Sludge is a sediment generated from the sewage or wastewater treatment process that contains suspended solids and active microorganisms. Sludge is categorized into raw sludge, surplus sludge, concentrated sludge, digested sludge, highly treated sludge, and chemical sludge.

(1) Raw sludge Raw sludge is sludge generated by the settling and flotation process of fine sand, organic matter, and suspended solids in the influent not removed from the settling basin ② Excess sludge Excess activated sludge from the final settling basin that is not required for wastewater management ③ Concentrated sludge Concentrated sludge is sludge that has been reduced in volume by concentrating the final sludge, ④ Digestion sludge Sludge generated from the anaerobic or aerobic digester treatment process ⑤ Highly treated sludge Sludge generated from the T-N and T-P treatment process to treat T-N and T-P in the influent, which has the same characteristics as surplus sludge, but has low density and settling properties ⑥ Chemical sludge Sludge that is agglomerated and precipitated using chemicals to facilitate biological treatment or wastewater that is difficult to be treated biologically, and differs depending on the characteristics of each wastewater.

The most widely utilized water treatment process worldwide is activated sludge. In general, in the activated sludge method, wastewater from households, factories, etc. is decomposed and purified by microorganisms in an abandonment tank and discharged through a final settling tank, and the excess sludge is concentrated and dewatered.A typical sewage treatment process is as follows.

Households: Wastewater from each household enters the sewage treatment plant through sewers.
(2) Sludge filter: The sludge filter is used to filter out dirt, sand, and other debris that comes in with sewage.
③ Influent pump: Sewage is pumped up by a large pump to treat sewage.
④ First settling cell: Sewage is settled by staying for a certain period of time, and the upper water is sent to the abandonment tank.
⑤ Aeration tank: Blowing air into the sewage, microorganisms are used to break down organic matter and multiply.
⑥ Final sedimentation cell: The activated sludge transferred from the abandonment tank stays for a certain period of time to settle the microorganisms and the upper part of the
Clear water is sent to the outfall.
⑦ Discharge: The cleaned water from the sewage treatment plant is discharged into the river after disinfection.

Depending on the type of operation, wastewater can be of different composition and contain many substances that interfere with biological treatment.
Therefore, wastewater treatment is pretreated according to the type of product and raw material, and biological
Biological treatment must be performed after removal of substances that affect the treatment. Pretreatment is performed to remove substances that interfere with the reduction of the pollution load.
The process is as follows.

Sludge generated during the treatment of sewage and wastewater is dewatered before its final disposal, i.e. landfill, drying, dumping or incineration. The process of dewatering, which is the process of reducing the volume of sludge, is very important for the final disposal of sludge. A typical sludge generation and treatment process is as follows.

Gravity settling of sludge from the water treatment process to reduce volume and increase concentration.

Using anaerobic digestion microorganisms, the input sludge is agitated in a sealed tank while maintaining a constant temperature to break down organic matter, resulting in sludge reduction and methane generation.

Dewatering reduces the volume and weight of sludge to a caked state, making it easier to transport and dispose of. The most important factor in dewatering is the water content. Moisture content is the percentage of water in the sludge, and there is a relationship between moisture content and volume.

For example, if the water content W1 = 991 TP3T and W2 = 801 TP3T, we can see that the volume ratio V1/V2 = 20, which means that if the water content is reduced to 801 TP3T by dewatering, the volume of the sludge is reduced by 20 times. This has the effect of reducing the cost of sludge treatment.