Biohydrogen generation from palm oil mill effluent using AnSBR-Mec Hybrid System

Abstract

The global dependency towards fossil fuels as an energy source has escalated resulting in rapid depletion of oil reservoir and global warming. The oil palm industry, the largest agricultural sector in the country, is expected to generate approximately 70-110 million tonnes of solid and liquid wastes by the year 2020. The large quantities of liquid waste, or palm oil mill effluent (POME), is highly polluting due to its high content of biological and chemical oxygen demand. Since POME is rich in substrates such as carbohydrates and fatty acids, this study investigated the application of bacteria from POME anaerobic sludge for hydrogen gas (H2) production in a hybrid system of anaerobic sequencing batch reactor (AnSBR) and microbial electrolysis cell (MEC). Five pre-treatment methods (heat-shock, acid, alkaline, chemical, and aeration) were tested for selecting H2-producers from POME anaerobic sludge. Out of the five methods, heat-shock showed the highest H2 production. Several factors influencing H2 production was determined using one-factor-at-a-time (OFAT) method. The results showed highest H2 production at initial pH 6.0, incubation temperature of 55.0°C, and 10.0% (v/v) inoculum size. Further study on the optimisation of H2 production using Box-Behnken Design (BBD) showed that all factors significantly contributed towards H2 production. The combination of initial pH (6.4), incubation temperature (58.0°C), and inoculum size (8.0% v/v) enhanced the H2 production up to 2.57-fold higher (239.0 mL) compared to before optimisation. Subsequently, these optimised conditions were applied in a 1.2 L AnSBR together with two factors: hydraulic retention time (HRT) and organic loading rate (OLR). The results showed that 12 h HRT and 36 g/L/d OLR were the best condition with maximum H2 production recorded at >2.80 L H2/L POME and almost 65% COD removal achieved in 16 cycles of operation. Single chamber MEC was then fed with the AnSBR treated POME. Voltage of 1.0 V was applied to operate the MEC with cumulative H2 production of 3.63 ± 0.16 L H2/L POME over a duration of seven days. The maximum Coulombic efficiency, CE (83.9 ± 3.3%), maximum H2 recovery, ???????? (99.1 ± 4.7%), and total energy recovery, ????+?? (103.9 ± 8.9%) were also recorded. To determine the microbial communities of the hybrid system, genomic bacterial DNA from AnSBR sludge and both bioanode of MFC and MEC were extracted and subjected to 16S rRNA gene amplicon sequencing. In the top 10 phyla of the three samples, phyla Firmicutes are predominant for AnSBR sludge while phyla Proteobacteria were dominant for both MFC and MEC bioanode, representing approximately 40% and 80% of the total gene fragment, respectively. Predictive functional metagenome profiles based on 16S rRNA marker genes showed that the most abundant metabolic functions in the three samples were related to central metabolisms and substrate utilisation, with “energy metabolism”, “carbohydrate metabolism”, and “amino acid metabolism” were the top three abundant pathways. Findings from this study showed effluent from POME AnSBR contains highly valuable volatile fatty acids (VFAs) that could be recycled and maximised to produce H2 in MEC. This study showed high potential of turning highly polluting POME as source to generate clean and renewable H2 energy.