Hi Engr.Chinenye Justin ...
Published by Marina A, Previously Key Account and Content Manager at AquaSPE AG
Hi Engr.Chinenye Justin Nwaogwugwu., coagulation and flocculation do go hand in hand but there is a significant difference between the two.
Coagulation is a water treatment process in which chemicals (coagulants) are added to the water.
It neutralises charges and forms a gelatinous mass to trap particles, forming a large mass that settles or is trapped in the filter.
Dissolved and suspended particles can be found in most of natural waters. These suspended materials mostly come from land erosion, dissolution of minerals and decay of vegetation and domestic and industrial waste discharges.
The water quality is deteriorated with their presence so they have to be removed.
Coagulation and flocculation are relatively simple and cost-effective, provided that chemicals are available and dosage is adapted to the water composition.
In colloidal suspensions the particles would settle very slowly or not at all, since they carry surface charge. Same charges repel each other. Coagulants are added in order to agglomerate the particles.
Coagulants can be classified into:
· inorganic coagulants (e.g., aluminum and ferric salts)
· synthetic organic polymers (e.g., polyacryl amide derivatives and polyethylene imine)
· natural coagulants .
All of them are very efficient at turbidity removal from water.
Coagulation with Aluminium- and Ferric-based coagulants, followed by disinfection by chlorinating is one of the most used methods of water treatment.
Water pretreatment by oxidation might be necessary when using them.
When the coagulants are added to water and stirred for a few minutes they dissolve.
Coagulation destabilises the charge of the particles.
Coagulants with charges opposite to those of the suspended solids are added to the water to neutralise the charges on dispersed non-settable solids such as clay and organic substances.
Once the charge is neutralised, the small-suspended particles stick together.
The slightly larger particles formed through this process are called microflocs and are invisible to the naked eye.
To achieve good coagulation and formation of the microflocs, a high-energy, rapid-mix to properly disperse the coagulant and promote particle collisions is needed.
Over-mixing does not affect coagulation, but insufficient mixing will leave this step incomplete. Proper contact time in the rapid-mix chamber is typically 1 to 3 minutes.
Flocculation is a process in which the colloids from a suspension form flocs of flakes.
During flocculation, the micro-particles and charged ions are attracted and attached to flocs. The impurities are adsorbed onto the coagulated flocs.
Flocs can further be removed by sedimentation , but only partially and for complete removal filtration may be necessary.
Gentle mixing stage, increases the size of submicroscopic microflocs to larger suspended particles.
With slow mixing, the microflocs are come into contact with each other. Collisions of the microfloc particles cause them to bond to produce larger, visible flocs. The floc size continues to grow through collisions and macroflocs are formed.
Separation is done (sedimentation, floatation or filtration) when the floc has reached its optimum size and strength.
Contact time for flocculation ranges from 15 or 20 minutes to an hour or more.
Hope this helps!