PRIMARY WASTEWATER TREATMENT

 PRIMARY WASTEWATER  TREATMENT

                                                                                                         Presenter name:

                                                                                                                KRISHNA T

Physical processes that remove non-homogenize solids and

Homogenize the remaining effluent.

The purpose of wastewater treatment is to remove the

contaminates from wastewater to that treated water can meet the

Acceptable quality standards.

• The quality standards usually depend upon whether water will be

reused or discharged into a receiving stream.

• These types of processes can be classified as

􀀀 Physical

􀀀 Chemical

􀀀 Biological

• The primary treatment of wastewater involves two essential steps:

Pretreatment and sedimentation.

• Pretreatment is a crucial phase where large suspended and

floating solids are removed from the wastewater.

• Flow measurement and prechlorination are sometimes included in

the pretreatment process to prevent odors.

• Sedimentation follows the pretreatment and aids in further

separation of particles from the wastewater.• Efficient pretreatment ensures smooth operation in the subsequent treatment processes. • The removal of suspended solids during pretreatment prevents interference with later treatment stages. • Prechlorination is applied as part of pretreatment to avoid any potential odors in the following processes.

Pretreatment in Wastewater Treatment

•Pretreatment involves two key steps: screening and grit removal. •Screens come in various sizes and shapes, tailored to the characteristics of the solids to be removed. •Cleaning methods include manual and mechanical approaches.

Screening Process

• Fixed bar screens are commonly used in domestic wastewater treatment facilities. • Fixed bar screens consist of parallel metal bars. •Apertures range from 20-60 mm for coarse screens, and 10-20 mm for medium and fine screens.

Coarse Screens •Coarse screens eliminate large solids before fine screening. • Helps prevent clogging issues in downstream processes. •Ensures effective functioning of the fine screen.

Enhanced Efficiency with Fine Screens

 • Fine screens remove smaller particles from wastewater. 

•Apertures range from 10-20 mm, targeting finer solids. 

•Preceded by coarse screens for optimal performance.

Grit Removal:

                        •Grit removal is a crucial aspect of pretreatment.

•Involves eliminating heavy particles like sand and gravel. 

•Ensures protection of downstream equipment from abrasion.

Sedimentation Process in Waste Treatment:

Sedimentation removes settleable solids from water using gravitational settling. 

• Key goal: separate sludge from clear liquid for effective waste treatment.

Tank Designs:

Three main tank designs: rectangular horizontal flow, circular radial flow, and vertical flow basins.

 • Each design has unique mechanisms for sludge removal and effluent clarification.

Rectangular Horizontal Flow Tanks:

Feed is introduced at one end, and overflow is collected at the surface. • Floating material scraped into a screen, and settled solids into a sludge hopper. • •Endless conveyor mechanism aids in the process.

Circular Radial Flow Tanks:

Feed enters through the center well, and clarified effluent is collected at the periphery. •Rotary sludge scraper forces settled sludge into a central hopper. •Surface skimming board removes scum efficiently.

Vertical Flow Tanks:

Suitable for small treatment plants. • Feed applied at tank bottom, effluent rises through suspension. •Sludge blanket maintained at the bottom, acts as a filter for small particles. •Careful control of sludge withdrawal and bleeding is necessary to avoid losing the blanket.

Importance of Sludge Blanket:

•Sludge blanket vital for effective particle filtration. • Maintains suspended particles, and acts as a filter layer. • Control measures needed to prevent blanket loss.

Sedimentation:

• Crucial process in water treatment and quality management. • Utilizes sedimentation basins or clarifiers. • Remove suspended particles and solids from water. • It results in improved water clarity and quality. • Clarifies water by allowing particles to settle.

Flotation:

Particles of density very close to that of water are very difficult to settle in normal sedimentation tanks and take a long time to separate. •In such cases, the separation can be specced up by a crating of the effluent whereby air bubbles are attached to the suspended matter. •The two types of flotation used commonly are a) dispersed-air flotation and b) dissolved-air flotation.

Dispersed-Air Flotation:

Air is introduced directly into the liquid. • Done through a revolving impeller or diffuser. • Generates 1 mm diameter air bubbles. • Turbulence from bubbles can break up fragile floc particles. • Less favored for municipal wastewater treatment due to turbulence. • Limited use in treating industrial wastes with oil, grease, and fine powders.

Dissolved-Air Flotation:

Air brought into contact with wastewater at high pressure. • Pressure reduced to release micron-sized bubbles. • Micron-sized bubbles carry suspended solids and oil to the tank surface. • Pressure systems can degrade flocculent suspensions or oil emulsions. • Recirculation of clear effluent prevents degradation.

Dissolved-Air Flotation:

Compressed air is introduced into the recycle pump discharge for contact. •Recycled flow mixed with influent for flotation. •Pressurization flow based on the required air-to-solids ratio. •Effective ratios range from 0.01 to 0.05, with design value often 0.02. •Residence time in the flotation tank is about 30 minutes.

REFERENCES:

Environmental Pollution and Control, 4th ed. by J. Jeffrey Peirce, P. Aarne Vesilind, Ruth F. Weiner •Environmental Pollution and Control,5th ed. by C S Rao •https://www.keeservices.com/dispersed-air-flotation-syst ems/

                                 

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