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The identification of microbial diversity is important to speed up the co-composting process of EFB and POME from the palm oil mills, as an effective and cost-efficient way, to treat the residues and reducing the greenhouse gas emission. This study was conducted to identify the microbial diversity from EFB-POME co-compost and to evaluate its potential mitigation of greenhouse gas emission. The physiochemical properties such as temperature (°C), pH and moisture content of the EFB-POME co-compost were measured. The microbial diversity was identified by 16S rRNA and 18S rRNA gene metagenomic sequencing analysis. The temperature, pH and moisture content recorded for the surface compost and inside compost were 30 °C, 7.43 and 58.76%, and 45 °C, 7.94 and 60.56%, respectively. Based on the 16S rRNA gene sequencing for the identification of bacteria, the dominant genera in the surface compost were Nitriliruptor, Delftia, Filomicrobium, Steroidobacter, and Ohtaekwangia; the dominant genera in the inside compost were Steroidobacter, Nitriliruptor, Anaeromyxobacter, Filomicrobium, and Truepera; and the dominant genera in the POME were Parabacteroides, Bellilinea, Levilinea, Smithella, and Prolixibacter. Based on the 18S rRNA gene sequencing for the identification of fungal, the dominant genera in the surface compost were Remersonia, Inonotus, Kluyveromyces, Chaetomium, Thermomyces, and Candida; the dominant genera in the inside compost were Remersonia, Inonotus, Saccharomycopsis, Chaetomium, and Saccobolus; and the dominant genera in the POME were Kluyveromyces, Inonotus, Kazachstania, Candida, andCystofilobasidium. The co-composting of EFB-POME is estimated to reduce up to 76% of greenhouse gas emission by avoiding the methane gas emission from the EFB open dumping and POME treatment ponds, as well as from the replacement of chemical fertilizers. The microbial diversity identified from EFB-POME compost and POME may enhance the effectiveness of co-composting due to the ability to synthesize ligninolytic and cellulolytic enzymes and reduce the emission of greenhouse gases from the oil palm plantations.

Hydrogen generation was studied using palm oil mill effluent (POME) as an agro-industrial waste obtained from the palm oil industry. POME was subjected to a dilute acid hydrolysis step by HCl (37% v/v) to release fermentable sugars from cellulosic content. POME hydrolysate obtained was used as a substrate for hydrogen generation. The composition of POME hydrolysate showed glucose and xylose were the main monomeric sugars liberated. Hydrogen production was performed in dark fermentation process, in which the new bacterial strain Clostridium acetobutylicum YM1 was cultivated on POME hydrolysate based on a central composite design (CCD). CCD was constructed by considering three pivotal process variables including incubation temperature, initial pH of culture medium and microbial inoculum size. An empirical model, namely second-order polynomial regression model was generated and adjusted to CCD data. The analysis of empirical model generated showed that the linear and quadratic terms of temperature had a highly significant effect on hydrogen generation (P < 0.01). Furthermore, the quadratic effects of initial pH value of culture medium and inoculum size had a significant effect on hydrogen production at 95% probability level (P < 0.05). The regression model also showed that the interaction effect between temperature and initial pH value of the culture medium on the hydrogen generation was highly significant (P < 0.01). The empirical model suggested that the optimum conditions for hydrogen production were an incubation temperature of 38 °C, initial pH value of 5.85 and inoculum size of 17.61% with predicting the production of a cumulative hydrogen volume of 334.2 ml under optimum conditions. In order to validate the optimum conditions determined, C. acetobutylicum YM1 was cultivated on POME hydrolysate in optimum conditions. Verification test results showed that a cumulative hydrogen volume of 333.5 ml and a hydrogen yield of 108.35 ml H2/g total reducing sugars consumed were produced.

Fermentative hydrogen generation was studied using palm kernel cake (PKC) as sustainable cellulosic biomass. PKC was subjected to an acid hydrolysis approach using dilute H2SO4 (7% v/v). PKC hydrolysate obtained was then diluted (70%) and used as a substrate for hydrogen generation. Chemical analysis showed that the main fermentable sugars in diluted PKC hydrolysate were glucose, xylose and mannose with the concentrations of 2.75 g/L, 2.60 g/L and 27.75 g/L, respectively. Hydrogen production was carried out by the cultivation of Clostridium acetobutylicum YM1 on PKC hydrolysate. The effect of incubation temperature, the initial pH of culture medium and microbial inoculum size on hydrogen production was studied using a statistical model. The analysis of the model generated showed that the initial pH value of the culture medium and inoculum size had significant effects on the hydrogen production. The study showed that the optimum conditions for the biohydrogen production were 30.57 °C temperature, pH 5.5 and 20% inoculum size. A verification experiment was performed in the optimum conditions determined. Experimental results of the verification test showed that a cumulative hydrogen volume of 1575 ml/L was generated with consuming 2.75 g/L glucose, 2.20 g/L xylose and 16.31 g/L mannose.

Microbial electrolysis cell (MEC) is a potentially attractive green technology to tackle the global warming and energy crisis, which employs electrochemically active bacteria to convert organic matter into hydrogen or a wide range of chemicals, such as methane, acetate, hydrogen peroxide, ethanol, and formic acid, without causing environmental pollution. Until now, probably the cleanest and the most efficient method of producing hydrogen has been MEC. However, this technology is still in its infancy period and poses various challenges towards up-scaling and widespread applications, such as such as lower hydrogen production rate (HPR), high internal resistance, complicated architecture, and expensive materials. New advances are needed in biofilm engineering, materials for electrodes and reactor configuration for successful real-world application of this technology. Thus, the present review deals with development of practical MEC technology and includes the following sections: firstly a general introduction to MECs; their operating principles, thermodynamics of MEC, and energy or voltage losses in the MEC system were provided. Followed by a section on the critical factors affecting MEC performance; microorganisms, anode, cathode, membrane or separator, fuel sources, the state-of-art MECs designs, other key operational factors, and its potential application in microbial production of value added products are discussed in detail. Afterwards, current challenges involved in developing practical MEC systems are highlighted, and outlooks for future development are also suggested. The review aims to assist researcher and engineers to gain fundamental understandings of MEC, and it also provides several future research directions and a road map on how to overcome the barriers, so the MEC technology can be further advanced and applied in larger scale.

This research aimed to study the fermentative production of hydrogen using de-oiled rice bran (DRB) as renewable biomass. DRB was obtained by the extraction of oil content of rice bran samples. DRB was then hydrolyzed by dilute H2SO4 (1% v/v) to obtain DRB hydrolyzate as substrate for hydrogen generation. The composition of DRB hydrolyzate showed that glucose and xylose formed the main fermentable sugars released from acid hydrolysis of DRB. Hydrogen production was performed by the cultivation of new strain Clostridium acetobutylicum YM1 on DRB hydrolyzate in an anaerobic fermentation process. The effect of pivotal operating variables (incubation temperature, initial pH of culture medium and microbial inoculum amount) on hydrogen production was studied using response surface methodology. A second-order polynomial regression model was generated to evaluate hydrogen generation trend under conditions tested. The model analysis revealed the high significance of linear effects of incubation temperature (P < 0.01) and inoculum amount (P < 0.05) on hydrogen generation. The process model also showed that the quadratic effects of the incubation temperature and pH value were significant at 99% probability level and 95% probability level, respectively. Similar results indicated that the interaction effect between incubation temperature and inoculum amount was highly significant (P < 0.01). The regression model suggested that the optimum conditions were an incubation temperature, pH value and inoculum amount of 35.2 °C, 5.5 and 11.6% (v/v), respectively. In order to validate the optimum conditions determined by the model, Clostridium acetobutylicum YM1 was cultivated on DRB hydrolyzates under optimum conditions determined. Verification experiment showed that a cumulative hydrogen volume of 572.5 ml and a hydrogen yield of 132.2 ml H2/g total sugars consumed were generated.