The use of design of experiments for the evaluation of the production of surface rich activated carbon from sewage sludge via microwave and conventional pyrolysis

Experimental design and response surface methodology were used for the preparation and comparison of activated carbon produced from sewage sludge by two types of pyrolysis: conventional furnace and microwave. The preparation method was performed following a full fractional factorial design (23), including pyrolysis temperature or power radiation, holding time and chemical activation agent, and specific surface area (SBET) of prepared activated carbon. The influence of these factors on the SBET of obtained carbon was investigated using an analysis of variance. Samples made by conventional pyrolysis showed overall higher SBET values than samples synthesised by the microwave method. The optimum parameters for the preparation of activated carbon using the conventional pyrolysis have been identified as: pyrolysis temperature of 500 °C, holding time of 15 min, and a ratio of ZnCl2:sludge of 0.5. Microwave pyrolysis is found to be optimal when operating at 980 W for 12 min. Under these conditions, SBET values of 679 and 501 m2g−1, respectively, have been obtained. The analysis of nitrogen adsorption/desorption isotherms revealed the presence of micro and mesopores in the activated carbon. The most important significant factor, according statistical analysis, in the variance in SBET for the conventional pyrolysis samples were the pyrolysis temperature and interaction between pyrolysis temperature, holding time and ratio of ZnCl2:sludge were the most important factors. The highest impact parameters for the microwave method were found for the interaction between power radiation and ratio of ZnCl2:sludge and the holding time.