5.3 Coagulation and flocculation
Coagulation is always considered along with flocculation and is used to remove particles which cannot be removed by sedimentation or filtration alone. These particles are usually less than 1 μm in size and are termed 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 either multivalent ions or colloids having an opposite (positive) charge. These are added as chemical coagulants.
Chemicals 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 (see below). In the precipitation reaction the colloidal particles are enmeshed within the precipitate and thus removed. The use of aluminium salts is not popular because of the (unproven) scare about Alzheimer's disease. The move away from aluminium salts was accelerated when aluminium sulphate was accidentally put into the treated water tank for the town of Camelford in 1988. Most plants now use ferric salts.
If there is inadequate alkalinity in the water, it can be added in the form of lime (calcium hydroxide Ca(OH)2) or soda ash (sodium carbonate Na2CO3).
In some waters, even with the optimum dose of coagulant, coagulation is poor and so it is necessary to add extra substances known as coagulant aids. These aids can be clay, silica or polyelectrolytes. Polyelectrolytes (Figure 22) are long-chain organic molecules with chemical groups attached along the length of the chain, which becomes charged when the molecule is dissolved in water. The negative colloidal particles are attracted to positively charged chemical groups on the polyelectrolyte.
As the coagulants are added, the water is mixed rapidly in a mixing chamber using a high-speed turbine. In small plants, coagulants are often added upstream of a weir in order to use the consequent turbulent motion to aid in mixing. Once coagulation has taken place, a very fine precipitate or floc will form. To aid this floc to coalesce with neighbouring particles and grow into larger flocs with more settleable masses, the water is gently stirred. The process of coalescence is known as flocculation. The gentle stirring can be achieved using paddles or baffles to induce a rolling motion in the water, and this continues for some 20–45 minutes. After this treatment, the water is passed for sedimentation.