Sludge Thickening process
Thickening of sludge is a process to increase its solids concentration and decrease its volume by removing some of the free water. The resulting substance is still fluid.
Thickening is employed before subsequent sludge-processing steps, such as digestion and dewatering, to reduce the volumetric loading and increase the efficiency of subsequent processes.
The most commonly used sludge thickening processes are gravity thickening, dissolved air floatation thickening, gravity belt thickening and rotary drum thickening. The thickening techniques are compared in Table 1.
The selection of a particular sludge thickening process sometimes depends on the size of the wastewater treatment plant and the downstream train chosen.
The following are the primary design variables of any thickening process:
_The supply stream’s solids content and flow rate
_Chemical demand and cost if chemical conditioning is applied
_Suspended and dissolved solids concentrations and flow rate of the clarified stream
_Suspended and dissolved solids concentrations and flow rate of the clarified stream
Table 1 Comparison of Thickening Methods [1]
| Method | Advantages | Disadvantages |
| Gravity thickening |
The last operation skill required | Large space required |
| Low operating costs | Odor potential | |
| Minimum power consumption | Erratic and poor solids concentration (2 to 3%) for WAS | |
| Ideal for small treatment plants | Floating solids | |
| Good for rapidly settling sludge such as WAS and chemical | ||
| Conditioning chemicals are typically not required | ||
| Dissolved air flotation thickening |
Provides better solids concentration (3.5 to 5%) for WAS than that ofggravity thickening | Operating costs are higher than for a gravity thickener |
| Require less space than a gravity thickener | Relatively high power consumes | |
| Will work without chemicals or with low dosages of chemicals | Moderate operator attention required | |
| Relatively simple equipment components | Odor potential | |
| Larger space requirements compared to other mechanical methods | ||
| Has very little storage capacity compared to a gravity thickener | ||
| Not very efficient for primarysludge | ||
| Requires polymer conditioning for higher solids capture or increased loading | ||
| Centrifugal
thickening |
Effective for thickening WAS to 4 to 6% solids concentration | High capital cost |
| Control capability for process performance | High power consumption | |
| Least odor potential and housekeeping requirements because of contained process | Requires moderate operator attention | |
| Less space required | Sophisticated maintenance requirements | |
| Requires polymer conditioning for higher solids capture | ||
| Gravity belt thickening |
Effective for thickening WAS to 0.4 to 6% solids concentration | Polymer dependent |
| Control capability for process performance | Housekeeping requirements | |
| High solids capture efficiency | Odor potential | |
| Relatively low capital cost | Moderate operator attention required | |
| Relatively low power consumption | Building commonly required | |
| Rotary drum thickening |
Effective for thickening WAS to 0.4 to 6% solids concentration | Polymer dependent and sensitive to polymer type |
| Less space required | Housekeeping requirements | |
| Low power consumption | Odor potential | |
| Moderate operator attention required | ||
| Building commonly required |
Centrifugal Thickening
Centrifugal thickening is the use of centrifugal force to speed up sedimentation. Solids settle by gravity in a gravity thickener.
A centrifuge applies a force 500 to 3000 times that of gravity, making it a highly effective gravity thickener.
For thickening wastewater sludge, centrifuges are often utilized.
Centrifuges are rarely used to thicken primary sludge since it frequently contains abrasive material that is harmful to the centrifuge.
Centrifuges, in addition to being particularly successful in thickening wastewater sludge, offer the additional benefit of requiring less space and having the least odor potential and housekeeping requirements due to the contained operation.
However, capital costs, as well as maintenance and power costs, might be significant. As a result, the technique is often restricted to big treatment plants.
Different types of centrifuges
There are three types of centrifuges: disk nozzle, imperforate basket, and solid bowl.
The feed sludge for disc nozzle centrifuges must be thoroughly prescreened and de-gritted.
They are appropriate for sludges with particle sizes of 400 m or less.
Imperforate basket centrifuges can only be utilized in batch mode, not in continuous feed and discharge.
They have a high bearing wear rate and necessitate extensive maintenance.
Solid bowl centrifuges are replacing disc nozzles and imperforate basket centrifuges for these reasons.
Solid bowl centrifuges (also known as continuous decanter scroll or helical screw conveyor centrifuges) are available in two main designs: Countercurrent and concurrent.
The primary differences between the two are the configuration of the conveyor (scroll) toward the liquid discharge end of the machine, and the location and configuration of the solids discharge port.
How does it work?
Sludge feed is introduced into the bowl by a concentric tube at one end of the centrifuge.
The discharge weir elevation relative to the bowel wall determines the liquid depth in the centrifuge. Typically, the weir is adjustable.
As the sludge particles are subjected to the gravitational field, they begin to settle out on the whirling bowl’s inner surface.
The lighter liquid collects above the sludge layer and flows toward the center outlet ports at the machine’s bigger end.
The spinning conveyor (scroll) transports the settled sludge particles on the inner surface of the bowl to the opposite end (conical section) of the bowl.
The fundamental distinction between a thickening centrifuge and a dewatering centrifuge is in the design of the conveyor and the conical portion of the bowl.
In a thickening centrifuge, the slope of the conical section is less steep.
The thickened sludge concentration (sludge cake concentration for a dewatering centrifuge) and solids recovery are used to assess centrifuge performance (often called solids capture).
Operational variables
Operational variables that affect thickening include:
Feed flow rate
_Feed sludge parameters such as particle size and shape, particle density, temperature and sludge volume index.
_The bowl’s rotational speed.
_The conveyor’s differential speed about the bowl.
_Depth of the liquid pool in the bowl.
_Polymer conditioning is required to improve performance.
One of the most important operational parameters of centrifuges is the factor of separation, which demonstrates how centrifugal forces are stronger than sedimentation forces by the following equation:
F=a/g, a=wr or F=r (n/g)
Where:
F =separation factor
a =speed of centrifugal force, m/s2
g = speed of sedimentation force, m/ s2
w = angle speed of bowl (rotor), min−1
r = inside radius of bowl, m
n = speed of bowl (rotor) rotation, min−1
The factor of separation can be increased by increasing the speed of the bowl (rotor) rotation.
The high bowl rotation speed, on the other hand, can reduce sludge particle sizes, increase polymer demand, and reduce flocculation efficacy.
Centrifuges normally run at speeds ranging from 1500 to 2500 RPM, with a factor of separation ranging from 600 to 1600.
Thickened sludge concentration and solids recovery values are lower for the lower factor of separation values.
References
[1] Wastewater Sludge processing / Izrail S. Turovskiy, P. K. Mathai, John Wiley & Sons, Inc., Hoboken, New Jersey,2006, p 81.
[2] L. K. Wang, Y. T. Hung, and N. S. Shammas (eds.), Physicochemical Treatment Processes. Humana Press, Totowa, NJ (2005) ,p 684.
[3] Metcalf & Eddy, Tchobanoglous, G., Burton, F. L., Stensel, H. D. “Wastewater engineering: treatment and reuse/Metcalf & Eddy, Inc.”, Tata McGraw-Hill, 2003.
[4] Wastewater sludge processing / Izrail S. Turovskiy, P. K. Mathai, John Wiley & Sons, Inc., Hoboken, New Jersey,2006, p 96.
