Effect of Microwave Pretreatment on BMP of Decanter Cake from Palm Oil Mill Factory

Corresponding Author: Thaniya Kaosol Environmental Engineering Program, Department of Civil Engineering, Faculty of Engineering, Prince of Songkla University, Songkhla, Thailand Email: thaniya.k@psu.ac.th Abstract: Anaerobic digestion is an effective method for treating decanter cake in the biogas production. Decanter cake usage is limited by its slow fiber degradation for anaerobic digestion process. The aim of this study is to study the possibility of using microwave pretreatment to enhance decanter cake digestibility. The power and heating time are the parameters that influence the biogas production and methane yield. The Biochemical Methane Potential (BMP) tests are used in this study to find the optimization setting of a microwave pretreatment for decanter cake solubilization and anaerobic digestibility. The results show that the microwave pretreatment provides a better performance in terms of COD solubilization and anaerobic digestibility. The increasing power and heating time can improve the biogas production and methane yield for microwave pretreated decanter cake under anaerobic digestion process. Finally, the optimum power and heating time used in the microwave pretreatment, found in this study, is 160 watts and 8 min for methane yield under the anaerobic digestion process. The highest methane yield is 309.9 mL CH4/g COD removed. The microwave pretreated decanter cake at 160 watts of power and 8 min of heating time can improve the methane yield with an improvement of 24.6% in comparison with the non-microwave pretreated decanter cake. It can be concluded that the microwave pretreatment of decanter cake from palm oil mill factory could be the rapid and environmental method for biogas production enhancement.


Introduction
Anaerobic digestion of organic wastes has been established due to the increasing investments in renewable energy production and in greenhouse gas emission reduction. Especially in the southern of Thailand, many biogas plants have been established by combining wastes from agriculture, agro-industry, industry and households for biogas. Fertilizer production is another product from such wastes which are recycled back on farmland and forestland. The anaerobic digestion is an oldest technology for organic waste stabilization (Lier et al., 2001;Malakahmad et al., 2012;. The oldest of anaerobic digestion is the sewage sludge treatment (Mata-alvarez et al., 2014). Biogas is produced from the biochemical breakdown of organic substrates by microorganisms in the absence of oxygen from the anaerobic digestion (Appels et al., 2011;Vergara-Fernández et al., 2008;Brennan and Owende, 2010;Rakmak et al., 2010). Biogas is mainly composed of methane, carbon dioxide and trace gases. Biogas application includes electricity, heating and cooking (Vergara-Fernández et al., 2008). In recent years, research into the potential feedstock for methane yield from anaerobic co-digestion has been conducted. Anaerobic co-digestion is an anaerobic digestion with two or more substrates for improving the economic viability of anaerobic digestion plants due to higher methane production (Mata-Alvarez et al., 2000).
To accelerate the anaerobic digestion, the pretreatment aims to raise the degree of degradation so that the organic substrates can be made better accessible for microorganisms (Lier et al., 2001). Pretreatment techniques include thermal, mechanical, biochemical and chemical methods (Lier et al., 2001). Many researchers reported the pretreatment techniques in terms of efficiency, energy balance, environmental sustainability as well as investment, operation and maintenance costs (Mata-Alvarez et al., 2000;Hendriks and Zeeman, 2009;Eggeman and Elanderb, 2005;Carballa et al., 2011;Dereix et al., 2006).
Anaerobic digestion for solid organic wastes is a very promising option because of its ability to transform organic substrates into biogas and its ability to reduce the amount of disposed final solid wastes (Appels et al., 2008). The final solid wastes from anaerobic digestion like fertilizer or soil conditioner. However, the anaerobic digestion for solid organic wastes is often limited by a very long retention time and a low overall degradation efficiency of 30-50% organic dry solids (Appels et al., 2008). Thus, the pretreatment techniques are required for reducing the time limit factor. Especially, the thermal pretreatment methods are effective at increasing methane production as thermal hydrolysis. Thus, heating the solid fraction of solid organic wastes improved methane production and volatile solids reduction (Mladenovska et al., 2006).
The waste selected for this study is a decanter cake. The decanter cake is a solid organic waste from the palm oil mill industry, containing high organic substrates such as COD and high moisture content. The estimated production and estimated methane production potential of decanter cake is 0.27 million tons/year and 19 million m 3 /year, respectively in Thailand (Paepatung et al., 2009). Thus, the biogas potential from decanter cake was estimated to be 16 ktoe and 5.6 MWe of electricity equivalent (Paepatung et al., 2009). Thus, decanter cake can improve the methane production when it is used as co-substrate for anaerobic co-digestion (Kaosol and Sohgrathok, 2012).
Lignocellulose is the major structural material of plant bodies which is a complex structure and also resisting to mechanical stress and enzymatic attack. Pretreatment of fiber to break down its complex structure is an effective way for enhancing anaerobic digestibility. However, it is hard to identify the suitable pretreatment for lignocellulose materials (Hahn-Hägerdal et al., 2006). Bobleter (1994) reported that lignocellulose is heated during thermal pretreatment. Parts of lignocellulose will start to solubilize at above 150-180°C (Bobleter, 1994). The thermal pretreatments can be applied to increase the methane yield thus it can improve the biogas production. Microwave, one of the thermal pretreatment techniques, is an electromagnetic radiation with an oscillation frequency for 0.3-300 GHz (Saifuddin and Fazlili, 2009). For cell lyses, the microwave method is very quick and has many potential applications (Sai, 1986). The main objective of this study is to study a methane production enhancement using microwave as a thermal pretreatment for decanter cakes.

Materials and Methods
Biochemical Methane Potential (BMP) test provides a measure of anaerobic digestibility of organic substrates. BMP test is used to estimate methane yield potential of organic substrates. Organic substrate is tested in a laboratory environment under an optimal condition for microorganism's growth, which is an inexpensive and repeatable method in order to relatively compare the anaerobic digestibility among various substrates (Owen et al., 1979).

Raw Materials and Sample Preparation
Decanter cake was collected from the palm oil mill industry in Krabi province, Southern of Thailand ( Fig.  1). Before using in the experiments, the decanter cake was stored at 4°C for stabilization components. The decanter cake characteristics are shown in Table 2. The reported results were the average value of three replicated results together with the standard deviation value. All analytical procedures were performed according to the APHA (1988).

Microwave Pretreatment for Decanter Cake
The experiments were conducted by varying the heating power and the heating times. The decanter cake (10 g) was diluted with 50 mL distilled water in the microwave vessel. The sample was in the slurry form. This homogeneous slurry was shaken. The decanter cake sample was taken to the microwave. The detail operation of microwave was shown in Table 1. When the microwave pretreatment was completed, the pretreated slurry sample was removed from the microwave oven. The sample was cooled to the room temperature. All experiments were run in duplicates. The parameters were analyzed in the pretreated slurry sample such as TCOD, SCOD, PCOD, TS, TDS, TVS and FVS. All analytical procedures were performed in accordance with APHA (1988).

Biochemical Methane Potential (BMP) Test
Pretreated decanter cake slurry sample (12 mL) using Microwave was mixed with 48 mL anaerobic seed. The anaerobic seed was collected from the anaerobic digestion sludge of the palm oil mill wastewater treatment process. The anaerobic seed contains 8.21±0.06 of pH, 23,916±522 mg L −1 of TCOD, 2,277±51 mg L −1 of SCOD, 4,320±0 mg L −1 of Alkalinity, 132±3 mg L −1 of VFA, 31,287±257 mg L −1 of TS and 18,000±165 mg L −1 of TVS. The substrate and inoculums ratio was 2:1 v/v base on total volatile solids.
The pH of mixed sample was adjusted to 7.2 using HCl or NaHCO 3 . The mixed slurry sample was placed in serum bottles and flushed with 99.99% of nitrogen gas for 3 min to create anaerobic condition before sealing.
The bottles were mixed twice a day by manually shaking the bottles. All reactors were run in duplicates. Two parameters were evaluated including (1) varying heating powers and (2) varying heating times.

Biogas Monitoring
The daily biogas content is recorded by inserting a gas syringe through the top of the anaerobic digester and letting the biogas displace the wetted barrel of the gas syringe. Biogas content is collected daily. The biogas sample was not collected when the biogas production was less than 5% of the maximum biogas production. The biogas was analyzed for methane using a Gas Chromatograph (GC) analyzer (GC7890A, Agilent technology, USA) with Thermal Conductivity Detector (TCD).

Calculating the BMP Results
The daily methane content is analyzed using the calculation of methane percentage from biogas production. The methane yield was calculated based on the proportion of the maximum cumulative methane and removed COD during the anaerobic digestion period, as shown in Equation 1: where, BMP is the methane yield (mL CH 4 /g COD removed); Max cumulative methane is the amount of cumulative methane at the end of the anaerobic digestion period (mL); COD removed is the removed COD at the end of the anaerobic digestion period (g COD removed).

Characteristics of Decanter Cake
The palm oil plant is an important economic plant grown in southern of Thailand. One of organic wastes from the palm oil mill process is decanter cake. The decanter cake is a fibrous material. The characteristics of raw decanter cake are shown in Table 2.
The decanter cake had the high moisture content and total volatile solids. It also had high biodegradability and nutrient-rich contents. The pH value of decanter cake was low as acidity. After microwave pretreatment of decanter cake, the pH was adjusted to neutral for starting the methanogenesis. The decanter cake contains high organic substrates in the forms of TCOD, SCOD and PCOD. The ratio of SCOD/TCOD was 17.3%. Thus, the majority of COD was in the form of Particulate COD (PCOD). The decanter cake can be a suitable low cost organic substrate for an anaerobic digestion study. Table 3 shows the concentrations of COD and solids of decanter cake after the microwave pretreatment process. All the microwave pretreatments release soluble COD and solids (R2-R9). The various forms of solids are changed after the microwave pretreatment process. The microwave pretreatment process of decanter cake rises up the TS, TDS, TVS and FVS in all reactors (R2-R9). Total Volatile Solids (TVS) analysis determines the total amount of organic matter in the substrate. Thus, the increasing of TVS can enhance the organic biodegradability and the biogas and methane productions for anaerobic digestion (Prabhudessai et al., 2013). Microwave pretreatment has proved to be an effective way to break down the structure of fiber and lignocellulosic materials such as rice straw and switch grass (Zhu et al., 2006;Hu and Wen, 2008). The SCOD provides an indication of a general extent of organic carbon solubility. The trends of SCOD, TCOD and PCOD with different heating power and heating times are correlated with the increasing power and heating time. The increasing SCOD can improve the biogas production of anaerobic digestion (Carrère et al., 2008) and reduce the Hydraulic Retention Time (HRT) (Graja et al., 2005).

Effect of Power and Heating Time on BMP test
Because the microwave pretreatment process of decanter cake induces an improvement of release yields, the biogas and methane production improvements can be expected. Anaerobic digestibility of microwave pretreated decanter cake was evaluated by the Biochemical Methane Potential (BMP) test. Heating power and heating time used in the microwave pretreatment process may influence the microwave pretreatment performance. At the end of the 45-day anaerobic digestion, the characteristics of both nonpretreated and pretreated decanter cake are shown in Table 4. The TS, TDS, TVS and FVS decrease in all reactors. After anaerobic digestion, the organic substrates in the forms of TCOD, SCOD and PCOD reduce significantly in all reactors. The SCOD/TCOD ratio at 160 watts of power and 8 min of heating time for microwave pretreated decanter cake rises up to 20.4% in comparison with the SCOD/TCOD ratio of the nonpretreated decanter cake. Table 5 shows the removal efficiencies of various forms of solids and CODs. On BMP test, the results show no significant removal efficiencies for both non-pretreated decanter cake (R1) and pretreated decanter cake (R2-R9). However, the TS, TVS and FVS removal efficiencies of microwave pretreated decanter cake are slightly higher than that of the non-pretreated decanter cake. Figure 2 showed the efficiency of TCOD, SCOD, PCOD and FVS removal of non-pretreated decanter cake (R1) and pretreated decanter cake (R2-R9) on BMP tests. Using the microwave pretreatment process of decanter cake, the removal efficiencies of TCOD, PCOD and FVS are increased. However, the SCOD removal efficiency trends to decrease after the microwave pretreatment process of decanter cake.   After the microwave pretreatment process, the TS and TVS removal efficiencies increase in comparison with that of the non-microwave pretreated decanter cake (Fig. 3). The results show that the TVS removal efficiency increases with the increasing of heating time and heating power.
The cumulative biogas from microwave nonpretreated and pretreated decanter cake shows in Fig. 4. As shown in Fig. 4, the microwave pretreated decanter cake has higher amount of cumulative biogas (R2-R9) than that of the non-pretreated decanter cake (R1). The higher cumulative biogas (293.4-336.3 mL) is achieved significantly for the microwave pretreated decanter cake than the cumulative biogas of non-pretreated decanter cake (278.3 mL). At the same power for microwave pretreatment, increasing of the heating time increases the cumulative biogas (R2-R5). Whereas at the same heating time (comparing between R2 and R8, R3 and R6), increasing the power also increase the cumulative biogas. However, the highest heating power does not provide the highest cumulative biogas because the pretreated decanter cake sample is too hot. The highest cumulative biogas is produced by the reactor containing the decanter cake pretreated with the 360 watts of heating power and 4 min of heating time (R6). The cumulative biogas increases with the increasing total amount of organic matter in the substrate after the microwave pretreatment process (Prabhudessai et al., 2013). Figure 5 shows the cumulative methane of pretreated decanter cake at the end of the 45-day anaerobic digestion. It also shows that the cumulative methane achieved from the pretreated decanter cake reactors (R2-R9) is higher than that achieved from the non-pretreated decanter cake (R1). The decanter cake pretreated with 360 watts of power and 4 mins of heating time in reactor R6 provides the highest cumulative methane. However, the rate of biogas and methane productions differs significantly according to the operation of microwave pretreatment for decanter cake at various powers and heating times. The cumulative methane also increases with the increasing of the total amount of organic matter in a substrate after microwave pretreatment (Prabhudessai et al., 2013).
The biogas and methane production on BMP test shows in Table 6. The methane composition of pretreated decanter cake in reactors R2-R9 is ranged between 47.6 and 50.5%. The non-pretreated decanter cake in reactor R1 provides 47.7% of methane composition. The methane composition of microwave pretreated decanter cake in reactors R2-R9 is higher than that of the non-pretreated decanter cake in reactor R1 with the exception in the reactor R4. The percentage of methane for these experiments is lower than the typical methane percentage for anaerobic digestion (55-75% of methane) (Rakmak et al., 2010;Karellas et al., 2010). The maximum methane composition is 50.5% in reactor R6 that contains the decanter cake with 360 watts of power and 4 min of heating time using microwave pretreatment process.
The methane yield is based on the removed COD during the anaerobic digestion. The methane yield of treated decanter cake in reactors R2-R9 is ranged between 258.4 and 309.9 mL CH 4 /g COD removed ( Table 6). The methane yield of the non-pretreated decanter cake in reactor R1 is 248.7 mL CH 4 /g COD removed which is the lowest methane yield in this experiment. The theoretical maximum methane production is 350 mL CH 4 /kg COD removed (Eskicioglu et al., 2006). Thus, the methane yields of the microwave pretreated decanter cake are lower than the theoretical methane production. At 160 watts of heating power, the biogas production increases with increasing heating time. That is, the methane yield of 258.4, 283.3 and 309.9 mL CH 4 /g COD removed, are observed from the reactors R3, R4 and R5 which uses 4, 6 and 8 min of heating time, respectively. At 2 min of heating time, the biogas production increases with the increasing power. That is, the methane yield of 262.3 and 272.9 mL CH 4 /g COD removed, are observed from reactor R2 and R8 which uses 160 and 640 watts of heating power, respectively. The influence of the heating power on the results shows the same trend. At 4 min of heating time, the biogas production increases with the increasing power. That is, the methane yield of 258.4 and 281.9 mL CH 4 /g COD removed are observed from reactors R3 and R6 which uses 160 and 320 watts of heating power, respectively. The decanter cake pretreated with 160 watts of power and 8 min of heating time, in the reactor R5 provides the highest methane yield. The highest methane yield in reactor R5 is 309.9 mL CH 4 /g COD removed. The highest methane yield in this study is near the theoretical methane production of 350 ml/g COD removed (Eskicioglu et al., 2006) which is provided the confidence in the result based on a COD balance. The microwave pretreatment of decanter cake at 160 watts of heating power and 8 min of heating time rises up the methane yield to 24.6% comparing with the nonmicrowave pretreated decanter cake in reactor R1. The methane production is correlated as the functions of heating power and heating time. In conclusion, the limitation of decanter cake is its slow organic degradation under the anaerobic digestion. Microwave pretreatment process is capable of improving the decanter cake biodegradability by increasing the organic solubility fraction or Soluble Chemical Oxygen Demand (SCOD).

Conclusion
Decanter cake solubilization process is the ratelimiting step for the anaerobic digestion. Microwave pretreatment is the thermal pretreatment method. The microwave pretreatment of decanter cake is applied to enhance the solubilization of decanter cake. The aim of this study is to optimize the methane yield using microwave pretreatment. The two parameters in this study are the heating power and the heating time. The study uses the BMP tests. The results show that the decanter cake structure is destroyed by the microwave pretreatment process. Microwave pretreatment of decanter cake enhances the COD solubilization. The increasing of heating power and heating time for microwave pretreatment process of decanter cake can improve the biogas production and the methane yield. The maximum methane yield is achieved in reactor R5 at 160 watts of heating power and 8 min of heating time for microwave pretreatment process. The least methane yield is 248.7 mL CH 4 /g COD removed which is observed from reactor R1 (non-microwave pretreatment). The highest methane yield is 309.9 mL CH 4 /g COD removed. The microwave pretreatment of decanter cake at 160 watts of heating power and 8 min of heating time can improve the methane yield to 24.6% comparing with the non-microwave pretreatment of decanter cake.
The results can be concluded that the heating power and heating time used in microwave pretreatment are optimized. The optimal values of heating power and heating time are 160 watts and 8 min for the best methane yield. The results obtained in this study show that the microwave pretreatment is beneficial to the solubility of the complex decanter cake particles.