Abstract
Biorefinery is a core strategy raised in 21st century to facilitate the circular economy concept. With combination of different chemical and biological methods, we are able to create a material recycling loop with much less waste generated. In this thesis, we were focused on development of novel biological waste treatment method and optimization of key fermentation processes.The first project aims to treat textile waste with a submerged fungal fermentation method. Textile waste is one type of municipal solid waste growing rapidly in recent years. In Hong Kong, 306 t of textile waste were produced daily in 2015 and more than 90% of these ended up in landfill. This is the first time we utilize textile wastes as substrate for cellulase production via submerged fungal fermentation. The resultant fungal cellulase was subsequently utilized in textile waste hydrolysis for recovery of glucose and polyester. Trichoderma reesei ATCC 24449 was selected with the highest cellulase activity (18.75 FPU g-1) after cultivation using textile blending cotton/polyester 40/60 as substrate. Cellulase production was upscaled in a 5-L bioreactor and the resultant cellulase was used in textile waste hydrolysis. Glucose recovery yield of 41.6% and 44.6% were obtained using fungal cellulase and commercial cellulase, respectively. These results suggest the proposed process has a great potential in treating textile waste and facilitating the recovery of glucose and polyester as value-added products.
The second project focused on biosurfactant production using food waste as feedstock. We investigated the production of the biosurfactant sophorolipids from several (food) waste streams, using the non-pathogenic yeast Starmerella bombicola. From a preliminary screen, restaurant food waste emerged as the most suitable feedstock compared to bakery waste, textile waste, used corn oil, animal fat and lipid fraction of hydrolysed food waste. Restaurant food waste was subsequently used for SL production in a laboratory-scale bioreactor. Food waste obtained from a local restaurant was subjected to enzymatic hydrolysis for 16 h, yielding a hydrolysate containing about 100 g L-1 glucose and 2.4 g L-1 free amino nitrogen. High sophorolipids process efficiency was achieved by fed-batch fermentation using the restaurant food waste hydrolysate as the complete batch medium, so without any supplementation of additional medium components such as vitamins, salts, nitrogen or phosphate. Controlled feeding of glucose and oleic acid to the culture was performed after the batch phase. A sophorolipid titer of 115.2 g L-1 was obtained in a fermentation time of 92 h resulting in an overall productivity of 1.25 g L-1 h -1. These results achieved for SL productivity using hydrolysed food waste are in the same order of magnitude as the reported values using traditional (complex) fermentation media. This indicates the suitability of the developed process using food waste towards the advancement of waste-based bio-processes for the production of sophorolipids.
After that, a robust production-separation system was developed for efficient sophorolipids production. In fed-batch-separation mode, an average volumetric productivity of 2.43 g L-1 h -1 and the overall SL yield of 0.73 g g-1 were achieved within 240 h. Moreover, the fermentation was carried out for 480 h to examine the potential metabolites inhibition in long-term fermentation. An average volumetric productivity and an overall SL yield of 2.39 g L-1 h -1 and 0.73 g g-1 were obtained, respectively. Notably, the highest cell dry weight of 117.2 g L-1 was also observed in fed-batch-separation, which was the highest cell density for Starmerella bombicola ever reported before. The concept of separation efficiency was also raised to quantitatively evaluate in-situ separation performance. In 480 h fermentation, an average separation efficiency of 74.3% and the overall separation efficiency of 93.0% were achieved, suggested that the separation design and process control was successful in laboratory scale fermentation. Based on these findings, this system could potentially be applied to an industrial scale.
The work presented in this thesis successfully verified the biorefinery concept on textile waste and food waste streams. With fermentation strategy and process optimization, its potential in projection to an industrial scale was also examined. This work positively contributes to the idea of waste conversion into high value-added products and promote the concept of circular economy.
| Date of Award | 4 Jul 2019 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Sze Ki Carol LIN (Supervisor) |