Production of porous carbonaceous adsorbent from physical activation of sewage sludge: application to wastewater treatment

With an objective of production of carbonaceous sorbent for industrial effluent treatment, physical activation by steam of biological sludge collected from the municipal wastewater treatment plant of Nantes (France) was studied and optimised using experimental design. Thus, this activation process consists of a carbonisation under N₂ atmosphere at 600 °C for 1 h, followed by a thermal oxidation using steam (760 °C, 0.5 h, 2.5 L/min). The global mass yield of the process is equal to 38%. The thermal treatment allows a specific surface area of up to 225 m²/g to be reached, the porous structure being composed of both micropores and mesopores. The content of acidic surface groups is 0.71 mEq/g whereas that of basic surface groups is 0.55 mEq/g. The adsorption properties of the sorbent made from sludge are estimated with regard to various pollutants representative of industrial pollution of wastewaters and compared with those of commercial activated carbon. Whereas the adsorption capacities of organic micropollutants are quite low because of proportionality to the microporosity, the important mesoporosity of the sorbent leads to interesting properties for macromolecules removal from aqueous solutions, such as dyes(q_m=175–200 mg/g). Furthermore, the surface functional groups and Ca^2 + ions within the materials allow high copper ion adsorption capacities of 140 mg/g to be obtained. Finally, a techno-economic approach shows that the sludge activation process seems to be economically competitive with regard to incineration.