Sewage sludge-based adsorbents: A review of their production, properties and use in water treatment applications

The imposition of more stringent legislation governing the disposal and utilisation of sewage sludge, coupled with the growth in its generation and the loss of traditionally accepted disposal routes, has prompted a drive for alternative uses for sewage sludge. One option that exhibits especial promise, due to its potential to valorise the sludge, is the conversion of the sludge into adsorbents. This paper seeks to review the published research in this field: it covers the means of production, the characteristics and the potential applications of sewage sludge-based adsorbents (SBAs). The literature has indicated that chemical activation utilising alkali metal hydroxides is the most effective technique for producing high surface area SBAs. In addition, acid washing is highly effective at raising the BET surface area of SBAs, especially when coupled with physical activation. Due to their relatively low microporosity, the phenol uptake of SBAs produced by physical activation is low, but through a combination of their favourable surface chemistry and relatively high mesoporosity, the best of these adsorbents can attain high uptakes of organic dyes. The SBAs produced by carbonisation, through their high cation exchange capacity, generally exhibit a high metal cation capacity. For further research, the following investigations are recommended: the utilisation of alternative chemical activation reagents; the optimisation of the most effective chemical activation techniques; the combined utilisation of different activation and surface chemistry modification techniques to produce application-specific adsorbents.