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Pyrolysis is one method for treating sewage sludge, particularly in remote areas or decentralised systems. The end product of pyrolysis, sludge-char, can serve as a soil improver. However, there is a lack of comprehensive data on the organic pollutants' behaviour in sludge-char. In our work, we focused on the behaviour of per- and polyfluoroalkyl substances (PFASs). Sludge was pyrolyzed at 200–700 °C to determine the minimum safe temperature for effective PFASs removal. It is important to note that PFASs may not only be mineralized but also cleaved to unanalyzed PFASs and other organofluorinated substances. To address this issue, we incorporated additional measurements of organic fluorine in the experiment using combustion ion chromatography (CIC). Due to the inherent heterogeneity of sludge, containing a variety of pollutants and their precursors, we conducted pyrolysis on artificially contaminated sand. This allowed us to assess and compare the behaviour of PFASs in a homogeneous matrix. Based on our analyses, we determined that a temperature greater than 400 °C is imperative for effective PFASs and organic fluorine removal. The results were verified by analyzing samples from a commercial sludge pyrolysis unit at the Bohuslavice-Trutnov WWTP, which confirmed our measurements. In light of these results, it becomes evident that sludge pyrolysis below 400 °C is unsuitable for PFAS removal from sewage sludge. Highlights: The minimum temperature for significant PFASs and organic fluorine removal was 400 °C. PFASs were part of primary pyrolysis gas: purification or combustion is necessary. Contamination by PFASs and their congeners could be monitored with organic fluorine. [ABSTRACT FROM AUTHOR]

With the growing emphasis on environmental protection, the ways of sewage sludge treatment are changing. In this review, we analyse different methods of sewage sludge treatment in terms of potential environmental risk and raw materials recovery. The review begins with a comparison and assessment of existing reviews on this topic. Then, it focuses on the properties and current utilisation of sewage sludge in agriculture and a brief description of sludge thermal treatment methods (mono- and co-incineration, pyrolysis, and gasification). The final part of the review is devoted to technologies for treating sludge ash from mono-incinerators to recover phosphorus, a substance listed as a critical raw material by the EU. Our results show that direct use of sewage sludge likewise composts containing sewage sludge should no longer be considered as a direct source of nutrients and organic matter in agriculture, because of its pollutant content. Co-incineration and landfilling represent a dead-end in sludge treatment due to the loss of raw materials, whereas pyrolysis is sustainable for remote locations with low heavy metal content sludge. Heavy metals also pose a problem for the direct use of sludge ash and must be therefore removed. There are already sludge ash processing technologies that are capable of processing ash to form a variety of raw materials such as phosphorus. These regeneration approaches are currently in their infancy, but are gradually being introduced. The sewage sludge treatment industry is rapidly evolving, and we have attempted to summarise and discuss the current state of knowledge in this review, which will provide a baseline towards the future of sewage sludge suitable treatment. [Display omitted] • Sewage sludge applied to soil can affect the surrounding environment. • Pyrolysis is suitable for sewage sludge with low heavy metal contamination. • Co-incineration results in the loss of valuable substances and ash contamination. • Mono-incineration of sewage sludge should be followed by ash processing. [ABSTRACT FROM AUTHOR]

The potential extractability, crop uptake, and ecotoxicity of conventional and emerging organic and metal(loid) contaminants after the application of pre-treated (composted and pyrolysed) sewage sludges to two agricultural soils were evaluated at field and laboratory scale. Metal(loid) extractability varied with sludge types and pre-treatments, though As, Cu, and Ni decreased universally. In the field, the equivalent of 5 tons per hectare of both composted and pyrolysed sludges brought winter wheat grain metal(loid) concentrations below statutory limits. Carbamazepine, diclofenac, and telmisartan were the only detected organic pollutants in crops decreasing in order of root > shoot > grains, whilst endocrine-disrupting chemicals, such as bisphenol A and perfluorochemicals were heavily reduced by composting (up to 71%) or pyrolysis (up to below detection limit) compared to raw sludges. As a consequence, no detectable concentrations were measured in soils 12 months after field application. This study highlights the potential advantages of processing sewage sludge before soil applications, especially in the context of reducing the mobility of emerging contaminants, though further studies are required on a broad range of soils and crops before land application can be considered. [Display omitted] • Pyrolysis removed endocrine-disrupting chemicals and PFAS from sludge. • Leaching of metal(loid)s from sludges was significantly influenced by the treatments. • Diclofenac and carbamazepine were detected only in wheat seeds after application. • Telmisartan and triclosan were the most abundant emerging pollutants in sludges. [ABSTRACT FROM AUTHOR]

Residual moisture (0–10 wt%) in the pyrolyzed fuel has a significant effect on the distribution and properties of pyrolysis products. It is due to the concentration of steam in the primary pyrolysis gas which is estimated to as much as 80 vol%. This is normally not taken into account in case of fixed-bed reactor studies at laboratory scale, where inert carrier gas is used to secure the absence of oxidizing agent, because the moisture is released within the beginning of heating and carried over from the hot zone before the pyrolysis starts. Consequently, the application of the results to large-scale technologies (working in the continuous sludge feeding regime) is not entirely valid because in industrial large-scale systems the steam passes through the hot pyrolysis zone and takes part in high-temperature reactions. In the experiments presented within this manuscript, we allowed the residual moisture typical for industrially dried sludge (10 wt%) to react within the pyrolysis process by incorporating it into the carrier gas (consisting finally of 70 vol% steam and 30 vol% He). This steam reacted within the pyrolysis process at temperatures 400–800 °C in a fixed-bed reactor. The results were thoroughly compared with the results of a previous study with pure helium atmosphere, within the same experimental setup. When comparing the experiments with and without steam in the carrier gas, a significant difference in the distribution and properties of the products was observed especially at temperatures above 600 °C. The use of steam as a carrier gas (instead of inert gas) resulted in a remarkably higher pyrolysis gas yield. The pyrolysis gas constituted of almost double the content of hydrogen with a decrease in the yield of the sludge-char. Moreover, particular changes in sludge-char properties were observed. Not only the steam enhanced textural properties of the sludge-chars, especially the specific surface area and the total specific pore volume, but it also resulted in a significantly lower content of C, H, N and S and lower heating value of the sludge-char. These parameters can be decisive in case of sludge-char utilization ranging from agronomic through adsorbents to fuel use. Our approach provides a simplified way to better predict the distribution and properties of pyrolysis products in large-scale units. This can be time and money saving and is crucial to make decisions about the applicability of pyrolysis technology to specific sewage sludge products. • Industrially dewatered and dried sewage sludge typically contains 10 wt% moisture. • This moisture normally is not reflected within laboratory fixed-bed experiments. • We simulated presence of moisture by including steam into carrier gas. • Significant effect of steam occurred at > 600 °C. • Steam enhanced the evolution of sludge-char porosity. [ABSTRACT FROM AUTHOR]