Feasibility of renewable bioenergy production from carbohydrate-rich waste streams using confectionery waste as a substrate

Ngwenya, Carol

Feasibility of renewable bioenergy production from carbohydrate-rich waste streams using confectionery waste as a substrate

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Chemical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Solid waste management has become a global concern due to increased industrial activities and subsequent disposal of waste to landfill. Sustainable solid waste management solutions are part of the United Nations 2030 Agenda for sustainable development. South Africa is working towards a more sustainable economy, including the establishment of a biobased economy and increased circularity to promote the use of waste material in biological processes to produce valuable biobased products. Readily available carbohydrate-rich waste streams from food and feed processing have, for many years, been disposed of by landfilling, a practice increasingly recognised as environmentally burdensome. Bioenergy recovery from biodegradable waste material is a sustainable approach to waste reduction through valorisation of waste streams to achieve resource efficiency. Furthermore, bioenergy as an alternative energy source has the potential to diversify South Africa's energy supply and minimise dependence on fossil fuels. In this project, conversion of carbohydrate-rich confectionery waste streams to bioenergy for resource efficiency and waste minimisation was studied. A comparative assessment of biogas and biofuels from confectionery waste was conducted to assess the feasibility of these substrates for bioenergy production. Zymomonas mobilis was selected for bioethanol owing to its high sugar fermentation capability compared to the commonly used Saccharomyces cerevisiae. Clostridium saccharobutylicum was selected for biobutanol production as a Clostridia spp with high affinity to sugar compared to C. acetobutylicum with higher affinity to starch. Biogas production from anaerobic digestion (AD) of confectionery waste was investigated for comparison. Cell acclimatisation strategies improved substrate conversion and bioenergy output. Acclimatised Z. mobilis cells on sucrose enhanced levansucrase secretion, improving sucrose hydrolysis and ethanol concentration from 3.94 g/L to 10.33 g/L. At bench scale, levan formation reduced ethanol yield but was mitigated by increasing temperature from 30 °C to 37 °C, increasing ethanol from 2.94 g/L to 9.09 g/L. Fed-batch studies showed sugar utilisation >70% and ethanol tolerance >5% (v/v). For biobutanol production studies, C. saccharobutylicum consumed sugars efficiently at small scale with average ABE solvents of 10 g/L and butanol up to 8.28 g/L on candy. Bench scale led to rapid acid accumulation and causing acid crash. A pH control strategy at 5.5 improved butanol to 6.18 g/L though residual acids remained high. MFA showed reduced acetic acid reassimilation and prolonged butyric acid production at bench scale, attributed to pH 5.5 favouring acidogenesis. The AD of the carbohydrates rich waste stream resulted into rapid hydrolysis caused VFA accumulation and process instability. An acetate-acclimatised inoculum improved methanogenic activity with CH₄ recovery of 5.53 L (MCW) and 4.63 L (biscuits). A comparative bioenergy assessment was conducted using experimental findings supplemented with literature data in a scenario based on 1.8 × 10⁸ kg/year of confectionery waste from a local manufacturer. Biofuels showed higher energy outputs than biogas, with biobutanol from starch-rich waste (1.22 × 10⁹ MJ/year, 76% waste reduction) and ethanol from sugar-rich waste (19.2% offset by S. cerevisiae, 10.6% by Z. mobilis) showing greatest potential. Although AD reported the lowest energy output, it produced the lowest CO₂ emissions and greatest process stability. The findings demonstrated the feasibility of bioenergy production from confectionery waste and highlighted the importance of matching microbial system with waste composition. Acclimatisation and process optimisation improved microbial performance, enhancing productivity and yields. The study provided new insights into sugar metabolism of confectionery waste by microbial systems and supports informed selection of WtE technologies based on bioprocess potential, resource efficiency, and environmental impact.

Keywords

solid waste management

renewable bioenergy

Reference:

Collections

PhD / Doctoral

Full item page