The main steps of sewage treatment

Wastewater treatment generally includes the following main steps:

pretreatment

Grille: A mechanical or manual grille is used to intercept larger solid debris in sewage, such as tree branches, plastic bottles, paper, etc., to prevent these debris from entering the subsequent treatment equipment and causing equipment blockage or damage.

Sand settling tank: Utilizing gravity, high-density inorganic particles such as sand and stones in sewage are settled to the bottom of the tank, thereby removing substances that may wear equipment or affect subsequent treatment processes.

Primary Treatment

Primary sedimentation tank: After the sewage enters the primary sedimentation tank, the suspended solids in the sewage gradually settle to the bottom of the tank under the action of gravity due to the slow water flow rate, which can remove some organic matter and suspended solids and reduce the load on subsequent treatment units.

Secondary treatment

Biological treatment: utilizing the action of microorganisms to decompose organic pollutants in wastewater into harmless substances such as carbon dioxide and water. Common biological treatment methods include activated sludge process, biofilm process, etc. Taking the activated sludge process as an example, sewage is mixed with activated sludge containing a large number of microorganisms. Under aeration conditions, microorganisms adsorb and decompose organic matter in the sewage, while also undergoing their own metabolism.

Secondary sedimentation tank: After biological treatment, the sewage enters the secondary sedimentation tank for solid-liquid separation, separating the activated sludge from the treated water. The settled sludge is partially returned to the biological treatment unit to maintain the concentration of microorganisms, while the remaining sludge is treated and disposed of.

tertiary treatment

Deep treatment: According to different effluent requirements, different deep treatment processes are adopted, such as filtration, adsorption, ion exchange, membrane separation, etc., to further remove residual organic matter, nitrogen, phosphorus, heavy metal ions and other pollutants in sewage, so as to achieve higher water quality standards, which can be used for reuse or discharged into more sensitive water bodies. For example, removing fine particles and residual organic matter from water through processes such as sand filtration and activated carbon adsorption; The use of reverse osmosis membrane technology can remove soluble salts and other small molecule pollutants.

Disinfection: Kill or remove pathogenic microorganisms to ensure that the microbiological indicators of the effluent meet the discharge standards. Common disinfection methods include chlorine gas disinfection, chlorine dioxide disinfection, ultraviolet disinfection, etc.

Sludge treatment and disposal

Sludge concentration: The residual sludge discharged from the secondary sedimentation tank is concentrated by gravity or mechanical methods to reduce its moisture content, decrease its volume, and facilitate subsequent treatment.

Sludge digestion: Under anaerobic or aerobic conditions, the concentrated sludge is subjected to biological treatment to decompose the organic matter in the sludge, further reduce the moisture content of the sludge, and generate recyclable energy such as biogas.

Sludge dewatering: After digestion, the sludge is mechanically dehydrated or naturally dried to reduce its moisture content to a certain extent, making it easier to transport and dispose of.

Sludge disposal: Dehydrated sludge can be disposed of through methods such as landfilling, incineration, composting, etc. The composted sludge can be used as soil amendments, and incineration can achieve the reduction and harmless treatment of sludge, and the heat generated by incineration can be utilized.

Different types of sewage treatment plants may make appropriate adjustments and combinations to the above steps based on the source of sewage, water quality characteristics, and treatment objectives.