INFLUENCE OF SOURCE AND TREATMENT METHOD ON THE PROPERTIES OF ACTIVATED CARBONS PRODUCED FROM SEWAGE SLUDGE

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4 ECSM 08 - European Conference on Sludge Management Liège, Belgium, 1&2 September 2008 INFLUENCE OF SOURCE AND TREATMENT METHOD ON THE PROPERTIES OF ACTIVATED CARBONS PRODUCED FROM SEWAGE SLUDGE S. Pullket, K. M. Smith, G. D. Fowler, N.J.D. Graham Environmental and Water Resource Engineering, Department of Civil and Environmental Engineering, Imperial College London, South Kensington, London SW7 2AZ, UK g.fowler@imperial.ac.uk Keywords: activated carbon, physical activation, sewage sludge, textural characterisation, surface chemistry The increasingly stringent legislation governing the use of sewage sludge as a fertiliser, coupled with the decline in the viability of existing sludge disposal routes such as landfill and incineration, have created a drive for more sustainable alternatives. The conversion of sludge into activated carbon by the combined process of pyrolysis and partial gasification (steam activation) is one option that, due to its valorisation of the sludge, is particularly attractive [1,2]. This type of treatment has been applied to other feedstock including sludge [3,4], but further work is required to enable a full understanding of the products and process parameters. This paper investigates the effect that both the geographical origins of sludge and the treatment method applied to it (anaerobic digestion, aerobic digestion, liming etc.) have on its properties as a feedstock for producing activated carbons. The approach taken has been, firstly, to utilise the response surface methodology (RSM) method to model the relationship between the activation conditions and the textural characteristics (i.e., the BET surface area, micro- and meso-porosity) of the carbons produced from two different UK sludges; these were a raw sludge and an anaerobically digested sludge. RSM is a tool that grants greater statistical power for fewer experiments enabling quicker experimental optimisation. The models generated have then been used to find the optimum activation conditions for maximising the BET surface area. Subsequently, these conditions have been employed to prepare activated carbons from a range of sludges collected from France and Poland. The activated carbons generated have been characterised both in terms of their textural characteristics and - with the aid of FTIR - their surface chemistry. The analyses indicated that there was minimal variation with sludge origin and treatment method in the activation conditions required to maximise the BET surface area. However, the size of the BET surface area and the nature of the surface chemistry were both found to vary significantly between sludge sources. This paper explores the reasons for this outcome and discusses the relationship between the characteristics, the sources and the treatment methods of the sludges. REFERENCES [1] Chen, X., Jeyaseelan, S., Graham, N., Physical and chemical properties study of the activated carbon made from sewage sludge, Waste Management., 22, (2002), [2] Graham, N., Chen, X.G., Jayaseelan, S., The potential application of activated carbon from sewage sludge to organic dyes removal, Water Science and Technology., 43, (2001), [3] Miguel, G.S., Fowler, G.D., Sollars, C.J., A study of the characteristics of activated carbons produced by steam and carbon dioxide activation of waste tyre rubber, Carbon., 41, (2003), [4] Pollard, S.J.T., Fowler, G.D., Sollars, C.J., Perry, R., Low-cost adsorbents for waste and wastewater treatment: a review. The Science of The Total Environment., 116 1, (1992),

5 ECSM 08 - European Conference on Sludge Management Liège, Belgium, 1&2 September 2008 INFLUENCE OF SOURCE AND TREATMENT METHOD ON THE PROPERTIES OF ACTIVATED CARBONS PRODUCED FROM SEWAGE SLUDGE S. Pullket, K. M. Smith, G. D. Fowler, N.J.D. Graham Environmental and Water Resource Engineering, Department of Civil and Environmental Engineering, Imperial College London, South Kensington, London SW7 2AZ, UK g.fowler@imperial.ac.uk Abstract This paper reports an investigation into the effects of the geographical origins of sludge and the treatment methods applied to it on its properties as a feedstock for producing activated carbons. The approach taken firstly utilises a response surface methodology (RSM) to model the relationship between the activation conditions and the textural characteristics of the carbons produced from two different UK sludges. Subsequently, the optimal conditions obtained were employed to prepare activated carbons from a range of French and Polish sludges. The activated carbons generated have been characterised in terms of their textural characteristics, chemical composition and their surface chemistry. Keywords: activated carbon, physical activation, sewage sludge, textural characterisation, surface chemistry INTRODUCTION The increasingly stringent legislation governing the use of sewage sludge as a fertiliser, coupled with the decline in the viability of existing sludge disposal routes such as landfill and incineration, have created a drive for more sustainable alternatives [1]. The conversion of sludge into activated carbon by the combined processes of pyrolysis and partial gasification (steam activation) is one option that, due to its valorisation of the sludge, is particularly attractive [2, 3]. Activated carbon is a high surface area adsorbent used to clean gases and liquids. Its retail price is up to 2000 pounds per tonne. It can be produced by two main methods, physical and chemical activation. This paper focuses on physical activation using steam as it is far more commercially viable, as well as being more environmentally friendly. Steam activated sludges and sludges mixed with other material have been examined before by many groups [4-6]. The adsorption properties of activated carbon are mainly dependent on two parameters: their physical and chemical properties. Physical property tests include nitrogen gas adsorption analysis and chemical property tests include elemental analysis, ash content, X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). 1

6 ECSM 08 - European Conference on Sludge Management Liège, Belgium, 1&2 September 2008 sludge could be due to the O-H with H bonding, which can be attributed to some moisture content within the sample. Except KAW_S, the peaks at cm -1 can be assigned to C=O functional groups[3]. KAW_S presents peaks at 1637cm -1 and 1456cm -1, which are outside the region for this functional group, and may be assigned to some oxygen functional groups that bond with the mineral content within the sludges since CHN analysis gave values that were near zero for this sludge (see table 3) and thus, functional groups containing CHN should not be formed. All the SBAs produce peaks at cm -1 which indicate the presence of Si-O-C and Si-O-Si functional groups[3, 18, 19]. For KAW_S, the presence of a peak at 916cm -1 may due to the formation of micas[18], which may bond with Ca. The other bands below this wave number (for all SBAs) are mainly contributions from mineral content, which include whitelockite, feldspars and calcite [18, 20]. Even though the surface functional groups of SBAs are very similar, the absorbance intensity of the surface functional groups, especially at cm -1, is obviously different. For example, DMAD, KAN and GOS have different peak to peak height ratios. Despite the ash content and CHN analyses being similar, the Si-O-Si content varies. This also indicated that after the digestion treatment, the sludges from different locations and origins yield a different ratio of mineral content within the ash. CONCLUSIONS In general, it can be said that sludges from different sources and treatment methods produce a range of activated carbons in terms of both their physical and chemical characteristics. Anaerobically digested sludges (without lime addition) yield the same physical properties (surface area), carbon and mineral content irrespective of source and treatment method after steam activation. The reason for these similarities may be attributed to the fact that the digestion process removes the same components from the carbonaceous content of the sludge. It can be said that the differences in carbonaceous content before digestion may be due to carbohydrates, fats and proteins which can vary among sources due to the different socio-economic conditions of the catchment areas connected to the wastewater treatment facility. The only significant difference between the digested sludges from different sources is the relative amount of minerals within the ash, which make a negligible contribution to the surface area of the SBA. However, the differing mineral content between sludges may have an effect on the adsorption properties of the SBA. The RSM model has been shown to be a useful tool for identifying the optimal physical conditions for processing precursor carbon feedstock. However, each type of sludge used in this study whether it is anaerobically digested (with or without lime addition) aerobically digested or raw sludge, displays slight differences in their activation behaviour and thus, for true optimisation a unique RSM model is required for each sludge type. Further, results suggest that sludges with lime added are least suitable for producing activated carbon due to their low carbon content and the acceleration of gasification reactions, which is attributable to their high calcium content. Except for sludges with lime added, it can generally be said that sludges are a good potential precursor for adsorbents and can produce surface areas up to 270m 2 /g by steam activation. The best surface area obtained was from raw sludge. ACKNOWLEDGEMENTS This work was funded by the EU REMOVALS programme under the FP6 Global Change and Ecosystems priority, contract number The authors would also like to thank Thames Water Plc and project partners at the University of Nantes and Lodz University for the provision of sewage sludge and details of the sludges. 7