Daud, Zawawi, et al. “Application of Response Surface Methodology (RSM) to Optimize COD and Ammoniacal Nitrogen Removal from Leachate Using Moringa and Zeolite Mixtures”. International Journal of Integrated Engineering, vol. 10, no. 1, May 2018, https://penerbit.uthm.edu.my/ojs/index.php/ijie/article/view/2294.