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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.

Lignocellulosic biomass (LCB) is a widely available and sustainable energy resource that can be directly or indirectly converted to biofuels and value-added bioproducts. In such LCB conversion, enzymatic saccharification is commonly regarded as a green alternative to chemical hydrolysis due to less energy-intensive, less toxic, and more environment-benign for efficient fermentable sugar recovery. However, enzymatic saccharification faces substantial challenges, since the complex polymeric matrices of LCB necessitates a variety of enzymes for complete and adequate saccharification. Empirical evidence on enzymatic saccharification has paved the way for optimizing the processes and design for enhancing the performance in LCB. This review examines the enzymatic saccharification of LCB, focusing on the important parameters affecting the process, such as pH, temperature, agitation, enzyme/substrate loading, residence time, and the enzymes required to degrade various LCB components. Various strategies have been reported to improve the performance in saccharification and to address the non-productive adsorption of enzymes. A preliminary economic competency valuation of enzymederived fermentable sugars is proposed. Wheat straw, sugarcane bagasse and corn stalk appear, in this case, to be the most economic competent LCBs for commercial enzyme-derived fermentable sugar production. Lastly, practical challenges and future research directions on the enzymatic saccharification of LCB are discussed.

The production of biohydrogen can be improved by focusing on the nutrients needed by fermentative bacteria like iron. Iron reacts with the [Fe-Fe]-hydrogenase enzyme within the mixed bacteria culture for optimum hydrogen release. Iron nanoparticles (NPs) are attractive due to its unique properties and high reactivity. It can be produced through green synthesis, a more eco-friendly and relatively lower cost process, by using iron salt as precursor and green coconut shell extracted by deep eutectic solvent (DES) as reducing agent. The coconut shell extract consists of phytochemicals that help in producing polydisperse magnetic iron oxide nanoparticles at ∼75 nm in size. The addition of optimum concentration of 200 mg Fe/L magnetic iron NPs resulted in the maximum cumulative hydrogen production, glucose utilization and hydrogen yield of 101.33 mL, 9.12 g/L and 0.79 mol H2/mol glucose respectively. Furthermore, the kinetic analysis on Gompertz model using the optimum magnetic iron NPs concentration showed that the hydrogen production potential (P) and hydrogen production rate (Rm) increased to 50.69 mL and 3.30 mL/h respectively and the lag phase time reduced about 7.12 h as compared with the control experiment (0 mg Fe/L). These results indicated the positive effects of magnetic iron NPs supplementation on fermentative biohydrogen production of mixed bacteria culture and proved the feasibility of adding the magnetic iron NPs as the micronutrient for enhancement of such hydrogen production system.

Optimum conditions for xylitol production from xylose based on the reliable kinetic models of the fermentative performance are important to design lignocellulosic xylitol production. The model was validated by comparing the simulation results with experimental data in batch culture of Kluyveromyces marxianus ATCC 36,907 under various concentrations of initial substrate (36 to 122 g/L) and agitation speed of the impeller (200 to 500 rpm) in a 3.6-L bioreactor. The model suggested that the kinetics of K. marxianus ATCC 36,907 using a modified Monod equation was regulated by the effects of xylose concentrations and oxygen levels during the fermentation. Xylitol production appeared favorable with maximum yield of 0.43 g/g corresponded to kLa value of 29.9 h−1 with low microbial growth. Sensitivity analysis on the obtained model parameters showed that the maximum specific growth rates (µmax) and substrate consumption constant (KS) are the most influential model parameters for xylitol production.

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.

In this study, a techno-economic assessment of an on-farm biogas system using an anaerobic biofilm reactor utilizing cow manure as a fermentation substrate was evaluated. A projection model was developed using Microsoft Excel software with three outputs, the size and dimension of a bioreactor, experimental microbial kinetic studies, and the economic studies based on the experimental results. Characterization analysis of cow manure wastewater showed the total solid (TS), total volatile solid (TVS), total carbohydrate (TC), chemical oxygen demand (COD), and pH values which were 10.95 g/L, 8.65 g/L, 6.65 g/L, 57.80 g/L, and 7, respectively. Using the modified Gompertz equation for the microbial studies, it was found that, at 37 °C and 20 days hydraulic retention time (HRT), the biogas yield was 934.54 mL/gVS, the volume of biogas produced was 11.28 m3/d, and 22.56 kWh of electricity was generated. The Gompertz prediction helps to determine the optimal HRT for the system so that the microorganisms are at their optimum stage to produce biogas. The economic analysis was done, and the results illustrated that, when the rate of cow manure produced was at 55 L/day.cow, the net present value (NPV) was RM 611,936.09, with a 13% internal rate of return (IRR), 0.14 return on investment (ROI), and 7.02 years of payback period (PP). By developing a techno-economic assessment that included all the necessary parameters such as sizing of the bioreactor, microbial kinetic studies, and economics of the plant, farmers could easily implement the system into their farms. This model showed that the anaerobic digestion system utilizing an attached biofilm with cow manure as a fermentation inoculum and substrate was applicable on an industrial scale to generate electricity and reutilize to the farm, at the same time generating additional income from the production of fertilizer.

Food processing wastewater (FPW) contains high levels of oil and grease (O&G), especially from factories that produce fast food such as nuggets, sausages, and burger patties from beef, poultry, and fish. Therefore, the feasibility of replacing conventional membranes for the treatment of FPW with low-cost dynamic membranes (DM) as an alternative was investigated in this study. An anaerobic DM bioreactor (DAnMBR) was operated for 90 days to evaluate the treatment performance using real FPW under different organic loading rates (OLR) of 3.5, 5.0, 6.5, and 7.0 g COD (chemical oxygen demand)/L day. Once the reactor had reached a steady-state of 90% COD removal, the feed to the reactor was supplemented incrementally with FPW from 10% to 90% as COD to allow the methanogenic bacteria to acclimate any potential inhibitory effects from its recalcitrant content. The bioreactor presented a stable performance at OLR 5.0 g COD/L day with 97.5% removal of COD and reached 20 mg/L total suspended solids (TSS) discharge. A significant correlation between COD fractions removed via acidogenesis and methanogenesis with different OLR was found, indicating that the increase in treatment performance is beneficial to the methanogenic archaea activity. The methane gas production yield achieved a maximum of 0.40 L methane/g CODadded at OLR 3.5 and 5.0 g COD/L day. The average permeate flux in these studies is around 60 L/m2 h. The DM fouled after 57 days (at flux 27.16 L/m2 h immediate drop to 2.16 L/m2 h) when operated at 3.5 g COD/L day. After it fouled, the membrane underwent physical cleaning, backwashed in-situ for 5 min, and reused again without any chemical cleaning. The improved filtration resistance is contributed by the occurrence of DM fouling induced by the soluble microbial products (SMP) and extracellular polymeric substances (EPS) release as well as the increased protein/carbohydrate (P/C) ratio in the mixed liquor.

Lack of good management practice of chicken slaughterhouse wastewater (CSWW) has caused pollution into water bodies. In this study, the potential of seed sludge acclimatised modified synthetic wastewater (MSWW) on bioreactor performance and energy recovery of CSWW treatment was investigated. Two sets of upflow anaerobic sludge blanket (UASB) reactors were employed. The seed sludge in UASB 2 was acclimatised with MSWW for 30 days. In UASB 1, no acclimatisation process was undertaken on seed sludge for control purposes. After the acclimatisation process of UASB 2, both reactors were supplied with CSWW under the same condition of organic loading rate (OLR = 0.5 to 6 gCOD/L/d) and mesophilic condition (37 °C). COD removal efficiencies of UASB 2 were >80% all through the steady-state of the OLR applied. Meanwhile, a drastic decrease in overall performance was observed in UASB 1 when the OLR was increased to 3, 4, 5, and 6 gCOD/L/d. Energy recovery from laboratory scale and projected value from commercial-scale bioreactor were 0.056 kWh and 790.49 kWh per day, respectively. Preliminary design of an on-site commercial-scale anaerobic reactor was proposed at a capacity of 60 m3.

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.

The inefficiency and lengthy treatment process of conventional open ponding method has posed challenges to palm oil industry in treating highly polluting palm oil mill effluent (POME), thus there is a need to look for a more efficient and sustainable alternative solution. Phytoremediation has been recommended as a potential method for wastewater treatment due to its economical and environmental advantages. However, research on phytoremediation of POME using Eichhornia crassipes (E. crassipes) is still limited. This study aims to determine the feasibility of applying phytoremediation using E. crassipes for POME treatment by investigating the effects of pH, plant: POME ratio and retention time on the biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solid (TSS) in POME. The highest BOD removal of 92.78% was achieved after 21 days retention time at pH 4 with plant: POME ratio of 1:20 kg/L. The highest COD removal of 25.24% was achieved after 14 days retention time at pH 6 with plant: POME ratio of 1:20 kg/L. Phytoremediation using E. crassipes was shown to be a promising eco-friendly technique for POME treatment, considering the shorter treatment time required and its effectiveness in reducing the BOD in POME to meet the discharge standard of 20 mg/L.

A mathematical model of minimum fluidization velocity (Umf) was developed based on thehydrodynamic characteristics of the fluidized bed reactors (FBR) with immobilised cellsattached to activated carbon at thermophilic biohydrogen fermentation. The maximumhydrogen productivity rate of 7.8 mmolH2/L.h and hydrogen yield of 2.2 molH2/mol of sugarconsumed was obtained when the HRT was shortened from 48 h to 6 h. The presence of theimmobilised cells enriched the biomass composition in the FBR from 4.9 to 7.1 g VSS/L andmaximum energy generated was 58.7 KJ H2/L.d. The FBR had to be operated at a high Umfof0.05e0.44 cm/s and a low terminal velocity of 2.11 cm/s to prevent the immobilised cellsfrom washed out from the FBR, hence achieved an adequate fluidization system. Ascreening of the microbial population by DGGE revealed that theT. thermosaccharolyticumsp. was dominant for all the HRTs, thereby indicating that this bacterium is resilient to-wards environmental disturbances.