According to Figure , the sludge settling rate was very high in 30 minutes and a small increment was observed after 5 hours with intense clarification of the supernatant liquid. This kind of settleability of the formed sludge from the oxidation of ferrous ions and selfdecomposition of the liquid ferrate into ferric hydroxides confirms the coagulating activity of this oxidant during AMD treatment at low pH values. By using a ruler, the initial length of the liquid in the measuring cylinder (25 mL) was deduced and found to be 9.70 cm (0.097 m) and sequence lengths are also provided in Table . During sludge settling process, four phases are subsequently expected to happen on every batch settling graph and each phase normally shows the variation in the settling behavior at the suspension-liquid interface . However, Figure indicates that in the first four (1, 2, 3 and 4) sequences (see Table .4), the sludge settling velocity was increasing (considered as a log phase) and started decreasing after in the next 5, 6 and 7 sequences (zone settling, transition and compression phases) and became stable after 5 hours . As the oxidation takes place, the sludge settling velocity increases due to the production of ferric ions and decreases during the coagulation process.