Generally, increasing adsorbent dose increases the adsorption percentage of adsorbate, which is attributed to the increased adsorbent surface area with raising the adsorbent dose, since more adsorption sites are available, which should facilitate the adsorption of adsorbate. in your case, an increase in adsorbent dose from 3 to 17 mg/L has no effect on the adsorption percentage of COD, except (5 mg / L). in my opinion, it is preferable to repeat the test of this dose (5 mg / L ) and add some doses before this point (e.g. 1 and 2 mg/L).