Understanding biopolymer clusters formation in microalgae systems for wastewater: application to membrane photo-bioreactors

Microalgae attract considerable interest due to their potential for the production of value-added products such as pharmaceuticals, nutraceuticals, animal feed, cosmetics and biodiesel. Although the applications for high value products are viable, applications for biodiesel production, with a lower value but much greater market potential, are still not economically viable due to the high cost of algal biomass production. The costs of growth medium and algal biomass harvesting have specifically been identified as the major contributions to the total cost of production and will have to be significantly reduced to enable widespread application. The use of wastewater as an algae growing medium has however been shown to be a sustainable low cost option as it provides the nutrients needed for algae growth while simultaneously delivering wastewater remediation. Applied to wastewater treatment, microalgae are proven to efficiently remove nutrients (phosphorus and nitrogen) to very low levels and also demonstrated potential to remove hazardous chemicals such as heavy metals and organic micro-pollutants. Essentially, microalgae can be used for wastewater pollution remediation whilst providing added value through the production of algal biomass. However, algae harvesting remains the major limitation and solving this problem will be the key to deliver the true potential of these technologies.
Membrane photobioreactors, which are integrated systems combining an algal photobioreactors with a membrane for direct separation of the algal biomass have been identified as promising alternatives to more conventional algae systems as they generally have the same advantages as typical algal photobioreactors but the integrated membrane provides complete retention of algae cells and decoupled biomass and hydraulic retention times. This enables increased biomass concentrations and consequently intensification of the process with significantly shorter contact times. While the membrane facilitates algal biomass harvesting, as for all membrane systems, fouling becomes the main limitation. The accumulation on the membrane of the algal biomass and any organic and inorganic compounds present in the water will affect its hydraulic performance and contribute to an increase in energy demand and costs. Membrane fouling is inevitable so it will be critical to control its formation through the implementation of mitigation measures to obtain sustainable operation and to make the technology economically viable.
Previous studies on membrane fouling by microalgae have highlighted the highly fouling nature of algogenic organic matter and more specifically the soluble biopolymers excreted by microalgae. Importantly, biopolymers have been shown to be the main contributors to irreversible fouling as they can penetrate in the pores of the membrane and block the channels, significantly affecting membrane filtration performance and cleaning requirements. Interestingly, biopolymers have also been shown to naturally aggregate in some systems. Promoting clustering would then enable to transfer the highly fouling compounds from the soluble to the particulate fraction in which case the biopolymer clusters formed can no longer enter the pores of the membrane and will only contribute to the formation of cake layer on the surface of the membrane, which is essentially reversible. This will then lead to a reduced impact on the filtration performance and decreased costs of operation therefore making the technology economically viable.
The aim of this research is then to develop a sustainable and economically viable algae based technology for wastewater treatment and algal biomass production for resource recovery by establishing the basis for controlled biopolymer clusters formation in membrane photo-bioreactors treating wastewater and demonstrating the beneficial impact of the particulate biopolymer assemblages on the reversibility of membrane fouling.

The main impact of this project will be to deliver a cost effective and sustainable system to grow and harvest algae while treating wastewater and consequently the main non-academic beneficiaries will be 1) the water industry, 2) the algal biomass industry and 3) the wider society.
1) With new regulations coming into place and stricter consents to be met for wastewater discharge and specifically for nutrients removal, water utilities are in need of sustainable treatment options. The findings from this project will enable the development of an integrated economically viable algae based system which will provide the treatment to meet the stricter nutrient discharge consents, while also providing added value through the production of algal biomass.
2) Microalgae are of major interest to a wide range of industries primarily because products such as pharmaceuticals, nutraceuticals, animal feed, cosmetics and biodiesel can be produced from algal biomass. However, the key limitation of algal biomass production systems is the high cost associated with the algae harvesting stage which limit their application to the production of low volumes, very high value products (pharmaceuticals, nutraceuticals). The new knowledge developed in this project will lead to the optimisation of algae harvesting with membranes and significantly reduce the costs of operation of the system therefore opening new opportunities for the lower value, large impact applications such as biodiesel production.
3) Finally, this project can lead to significant environmental and societal impacts. The development of technologies providing sustainable removal of nutrients from wastewater effluents before discharge into the environment will allow improving water quality in rivers and lakes, contributing to environmental protection and sustainability, which will be of benefit to all. Also, supporting the development of biodiesel production from algal biomass by providing economically viable technologies for algae harvesting can be a major step towards reducing dependence on fossil fuels and ultimately leading to environmental sustainability and economic growth.