Thickening is a process for reducing the water content in sludge and concentrating solids in industrial wastewater treatment and sewage sludge treatment. It is used to reduce the volume of the sludge content, thereby increasing the efficiency of subsequent dewatering and drying processes and reducing disposal costs. Thickening is used in various industries such as municipal wastewater treatment, the food and beverage industry, the paper and pulp industry and the chemical and pharmaceutical industries.
Table of contents
Technical principles of thickening
Thickening is carried out either by gravity sedimentation or flotation. In this process, solid particles and water are separated from each other due to their different density and physical properties. The process concentrates the dry matter (DM), i.e. the solids content in the sludge, in order to make further processing more economical and efficient.
Types of thickening
Gravity sedimentation:
- Gravity is used to separate the solids in the sludge. In a cylindrical container, the heavier particles settle to the bottom and form a compact sludge layer, while the water flows off at the top. This method is suitable for sludge with a high concentration of solids and a relatively coarse particle size.
Flotation thickening:
- In this process, air is introduced into the sludge. The resulting air bubbles attach themselves to the solid particles and cause them to rise to the top. The particles are then removed as a surface layer while the water drains downwards. Flotation thickeners are particularly suitable for sludge with finer particles or low density.
Belt and slice thickening:
- Mechanical thickeners use moving belts or disks to separate the water from the sludge. The belt compacts the sludge by continuously pressing it. This method is particularly space-saving and is often used in industries with limited space.
Centrifugal thickening:
- Centrifugal technology uses centrifugal force to thicken the solids. High rotation speeds separate solids from water, which enables particularly efficient separation. This method is suitable for sludge with fine particles, but is more energy-intensive.
Thickening process
The thickening process involves several steps:
Discharge of the sludge:
- The sludge is fed into the thickener. Depending on the type of sludge and the type of thickener, the retention time and other parameters are adjusted to achieve optimum separation.
Sedimentation or flotation:
- In a gravity thickener, the particles settle to the bottom, while in a flotation thickener the particles rise to the top through the air bubbles. This step is used for the actual concentration of the solids content.
Removal of the thickened sludge:
- The thickened sludge is removed from the thickener. The dry matter is now significantly increased and the remaining volume of sludge is reduced, which facilitates disposal or further treatment.
Drainage of the clear water:
- The clarified water is separated and can be further treated or recycled depending on the degree of contamination. This reduces the amount of water in the sludge and relieves the sewage treatment plant.
Photo: ALMA NeoDAF flotation plant with precipitation stage
Areas of application for thickening
Thickening is essential for reducing sludge volumes in a variety of industrial processes. Typical fields of application are
Municipal wastewater treatment:
- Thickeners are used in wastewater treatment plants to thicken primary and secondary sludge. They enable more efficient dewatering and a reduction in volume, which facilitates subsequent treatment.
Paper and pulp industry:
- Thickening is used to treat wastewater containing fibers and to separate the solids from the wastewater. This reduces the volume of wastewater and lowers disposal costs.
Food and beverage industry:
- Food processing produces sludge with a high organic content, which is concentrated by thickeners and prepared for further treatment. This is crucial in order to minimize transport and disposal costs.
Chemical and pharmaceutical industry:
- Thickeners are used to thicken wastewater contaminated with solids and to collect toxic or poorly degradable substances in a concentrate that must be specially disposed of.
Mining and metal processing:
- Sludges with high concentrations of heavy metals and abrasive particles are produced in mining and metal processing. Thickening reduces the volume and facilitates further treatment of the residual materials.
Advantages of thickening
Reduction of sludge and disposal costs:
- The reduction in volume significantly reduces disposal costs, as less sludge has to be transported and processed.
Improved efficiency of subsequent dewatering:
- The reduced water content improves the efficiency of subsequent dewatering and drying processes, as the solids concentration is increased.
Water reuse:
- In many cases, the separated water can be returned to the production process after minimal treatment, which increases resource efficiency.
Compact design:
- Thickeners are space-saving components that are available in various sizes and designs and can be flexibly adapted to the available space conditions.
Technical challenges and requirements
Sludge composition and solids content:
- The composition and solids content of the sludge varies depending on the industry and process. Thickeners must be adapted to the specific sludge composition to ensure effective thickening.
Scaling and deposits:
- Deposits and blockages can impair functionality, especially with hard or heavily soiled sludge. Regular maintenance and cleaning are essential.
Energy requirement and costs for mechanical thickening:
- Mechanical thickening processes, such as centrifugal technology, are more energy-intensive. The choice of thickening process should therefore be weighed up economically.
Maintenance of segregation:
- Correct operational management is crucial to maintain segregation in the thickener. Insufficient segregation can lead to performance losses and impair thickening efficiency.
Conclusion
Thickening is an indispensable process for reducing the volume and pre-treatment of industrial sludge. Thanks to the different processes such as gravity sedimentation, flotation and mechanical thickening, it offers a flexible solution for many industrial requirements. Thickening ensures lower transportation and disposal costs and enables more efficient further treatment of the sludge. It is an important basis for economical and resource-saving wastewater and sludge treatment and has become indispensable in many industries.
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