Analysis of Sewage Treatment Processes in Urban Wastewater Treatment Plants

1. Origin and Development of Screw Press Sludge Dewatering Machine

The screw press filter, also known as the screw extrusion dewatering machine, first emerged in Germany in the 1960s. Subsequently, countries such as the former Soviet Union, Sweden, and the United States also developed this type of filter, initially applying it to oil extraction, fish meat pressing after grinding, and filtering fish and shrimp waste. In recent years, this technology has been widely used in sludge dewatering and other industrial dewatering processes.

With the advancement of screw filtration technology, to prevent clogging issues, Germany and other European countries developed a relatively new filter mesh structure in the 1970s, such as bar screen structures and moving-fixed ring filter structures. Among them, the moving-fixed ring filter mesh consists of multiple rings mounted on a screw shaft, categorized into fixed and moving rings. These rings are arranged alternately with specific filtration gaps. The friction between the moving and fixed rings enhances material extrusion while minimizing obstruction, thereby improving filtration and separation efficiency and preventing clogging in high-viscosity materials.

For example, institutions such as Darmstadt University of Technology in Germany, FAN Company, and LYCO Company in the United States developed early screw press dewatering machines using moving-fixed ring structures. These devices, initially used in fisheries and farms, effectively addressed clogging problems. Consequently, they gained popularity in seafood and food-processing industries in Japan, South Korea, and other countries.

In recent years, the screw press sludge dewatering machine has also been developed, innovated, and widely used in China. Due to its ability to separate materials efficiently without clogging, it has found extensive applications in various industries, including the petrochemical sector.

2. Characteristics and Current Status of Petrochemical Sludge Dewatering

2.1 Sources of Petrochemical Sludge

Petrochemical sludge refers to the oily solid waste generated during oil extraction, transportation, refining, and wastewater treatment. It originates mainly from ground treatment systems during crude oil extraction and from produced water treatment in oilfields. Other sources include bottom sludge from oil storage tanks, sedimentation tanks, wastewater tanks, and oil separation pits at transfer stations and joint stations. Additionally, sludge may form due to drilling operations, pipeline leaks, and refinery processes, accumulating in facilities such as light hydrocarbon plants and natural gas purification units.

The sludge generated from wastewater treatment, known as the "three types of sludge," includes oil separation tank (or tank) bottom sludge, flotation tank scum, and excess activated sludge from biochemical treatment processes.

2.2 Characteristics of Petrochemical Sludge

Oily sludge has a large volume and, if discharged untreated, not only occupies vast land resources but also emits foul odors, severely polluting surrounding soil, water bodies, and air. The sludge contains harmful components such as pathogenic bacteria, parasite eggs, heavy metals (e.g., copper, zinc, chromium, mercury), salts, polychlorinated biphenyls (PCBs), dioxins, and radioactive isotopes, many of which are persistent toxic pollutants. In particular, oilfield sludge has been classified as hazardous waste and is subject to strict hazardous waste management regulations.

Moreover, petrochemical sludge exhibits high oil content, severe emulsification, fine particle sizes, and high viscosity, making traditional dewatering equipment ineffective.

2.3 Current Status of Petrochemical Sludge Dewatering

Currently, sludge dewatering in the petrochemical industry primarily relies on plate-and-frame filter presses, belt filter presses, and centrifuges. However, each of these methods has its limitations:

(1) Plate-and-frame filter presses operate in batch mode, have low efficiency due to filter cloth clogging from oily sludge, and require frequent maintenance.

(2) Belt filter presses are prone to clogging, require excessive water for cleaning, and suffer from poor filter fabric cleanliness and low dewatering efficiency.

(3) Centrifuges struggle with separation efficiency, have high energy consumption, and generate significant noise, making them unsuitable for certain working environments.

As a result, none of these conventional dewatering machines effectively handle high-viscosity or high-oil-content sludge. With increasingly stringent environmental regulations, the demand for harmless, reduced, and resource-efficient treatment of oily sludge has become a pressing issue, necessitating more advanced dewatering technologies in the petrochemical industry.

3. Screw Press Sludge Dewatering Technology for Petrochemical Sludge

3.1 Screw Press Sludge Dewatering Equipment

The screw press sludge dewatering machine consists of multiple fixed and moving rings and a screw shaft forming the filtration unit. The machine features a compact structure, easy operation, and stable performance.

The dewatering machine comprises two main sections: the concentration unit and the dewatering unit. As sludge enters the filter body, the relative movement between the fixed and moving rings allows filtrate to be rapidly discharged through the gaps, facilitating sludge thickening. The sludge then moves toward the dewatering section, where the chamber volume decreases, increasing internal pressure. The back-pressure plate at the sludge discharge outlet further enhances dewatering, resulting in continuous dry sludge discharge.

Unlike conventional filter presses, the screw press dewatering machine replaces filter cloth with a moving-fixed ring mechanism. The continuous movement of the moving rings against the fixed rings prevents clogging, ensuring a self-cleaning operation.

Additionally, the dynamic impact of the moving-fixed ring structure adjusts with sludge viscosity variations. The higher the viscosity, the greater the enhancement in flow velocity, leading to improved dewatering efficiency. As a result, the screw press sludge dewatering machine exhibits superior resistance to oily sludge, does not clog, does not require high-pressure water cleaning, and prevents secondary pollution, making it particularly suitable for dewatering viscous sludge in the petrochemical industry.

3.2 Specialized Screw Press Dewatering Equipment for the Petrochemical Industry

To address the challenges posed by high oil content, high viscosity, fine particles, and difficult dewatering in petrochemical sludge, some domestic companies have independently developed industry-specific screw press sludge dewatering machines with the following features:

(1) Specialized screw shaft design tailored for conveying viscous petrochemical sludge.

(2) Enhanced drive system to accommodate high-resistance oily sludge.

(3) Corrosion-resistant moving-fixed rings suitable for high-chloride petrochemical environments.

(4) Explosion-proof design meeting ExdIIBT4 standards, ensuring compliance with petrochemical safety regulations.

(5) Containerized system designed for offshore oil platforms.

(6) Specialized flocculation dosing system to overcome difficulties in sludge flocculation and settling.

(7) Proprietary flocculant technology reducing the specific resistance of oily sludge.

(8) Compact footprint with high dewatering efficiency for cost-effective sludge management.

4. Conclusion

(1) Proven operational results demonstrate that screw press sludge dewatering technology operates stably and effectively, achieving sludge moisture content of 75%-80% post-dewatering. The system overcomes the clogging issues of belt filter presses and plate-and-frame presses while surpassing centrifuges in separation efficiency. This maximizes the benefits of sludge reduction, harmless treatment, and resource recovery, significantly lowering costs related to dewatering, transportation, drying, and incineration while ensuring treated water meets discharge or reuse standards.

(2) Excellent economic, environmental, and social benefits have led to widespread adoption of screw press sludge dewatering technology in petrochemical enterprises. Based on field experience, this system presents a highly promising application for sludge management in the petrochemical industry.