Sludge Dewatering
Any system for water treatment may be visualized as a pollutant concentrator. In wastewater treatment the pollutants are agglomerated and precipitated in form of flocs, called “sludge”. Typically, sludge from a final liquid-solids separation unit may contain from 1 to 5% total suspended solids. Because of the costs saving associated with handling smaller volumes of sludge, there is an economic incentive to remove additional water. Dewatering equipment is designed to remove water in much shorter time than nature would by gravity. An energy cost is required for as well as a conditioning of the sludge prior to the dewatering step. Conditioning is necessary due to the nature of the sludge particles, consisting of colloidal (less than 1 micron), intermediate and large particles (gather than 200 micron). Large particles are usually compressible, under an energy gradient. The small ones also participate in this mechanism, plugging the pores of the sludge cake. The pressure drop through the sludge cake, due to the decrease in porosity and pore sizing, exceeds available energy and dewatering ceases. The purpose of sludge conditioning is to provide a rigid sludge structure of porosity and pore size sufficient to allow drainage.
Water is present in various forms into the sludge:
-
Inter-particles water: is that one contained through the flocs and represents the most relevant content in percentage. Also this type of water is the most easily to remove, using pre-thickening units.
-
Bonded water: is the external water and belongs to the single particles – moving together with them – and can be classified as “colloidal water “ and “capillary water”; this water can be removed, after sludge stabilization or conditioning, via dewatering processes. In any case this water cannot be removed completely.
-
Inside water: is the smaller fraction of water and is strictly inside the particles. The removal of this water is feasible only using thermal processes.
For example, the following table shows the water distribution in a typical exceeding sludge extract from an activated sludge biological plant:
|
Type of water |
Content % (in volume) |
Content in the fraction % |
|
Inter-particles water |
75 |
|
|
Bonded water |
22 |
colloidal water 20% capillary water 2% |
|
Inside water |
2.5 |
|
|
Total |
99.5 |
The content of solid is equal to 0.5% |
There are few elements that make different sludge originated from wastewater treatment with respect to several products that are normally processed for water removal (dewatering processes):
- Sludge originated from wastewater plant are often heterogeneous;
- They are frequently affected by humidity variation not priory detectable and difficult to handle;
- Sludge originated from civil wastewater treatment plants, as well most of those produced from industrial wastewater treatment plants, have a “plastic” behaviour so the viscosity increase to much especially in period of low rainfall. This problem could be solved recycling a quantity of dried solids in the incoming flow in order to increase the total content of dried solid for the dewatering processes.
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