The correct dosage must be ...

 The correct dosage must be evaluated in a test of treatability. In your case, if you want lower Ecoli values, you should perform the test in the laboratory. How to make? In a sample composed of effluent, separate into, for example, 5 equal vials. Apply different dosages to each sample. Shake vigorously. Wait at least 30 min. Measure results. The best result to apply in the treatment. Not satisfied to repeat with new dosages.