Microalgae-Based Wastewater Treatment Processes and Biorefineries

Dear Colleagues,

We are pleased to launch a Special Issue related to the integration of microalgae cultivation in wastewaters for sustainable nutrient removal/recovery, bioenergy production, and CO₂ biofixation.

Conventional wastewater treatment plants (WTTP) typically rely on biological processes to remove nutrients and other contaminants from wastewaters due to their high efficiency and low cost compared to other treatments. These systems can achieve high removal efficiencies for biodegradable organic matter, inorganic nitrogen, and phosphorus compounds. However, the high energy requirement for the oxygenation of mixed liquors in activated sludge processes, and the emissions of large quantities of greenhouse (CO₂, N₂O) and toxic (NH₃) gases, has led to an increasing interest in the use of microalgae-based wastewater treatment processes and biorefineries. The main advantage of integrating microalgae in WWTPs is the possibility of exploiting synergic consortia among microalgae and aerobic bacteria, thus leading to simultaneous nutrient removal, CO₂ biofixation, and biomass production. The algal biomass can be valorised in different ways, including the production of biofuels (biogas, bio-biodiesel, bio-hydrogen, among others) and bio-based materials: i) bioplastics; ii) soil biofertilizers and biostimulants; iii) feed for animal consumption; iv) biochar and other bio-absorbent materials. However, several bottlenecks must still be addressed in order to apply this technology at an industrial scale.

This issue aims to collect recent advances and relevant research experiences in microalgae cultivation using municipal and industrial wastewaters based on a biorefinery concept. The Special Issue will cover the following topics:

• Process validation at the pilot and demonstrative scale (assessment of biomass productivity, nutrient removal/recovery efficiency, CO₂ fixation, and biogas upgrading in open systems or closed photobioreactors);
• Process modelling (definition, calibration, and experimental validation of algae–bacteria growth models);
• Process optimization (evaluation of optimal management strategies to reduce environmental impacts and/or to increase biomass productivity and nutrient removal/recovery);
• Advanced monitoring of algal–bacterial populations (respirometry, PAM fluorometry, flow cytometry, next-generation sequencing);
• Evaluation of optimal working conditions for wastewater-treating photobioreactors (including nutrient availability/inhibition and environmental/operational conditions);
• Life cycle and environmental impact assessment (LCA studies and assessment of gaseous emissions);
• Energetic and techno-economic assessment (energy balances, mass flows, and economic evaluations);
• Biomass harvesting (assessment of low-cost and low-energy processes, use of photo-granules or floccular biomass);
• Biomass valorisation (such as biofertilizers or biostimulants, bioplastics, biofuels, animal feed, and other valorisation routes).

The submission of all types of articles is highly encouraged, including experimental research, case studies, modelling, and review articles. We look forward to receiving your contributions to draw attention to the sustainability of microalgal cultivation in wastewaters and to enhance the knowledge related to this interesting and promising biorefinery concept.

Dr. Simone Rossi
Dr. Francesca Casagli
Dr. Micol Bellucci
Guest Editors

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