POLYAROMATIC HYDROCARBONS IN HYDROCHARS : HYDROTHERMAL CARBONIZATION OF SEWAGE SLUDGE

Abstract: Sewage sludge is an inevitable by-product from waste water treatment plants. Sludgemanagement is a difficult task since sludge has a high moisture content, poordewaterability and generally contain heavy metals, pharmaceutical residues,polyfluorinated alkyl substances (PFAS) and organic pollutants. Recentlyhydrothermal carbonization (HTC) has been getting attention for its suitability to treatsewage sludge. HTC is a flexible process which can handle feedstocks with highmoisture content. The process stabilizes the organic content in a solid residue calledhydrochar which has an increased energy content and decreased moisture contentcompared to the starting material. Hydrochar can be used as soil amendment materialsince it has a good nutrient content and can prevent leaching of fertilizers. However,there is a concern for the risk of high levels of poly aromatic hydrocarbons (PAHs) inthe hydrochar which might limit its applicability in soil applications. PAHs are acomplex group of organic pollutants that can be toxic and even cancerogenic tohumans. 16 PAHs have been described as priority pollutants by the US EnvironmentalProtection Agency due to their toxicity and risk for human exposure. In this studyHTC was performed, with municipal sewage sludge as feedstock, at different processconditions to investigate its influence on the PAH content in the produced hydrocharsand aqueous residues. Ten experiments were performed with varying reactiontemperature (ranging from 180 °C to 260 °C fixed at 1 h residence time) and varyingresidence time (ranging from 1 h to 5 h fixed at 200 °C and 260 °C). The PAH contentof the products was analysed using GC/MS. The hydrochars suitability for soil amendment was investigated. At a 1.7wt.%amendment level (approximately 60 tons/hectare) the results showed that the PAHcontribution from the hydrochars to the soil was well below the SwedishEnvironmental Protection Agency (SEPA) contamination guidelines. No significantPAH content in the products, that might limit the hydrochars applicability for landusages, was observed. The content of the 16 priority PAHs (PAH 16) in thehydrochars were below the international biochar (IBI) limit (6mg/kg TS) and theEuropean Biochar Certificate (EBC) EBC-Agro limit (6mg/kg TS). The PAH contentof the filtrates where below the Swedish Petroleum institute (SPI) irrigationguidelines, except for the filtrate produced at 260 °C, 5 h. In the raw material no PAHspecies was found, however, the reporting limit was higher compared to thehydrochars. Overall, the content of PAH was lower in the filtrates compared to thehydrochar, e.g. at 200 °C, 1 h the content PAH M were 0.05 and 0.85 mg/kg sludgeTS for the filtrate and hydrochar respectively. This was expected since PAHsgenerally has a low solubility in water. To assess the toxicity of the hydrochars thetoxic equivalent quantity (TEQ) was used. The TEQ for the hydrochars wereapproximately ~0.0 to 0.68 mg/kg TS which is below the IBI limit level at 3.0 mg/kgTS. Temperatures around 200 °C to 220° C were found to be favourable in terms ofPAH content in the hydrochar, at 220 °C and 1 h the sum of PAH16 is lower than forall other samples, for both the hydrochar and the filtrate (<R.L and 0.01mg/kg sludgeTS for the hydrochar and filtrate respectively). No significant correlation betweenresidence time and PAH content was observed, except at 260 °C where the PAH16content increased significantly in the filtrates between 1 h and 5 h residence time(0.05 to 0.85 mg/kg sludge TS). In conclusion, HTC was found to be a promisingprocess for utilizing sewage sludge and the results indicates that the risk of high levelsof PAH content of the hydrochars are relatively low.