Samenvatting

This study investigates the process conditions required for the scalable production of NPK 5-10-30 using sewage sludge ash as a recycled phosphate source for fertilizer production. Although sewage sludge ash offers strong potential for circular phosphate recovery, its industrial implementation remains technically challenging and is yet to be adopted at commercial scale. To address these challenges, laboratory and plant-scale granulation trials were conducted to evaluate the influence of key operating parameters on process stability and product quality. Moisture content was identified as the dominant parameter governing granulation behaviour. Low moisture levels resulted in under-granulation, while excessive moisture caused over-wetting, over-granulation, and unstable operation. Stable and controlled granulation was consistently achieved at 7% moisture, producing granulation efficiencies above 70% and a dominant product fraction of 2-5mm. Drying performance was also identified as a critical limitation, as insufficient drying led to wet granules and subsequent process instability due to clogging. This was successfully mitigated through the addition of 1.5% Agent M, enabling stable drying and reliable downstream handling. At plant scale, similar process sensitivities were observed, with stable operation achieved at moisture levels between 5.6% and 6.9%, confirming the scalability of the laboratory findings. In addition, stable operation depended on maintaining sufficient recycle flow, with recycle rates below \~ 60 t/h, leading to rapid destabilisation and shutdown. Overall, these results demonstrate that SSA can be successfully implemented as a recycled phosphate source in industrial NPK fertilizer production, while confirming pilot-scale experimentation as a reliable approach for process development and scale-up of circular fertilizer technologies.