Mechanical sludge drying: comparison of available technologies

• 1. Sludge production in water treatment plants
• 2. Mechanical drying of sludge
• 3 .Technologies for mechanical sludge drying
• 3.1. Centrifugation
• 3.2. Band filters
• 3.3 Filter presses
• 3.4. Screw presses
• 4. Conclusion

1. Sludge production in water treatment plants

Wastewater, both industrial and urban, usually has high levels of suspended solids carried by the effluent, which are either produced by chemical reactions (precipitates) or appear in biological treatments (excess sludge) from the digestion of the organic matter (BOD) present.     

Pre-treatment

Effluent pretreatment processes include simple screens and sieves for larger solids. These basic processes allow for:

• Eliminate clogging problems in circuits or pump groups and, in certain cases, recover raw materials or product residues from washing and leaks.
• Reduction of biodegradable materials which, when separated at the beginning of the process, simplify and reduce the size and operating costs of the subsequent stages of the wastewater treatment plant.

Primary sludge

Small suspended solids, even in colloidal form, that escape pretreatment systems must be separated to optimize subsequent purification stages.

The most common processes at this stage are:

• Physicochemical treatment using appropriate reagents (neutralization, coagulation/flocculation)
• Separation techniques by clarification, including decantation and dissolved air flotation, the latter being the most efficient as it significantly reduces the volume of separated sludge and separates a much greater amount of solids, oils, and fats than simple sedimentation.

Primary sludge is usually thicker and easier to dry than secondary sludge.

Secondary sludge

Secondary sludge appears during the digestion stages of organic matter through aerobic or anaerobic biological treatments. The bacteria used in a biological reactor develop their life cycle in a medium (mixed liquor) that is maintained in conditions suitable for their survival (concentration,_O2, temperature) within the biological reactors. The result is purified water and masses of bacteria, part of which is purged as excess sludge, which is extracted by clarification (DAF systems) for disposal.

Treatment of separated sludge

Depending on the destination of the sludge, it is concentrated to facilitate subsequent handling and transport.

When sludge contains suspended solids with commercial value, it is recommended that it be dewatered to high dry matter content using thermal treatments capable of achieving very high dryness levels (≥ 90%). This same process is applicable to highly polluting sludge, whose volume must be reduced to a minimum before being sent to special landfills, where management costs are significantly high.

Most sludge from wastewater is sent to standard landfills, where it is selected based on criteria such as composition, stability, and concentration (≥30% dry matter). In these cases, mechanical sludge drying is the most efficient option.

2. Mechanical sludge drying

Mechanical drying is a physical process whereby free water and part of the bound water are separated from the sludge by applying mechanical means.

The most commonly used equipment in wastewater treatment is:

• Centrifugal Decanters
• Band filters
• Filter presses
• Screw presses

They are robust, easy-to-operate devices with moderate energy consumption.

These systems allow dry matter contents of between 18% and 40% to be achieved, depending on the type of sludge, its chemical conditioning, and the technology used.

The sludge to be treated must have a minimum dry matter content of between 3% and 5% in order to be managed by mechanical drying. Dissolved air flotation (DAF) regularly achieves these values, eliminating the need for prior concentration processes such as sludge thickeners.

Chemical conditioning

To maximize the efficiency of mechanical drying, the sludge is first conditioned with coagulants and flocculants. This treatment facilitates the release of interstitial water and increases the performance of the equipment, while also reducing mechanical wear and energy consumption.

The correct selection of these reagents and their dosage is a determining factor in both the quality of the dry sludge and operating costs (OPEX).

The main reagents used are as follows:

• Flocculants ( the most common). Organic polymers are used, which can be cationic, anionic, or nonionic.
• Coagulants ( _FeCl₃,_Al₂(_SO₄)_₃, PAC, and_FeSO₄) neutralize polarity and improve the performance of flocculants.
• Lime (CaO, or Ca(OH)₂). It increases the concentration and filterability of the sludge and stabilizes it, but has the disadvantages of increasing sludge production and having an abrasive effect.
• Special materials. Such as cellulose, diatoms, perlite. These types are specific to the chemical industry.                      

Technical and economic advantages

From a technical standpoint, mechanical sludge drying offers numerous advantages, such as reducing volume to levels acceptable for landfill disposal or reuse in composting. Transportation, handling, and energy costs are generally moderate.

Common applications

Mechanical drying is applicable in:

• Urban wastewater treatment plants: treatment of biological, physicochemical, or mixed sludge.
• Industrial effluents:                                                                                         
  • Chemical and pharmaceutical industry.
  • Agri-food industries (slaughterhouses, dairy, canning) and beverages.
  • Mining.
  • Ceramic industry
  • Paper industry.
  • Facilities that prepare sludge for energy recovery, composting, or landfill disposal.
  • Other activities such as tanneries, industrial laundries, marble sawmills, etc.

TYPICAL SLUDGE LINE OF A WWTP

Mechanical sludge drying is an efficient, sustainable, and economically competitive process. Its correct implementation allows compliance with environmental regulations and significantly improves the overall profitability of the facility.

3. Technologies for mechanical sludge drying

Mechanical sludge drying involves a set of solid-liquid separation technologies aimed at reducing the water content in sludge through the application of mechanical solutions. The choice of the most suitable process depends on the nature of the sludge, the flow rates to be treated, the degree of dryness required, and the investment and operating costs.

3.1 Centrifugation

Centrifugation is a process based on the separation of solid and liquid phases through the application of centrifugal force, which is generated by a rotating drum spinning at high speed (4,000–6,000 rpm). It is a continuous process in which the sludge, previously conditioned with polymer, is introduced into the centrifugal decanter, separating the solid in the central zone due to differences in density.

Common applications

• Urban WWTPs with continuous operation.
• Installations with variations in flow and load.                       
• Plants with a high degree of automation.

Advantages

• Compact design
• High operational availability
• Easy integration with existing lines

Disadvantages

• Reduced degree of dryness in sludge (20–28% dry matter)
• Mechanical wear
• Noise pollution (≥80 dB (A)
• High energy consumption (20–40 kWh/t DM)

3.2. Band filters

Continuous operation technology in which sludge pressing is carried out between filter belts that are tensioned by a central pneumatic compressor unit. These filter cloths move, compressing the sludge between rollers that advance it as it is dewatered by drainage.

The sludge is pre-treated with specific flocculants, particularly polyelectrolytes.

Common applications

• High flow rates in urban WWTPs
• Stabilized biological sludge
• Low requirements for dehydrated sludge concentration

Advantages

• Robust and proven technology
• Moderate energy consumption (5–10 kWh/t DM) and maintenance costs
• Affordable installation costs (CAPEX)

Disadvantages

• Reduced degree of dryness in sludge (18–25% dry matter)
• High water consumption for fabric washing
• Cloudy filters
• Foul odors
• Sensitive to specific imbalances in sludge conditioning
• Large space occupied

3.3 Filter presses

Dehydration is carried out by high-pressure filtration in closed chambers, which are made up of plates with filter cloths. The process is discontinuous and achieves high levels of dryness. The most commonly used type of filter press is the plate and frame/membrane type.                                                 

The sludge is pre-treated by adding controlled amounts of coagulants, flocculants, and in many cases lime, as its robustness and resistance are used to increase the concentration and consistency of the dehydrated cakes.

Common applications

• Industrial sludge
• Sludge that resists dehydration
• Maximum volume reduction in sludge is sought

Advantages

• High final dryness (35–40% dry matter)
• Small footprint required
• Excellent filtration clarification
• Moderate energy consumption (3–8 kWh/t DM)

Disadvantages

• Discontinuous regime
• Requires supervision and maintenance, although it can be automated
• High installation cost (CAPEX)
• You need to wash the fabrics.
• Risk due to high pressure

3.4. Screw presses

Continuous process in which sludge is progressively compressed by a screw conveyor housed in a filter drum. The pressure increases gradually, allowing for continuous and controlled separation. Flocculants are often used to condition the sludge.

Common applications

• Biological and mixed sludge.
• Small and medium sludge production.
• Specific industrial applications
• Pre-filtered

Advantages

• Simple and stable operation.
• Low power consumption (3–6 kWh/t DM)
• Simple maintenance
• Low noise pollution
• Reduced installation cost
• Reduced space requirement

Disadvantages

• Low degree of dryness in sludge (18–25% DM)
• Wear on rings and screws
• Continuous washing (water consumption)
• Problems with fats and fibers

  COMPARISON OF MECHANICAL SLUDGE DRYING TECHNIQUES

Technology	% DM	capital expenditure	OPEX	main differential
Centrifuge	20–28	Medium/High	Medium/High	Continuous operation, flexibility, and automation
Band filter	18–25	Medium	Medium	Continuous operation, high flow rates
Filter press	30–40	High	Low/Medium	Discontinuous operation, maximum dryness
Screw press	18–25	Medium/Low	Under	Continuous operation, low power consumption, and simplicity

3. Conclusion

Wastewater treatment generates sludge that must be transformed into solid, stable waste for handling and management. Authorities regulate its disposal based on its composition and dryness.

Effluent treatment plants often incorporate sludge drying systems in order to meet these requirements. Some industries, instead of installing these facilities, choose to send their liquid waste to external waste managers, which, although it does not involve installation or maintenance costs, does imply a higher cost per ton of sludge treated.

Mechanical sludge drying processes offer solutions that can be adapted to each operating scenario, from urban WWTPs to complex industrial applications. The correct selection of technology allows for the optimization of the balance between investment, operating costs, dryness achieved, and final destination of the sludge, constituting a key stage in the efficient and sustainable management of sludge.

Bibliography and references.

Wastewater Treatment and Purification (Metcalf–Eddy). Ed. Labor

Filter press for sludge dewatering

68 Sludge Dewatering | PDF | Filtration | Water