Methods and compositions for controlling biofouling

A method of monitoring and controlling biocides in an aqueous medium, comprising:

• premixing a first oxidizing biocide with a second oxidizing biocide to form a composition;
• adding the composition to the aqueous medium at a first injection point upstream of a RO membrane inlet;
• measuring an oxidation-reduction potential (ORP) at a first location downstream of the first injection point but upstream of the RO membrane inlet;
• and
• adjusting an amount of the composition being added to the aqueous medium to maintain the ORP at about 500 mV or less.

A method of monitoring and controlling biocides in an aqueous medium, comprising:

• adding a first oxidizing biocide at a first injection point upstream of a RO membrane inlet;
• adding a second oxidizing biocide at a second injection point upstream of the RO membrane inlet;
• obtaining a sample of the aqueous medium from a first location downstream of the first injection point and upstream of the RO membrane inlet;
• measuring a free residual oxidant concentration of the sample using a reagent comprising DPD;
• and
• adjusting addition of the first oxidizing biocide so that the free residual oxidant concentration in the sample is less than about 0.1 ppm as Cl2.