Can coffee grounds be considered as a potential for

2018 4th International Conference on Renewable Energies for Developing Countries (REDEC)

Can coffee grounds be considered as a potential for green energy production? Jean H. El Achkar, Ali Baydoun, Dominique Salameh, Nicolas Louka, Zeina Hobaika, Richard G. Maroun Centre d’Analyses et de Recherches, Unité de recherche Technologies et Valorisation Alimentaire, Faculté des Sciences, Université Saint-Joseph de Beyrouth, Liban Coffee is one of the most popular beverages in the world. Over 50% of this mass is discarded after use, becoming a significant waste source known as spent coffee grounds (SCG). SCG usage as a raw material for biogas production emerges with great potential. In this study, we investigated the biochemical composition of SCG and its potential as a green energy source. We demonstrated that SCG can produce 0.1559 Nm3 CH4 / kg COD and that the extraction of polyphenols from the substrates can increase this production by almost 100%. In contrast, acid treatment using HCl showed no effect on gas production from this biomass. Key words: Spent coffee grounds, Anaerobic digestion, Biogas production, Polyphenols, Acid pretreatment

I. INTRODUCTION Worldwide, the global coffee consumption has been estimated at 151.3 million bags of 60kg of coffee during 2016. With an annual growth of 1.3%, the consumption of coffee is increasing steadily which also means that the spent coffee grounds (SCG), the remaining residue of coffee brewing, are also increasing. Most of the SCG produced are being unexploited since thousands of tons are disposed in landfill sites instead of being treated or valorized. With the increase of the organic waste production in the world, particularly in Lebanon which suffers from a waste crisis since 2014, green solutions must be proposed. Given its high organic matter content, the potential of using SCG to produce green energy is very promising. Many recent studies have investigated the conversion of SCG into biofuels, such as biodiesel, ethanol, and biogas using physicochemical or biological processes. For example, anaerobic digestion (AD), which mineralizes organic compounds to methane and carbon dioxide gases, is considered a practical method for recovering energy from an organic waste. The main objectives of this study were to define the main chemical components of SCG (Dry Matter (DM), Volatile Matter (VM), total Chemical Oxygen Demand (COD), cellulose, hemicellulose, lignin, total soluble compounds, polyphenols and oil content) and to evaluate the maximal methane production of the samples in batch mode. In addition, the effects of acid pretreatment with HCL and polyphenols extraction on methane production from our biomass were investigated.

II. MATERIALS AND METHODS 1.

Materials The spent coffee grounds were obtained from a local coffee factory in Beirut. The raw material was stored in the laboratory at -20°C until use. The inoculum was obtained from a local farm in Beckfaya. The particulate matter (>0.5 mm) was removed from the inoculum by passing through a sieve for a better homogeneity. 2. Methods a) Dry matter: The dry matter (DM) content were determined by dry weight in oven at 105 °C until constant weight. Dry matter is calculated using this equation: DM (g/kg) = M105°C / Msample M105°C: mass of the recipient and the dry residue obtained (g) Msample: mass of the initial sample placed inside the recipient (kg) b) Volatile matter: The volatile matter (VM) content were determined after calcination of the dry residue obtained, in a muffle furnace at 550 °C for 4 hours. VM is calculated using this equation: VM (g/kg) = (M105°C - M550°C) / Msample M105°C: mass of the recipient with the dry residue (g) M550°C: mass of the recipient with the calcinated residue (g) Msample: mass of sample (kg) c)

Chemical oxygen demand (COD)

The COD represents the consumption of oxygen necessary to completely oxidize the organic material of the substrate. It is expressed in grams of oxygen per gram of raw material. The chemicals added to the diluted organic suspension to test for COD include: • Oxidizing solution - mercuric sulfate: 100g/L

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silver sulphate: 1g/L potassium dichromate: 1.226g/L

• Titrant solution (FAS) - iron and ammonium sulphate: 9.8g/L - sulphuric acid: 20 mL A suspension of SCG was diluted to 0.3 g/L. 10 ml of the sample was mixed with 10 ml of oxidizing solution and 25 ml of the sulphuric acid solution. The mix is then placed in a digestion tube. The tubes were placed in a digester at 150°C for 2 hours. 100 ml of water was added to the mixture with 1 drop of ferroin. After that, the obtained solution was titrated with the titrant solution until a change in color was observed. The same procedure has been conducted with the blank solution. COD can be calculated using this equation: Figure 1: Van-Soest fractionation COD = [(A - B) * M * 8] / g of sample A: Volume of FAS used to change color for the blank B: Volume of FAS used to change color for the sample M: molarity of FAS 8000: milli equivalent weight of oxygen (8) ×1000 mL/L. d) Biochemical fractionation by the Van-Soest method Van Soest method allows the fractionation of lignocellulosic biomass fibers by successive chemical extractions and quantifies by gravimetry the dry insoluble fibers recovered after filtration[1]. This method (Figure 1) allows the extraction of the soluble fraction (containing nonstructural carbohydrates, pectin, mucilage, soluble tannins at neutral pH, lipids and soluble proteins) with an excess neutral detergent solution acting on the dry sample for 1 h at 100 °C, with a pH of 7 ± 0.05. This solution is composed of disodium phosphate (4.5 g/L), sodium tetraborate (6.81 g/L), α-amylase (0.25 g/L), Sodium EDTA (18.6 g/L), sodium lauryl sulfate (30 g/L) and sodium sulfite (5 g /L). The extracted fraction is then separated from the neutral detergent insoluble fiber (NDF) by filtration. An excess solution of acidic detergent (20 g/L of cetyltrimethyl ammonium bromide and 98 g/L of H2SO4) for 1 hour at 100 °C allows the extraction of hemicellulose. The extracted fraction is separated from the insoluble acid detergent fibers (ADF) by filtration. The cellulose is extracted by treating the ADF fraction with sulfuric acid (1317 g/L) for 3 hours at room temperature. The filtration residue, named ADL, corresponds to the lignin associated with the inorganic elements. The hemicellulose and celluloses contents were calculated as the difference between the NDF and ADF fractions, and the ADF and ADL fractions, respectively. The lignin content is equal to the difference between the ADL fraction and the mineral content determined after calcination at 550 °C.

e)

Determination of total polyphenols

The solid-liquid extraction process of the polyphenols was carried out with a solid-liquid mass ratio of 1/10, in a 70% ethanol/water solvent, at 50 °C. and for 3 hours. The total phenol content is then determined by the Folin-Ciocalteu colorimetric assay[2]. In practice, 200 μl of extract were mixed with 1 ml of Folin-Ciocalteu reagent (diluted ten times). Then, 800 μl of sodium carbonate are added (75 g/L) and the mixture is incubated for 10 minutes at 60 °C and then cooled to room temperature. The absorbance at 750 nm is then measured by a UV spectrophotometer. The total phenol compound contents are expressed as concentration per mg of gallic acid equivalent relative to the initial dry matter (DM) (gGAE/kg DM). f)

Determination of oil content

Oil content was determined by solid-liquid extraction using hexane as the solvent for 30 minutes at room temperature. Hexane is described as giving the highest oil yield when using it to extract oil from SCG[3]. 1g of SCG was dissolved in 25ml of hexane. After 30 minutes at room temperature, the contents were filtrated, and the residue obtained was weighed. Oil content is calculated using this formula: %Oil = (1 – A) * 100 A: Mass obtained after treatment with hexane g) Determination of Alkalinity and TVA Alkalinity and Total Volatile Acidity (TVA) were measured, as described by EL Achkar, 2017, at the end of anaerobic digestion process to verify the stability of the bioreactors.

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h) SCG acid pretreatment The effects of acid treatment on methane production were evaluated using HCl. The pretreatment was carried out in 250 mL flasks, closed with rubber septa. In each flask, SCG samples are introduced with a total solid load of 50 g L−1. Samples were soaked in chemical agent concentration of 6% and 10% HCl. To ensure sufficient mixing, the flasks were continuously shacked at ambient temperature (20 ± 1 °C), for 24 h. At the end of the treatment, the pretreated samples were neutralized with a concentrated NaOH solution to pH 7.5 prior to BMP test. i)

Kinetic study

The methane potential test can be used to estimate the overall hydrolysis constant k. In descriptive terms, this reaction step is typically simplified and reduced to first-order kinetics [6]. B = B0 × [1 − exp (−k · t)] Where B is the cumulative methane yield, B0 is the maximum methane yield of the substrate, k (days−1) is the apparent kinetic constant and t (days) is the time. The adjustment by non-linear regression of the pairs of experimental data (B, t) using GraphPad Prism® (GraphPad, San Diego, CA; USA) allows the calculation of the apparent kinetic constant k and R2. j)

Figure 2: Batch anaerobic digesters unit implemented at Saint Joseph University of Beirut.

Measurement of methane production

This anaerobic digestion unit contains three mains parts. The incubation item consists of a thermostatically controlled (37°C) water bath that contains 500 mL hermetically sealed digesters. Each digester is shaken manually twice a day for one minute to ensure sufficient mixing. The biogas produced passes through a tube connecting the incubation unit to the CO2 fixation unit. The latter consists of bottles containing an alkaline solution fixing the CO2 (80 ml of NaOH at 240 g/L) and thymolphthalein as a colored indicator of pH. The third unit allows the measurement of the CH4 then released, by displacement of the water inside the graduated test tubes. Regarding the inoculum, fresh cow manure was used. It was sieved with a 500 μm sieve to eliminate large particles and to ensure better homogeneity. Then, it was diluted with water, and added to the digesters in a ratio (g DM substrate/ g DM inoculum) of 1/3. The net value of methane production is obtained by subtracting the endogenous production from the bottles containing the inoculum alone.

III. RESULTS 1.

Biochemical fractionation

SCG present a high amount of DM (97.30 %) with high amounts of Volatile solids (93.51 % of DM) (Table 1). Other publications showed similar results regarding dry matter which varied between 92.2 and 94.4% [8], with a VM content of almost 98.57% [9]. Component

Spent Coffee Grounds

Dry Matter (DM) (%)

97.30 ± 0.06

Volatile Matter (VM) (% of DM)

93.51 ± 0.1

Mineral Matter (MM) (% of DM)

6.49 ± 0.05

Table 1: DM, VM and MM of SCG On the other hand, our results show that the soluble compounds represent the most abundant fraction in SCG with almost 52.57 % of DM (Table 2); similar results were obtained in another publication (58.44%) [9]. Hemicellulose constitutes 22.99% of DM, while other publications showed values of 16.3% and 36.7 % [9,10]. Cellulose represents 22.75% of the DM; this result is similar to a publication showing that cellulose represents almost 26.81% of the SCG dry matter [9]. SCG also showed a lignin content of 1.79% with respect to dry matter. Lignin is known to be nonbiodegradable in anaerobic environment [11]. In theory, this would negatively affect the methane production, but since its content is very low in our biomass, pre-treatment does not seem to be necessary for its elimination. SCG had a COD of 0.6 gO2/Kg of raw matter (Figure 2) which indicates that the biomass contains high amount of degradable components and would be a good potential for methane production. Oil content was 20.2 %, higher than the usual values found in literature 15-16%[8][3].

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Furthermore, SCG presents high polyphenol content (31.62 gGAE/kg DM). It is worth noting that polyphenols are generally known as inhibitors of AD systems by limiting the microorganism’s activity as consequence of biostatic effects[13]. Their negative effects on anaerobic microorganisms were observed in many anaerobic systems: treating wine distillery wastewater [14], olive mill wastes [15] and coal gasification wastewater [16]. Thus, the removal of polyphenols from SCG seems to be essential to prevent the system failure and inhibition. COD (gO2/Kg)

0.605

Soluble matter (% of DM)

52.57 ± 3.79

Hemicellulose (% of DM)

22.99 ± 1.32

Cellulose (% of DM)

22.75 ± 2.60

Lignin (% of DM)

1.79 ± 0.01

Oil content (% of DM)

20.00 ± 2.1

Polyphenols (gGAE/kg DM)

31.62 ± 1.72

Table 2: Biochemical fractionation of SCG

2.

Methane production from raw SCG

The cumulative methane production (Nm3 CH4·kg COD-1) for untreated SCG samples is shown in Figure 3. It increases exponentially during the initial phase until day 14 to 16 after the beginning of the experiment. This is the first phase of methane production, corresponding to organic material that is readily biodegradable. After approximately 20 days, a plateau is reached, which indicates that the biomass has been depleted. The cumulative methane yield at the end of test were 0.1559 Nm3 CH4 / kg COD for untreated SCG. The comparison of the obtained results provides detailed information about methane production from these substrates. In many literature studies, batch trials were carried out to assess the methane potential of different types of SCG and the results showed a specific methane production ranging from 0.08 to 0.2 Nm3 CH4 / kg COD [19, 20].

Figure 3: Cumulative methane production of the untreated SCG samples A kinetic study using the first order model was conducted to estimate the overall hydrolysis constant k and the coefficient of determination R2. The untreated SCG had a k of 0.3320 day-1 and an R2 of 0.97. Veeken and Hammelers evaluated the rates of hydrolysis for six components of biowaste (whole wheat bread, leaves, bark, straw, orange peelings and grass)[21]. The first-order hydrolysis kinetic constants ranged from 0.003 to 0.47 day-1 which is consistent with that observed in our study. 3.

Effect of acid pretreatement and polyphenol extraction on methane production from SCG

To intensify the methane production of SCG, an acid pretreatment using 6% and 10% HCl was used. The Van Soest fractionation of the SCG treated with acid solution showed that a treatment with 10% HCl is the most suitable to degrade hemicellulose which was decreased from 22.99% of DM to 11.6% of DM (Table 3). No significant effect was observed when using 6% HCl, in comparison to untreated SCG. SCG treated with 6% HCl

SCG treated with 10% HCl

Soluble matter (%)

50.8 ± 2.1

62.2 ± 1.5

Hemicellulose (%)

21.5 ± 1.3

11.6 ± 0.8

Cellulose (%)

22.8 ± 1.1

22.6 ± 1.9

Lignin (%)

0.0379 ± 0.0012

0.00378 ± 0.0016

Table 3: Van Soest fractionation of SCG treated with 6% and 10% HCl

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Our results showed that the extraction of polyphenols from our biomass resulted in methane production of 0.3148 Nm3 CH4 / kg COD, with a nearly 100% increase in gas production compared to raw SCG. IV. CONCLUSION

Figure 4: Cumulative methane production of SCG treated with 6% and 10% HCl The cumulative methane yields at the end of test were 0.1534 and 0.1574 Nm3 CH4 / kg COD for SCG treated with 6% HCl and 10% HCl, respectively. This treatment did not affect the methane production yields in comparison with the raw SCG. Similar results were obtained when treating maize plants with 2% HCl, for 24 hours at 20 °C [23]. However, some studies have showed that HCl can have a limited positive effect on methane production from sugarcane bagasse and coconut fibers [22]. On another note, polyphenols, as specified previously, are inhibitors of AD systems by limiting the microorganism’s activity. Hence, polyphenols extraction before the AD process of SCG may improve the methane production.

This study was conducted to highlight the potential of SCG as an alternative energy source. SCG were tested for both methane production and physico-chemical characterization. The batch anaerobic digestion test demonstrated that SCG can be considered an important energy source via anaerobic digestion. In addition, we underlined the great effect of polyphenols extraction in increasing methane production by almost 100%, compared to the raw substrates. Polyphenols extraction would be of great interest because these molecules are widely known for their medical uses and antioxidant powers, protecting the body's tissues against oxidative stress and various associated pathologies such as cancer, coronary heart disease and inflammation. On the other hand, acid pretreatment using HCl did not display any significant effect on methane production, which remained constant. As a perspective for this work, an interesting aspect to study is the co-digestion. That is why coupling SCG with different types of organic wastes could increase the performance of the anaerobic digesters by improving the economic balance of the process. REFERENCES [1] [2]

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