Growth of microalgae for the purposes of producing biofuels and/or other biocommodities is an emerging technology and brings numerous challenges. For the algae industry to be viable, improvements need to be made across the entire technology train, starting at algal growth all the way through to energy recovery.
In this thesis, two aspects of the algal industry were investigated; the biomass growth and accumulation stage, and the anaerobic digestion of the biomass to generate methane. Hence the aims of this thesis were twofold. The first aim was to enhance the understanding of the factors that influence algal photosynthetic activity, particularly the influence of dissolved oxygen and inorganic carbon on photosynthetic oxygen production. The second aim was to enhance methane yield and nutrient recycling potential from anaerobically digested algal biomass through high pressure thermal hydrolysis (HPTH) pre-treatment.
A feature of the PhD was the invention of a novel electrochemical system for oxygen control (ESOC), which was developed to assist in addressing the first aim of the thesis. This ESOC enabled dissolved oxygen (DO) control and quantification of net algal photosynthetic oxygen production, allowing for the determination of the algal photosynthetic activity whilst monitoring the variables of interest. The ESOC was used to determine the inhibitory relationship between algal photosynthetic activity and dissolved oxygen concentration in an algal culture, which was previously difficult to achieve without such a system. The inhibition relationship followed a Hill inhibition model with a 50% reduction in photosynthetic activity at 27.2 mgO2 L-1 and a Hill coefficient of 0.22 L mgO2-1. The second use of the ESOC was to assist in the development of a generic model which can predict algal photosynthetic activity as a function of inorganic carbon and pH, using variables that can be easily measured (pH and TIC), to assist in design and operation of algal culture systems.
The second aim of the thesis was addressed through the development and use of a high pressure thermal hydrolysis (HPTH) reactor for the pre-treatment of both raw algae and lipid extracted algae prior to anaerobic digestion. It was found that the HPTH did enhance the methane yield of both substrates and also improved the nutrient recovery potential of the anaerobic digestate.