Why is sludge treatment of Sewage-treatment Plant so important?
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Why is sludge treatment of Sewage-treatment Plant so important?

Views: 37     Author: Ryde Environment     Publish Time: 2019-05-17      Origin: Ryde Environment

The treatment of Sludge from Sewage-treatment Plant is becoming a thorny problem. 

The reason why the sludge problem is becoming more and more prominent is as following:

  • After long-term exploration and testing, A number of sewage treatment plants built in the early stage have still not found a good disposal plan.

General sludge treatment process of Sewage-treatment Plant : Sludge concentration -> Sludge dewatering -> Sludge drying -> Sludge transportation -> Sludge landfill

Mud scraper and concentrator are usually used in concentrating stage. Multi disc screw press and belt filter press are usually used in dewatering stage. Sludge dryer is usually used in drying stage. 

  • There are fewer and fewer resources for storage, disposal and landfill.

  • Enhanced supervision by environmental protection authorities around the world has led to unprecedented growth and expansion of sewage treatment worldwide.

According to China's urban population base, even if only 100 million people's sewage is treated, 25,000 tons of sludge cake with 20% solid content will be produced every day.

If these mud cakes are stacked at a maximum of 2 meters, 600 international standard football stadiums are required each year. For the city, the surrounding land resources have been difficult to meet the needs.

Therefore, the reasonable disposal of sludge must be carried out sooner or later.

We will discuss this issue from the following aspects:

① Where does the sludge come from?

② How can sludge be produced from wastewater?

③ According to sewage treatment process, how to classify sludge?

④ How does dewatered sludge come from?

⑤ What is the water content of dewatered sludge?

⑥ Is sludge harmful to human body?

Where does the sludge from Sewage-treatment Plant come from?

Sludge is a solid sediment produced by the process of water and sewage treatment. Because the properties of sludge vary greatly, it is necessary to classify them, and their treatment and disposal are also different.

According to its source, it can be divided into:

  • Municipal sludge, also known as sewage sludge or civil sludge, mainly refers to sludge from sewage plants, which is the largest type of sludge. In addition, sludge from waterworks comes from municipal facilities and can be classified into this category.

  • Pipeline sludge, sludge from drainage collection system.

  • River and lake silt, which comes from rivers and lakes.

  • Industrial sludge is a mixture of solid and water, oil, chemical pollution and organic matter produced by various industrial processes. In non-specific environment, sludge generally refers to municipal sewage sludge.

How can sludge be produced from wastewater?

Wastewater treatment consists of a series of physical, chemical and biological processes:

  • Sedimentation (using or not using chemical flocculants), filtration, filtration

  • Organic complexes are produced by aerobic and anaerobic treatment of microorganisms.

  • Biochemical denitrification and dephosphorization

  • In the process of wastewater purification, pollutants in wastewater are centrally removed by biochemical degradation.

  • Biological treatment can degrade most organic pollutants into water and gas (aerobic treatment produces CO2 and O2, anaerobic treatment produces CH4-based gas), while metal pollutants (including heavy metals) cannot be treated and concentrated in sludge.

Sludge is a solid substance produced by sewage treatment at all levels, and it is an inevitable by-product of sewage treatment plants.

According to sewage treatment process, how to classify sludge?

Sludge from sewage treatment plants can be divided into the following categories according to the different treatment process levels:

  • Primary sludge: sludge is treated only physically and chemically

  • Secondary sludge: sludge is biotreated

  • Tertiary sludge: sludge treated by dephosphorization or denitrification

According to the nature of sludge, it can be divided into:

  • Undigested raw sludge

  • Digested sludge digestion can be divided into aerobic and anaerobic digestion.

  • The physicochemical properties of sludge at different levels are different, and the properties of digested and undigested sludge are more different.

Many back-end treatment processes must understand the characteristics of the front-end sludge to determine its treatment mode.

How does dewatered sludge come from?

Pollutants and nutrients in sewage accumulate under the action of a large number of bacteria and chemicals.

Gradually increasing aggregate structure eventually settles down in water and forms sludge. Adding macromolecule flocculant and concentrating by physical method can remove most or part of the so-called free water and form the dewatered sludge that we see.

Therefore, the organic composition of sludge obtained by biological treatment is mainly composed of these microorganisms and bacteria. Due to the different composition of sludge, the organic matter content of undigested municipal sewage sludge may account for 60%-75% of dry matter.

After high-efficiency digestion, it is halved.

Organic nitrate is the main active ingredient in sludge. When applied to the soil, nitrate can be biodegraded to improve the soil.

What is the water content of dewatered sludge?

Sludge from sludge dewatering workshop of sewage treatment plant has strong fluidity because of its high water content, which is generally between 75% and 85%.

This is determined by the nature of the sludge itself.

According to the analysis, sludge and water molecules are closely bound and have different phases:

  • Free water: removal by gravity precipitation and mechanical action

  • Physically bound water: more energy is needed to remove (e.g. heating)

  • Capillary/interstitial water

  • Colloidal/Surface Adsorbed Water

  • Chemically bound water: it be removed only by breaking chemical bonds. It is known as "balanced water"

  • Intracellular water

  • Molecular water

water content of dewatered sludge

Is sludge from Sewage-treatment Plant harmful to human body?

In the process of sewage treatment, bacteria and most parasitic organisms remain in the sludge. Viruses can be adsorbed on the particles in the sewage and deposited into the sludge with the precipitation of particles.

The number of pathogenic bacteria in raw sludge is billions per gram. These microorganisms include: coliform bacteria, coliform fecal bacteria, Streptococcus faecalis,

Bacteriophages, Salmonella, dysentery bacteria, Trichoderma aeruginosa, parasitic eggs/larvae, ascaris, whipworm, colonial whipworm, Toxoplasma, Hymenoptera larvae, enteroviruses, etc.

Because the sources of municipal sewage are human living environment, coliform bacteria, coliform fecal bacteria, Streptococcus faecalis and so on, which are the normal excreta of mammalian rectum, their quantity is basically constant in sewage and sludge.

The proportion of other pathogens such as Salmonella, dysentery, enteroviruses (e.g. poliovirus, coxsackievirus, hepatitis virus, rotavirus) and parasites (e.g. ascaris, whipworm and entamoeba) in sewage/sludge is related to the prevalence of local infectious diseases.


Obviously, if the sludge after mechanical dewatering (such as belt filter press ) is not properly disposed of and enters the human food chain, it will inevitably lead to the spread of disease.


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