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Activated Sludge Process & AERATED LAGOON

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Activated Sludge Process

A biological treatment process in which a mixture of sewage and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated sewage by settlement and may be re-used. A common method of disposing of pollutants in wastewaters.

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Flow sheet of ASP

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Why we use ASP?

Secondary treatment usually biological, tries to remove the remaining dissolved or colloidal organic matter. Generally, the biodegradation of the pollutants is allowed to take place in a location where plenty of air can be supplied to the microorganisms. This promotes formation of the less offensive, oxidized products.

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Types of biological treatment process

There are two major types of biological treatment processes:

• Attached growth Microorganisms grow on a surface, such as rock or

plastic i.e. trickling filters.• Suspended growth Microorganisms are suspended i.e. activated sludge

process

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Classification of Secondary Treatment Process

Classification of secondary treatment processes is often based on the nature of microbial growth. Organisms can be suspended in wastewater, or they can be attached to an inert surface.

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Suspended Growth Processes

• Attached Growth Processes

Lagoons

Activated Sludge (most common)

Trickling filter Mechanically aerated

Conventional (tapered aeration),

Rotating disks Waste stabilization ponds

Step aeration Biological towers

Contact stabilization

Extended aeration

High purity oxygen

Oxidation ditch.

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Steps of ASP

• Mixing the activated sludge with the waste water to be treated (mixed liquor),

• Aeration and agitation of this mixed liquor for the required length of time

• Separation of the activated sludge from the mixed liquor, in the final clarification process

• Return the proper amount of activated sludge for mixing with the wastewater, and Disposal of the excess activated sludge.

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Components of ASP

Components of ASP can be listed as follows• Equalization basin• Primary settling tank• O2 supply (aeration tank)• Secondary settling tank• Sludge recycle line

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Equalization basin

Raw waste water is homogenized in an equalization basin to reduce variations in the feed, which may cause process upsets of the microorganisms and diminish water treatment efficiency.

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Primary settling tank

Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Grease and oil from the floating material can sometimes be recovered for saponification.

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Screening

The influent sewage water passes through a bar screen to remove all large objects like cans, rags, sticks, plastic packets etc. carried in the sewage stream. This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, whilst in smaller or less modern plants, a manually cleaned screen may be used.

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Grit removal

Pre-treatment may include a sand or grit channel or chamber, where the velocity of the incoming wastewater is adjusted to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other equipment.

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Aeration tank

In the aeration tank aerobic bacterial population is maintained in suspension in the mixed liquor and oxygen, as well as nutrients are provided. Oxygen is supplied either by mechanical or diffused aeration, which also aids in keeping the microbial population in suspension. The mixed liquor is continuously discharged from the aeration tank into a secondary clarifier in waste water treatments.

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Secondary settling

The final step in the secondary treatment stage is to settle out the biological floc or filter material through a secondary clarifier and to produce sewage water containing low levels of organic material and suspended matter.

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Advantages

• Allows good nitrification since COD is uniformly low • Able to handle peak loads and dilute toxic substances • Used in smaller systems, like package plants

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Disadvantages

• Larger volume, high aeration costs • Not much operational flexibility • Associated with biomass instabilities like sludge

bulking

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Aerated lagoon

An aerated lagoon or aerated basin is a holding and/or treatment pond provided with artificial aeration to promote the biological oxidation of waste waters.

The aeration segment in these systems is the most critical component and is the core of their biological treatment process. A lagoon systems ability to aerate the incoming sewage has a direct impact on the level of wastewater treatment it achieves.

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Types of lagoons

Aerated lagoons are of two types depending on how the microbial mass of solids in the system is handled. The main difference in most cases is the depth.

• Suspended Growth Aerated lagoons• Facultative Aerated Lagoon

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Suspended Growth Aerated lagoons

• Suspended growth aerated lagoons are relatively shallow earthen basins of depth varying from 2 to 5 m provided with mechanical aerators on floats or fixed platforms.

• Mechanical aerators are used to provide oxygen for biological treatment of wastewater as well as keep the biological solids in suspension. They are fully aerobic from top to bottom as the aeration power input is sufficiently high to keep the solids in suspension with complete mixing.

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Facultative lagoons

• They are only about 4-7 feet deep.• some solids leave along with the effluent stream and

some settle down in the lagoon since aeration power is sufficient for oxygenation and not for keeping solids in suspension. As the lower part of such lagoons may be anaerobic while the upper layers are aerobic, the term facultative is attached to the aerated lagoon. Suspended solids settle to the bottom of the pond where there is less dissolved oxygen.

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Facultative lagoons

• Facultative aerated lagoons can provide 70 – 80% BOD removal from readily degradable wastes such as domestic sewage

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Principle of Aerated Lagoon

• An aerated lagoon is a suspended-growth process treatment unit.

• It is provided with inlet at one end and outlet at the other end to enable the wastewater to flow through and to retain for the specified detention time.

• Initially, the population of microorganisms in an aerated lagoon is much lower than that in an ASP because there is no sludge recycle.

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• Therefore, a significantly longer residence time is required to achieve the same effluent quality. However, this longer residence time may be an advantage when complex organic chemicals are to be degraded.

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Principle of Aerated Lagoon

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Advantages of aerated lagoons

• Lagoon systems construct in areas where land is inexpensive.

• They use less energy than most wastewater treatment methods.

• They are simple to operate and maintain and generally require only part-time staff.