# Slow Pyrolysis as a Method of Treating Household Biowaste for Biochar Production
## 1 Introduction  
The continuous rise in the volume of waste generated by modern society has created an urgent demand for the development of improved and more efficient waste treatment methods. Within the European Union, annual biowaste generation is estimated to range between 118 and 138 million tons, with over two-thirds originating from municipal sources and the remainder primarily from the food industry [1]. In accordance with the Waste Framework Directive, biowaste is defined as \"biodegradable garden and park waste, food and kitchen waste from households, offices, restaurants, wholesale, canteens, cateres and retail premises, and comparable waste from food processing plants\" [2]. Based on this definition, biowaste is generally categorized into two main fractions: garden and park waste, and food and kitchen waste [3]. Consequently, household biowaste can be broadly considered a combination of these two components [4]. Aligned with the principles of the circular economy, biowaste is increasingly being viewed as a valuable resource for the recovery of nutrients, organic matter, and energy [5]. Despite its high recycling potential, a significant portion of biowaste generated annually in Europe continues to be disposed of through landfilling and incineration [6]. The environmental consequences of landfilling biowaste are particularly severe. During decomposition in landfills, biodegradable materials release greenhouse gases such as methane, carbon dioxide, and hydrogen sulfide [7]. According to the European Environment Agency (EEA), biodegradable waste--biowaste included--is a major contributor to landfill-related greenhouse gas emissions, accounting for approximately 3% of the EU's total emissions [8]. This highlights the critical need to ensure the proper collection of unavoidable biowaste and to adopt sustainable treatment strategies [6]. Up to now, various biological and thermochemical methods have been investigated for the treatment of biowaste [7]. Currently, the most widely applied biological processes include composing (aerobic treatment) and anaerobic digestion [8]. The selection of an appropriate treatment method is largely influenced by the composition of the biowaste and the characteristics of the source separation system. Effective source separation is essential for producing high-quality end products [4]. In contrast, biowaste that is not separately collected and is instead mixed with municipal solid waste (MSW) is typically subjected to incineration [5]. Biowaste typically has a high moisture content, often exceeding 60% [1], which has historically limited the use of thermochemical processes such as pyrolysis and gasification for its treatment. Before thermochemical treatment, the biowaste must be dried, which consumes a lot of energy [7]. Meanwhile, different ways of drying waste like solar drying in a greenhouse to reduce the moisture content of kitchen waste are currently being developed [9; 10]. Pyrolysis has emerged as a promising and sustainable method for biowaste valorization, capable of producing significant quantities of renewable bioenergy products such as biochar and bio-oil, while simultaneously reducing greenhouse gas emissions and other environmental pollutants [11]. A key advantage of pyrolysis is the ability to transform low-energy-density feedstocks into biofuels with higher energy density [8]. Depending on the process conditions, several types of pyrolysis can be distinguished, which differ in terms of temperature, duration, and end products. Slow pyrolysis involves gradually heating biomass in an oxygen-free environment, usually at temperatures ranging from 300 to 550 \({}^{\circ}\)C [12]. The process lasts from several dozen minutes to a few hours. The main product is biochar--a solid, carbon-rich material that can be used as a soil amendment. Fast pyrolysis takes place at higher temperatures (450-550 \({}^{\circ}\)C) but in a much shorter time, often less than 2 s [12]. Its primary product is bio-oil, which can be used as an energy source or fuel [13].