Environmental pollution caused by the palm oil industry is a severe problem. In palm oil production, liquid waste is obtained as palm oil mill effluent containing COD and phosphate, which can pollute aquatic ecosystems and soil. Phytoremediation technology with modification using ammonium-based polymer adsorbents effectively reduces palm oil mill effluent contaminants. This study aimed to test the effectiveness of phytoremediation and adsorption modification technology using ammonium-based polymer adsorbents to consider environmentally friendly and economical reducing pollutants in palm oil mill effluent. Vetiveria zizanioides plants were cultivated in floating treatment wetlands, and the planting media was varied using ammonium-based polymer to treat palm oil mill effluent with a volume of 3 L. The performance of the modified reactor in degrading COD and phosphate was examined by monitoring the floating treatment wetlands for 9 days and measuring the physiochemical parameters. Variations in ammonium-based polymer showed optimal performance using a lower ratio of 0.3. This combination of technologies removed COD by around 77.3% with an adsorption capacity of 558.4 mg/g and phosphate by 59.5% with an adsorption capacity of 2.77 mg/g within nine days. These results could be elaborated on with tertiary treatment for future treatment to enhance removal according to the quality standards of the Ministry of Environment and Forestry in Indonesia.