The objectives of the current work were a) to identify the correlation between sewage sludge conditioning / dewatering processing and fate of a major sludge organic pollutant (Nonylphenol, NP) in sludge-amended soils and b) to provide, if possible, clues about the mechanism driving this correlation. In view was a contribution to predicting and controlling the fate of organic xenobiotics in the soil-sludge environment, by examining an until now not adequately studied, but nevertheless potentially determinant factor, i.e. the sludge treatment. The fate of a radiolabeled isomer of NP was compared in soils amended with sludges which had been treated by different conditioning and dewatering methods in the lab. Under the conditions tested, the experiments indicated:• a major impact of sludge centrifugation, leading to a considerably increased persistence of NP in soil• a moderate impact of freeze-thaw conditioning, leading to a moderate increase of the persistence of NP in soil• a minor effect of sludge liming, enhancing the formation of bound residues in soil with a concomitant lower mineralization rate of NP, as well as a slight increase in NP leaching potential• a minor effect of conditioning by a cationic acrylamide-based polymer, expressed as lower mineralization rates for prolonged incubation periods, as well as a slightly lower leaching potential of NP• a negligible impact of ferric chloride conditioning on the fate of NP in soil.For interpreting the effect of sludge treatment on NP fate in soil, it was attempted to link its transformation rates with specific parameters, either determined in the same systems (availability of NP to the aqueous phase, nutrients content, water content of sludge, profile of bacterial populations), or controlled during especially designed experiments (concentration of the pollutant in sludge, localization of the pollutant, soil or sludge biomass). Moreover, related data from the literature were co-evaluated. The increased persistence of NP in centrifuged sludge was attributed to its entrapment in remote sites / compact structures of the condensed matter in sludge aggregates, where it is not easily accessible to soil and sludge microorganisms, and where oxygen cannot easily penetrate either. Decreased availability of the NP located in the less permeable flocs of freeze-thawed sludge was similarly proposed as the most probable mechanism explaining the moderate increase of NP persistence in soils amended with this type of sludge. By addition of lime, the increased pH (primarily of sludge, but also of the amended soil) was assumed to be a factor favouring a) binding of oxidation products of NP to the soil or sludge humic substances – thus hindering further oxidation to CO2 – as well as b) a slightly higher degree of deprotonation of NP – hence increasing its mobility towards the aqueous phase and groundwater bodies. A effect on soil and sludge populations involved in NP transformation processes was underlined as an equally alternative – or complementary - hypothesis for the lime-induced effects. Large flocs with dense cores appealed as a potential fact responsible for the declining mineralization rates for NP in sludge conditioned with a cationic polymer. The fraction of NP sorbed in these cores was probably less available to water, and thus less amenable to leaching. The results highlighted the physicochemical properties of sludge (e.g. size and compactness of sludge flocs) as more determinant to the fate of non-polar organic pollutants in soil, in comparison to its biological profile. As a consequence, application of advanced treatment techniques for the hygienization of sludge is not expected to have considerable drawbacks regarding the biological degradation of hydrophobic pollutants in the soil environment.The results furthermore propose sludge dewatering as one determinant process for the fate of organics in amended soils. Liquid sludge amendment showed to provide a means for significantly reducing risks associated to the persistence of NP and probably other pollutants in these systems, although in a real scenario extrinsic factors might interfere with the effect of sludge treatment. For mitigating the respective risks associated with the application of dewatered sludge, use of dewatering techniques relying on the removal of water rather than the flocculation of solids (e.g. bed drying) or an elimination of its pollutants whilst in liquid form (e.g. by bioaugmentation) were proposed. Finally, when decisions are to be made about the best available sludge treatment technique, a holistic approach has to be followed, taking into account, apart from organic pollutants related aspects, all beneficial, risk-related and economical factors associated with the recycling of sludge to the terrestrial environment.