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Assessment & design of perfused microalgal biofilm cultivation processes

Microalgae are emerging as a promising technology for photosynthetic recycling of carbon dioxide (CO₂) into industrially useable biomass. However, the efficiency of current suspension cultivation systems is limited by low biomass concentration and high dewatering costs. In this thesis I investigated perfused microalgal biofilms as an alternative cultivation process. I designed and built a prototype cultivation system and process, developed a laboratory method for assessing basic cultivation parameters and synthesised the knowledge gained into a mathematical simulation of productivity and power costs of microalgal biofilm cultivation for the production of concentrated feeds in aquaculture. 
 
Microalgae (including cyanobacteria) are a diverse group of microscopic aquatic organisms, many of them capable of photosynthesis. These traits allow the cultivation of microalgae in intensive bio-processes, independent of arable land, using light as an energy source to convert CO₂ into biomass. This makes microalgae potential candidates for mitigating globally increasing CO₂ levels and allows for promising synergies with waste water remediation. The microalgal biomass can be used as feedstock for a wide range of applications, such as biofuels, animal feeds in agriculture and particularly aquaculture and as raw material for green chemistry and pharmaceutical processes. Yet, despite this potential, microalgal cultivation has struggled to gain widespread industrial use. A key challenge is the low cell concentration in current suspension systems, which results in high water requirements and dewatering costs that are economically prohibitive. 
 
Biofilm-based cultivation can provide an answer to this challenge, by growing microalgae at high cell concentration attached to surfaces. This approach promises increased light availability, higher gas exchange rates and overall lower water requirements and substantially reduced dewatering costs. In Chapter 2, I reviewed existing microalgae biofilm cultivation systems and classified them into three distinct groups based on the interaction between surface and cultivation medium: Constantly submerged systems, where the biomass is always covered by a thin film of liquid, intermittently submerged systems, where the biofilms move in and out of the liquid phase and perfused systems where the biofilm is directly exposed to the surrounding atmosphere. 
 
A system of this last group was the focus of Chapter 3, which describes the design and development of a perfused membrane photo-biofilm reactor and its assessment under tropical greenhouse conditions. A biofilm of Mesotaenium sp. was successfully cultivated and growth curves were obtained, showing a maximal biomass productivity of up to 1.7 g m⁻² d⁻¹ (dry weight) and a maximal biomass yield of 21.25 g m⁻² (dry weight). Spatial variations in growth were correlated with high temperatures (above 39 °C) and a corresponding drop in relative humidity which led to dehydration of part of the biofilms. This represents a new finding not previously described in perfused biofilm cultivation in temperate conditions and will influence the design of future cultivation processes, especially in the tropics. 
 
The findings from the literature and the experience with the prototype also highlighted the need for simpler test methods to allow for standardised testing of basic cultivation parameters under reproducible laboratory conditions. In Chapter 4, a petri-dish assay was developed which allows the cultivation of perfused biofilms on different materials at low cost and technical requirements. The use of this assay was successfully demonstrated by investigating the growth behaviour of different microalgal species, as well as growth on different surface materials and under different light conditions. This assay was also used in a related Honours thesis project, which showed the potential use of Isochrysis aff. glabana biofilms as concentrated aquaculture feed and replacement for commercial algal paste. 
 
The results of these investigations were applied in Chapter 5. The growing aquaculture industry has led to an increasing demand for high-quality microalgal concentrates. Cultivation of microalgae in suspension and biofilm cultivation systems was simulated, at the scale of an aquaculture hatchery, in order to compare productivity and power costs in batch and semi-continuous growth modes. Biofilm cultivation was shown to be feasible at similar scale to suspension cultivation, but still in need of improvement to be competitive. This simulation could be further expanded into a techno-economic analysis by incorporating data from existing industrial processes. 
 
The overarching aim of this thesis was to investigate and develop new approaches for microalgae cultivation in order to open up new, sustainable industrial production pathways. This was achieved by assessing a rapidly developing field, identifying the potential of perfused biofilm technology and by designing cultivation processes and systems based this approach. The final productivity and power simulation relates the knowledge gained to a promising industrial application with short and long term potential in Australia and worldwide. In a broader context this work provides a summary off the current state of applied microalgal biofilm research, highlights the importance of experiments under relevant environmental conditions, and provides analytical and computational research tools for improved assessment of applied microalgal biofilm cultivation.