Coagulation–Flocculation as an Alternative Way to Reduce the Toxicity of the Black Liquor from the Paper Industry: Thermal Valorization of the Solid Biomass Recovered

Pulp and paper mills generate a huge amount of wastewater and some of them, especially non-wood paper mills, do not possess recovery systems. In this research work, the effectiveness of coagulation–flocculation using poly (ethylene oxide), poly aluminium chloride and poly (dimethyl diallyl ammonium chloride) for the treatment of a black liquor obtained from a pulp mill (with wheat straw as the raw material) is reported. Furthermore, the calorific potential of the solid fraction generated after these treatments was investigated with the aim of enhancing this residue. Chemical oxygen demand (COD), turbidity and total solids (TS) were determined in the supernatants obtained from the different coagulation–flocculation processes. The treatment with poly (ethylene oxide) at pH 2.0 achieved the maximum reduction of TS (66%), COD (75%) and Turbidity (95%) in the black liquor and the precipitated solid fraction with the highest calorific value (19.9 MJ/kg).