Primary wastewater treatment
Primary wastewater treatment is the initial step in the wastewater treatment process, which focuses on the removal of large solid and organic particles from sewage or industrial effluent. This stage is crucial in order to minimize the burden on the following treatment steps and enhance the efficiency of the treatment system. Here’s an overview of the primary wastewater treatment process and its components:
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Goals of Primary Sewage Treatment
- Removal of Suspended Solids: To remove large particles and sediments from the wastewater before it is treated.
- Reduction of Biochemical Oxygen Demand (BOD): To reduce the amount of the organic matter that needs oxygen to decompose in the next stages of water treatment.
- Preparation for Secondary Treatment: To prepare the wastewater for biological treatment processes that are carried out after primary treatment.
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The following are the key components of primary wastewater treatment:
- Screening
- Purpose: To filter out large particles like rags, sticks, plastics and any other solid materials that may cause damage to the equipment or hinder the treatment processes.
- Method: Wastewater flows through bar screens of varying mesh sizes. Coarse screens are used to remove large objects while fine screens are used to trap small objects.
- Equipment: Bar screens, drum screens, and step screens.
- Grit Removal
- Purpose: To remove materials such as sand, gravel, and small stones that may cause abrasion and wear on equipment.
- Method: Wastewater passes through grit chambers where the velocity is low and heavy particles settle down while the lighter organic matter floats.
- Equipment: Grit chambers and aerated grit tanks.
- Primary Sedimentation
- Purpose: To remove out the suspended solids and to decrease the BOD of the wastewater.
- Method: Wastewater is stored in large containers known as primary clarifiers where the heavy particles sink to the bottom forming sludge while the lighter particles such as oils and grease float on the surface.
- Equipment: Primary clarifiers or sedimentation tanks equipped with scrapers and skimmers.
- Skimming
- Purpose: To skim off floating materials like oils, grease, and scum from the surface of the wastewater.
- Method: Surface skimmers gather floating debris, which is then scooped and disposed of.
- Equipment: Floating skimmers and oil separators.
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Process Overview
- Influent Screening: Raw sewage flows into the treatment plant and goes through a preliminary step of screening.
- Grit Removal: The screened water is then discharged to grit chambers where sediments with high densities sink to the bottom.
- Primary Sedimentation: The wastewater then flows to primary clarifiers where the suspended solids settle at the bottom as sludge and scum floats on the surface.
- Sludge and Scum Removal: The settled sludge is pumped and transferred to the sludge processing stations while the surface scum is also removed for further treatment.
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Outcomes of Primary Treatment
- Reduction of Solids: Reduces the concentration of suspended solids in the wastewater by 50-70%.
- Lowered BOD: Reduces the BOD by 25-40% and enhances the quality of water that is fit for secondary treatment.
- Preparation for Biological Treatment: Treats the wastewater for biological processes that further decompose the organic materials.
Conclusion
The primary wastewater treatment is an important initial stage of wastewater treatment. It helps to reduce the organic load by taking out large solids, grit, and scum and thus makes the wastewater ready for further treatment. Primary treatment plays an important role in the overall performance of the wastewater treatment system, and thus public health and the environment are safeguarded.