From sludge to resources through biosolids

The paper gives an overview of the possibilities to achieve a more sustainable sludge management strategy by recovering and reusing valuable products as much as possible. Discussion separately deals with the recovery of products suitable for “material” uses and those for “energy” ones. Discussion on material recoveries includes organic matter, nutrients, carbon source, coagulant, bricks, pumice, slag, artificial lightweight aggregate (ALWA) and Portland cement. Regarding energy recovery, (i) conversion processes, including thermo-chemical conversion of dry sludge to oil, thermo-chemical liquefaction of wet sludge to oil and conversion/combustion processes, (ii) deep shaft wet air oxidation, and (iii) gasification processes, including starved air combustion, are outlined. Only general indications are given because the selection of an appropriate system for sludge management is strongly influenced by many other important factors, such as local economy and geography, climate, land use, regulatory constraints and public acceptance of the various practices. Also, the conventional and more traditional recycling options, like agricultural and other land uses, and incineration with energy recovery, are not dealt with because they are well known and extensively discussed elsewhere.