As Svein mentioned earlier, ...

As Svein mentioned earlier, this is a question that can't be answered simply.  There are many organic coagulants as well as inorganic/organic blends from which to choose.  The choice and dosage depends on many factors (type and level of contaminant, mixing energy, contact time, etc.).  So, choosing the correct polymer and dosage should be done initially by jar testing, followed by a pilot/trial.  There are also many good chemical companies that provide polymers, but I would go with one that is well established and has a well trained sales force since determining the correct treatment protocol is partly science, partly art.  Lastly, be careful with some of the organic polymers if used prior to a media filter.  Some polymers (DMA/EPI's, as an example) can potentially foul the media if grossly overfed.  This is especially true for organic flocculants, if you are using them in conjunction with the coagulant.

Good Luck.