River water treatment - Ecoli removal

We gathered water samples from a certain river and according to the results‚ more than 1 000 000 units of the indicator microbe Escherichia coli were present per 100 ml of water. The health risk is high for people coming into full or partial contact with water having more than 2000 colony forming units per 100 ml. Untreated or partially treated sewage also contains high concentrations of phosphate and nitrate, which contribute to the eutrophication problems. Which method can be recommended when coming to the treatment of this water?