Selective release of phosphorus and nitrogen from waste activated sludge with combined thermal and alkali treatment

•Thermal-alkali treatment released 94%, 76%, and 49% of T-P, T-N, and COD from sludge.•Release rate increased with NaOH concentration.•Temperature increased release rate only at low NaOH concentration.•The ratio of T-N and COD to T-P that released were 0.6–0.9 and 0.2–0.6.•NaOH created cavities on sludge, and these cavities reduced P selectivity. Selective release characteristics of phosphorus and nitrogen from waste activated sludge (WAS) were investigated during combined thermal and alkali treatment. Alkali (0.001–1.0N NaOH) treatment and combined thermal-alkali treatment were applied to WAS for releasing total P(T-P) and total nitrogen(T-N). Combined thermal-alkali treatment released 94%, 76%, and 49% of T-P, T-N, and COD, respectively. Release rate was positively associated with NaOH concentration, while temperature gave insignificant effect. The ratio of T-N and COD to T-P that released with alkali treatment ranged 0.74–0.80 and 0.39–0.50, respectively, while combined thermal-alkali treatment gave 0.60–0.90 and 0.20–0.60, respectively. Selective release of T-P and T-N was negatively associated with NaOH. High NaOH concentration created cavities on the surface of WAS, and these cavities accelerated the release rate, but reduced selectivity. Selective release of P and N from sludge has a beneficial effect on nutrient recovery with crystallization processes and it can also enhance methane production.