What is the Application of Urban Sewage Sludge?

 Introduction

Urban sewage sludge is a semi-solid or solid waste generated after the treatment of our domestic wastewater. It contains a large amount of potential pollutants such as organic matters, microorganisms, and heavy metals. If we do not treat it, the harmful substances in the sludge may seep into the soil, affecting the growth of plants, or cause eutrophication of water bodies. People may also experience symptoms such as difficulty in breathing, headache, and nausea after coming into contact with this sludge. Today, let's take a look at how to properly handle this sludge and what we can do with it.

 

How is Urban Sewage Sludge Generated?

Domestic sewage, commercial wastewater, and industrial wastewater can all generate sludge after complex treatment. But how exactly do we obtain this sludge?

(1) Pretreatment: The formation of urban sewage sludge runs through the entire sewage treatment process chain. In the pretreatment stage, we use physical methods such as grid interception and sand settling tank sedimentation to remove about 30% of the suspended solids in the sewage. This includes small twigs, plastic bags, and sanitary products in the water.

(2) Primary Sedimentation Tank Sedimentation: Next, the sewage will flow into the primary sedimentation tank. Under the action of gravity, the denser suspended particles in the sewage gradually settle to the bottom of the tank, forming primary sedimentation sludge. This process can remove about 50% - 70% of the suspended solids in the sewage.

(3) Biological Treatment: The sewage that has passed through primary sedimentation will continue to enter the biological treatment unit, such as the MBBR Bio Filter Media and Activated Sludge system. In the MBBR system, a large number of microorganisms feed on the organic matters in the sewage and decompose and transform them through metabolism. In addition, these microorganisms will aggregate to form flocs during the process of growth and reproduction. These flocs, together with other impurities in the sewage, constitute the activated sludge.

(4) Secondary Sedimentation Tank Separation: Here, the activated sludge is separated from the treated water. Part of the sludge that settles to the bottom of the secondary sedimentation tank is returned to the biological treatment unit to maintain the concentration of microorganisms. The other part becomes the excess sludge, which, together with the primary sedimentation sludge, forms the final sludge through processes such as filtration and dewatering by sludge dewatering machines.

What Does Sewage Sludge Contain?

Due to differences in sources and treatment processes, the composition of urban sewage sludge may have slight differences, but their main components basically include:

 Organic Components (40-70%): Microbial zoogloea, cellulose, protein, etc.

 Inorganic Minerals (20-50%): Silicates, carbonates, metal oxides

 Nutrient Elements (TN 2-8%, TP 1-5%): Essential nutrients for plants such as nitrogen, phosphorus, and potassium

 Trace Pollutants: Heavy metals (Zn, Cu, Cr, etc.), persistent organic matters (polycyclic aromatic hydrocarbons, dioxins)

 Pathogenic Microorganisms: Escherichia coli, parasite eggs, etc.

What Can We Do with This Sludge?

Soil Conditioner

The sludge contains rich organic matters and nutrient elements such as nitrogen, phosphorus, and potassium, which can increase the fertility of the soil. These components make the soil better at retaining water and fertilizers, thus promoting plant growth. For example, we can use this sludge to increase the crop yield in poor farmland or degraded land.

 

Land Reclamation

Some lands in the city may become uneven due to activities such as mining and construction. We can mix the sludge with other filling materials and fill it into the abandoned mine pits or collapsed areas of these lands. The elements such as nitrogen, phosphorus, and potassium in the sludge can provide conditions for the growth of vegetation and promote ecological restoration.

 

Brick Making

After processes such as dewatering, drying, and mixing with additives, the sludge can be used as a raw material for brick making just like clay. The sludge has a higher density, and the bricks made from it are strong and have good heat insulation properties. This can reduce people's resource dependence on natural clay and production costs.

 

Cement Production

We can also add an appropriate amount of pretreated sludge in cement production to replace some raw materials, such as clay and limestone. The organic matters in the sludge can burn to provide heat during the cement calcination process. The mineral components it contains can also participate in the formation reaction of cement clinker, improving the quality of cement.

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Biochar Production

Of course, this sludge can also be used to prepare biochar through processes such as pyrolysis. In the agricultural field, biochar can adsorb pollutants in the soil due to its large number of pore structures. At the same time, it can also serve as a slow-release carrier for fertilizers, prolonging the action time of fertilizers and improving the utilization rate of fertilizers.

Conclusion

Although urban sewage sludge is a product of sewage treatment, through reasonable treatment and utilization, it can be turned into a valuable resource. We can use it for purposes such as soil improvement, land filling, brick making, cement production, and biochar production. AquaSust has been providing sludge solutions for wastewater with medium and high solid content since 2013. If you are interested in this, please let me know in the comments or contact our experts: pro@aquasust.com.