Sustainable phosphorus recovery from sewage sludge ash through microwave-assisted thermochemical treatment for next-generation fertilizers

Phosphorus is an essential but finite resource whose EU supply depends heavily on imported phosphate rock. Sewage sludge ash (SSA), produced by the mono-incineration of wastewater sewage sludge, represents a promising phosphorus reservoir, yet its direct agronomic use is limited by the presence of poorly soluble Ca–Mg–Fe phosphates. This study develops a microwave-assisted thermochemical process for converting SSA into a new generation of slow-release, plant-available phosphate fertilizers. Two SSA samples from a municipal wastewater treatment plant were treated with sodium- or potassium-based additives in the presence of anthracite. Microwave hybrid heating (800–1000 °C) promoted rapid solid-state reactions, transforming whitlockite and hydroxyapatite into mixed alkali–calcium phosphates. Sodium bicarbonate proved the most effective additive, yielding a buchwaldite-rich product with >99 % phosphorus solubility in neutral ammonium citrate, fulfilling EU regulatory criteria for agronomic effectiveness. The plant-available fraction was preliminarily assessed using NAC-derived extracts in germination and greenhouse assays with Sorghum vulgare and Brassica rapa . At intermediate fertilizer dilutions, biomass increased by up to 30% and root length by approximately 25%, highlighting the agronomic potential of the SSA-derived formulation. Sustainability assessment using the ESCAPE method demonstrated that the microwave process can achieve a lower embodied energy and carbon footprint than the conventional AshDec route, especially when powered by renewable electricity. Overall, the microwave-assisted thermochemical treatment provides an efficient, sustainable, and regulation-compliant strategy for recovering phosphorus from SSA and producing high-quality, next generation fertilizers that support circular nutrient management.