Coagulation Theory has to do ...

Coagulation Theory has to do with electronic charges and attraction and repulsion of particles in suspension. It works because particles, during Brownian motion, become closer than a certain radius that determines that they will be attracted to each other. The number of collisions is related to the amount of turbidity. The concentration of particles in the effluent is what determines how safe the water is. If it is too high, the particles may either carry, or indicate the presence of, or inhibit the disinfection of protozoans and other biological agents. You can ask the question about whether it is effective in some water or another but, in the end, jar testing and sampling of the effluent is what determines effectiveness. There are polymers, cationic agents and anionic agents and even bentonite clay that I have seen work to improve the surface charge or particle density of the solution. Usually you want to try out Alum or ferric chloride for cheapness and some specialty chemical polymer for efficiency.