Polyelectrolytes:

Polyelectrolytes are either derived from natural sources or are chemically synthesized. They may carry electrical charges and are classified as either cationic (positively charged) or anionic (negatively charged). Most naturally occurring polyelectrolytes are ampholytic.  In water works conditions where alum is being used as primary coagulant, polyelectrolytes provide a powerful ancillary bridging and linking action to promote more rapid settlement. It is normally advantageous to put them into the waterafter the alum. Easier treatment of sludge is claimed from the use of polyelectrolytes, but is still not proven.