Primary Treatment of Wastewater

Primary treatment of wastewater is the second stage. It comes after preliminary treatment. Primary treatment removes solid materials and organic matter from wastewater.

This stage uses processes like

• Sedimentation
• Flotation

Sedimentation

Sedimentation uses gravity to separate solids from wastewater. In primary wastewater treatment, it happens in a large tank called a sedimentation tank. Wastewater enters this tank, where solids settle. The tank can be rectangular or circular, depending on the needs of the treatment facility.

The tank slows the flow of wastewater. This process allows heavier solids like sand, grit, and organic matter to settle at the bottom. The speed of settling depends on factors like particle size, shape, density, water temperature, and composition.

In the tank, heavier particles sink to form sludge, while lighter particles rise to form clarified water. The clarified water is removed from the top for further treatment. The sludge is collected from the bottom and treated separately.

Sedimentation removes many suspended solids, organic matter, and large particles. However, it does not remove dissolved pollutants like nitrogen, phosphorus, bacteria, or viruses. Therefore, wastewater typically goes through secondary treatment using biological and chemical methods to remove these contaminants.

Flotation

Flotation is a method of primary wastewater treatment that uses air to float solids to the surface for removal. This process, called Dissolved Air Flotation (DAF), creates air bubbles. These bubbles attach to solids and cause them to rise to the surface.

Steps of flotation process in primary treatment of sewage

1. Coagulation or Flocculation: A coagulant or flocculant is added to the wastewater to help solids form larger particles.
2. Introduction to Flotation Tank: The wastewater enters a flotation tank, where it is saturated with air under pressure.
3. Formation of Bubbles: Air is released, creating small bubbles that attach to solids in the water.
4. Solid Separation: Solids float to the surface, forming a sludge layer. This sludge is skimmed off.
5. Clarified Water Removal: The clear water is removed from the bottom of the tank and sent for further treatment.
6. Sludge Treatment: The sludge is treated separately to remove any remaining solids.

Advantages of Flotation

• Removes fine particles and light solids that sedimentation cannot.
• Effective at removing fats, oils, and grease.
• Reduces the load of solids and organic matter, improving downstream treatment efficiency.

Limitations

• More complex and expensive than sedimentation.
• Requires specialized equipment to dissolve air and skim solids.

The choice between flotation and sedimentation depends on factors like cost, complexity, and the type of solids in the wastewater. Flotation is often used when sedimentation is less effective.

Effluent Quality: After primary treatment of wastewater

Effluent quality refers to the condition of treated wastewater discharged from a primary treatment plant. It depends on factors such as plant design, operation, influent quality, and treatment methods.

Key Parameters to Measure Effluent Quality:

1. Suspended Solids (mg/L):
   Indicates how well sedimentation or flotation has removed solid particles.
2. Biochemical Oxygen Demand (BOD) (mg/L):
   Measures organic matter content, reflecting how effectively the plant removes pollutants.
3. Nitrogen and Phosphorus:
   Regulated nutrients that can cause environmental issues like eutrophication.
4. pH:
   Affects pollutant breakdown and impacts corrosion or scaling in infrastructure.
5. Pathogens:
   Includes bacteria and viruses that pose risks to health and the environment.

Effluent Quality Standards

Effluent quality must meet regulatory limits set in discharge permits. These permits define the maximum pollutant concentrations to avoid fines and environmental damage.

Primary Treatment Efficiency

• Removes: Up to 60% of suspended solids and 30% of BOD.
• Limitations: Does not effectively remove dissolved pollutants, pathogens, or nutrients.

Additional Processes for Better Effluent Quality

• Secondary Treatment: Further removes pollutants.
• Advanced Technologies: Methods like ultraviolet (UV) disinfection eliminate pathogens before discharge.

This ensures the effluent is safer for release into the environment and meets regulatory requirements.

Design and operation of primary clarifiers

Primary clarifiers (sedimentation tanks) play a vital role in primary wastewater treatment. Their design and operation directly influence the treatment’s efficiency.

Design of Primary Clarifiers

1. Tank Shape and Size:
   • Rectangular tanks are common, though circular tanks are also used.
   • Tanks must be large enough to provide sufficient retention time for solid settling.
2. Inlet and Outlet Design:
   • The inlet structure evenly distributes wastewater across the tank.
   • The outlet structure is placed opposite the inlet to remove clarified water without disturbing settled solids.
3. Baffles:
   • Installed to promote uniform flow and prevent short-circuiting.
4. Sludge Removal:
   • Includes mechanisms like scrapers to remove settled solids (sludge) from the bottom of the tank.

Operation of Primary Clarifiers

1. Wastewater Flow Rate:
   • Must be controlled to prevent short-circuiting and allow sufficient settling time.
2. Retention Time:
   • Optimized to maximize the settling of solids.
3. Sludge Removal:
   • Regularly remove sludge to prevent accumulation.
4. Inlet Control:
   • Ensures wastewater is evenly distributed across the tank.
5. Baffle Maintenance:
   • Regular inspection ensures proper functioning and good condition.
6. Clarifier Performance Monitoring:
   • Measure influent and effluent flow rates and suspended solids concentrations to ensure compliance with effluent quality standards.

Importance of Primary Clarifiers

Proper design and operation maximize the removal of suspended solids and pollutants. This improves effluent quality, reduces stress on secondary processes, and ensures environmental protection.

Summary

Primary treatment removes solid materials and organic matter from wastewater, reducing the load on downstream processes. It sets the stage for secondary and tertiary treatments, which handle dissolved pollutants, pathogens, and nutrients for comprehensive wastewater management.

FAQs

Read Also:

Wastewater Treatment Process Overview

Preliminary Treatment of Wastewater

Fluid Flow Operations