Study of the pH Effects on the Phosphorus Removal Mechanism in Lab-Scale Electric Arc Furnace Slag and Limestone Filters in Synthetic Wastewater

摘要

Eutrophication as a result of uncontrolled phosphorus (P) concentration that is released in wastewater has emerged as a major problem nowadays. Treatment of P demands high costs specifically to its chemical and maintenance needs. A lot of efforts were undertaken to find the most economical material that can treat P such as Electric Arc Furnace Slag (EAFS), a by-product from steel industry and Limestone (LS), and a natural resource that can be easily obtained from sedimentary rock. Despite numerous study conducted previously, the mechanism of P removal between these two materials have not been explored yet in detail. Therefore, an experimental work had been designed to evaluate the performance of P removal mechanism between the EAFS and LS lab-scale filter systems which can offer the best removal in overall. In this study a column lab scale of vertical rock filters in 100 mm diameter × 400 mm height were constructed for both EAFS and LS filters. The observation period was conducted for 2.5 months utilizing synthetic phosphorus concentration of 25 mg/L as its initial concentration. Working condition of the experiment was adjusted at pH 4.5 to 7.5 under 26.8±0.64°C. X-ray fluorescence (XRF) analysis was also performed in order to determine chemical composition of EAFS and LS media. Results revealed that EAFS showed a significant effect on the removal of P mechanism compared to the LS filter system. The highest removal of P for both filter systems were observed to achieve 90% (pH 5) in the EAFS compared to only 68% (pH 4.5) in the LS system. This may be associated with the dissolution of Ca, Fe and Al elements in the EAFS that promotes precipitation process and hence gave higher removal compared to the LS element (only Ca has the highest percentage = 91%). At lower pH (acidic) condition the soluble metal salts react with phosphate ion to form phosphate hydrolysis product thus promoting precipitation in the system. In conclusion, it is predicted that precipitation may occur within the filter systems predominantly in the EAFS filter as pH was change from acidic to alkaline (4.5 – 7.5) due to dissolved (Al~(3+), Ca~(2+), Fe~(3+)) within the filter system. Nevertheless, it is recommended that future study should be carried out on the precipitates salts through X-ray diffraction (XRD) analysis so as the existence of phosphate and metal salts as a result of precipitation can be finally confirmed.