248 Organic waste recycling: technology and management
5.3.3 Flocculation and flotation
Flocculation is a process of floc formation through slow mixing so that the sizes
of flocculent materials are large and heavy enough to be settled in a
sedimentation tank. In water and wastewater treatment, flocculation is normally
preceded by coagulation in which coagulant materials such as alum, lime, ferric
chloride or polymers are added individually or in combination and rapidly
mixed to enhance floc formation. Flotation is a physical process in which solid
particles float to the water surface through some kinds of buoyant forces (such
as dissolved-air flotation or foam flotation), and the floated particles can be
skimmed off from the water surface, learning the clear liquid at the lower
portion of the flotation tank. The application of coagulation process preceding
flotation is expected to be beneficial to solids removal as the floated particles
will be larger in size, easier to entrap or absorb air bubbles and be buoyed up by
dissolved air so that they can be effectively skimmed off from the water body.
Coagulation-flocculation
Based on the above information it appears that higher efficiency of solids
removal by flocculation or flotation can be achieved with the aid of coagulation.
For the case of algal flocculation using alum as coagulant, the pH range between
6.0-6.8 (6.5 optimum) gave good algal removal efficiency (Golueke and Oswald
1965). The same result was also found by Batallones and McGarry (1970) when
they studied jar test by using a fast mixing speed of 100 rpm for 60 sec for
coagulation and a final slow mixing speed of 80 rpm for 3 min for flocculation.
They found the most efficient alum dose for algal flocculation to be between 75-
100 mg/L, while Golueke and Oswald (1965) found the alum dose to be 70
mg/L.
Besides alum, other polyelectrolytes or polymers can also be used as
coagulant aid materials. Only cationic polyelectrolytes should be used in algal
flocculation because the algal cells act like a negative charge. Batallones and
McGarry (1970) found the cost of harvesting by alum alone at algal
concentrations below 30 mg/L to be rather expensive and, to reduce the
chemical cost, suggested the use of alum in combination with some cationic
polyelectrolyte. They reported that if polyelectrolyte (Purifloc-C31) was used to
aid alum, the most economic doses were 40 mg/L of alum together with 2-4
mg/L of Purifloc-C31.
It is well known that the efficiency of coagulation-flocculation is dependent
on several environmental parameters such as pH, alkalinity, temperature, and
turbidity, etc. Therefore laboratory or pilot-scale experiments on individual
HRAP water should be conducted, wherever possible, to select the appropriate