Preparation of Adsorbents from Sewage Sludge by Steam Activation for Industrial Emission Treatment

Abstract

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) is studied. This work focuses on the optimization of the carbonization and activation conditions on physico-chemical and adsorptive properties of adsorbent materials produced. The carbonization is performed in a vertical pyrolysis furnace. Its duration is fixed at 1 h, based on TGA analysis. The carbonization temperature is optimized between 400 and 1000°C to improve the BET specific surface area, the micropore volume and the quantity of surface functional groups of the char. These parameters seem to be improved by a temperature increase, the specific surface area varying from 20 to 96m2g−1. An intermediate temperature of 600°C is selected, in order to obtain an interesting specific surface area (60m2g−1) and satisfying mass yield. The activation conditions are optimized in terms of temperature (750–850°C) and duration (30–90 min). A factorial design is carried out, based on the following responses: mass yield, porosity (BET surface area, mesopore and micropore volume), surface chemistry and adsorption properties for current pollutants (phenol, copper ions, dyes and COV). The activation step shows an improvement of pore and adsorption characteristics of the adsorbent, with specific surface area reaching 230 m2g−1 and equilibrium adsorption capacities of phenol and copper equal to 50 mg g−1, and 80 mg g−1, respectively. The adsorption of micro-organic compounds and dyes may be related respectively with micro- and meso-porous properties, whereas copper adsorption efficiency is due to an ion-exchange mechanism as demonstrated by exchange with Ca2+ ions contained in the raw water sludge. Experimental design responses are optimized using surface response methodology and are validated experimentally. Finally, techno-economical analysis of the process shows a higher cost than farmland re-use but steam activation of sludge seems to be competitive towards incineration.