The use of Phragmites australis (common reed) in biofuel production : Effects of chemical and mechanical pre-treatments, harvesting season and co-digestion with food waste on biogas production.

Al-Iraqi, Ahmed R. and Semple, Kirk and Folkard, Andrew and Barker, Philip (2024) The use of Phragmites australis (common reed) in biofuel production : Effects of chemical and mechanical pre-treatments, harvesting season and co-digestion with food waste on biogas production. PhD thesis, Lancaster University.

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Abstract

Lignocellulosic materials possess various features that make them exceptionally attractive as organic substrates in anaerobic bioreactors. Among these substrates, wetland plants such as common reed (Phragmites australis) are highly interesting as a promising feedstock for biogas production due to their vast abundance, sustainable nature, and lack of competition with arable land and food and feed production. However, the main obstacle lies in the recalcitrance of these feedstocks to biological conversion due to their high lignin content, impeding their full-scale adoption for biogas production. Therefore, additional steps are necessary to overcome this challenge and maximize their biodegradability. In the present work, a series of screening assays were conducted using microcosm vessels (125 mL) operating at mesophilic conditions to evaluate the potential of utilising alkaline pre-treatment (using NaOH) and co-digestion with food waste for augmenting the biogas production from P. australis. And determining the parameters that promote P. australis degradation efficiency and anaerobic digestion process stability, such as P. australis particle size, NaOH concentration, incubation time of pre-treatment process, inoculum to substrate ratio, and substrates mixing ratio. Furthermore, two batch experiments were performed using a 1 L bioreactor to investigate the effect of P. australis harvesting time, co-digestion with food waste and mechanical pre-treatment (P. australis biomass grinding to smaller than 0.4 mm) compared to chemical pre-treatment on the chemical composition of P australis and enhancing methane production. The results revealed inverse correlation between the biogas production level and the particle size of P. australis (10, 5, 2, and <1 mm), achieving the highest biogas production at particle size <1mm. As well as it was found that the pre-treatment of P. australis substrate with 0.5, 1, 2, and 4% NaOH positively affected biogas production, causing an increase by 2.4, 3, 3.6 and 3.9-fold than biogas produced from untreated P. australis, respectively. Besides, the P. australis substrate pre-treated with NaOH for long periods (72-120 h) showed improved biogas production compared to the P. australis substrates treated for shorter periods (12-48 h). Regarding the inoculum-to-substrate ratio (ISR), it was found that the increasing P. australis substrate proportion (ISR of 1:2 and 1:4) resulted in higher biogas production than that produced from ISR with low P. australis proportion (4:1, 2:1, and 1:1). On the other hand, this study revealed that the co-digestion of pre-treated P. australis with synthetic food waste at a high substrate ratio (ISR of 1:4) and high synthetic food waste proportion at ISR 1:2 (synthetic food waste to P. australis mixing ratios of 50:50 and 75:25) resulted in systems acidification and inhibition. In contrast, the systems stability and biogas production increased at a lower substrate ratio (ISR of 1:1), with the highest biogas production achieved at synthetic food waste to P. australis mixing ratio of 75:25. Finally, the change in the harvest season showed a remarkable influence on P. australis composition, cellulose degradation, and methane production. The monodigestion and co-digestion of pre-treated P. australis harvested during the summer and autumn seasons showed higher methane production than P. australis harvested in the winter. Overall, the chemical pre-treatment of P. australis using NaOH improved methane production more than mechanical pre-treatment using the grinding method. As well as the co-digestion of P. australis with food waste contributed significantly to increasing the efficiency of the mechanical pre-treatment process in enhancing the biodegradability of P. australis and increasing methane production.