Recovery of phosphate from sludge in fecal sludge treatment plants using gypsum waste

Environmental Engineering, Sepuluh Nopember Institute of Technology, Surabaya 60111, Indonesia

^* Corresponding author: soedjono@enviro.its.ac.id

Abstract

This study evaluates the use of industrial gypsum waste as a seed material for phosphate recovery from fecal sludge treated in Fecal Sludge Treatment Plants (FSTPs). The objective is to assess the effects of pH and agitation speed on the quantity of recovered phosphate crystals. Industrial gypsum is compared with commercial-grade gypsum, and calcium dosages are based on stoichiometric requirements. Experimental conditions include pH levels of 7, 8, and 9, and agitation speeds of 100, 120, and 150 rpm, with pH adjusted using NaOH or H₂SO₄. Phosphate was measured using spectrophotometry following the extraction of sludge into a phosphate-rich solution. Extraction at pH 2 for 5 hours yielded recovery efficiencies of at least 65%. The results show that industrial gypsum waste supports crystal formation and achieves phosphate removal efficiencies exceeding 98% at pH 8–9 and stirring speeds of 100–150 rpm. Compare to amount of crystal formed, this research results suggest that best choice is reactor LG8-120, with phosphate removal 98.79% and crystal formed were 0.9903 grams. These findings suggest that industrial gypsum waste is a cost-effective and sustainable alternative for phosphate recovery, offering potential for nutrient recycling in resource-limited settings.