Simply put, sludge thickening mainly reduces sludge volume and increases solids concentration, while sludge dewatering goes a step further by removing more water and producing a sludge cake that is easier to transport, store and further process.

Both involve water removal, but they solve different problems.

Sludge Thickening: Making Sludge More Concentrated

Freshly generated sludge usually contains a large amount of water, particularly low-concentration sludge from settling tanks, dissolved air flotation systems and other treatment processes. If this sludge is sent directly to downstream dewatering equipment, a large proportion of what the equipment has to handle is water rather than solids.

The main purpose of sludge thickening is to remove part of the free water before dewatering and increase the solids concentration without significantly changing the amount of dry solids.

For example, after low-concentration sludge is thickened, its volume may be significantly reduced while the amount of dry solids remains largely unchanged. This means that the downstream dewatering equipment has less sludge volume to process.

Thickening therefore focuses on:

Reducing volume and increasing feed solids concentration.

Common sludge thickening methods include gravity thickening, belt thickening, rotary drum thickening and other mechanical methods. The most suitable method depends on factors such as sludge characteristics, feed concentration, flow rate and the operating conditions of the overall treatment system.

Sludge Dewatering: Further Separating Water from Sludge

After thickening, the solids concentration of the sludge has increased, but this does not necessarily mean that the sludge is ready for final transport or disposal.

The purpose of sludge dewatering is to further separate water from the sludge through mechanical processes, producing a relatively stable sludge cake.

Common sludge dewatering equipment includes belt filter presses, multi-disc screw presses, centrifuges and plate-and-frame filter presses. Different machines use different solid-liquid separation principles and have different levels of suitability for various sludge characteristics, flow rates and target cake moisture contents.

Dewatering therefore focuses on:

Further reducing moisture content and forming a sludge cake.

Why Do Some Systems Need Thickening While Others Can Dewater Directly?

For some low-concentration, high-water-content sludges, sending the sludge directly to dewatering equipment means that the equipment has to handle a large amount of water. This may affect processing capacity and operating stability.

In such cases, adding a thickening stage upstream can reduce the hydraulic load on the downstream dewatering equipment.

However, not every sludge treatment system requires a separate thickening unit.

Some equipment can perform both thickening and dewatering to a certain extent. For example, a multi-disc screw press uses a screw shaft to convey sludge while separating solids and water through the filtration gaps between the moving and fixed rings. It can handle certain types of low-concentration sludge and, under suitable operating conditions, may reduce the need for a separate thickening stage.

Therefore, when designing a sludge treatment system, it is not correct to assume that more thickening equipment is always better, nor that all sludge should be sent directly to dewatering.

The following factors need to be considered:

• Sludge source and characteristics
• Feed concentration and flow rate
• Sludge settling and flocculation properties
• Target sludge cake moisture content
• Subsequent transport, disposal or resource recovery methods
• Space, energy consumption and operation and maintenance requirements of the overall system

For a sludge treatment system, thickening and dewatering are usually not simply an either-or choice.

Some applications are better suited to:

Thickening → Dewatering

The sludge is first thickened to increase its solids concentration before being sent to the dewatering equipment.

Other applications may use:

Thickening + Dewatering

A single machine with both thickening and dewatering capabilities can provide continuous treatment.

For certain types of sludge, additional pre-treatment, conditioning or solid-liquid separation processes may also be required to achieve stable dewatering performance.

Therefore, equipment selection should not be based solely on the specifications of an individual machine. The decision should be made by considering the entire process, from sludge characteristics and feed conditions to the final disposal or treatment method.

Removing Water Is Not the Only Question

Water removal in sludge treatment is actually a continuous process.

Upstream thickening helps reduce the amount of water that needs to be handled and increases the sludge solids concentration. Downstream dewatering then further separates water from the sludge structure and produces a more manageable solid. There is no absolute rule that one must always come before the other – what matters is how the two processes fit into the overall sludge treatment system.

If you are designing a new sludge treatment system, or if your existing system has issues with feed concentration, processing capacity or dewatering performance, the first question should not simply be “Which machine should I buy?“

Instead, ask:

“What condition is this sludge in? Does it need to be thickened first, or can it be dewatered directly?“

Starting with the sludge itself is the key to finding a more suitable solid-liquid separation solution.

Haibar specialises in solid-liquid separation and sludge treatment equipment, providing sludge thickening, sludge dewatering and supporting systems for different operating conditions, including belt thickeners, rotary drum thickeners, multi-disc screw presses and belt filter presses. If you are evaluating a sludge thickening or dewatering solution, feel free to contact us and share your operating conditions and requirements. We would be happy to discuss a suitable treatment approach with you.