**4. Conclusions**

The current work evaluated the use of POME as an agro-industrial waste for landfill leachate treatment. Biological treatment using an aeration process for different mixing ratios between leachate and POME was performed. The optimization of the removal efficiencies for COD, TSS, color, and NH3-N for the aeration treatment process of old leachate was investigated in this study. The optimum operational conditions were obtained at 188.32 mL of POME added to 1 L of leachate for 21 days of aeration time. The respective percentage removal efficiencies were 89.83%, 66.7%, 91.7%, and 94% for COD, TSS, color, and NH3-N. Moreover, the treatment process reported an efficient removal of some heavy metals from landfill leachate. The results revealed that POME-based agricultural waste can be effectively used for organic removal from stabilized leachate.

**Author Contributions:** Formal analysis, T.J.H.B.; writing—original draft preparation, T.J.H.B.; writing—review and editing, M.M.H., S.S.A.A., A.F.M.A., and M.H.; supervision, M.M.H. and S.S.A.A.; funding acquisition, M.M.H. All authors read and agreed to the published version of the manuscript.

**Funding:** This research was partly funded by UKM research grants (DIP-2019-001 and MI-2020-005). **Acknowledgments:** Marlia M. Hanafiah was supported by the Ministry of Education Malaysia (FRGS/1/2018/WAB05/UKM/02/2) and a UKM research gran<sup>t</sup> (DIP-2019-001).

**Conflicts of Interest:** The authors declare no conflict of interest.
