Biomethane potential (bmp) of arrowroot powder processing waste and effect of alkaline pre-Treatment

Vietnam Journal of Science and Technology 56 (2C) (2018) 171-177 BIOMETHANE POTENTIAL (BMP) OF ARROWROOT POWDER PROCESSING WASTE AND EFFECT OF ALKALINE PRE-TREATMENT Nguyen Pham Hong Lien1, *, Tran Dac Chi1, Hidenori Harada2, Shigeo Fujii2 1School of Environmental Science and Technology, Hanoi University of Science and Technology, 1 Dai Co Viet, Ha Noi, Viet Nam 2Graduate School of Global Environmental Studies, Kyoto University, Yoshidahonmachi, Sakyo Ward, Kyoto, 606-8501, Japan *Email: lien.nguyenphamhong@hust.edu.vn Received: 16 May 2018; Accepted for publication: 22 August 2018 ABSTRACT Arrowroot waste has been being discharged without treatment in powder/starch processing villages in Viet Nam causing serious environmental problem. This waste is degradable and might have good Biomethane Potential (BMP) which leads to a possibility to treat them by anaerobic digestion. Therefore, the study aimed to find out the BMP of the waste and to find out if alkaline pre-treatment would improve it. Solid waste from arrowroot powder processing was collected in Duong Lieu village, Ha Noi, and different samples were going through BMP test: the untreated sample and NaOH pre-treated samples at different NaOH dose from 3 to 9 wt. % (dry weight based). BMP was determined in 590 mL bottles at 37 oC for 50 days. As the results, BMP of the original arrowroot waste sample was 253 NmLCH4/gVS and alkaline pretreatment increased BMP of the waste 21.9 % at 7 % NaOH. The reduction of lignin content and hemicellulose content at this pretreatment were 7.2 % and 9.4 %, respectively. The results show that the waste has potential for methane recovery and alkaline pre-treatment by NaOH would slightly improve its biodegradability. Keywords: biomethane potential, arrowroot waste, alkaline pre-treatment. 1. INTRODUCTION Currently in Viet Nam, many trade villages with small-scale food processing industry is facing serious environmental problem associated with un-treated waste residue. Particularly, for starch production in the well-known Duong Lieu village (Hoai Duc, Ha Noi), waste output of arrowroot waste powder/starch processing is very high, at 2 tons of solid waste per 1 ton of product [1]. The waste has been being disposed uncontrollably, leading to an urgent need to find out a suitable waste treatment and disposal. During the last decades, applications of anaerobic digestion has become very popular for production of renewable energy because of its known energy potential, low maintenance costs Nguyen Pham Hong Lien, Tran Dac Chi, Hidenori Harada, Shigeo Fujii 172 and, primarily, to its environmental benefits such as the bioconversion of organic waste into organic fertilizers and biogas. Converting waste into biogas is an attractive solution for many types of organic wastes including lignocellulose wastes (corn cob, rice straw, wheat straw, sugarcane bagasse, cassava residue, etc.) and intensive researches on it have been being carry out [2,3]. One of research direction is to find out Biochemical Methane Potential/Biomethane Potential (BMP) of various kind of waste [3 - 6] and how to improve this value [3]. BMP (NmLCH4 /gVS of substrate) is an important parameter which measures anaerobic digestion ability (in a specific condition) of organic substance and is determined by BMP assay [5]. While BMP values have been reported for various wastes in general and agro-waste/lignocellulose wastes in particular, the information on BMP of arrowroot waste is not available in the literature. As a type of lignocellulosic substrate, the methane production of arrowroot waste depends on their complex structure, which might limit their biodegradability. The structure of lignocellulosic materials is mainly composed of cellulose, hemicellulose, and lignin, strongly linked to each other’s. Cellulose and hemicelluloses are quite easily degradable by anaerobic microorganisms and can be converted to methane. However, lignin limits their accessibility to hydrolytic enzymes, reducing their degradation [3]. Various pre-treatment methods could make change in physical and chemical composition of lignocellulose materials by breaking down the linkage between polysaccharides and lignin. Pre-treatments include mechanical, chemical (alkaline or acidic), thermal, and biological processes or a combination of them. In many cases, alkaline pre-treatment exhibits as the cost-effective, easily applicable method in comparison with acidic or thermal pre-treatment [3]. Common substrates for alkaline pretreatments shown in literature are corn by products, rice straw, soybean straw, sunflower stakes, barley waste, etc. but effect of a pre-treatment method depends on the substrate to which they are applied [3]. Effect of alkaline pre-treatment of arrowroot waste on its anaerobic biodegradability is still unknown. The objectives of the study were (1) to determine the Biomethane Potential (BMP) of the arrowroot waste sample collected in Duong Lieu village and (2) the effect of alkaline pre- treatment by sodium hydroxide on composition and anaerobic biodegradability of the waste. 2. MATERIALS AND METHOD 2.1. Substrate and analysis Samples of solid waste from arrowroot powder processing were collected in Duong Lieu village, Ha Noi, Viet Nam. The samples were sorted manually for eliminating visible inert materials, grinded and mixed using a blender, then were analysed in terms of total solids (TS), volatile solids (VS) (according to APHA 2006), organic carbon content and nitrogen content (according to TCVN 6498: 1999, TCVN 6644: 2000). Lignin content, cellulose content, and hemicellulose content of the samples was analysed according to TAPPI T17, TAPPI T204 and TAPPI T222. The samples were stored for about 2 days in 5 oC refrigerator before alkaline pre- treatment and BMP test. 2.2. Alkaline pre-treatment A part of a sample, then, went through alkaline pre-treatment using Sodium Hydroxide. The pretreatment was performed in 590 mL Duran bottles in batch mode and at total solid content of 50 gTS/L. In each bottle, the sample was soaked in the NaOH solution at the dose of 3, 5, 7, 9 % gNaOH/gTS (wt.%). The bottles then were closed and kept at 37 oC in an incubator Biomethane potential (BMP) of arrowroot powder processing waste and effect of 173 for 120 hours, with daily manual stirring. After pre-treatments, samples were dried at 80 oC during 48 h for analysis of the above parameters. 2.3. Biochemical methane potentials (BMP) BMP experiment: The BMP was determined in anaerobic batch reactor of 590 mL DURAN bottles (BMP reactor) with hermetically sealed stopper and controlled gas opening valves. For each reactor, 5 g VS of substrate and 1mL nutrient solution – which is prepared according to literature [5] - was added. The effective volume was maintained at 490 mL by adding inoculums (obtained from a lab reactor; TS of 8 %WW and VS of 67 %VS) leaving 100 mL headspace for gas phase. The headspace was flushed with gas mixture of 80 % N2 and 20 % CO2. The reactor, then, was kept at a temperature controlled mechanical shaker operating at 37 °C and 100 rpm mixing. Biogas is withdrawn every 2 to 5 days. Methane volume measurement was conducted by liquid displacement method after the biogas passing through 5 % NaOH solution in order to absorb CO2. In addition to reactors for substrates, a blank reactor was set up with deionized water instead of substrates, and a reactor for pure cellulose was set up as a control reactor. Cumulative methane volume for each reactor was recorded and the net methane volume of a substrate was obtained by subtract the methane volume of substrate reactor from that of the blank reactor. Finally, the net methane production will be converted to a value at standard temperature and pressure per gram volatile solid of substrate (NmL/gVS). Estimation of ultimate methane production (uBMP) and kinetic constant (k): Degradation of each substrate can be assumed to follow a first-order rate of decay: BMP = uBMP [1 - exp (-k×t)] where: BMP (NmL of CH4/gVS) is the cumulative methane volume at time t (day); uBMP (NmL of CH4/gVS) is the ultimate methane production and; k (day-1) is the first-order kinetic constant. uBMP and k was estimated using sigmaplot software. 3. RESULTS AND DISCUSSION 3.1. Characterization of untreated arrowroot waste Proximate analysis of arrowroot waste samples showed that the waste has low TS of 5 %WW, and high VS of 94 %TS, which is comparable with the other data (83-96 %) [7]. High moisture content and VS content of the waste should be favorable for biological treatment. Ultimate analysis resulted in C to N ratio of 69 which is rather high compared to optimum value for anaerobic digestion, but this will be adjusted by nutrient addition in BMP experiment. The arrowroot waste sample consisted of cellulose: 28.9 %TS, hemicellulose: 37.4 %TS, lignin: 13.9 %TS, confirming the lignocellulose characteristic of the waste. The lignin content is quite high, which is slightly higher than that of rice straw: 7.4 % [8]; corn straw: 7.5 % [9], wheat straw: 6.5 % [10], but much lower than peanut hull: 44.3 % [11]. High lignin content is considered is one of the obstacle for biogas conversion. 3.2. BMP of untreated arrowroot waste BMP curves during 50 days of experiment for cellulose control sample and for untreated Nguyen Pham Hong Lien, Tran Dac Chi, Hidenori Harada, Shigeo Fujii 174 arrowroot waste samples are shown in Figure 1a. Net methane production tends to stop increasing at the end of experiment. The first order kinetic model describes rather well the anaerobic degradation of the substrate (Figure 1b) with R2 above 0.97. Cellulose sample has ultimate BMP of 419.3 NmL/gVS which is quite close to the theoretical data. Other authors reported uBMP of cellulose control around this theoretical value [4, 6]. The result of cellulose sample demonstrates the good response of inoculums used in the test. (a) (b) Figure 1. BMP curve of original arrowroot waste samples and cellulose control (a): net methane cumulative curve, (b): kinetic curve for arrowroot waste (duplicate). Ultimate BMP (NmLCH4/gVS) of untreated arrowroot waste was 250 and 256 for duplicate reactors and 253 in average, showing the good repeatability of experiment (CV of 2 %). This BMP value indicates that that treatment by anaerobic digestion will possibly results in a significant methane recovery. It is higher than those of some lignocellulose wastes such as yard wastes (123-209 NmLCH4/gVS) [4], corn straw (100 NmLCH4/gVS) [9] and comparable to our reported value for pig manure (251 NmLCH4/gVS), a common substrate for anaerobic digesters in rural area of Viet Nam [12]. It is, however, lower than BMP of organic fraction of municipal solid waste in Ha Noi - input material of a composting factory (336 NmLCH4/gVS) [6], fruit and vegetable waste (288-516 NmLCH4/gVS) [13], and lignocellulose wastes such as rice straw (430 NmLCH4/gVS) [2]. 3.3. Effect of NaOH pre-treatment on substrate composition and biomethane potential As BMP of original arrowroot waste was not very high, it was expected to see the increase in biodegradability of the waste after alkaline pre-treatment. This possibly can be achieved by changing raw material property, remove or dissolve lignin and hemicellulose, and/or reduce the crystallinity of cellulose [3]. Figure 2 shows the change in cellulose, hemicellulose and lignin content of the pre-treated samples and Table 2 show % reduction of them from those of the untreated sample. It can be seen that as NaOH dose increased, cellulose, hemicellulose and lignin contents gradually decreased. Maximum reduction of lignin was 8.6 % (from 13.9 % down to 10.9 %TS) at highest NaOH dose of 9 %. Maximum reduction of hemicellulose and cellulose contents were 21.6 % and 13.8 %, respectively at the same NaOH dose. Changes of 0 50 100 150 200 250 300 350 400 450 -10 10 30 50 C u m u la tiv e m et ha n e pr o du ct io n (N m l / gV S) Time (day) cellulose - control cellulose - theoretical arrowroot waste arrowroot waste duplicate Biomethane potential (BMP) of arrowroot powder processing waste and effect of 175 main composition trend by alkaline pre-treatment is quite similar to that for rice straw, corn straw reported literatures [8, 9]. Figure 2. Change in composition of arrowroot waste after NaOH pretreatment. Result of BMP test (uBMP and k) for NaOH pre-treated samples are shown in Figure 3 and Table 1. uBMP of the pre-treated also increased as NaOH dose increased from 3 % to 7 % but started to reduce when NaOH dose reached 9 %. The highest BMP was at NaOH 7 % with the BMP of 315.7 NmLCH4/gVS, corresponding to an increase of 21.5 %. The effect of treatment is quite similar to one study for rice straw at the point that too high alkaline does not result in the highest BMP [8]. It was suggested that the removal of lignin, to some extent, increase the accessibility of the microorganism to cellulose and hemicellulose. Similarly, the removal of hemicellulose has a positive effect on the degradation of cellulose because it serves connection between the lignin and the cellulose fibers and gives the whole cellulose-hemicellulose-lignin network more rigidity [3]. However, too much NaOH dose might not result in further positive effect because of one or combination of the following reasons: (1) the higher loss of cellulose and hemicellulose for degradation, (2) inhibition caused by more soluble lignin content [8,9], (3) toxicity caused by the left over NaOH [14], etc. In another hand, regarding positive effect of the pre-treatment, there were possible effects that could not be seen from the changing of composition such as saponification of the uronic bonds between hemicelluloses and lignin, swell fibers and increase pore size, facilitating the diffusion of the hydrolytic enzymes [3] which might play important roles in the pretreatment. Table 1. Composition and Biomethane Potential of pretreated arrowroot waste in comparison with untreated arrowroot waste (Reduction/Increase are in percentage of untreated values). NaOH dose (dry weight based) Cellulose Reduction (%) Hemicellulose Reduction (%) Lignin Reduction (%) uBMP (Nml CH4/ gVS) k (1/d) Increase uBMP (%) NaOH 3 wt.% 3,1 0 0,8 264,4 0,128 1,8 NaOH 5 wt.% 7,3 7,2 0,8 294,6 0,137 13,4 NaOH 7 wt.% 11,4 9,4 7,2 315,7 0,152 21,5 NaOH 9 wt.% 13,8 21,6 8,6 311,8 0,124 20,0 0 10 20 30 40 Cellulose Lignin HemicelluloseCe llu o se /H em ic el lu os e/L ig n in Co m po si tio m (% TS ) Untreated NaOH 3 wt% NaOH 5 wt% NaOH 7 wt% NaOH 9 wt% Nguyen Pham Hong Lien, Tran Dac Chi, Hidenori Harada, Shigeo Fujii 176 Figure 3. BMP of pre-treated arrowroot waste (left: cumulative curves, right: ultimate BMP). 4. CONCLUSIONS The ultimate BMP of untreated arrowroot waste was 253 NmLCH4/gVS showing that the waste has a moderate biodegradability. Pre-treatment by NaOH (from 3 to 9 wt.%) resulted in 1.8 % - 21.5 % more methane yields and the highest yield of 314 NmLCH4/gVS was achieved at NaOH dose of 7 %. 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