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Anaerobic digestion of pineapple waste appears to be an effective method for non-renewable energy substitution through biogas production. The potential power generation from the exploitation of pineapple waste as fuel is estimated to be roughly 20.8 MW. Nevertheless, the intricate composition of pineapple waste, characterized by the complex arrangement of its structure, poses a significant challenge in attaining a substantial amount of biogas production. This study pretreated pineapple waste with subcritical water to increase biogas production. Two temperature settings (120⁰C and 200⁰C) were used for pretreatment. Combined pre-treatment at low temperatures and short time (120⁰C, 5 minutes, 10 water to solid ratio) resulted in 31.6% higher biogas production than untreated. However, pretreatment at high temperatures and longer reaction time (200⁰C,25 min) reduced the biogas production by 9% as compared to untreated. Using the Modified Gompertz kinetic model, pretreatment improved the lag phase and increased biogas production to 14.41 mL/day. The lignocellulosic composition of pre-treated pineapple waste decreased, while process parameters such as total ammonia nitrogen removal and pH improved after the pretreatment. Subcritical water pretreatment, particularly when conducted at high temperatures, did not yield any enhancements in the anaerobic digestion of pineapple waste. As a result, it is not advisable to employ this method for these purposes.

This work reviews hydrothermal subcritical water pretreatment to enhance biogas production through anaerobic digestion. The complexity of the lignocellulosic structure has been the main limitation contributing to unsatisfactory biogas production throughout the anaerobic digestion. The high resistance of the structure to biological hydrolysis has increased the interest in applying pretreatment prior to anaerobic digestion to facilitate hydrolysis. Hydrothermal subcritical water technology, an environmentally friendly pretreatment that uses water as the main medium, is gaining prominence in biogas enhancement. However, the subcritical water pretreatment influence on structural properties, biogas production, and the production of anaerobic process inhibitors signifies a knowledge gap and needs an evaluation. This review presents the need for pretreatment reaction and properties in the subcritical water region, biogas production from subcritical water pre-treated waste, production of inhibitors, and its challenges are discussed. This pretreatment could be a promising option and further enhance biogas production throughout the anaerobic digestion process.

The co-digestion of lignocellulosic waste with manure has been seen to improve the C/N ratio required by the systems to produce higher methane yield compared to mono digestion. Despite the success of co-digestion systems, the complex structure of pineapple waste slows down the digestion process. Subcritical Water (SCW) pre-treatment was introduced to improve the digestibility of pineapple waste. This study aims to evaluate suitable organic loading and mixing ratio of pineapple waste and cow dung for co-digestion process. Then, the SCW pre-treatment conditions of pineapple waste such as temperature, time and water to solid ratio were optimized using Response Surface Methodology (RSM). The impact of untreated and pre-treated pineapple waste on the physicochemical and morphology properties, methane production, its kinetic, and performance in an up-scaled anaerobic digester were further assessed at optimal conditions. Based on volatile solid (VS) contents, five different cow dung to pineapple waste ratios were assessed. The highest biogas and methane yield were obtained at ratio 1:3 with 179.08 mL/g VS and 142.89 mL CH4/g VS. The co-digestion improved the C/N ratio, total ammonia nitrogen (TAN), VS removal, and pH. The kinetic study using the modified Gompertz model showed that the co-digestion process had shortened the lag phase, and the highest biogas production rate was observed at 12.80 mL/g VS. day. For optimization process, the model fitting showed the coefficient of determination (R2) value of 0.9825. The RSM model predicted a maximum biogas yield of 144.17 mL/g VS for pre-treatment conditions at the water to solid ratio of 7.5:1, the temperature at 160°C and the reaction time of 15 minutes. The optimal condition of SCW pre-treatment was at 128.51°C, water to solid ratio of 5.67:1, and reaction time of 5 minutes. The compositional analysis found that the pre-treated pineapple waste reduced lignin, hemicellulose, and cellulose contents. Pre-treated samples yielded

The current study compares the biogas production via anaerobic co-digestion of pineapple waste and cow dung to mono digestion under mesophilic condition, with the goal of modifying the substrate ratio and increasing the system stability. Five different cow dung to pineapple waste ratios (1:1, 1:2, 1:3, 2:1 and 3:1) were assessed based on volatile solid (VS) contents. The highest biogas and methane yield were obtained at a ratio of 1:3 with 179.08 mL gas/g VS and 142.89 mL CH4/g VS, respectively. When compared to mono digestion of pineapple waste, the co-digestion process improved process stability in terms of C/N ratio, total ammonia nitrogen, VS removal and pH. Biogas refinement using carbon dioxide remover was able to reach high methane percentage while lowering carbon dioxide percentage to below 3%. The kinetic study using modified Gompertz model showed that the co-digestion process had shortened lag phase, and the highest biogas production rate was observed at 12.80 mL/g VS day. This suggests that the co-digestion process could increase biogas production rate, enhance process efficiency and significantly reduce fermentation time. Co-digestion of pineapple waste with cow dung appears to be a promising approach for bioenergy recovery.

The abundance of agricultural wastes produced from pineapple processing and livestock industries has resulted in the difficulties of disposing of a large amount of waste. Anaerobic digestion is a way to reduce waste and generate renewable energy sources including biogas. In this study, pineapple waste is co-digested with cow dung in batch experiments under mesophilic temperature at 38±1°C at a working volume of 100 ml in 125 ml serum bottle. The effects of the total solid on methane yields are investigated at a different substrate ratio. The batch study is conducted at 3 different total solid which are 12%, 20% and 28% and at three different substrate ratio cow dung to pineapple waste (CD: PW) (1:1, 1:2 and 1:3). Daily biogas collection for 28% total solid at 1:1 ratio results in the highest cumulative biogas production of 313 ml, followed by 28% total solid at 1:3 ratio with 246 ml biogas yield. The highest methane yield is achieved at 12% total solid with a 1:2 ratio (17.19 CH4/g VS). Results show that at 12% total solid produces the highest methane yield at all ratios compared to other total solid percentages. Moreover, methane yield decreases as the total solid percentage increases from 12% to 28%. Overall, the production of methane from pineapple wastes co-digested with cow dung is proven to be a good strategy to minimise solid wastes.