Integrated Biorefinery Strategies for Valorisation of Food and Textile Wastes


Student thesis: Doctoral Thesis

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Award date3 Aug 2018


There is a growing demand to establish biotechnology-based processes for material, chemical and fuel production which can decrease our dependency on dwindling oil reserves and reduce greenhouse gas emissions. These novel bioprocesses are incorporated into biorefineries, which are integral units converting different renewable-derived feedstocks into a range of useful products as diverse as those from petroleum using environmentally friendly technologies. A number of agricultural-based biorefinery processes have been developed and realised at commercial scale since the past decade. At the same time there are growing concerns over organic waste generation and insufficient resource supply due to the everincreasing global population and the cradle-to-grave system of material flows in the linear economy. Therefore, the research in our team aimed at valorising organic waste materials through bioconversion processes to recover their inherent nutrients for transformation into value-added products. Waste-based biorefinery not only provides a means for waste treatment, but also promotes the development of a circular economy by valorising wastes into high-value products. As part of this goal, the three projects reported in this thesis focused on the development of integrated biorefinery strategies to valorise food and textile wastes. Due to the different characteristics of the waste materials, the bio-processes were specifically designed and demonstrated in laboratory- and pilot-scales, accompanied with techno-economic assessment for evaluation of technical and economic feasibility. 

The first part of this thesis presents a bioconversion process which was developed to valorise the leftovers from restaurants and bakeries for lactic acid (LA) production. The process includes two main steps: (1) upstream processing to convert food waste to a generic fermentation feedstock through fungal hydrolysis using Aspergillus awamori and Aspergillus oryzae; (2) LA production using food waste-derived hydrolysates as the medium in bacterial fermentation. Meanwhile, a decentralised approach was proposed by onsite pre-treatment of the leftovers into dried powder in order to facilitate efficient waste collection in urban areas. In general, the hydrolysates obtained from the leftovers were rich in glucose (80.0-100.2 g L-1), fructose (7.6 g L-1) and free amino nitrogen (947-1081 mg L-1). The results of the subsequent batch fermentation with Lactobacillus casei showed that the carbon and nitrogen sources derived from the leftovers can be used for LA production and cell growth. Around 82.6-94.0 g L-1 of LA were produced within 36 hours, which corresponds to the productivity of 2.50-2.61 g L-1 h-1 and yields of 0.92-0.94 g g-1. In the study carried out by another research team member, the LA was purified by a novel solvent extraction method and processed into poly(lactic acid) (PLA) via ring-opening polymerisation. A techno-economic study wasconducted with the aid of the simulation software Super-Pro Designer® to evaluate the bioconversion of dried leftover powder to LA, lactide or PLA using a decentralised approach. Mass balance showed conversion yields of 3.1 metric tons (MT) of LA, 1.7 MT of lactide, and 1.3 MT of PLA from 10 MT of dried powder from leftovers. The processes were economically feasible, but production of LA had the highest annual net profits (US$16,736,218), internal rate of return (33.0%), net present value (US$152,243,489) and the shortest payback period (4.9 years) at a discount rate of 5%. Sensitivity analysis showed that the prices of LA, lactide and PLA were the largest determinants of the profitability in the process while the sales of coproducts, which are the unhydrolysed solids and crude lipids recovered by fungal hydrolysis, were also critical to the economics. 

The second part of this thesis presents another biorefinery strategy for the valorisation of expired food and beverage (F&B) wastes to fructose syrup, which was a study invited and sponsored by PepsiCo. The process proposed involves seven main steps, namely saccharification, solid-liquid separation, adsorption, ion-exchange chromatography, isomerisation, glucose-fructose separation using a simulated moving bed system, and evaporation of sugar syrups. A kinetics study was performed and confirmed that the preservatives and additives in the F&B wastes (benzoic acid, sorbic acid and caffeine) do not inhibit glucoamylase and sucrase during saccharification. High levels of glucose (228.1 g L-1) and fructose (55.7 g L-1) were efficiently produced within 12 hours at a solid-to-liquid ratio of 37.5% (w/v) but the hydrolysate also contained trace preservatives, caffeine, colourants, ions and soluble proteins. Over 99% of these impurities were removed by adsorption and ion exchange chromatography, followed by enzymatic isomerisation where glucose was converted to reach a 50% fructose content. Pilot-scale downstream processing also indicated more than 89% of sugars recovered from the hydrolysate and complete removal of the impurities. Similarly, techno-economic analysis was conducted to evaluate the technical and economic feasibility of the production of fructose syrup as well as other possible sugar syrups (high fructose syrup-42 and glucose-rich syrup). Mass balance showed conversion yields of 0.24 MT sugar syrups per MT of F&B waste, while lipids (0.07 MT per MT of F&B waste) and insect feed (0.44 MT per MT of F&B waste) were the co-products proposed to be used for other industrial biorefinery processes. Generally, the processes were economically self-sustainable with net profit generation (US$11-26 million year-1) and positive net present values (US$92- 294 million). Along with the net production costs (US$443-665 MT-1), the sugar syrups derived from the F&B waste have relatively low minimum selling prices of US$157-747 MT-1 at a 5% discount rate, as compared to their corresponding market prices. Sensitivity analysis was performed which found that the prices of sugar syrups were the largest determinants of their profitability. 

Lastly, the third part of this thesis describes a preliminary study on textile waste recycling via a biological method at pilot scale. A biorefinery strategy was proposed to recover the glucose and polyester from cotton-polyester blend waste textile, which is a typical pre-consumer waste provided by the project sponsor (H&M Hennes & Mauritz (Far East) Ltd.). The bioprocess comprised thermo-chemical pre-treatment, enzymatic hydrolysis, downstream processing of the hydrolysate into glucose syrup, and melt-spinning of polyester into fibre. The results from both 5 L bench-top and 50 L pilot reactors show that over 98% of cotton was removed from the textile waste by hydrolysis using commercial cellulase and β-glucosidase. However, the freezing NaOH/Urea pre-treatment method yielded a higher glucose recovery with 75.2-81.5% than the dilute H2SO4 pre-treatment where only 29.1% of glucose was recovered and the unhydrolysated cotton was precipitated in a powder form. After the downstream processing, the hydrolysate was purified into glucose syrup which could be used as a carbon source for various fermentation processes. Meanwhile, the polyester recovered after hydrolysis was pure with 1.9-2.5% of insoluble content and then subjected to melt-spinning for producing fibre for further application in the textile industry. However, techno-economic analysis indicated the process deficit due to the high operating cost which was mainly contributed by the commercial enzyme. The annual deficit was US$907,711,298 per year. Further study should focus on improvement of the hydrolysis conditions for reducing the enzyme costs and modification of the reactor design for recovering the polyester from the reactor after the hydrolysis. 

This thesis presents three independent showcases on the development of integrated biorefinery strategies for the valorisation of food and textile wastes by laboratory- and pilotscale studies. These examples provide a novel approach to waste valorisation to replace the current concept of waste treatment and facilitate the transformation from a linear to circular economy for global industries and society. Further study focused on evaluating the environmental performance (e.g. greenhouse gas emission and global warming potential) and valorisation of other organic wastes will help in the incorporation of these biorefineries in different industries. This will be an important contribution towards circular economy development.