect of thermal hydrolysis on the reutilization of three phases in KW. Thermal hydrolysis could regulate the nutrition level in solid phase thus the nutrients could be guaranteed when used as the raw materials for pig feed. Anaerobic biodegradability for liquid phase could be improved after thermal pretreatment. Composition analysis of FA after thermal hydrolysis indicates that the main ingredients are MUFA, which are primarily C18:1. Biodiesels obtained from KW pretreated at 120
C and 50 min showed higher acid value while others could satisfy the requirements in standard of UNE-EN 14214, 2003. Overall, KW treatment plants with the thermal pretreatment could achieve higher profits. Acknowledgments The authors acknowledge the financial support by the State Key Joint Laboratory of Environmental Simulation and Pollution Control, China Twelfth Five-Year plan of Scientific and Technical program and BMEI Co., Ltd. References [1] Tsai WT, Lin CC, Yeh CW. An analysis of biodiesel fuel from waste edible oil in Taiwan. Renew Sust Energ Rev 2007;11:838e57. [2] Ma J, Duong TH, Smits M, Verstraete W, Carballa M. Enhanced biomethanation of kitchen waste by different pre-treatments. Bioresour Technol 2011;102: 592e9. [3] Zhong Y, Roman MB, Zhong Y, Archer S, Chen R, Deitz L, et al. Using anaerobic digestion of organic wastes to biochemically store solar thermal energy. Energy 2015;83:638e46. [4] Browne JD, Murphy JD. Assessment of the resource associated with biomethane from food waste. Appl Energ 2013;104:170e7.
land M, et al. Biohydrogen [5] Zhu H, Parker W, Basnar R, Proracki A, Falletta P, Be production by anaerobic co-digestion of municipal food waste and sewage sludges. Int J Hydrogen Energ 2008;33:3651e9. [6] Dyk JSV, Gama R, Morrison D, Swart S, Pletschke BI. Food processing waste: problems, current management and prospects for utilisation of the lignocellulose component through enzyme synergistic degradation. Renew Sust Energ Rev 2013;26:521e31. [7] Ariunbaatar J, Panico A, Esposito G, Pirozzi F, Lens PN. Pretreatment methods to enhance anaerobic digestion of organic solid waste. Appl Energ 2014;123: 143e56. [8] Sasaki K, Aizaki H, Motoyama M, Ohmori H, Kawashima T. Impressions and purchasing intentions of Japanese consumers regarding pork produced by ‘Ecofeed,’ a trademark of food-waste or food co-product animal feed certified by the Japanese government. Anim Sci J 2011;82:175e80. [9] Chen T, Jin Y, Qiu X, Chen X. A hybrid fuzzy evaluation method for safety assessment of food-waste feed based on entropy and the analytic hierarchy process methods. Expert Syst Appl 2014;41:7328e37. [10] Cheng JYK, Lo IMC. Investigation of the available technologies and their feasibility for the conversion of food waste into fish feed in Hong Kong. Environ Sci Pollut Res 2015. http://dx.doi.org/10.1007/s11356-015-4668-3. [11] Karmee SK, Linardi D, Lee J, Lin CSK. Conversion of lipid from food waste to biodiesel. Waste Manage 2015. http://dx.doi.org/10.1016/ j.wasman.2015.03.025. [12] Cammarota MC, Teixeira GA, Freire DMG. Enzymatic pre-hydrolysis and anaerobic degradation of wastewaters with high fat contents. Biotechnol Lett 2001;23:1591e5. [13] Lin CSK, Pfaltzgraff LA, Herrero-Davila L, Mubofu EB, Abderrahim S, Clark JH. Food waste as a valuable resource for the production of chemicals, materials and fuels. Current situation and global perspective. Energ Environ Sci 2013;6: 426e64. [14] Opatokun SA, Strezov V, Tao K. Product based evaluation of pyrolysis of food waste and its digestate. Energy 2015;02:098. http://dx.doi.org/10.1016/ j.energy.2015. [15] Zisopoulos FK, Moejes SN, Rossier-Miranda FJ, Van der Goot AJ, Boom RM. Exergetic comparison of food waste valorization in industrial bread production. Energy 2015;82:640e9. [16] Xu C, Shi W, Hong J, Zhang F, Chen W. Life cycle assessment of food wastebased biogas generation. Renew Sust Energ Rev 2015;49:169e77. [17] Zhang C, Su H, Baeyens J, Tan T. Reviewing the anaerobic digestion of food waste for biogas production. Renew Sust Energ Rev 2014;38:383e92.
Influence of fatty acid
rez A. [18] Ramos MJ, Fern
andez CM, Casas A, Rodríguez L, Pe composition of raw materials on biodiesel properties. Bioresour Technol 2009;100:261e8.

Y. Li et al. / Energy 98 (2016) 155e167 [19] APHA. Standard methods for the examination of water and wastewater. 20th ed. Washington, DC: American Public Health Association; 1998. [20] Methodenbuch, Band III . Die chemische Untersuchung von Futtermitteln. Erg€ anzungslieferung. Darmstadt: VDLUFA, Verlag; 1993. [21] AOAC. Official methods of analysis. 15th ed. Washington, DC: Association of Official Analytical Chemists; 1990. [22] Noblet J, Perez JM. Prediction of digestibility of nutrients and energy values of pig diets from chemical analysis. J Anim Sci 1993;71:3389e98. [23] Noblet J, Van Milgen J. Energy value of pig feeds: effect of pig body weight and energy evaluation system. J Anim Sci 2004;82:229e38. [24] Noblet J. Digestive and metabolic utilization of dietary energy in pig feeds: comparison of energy systems. In: Garnsworthy PC, Wiseman J, Haresign W, editors. Recent advances in animal nutrition. Nottingham, UK: Nottingham University Press; 1996. [25] Wu GG, Davis PK, Flynn NE, Knabe DA, Davidson JT. Endogenous synthesis of arginine plays an important role in maintaining arginine homeostasis in postweaning growing pigs. J Nutr 1997;127:2342e9. [26] Wu M, Wu G, Han L, Wang J. Low-temperature fluidity of bio-diesel fuel prepared from edible vegetable oil. Pet Process Petrochem 2005;35:57e60. [27] Campbell L, Raikos V, Euston SR. Heat stability and emulsifying ability of whole egg and egg yolk as related to heat treatment. Food Hydrocolloid 2005;19:533e9. [28] Arseneau DF. Competitive reactions in the thermal decomposition of cellulose. Can J Chem 1971;49:632e8. [29] Wang FX. Application of net energy system in formulating low protein dietary during pig breeding production. Guangdong Feed 2012;9:35e9. [30] Ewan RC. Predicting the energy utilization of diets and feed ingredients by pigs. In: Van Der Honing Y, Close WH, editors. Energy metabolism of farm animals. Wageningen: Pudoc; 1989. p. 215e8. [31] May RW, Bell JM. Digestible and metabolizable energy values of some feeds for the growing pig. Can J Anim Sci 1971;51:271e8. [32] Black JL. Bioavailability: the energy component of a ration for monogastric animals. In: Moughan PJ, Verstegen MWA, Visser-Reyneveld MI, editors. Feed evaluation: principle and practice. Wageningen, The Netherlands: Wageningen Press; 2000. [33] National Research Council (NRC) (US). Subcommittee on swine nutrition. Nutrient requirements of swine (tenth revised edition). National Academy of Sciences; 1998.

167

[34] Chinese standard GB/T 5915-2008. Formula feeds for starter and growingfinishing pigs. Beijing, China: Standards Press of China; 2008. [35] National Institutes of Health. Open formula laboratory swine ration. USA: NIH; 2004. www.ors.od.nih.gov/sr/dvr/drs/nutrition/Documents/SpecsDiets/11142g.pdf.
rinel I. On calculating sterility in thermal [36] Mafart P, Couvert O, Gaillard S, Legue preservation methods: application of the Weibull frequency distribution model. Int J Food Microbiol 2002;72:107e13. [37] Kvesitadze G, Sadunishvili T, Dudauri T, Zakariashvili N, Partskhaladze G, Ugrekhelidze V, et al. Two-stage anaerobic process for bio-hydrogen and biomethane combined production from biodegradable solid wastes. Fuel Energy Abstr 2012;37:94e102. [38] Zhang HR, Wang Q, Fan E. Stability profile of fatty acids in yak (Bosgrunniens) kidney fat during the initial stages of autoxidation. J Am Oil Chem Soc 2009;86:1057e63. [39] Lin CY, Lin HA, Hung LB. Fuel structure and properties of biodiesel produced by the peroxidation process. Fuel 2006;85:1743e9. [40] Knothe G. “Designer” biodiesel: optimizing fatty ester composition to improve fuel properties. Energ Fuel 2008;22:1358e64. [41] Nainwal S, Sharma N, Sharma AS, Jain S, Jain S. Cold flow properties improvement of Jatropha curcas biodiesel and waste cooking oil biodiesel using winterization and blending. Energy 2015;89:702e7. [42] Mittelbach M, Remschmidt C. Biodiesel: the comprehensive handbook. Vienna: Boersedruck Ges. M.B.H.; 2004. [43] UNE-EN 14214. Automotive fuels. Fatty acid methyl esters (FAME) for diesel engines. Requirements and test methods. 2003. [44] ASTM. Standard specification for biodiesel fuel (B100) blend stock for distillate fuels. In: Annual book of ASTM standards. West Conshohocken: ASTM Press; 2008. Method D6751. € [45] Uzuna BB, Kılıça M, Ozbayb N, Pütüna AE, Pütünc E. Biodiesel production from waste frying oils: optimization of reaction parameters and determination of fuel properties. Energy 2012;44:347e51. [46] Chen KS, Lin YC, Hsu KH, Wang HK. Improving biodiesel yields from waste cooking oil by using sodium methoxide and a microwave heating system. Energy 2012;38:151e6. [47] S
anchez BS, Mendow G, Levrand PG, Querini CA. Optimization of biodiesel production process using sunflower oil and tetramethyl ammonium hydroxide as catalyst. Fuel 2013;113:323e30.