Converting wastewater cellulose to valuable products: A techno-economic assessment

Wastewater contains about 25% of chemical oxygen demand as cellulose. The source of this cellulose is mainly toilet paper which can be recovered and converted into useful products. The recovered cellulose can be pretreated and pyrolyzed to produce biochar, bio-oil, and pyroligneous acid. A techno-economic assessment was performed on the entire value chain including the char activation process. The data on mass and energy balance and investment cost was obtained from the pilot plant developed at the sludge treatment plant in Ede, the Netherlands. The minimum selling prices estimated for each of the products while considering other co-products as revenue are respectively equal to €1.06, €0.25, and -€0.11/kg. The negative value of pyroligneous acid is due to the high revenue obtained from selling the other two co-products (biochar and bio-oil) at their assumed market prices. It indicates that the revenue obtained covers the annual capital and operating expenditure. Char activation is necessary to expand its applications and thus both physical and chemical activation were considered. The physical activation method resulted in a price higher than the market price whereas the chemical activation method is most suitable considering its high yield and assumed quality. The chemically activated char can be produced at €2.3/kg which is 42% lower than the assumed market price. However, the difference in quality would be significant due to the nature and source of the feedstock (cellulose). This value chain is promising under certain conditions and might be a promising business case to investigate for a sludge treatment facility.