Suitability of adsorption isotherms for predicting the retention capacity of active slag filters removing phosphorus from wastewater

摘要

Active slag filters are an emerging technology for removing phosphorus (P) from wastewater.nA number of researchers have suggested that adsorption isotherms are a useful tool fornpredicting P retention capacity. However, to date the appropriateness of using isotherms for slagnfilter design remains unverified due to the absence of benchmark data from a full-scale, field filternoperated to exhaustion. This investigation compared the isotherm-predicted P retention capacitynof a melter slag with the P adsorption capacity determined from a full-scale, melter slag filternwhich had reached exhaustion after five years of successfully removing P from waste stabilizationnpond effluent. Results from the standard laboratory batch test showed that P adsorptionncorrelated more strongly with the Freundlich Isotherm (R 2 ?0.97, P , 0.01) than the LangmuirnIsotherm, a similar finding to previous studies. However, at a P concentration of 10 mg/L, typicalnof domestic effluent, the Freundlich equation predicted a retention capacity of 0.014 gP/kg slag;nmarkedly lower than the 1.23 gP/kg slag adsorbed by the field filter. Clearly, the result generatednby the isotherm bears no resemblance to actual field capacity. Scanning electron microscopynanalysis revealed porous, reactive secondary minerals on the slag granule surfaces from the fieldnfilter which were likely created by weathering. This slow weathering effect, which generatesnsubstantial new adsorption sites, is not accounted for by adsorption isotherms rendering themnineffective in slag filter design.