Torrefaction of sewage sludge: An approach to nutrient recycling and contaminant reduction in agriculture

The disposal of sewage sludge, a byproduct of wastewater treatment, presents both opportunities and challenges in agricultural applications due to its rich nutrient content and potential pollutant load. This study investigates the effects of torrefaction, a low-temperature thermochemical treatment, on the elemental composition and organic pollutant content of sewage sludge and its impact on maize (Zea mays L.). Sewage sludge from a municipal wastewater treatment plant was torrefied at 320 C for 3 h, then applied to soil in a field experiment. The torrefaction process concentrated mineral nutrients such as phosphorus and potassium but reduced nitrogen and sulfur content. The content of available phosphorus and potassium decreased by only 28 % and 24 %, respectively. Importantly, torrefaction significantly decreased the concentrations of pharmaceuticals, synthetic musk compounds, and endocrine-disrupting compounds in the sludge, with reductions of over 90 % for most compounds. Persistent organic pollutants, including polychlorinated biphenyls and organochlorine pesticides, also showed significant reductions (99 and 95 %, respectively), although polycyclic aromatic hydrocarbons (PAHs) increased by 20 % due to torrefaction. Despite this, the PAH content in torrefied sludge (Σ16 PAH <3 mg/kg) remained below regulatory limits set for soil application. At higher application doses, the effect of torrefied sludge on agronomic performance of maize did not differ from that of dried sewage sludge. Torrefaction has demonstrated promising results in improving the safety and efficacy of sewage sludge as a fertilizer by reducing organic contaminants and increasing phosphorus and potassium contents without significantly diminishing their availability. However, the limited effectiveness of torrefaction in reducing PAH content must be carefully considered when evaluating its suitability for soil applications. Furthermore, the condensate produced during the torrefaction process was not analyzed in this study and may represent a potentially hazardous by-product due to its pollutant content.