Application of response surface methodology for optimization of phosphate and ammonia removal from domestic wastewater by modified steel slag

Domestic wastewater is one of the significant sources causing phosphorus and nitrogen pollution in natural water. Though steel slags have been applied for the adsorption of phosphate, ammonia, and both in various synthetic and real wastewaters, no report has been found for the application of response surface methodology (RSM) to investigate the main and interaction effects, and optimization of various factors on phosphate and ammonia adsorption by steel slag. The current study aims to evaluate the removal of phosphate and ammonia from domestic wastewater using adsorbents of modified steel slags. Steel slags from two companies were modified, characterized, and preliminarily applied for synthetic wastewater treatment to select the suitable type of steel slag. Optimization experiments were then designed using RSM to find the significant factors, their interactions affecting the treatment processes, and optimum conditions. The results of energy-dispersive X-ray spectroscopy (EDS) and Brunauer–Emmett–Teller (BET) analyses showed that thermal treatment affected the mass percentages of elements and increased the surface areas of the materials. The adsorption kinetics and isotherm studies demonstrated that the phosphate adsorption process followed pseudo-second-order reaction kinetics, and the Langmuir model was appropriate to the present experimental results. The results from RSM suggested that output phosphate concentration was significantly affected by the modified temperature, steel slag dosage, contact time, and the modified temperature-dosage interaction, while the modified temperature and steel slag dosage were significant factors for output ammonia concentration. Validation of the domestic wastewater treatment at the optimum conditions including the modified steel slags at 776°C, contacting time of 15.7 h, and dosage of 0.87 g/L confirmed the high removal efficiency for phosphate (81.7%), while ammonia and COD were eliminated at the efficiencies of 31.6% and 32.7%, respectively. This study hence proposed the application of the modified steel slag as a promising and cheap adsorbent for nutrient removal in domestic wastewater.