Mechanical Dewatering of Sludge

With the gradual strengthening of modern civilization and environmental awareness, issues such as traffic and environmental problems caused by construction projects are increasingly receiving attention. Utilizing horizontal directional drilling (HDD) technology for construction is the best choice for laying gas pipelines and optical cable pipelines in urban areas, as well as for crossing roads, rivers, mountains, and non-demountable buildings. Choosing HDD for construction offers advantages such as a small footprint, high work efficiency, and minimal landscape disruption.

Before drilling, we need to prepare a certain amount of slurry, which serves functions such as cooling and lubricating the drilling tools, carrying cuttings, and preventing wall collapse. We often refer to it as the “blood” of HDD. Different slurry preparation schemes should be selected based on various factors such as the location, environment, key points, and length of the project. With the increase in environmental awareness, the treatment and recycling of slurry are particularly important. This can significantly reduce the amount of slurry needed, as well as the consumption of water and bentonite, thereby lowering the construction costs. Moreover, from an environmental perspective, it reduces the footprint of slurry pits and the storage and overflow of large amounts of slurry, which can cause significant problems for environmental protection and subsequent slurry disposal. Slurry recovery systems will be increasingly used in future HDD construction projects.

  1. General Overview of Mechanical Dewatering of Sludge

Mechanical dewatering of sludge mainly includes methods such as belt filter press dewatering, centrifugal dewatering, and plate-and-frame filter press dewatering. The main types and characteristics of mechanical dewatering processes are as follows:


Belt Filter Press Dewatering: Low noise, low power consumption, but large footprint and high wash water usage, with poor workshop environment. The required sludge feed moisture content is generally below 97.5%, and the output sludge moisture content can generally reach below 82%.

Centrifugal Dewatering: Small footprint, no need for wash water, good workshop environment, but high power consumption and high noise. The required sludge feed moisture content is generally between 95% and 99.5%, and the output sludge moisture content can generally reach 75% to 80%.

Plate-and-Frame Filter Press Dewatering: Low cake moisture content, but large footprint and high wash water usage, with poor workshop environment. The required sludge feed moisture content is generally below 97%, and the output sludge moisture content can generally reach 65% to 75%.

Screw Press and Rolling Press Dewatering: Small footprint, low wash water usage, low noise, good workshop environment, but small single-machine capacity and high supernatant solid content. The required sludge feed moisture content is generally between 95% and 99.5%, and the output sludge moisture content can generally reach 75% to 80%.

  1. Pretreatment Before Mechanical Dewatering of Sludge

The purpose of pretreatment before mechanical dewatering of sludge is to improve the sludge dewatering performance and enhance the production capacity and dewatering effect of mechanical dewatering equipment. Pretreatment methods include chemical conditioning, elutriation, thermal treatment, and freezing methods, with chemical conditioning and elutriation being the most commonly used methods.


According to the “Outdoor Drainage Design Code” (Clause 7.4.1):

  1. The type of sludge dewatering machinery should be selected based on the sludge’s dewatering properties and dewatering requirements after a technical and economic comparison.
  2. The sludge moisture content before entering the dewatering machine should generally not exceed 98%.
  3. Digested sludge can be elutriated before dewatering based on sewage properties and economic benefits.
  4. The layout of the mechanical dewatering room should comply with relevant provisions in Chapter 5 of the code and consider sludge cake transportation facilities and passages.
  5. Dewatered sludge should be stored in sludge yards or silos, with the capacity determined by sludge disposal and transportation conditions.
  6. Mechanical dewatering rooms should be equipped with ventilation facilities, with an air change rate of at least six times per hour.
  7. Filtration Dewatering of Sludge

The basic principle of sludge filtration dewatering is using the pressure difference across the filter media to drive dewatering. There are four methods to create the pressure difference:

  1. Relying on the static pressure of the sludge’s own weight (e.g., drying bed dewatering).
  2. Creating a vacuum on one side of the filter media (e.g., vacuum filtration dewatering).
  3. Pressurizing the sludge to squeeze water through the media (e.g., filter press dewatering).
  4. Creating centrifugal force (e.g., centrifugal dewatering).

Solids control manufacturer

Sludge filtration dewatering includes plate-and-frame filter press and belt filter press. According to the “Outdoor Drainage Design Code”:

  1. For plate-and-frame filter press and box filter press design (Clause 7.4.5):

– Filtration pressure should be between 400 and 600 kPa.

– Filtration cycle should not exceed 4 hours.

– Each filter press can be equipped with one sludge feed pump, preferably a plunger pump.

– Compressed air volume should be no less than 2 m³/min per cubic meter of filter chamber.

  1. For belt filter press design (Clauses 7.1.4 and 7.4.4):

– The number of sludge treatment structures should be no less than two, designed for simultaneous operation, with one sludge dewatering machine as a backup.

The filter press should be equipped with an air compressor, with at least one backup.

– Wash pumps should be configured with a pressure of 0.4 to 0.6 MPa and a flow rate calculated based on 5.5 to 11 m³ per meter of belt width per hour, with at least one backup.

– Sludge dewatering load should be determined based on test data or similar operational experience, with a sludge cake moisture content of 75% to 80%.

  1. Centrifugal Dewatering of Sludge

The basic principle of centrifugal dewatering of sludge is using the rotation of the centrifuge to separate the solid and liquid in the sludge. According to the “Outdoor Drainage Design Code” (Clause 7.4.7), for sewage sludge using horizontal screw centrifuge dewatering, the separation factor should be less than 3000g (g being the acceleration due to gravity). Clause 7.4.8 specifies that a sludge cutter should be installed before the centrifugal dewatering machine, with the cut sludge particle size not exceeding 8 mm.

Method 1: Cement Solidification Treatment

The cement solidification treatment method merely delays the environmental pollution rather than eliminating the secondary pollution problem entirely. Many local environmental protection departments have explicitly banned this method for treating oily sludge.

Method 2: Chemical Reagent Desorption Treatment

The chemical reagent desorption treatment method is not effective for sulfonated oily sludge systems. Theoretically, it is feasible for non-sulfonated oily sludge systems, but it is challenging to implement in practice. The success of this method heavily depends on the technical skill and dedication of the operators, and it carries significant risks of secondary environmental pollution.

Method 3: Oily Sludge Incineration Treatment

The incineration treatment method for oily sludge leverages the heat value of the oily sludge itself, along with other auxiliary fuels, to achieve co-combustion. In this process, all the organic substances, pathogens, and other toxic and harmful materials in the oily sludge are oxidized and pyrolyzed at high temperatures ranging from 850°C to 1000°C, thoroughly destroying their toxic properties. This method is currently considered a thorough approach for volume reduction, harmlessness, and resource recovery. However, it has high treatment costs and poses potential risks of dioxin pollution during the incineration process.

Method 4: High-Temperature Anaerobic Thermal Desorption Extraction Technology

The high-temperature anaerobic thermal desorption extraction method uses the principle of the critical weight loss temperature drift characteristic of the thermal instability of organic substances. In an oxygen-deficient environment, the oily sludge is heated to break the bonds of organic substances, rendering them non-toxic. Initially, light crude oil and water evaporate at low temperatures. Subsequently, heavy crude oil undergoes thermal degradation in the high-temperature zone, transforming into light crude oil, which then evaporates from the solid phase of the oily sludge. Different organic components have distinct thermal weight loss zones. Through continuous thermal desorption from low to high temperature zones, the method ultimately separates crude oil and other organics from the sand and gravel particles. The gaseous phase produced in this process, after cooling, forms three phases:

– Non-condensable gas phase includes combustible gases like hydrogen, methane, and carbon monoxide.

– Liquid phase includes substances like gasoline, diesel, wax hydrocarbons, tar, and water.

– Solid phase consists of harmless sand and gravel particles.

This entire thermal desorption extraction process does not produce secondary pollution.

Guanneng Company can provide complete equipment for treating oily sludge, enabling the recovery of oil and water. The hot wash technology for treating oily sludge can achieve lower disposal costs for solids while maximizing oil recovery.

Adding surfactants to oily sludge materials can alter the surface tension between the oil, water, and sludge particles, thereby reducing the solution’s viscosity and making the separation of materials easier. The hot washing method offers several advantages, including good treatment efficiency, low energy consumption, low treatment costs, and short processing cycles. These attributes make it a leading trend in the separation of oily sludge.

Given the varying characteristics of oily sludge in different oil fields domestically, it is crucial to select and blend stable and efficient surfactants. Additionally, determining the appropriate process conditions and designing corresponding equipment for treating oily sludge are essential for effective treatment.