# Urban Biorefinery Demonstration: Production of Polyhydroxyalkanoates from a Municipal Solid Waste
## 1 Introduction  
With the growth of the global population, the consumption of materials, foods, and energy is drastically increasing [1]. This generates a significant increase in the amount of waste generated by the society [2, 3, 4]. Currently, 2.01 billion tonnes of municipal solid waste (MSW) are generated in the world every year [5]. Specifically, in Europe, each inhabitant generates, on average, 0.5 tonnes of MSW per year, increasing by an annual rate of 10% [6,7]. Around 40-50% of this corresponds to organic waste, highly rich in lipids, proteins, and carbohydrates [7]. The accumulation of MSW poses serious environmental and economic challenges, particularly regarding waste management, greenhouse gas emissions, and the depletion of natural resources [8]. Conventional waste disposal methods, such as landfilling and incineration, not only contribute to environmental pollution but also represent an inefficient use of valuable organic materials. Consequently, there is a growing interest in the development of sustainable waste valorization strategies that align with circular economyprinciples, aiming to convert waste into valuable bioproducts [8; 9]. The valorization of these waste or by-products may decrease environmental pollution, oil dependence, and contribute to the implementation of a renewable circular economy scenario [10; 11]. The present work aligns with a circular economy model as it focuses on demonstrating the valorization of the Organic Fraction of Municipal Solid Waste (OFMSW) in a real environment into a biodegradable bioplastic, such as polyhydroxyalkanoate or PHA, on a large scale with the aim of studying its technical viability [6]. PHAs are natural thermoplastic polyesters with interesting environmentally friendly applications [12]. More than 160 types of PHAs, produced using different monomers, have been identified [12; 13]. Individual monomers of PHAs can be sorted as short-chain-length (scl) (4 to 5 carbons), medium-chain-length (mcl) (6 to 12 carbons), and long-chain-length (lcl) (more than 14 carbons) [12; 14; 15]. PHAs can exist as homopolymers or copolymers, consisting of both mcl- and scl-PHAs [16]. Scl-PHAs include poly(3-hydroxybutyrate) (P3 HB), poly(3-hydroxybutyrate-co-3-hydroxyalerate) (PHBV), poly(4-hydroxybutyrate) (P4 HB), copolymers of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (PHB-4 HB), copolymers of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), and poly(3-hydroxyocanoate) (PHO) [12; 17]. Nevertheless, poly-(3-hydroxybutyrate) (PHB), a homopolymer with four carbon subunits of 3-hydroxybutyrate (3 HB), is the most common and extensively characterized member of the PHAs family [12; 18; 19]. In this regard, the presented research shows the development of short-chain-length PHAs (scl-PHA) using pretreated OFMSW as a carbon source through a mixed microbial culture (MMC) on a semi-industrial scale. The OFMSW pretreatment consisted of anaerobic digestion to produce a volatile fatty acid (VFA) rich digestate coming from a hydrolytic reactor. The current study contributes to addressing a critical gap in the field of larger-scale biopolymer production than at laboratory or pilot scale by demonstrating under real conditions the feasibility of utilizing the OFMSW as a carbon source for short-chain-length polyhydroxyalkanoate (scl-PHA) biosynthesis on a demonstrative scale. Two of the main challenges of PHA industrial production are the technical and economic viability of the process, particularly due to the high cost of conventional carbon sources. The origin of the carbon source is crucial for PHA production and represents a significant cost [12; 20; 21]. The carbon source is metabolized to PHAs, and the chain length of the carbon source has a huge influence on the type of PHA synthesized. The resulting process consists of the following: (1) Transformation of the substrate into VFA; (2) enrichment of PHA accumulating bacteria; (3) PHA accumulation; and (4) extraction [22; 23; 24].