As Nikolay stated, floculants ...

As Nikolay stated, floculants cannot be used at any time. With respect to ferric coagulants, the best dosage is zero, they must be avoided as well. The only reason for adding them is when you have a lot of colloids and the double stage granular filtration cannot reduce the SDI down to a safe value. It's better to operate a Plant without chemicals with a higher SDI than adding them to get a lower one. All the Plants I have operated work with the concept of zero chemicals.(No coagulants, no flocculants, no acid, no chlorine, no anti scalant) and they suffer little fouling (One CIP every 12 or 15 months) Only one Plant gets iron because of the high amount of colloids in the feed water. In case you decide to add iron, do not use more than 2 ppm of commercial ferric chloride, 0.5 or 1 ppm are preferred. Remenber, All what you studied in the University is applicable only to potable or industrial waters. RO has a big difference: At the end of the system there is an absolute filter: the membrane, that's going to stop everithing you have added and not previously removed before entering in the pressure vessel. This means fouling. That's the reason because jar test is not applicable to RO The less chemicals you use, the better operation of the Plant you get.