# Paper-Mill Wastes for Bioethanol Production in Relation to Circular Economy Concepts: A Review
## 1 Introduction  
The increase in world population, urbanization, industrialization, and remote commerce has resulted in a higher demand for energy, water, land, as well as waste and pollution production. Currently, most municipal solid wastes are either landfilled or disposed of in open dumps [1, 2], an environmentally harmful practice that is being criticized in many countries. Figure 1 shows the total wastes being disposed of by landfilling and open dumping out of the total waste generation for different geographical regions around the world. Reaching negative carbon emissions in a decade will require the adoption of strategies such as the reduction of fossil fuels like oil and natural gas, implementation of carbon capture and storage (CCS), implementation of bioenergy with carbon capture and storage (BECCS), and an increase in the use of renewables and biofuels [3]. Paper and cardboard wastes originate from wood materials and are composed of 40-80% cellulose, 5-15% hemicellulose, and minor traces of polyphenolic lignin [4]. The high cellulose content of these wastes suggests they can be vaporized for many applications. Different valorization options have been suggested, including their use as feedstock for nanocellulose, building materials supplementation, or bio-gas production [5]. Nevertheless, none of these solutions were viable, resulting in most of the world's paper and cardboard wastes being landfilled or discarded in open dumps, causing significant environmental issues such as groundwater contamination, greenhouse gas emissions, and acceleratedlandfill filling [6]. The P&P industry is one of the major consumers of woody biomass, and consequently, major producer of cellulose-rich wastes. Thus, wastes originated from the industry, such as kraft pulp and sludge [7], are suitable for bioethanol production despite their low quality. Figure 1: (**A**) Worldwide landfilling and open-dump disposal rates. The landfilled fraction of the wastes for each geographical region is represented by the blue color (**B**) Global waste generation, based on data from [2]. Recycled paper sludge (RPS), a paper production by-product, is an excellent lignocellulosic biomass source for bioethanol production due to its high cellulose content and negative cost [5]. Converting RPS to bioethanol aligns with circular economy (CE) concepts and is key in achieving Agenda 2020 for America's forest, wood, and paper industries [8; 9]. Paper is a well-explored material, including its production process, waste product, and properties. However, there has been a decline in the number of publications specifically addressing paper and paper-waste materials in 2022. The CE topic, however, has increased dramatically in publications and research since the mid-century, as observed in the Web of Science and Scopus databases (Figure 2). Models and ideas for adapting circularity concepts in industries are in line with sustainability. The valorization of RPS cellulose requires a pre-treatment stage to release the cellulose fragments. This stage is commonly viewed as the limiting factor in the process, both in terms of cost efficiency and sustainability. Ozonation has been shown to be an effective and eco-friendly method for pre-treating paper waste in the production of bioethanol [10; 11], and it has been applied in various stages of paper production and advanced treatment of wastewater [12]. Since RPS cannot be recycled, it can be used as a source for waste-to-energy (WtE) as the preferred option. Moreover, WtE does not have to compete with recycling but rather with landfill, which is lower in the waste hierarchy [13]. As current economy is mostly based on a linear and open-ended system, it compromises sustainability and the ability to meet future needs. CE addresses this sustainability issue, proposing a cyclical material and energy flow model. Biorefining for ethanol production is a crucial step towards CE, as it allows closing the loops for biomass, water, and carbon and maximizing valorization.