Phosphorus recovery from sewage sludge ash: leaching and precipitation conditions for high-value fertilizer production

The increasing demand for phosphorus in fertilizers industries put the natural rock reserves under strong pressure. Thanks to similar phosphorus content of medium grade ore, sewage sludge ash looks a promising secondary raw material, where most of nutrient accumulates. The present works investigates (i) the influence of combustion process conditions both on the physical characteristic of sewage sludge ash (e.g., loss on ignition) produced in a pilot-scale furnace and on the phosphorus extraction efficiency, (ii) the precipitation conditions (pH, type of reagent, aging period) for the recovery of a valuable product to be used as fertilizer. Experimental results underlined the influence of combustion process conditions on phosphorus extraction suggesting the need of an optimized combustion process. Overall, tests conducted both on bottom and cyclone ash indicated that acid leaching is an effective alternative for phosphorus recovery (70-85%), while operating conditions set for the subsequent precipitation process confirmed pH as the key-factor for phosphorus precipitation. 90% of phosphorus precipitates indeed at lower stage of pH (3.5) for all tests, resulting in concentrated recovered material (17 wt.%) with phosphorus content comparable to commercial fertilizers.