Figure 1: Examples of creaming in settling tanks
When working with small particle removal, experience has shown that the addition of chemical coagulants or flocculants is usually a necessity. The question is what is the best method of applying this system?
First, all the applicable terms need to be defined:
The first stage in the clarification of water is coagulation. When small particles are involved, coagulation usually has to be achieved through the addition of chemical aids. The next stage is flocculation and whether or not another chemical is required here is dependent on the particles and the desired speed of floc formation. The final stage is separating the flocs from the water.
First, all the applicable terms need to be defined:
- Clarification: this term can be used to describe the overall process of removing fine particles from water
- Coagulation: is the act of neutralizing the charges of particles that allow them to repel each other.
- Flocculation: occurs when neutralized (coagulated) particles are brought together to form larger compounds called ‘flocs’
- Sedimentation: separating solids from water through settling
- Creaming/Flotation: separating solids from water by floating them to the surface
- Dewatering: separating solids from water through filtration
The first stage in the clarification of water is coagulation. When small particles are involved, coagulation usually has to be achieved through the addition of chemical aids. The next stage is flocculation and whether or not another chemical is required here is dependent on the particles and the desired speed of floc formation. The final stage is separating the flocs from the water.
Figure 2: Coagulation, flocculation, creaming, sedimentation, and dewatering
Floc separation can occur three different ways: sedimentation, creaming, or dewatering. The method chosen will depend on the type of flocs as some will naturally sink and others will prefer to float. It is important to match the coagulant and flocculant chemicals, floc behaviour, and method of separation to make a clarification system operate properly and efficiently.
Each separation method has its own challenges including infrastructure needs. Sedimentation is most commonly completed using a settling tank. The separated solids will need to be scooped out regularly so the tank has sufficient room to collect solids. Creaming is associated with air flotation where solids either naturally float or are aided with dissolved air carrying solids upwards. The solids are then skimmed off the water surface. Dewatering uses filters to collect the solids behind a membrane. The membrane will require regular cleaning or replacement when the solids clog the pores. Water used to clean the membranes will also need to be considered for further treatment.
References
- Tramfloc, Inc. (2014). Selecting Polymers, Jar Testing Procedures. In Flocculants. Retrieved March 14, 2016, from http://tramfloc.com/polymers-selection-jar-testing-procedures/
- GE Power & Water. (2012). Chapter 05 - Clarification. In Handbook of Industrial Water Treatment. Retrieved March 14, 2016, from http://www.gewater.com/handbook/ext_treatment/ch_5_clarification.jsp
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