Coagulants and Flocculants

Introduction

 

Coagulants

Naturally occuring waters will almost always contain suspended material .  It is this suspended material that is generally what the water treatment operator is trying to remove to cause clarification.   In order to remove this suspended material the charge on the particles must be neutralized.  In almost every case naturally occuring waters will  only contain particles which are negatively charged or anionic.  The negatively charged particles repel one another due to their charge.  The neutralization of the particles will cause them to attract one another and form larger particles.  The product added to cause neutralization must be positively charged, or cationic.  The cationic particles get between two anionic particles and cause them to come together. 

 

There are many products used as cationic coagulants.  Inorganic products such as lime, aluminum salts, and iron salts are the most common.  Organic coagulants such as polydadmacs and polyamines cause neutralization as well but they have a large molecular structure which can cause larger particles to form.

 

Flocculants

 

Flocculants are long chain organic molecules and can also be referred to as polymers.  They are added after the coagulation step.  They cause coagulated particles to come together and become part of the large molecular structure of the floccuant.  They are also bound by electrostatic forces, i.e. positive and negative charges.  Flocculants can be cationic and anionic as well.  The most common use of cationic polymers in water treatment is for the coagulation and flocculation of sewage treatment sludge in the dewatering step.  No separate coagulant is needed in many cases which actually means the polymer is both a coagulant and a flocculant.  Anionic polymers are used commonly to treat water which has gone through a coagulation step with alum.  Very small amounts, in the neighborhood of .25 ppm to 50 ppm are used.  Overfeeding an anionic polymer can coat and blind a filter in a hurry.  Care must be taken to avoid blinding of filters. 

 

There are some products which display properties of both coagulants and flocculants.  As mentioned above, cationic polymers in sludge dewatering are a good example of this.  The newer products on the market, the polyaluminum chlorides, can be considered both a coagulant and a flocculant because they have aluminum in their structure which acts as a coagulant as well as a long chain molecule also made up of aluminum capable of forming larger molecules.  They are often used by themselves with no separate polymer added.   

Conclusion

 

While it seems that coagulants and flocculants are the same in that they both aid in particle enlargement for good settling they are very different in the way they achieve that enlargement.

Overfeeding is always something to be kept in mind also,  Overfeeding a coagulant can cause the reverse of what is desirable in that the suspension becomes predominantly positively charged which again causes partices to repel one another.  Overfeeding a flocculant can cause similar issues in that the particles formed during coagulation will not bind together with the flocculant and poor sedimentation results.  Over mixing the flocculated mixture can cause shearing of the particles and this will result in poor sedimentation as well. 

 

A good understating of how each product works is important in achieving the desired result which is good sedimentation.