Coagulation and Flocculation
The Basics
Coagulation and Flocculation
Coagulation and flocculation in water treatment, are used to remove particles which cannot be removed by sedimentation or filtration alone. These particles are usually less than 0.45 μm in size and are therefore often defined as colloids. They have poor settling characteristics and are responsible for the colour and turbidity of water. They include clays, metal oxides, proteins, micro-organisms and organic substances such as those that give the brown coloration to water from ‘peaty’ catchment areas. The important property which they all have is that they carry a negative charge and this, along with the interaction between the colloidal particles and the water, prevents them from aggregating and settling in still water. The particles can be aggregated by adding multivalent ions having an opposite (positive) charge. These are added as chemical coagulants.
Chemicals most commonly used as coagulants in water treatment are aluminium and ferric salts which are present as the ions Al3+ and Fe3+. These positively charged multivalent ions neutralise the naturally occurring negatively charged particles, thus allowing the particles to aggregate. At high concentrations of aluminium or ferric salts, and in the presence of sufficient alkalinity, insoluble hydroxides of aluminium or iron are formed. In the precipitation reaction the colloidal particles are enmeshed within the precipitate.
There are many other coagulants, some highly sophisticated like Poly Aluminium Sulphate (PAS), a pre-polimerised aluminium solution, some organic, like creatine recovered from crabs and others like concentrated brine simply because they are available. However, Sulphates and chlorides of iron and aluminium are by far the most common and we will descuss them in detail on the next page.