Among existing green technologies, subcritical water was utilized as the primary tool for valorizing the algae industry waste stream. Using optimization principles, the key operating parameters to achieve maximum efficiency and reproducibility were systematically evaluated and refined. The primary focus was on extracting residual crude agar from the algae industry waste stream by varying key operating conditions such as temperature, pressure, extraction time, and the ratio of algae to water.
The use of subcritical water hydrolysis reduced the environmental impact by substituting hazardous chemicals with water. It is widely available, cost-effective, recyclable, and capable of significantly reducing downstream processing costs by eliminating harmful organic solvents from the final product. Optimizing the cascade biorefining concept indirectly considered the impact of water at subcritical conditions on product yield, properties, texture, and purity, while also minimizing environmental impact.
Conducting pilot-scale experiments was crucial to validating the feasibility and performance of the optimized process under real-world conditions. This approach enabled the monitoring of key performance indicators (KPIs) such as product yield, energy consumption, and process stability.
The ALGWAS-BIOR project has primarily identified potential environmentally friendly substitutes for hazardous chemicals, with a preference given to water as the most environmentally benign solvent. By promoting conditions that minimize water usage in subcritical formulations by increasing solid-to-liquid ratios up to 10%, higher extraction efficiency was ensured. Statistical analysis also confirmed a negative relationship between the severity factor (time and temperature-dependent) and the agar gel texture parameters. Under optimal recovery conditions, the recovered agar exhibited typical texture characteristics, with desirable parameters, such as hardness, adhesiveness, cohesiveness, springiness, and gumminess.
Mild extraction conditions enabled the recovery of the majority of residual agar without significantly affecting other functional molecules such as lignocellulose, proteins, antioxidants, polyphenols, and minerals. In the context of circular economy and industrial symbiosis, two concepts promoted by the ALGWAS-BIOR project, subcritical water extraction selectively targets molecules, facilitating a full cascade biorefining process of algae waste.
The sustainable applications of the recovered agar have been investigated with the design of bilayered composite films made of a first layer of agar/chitosan and a second layer of citric acid-crosslinked agar/polyvinyl alcohol (PVA) using an efficient layer-by-layer casting technique.
Additionally, various high-pressure processing technologies in combination, including subcritical water, high-pressure homogenizers, and high-pressure microfluidizers, have been explored in green nanotechnology. Their synergistic action has significantly improved conversion efficiency in terms of cost, energy efficiency, and thermal stability of products.
The ALGWAS-BIOR project has established a process and engineering design through the comprehensive development of process flow diagrams (PFDs) and piping and instrumentation diagrams (P&IDs).
