Orthophosphate removal from domestic wastewater using limestone and granular activated carbon

The discharge of excessive concentration of orthophosphate (PO₄-P) ions into the receiving water causes environmental problems such as "eutrophication." The aim of the present study was to investigate the adsorption behavior of limestone (LS), granular activated carbon (GAC) and the mixture of both adsorbents for orthophosphate removal from domestic wastewater. The range of initial concentration of PO₄-P throughout the study was between 9 and 25mg/L. Effects of contact/settling times, pH, adsorbent dosage, initial concentration, adsorption isotherm models and kinetics were studied in batch-scale experiments while for the column experiments, the effects of flow rate, pH and initial concentration were studied. Limestone alone was shown to be an effective adsorbent which has potential to remove over 90% orthophosphate at optimum conditions. The lower initial concentration (2.5mg PO₄-P/L) yielded the maximum removal (94%) compared to the higher concentration (80% removal at 100mg PO₄-P/L). Freundlich and Langmuir isotherms provided good correlation coefficient for PO₄-P and the data agreed with the pseudo-second-order kinetics model (R²>0.95). In the up-flow column study, higher flow rate, alkaline pH and higher initial concentration yielded shorter column saturation time.