Water pollution is a major global problem and accounts for more than 14,000 deaths per day. By 2025, 1.8 billion people will be living in areas with absolute water scarcity and two thirds of the world faces drinking water shortages. Mining contributes greatly to water pollution through generation and uncontrolled release of acid mine drainage (AMD), a highly polluted water runoff from operating and abandoned mines.
Affordable and sustainable energy provision is another global challenge. Biofuels are promoted as modern alternatives however current-generation biofuels have limited environmental benefits while putting pressure on land and water resources. Algal biofuels could overcome many drawbacks of terrestrial plant-based biofuels but are currently more energy-intensive and expensive.
Thus far, algae have been trialled to remediate AMD but these systems are not competitive with chemical treatment (rate-wise) or compost wetlands (cost-wise). Additionally, challenges in algal fuel production include low growth rates, high costs of drying and lipid extraction.
The current treatment plant at Wheal Jane pumps AMD directly from the flooded mine workings into an active treatment system. Lime and flocculants are added and the metals removed as high density sludge (HDS). Treated water enters the local water course while the HDS is sent to a tailings storage facility.
AVaRICE diverts AMD into an algal bioreactor system (i) which produces algal biomass and removes metals in the process. The Wheal Jane AMD contains high concentrations of dissolved CO2 which may alleviate the need for CO2 injection.
The algal biomass is harvested (ii) and the liquid returned to the AMD treatment loop. Provided water quality standards are met, this may be discharged directly into the environment.
The biomass is processed by hydrothermal liquefaction (iii) to produce an aqueous phase, a bio-crude and a char. The nitrogen and phosphorus (NP) in the aqueous phase may be recycled to the algal bioreactor, closing the loop, or may be sold as a liquid fertiliser to provide another revenue source.
The bio-crude can be used as a heavy heating oil, or be hydroprocessed to yield ‘drop in’ advanced biofuels that can be used in the current infrastructure with minimal disruption.
Valuable metals are recovered from the char before the final process residue is treated to remove organic carbon and disposed of; potentially with the HDS from the current active treatment plant.