Optimisation of nutrients and metals release from municipal sewage sludge by chemical extraction using Box-Behnken design

In this study, H2SO4 and citric acid using Box-Behnken design were utilised in order to release the nutrients and metals fixed in the anaerobic digested and dewatered sludge. For the dewatered sludge, release of 16.27 mg/L P and 120.74 mg/L NH4+ were obtained at optimised conditions with citric acid 0.3 M, H2SO4 6% and reaction time 105 min, respectively. For the digested sludge, release of 764.1 mg/L P and 388.3 mg/L NH4+ were achieved at optimised conditions with citric acid 0.1 M, H2SO4 6% and reaction time 180 min, respectively. In all experimental conditions, Fe and Zn were released together with nutrients. For digested sludge, a minimum Fe and Zn release was achieved at optimised conditions with citric acid 0.5 M, H2SO4 2% and reaction time 105 min, respectively. Thus, this test conditions are thought to be more appropriate for the nutrients recovery from digested sludge.