Invasive aquatic macrophytes choke tropical and subtropical waters, yet their harvested biomass is a large and untapped carbon resource. This thesis
determines how four such species can be converted into fuels and bioproducts, and which conversion route best suits each. Water hyacinth (Pontederia crassipes) was the primary feedstock, with common duckweed (Lemna minor), water fern (Azolla) and water lettuce (Pistia stratiotes) as
secondary feedstocks. All four were characterised under a single analytical protocol and then processed through acid-catalysed butanolysis, hydrothermal liquefaction and slow pyrolysis, with anaerobic digestion as a biochemical benchmark. Characterisation showed a common signature of high ash (18.8 to 31.8 wt%), moderate carbon and low calorific value (12.5 to 14.9 MJ kg-1), with biochemical composition varying most widely and driving conversion behaviour. Acid-catalysed butanolysis of water hyacinth, optimised here for the first time through a four-factor Box-Behnken design, reached a butyl levulinate yield of 11.14 wt% at 206.5 °C and closed a total-carbon balance at approximately 96%, although the product slate was dominated by solvent-derived dibutyl ether. Hydrothermal liquefaction gave its highest-quality biocrude at 350 °C with 1 wt% potassium hydroxide (30.1 MJ kg-1, 36.0% energy recovery), while slow pyrolysis at 550 °C produced a bituminous-grade biochar retaining 52.3% of the feedstock energy. Measured biomethane potentials spanned 29.1 to 199.8 mL CH₄/g VS, placing anaerobic digestion ahead only for the readily biodegradable water lettuce.
The central finding is that composition dictates conversion. On an energy basis the protein-rich duckweed is matched to hydrothermal liquefaction, the readily biodegradable water lettuce to anaerobic digestion, and water hyacinth and the recalcitrant water fern to slow pyrolysis. Butyl levulinate yield tracks the structural carbohydrate content, so butanolysis suits water hyacinth and water lettuce where a levulinate ester is the target product rather than energy recovery. These results are drawn together into a feedstock-to-process allocation framework that recovers approximately 36 to 52% of the feedstock energy in a usable product across the species range, providing a defensible basis for coupling the ecological management of these invasive weeds with the recovery of value.
