Investigating the effects of pre-treatment of tropical seaweeds and freshwater macrophytes for bioenergy production

Student thesis: Doctoral Thesis

Abstract

The use of third-generation feedstocks as potential sources for biofuels production is currently attracting considerable global interest. This is mainly because both marine and freshwater species, especially seaweeds, water hyacinth and water lettuce are rich in polysaccharides, and generally have high growth. This study aimed at investigating the potential for the production of bioenergy from selected tropical seaweed and freshwater macrophytes in Nigeria through two biochemical processes: as anaerobic digestion (biogas production) and fermentation (bioethanol production).
The study was carried out in four main stages.

The first stage investigated the effect of saline inoculum in the anaerobic digestion of selected aquatic plants. The use of sewage anaerobic sludge was used as a reference. Findings show that using the non- adapted anaerobic fresh sludge produced higher methane in L. digitata 378 ml/g VS added followed by E. crassipies 275 ml/g VS added, P. stratiotes 232 ml/g VS added, and S. fluitans 139 ml/g VS added, respectively. Similar pattern was observed in adapted saline reactors. However, the methane yields were lower in comparison to non-adapted fresh sludge reactors. L. digitata 293 ml/g VS added, E. crassipies 253 ml/g VS added, P. stratiotes 198 ml/g VS added, and S. fluitans 122 ml/g VS added, respectively.

The second stage involved investigating the effects of thermo-chemical pre-treatment on the hydrolysis of the aquatic plants, measured through the release of reducing sugars. Different acids/alkalis were used to investigate the effects of acid concentration, temperature, and residence time. Findings show that 1% HNO3 at 30 minutes and 120ÂșC was effective in solubilising these plants. Results show a reducing sugar concentration of L. digitata 71 mg/g dry weight, E. crassipies 152 mg/g dry weight, P. stratiotes 118 mg/g dry weight and S. fluitans 120 mg/g dry weight, respectively.

The third stage investigated, the bioethanol potential of these plants, using the hydrolytic by-products derived from the stage two study. This study also investigated the effectiveness of different types of yeast strain to ferment a wide range of sugars found in these aquatic biomasses. The different strains of yeast namely, Saccharomyces cerevisiae, Pichia stipitis (aka Scheffersomyces stipitis) and Kluyveromyces marxianus. Findings show that S. stipitis yeast strain demonstrated the highest affinity to ferment the sugar rhamnose which was the dominant hydrolysate in seaweeds to produce 5.9 g/Land 1.8g/L ethanol in L. digitata and S. fluitans, respectively. S. cerevisiae yeast strain showed highest affinity to glucose fermentation, to produce 4.7g/L and 4.1 g/L ethanol in E. crassipes and P. stratiotes, respectively.

The fourth stage involved investigating the effects of both acid and enzymes on the anaerobic digestion of seaweeds and freshwater macrophytes. Findings showed that pre-treatment of these feedstocks for AD process can be affected by the type and composition of the aquatic biomass. The study further revealed that methane yield from these macrophytes could enhanced or decrease depending on the aquatic plants type. Results show methane yield decrease in L. digitata (189 ml/g VS added) and E. crassipies (221 ml/g VS added) reactors after both acid and enzymatic pre-treatments. While methane yield was enhanced in P. stratiotes (234 ml/g VS added) and S. fluitans (195 ml/g VS added) reactors after pre-treatment.

The findings of this research could be used to develop a strategic operational framework for the application of bioenergy technology from these aquatic plants in Nigeria.
Date of Award31 May 2021
Original languageEnglish
SponsorsPetroleum Technology Development Fund (PTDF)
SupervisorJoseph Akunna (Supervisor) & Graeme Walker (Supervisor)

Keywords

  • Anaerobic digestion
  • Tropical seaweed
  • Bioenergy production
  • Freshwater macrophytes

Cite this

'