ACTIVATED SLUDGE

There are many different designs, but in general, all ASP have three main components: an aeration tank that serves as a bio reactor; a settling tank for separating AS solids and treated waste water; and a return activated sludge (RAS) equipment that transfers settled AS from the clarifier to the aeration tank's influent.

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Activated sludge process (ASP) is a process that was developed around 1912-1914 for the treatment of sewage and industrial wastewaters.

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The concentration of biodegradable components in the influent is reduced in all activated sludge plants due to biological (and sometimes chemical) processes in the aeration tank. Different boundary conditions, such as the hydraulic residence time (HRT) in the aeration tank, which is defined as the aeration tank volume divided by the flow rate, control the removal efficiency.

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The activated sludge process in wastewater treatment involves injecting oxygen or air into raw, unsettled sewage. The solids are smashed during this process. The sewage is bubbled, and the sewage liquor is discharged into a chamber with activated sludge. Live bacteria sink to the bottom of the tank, while dead bacteria float to the surface. While the live bacteria return to the digestion chamber, clean water is discharged into a soak away or watercourse.

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It is critical to understand what activated sludge is in order to comprehend how the entire activated sludge system operates. Because the particles are actively swarming with beneficial bacteria that digest the sewage, the sludge is considered activated.

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The benefits of activated sludge treatment:

The activated sludge treatment process has numerous significant advantages over other alternatives. Benefits are:

· The quantity of unwanted sludge is reduced.

· Beneficial bacteria reseed themselves in sewage treatment plants.

· The activated sludge treatment process allows for longer emptying intervals.

· The procedure is extremely dependable.

· The procedure is less complicated.

· Costs are being reduced.

· The method can be odorless.

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Disadvantages of the activated sludge treatment process:

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The activated sludge treatment process has some drawbacks and may not be suitable for all applications. Some disadvantages of activated sludge include:

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· Some facilities may be discouraged from using this method due to the initial high capital and operating costs.

· The activated sludge system must be designed and built by professionals.

· Skilled personnel are required to operate and maintain the treatment of activated sludge.

· Electricity must be used continuously, which increases the energy consumption of wastewater treatment.

· Parts and materials may not be available locally.

· Sludge and effluent may necessitate additional treatment or discharge.

Excess sludge eventually accumulates in the aeration tank beyond the desired MLSS concentration due to biological growth (and solids present in raw waste water that are only partially degraded). A portion of organic matter is synthesized into new cells, while the remainder is oxidized to CO2 and water to generate energy.

The three most important variables in the activated sludge process are the mixing regime, loading rate, and flow scheme.

 

Mixing Regime

In the activated sludge process, two types of mixing regimes are of particular interest: plug flow and complete mixing. Mixed liquor may be mixed laterally, but there is no mixing along the flow path.

The contents of the aeration tank are thoroughly mixed and uniformly distributed during complete mixing. As a result, at steady state, the effluent from the aeration tank has the same composition as the contents of the aeration tank.

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Flow Scheme

The flow scheme includes:

· the sewage addition pattern

· the sludge return to the aeration tank pattern

· the aeration pattern

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Sludge can be returned to the aeration tank directly from the settling tank or via a sludge re-aeration tank. From the aeration tank's head to the tank's end, aeration can be uniform or variable.

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Two engineers, Edward Arden and W.T. Lockett, discovered the activated sludge process in the United Kingdom in 1913 while conducting research for the Manchester Corporation Rivers Department. Experiments with sewage treatment in a draw-and-fill reactor (the forerunner of today's sequencing batch reactor) produced highly treated effluent. Because it was thought that the sludge had been activated, the process was named activated sludge (similar to activated carbon). It wasn't until much later that it was discovered that what had actually occurred was a method of concentrating biological organisms by decoupling the liquid retention time (which should be low for a compact treatment system) from the solids retention time (ideally, fairly high, for an effluent low in BOD-5 and ammonia.)

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