The process of removing solid material from an
effluent stream is done in a device called a
clarifier. This process is also known as sedimentation. Sedimentation can be carried out in any pond or vessel that provides sufficient holding time to allow the solid particles to settle to the bottom by gravity. This is the most common type of solids removal in the pulp and paper industry.
Generally clarifiers are large round tanks with sloping bottoms. Inside is a moving rake mechanism to move the settled solids to a center well where the solids are drawn out the bottom.
The removed solids are usually 3 to 6 percent of the total volume. These solids are forwarded to other treatments such as filters and presses to remove as much water as possible. In a pulp and paper mill the primary solids are usually mainly composed of wood fibre. Attempts are made in the mill to screen the
effluent to recover usable
fibre before the effluent goes for treatment. Primary solids, after de-watering, are sent on and mixed with sawdust and secondary
sludge and further
de-watered in presses. This pressed mixture is burned as fuel in our No.6 Power Boiler.
The clarified liquid from the primary clarifier overflows a rim at the outer edge of the clarifier into a collection trough and passes on to the next process, which is secondary treatment.