Admitting Flocculant to the Decanter

The laboratory tests will advise the operator as to the quantities of polymeric flocculant that will be required. The equipment will be set up capable of supplying in excess of this quantity by at least 50% and perhaps 300% when dry solids operation is planned. It will be seen in subsequent chapters that three times the normal amount of polymer can be used in dry solids operation when the driest cakes are required. 

When applying the flocculant to the sludge, there has to be good intimate mixing, otherwise excess polymer will be needed to ensure full flocculation. Sufficient time has to be allowed to ensure optimum flocculation before settlement commences, to maximise the use of the sedimentation and compaction zones in the decanter. Flocculation must not be too early, such that major breakage of the floes occurs before sedimentation commences. 

As another example, anionic polymers tend to be slower acting than the cationics and. even with the high-speed centrifuges, often have to be added upstream of the centrifuge. 

The cross-linked polymers have the ability of reforming floes, and do not greatly exhibit the phenomenon of overdosing. These can often be added upstream. Some operators prefer adding this polymer upstream, because it is claimed that some breakage of floes on entry to the centrifuge is beneficial in releasing more water. This is confirmed by a lot of work [9-12], which has 

The value of extra dryness varies from plant to plant and country to country. When thermal dewatering is required after centrifuging, it pays to dewater as much as possible mechanically in the decanter, which is much cheaper than thermal costs. However, if the cake is to be incinerated then there is no economic advantage in drying in the decanter much beyond its dryness when it will burn autothermically. 

---