Disposal and Comprehensive Utilization of Sludge in Urban Sewage Treatment Plants

1. Introduction

With the rapid development of modern cities, the composition of industrial and domestic wastewater has become increasingly complex, leading to a greater variety of harmful substances in the sludge remaining after sewage treatment. If sewage treatment plant sludge is not properly disposed of, it can not only affect the living environment and endanger public health but also occupy valuable social resources. Therefore, achieving harmless and resource-efficient sludge disposal has become a crucial issue in urban development.

2. Goals of Sludge Disposal

The ultimate goal of sludge treatment is to meet national standards and minimize its impact on the living environment. The process follows fundamental principles to achieve disposal objectives. The primary goals of sludge treatment and disposal include the followings.

2.1 Minimization

Sludge volume reduction is achieved by dewatering mechanisms that separate free water, bound water, and capillary water from the sludge, making subsequent processing easier. The sludge generated during sewage treatment, particularly primary sludge, can have a water content as high as 90% to 95%. Without dewatering, downstream sludge treatment systems would face excessive burdens, leading to resource wastage. Volume reduction not only decreases sludge size but also facilitates storage, transportation, and further processing.

2.2 Stabilization

Urban sewage treatment often involves biological methods where microorganisms break down organic pollutants under specific conditions, purifying the sewage. However, the treated sludge still contains many highly active microorganisms, increasing the likelihood of sludge degradation during processing. Therefore, residual sludge must undergo stabilization treatments to prevent spoilage during storage, making subsequent processing more manageable.

2.3 Harmlessness

Ensuring that sludge is rendered harmless is essential for both resource utilization and sanitary landfilling. Even after dewatering, sludge contains a high concentration of pathogenic bacteria from municipal sewage. If left untreated, it can cause disease transmission and environmental pollution. By achieving harmlessness and meeting sanitation standards, sludge can be used in various applications.

2.4 Resource Utilization

Properly treated sludge has significant potential for resource utilization. It contains organic materials such as nitrogen and phosphorus, which are essential nutrients for plant growth. Processed sludge can be turned into a high-quality fertilizer. Additionally, anaerobic digestion of sludge generates biogas, while dried sludge cakes can be used as fuel, fulfilling the goal of turning waste into valuable resources. The most desirable sludge disposal method is to convert it into reusable resources.

3. Common Sludge Disposal Methods

After undergoing several sludge treatment processes, disposal is the final and crucial step. Choosing a sludge disposal method requires consideration of both technical and economic factors. The preferred approach prioritizes resource utilization, followed by energy recovery through incineration, and finally, sanitary landfill for non-recyclable sludge.

3.1 Fertilizer Utilization

Sludge disposal should first consider fertilizer production through composting. Composting involves anaerobic digestion of sludge under high temperatures to break down organic matter into humus, making it useful as a soil conditioner. The anaerobic digestion process also produces biogas, which can be used as an energy source. Furthermore, the high temperatures involved in composting effectively eliminate pathogenic bacteria and parasites, ensuring both harmlessness and resource utilization.

3.2 Use in Building Materials

Sludge can be processed into construction materials such as bricks, cement, and tiles through dewatering, drying, and incineration. This method not only promotes resource utilization but also reduces environmental pollution caused by direct landfill disposal. However, due to the high technical requirements and production costs of sludge-based construction materials, economic feasibility should be carefully evaluated.

3.3 Incineration

Sludge incineration converts sludge into energy, making it a widely adopted method worldwide. It effectively reduces sludge volume, eliminates pathogens, and stabilizes sludge properties. Additionally, the energy released during incineration can be used for electricity generation. However, incomplete combustion due to insufficient oxygen supply can produce toxic gases such as nitrogen oxides and sulfur dioxide. Moreover, heavy metals, ash, and smoke generated during incineration pose environmental challenges. Incineration requires advanced sludge dewatering technology, making it an expensive disposal method that must be assessed based on economic and environmental considerations.

3.4 Sanitary Landfilling

Compared to other methods, sanitary landfilling is a simpler and more cost-effective sludge disposal method, though not an ideal solution. It involves reducing, stabilizing, and rendering sludge harmless before burying it in designated landfill sites. Despite its simplicity and low cost, landfilling is constrained by land availability, particularly in regions with limited land resources. Additionally, potential groundwater contamination must be considered when selecting landfill sites.

4. Recommendations for Sludge Disposal Development

Both domestic and international research efforts are advancing sludge disposal techniques, yet practical implementation still faces significant challenges. To ensure steady progress in sludge disposal, the following recommendations are proposed:

(1) Improvement of Laws and Regulations – Establishing comprehensive legal frameworks and policies is essential for standardized sludge treatment and enforcement against improper disposal practices.

(2) Increased Investment – Achieving ideal sludge treatment standards requires financial investment, not only in advanced equipment but also in skilled personnel.

(3) Technological Innovation – Existing sludge treatment and disposal technologies have limitations; therefore, continuous technological advancements are needed to enhance efficiency.

(4) Public Awareness and Education – Leveraging media platforms such as television, the internet, and mobile applications to promote sludge reuse can help eliminate public misconceptions and encourage widespread adoption.

5. Conclusion

As cities continue to expand, the volume of sewage and sludge requiring treatment increases. Various sludge disposal methods are available worldwide, each tailored to local conditions. However, the ultimate goal remains the same: to stabilize and render sludge harmless while identifying cost-effective and environmentally sustainable solutions. When selecting a disposal method, both technical feasibility and regional economic and geographic conditions must be considered. The best approach is to transform sludge from waste into a valuable resource, ensuring its beneficial reuse for society.