## 1 Introduction  
Used disposable hygiene products make up 1.5-6.3% of the municipal organic wastes stream [1, 2]. Disposable nappies (DNs) typically consist of an absorbent core, with its main components being cellulosic fibers and a super absorbent polymer (SAP), usually sodium polyacrylate, while the exterior is made from various types of plastics. The composite nature of DNs is detrimental for their treatment and valorization after their use, being part biodegradable material (cellulosic fibers, excreta, and urine up to 88% [1]) and part recyclable plastics and absorbent. As nappies are usually led for incineration or landfilling due to their complex structure, many environmental issues arise, such as plastics accumulation in landfills, methane release and potential pollution due to virus-infected excreta [3]. A treatment method has been employed by the authors that facilitates the separation of the different DN fractions and their valorization [4, 5, 6], according to which the biodegradable fraction is co-digested with expired food products (EFPs). EFPs consisting of fruits and vegetables have already been tested by the authors with very promising results [5]. The simultaneous food waste and used DNs treatment is suggested, as in most European countries these are the main waste streams consisting of organic matter (partially or entirely) and no specific regulations or strategies of waste management are pursued. Dark fermentation is the bioconversion of an organic substrate to hydrogen and other metabolic products, like volatile fatty acids (VFAs) and ethanol. Biohydrogen can be used to produce heat and electricity, either through its combustion, or through electrochemical processes like fuel cells. The increasing energy demand of human activities and the deterioration of the environment has led to an increasing need for alternatives to fossil fuels, with hydrogen produced through dark fermentation of wastes being an excellent candidate [7]. The metabolic products that are co-produced during dark fermentation can also be employed as substrate for processes that produce valuable materials and energy [8]. VFAs can be directly converted to electricity through microbial fuel cells (MFC), where acetic acid is anaerobically converted to CO2 (anode), with simultaneous production of protons and electrons. Through a proton exchange membrane, the protons react with oxygen in an aerobic chamber (cathode), while the electrons are transported from the anode to the cathode through an external circuit producing electricity [9]. Biodiesel can also be produced from VFAs by utilizing bacteria, yeasts, or microalgae [10], with studies demonstrating an accumulation of 20% _w/w_ in the culture biomass as lipids [11]. Finally, the simplest method for the conversion of VFAs is their anaerobic digestion, with a typical methanogenic reactor treating the effluent of the dark fermentation process, in a two-step anaerobic digestion system. This kind of systems have been tested by the authors with similar substrates as the one examined in this study, proving the synergy of the two processes [4]. The literature regarding the valorization of used DNs for the recovery of materials and energy is limited [1, 12, 13, 14]. [PERSON] et al. [1] examined the co-composting of the biodegradable fraction of used DNs with separately collected biodegradable municipal waste, proving that the occurring compost had very small differences compared to compost produced solely from separately collected biodegradable municipal waste. [PERSON] et al. [13] examined the co-digestion of the biodegradable part of used DNs with waste activated sludge, with 3.6 M] being produced from 1 kg of VS of the biodegradable part of used DNs. [PERSON] et al.