## 1 Introduction  
The rapid increase in waste generation worldwide, according to the World Bank Group (global waste generation is expected to grow to 3.4 billion t. per year, a 70% increase compared to 2016 levels) [1] forces the scientific community and the society to find sustainable solutions, primarily reducing the volume of the produced residues and, consequently, utilizing these materials under eco-friendly processes. The reduction of wastes, combined with their valorization for production of new added-value products, benefits both the environment and the industry [2,3]. The majority of solid or semi-solid byproducts' streams produced by virtue of several agro-industrial activities (mostly we refer to the -food-production and food-processing sector residues) is currently utilized as animal feed, fertilizers, and substrates for composting [3]. Moreover, in several other cases, an important number of food-processing residues is directly landfilled, burnt, or discarded untreated in the aqueous environments (in the latter case, we refer to the liquid waste streams) with evident very important negative impact on the ecosystem [2], whereas c. 33% of all produced foodstuffs, totaling about 1.3 \(\times\) 10\({}^{3}\) million t. per annum, is completely wasted around the world (Food and Agriculture Organization of the United Nations 2019 report; see: [3]). From all the above-mentioned analysis, therefore, it is evident that all aspects dealing with food waste valorization processes are widely studied, but unfortunately, many of the provided solutions are not yet applied by the majority of the implicated industries. Therefore, the continuously increasing food waste accumulation connected with the potential exhaustion of conventional fuels and the various resulting environmental issues (increase of CO\({}_{2}\) emission, global warming, climate change, eutrophication, etc.) [2] makes it increasingly necessary for industries to apply sustainable utilization of by-products and, consequently, to enter in the \"meta-industry\" era. Meta-industry, for the first-time reported as a term, describes the transition from classical industrial practice of product manufacture and waste disposal to an innovative next-generation industry, in which the by-product valorization would be integral part of industrial practice, in terms of circular bioeconomy (\"meta\" in ancient (and modern) Greek means \"after\"; therefore \"meta-industry\" would refer to the industrial applications after the synthesis of the principal industrial product, viz. to the industrial applications in the waste and residue streams generated). Green and sustainable development of a meta-industry could enhance profit increase, through the production and sale of new added-value products, while protecting the environment and acting as an antidote to climate change. The dairy and cheese-making industry have always been pillars of the Greek economy, with a variety of products being widely accepted by the consumers worldwide. Cheese-making practice, however, leads to the production of cheese whey (CW), after milk coagulation and curd drainage [4]. According to [PERSON] et al. [5], approximately 160 \(\times\) 10\({}^{6}\) t. of CW are, annually, produced worldwide, of which 45 \(\times\) 10\({}^{6}\) t. are produced within the EU level. CW characteristics depend on milk type and cheese making procedure, containing (per weight) 93-94% moisture, 5-6% lactose, 0.6-1.0% whey proteins and low quantities of mineral salts [6]. In Greece, goat's and sheep's milk is the most widely used [7] for production of cheeses such as Feta and Graviera, and the total CW production is about 700,000 t. per year, with 250,000 t. being used for whey cheeses production, such as Mizithra, Anthotyros, Manouri, and Xynomizithra [8]. CW is a by-product with a high organic load (COD between 50 and 80 g/L and BOD\({}_{5}\) between 35 and 60 g/L) [4; 9].