The potential utilisation of contaminated biomass following phytoremediation

Bioenergy and bioproducts are widely regarded as one of the key contributors to reducing global greenhouse gas emissions. Utilisation of wastes or pests could provide an alternative feedstock to avoid land use complications and reduce costs. An example of a potential feedstock is the invasive aquatic macrophyte water hyacinth (WH), however, due to its heavy metal content, moisture content and cost of removal, it has yet to be valorised or used on a large scale. The aim of this thesis was to demonstrate the potential utilisation of WH in a biorefinery concept, to produce bioproducts, with a focus on protein and bioenergy.
This work focussed on the comparison of biomass composition across various locations, pollution sources and degrees, and timescales. The results demonstrated that water hyacinth has a high variability across locations with particular impact from the residence time of the waterbody: a lower residence time appeared to increase plant contamination despite reduced pollutant concentration within the water. The phenological study demonstrated that if a growth period can be identified, then clear trends can be observed, in particular where growth methods by WH are identified. However, all sites demonstrated increased heavy metal content at the same time as reduced protein content, suggesting that either protein recovery or heavy metal removal should be prioritised when harvesting plants.
When cultivated under controlled conditions, it was evidenced that WH growth rate had a strong linear relationship with water nitrogen concentration but that other factors such as plant density and temperature did have an impact. The source of nutrients did have a significant impact on growth, but all nutrient sources fit within a 95% confidence of the linear relationship. However, the relationship identified here resulted in a lower growth than literature, suggesting that optimisation was still possible.
The potential for protein extraction from water hyacinth was examined, utilising an alkali acid extraction. This demonstrated that a safe protein precipitate could be produced from water hyacinth leaves, but the yields were lower than other feedstocks. The fibrous residue, from the alkali extraction, demonstrated an improved biogas yield as compared with the raw biomass. The data was utilised in various scenarios, it was demonstrated that a WH biorefinery was not economically viable on a large scale due to the high running costs, but on a small scale could provide enough biogas to replace 1- 6 cylinders of liquid petroleum gas, 14.2 kg cylinder for a single family. Whilst optimisation could improve the viability of the large-scale systems, the value of digestate would be the most significant change, whilst non-direct benefits from harvesting should be investigated further.