Exploring the recovery of potassium-rich struvite after a nitrification-denitrification process in pig slurry treatment

The integration of biological nitrogen (N) removal with struvite-type material recovery, which contained phosphorus (P) and potassium (K), was proved to be technically feasible in pig slurry treatment. Phosphate (PO4) salts were precipitated by raising the pH-value, using denitrified effluent and waste sludge purged from the bioreactor. When P was limiting, the unbalanced composition of the denitrified effluent resulted in low K-removal efficiency from the liquid phase; 10 % maximum when the initial pH-value was adjusted to 11.5 (93 % PO4-P recovery). By processing the waste sludge in two steps, using first ethylenediaminetetraacetic acid (EDTA) as an acidifier to release PO4 while preventing calcium interference, the K-removal efficiency reached 25 % (75 % PO4-P recovery). When K was limiting, the addition of newberyite particles resulted in the highest K-removal efficiency, up to 90 % (under online pH control to 10.5). Overall, new opportunities are envisaged for producing second-generation fertilizers potentially containing 0–1 % N, 11–17 % P and 6–8 % K.