The assessment of phosphorus recovery potential in sewage sludge incineration ashes — a case study

A sewage sludge incineration ash contains large amounts of phosphorus, which are considered as a novel anthropogenic waste-based substitute for phosphorus natural resources. Phosphorus is accumulated at most in phosphate minerals of whitlockite structure, that contain Fe, Ca, and Mg and in the matrix composed of Si, Al, Fe, Ca, P, Mg, K, Na in various proportions. The goal of this study was to estimate phosphorus recovery potential. A four-stage sequential extraction, following the modified Golterman procedure, was applied. Separation of four independent fractions enabled to understand better the manner of phosphorus occurrence in the studied ash. The results of the extraction indicated the greatest release of phosphorus combined with organic matter using sulfuric acid. The release was on average at the level of 64%. The chelating Na-EDTA compound indicated lower ability to extract phosphorus (at the level of 35%), and the highest ability to extract heavy metals and potentially toxic elements (As, Zn, Mo). The sequential extraction led to the total recovery of phosphorus of around 40-60.