Design and optimisation of a small, robust outdoor duckweed bioreactor for operation under ‘on-site’ farmyard conditions
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Date
2024
Authors
O'Sullivan, Grace
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Publisher
University College Cork
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Abstract
Duckweed (Lemnaceae) is a small, aquatic plant that is ubiquitous across Ireland. It grows remarkably fast, with a doubling time of one to two days. Duckweed is typically found on slow moving water bodies, but it can grow on agricultural waste
streams such as yard washings. Usefully, the plant removes excess nutrients (nitrogen and phosphorus) from wastewater to produce a high protein crop. Duckweed dry weight can contain up to 40% protein. Duckweed is currently being investigated for use in human nutrition, biofuel production, and the remediation of waste streams (Appenroth et al., 2017, Xu et al., 2012, Zhou et al., 2023).
The rising need for sustainable agriculture systems magnifies the importance of a circular economy approach to farming. This project involves the design and optimisation of an outdoor duckweed growth system suitable for use on farms. It offers
farmers a plausible solution to managing the large volumes of nutrient-rich wastewater produced on farms daily, whilst also generating high-value biomass. Hence, it can help provide a circular economy approach to sustainable agriculture. Currently, two main styles of growth systems are used in the production of duckweed: ponds and raceway systems. A literature survey (undertaken as part of this research) reveals that limited studies have been completed on the effect of system design on duckweed cultivation.
The approach undertaken in this project involves a systematic analysis of fluid flow using Computational Fluid Dynamics (CFD) and tracer response experiments to inform the design of a system optimal for duckweed production. ANSYS Fluent is
used to optimise the system geometry and interpret the effects of the geometry on nutrient distribution. Relative growth rate and nutrient uptake experiments were also employed to gain experiential knowledge and thus a holistic understanding of the impact of design on duckweed growth. From this study, it can be concluded that low levels of mixing within a cultivation system have a positive effect on both the relative growth rate of duckweed and its remediation ability. However, at higher flow rates the growth of duckweed is negatively impacted. The results also feature the design of a medium-scale outdoor duckweed cultivation system, informed by the literature review
and experimental work.
This work will inform stakeholders on the most suitable system for duckweed production in a farm setting. In addition to this, it will provide guidance on the style of system that is most suitable for large scale duckweed production sites worldwide.
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Keywords
Duckweed , Circular economy , Computational fluid dynamics , Biorector design , Farmyard effluent , Animal feed
Citation
O'Sullivanr, G. 2024. Design and optimisation of a small, robust outdoor duckweed bioreactor for operation under ‘on-site’ farmyard conditions . MRes Thesis, University College Cork.