Wastewater treatment is the process of restoring desirable quality to water that has been utilized and/or contaminated by humans or nature.
Chemical, biological, or physical processes, or a combination of these, may be used in treatment.
Water can be treated to any desired level of purity; but, as purity increases, so does the cost of achieving that purity.
Sludge is the residue that accumulates in sewage treatment systems (or biosolids).
Sewage sludge is a solid, semisolid, or slurry residual material produced by wastewater treatment procedures.
What is the meaning of the sludge dewatering process?
Dewatering separates a considerable amount of the sludge’s water content, resulting in a concentrated sludge product with a DS concentration of 15 to 45 percent.
The result, known as a cake, is not free-flowing at this concentration and instead forms lumps that can only be delivered via a conveyor belt, motorized earth-moving equipment, or a spade.
Dewatering techniques use a substantial amount of mechanical power to remove more water than thickening alone.
Sludge dewatering is the process of separating sludge into liquids and particles to reduce waste.
Sludge dewatering can be accomplished using a variety of methods, including plate & frame and belt filter presses, centrifuging, screw pressing, and geomembranes.
Other alternatives are available in addition to these.
The remaining moisture content must be decreased, which will be accomplished by the sludge dewatering process.
A sludge drying bed is a system employed in this procedure.
Both the amount of sludge and the moisture content is lowered throughout the dewatering process.
It is important to note that dewatering does not treat the sludge or liquid; rather, it separates the solid and liquid components so that the two phases can be handled more simply and economically for eventual disposal.
Both the solid and liquid components of the sludge may include contaminants that must be handled individually once it has been dewatered.
How do sludge thickening and sludge dewatering differ?
Thickening of sludge
It is the initial step in the sludge treatment process.
The primary goal of thickening is to lower the sludge’s moisture content.
The moisture content can be lowered by up to 90% in this phase.
It consistently creates thickened sludge and keeps sludge pumps from becoming clogged.
An anaerobic digester lowers polymer consumption while increasing biogas production.
The main advantage is that it improves the plant’s operating aspects.
Dewatering of sludge
It happens later in the sludge treatment process.
The major goal is to lower the quantity of moisture in the sludge as well as the number of organic materials.
It contributes to the reduction of costs.
Because of the lower polymer use and the ease with which sludge can be disposed of, it helps to save money.
The main advantage is that it saves money and reduces the quantity of dangerous high-content organic materials.
Methods of sludge dewatering
Filter Press with Plates and Frames
To separate the liquids from the solids, plate and frame filter presses employ recessed-chamber filter plates.
Pressurize the slurry and squeeze the filtrate out through the chamber’s filter cloth by joining two plates together to form a chamber.
The filter cake is gathered on the filter plate cloth’s surface and deposited on a horizontal conveyor.
Filter presses are very efficient, producing filter cake with a solids content of 40 to 70%. It’s also advantageous since it generates the most solid material, which makes transportation and disposal more cost-effective.
The use of a plate and frame filter press has several advantages, including the fact that it does not require any particular license and produces stackable, cake-like solids that are easier to carry.
Centrifuge
A centrifuge is a device that separates components with various densities by spinning at a high speed.
The buoyancy concept underpins the operation of a centrifuge.
A component with a greater density will sink to the bottom of a mixture, whereas a component with a lower density would float above it.
Between 25% and 35% of solids by weight are produced using centrifuges.
The speed of the procedure is the centrifuge’s greatest advantage.
This method is often used when oil recovery is the primary focus and residual solids quality is less of an issue.
The following applications make centrifuges particularly useful
Dewatering of a Heavy Oil Sludge
Dewatering of Sludge from Municipal Wastewater Treatment
When a high moisture content in the filter cake is allowed, the belt press is the most commonly utilized method. It yields 18% to 25% solids by weight on average.
It is effective on high-volume waste streams where a substantial generation of the low-quality filter cake is tolerated because it is a continuous process. Municipal wastewater treatment plants frequently employ this method.
The belt press comes in use for the following tasks:
Sludge from a paper mill
Municipal Wastewater Treatment Sludges are made up of river silt.
Aids for sludge dewatering
Before thickening and dewatering, sludge is usually conditioned.
To improve the sludge’s treatment ability, two types of conditioning chemicals are used:
Iron salts and lime, for example, are mineral chemicals. In filter press applications, these compounds are often found.
Coagulants and flocculants are examples of organic compounds. Cationic flocculants are the most prevalent form of flocculent found.
Mineral chemicals
Iron salts
Ferric chloride and iron chloro-sulfate are commonly employed with lime to treat sludge before filtering.
They improve filterability by coagulating colloids (reducing linked water content) and micro-flocculating precipitates (hydroxides).
Depending on the quality of the sludge, iron salts doses range from 3% to 15% of dry content.
In comparison to the traditional iron salts + lime procedure, there is a tendency to combine iron salts with organic flocculants (cationic) to reduce the volume of sludge generated.
Lime
Lime is exclusively utilized in filter press applications as a conditioning agent in combination with iron salts.
It gives the sludge a mineral appearance and improves its mechanical qualities (higher specific resistance to filtration).
Lime is applied in amounts ranging from 15% to 40% of the dry weight.
Organic compounds
Sludge flocculation mechanism
Sludge flocculation is the process of agglomerating destabilized particles into aggregates termed flocs.
Flocculants have very large molecular weights (long chains of monomers) and have a high surface area.
Destabilized particles are fixed on their chains by varying ionic charges.
During the flocculation process, the particle size in the aqueous phase will rise.
The production of flocs causes the water to be released. As a result, this water will be simply removed during the phase of dewatering
Destabilized particles
The source of destabilized particles is complex and mostly determined by the sludge’s composition.
The sort of destabilized particles in the sludge to be treated will determine the charge the flocculent will deliver.
As a result, the type of sludge will play a role.
References
[1] Wastewater sludge processing / Izrail S. Turovskiy, P. K. Mathai, John Wiley & Sons, Inc., Hoboken, New Jersey,2006, p 106.
[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.
