preliminary treatment...preliminary treatment operations to eliminate large objects, grit and grease...

Wastewater TreatmentWastewater Treatment

Preliminary TreatmentPreliminary TreatmentPreliminary TreatmentPreliminary Treatment

Preliminary TreatmentPreliminary TreatmentOperations to eliminate large objects, grit and grease along Operations to eliminate large objects, grit and grease along with flow measurement are referred to as preliminary with flow measurement are referred to as preliminary treatment systems. If these constituents are not removed, treatment systems. If these constituents are not removed, their presence would interfere with subsequent treatment their presence would interfere with subsequent treatment processes or mechanical equipment. processes or mechanical equipment.

Operations under preliminary treatment system include:Operations under preliminary treatment system include: Operations under preliminary treatment system include:Operations under preliminary treatment system include: ScreeningScreening

ComminutingComminuting

Grit removalGrit removal

Skimming tanksSkimming tanks

Floatation andFloatation and

PrePre--aerationaeration

Screening devices are used to remove coarse solids such as Screening devices are used to remove coarse solids such as sticks, rags, boards and other large objects from wastewater. sticks, rags, boards and other large objects from wastewater. The primary purpose of screen is to protect pump and The primary purpose of screen is to protect pump and other mechanical equipment and to prevent clogging of other mechanical equipment and to prevent clogging of valves and other appurtenances in the wastewater plant.valves and other appurtenances in the wastewater plant.

ScreeningScreening

valves and other appurtenances in the wastewater plant.valves and other appurtenances in the wastewater plant.

The screening element may consist of parallel bars, rods or The screening element may consist of parallel bars, rods or wires, grating, wire mesh or perforated plate. Solids retained wires, grating, wire mesh or perforated plate. Solids retained by coarse screens are usually removed by manual raking in by coarse screens are usually removed by manual raking in small plants, while mechanically cleaned units are used in small plants, while mechanically cleaned units are used in large plants. Fine screens are seldom used in municipal large plants. Fine screens are seldom used in municipal wastewater treatment. Their major use is in treatment of wastewater treatment. Their major use is in treatment of industrial wastes. They should be mechanically cleaned on a industrial wastes. They should be mechanically cleaned on a continual basis.continual basis.

ScreeningScreening

The quantity of solids removed by screening depends The quantity of solids removed by screening depends on screenon screen--opening size. Screened solids are coated with opening size. Screened solids are coated with organic materials of a very objectionable nature and organic materials of a very objectionable nature and should be promptly disposed off to prevent a health should be promptly disposed off to prevent a health hazard and/or nuisance conditions. Disposal in a hazard and/or nuisance conditions. Disposal in a

ScreeningScreening

hazard and/or nuisance conditions. Disposal in a hazard and/or nuisance conditions. Disposal in a sanitary landfill, grinding and returning to the sanitary landfill, grinding and returning to the wastewater flow and incineration are the most common wastewater flow and incineration are the most common disposal practices.disposal practices.

Cutting large suspended solids of wastewater into smaller, Cutting large suspended solids of wastewater into smaller, more uniform size is known as comminuting. The chopped more uniform size is known as comminuting. The chopped solids are then removed in sedimentation processes.solids are then removed in sedimentation processes.

Many kinds of Many kinds of comminutorcomminutor are available. Basic parts are available. Basic parts include a screen and cutting teeth. The screen may be a include a screen and cutting teeth. The screen may be a

ComminutingComminuting

include a screen and cutting teeth. The screen may be a include a screen and cutting teeth. The screen may be a slotted drum that rotates in the vertical plane. Stationary slotted drum that rotates in the vertical plane. Stationary teeth then shred material that is intercepted by the screen. teeth then shred material that is intercepted by the screen. Other types use a stationary semicircular screen and Other types use a stationary semicircular screen and rotating or oscillating cutting teeth. Another device, called a rotating or oscillating cutting teeth. Another device, called a barminutorbarminutor, uses a vertical bar screen with a cutting head , uses a vertical bar screen with a cutting head that travels up and down the rack of bars, shredding the that travels up and down the rack of bars, shredding the intercepting material.intercepting material.

ComminutorComminutor

Grit consists of pebbles, sand, silt, egg shells, glass, metal Grit consists of pebbles, sand, silt, egg shells, glass, metal fragments, fragments, bonechipsbonechips, seeds, coffee grounds and large , seeds, coffee grounds and large organic particles such as food wastes. organic particles such as food wastes.

Most of the substances in grit are abrasive in nature and Most of the substances in grit are abrasive in nature and

Grit RemovalGrit Removal

Most of the substances in grit are abrasive in nature and Most of the substances in grit are abrasive in nature and will cause accelerated wear on pumps and sludge handling will cause accelerated wear on pumps and sludge handling equipment which it comes in contact. Grit deposits in equipment which it comes in contact. Grit deposits in pipelines, channels and conduits. Grit occupies valuable pipelines, channels and conduits. Grit occupies valuable space in sludge digesters.space in sludge digesters.

Grit removal facilities basically consist of an enlarged Grit removal facilities basically consist of an enlarged channel area where reduced flow velocities allow grit to channel area where reduced flow velocities allow grit to settle out. Many configurations of grit tanks are available, settle out. Many configurations of grit tanks are available, with the most recent installations usually being channelwith the most recent installations usually being channel--type or aerated rectangular basins. The deposited grit is type or aerated rectangular basins. The deposited grit is


type or aerated rectangular basins. The deposited grit is type or aerated rectangular basins. The deposited grit is removed by mechanical scrappers. The most common removed by mechanical scrappers. The most common method of grit disposal is as fill, covered if necessary to method of grit disposal is as fill, covered if necessary to prevent objectionable condition. In some large plants, grit prevent objectionable condition. In some large plants, grit is incinerated with sludge.is incinerated with sludge.

Grit Removal Design (Example)Grit Removal Design (Example)


R ≤500

A skimming tank is a chamber so arranged that floating A skimming tank is a chamber so arranged that floating matter rises and remain on the surface of the wastewater matter rises and remain on the surface of the wastewater until removed, while the liquid flows out continuously until removed, while the liquid flows out continuously through deep outlets or under partitions, curtain walls or through deep outlets or under partitions, curtain walls or deep scum boards. The material collected on the surface deep scum boards. The material collected on the surface

Skimming TanksSkimming Tanks

deep scum boards. The material collected on the surface deep scum boards. The material collected on the surface of skimming tanks includes oil, grease, soap, piece of cork of skimming tanks includes oil, grease, soap, piece of cork and wood, vegetable debris and fruit skins. The floating and wood, vegetable debris and fruit skins. The floating matter is usually removed by continuous mechanical matter is usually removed by continuous mechanical skimming.skimming.

Floatation is used for the removal of suspended solids, oil and Floatation is used for the removal of suspended solids, oil and grease from wastewaters and for separation and concentration grease from wastewaters and for separation and concentration of sludge. Separation is brought by introducing fine gas of sludge. Separation is brought by introducing fine gas (usually air) bubbles into the liquid phase. The bubbles attach (usually air) bubbles into the liquid phase. The bubbles attach to the particulate matter and the buoyant force of the to the particulate matter and the buoyant force of the

FloatationFloatation

to the particulate matter and the buoyant force of the to the particulate matter and the buoyant force of the combined particle and gas bubbles is great enough to cause combined particle and gas bubbles is great enough to cause the particle to rise to the surface. The principal advantage of the particle to rise to the surface. The principal advantage of floatation over sedimentation is that very small or light floatation over sedimentation is that very small or light particles that settle slowly can be removed mere completely particles that settle slowly can be removed mere completely and in a shorter time. Once the particles have been floated to and in a shorter time. Once the particles have been floated to the surface, they can be collected by a skimming operation. the surface, they can be collected by a skimming operation.

Types of floatation:Types of floatation: Dissolved air floatation: Injection of air in which the Dissolved air floatation: Injection of air in which the

liquid is under pressure, followed by release of the liquid is under pressure, followed by release of the pressurepressure

FloatationFloatation

pressurepressure

Air floatation: Aeration at atmospheric pressureAir floatation: Aeration at atmospheric pressure

Vacuum floatation: Saturation with air at atmospheric Vacuum floatation: Saturation with air at atmospheric pressure, followed by application of a vacuum to the pressure, followed by application of a vacuum to the liquidliquid

PrePre--aeration is generally consists of expanding aerated grit aeration is generally consists of expanding aerated grit chambers to provide retention time of about 30 minutes. chambers to provide retention time of about 30 minutes. Aeration of sewage before primary treatment may offer a Aeration of sewage before primary treatment may offer a number of advantages. Grease removal may be modestly number of advantages. Grease removal may be modestly improved, odors from septic sewage will be reduced, a improved, odors from septic sewage will be reduced, a

PrePre--aerationaeration

improved, odors from septic sewage will be reduced, a improved, odors from septic sewage will be reduced, a degree of flocculation may occur, solids will be kept in degree of flocculation may occur, solids will be kept in uniform suspension, BOD will be reduced.uniform suspension, BOD will be reduced.

preliminary treatment...preliminary treatment operations to eliminate large objects, grit and grease...

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