Another category of paper mill sludge (PMS) management is waste-to-energy (WtE), which groups several methods [96]. Thermal processes, like pyrolysis, provide an option for thermally upgrading the PMS to higher calorific value fuels. The bio-oils and charcoal produced from paper sludge pyrolysis have the potential to provide marketable feedstock and sources of energy. Charcoal derived from biomass was traditionally used as metallurgical fuel and is also being considered as a soil amendment and fertilizer replacement. The bio-gas product of waste pyrolysis has sufficient calorific energy and can be combusted to provide the required internal heat of pyrolysis, thus closing the circular process and reducing the external heat supply [91,118]. Other thermal processes include combustion incineration, steam reforming, wet oxidation, gasification, and more [99].

Ethanol (or EtOH, C₂H₆O) is a colorless liquid with a slight odor, soluble in water, flammable, and volatile. Ethanol, unlike fossil fuels, is a renewable energy source produced through the fermentation of sugars. As a high-octane fuel (98), it has replaced lead as an octane enhancer in petrol. Blending ethanol with gasoline fuel benefits the engine, burns more completely, and reduces polluting emissions [119]. Ethanol can also be used as a fuel for power generation, such as in solid oxide fuel cells, and as an ingredient in the chemicals industry [120,121].

Bioethanol is produced from natural matter. first-generation bioethanol feedstock is mainly edible food crops such as rice, corn, sugarcane, and vegetable oils like soybean oil and olive oil. Production of first-generation bioethanol has its disadvantages, competing with the food supply and land utilization. However, this form of biofuel is commercially available and known for its yield and production process [14].

Second-generation bioethanol refers to ethanol produced from nonedible feedstocks such as woody biomass, herbaceous biomass, solid waste, and animal fat. Compared to the first generation, the amount of energy that can be produced per land area is much bigger. Some drawbacks, however, are higher capital costs in production due to sophisticated processing equipment and lower energy density vs. first-generation bioethanol [122].

The conversion of lignocellulosic biomass into bioethanol generally starts with feedstock preparation that involves cleaning and size reduction. This is done by milling, grinding, or chopping and is crucial for the removal of impurities and increasing surface area; however, this step is energy-consuming [123]. The process is followed by four steps: pretreatment for the degradation of the lignocellulosic matrix, hydrolysis of cellulose in the lignocellulosic materials to fermentable reducing sugars, fermentation of the sugars to ethanol, and lastly, distillation and recovery of the produced ethanol (Figure 1).

Pre-treatment is needed to reduce the lignin and hemicellulose amount, as they hinder hydrolysis by impeding the access of cellulase enzymes to cellulose [124,125]. The pre-treatment should also reduce cellulose crystallinity and increase its porosity or surface area, which can significantly improve hydrolysis. In addition to being cost-effective, it is required to prevent the loss of carbohydrates and the formation of hydrolysis-inhibiting by-products [126,127]. This process has been intensively studied as it is a major technical and economical bottleneck in the bioconversion of lignocellulos to bioethanol, accounting for 40% of total costs [128], hence enabling scaling up production [129,130]. During pre-treatment, inhibitory compound may be generated. These compounds have a negative effect on enzymes and microorganisms, affecting catalytic processes and reducing ethanol production yield. Table 5 presents current industrial pre-treatment methods.

Hydrolysis is usually catalyzed by cellulase enzymes, and fermentation is carried out by yeast or bacteria. Factors that have been identified to affect the hydrolysis of cellulose include the porosity (accessible surface area) of the feedstock, cellulose fiber crystallinity, and lignin and hemicellulose content [10].

Ethanol is produced by fermenting soluble sugars using a variety of microorganisms. The fermentation is conducted in anaerobic conditions, with the maximum theoretical yield being 0.51 kg of ethanol and 0.49 kg of CO₂ per kg of glucose [133]. There are several possibilities for the integration of hydrolysis and fermentation steps. These include separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and consolidated bioprocessing (CBP) [134]. SHF consists of two separate hydrolysis and fermentation steps. In SSF, both steps are conducted simultaneously in the same vessel. In this method, soluble sugars are immediately fermented into ethanol, improving both enzymatic hydrolysis efficiency and ethanol yield. In CBP, enzymes are produced alongside hydrolysis and fermentation in a single step [22].

Generally, at the end of the fermentation step, the bioethanol value stands at 5% wt. This is a lower value compared to first-generation ethanol, which stands at 12% wt. The ethanol broth is distilled in a stripper column to reach a 20% wt. concentration and then further concentrated in a rectifier column to no higher than 95.6% wt. ethanol in water. Distillation is an energy-demanding step, accounting for 60–80% of the total separation cost of bioethanol from water [22].

Paper wastes are considered a good lignocellulosic biomass source for bioethanol production because of their high cellulose content (Table 2). Paper is usually recycled 3–4 times [135], and after that, most fibers become too short and incompatible for papermaking. Using such fibers may negatively affect paper product properties; therefore, they are rejected as waste. Since the recovery of recycled paper cannot solely be in the papermaking market, RPS may also be used for ethanol production, as the process waste is beneficial [136].

Both RPS and other PMS may have a negative cost impact from a circular economy standpoint, as they are an environmental burden and typically landfilled by default. PMS and RPS were evaluated and demonstrated as bioethanol feedstock, presenting successful conversion (Table 3). The integral utilization of lignocellulosic biomass, namely paper sludge, has been considered within the bio-refinery concept and can be evaluated by circular economy monitors for environmental, economic, and even social aspects. The common evaluation methodology is the life cycle assessment (LCA), standardized in the ISO 14040 series [142].

Several PS valorization routes have already been suggested and explored. Many different products have already been produced from sludge, including: nanocellulose [59], lactic acid [156], cellulase [8], isoprene [157], microbial lipids [158], and building materials supplement [159]. In addition, various WtE routes have also been suggested for a variety of energy products, including: hydrogen [160], bio-gas from anaerobic digestion (AD) [161], butanol through acetone-butanol-ethanol fermentation [162], bio-oil [163], bio-methanol through gasification [164], and bioethanol. These solutions have yet to be found sustainable, resulting in PS being landfilled for the most part. The conversion of sludge to ethanol holds several economic benefits, and even negative costs, since using PS to produce other products reduces landfilling and shipping costs, environmental fees, and eliminates purchasing costs without the need to purchase the original source material. In addition, other by-products from the papermaking process, like kraft pulp and Spent sulfite liquor, have also been suggested for ethanol production, [22]. Table 4 shows different studies on bioethanol production from papermaking process waste by-products.

A large amount of water is required in the papermaking process for both the reaction media and for use as wash water. Depending on the type of raw material input and the process conditions, there is great variability in PS properties, such as organic content, ash content, and pH [22]. In addition, there is a large variation in the chemical composition of the PS produced in different pulp mills. Previous work showed differences in the chemical composition of 37 PS samples that originated from different mills, depending on the feed material and the upstream processing in papermaking [152].

Although PS shows great promise, a major barrier in PS conversion to ethanol is its high ash content. The ash, mostly CaCO₃, absorbs enzymes and increases sludge pH levels and suppresses enzymatic activity. Acidic treatments can be used for neutralization to minimize this limitation [172]. Moreover, high ash content limits the total solid load, eventually increasing processing costs. PS also has high water-holding capacity and viscosity, resulting in inefficient mixing and poor mass transfer [22].

Paper waste management poses a major economic burden on paper and cardboard producers and recyclers. This burden has dramatically increased in recent years, due to the increase in e-commerce and shipments and the growing need for cardboard boxes and containers [59]. These challenges are also accompanied by major fluctuations in paper and cardboard prices, initially started by a major reduction of more than 300% in cardboard prices, due to the high-volume importation of cheap cardboard from China in recent years, which left the paper recycling sector “in a crisis situation” [173]. A more recent case study in Catalonia, Spain, showed a major drop of 73% in paper and cardboard prices from 74.5 € to 20 € during 2018–2019 [174]. Recently, cardboard prices have been moving up, which is explained by the end of inventory hoarding that characterized the post-pandemic recovery. However, it is too soon to determine if these price increases will stay and be repeated [175].

Furthermore, the necessity for waste handling and shipments to landfills/treatment sites, as mostly practiced, contributes substantially to the economic burden of the paper industry. The necessity for the paper and carboard industry to shift toward a CE is demonstrated by the cost of unit cargo for truck shipments that can be estimated to range between 40–180 USD/ton of waste (for a 1000 km distance), according to the truck fullness ratio and additional tax fees [176]. A recent publication by the Ministry of Land, Infrastructure, Transport and Tourism of Japan issued a transportation rate of 7.63 USD/km for a 20-ton truck (with a volume capacity of 75 m³) [177]. This has led to several works evaluating full life cycle assessments (LCAs) in the paper industry, analyzing water and raw materials and end waste-products for the possibility of further use, thus increasing economic feasibility [60,178,179]. Moreover, PS valorization into valuable energy products holds high potential for environmental benefits. A case study conducted on a virgin pulp mill has shown that the use of PS bioenergy for ethanol and bio-gas production has the potential to reduce energy demand by 10%, while reclaiming 82% of the water from the PS. As a result, greenhouse gas emissions (GHGs) will be reduced by three times, and solids suitable for land spreading will be produced as well [180]. To date, carbon taxes and emissions trading system (ETSs) cover 20% of global emissions. Since these tools increasingly take central places in GHG emissions regulations, this approach can be much more beneficial economically, saving high carbon fees [181].

Integration of ethanol production may transform P&P mills into biorefineries, allowing them to diversify production and increase profitability. For example, several works have assessed the techno-economic potential for repurposing kraft mills into ethanol production plants [182,183]. Although sulfite pulps correspond to only 2% of annual wood pulps [22], additional papers have suggested the conversion of sulfite mills into integrated biorefineries [184,185]. Bioethanol production integration into existing paper mills can maximize the use of raw material and reduce operational costs. The economics for the conversion of PS to ethanol in kraft pulping mills were evaluated to be a low-cost opportunity, due to the negative cost of feedstock material and the simplicity of the process [186]. Nevertheless, industrial-scale production of ethanol from PS and from lignocellulosic material is limited due to high capital investment and technical risks [22].