A method for purifying contaminated liquids, especially waste water with high TKN/BOD concentrations, by first introducing, into the contaminated liquid, an acidic coagulant such as a metal salt and then introducing a second alkaline coagulant, into the contaminated liquid which dramatically increases the speed and effectiveness of subsequent coagulation and flocculation and thereby removal of TKN/BOD. By comparing the total (molar amount of acidic coagulant cation+alkaline coagulant cation) with an equal molar amount of the acidic cation, superior removal of TKN/BOD are achieved due to a synergy between the acidic and alkaline coagulant versus using just an acidic metal coagulant.