Sludge Volume Reduction in Phosphate Removal Process from Wastewater with Lime

Abstract

Chemical precipitation with lime is an effective phosphate removal method when the concentration is relatively high such as industrial wastewater. However, lime treatment in its conventional form is not favored as a process for removing phosphate from wastewater for the following reasons. Since a high dosage of lime is required to raise the pH up to 10-42and large quantities of sludge are produced. The purpose of this study is to prepare a small volume of sludge and to precipitate calcium phosphate in the pH 8-9 region. Experimental results showed that the stepwise addition of lime was usefull to reduce sludge volume. A new process consists of two stage lime addition was proposed. For the first stage, the addition of slaked lime was done at a pH of below 6.5. The second stage was increased the pH to 8---9. The process reduced sludge volume to one-tenth compared with the one-time rapid addition of lime in the conventional process. A 10.6 mmol⋅dm^-3 Ca(OH)₂ dosage to 7 mmol⋅dm^-3 H₃PO₄ removed 98% of the phosphate at pH 8.7 and 99.5% of the phosphate in the presence of 1 mmoldm^-3 CaC1₂ at pH 8.5. The reduction in lime dose and sludge volume indicates a potentially improved conventional process.