## 1 Introduction  
In the last decade, in developed countries, the industrial practice shifted from the mere management of biomass constituted by industrial residues, by-products, and waste to their full valorisation according to biorefinery schemes. While at the turn of the millennium, in most countries, the correct landfilling of waste was considered a fully acceptable practice, nowadays the full implementation of a circular economy demands a complete valorisation of every raw material, and especially for those of biological origin [1]. A giant step in the right direction has been the adoption by the EU of the \"Circular Economy Action Plan\" [2] that is destined to shape, over the next decades, the European economy towards a \"cleaner and more competitive future\". In many industrial chains, applied research has allowed the potential complete valorisation of several feedstocks (e.g., orange [3, 4], avocado [5], and lignocellulosic biomass [6, 7]). One of the most interesting ways to implement the circular economy approach is the \"blue economy\", a paradigm founded on the sustainable exploitation of marine resources that allow the preservation of the marine environment, conceived as one of the key factors of global prosperity [8]. As an example of a \"blue economy\" process, this paper aims to demonstrate, at the laboratory level, the potential of a biorefinery scheme applied to anchovies. Several anchovies species are captured by fisheries all over the world; according to official data [9] in the decade 2010-2019, the capture ranged between 5.5 and 11 million tonnes per year. In addition to the direct consumption as fresh fish, anchovies are transformed either by brine-fermentation or canning (preserved in oil). These processes cause the loss, as residues (heads, viscera, and spines), of about 50% of the original fish biomass. In the conventional industrial processes, these residues are not lost but used to produce fishmeal destined for indirect human consumption (e.g., aquaculture) and, more rarely, fish oil [10; 11]. Indeed, omega-3 production still strongly depends on fish oil extraction of fresh fish (especially anchovies and sardines), so much that it is competing with its aforementioned direct human consumption [12]. In this paper, the advancement of a recently proposed process [13] for the full valorisation of anchovies aimed at the extraction of fish oil (to be used as an omega-3 source) by their residues and the production of biomethane through anaerobic digestion is presented (Figure 1). The produced biomethane can be converted to energy by very efficient combined heat and power schemes, allowing not only to cover the energy demand of the process itself but also to sell surplus energy. The proposed extraction process uses, as a green-solvent, \(d\)-limonene, namely a widely available by-product of the citrus industry [3]. As a by-product, an anchovy sludge (AS) is obtained in considerable amount (97% \(w/w\) of the total residue quantity) [10]. AS is expected to be rich in \(d\)-limonene and poor in nutrient elements after oil extraction and, thus, unsuitable for animal feeding or other similar forms of valorisation. The advancement proposed and presented in this paper is the evaluation of the potential of by-products derived from fish oil extraction (AS), to produce biomethane by anaerobic digestion, both by batch and by semi-continuous experiments without any previous pre-treatment. In particular, semi-continuous experiments afford for a preliminary optimization of the process, solving, at least partially, two of the problems linked to the use of AS a substrate for anaerobic digestion: the high presence of \(d\)-limonene (whose inhibition capacity during anaerobic digestion is well known [14; 15]) and the unbalanced C/N ratio. As discussed below, granular activated carbon (GAC) was used to reclaim and improve the anaerobic digestion processes in a reactor showing clear signs of inhibition. In fact,  
Figure 1: Biorefinery scheme for the full valorisation of anchovy waste.