A method for treating tailings substrate from mining or oil sands separation process, the tailings substrate comprising an aqueous phase with suspended solid particulate material. In the method, a flocculating agent is added to the tailings substrate, flocs are allowed to form and the formed flocs are separated from the aqueous phase. The flocculating agent comprises a degraded polyacrylamide comprising at least anionic units and having a process water viscosity reduced by 25-99%, preferably 50-99%, compared to a substantially similar polyacrylamide that has not been degraded by a degradation agent, measured at a shear rate 1 s−1 as a 0.4-0.8 weight-% solution of polymer in process water.