Enhancing biogas yield during anaerobic co-digestion of organic fraction of municipal solid waste and garden waste
Creators: name: Robles, Gerianne
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400402
subject: Chemical and thermal processes in energy and combustion
classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 401103
subject: Global and planetary environmental engineering

description: In Victoria, Australia, organic waste makes up approximately 50% of the average household municipal solid waste. The organic waste is comprised of both food waste, commonly referred to as the organic fraction of municipal solid waste (OFMSW) and lignocellulosic materials, such as garden waste (GW). With growing concern regarding anthropogenic climate change and increasing energy demand, there has been a shift from traditional fossil fuel power generation towards an adoption of renewable alternatives such as biogas generated through anaerobic digestion (AD). AD is a natural process that takes place in landfills, however by moving it from a landfill into a reactor it allows for greater control over each stage. The outcome is process optimisation, thereby giving faster production and higher quality biogas. The potential for OFMSW as an energy source has only recently been recognised in Australia, with source segregation taking place in some council areas, and the commissioning of a new Waste to Energy facility in Melbourne’s North. The process of co-digestion is used to enhance biogas yield and is the ability to balance the concentration of carbon (C), nitrogen (N), and other trace nutrients entering an AD system by altering the feed waste. This can be carried out by mixing two different waste streams, such N-rich OFMSW with C-rich GW.<br><br> <br><br>GW’s complex lignocellulosic structure limits its degradability during AD therefore an effective pretreatment is required to increase the bioavailabilty of C thus enhancing biogas yield and digestate quality. After identifying alkaline-autoclaving (AA) as the most feasible treatment based on published literature, it was performed on Victorian GW investigating particle size, chemical concentration, and temperature. Its success was assessed by analysing changes in cellulose crystallinity, structural composition, and surface morphology. This was done in Phase A of the experimental work, and the results showed AA treatment to enhance biogas yield during the biochemical methane potential (BMP) tests. The highest increase found within the study was achieved in GW ground &lt;10mm using 1.0% NaOH to increase to 213.70L CH4/kg VS, corresponding to a 214.2% increase when compared to untreated GW. When the results from the BMP were compared with the results during the compositional analysis, it was found that enhanced yield was exhibited in conjunction with significant lignin removal as well as increases in relative glucan and xylan fractions. Furthermore, analysis of the surface morphology of AA treated GW showed the formation of pore holes allowing an increase in accessible surface area for microorganisms during AD and thus enhancing biogas yield.<br><br> <br><br>In Phase B of the experimental work, mixtures of synthetic food waste (SFW) and Victorian GW were applied in BMP tests at low total solids loading (TSL) investigating co-digestion mixtures and the role of fresh and acclimatised. The co-digestion of SFW with GW showed enhanced biogas yield when compared to single substrate digestion. The highest methane yield was achieved in the co-digestion mixture 15%GW using acclimated seed with a final cumulative methane yield of 375.12L CH4/kg VS. Using seed which was first acclimatised to its feed, as opposed to fresh seed, resulted in faster digestion rates, with peak methane production in AD reactors using acclimated seed occurring 7 days earlier than its fresh seed counterpart.<br><br> <br><br>As part of Phase C, these co-digestion BMP were again performed in Sweden simulating high TSL AD, a process more efficient in large scale industry, investigating low and high TSL, psychrophilic and mesophilic temperature conditions, and substrate/inoculum (S/I) ratio of 0.5, 1.0, and 2.0. The effect of pretreatment of GW during co-digestion of Swedish GW with SFW was also investigated. The results overall showed that with increasing S/I ratio (to values greater than 0.5), cumulative methane yield decreased, and more commonly to complete inhibition within 1-2 weeks of digestion. This occurred in AD reactors loaded with SFW, and only reactors digesting GW as a single substrate did not experience severe inhibition. While mesophilic temperatures resulted in more rapid digestion rates, similar cumulative methane yields could be achieved by psychrophilic temperatures over a longer digestion period. AA treatment had a negative impact on biogas yield from Swedish GW and insignificant effect when compared to co-digestion with SFW. Increasing the TSL in the AD reactors commonly caused a reduction in cumulative methane yields causing early inhibition as mixing within the reactors became challenging.