Hi George,
Here are a number of published definitions that may help you:

Coagulation: a process that alters the surface charge on dispersed colloidal particles in a liquid so that they are able to agglomerate; the first stage in floc formation

Coagulation: The destablization and initial aggregation of finely divided suspended solids by the addition of a polyelectrolyte or a biological process.

Coagulation: Clumping of particles in wastewater to settle out impurities, often induced by chemicals such as lime, alum, and iron salts.

Flocculation: the agglomeration of coagulated particles to form a floc which can settle or float; may be assisted by biological, chemical or mechanical means

Flocculation: Gentle stirring or agitation to accelerate the agglomeration of particles to enhance sedimentation or flotation.

Flocculation: Process by which clumps of solids in water or sewage aggregate through biological or chemical action so they can be separated from water or sewage.

normally which one is required more? floculants or coagulants? If I see in my settling tank prior to discharge, there are lots of coagulated particles won't settle down, does this mean I dont add enough floculants?

Dear George, Dear Wan

flocculation is mostly carried out with polymeric flocculants, in most cases with cationic polyelectrolytes, given that particle surfaces have a net negative surface charge.

Coagulation is a preferred method for water treatment, in wastewater treatment it is mostly used when finely dispersed colloids with lower mass concentration have to be removed. On the other end for sludge dewatering flocculation is preferred.

Poor settling, besides other operational parameters, might be caused by using the wrong flocculant, by under and overdosing as well.

Kind regards
T. Sobisch

Organic sulfides can be strong chelators of metals.  So, chemicals that are dithiocarbamate types chelate and precipitate soluble metal ions such as Cu, Ni, Pb and Zn.  The sulfides typically do not work with soluble trivalent metals like Al.

I like to think of coagulation as getting material that would not be removed by sedimentation/filtration to lump together so that sedimentation/filtration is possible. Flocculation I then regard as getting that coagulated material to form larger clumps that can be economically removed by settlement (or filtration).

So coagulation *needs* chemical dosing to allow the material to coalesce. Flocculation does not *require* chemical dosing, but does need to provide conditions to encourage the small flocs to join together to form large flocs, and to ensure that the large flocs are not then broken up.