Phosphate recovery from wastewater of fertiliser industries by using gypsum waste

One of the fertiliser industries in Indonesia produces wastewater from the production process of phosphoric acid, which has a pH of 1.6 to 3, and phosphate concentration of 1,000 - 6,000 mg/L. This waste has been processed by the neutralisation system for phosphate precipitation with the addition of lime (CaO). The effluent from this treatment process still contain high concentrations of phosphate, further process is required to reduce it. The production processes in this fertiliser industry generates solid waste gypsum from the production process. Currently, this solid waste is dried out and has never been harnessed. The calcium (Ca) contained in the gypsum can be utilised to eliminate the phosphate by chemical precipitation. This study was conducted to identify the use of gypsum waste from the waste of fertiliser industry as a phosphate precipitation agent. Effect of molar ratio and pH to reduce phosphate are the main factors that were analysed in this study. The samples were taken from the effluent basin of the sewage treatment process for phosphoric acid production. At this stage the waste still contains 3,826 mg/L of PO₄ ^3-. The gypsum waste was taken from the waste drying facility. It was analysed by using SEM-EDS method to determine the percentage of Ca and other elements in the waste. The variables in this study were the molar ratio of [Ca²⁺]: [PO₄ ^3-], pH and settling time. The study was conducted on a laboratory scale using mechanical stirring (jar test) with the addition of gypsum waste at a speed of 200 rpm for ± 60 min. The precipitation was then carried out, and subsequently the supernatant was taken and the residual phosphate was analysed. The software PHREEQC v 2.8 was applied to predict the precipitation process and the mineral generated from the process. This software works with thermodynamic principles, and is an open-source software that can be downloaded and modified for additional database. The result of this study shows that the gypsum waste contains 29.1 % of Ca. These results were used to determine the amount of gypsum to be added for a variety of molar ratios. Based on the modelling by using PHREEQC software, the dominant precipitate was hydroxyapatite. The optimum reduction of phosphate occurred in the molar ratio [Ca²⁺]: [PO₄ ^3-] of 2 : 1. The percentage of phosphate removal was 99 %, obtained at pH 8.5. The precipitation was going well with the settling time of 60 min. The main mineral generated from this process was hydroxyapatite. This parameter can be used to design further treatment for phosphate reduction, which requires the addition of precipitation and sedimentation unit, in addition to the neutralisation process that have been available in this factory.