What Is the Process of Municipal Sewage Treatment?

 Introduction

Municipal sewage treatment mainly refers to the domestic, commercial, and industrial wastewater that flows into the sewers. Of course, it also includes rainwater and meltwater from snow. These waters contain a large amount of food residues, nutrients such as nitrogen and phosphorus, as well as various microorganisms and pathogens, etc.

For the sustainable utilization of water resources and ecological protection, we generally treat this wastewater before discharging it into rivers and oceans. Today, we will explain in detail the process flow of municipal sewage treatment.


（1）Pretreatment

Pretreatment is the first step in sewage treatment, which includes:

  • Screening Treatment: The screen can intercept large pieces of garbage to prevent them from clogging the water pumps.
  • Pump Station Lifting: The pump station lifting can increase the water head of the sewage, allowing the sewage to flow through subsequent treatment structures by gravity.
  • Grit Removal Treatment: Grit removal treatment can remove sand and stones in the sewage, reducing their sedimentation in subsequent structures. This can avoid equipment blockage caused by sand deposition. Generally speaking, the grit chamber can intercept sand particles with a particle size of 0.15 - 0.2 millimeters or more. The aerated grit chamber can also keep the organic particles suspended while removing the sand and scrub off the organic pollutants on the surface of the sand particles.


（2）Primary Treatment

Primary treatment mainly refers to the primary sedimentation tank. It can reduce the degree of sewage putrefaction through sedimentation, filtration, or appropriate aeration. With it, we can remove about 50% of the suspended solids and about 25% of the BOD5 (biochemical oxygen demand), as well as most of the particulate matter with a particle size of more than 100 micrometers. It should be noted that the sewage treated by primary treatment still does not meet the discharge standards and requires secondary treatment.


（3）Secondary Treatment

Secondary treatment is mainly composed of an aeration tank and a secondary sedimentation tank. Among them:

  • Aeration Tank: It uses an aeration fan and a special aeration device to supply oxygen to the tank, and through the metabolism of microorganisms, most of the pollutants in the sewage are converted into CO2 and H2O. This is sometimes also called the aerobic technology.
  • Secondary Sedimentation Tank: After that, the water mixed with the microorganisms in the aeration tank flows into the secondary sedimentation tank after the reaction. They settle at the bottom of the tank and are sent back to the front of the aeration tank through pipes and pumps to mix with the new sewage. The clarified treated water on the top of the secondary sedimentation tank flows out of the sewage treatment plant through the effluent weir. This step is mainly used to remove the organic pollutants in the water in the form of colloids and dissolved substances.


（4）Advanced Treatment

Advanced treatment is carried out to meet high water discharge standards and the reuse of industrial wastewater. It can remove the refractory organic substances, nitrogen, phosphorus, etc. remaining after secondary treatment to prevent eutrophication of water bodies. Common advanced treatment includes coagulation sedimentation and filtration, and a chlorination step and a contact tank are also designed at the end.


（5）Sludge Treatment

Sludge treatment mainly consists of steps such as thickening, digestion, dewatering, composting, or domestic landfill. In this process, we press the sludge into sludge cake through sludge dewatering machine or gravitational means to reduce its water content. We also generate biogas through anaerobic mesophilic digestion, and this biogas can be burned as an energy source or used for power generation. However, for the sludge with excessive heavy metal content, we generally carry out landfill or incineration.


（6）Discharge

At this time, the effluent quality of municipal sewage can meet some common discharge standards. They can be discharged into natural water bodies, used for farmland irrigation, industrial water, etc.

If you are interested in some of the treatment links mentioned in the article, such as the principle of a certain technology, the applicable scenarios of different processes, etc., or if you want to learn more about other aspects of the sewage treatment industry, please feel free to post in the comments below or contact us. As a leading solution provider in the water treatment industry, AquaSust is confident to answer your questions. 

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