Effect of Thermal Pretreatment of Pineapple Peel Waste in Biogas Production using Response Surface Methodology

This study aims to determine the effect of thermal pretreatment from pineapple peel waste on biogas production using a batch anaerobic digestion process. The experimental process was carried out on various variables, including observation time (30 days), operating temperature (25 – 35 ), the ratio of starter and sample (1:1), also applied by two treatments, namely the anaerobic digester process without pretreatment and pretreatment using a hot water bath with a temperature of 60 , 80 , and 100 , with a time duration of 25, 45, and 65 minutes. The results showed that the thermal pretreatment given to pineapple peel waste accelerated the biogas production process and reduced the lag phase in the anaerobic digestion process. The highest biogas production volume was obtained from pineapple peel waste, which was 616.33 mL (357.190 mL/g volatile solids), pretreated for 25 minutes at 60  (variable B3).  The lowest biogas production was obtained from pineapple peel waste without pretreatment (variable A), which was 384.33 mL or 219.619 mL/g of volatile solids. The optimum % yield value of CH₄ gas content reached 67.27%, which was achieved in the pineapple peel hot water bath pretreatment at a temperature of 100  with a water bath time of 25 minutes. Meanwhile, pineapple peel waste without a pretreatment hot water bath obtained a % CH₄ yield of 60.19%. The lignocellulose analysis results with the highest hemicellulose and cellulose content were found in pineapple peel waste, pretreated for 45 minutes at a temperature of 80  (B9 & B10), with 22.1% and 55.2%, respectively. The B9 and B10 samples obtained the lowest lignin content of 0.41% for both samples.

Anaerobic digestion; Biogas; Pineapple peel waste; Thermal pretreatment

    Animal waste includes a large amount of nitrogen (N) and phosphorus (P), causing nutrient imbalance and environmental degradation if not effectively managed (Chakravarty, 2016; Deressa et al., 2015; Hamzah et al., 2020). Therefore, combining pineapple peel waste and animal waste could produce biogas with high content of carbohydrates and methane gas while also be the solution for the environmental issues. The gas composition in biogas fuel has a generally higher percentage of CH₄ than CO₂, N₂, O₂, and H₂S. According to (Alvarez & Lidén, 2009), cow dung and the mixture of agricultural waste yielded CH₄ fertilizer in 47% and 47 – 55%, respectively. Thermal pretreatment is one of the processes to increase biogas production. It is used to help the compound contained in plants be easily broken so that microorganisms can easily convert polymers in cellulose and hemicellulose into biogas (Budiyono et al., 2017; Darwin et al., 2016). A method that can be used to produce biogas is the anaerobic fermentation method using a biogas reactor (biodigester). Biogas is a gas mixture consisting mainly of methane and carbon dioxide. Biogas is produced anaerobically through the following three stages: hydrolysis, acidogenesis, and methanogenesis (Biarnes, 2016; Prasetyo et al., 2017). Various kinds of organic wastes such as animal manure, municipal solid waste, agricultural residues, and industrial waste can be used as a substrate in biogas production. Other substrates are kitchen, garden, cow dung, and domestic waste. Different biomass sources (waste) will produce different quantity of biogas, e.g., two liters tapioca waste water could produce 2458 mL of biogas (Budiyono et al., 2018a; Nwokolo et al., 2020; Sumardiono et al., 2015). The biogas production system has several advantages, such as: reducing the effect of greenhouse gases, reducing unpleasant odor pollution, as fertilizer also producing power and heat, creates a healthier environment by converting waste to biofuel, also compost sludge and liquid fertilizer can be made from biogas (Koopmans & Consultation, 1999; Pertiwiningrum et al., 2017).

The pineapple peel is the outer part of the pineapple fruit (Ananas comosus), it’s also the biomass source that is usually thrown away (Sianipar, 2006). Characterization of pineapple peel waste with C and N content according to (Fu et al., 2016), namely C in the amount of 41.02% and N in the amount of 0.79%. The composition between the C and N content in the pineapple peel waste was reported to affect the biogas CH₄ production (Arifan et al., 2018; Laopaiboon et al., 2010). The pre-treatment and hydrolysis processes are essential processes that affect the yield of biogas. Large organic molecules cannot be directly absorbed and used by microorganisms as a substrate source and yield methane (Schnurer & Jarvis, 2010; Verma, 2002). The pretreatment process is carried out to condition lignocellulosic materials both in terms of structure and size. The pre-treatment process directly affects biogas production by breaking down the lignin content (Ghatak & Mahanta, 2017; Sumardiono et al., 2017). Research conducted by (Basaria & Priadi, 2016) and (Arifan et al., 2021a) shows that the pretreatment process can improve the performance of anaerobic digestion (AD) by increasing the contact between the substrate and microorganisms resulted in the higher amount of methane yields produced of 0.229 L CH₄/g VS. According to (Casabar et al., 2019), the purpose of pretreatment is to open the lignocellulose structure and to make the cellulose more accessible to enzymes that break down saccharide polymers into sugar monomers. Pre-treatment provides easier access to the enzymes to increase glucose and xylose yield. According to (Harmsen et al., 2010) the pretreatment process in which hemicellulose hydrolysis may occur, includes physical pretreatment (heated, crushed, milled, sheared), chemical pretreatment (hydrolysis of weak acids, strong acids, alkalis), a combination of physical and chemical pretreatments (steam explosion, CO₂ explosion, ammonia fiber explosion (AFEX)), and biological pretreatment (enzymes and microorganisms).
    An advantage of thermal pretreatment is its cost-effectiveness, as it dissolves the biomass waste's high lignin concentration without the use of sodium hydroxide, which is typically required for other pretreatment methods. The research objective is to determine pineapple peel waste thermal effect on biogas production using a batch anaerobic digestion process. As a result, this research will be used to the problem of finding solutions for renewable energy.

This study used thermal pretreatment using a hot water bath with pineapple peel waste as the raw material. The results indicate that pretreatment of pineapple peel waste with hot water bath significantly affects the CH₄ content in biogas. In contrast to the analysis of lignocellulose in lignin, hemicellulose, and cellulose in variable A (without pretreatment), which revealed a high lignin concentration, the lignin content decreased in variables B1 – B10 with pretreatment. This result indicates that the pretreatment process affects lignin yields, affecting the amount of cellulose and hemicellulose digested by microorganisms. The CH4 gas content in hot water pretreated bath (B1 – B10) resulted in a higher % CH4 than those without pretreatment (A). The optimum results of % CH₄ reached were 67.27%, which was achieved in pretreated hot water bath of pineapple peel waste at a temperature of 100° in 25 minutes. Future works may consider comparing the process and method of thermal pretreatment with other pretreatments such as mechanical and chemical pretreatment. The results can determine which pretreatment method is the best pretreatment to increase the yield of production and obtain the purest CH_{4 ­}content without impurities in the produced biogas.

The authors would like to thank the Industrial Chemical Engineering of Vocational School UNDIP, and Department of Animal Science UNDIP for providing the laboratory for conducting this research. This work was supported by the PNBP Universitas Diponegoro [185-59/UN7.6.1/PP/2021].

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