[External Phosphorus Adsorption and Immobility with the Addition of Ignited Water Purification Sludge]

The cumulative adsorption of external phosphorus (P) by water purification sludge after ignition under different addition methods (mix and cover) was investigated along with the adsorption isotherm and the release of internal P after external P adsorption. The cumulative adsorption of external P was 11.496 mg (mix) and 11.042 mg (cover) and these values were higher than those in the control (7.219 mg). The maximum sorption capacity (S_max) increased under the mix and cover and the former (7.795 mg·g^-1) was higher than the latter (6.807 mg·g^-1). However, the zero equilibrium P concentration (EPC₀) in the mix was 0.83 mg·L^-1, higher that in the cover (0.64 mg·L^-1). The result suggests that the internal P was easily released in the mix method, compared to the cover method. Under anaerobic conditions, the release of the internal P was 0.93 mg in the mix, lower than that in the cover (1.49 mg) and in the control (7.76 mg). In addition, the specific release rate in the cover method was 0.00614 (mean), higher than that (0.00396) in the mix method. Noticeably, these data were lower than those in the control, indicating that the retention of P under the mix method was higher, compared to the cover and the control and it is challenging to evaluate the P retention with EPC₀.