Pulp-paper mill sludge, or PPMS, is the concentrated solid or semi-solid residue generated by the wastewater treatment plants that serve paper mills. It is a heterogeneous material, a mixture of cellulosic pulp fibers that escaped the manufacturing process, inorganic fillers such as calcium carbonate, kaolin, and titanium dioxide, lignin byproducts, and organic compounds. Researchers have even detected microplastic fragments, with polyethylene and polypropylene the most prevalent polymers. The sludge comes in several distinct varieties: primary sludge settled out in the primary clarifier, secondary or biological sludge from the activated sludge process, deinking sludge from recycled paper processing, and mixed sludge combining the primary and secondary streams. Each type differs in composition, moisture, and ash content, and each presents its own challenges and opportunities for bioconversion.

The conventional disposal options for this waste are increasingly untenable. Sludge disposal can account for approximately 60 percent of the total operating expenses of wastewater treatment plants in numerous mills. Landfilling releases methane, a potent greenhouse gas, and produces leachate that can contaminate soil and water. Land application as a soil amendment risks introducing heavy metals and refractory organic contaminants into food chains. Composting demands energy inputs and can spread odors and pathogens. Incineration, meanwhile, is energy-intensive and frequently requires supplemental fuel, because the high moisture and ash content of the sludge depress its calorific value, promote equipment corrosion, and leave behind residual ash that itself requires disposal, all while emitting greenhouse gases along with sulfur and nitrogen oxides.

What makes sludge attractive as a fuel feedstock is its cellulose. Primary sludge from kraft pulp mills has been found to contain roughly 58 percent cellulose, 12 percent hemicellulose, and 20 percent Klason lignin by weight. Cellulose is a linear polymer of cellobiose units linked by beta-1,4-glycosidic bonds, and its chains are bound together by hydrogen bonds and van der Waals forces into strong microfibrils. Hemicellulose branches weave between the microfibrils, while lignin encases the whole structure in a rigid matrix. This recalcitrant architecture resists both chemical and biological attack, which is precisely why pretreatment sits at the heart of any strategy to unlock the fermentable sugars, chiefly glucose, trapped inside the fibers.

The review highlights anaerobic digestion as one of the most mature routes for converting sludge into energy. In this oxygen-free microbial process, hydrolytic bacteria and fungi first break down complex macromolecules, acidogenic bacteria then convert the products into volatile fatty acids and hydrogen, acetogens generate acetate, carbon dioxide, and hydrogen, and finally methanogenic archaea produce biomethane, which makes up 50 to 75 percent of the resulting biogas. Yet industrial adoption has lagged because yields are low and digestion times long. In semi-continuous trials, primary sludge yielded 190 to 240 normal liters of methane per kilogram of volatile solids over 23 to 32 days, while blends with secondary sludge dropped to 150 to 170 liters over shorter retention times. Toxic compounds such as chlorolignin, mineral oil, and certain metals, together with high ash content, suppress methane potential, and hydrolysis remains the rate-limiting step.

Pretreatment can dramatically change that picture. When researchers applied thermal pretreatment at 170 degrees Celsius to secondary sludge from kraft and sulfite mills, the methane production rate from kraft sludge improved by 300 times and the yield by 280 percent. Autoclaving biosludge at 121 degrees Celsius for 20 minutes enabled stable digestion at a shortened 10-day hydraulic retention time, delivering 138 normal liters of methane per kilogram of volatile solids where untreated sludge failed. Thermal treatment at 140 degrees Celsius alone boosted methane yield by 170 percent, making added chemical steps unnecessary. Hydrothermal pretreatment at 150 degrees Celsius for 10 minutes raised yields by 31 percent, while alkali dosing with sodium hydroxide disrupted the floc structure and lifted soluble chemical oxygen demand by up to 83 percent. Even black liquor, a pulping byproduct, enhanced methane yields by 7 to 30 percent more cheaply than sodium hydroxide.

Biohydrogen offers another compelling pathway, prized because its combustion emits no pollutants. Under optimized conditions of a 32-hour solids retention time, anaerobic fermentation of paper mill sludge delivered 620.8 milliliters of hydrogen per gram of chemical oxygen demand. Co-digesting pulp-paper sludge with food waste in a two-stage mesophilic-thermophilic process achieved a hydrogen yield of 64.48 milliliters per gram of volatile solids fed alongside a methane yield of 432.3 milliliters per gram, without any accumulation of inhibitory volatile fatty acids. The extreme thermophile Caldicellulosiruptor saccharolyticus has also fermented paper sludge hydrolysate into hydrogen, although inhibitory substances in the hydrolysate limited production rates. Supplementing fermentation with the cellulolytic bacterium Clostridium thermocellum improved holocellulose degradation by nearly 33 percent and hydrogen yield by almost 97 percent under thermophilic conditions.

Bioethanol may be the most commercially advanced option. Production proceeds either through separate hydrolysis and fermentation, in which cellulase enzymes release sugars before yeast fermentation, or through simultaneous saccharification and fermentation, which merges the two steps and cuts enzyme use, energy demand, and cost. The central obstacle is ash: calcium carbonate filler binds enzymes more readily than fibers, with roughly 3 to 5 milligrams of enzyme lost per gram of acid-insoluble ash, and hydrolysis yields have been as low as 8 to 32 percent glucose per gram of cellulose. Washing sludge with dilute hydrochloric acid converts insoluble calcium carbonate into soluble calcium chloride, cutting ash from 27 percent to 0.5 percent in one study and raising sugar conversion from 20 to 88 percent in another. With surfactant-assisted enzymatic hydrolysis reaching 74.4 percent efficiency, Saccharomyces cerevisiae then produced 9.7 grams per liter of ethanol at a 92 percent yield, and techno-economic modeling suggests sludge-derived ethanol is about 20 percent cheaper to produce than ethanol from corn stover.

The portfolio extends further. The oleaginous yeast Cryptococcus vishniaccii converted ultrasonicated sludge extract into neutral lipids with 53.4 percent intracellular lipid content, suitable for biodiesel after transesterification. An engineered Clostridium tyrobutyricum strain co-fermented sludge sugars with corn steep liquor to yield 16.5 grams per liter of biobutanol, a fuel with higher energy density and lower corrosiveness than ethanol. Perhaps most striking is sustainable aviation fuel: a life-cycle assessment found that catalytic sugar upgrading of paper sludge yields fuel with a carbon intensity of 35.7 to 41.8 grams of carbon dioxide equivalent per megajoule, falling to 5.1 to 11.1 when ash is recycled as cement substitute, and that the approach could produce over 330 million gallons of aviation fuel annually while cutting emissions by 2 to 7 million tonnes of carbon dioxide equivalent, a far better climate outcome than landfilling.

Significant hurdles remain before these laboratory successes translate into industry. The heterogeneity of sludge complicates standardized processing, pretreatment methods consume large quantities of chemicals and energy, and inhibitors such as phenolics, furfural, and ligno-carbohydrate complexes continue to hamper microbial fermentation. Almost all published work remains at laboratory scale, and the reviewers call for pilot and field studies, robust microbial strains tolerant of sludge variability, deeper understanding of sludge chemistry, and techno-economic and life-cycle analyses to guide commercialization. Still, the direction is clear: a waste stream that mills once paid to discard is emerging as a low-cost reservoir of fermentable sugars and a credible pillar of renewable energy, one that aligns with the United Nations Sustainable Development Goals and could help close the loop on one of the world’s oldest industries.

Subject of Research: Biofuel production from pulp and paper mill sludge through pretreatment and biorefinery methods

Article Title: A critical review on biofuels generation from pulp-paper mill sludge with emphasis on pretreatment methods: renewable energy for environmental sustainability

Article References: Kumar, V., Verma, P., de Freitas, F. A., Srivastava, P. K., Vashishth, A., & Américo‑Pinheiro, J. H. P. (2025). A critical review on biofuels generation from pulp-paper mill sludge with emphasis on pretreatment methods: renewable energy for environmental sustainability. BMC Environmental Science, 2(1), Article 2. https://doi.org/10.1186/s44329-024-00016-0

Image Credits: AI Generated

DOI: 10.1186/s44329-024-00016-0

Keywords: pulp and paper mill sludge, biofuels, pretreatment, anaerobic digestion, biomethane, biohydrogen, bioethanol, biobutanol, biodiesel, sustainable aviation fuel, circular economy, biorefinery