Coagulation and flocculation ...

Coagulation and flocculation processes are used to facilitate the elimination of suspended solids and colloids by gathering these together to form the floc. Sedimentation process, flotation and/or filtration systems are used after coagulation and flocculation to separate the floc.

Factors that influence destabilisation of colloidal particles are type of coagulant added to water and velocity gradient, G. In order to be effective, coagulant must be dispersed immediately and therefore optimum G factors range from 400 – 500 s-1 and up to 1000 s-1.

Velocity gradient is determined by equation

G =

Where

G= mean velocity gradient (s-1)

P- power actually dissipated, W

V -  fluid volume, m3

µ - dynamic viscosity (Pa s)

Optimum retention time is from 30 – 60 s.

After particles neutralization and agglomeration of particles, floc is formed.

The optimum G values in flocculation should be up to 100 s-1. It is advisable to have at least 3 stages of flocculation, where the G value will be lowered from I stage to III stage to (20 – 30 s-1.). To further improve the flocculation process, it is good to have variable speed mixers with possibility to change G value. Special types of mixers are needed for good flocculation process.

The overall retention time in all stages of flocculation should be not less then 20 min but could be longer (up to 30 minutes).

Regarding the coagulants, one of the most applied coagulant is alum sulpahte, which is manufactured in Saudi Arabia so there will not be much problem to use it.

As for the flocculation, various types of polyelectolytes are available.

There are some products which present combination, so called aluminium polymers PAC, PACS, etc.

In addition, pH of water plays significant role in coagulation and flocculation process. For optimum consumption of alum salts it should be in the range of 6.0 – 7.2 in coagulation process.