Sludge Treatment and Disposal Technology (2)

6. What Are the Technical Parameters of Sludge That the Drying Process Focuses on?

The composition of sludge changes according to the water quality of the sewage treatment plant, and of course it will also be largely affected by the sewage treatment process. The factor that will ultimately have a significant impact on the drying process is the flocculant dosage. For the drying process, the following factors are worth noting:

(1) The flocculant content in the sludge.

(2) The viscosity and elasticity of the sludge.

(3) The proportion of organic matter in the dry matter.

(4) The proportion of abrasive components (such as sand, stone, etc.).

(5) The concentration of corrosive components (such as chlorine, sulfur, etc.).

(6) The percentage of oily substances.

7. How Does Dewatered Sludge Come From?

The pollutants and nutrients in sewage are aggregated under the action of a large number of proliferating bacteria and chemical agents. The gradually enlarged particles finally settle in the water to form sludge. Adding polymer flocculants and using physical methods for concentration can remove most or part of free water, and form the dewatered sludge.

Therefore, the organic matter in the sludge after biological treatment is mainly composed of these microorganisms and bacteria.

Due to different sludge components, the organic matter content of undigested municipal sewage sludge may account for 60% to 75% of the dry matter. It will be half after efficient digestion treatment.

Organic nitrates are the main active ingredients in sludge. When applied to the soil, nitrates can improve the soil through biodegradation.

8. Why Does Dewatered Sludge Have a High Moisture Content?

The sludge produced from the sludge dewatering workshop of the sewage treatment plant has strong fluidity. This is because its moisture content is very high, generally 75% to 85%. According to analysis, the sludge is very tightly bound to water and has different phases:

(1) Free water: can be removed by gravity sedimentation and mechanical action.

(2) Physically bound water: requires more energy to remove (such as heating).

① Capillary/interstitial water.

② Colloidal/surface adsorbed water

(3) Chemically bound water: can only be removed by breaking chemical bonds, called "equilibrium water".

① Water in cells.

② Molecular water.

9. Is Sludge Harmful to the Human Health?

During the sewage treatment process, bacteria and most parasites still stay in the sludge. Viruses adsorb on the particles in the sewage and deposit to the sludge as the particles precipitate.

The number of pathogens in the sludge is in the billions per gram. These microorganisms include: Escherichia coli, coliform bacteria, fecal streptococci, bacteriophages, Salmonella, Shigella, Pseudomonas aeruginosa, parasite eggs/larvae, ascaris, whipworms, whipworms, toxocara, hymenoptera larvae, enterovirus, etc.

Since the source of municipal sewage is the human living environment, Escherichia coli, coliform bacteria, fecal streptococci, etc. are normal discharges from the rectum of mammals, and their numbers remain basically constant in sewage and sludge. The ratio of other pathogens such as salmonella, dysentery bacteria, enteroviruses (e.g. poliovirus, coxsackievirus, hepatitis virus, rotavirus) and parasites (e.g. roundworms, whipworms, and amoeba) in sewage/sludge is related to the prevalence of local infectious diseases.

Obviously, if the sludge after mechanical dewatering is not properly disposed and enters the human food chain, it will inevitably lead to the spread of diseases.

10. What Are the Characteristics of Industrial Sludge?

According to different sources, there are significant differences between various types of industrial sludge. These differences are mainly reflected in its viscosity, hygroscopicity, pollutant properties, oil content, moisture content, organic matter ratio, inorganic matter ratio and other aspects.

Compared with municipal sludge, its high viscosity, high moisture content and high inorganic matter ratio sometimes make it more difficult to dispose.

Sludge from the chemical and pharmaceutical industries must be properly disposed because of its high concentration of pollutants. Sludge from the petroleum, metallurgy, leather making, fermentation, food, slaughtering and other industries can be disposed separately and recycled.