Final effluent from the palm oil industry still contains residual oil with potential for use as a value-added alternative material; however, its utilization remains limited. This study aims to investigate the use of final effluent oil as an additive in solid soap production and to evaluate the physical characteristics and chemical quality of the resulting product according to SNI 3532:2021. Residual oil extracted from the final effluent was purified and formulated into solid soap using the hot process method, followed by analyses of pH, moisture content, foam height and stability, and free alkali content. The results indicate that the addition of purified final effluent oil produced solid soap with favorable physical characteristics and most chemical quality parameters meeting standard requirements, particularly pH, moisture content, and foam properties. However, the free alkali content of soap formulated with purified final effluent oil does not fully comply with the requirements of SNI 3532:2021, indicating that optimization of formulation and process conditions is necessary to enhance the saponification reaction. These findings demonstrate that final effluent has significant potential as a value-added alternative resource for sustainable and environmentally responsible liquid waste management and support the broader implementation of circular economy principles in the palm oil industry.
