Microalgae cultivation on secondary resources: A review.

Abstract

Microalgae are widely investigated for wastewater treatment, nutrient removal, carbon dioxide sequestration and bioproduct production from side resources, such as industrial by-products, hydrolysates, organic wastes and industrial agriculture wastewater. These substrates are used either alone or in combination with standard mineral media for microalgae growth. pH, culture volume, temperature, and light availability are applied depending on the nature of waste streams. One of the key challenges associated with using secondary resources is managing physical factors (turbidity, color, and salinity) and chemical factors (nutrient imbalances such as the ratio of C: N: P and the concentration of inhibitory substances including free ammonia, but also bacteria) at relatively low operational costs. In most cases, microalgal production efficiency from such secondary resources is significantly higher compared to autotrophic cultivation, because these materials provide readily assimilated organic carbon as well as the required inorganic nutrients for growth. But this may also require higher oxygen demands from cultivated cells which stimulate bacterial growth without an appropriate system design and may lower quality of biomass produced and be less stable. In the present review, we provide a summary and analysis on secondary resources, microalgal strains, types of culture methods (autotrophic, mixotrophic, hybrid cultivation), process technologies (pretreatment methods and operating parameters) for microalgal production and wastewater treatments. Special attention is paid to the relation between substrate characterization, productivities and pollution removal efficiencies. Limits for scale-up and future prospects are considered such as standardized feedstocks and low-cost harvesting technologies, a modular design of cultivation system, and wastewater preprocessing before using as a culture medium.