BIO-HYDROGEN PRODUCTION FROM ORGANIC WASTE AND WASTEWATER BY MICROBIAL ELECTROLYSIS CELL

Abstract

The persistent socioeconomic progress in terms of urbanization and industrialization demand increased energy. This can be supported by a fact that oil budget has gone up from 9000 million tonnes of oil equivalent (Mtoe) in 1990 to a 14,000 Mtoe in 2018, with projections of about 20,000 Mtoe by 2050. In a study it is estimated that 80+ % of global energy demand is dependent on fossil fuels, but these fuels led to increased carbon dioxide emissions and greenhouse gas emissions (GHG) causing global warming, and different types of pollution. The need for sustainable, renewable and carbon-neutral fuel sources to address the depleting finite fossil reserves and the associated pollutions. Biomass-based renewable energy sources are promising e.g. bio-hydrogen, bio-ethanol, bio-methane etc. Among these, hydrogen is considered as “Fuel of the Future” as it produces water vapours upon combustion and do not contribute to any greenhouse gas emission, ozone depletion, or acid rain, which leads to climate change. Hydrogen gas is colourless, tasteless, odourless, light, and non– toxic. The high heating value of hydrogen (∼120 kJ/g) and low density make it a competitive energy carrier. The present hydrogen production methods, like steam reforming, electrolysis of H2O, pyrolysis, gasification, etc., are energy extensive processes and also requiring high temperatures also linked to various greenhouse emissions. To counter this, biological route i.e. microorganisms assisted hydrogen production from renewable resources such as biomass, is in focus for sustainable development and waste minimization. The disposal of the waste and wastewater directly to the land or into the waterbodies is detrimental. Moreover, municipal solid waste generation in 2016, reached 2.0 billion tonnes and is expected to reach 3.4 billion tonnes by 2050. Therefore, use of waste and wastewater for energy extraction would reduce the cost of the treatment. Biological hydrogen production using these wastes is a promising alternative as this simultaneously address the energy crisis and reduce carbon footprints. Also, in India, burning of the agricultural wastes (stubble/PARALI) cause choking of the Delhi-NCR region. If, these biomasses could be used as a feed for microbial digestion, problem of stubble burning can be reduced. Microbial electrolysis cell (MEC) is a significantly sustainable bio-electrochemical system for which can use wide range of the feed. This technology involves the oxidation of organic matter at the anode and the reduction of proton at the cathode under the nominal external voltage supply.None of the studies have claimed 100% COD removal in these bio-electrochemical systems, that means there is organics left in these reactor digestate. This thesis has focused on the hydrogen production from the organic biomass in the MEC and its reactor digestate valorisation. Additionally, it sheds light on the different kinds of MECs, its configurations, microorganisms, methanogens and their inhibition, electrode material, membranes, and substrates, Also, it delves into the technicality of the MEC, thermodynamics involved, its working principle, the performance evaluating parameters and indices are also discussed. As a part of this thesis, an integrated Bio-Electrochemical System was studied for the assessment of different organic substrates for Bio-Electricity and Bio-Hydrogen generation in Microbial Fuel Cell (MFC) and Microbial Electrolysis Cell (MEC) respectively. Firstly, Sewage sludge (SS), Compost leachate (CL), and Mess Food waste (FW) were fed to the dual-chambered H type MFC, then MFC reactor digestate was fed to the MFC modified to MEC for biological hydrogen production. The maximum open-circuit voltage of 595 mV, 505 mV, and 690 mV and their corresponding peak power densities of 1.8 ± 0.1 W/m2, 1.3 ± 0.1 W/m2 and 2.4 ± 0.1 W/m2 were recorded in SS, CL, and FW fed MFC respectively. The batch reactors had shown COD removal of 62.37 %, 66.8 %, and 50.19 % in MFC fed with SS, CL, and FW respectively. The digestate fed MECs had further shown further COD removal of 55.26 %, 64.40 % and 46.61% MFC’s reactor waste. Therefore, the overall COD removal efficiency of bio-electrochemical systems (BES) was 83.25 %, 88.23, 73.41 for SS, CL and FW were achieved. The corresponding volume of gas produced per gram of COD removed were reported as 23.8, 18.1 and 55.1 for SS, CL and FW respectively. The high OCV and volume of bio-hydrogen per gram of the COD removed has suggested the suitability of the FW as the best fit organic waste for both MFCs and MECs. The comparative higher performance of FW as feeds has established it as a potent choice for bio-electricity and bio-hydrogen production. In developing nations like India, there is an issue of burning of the stubble (Parali), a left out portion of the crops after harvesting. Farmers usually burn these lignocellulosic biomass directly in the field, which cause lots of air pollution leading to 0.5 million deaths per year. These biomasses have potentially to be used as energy resources. It is estimated that burning of parali causes around 150 million tonnes of carbon di oxide, 9 million tonnes of carbon monoxide, also produces SOx, NOx and particulate matter. These gas are responsible forclimate change, global warming and other associated environmental issues. The relevance of sustainability through circular economy has focused on exploring biomass for biofuels and subsequent reactor digestate waste valorisation for subsequent energy extraction. Also, the double chambered MECs are known to contribute higher over-potential and fouling of the proton exchange membrane in comparison to the single chamber MECs. The present noble study has investigated the bio-hydrogen production from sugarcane bagasse fed membrane-less single-chambered microbial Electrolysis Cell (SC-MEC). Also, the consequent biochar production from the MEC reactor digestate to promote maximum energy extraction from waste was explored. The bagasse in MEC resulted in 0.25 m3 of hydrogen/m3/day at an externally applied voltage supply of 0.8 V, with coulombic efficiency (CE) of 82.87 ± 2 % and electrical energy efficiency of 97.47 ± 2%. The cathode and the Shewanella sp. enriched bioanode were arranged for overpotential reduction, resulting in a high current density of 62 A/m2. The bio-remediation efficiency of SC-MEC was estimated in terms of COD removal reported as 31.16 ± 0.5 %, nitrate removal as 28.18 ± 0.2 %, and sulphate removal as 41.2 ± 0.2 %. The reactor digestate was pyrolyzed, and the biochar yield was calculated as 17.8 ± 0.2 %, with BET surface area of 8.903 m2/g. The fixed carbon content of digested bagasse was reported as 22.9 ± 0.2, while the total carbon lost during microbial digestion in SC-MEC was around 32 ± 0.2 %. This study paves a route for circular economy by practicing clean renewable energy and using the waste for subsequent biochar production. The escalation of geopolitical strains has put forward a need for sustainable biofuels with high calorific values. Also, there is an increased concern over the circular economy aspects for maximum extraction of energy. Therefore, waste to wealth and waste to maximum energy concept start emerging. The bio-electrochemical systems have conversion efficiency in a range of 60-70%, which means there is still organics left inside the reactor. When disposed untreated, it causes land pollution, air pollution and other associated environmental concerns. The current study has explored a two staged energy extraction strategy. Here, hydrogen production in single chambered Microbial Electrolysis Cell and the usage of its reactor digestate for the algal biomass production to promote maximum resource recovery is aimed. A heat-pre-treated sugarcane bagasse fed MEC resulted in 2.1 ± 0.02 m3 of hydrogen/m3/day at an applied voltage of 0.8 V, with columbic efficiency of 57.6 ± 0.5 % and electrical energy efficiency of 70.16 ± 2%. This study on exo-electrogens Shewanella oneidensis attached bioanode corresponds to the current density of 48 A/m2. COD removal efficiency of 69.1± 2% and the corresponding HPR for the MEC was reported as 1.85 ± 0.02 m3/m2/d. For reactor digestate valorisation for circular economy the solid digestate of MEC reactor was dried, powdered and supplied to the algal growth batch reactor, resulted into significant biomass growth. The solid feed digestate residue produced biomass productivity of 0.95 g/l and liquid feed digestate filtrate produced the biomass productivity of 0.65 g/l of dry algal biomass. The study is a step towards maximum energy recovery for circular economy through reactor digestate valorisation. As discussed, overpotential in MECs leads to energy losses and ohmic losses. These losses need to be minimized. There are few approaches to take on the this overpotential e.g. reduction in the path length by reducing the distance between electrodes i.e. the anode and the cathode. In the present study, electrode modification is practiced aiming to reduce the overpotential for enhanced biofilm formation. The zeta potential of graphene oxide is somewhere around negative 30 mV, if used to coat over the carbon cloth as anode will cause electrostatic interaction between electrode surface and the exo-electrogenic biofilms. In this chapter, graphene oxide modified electrodes as anode is investigated for overpotential reduction. Biological electro-hydrogenesis for hydrogen production through microbial electrolysis cell (MEC) is an emerging technology for answering energy security concerns. MECs can derive energy from waste organic materials which are by-products of the agricultural and food/drinks processing industries. The efficiency of the hydrogen evolution reaction (HER) at the cathode is influenced by the electron transfer and biofilm formation over the anode. The selection of efficient electrode material for the effective biofilms and HER are crucial for MECs. Carbon is the most widely used electrode material and better performance can be achieved through electrode modification. The present study is focused on anode electrode modification using graphene oxide, nafion and carbon black. These modified electrodes were inoculated with pure culture of Pseudomonas species for biofilm formation. Performance of the modified electrodes were compared with unmodified electrodes and analyzed for surface study using SEM techniques and microscopy. Cyclic voltammetry electrochemical characterization was performed and analyzed. These electrodes are intended to be used in microbial electrolysis cell for improved performance and overpotential reduction.