Activated Sludge Process

Pakistan Council of Scientific & Industrial Research Laboratories Complex, Karachi

Off: University Road, Karachi-75280.

ACTIVATED SLUDGE

PROCESSES…

Pakistan Council of Scientific & Industrial Research Laboratories Complex, KarachiOff: University Road, Karachi-75280.

The most common suspended growth process used for municipal wastewater treatment is the activated sludge process.



Activated sludge plant involves:

1. wastewater aeration in the presence of a microbial suspension,

2. solid-liquid separation following aeration,3. discharge of clarified effluent,4. wasting of excess biomass, and5. return of remaining biomass to the aeration tank.



Process

The process involves air or oxygen being introduced into a mixture of primary treated or screened sewage or industrial wastewater combined with organisms to develop a biological floc which reduces the organic content of the sewage.

The combination of wastewater and biological mass is commonly known as mixed liquor.

In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge.



Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank.

This fraction of the floc is called return activated sludge (R.A.S.). Excess sludge is called surplus activated sludge(S.A.S.) or waste activated sludge(W.A.S).

S.A.S is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance.

S.A.S is stored in sludge tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.



Advantages

Diverse; can be used for one household up a huge plant

Removes organicsOxidation and Nitrification achievedBiological nitrification without adding chemicalsBiological Phosphorus removalSolids/ Liquids separationStabilization of sludgeCapable of removing ~ 97% of suspended solidsThe most widely used wastewater treatment process



Disadvantages Does not remove color from industrial wastes and may

increase the color through formation of highly colored intermediates through oxidation

Does not remove nutrients, tertiary treatment is necessary

Problem of getting well settled sludge

Recycle biomass keeps high biomass concentration in aeration tanks



Types of Activated Sludge Processes

Plug Flow wastewater is routed through a series of channels

constructed in the aeration basin. Wastewater Flows to tank & is treated as it winds its way

through the tank. As the wastewater goes through the system, BOD and

organics concentration are greatly reduced.



Variations to this method include:

adding return sludge and/or in decreasing amounts at various locations along length of the tank;

wastewater BOD is reduced as it passes through tank,

air requirements and number of bacteria required also decrease accordingly.



Complete Mix wastewater may be immediately mixed throughout the

entire contents of the aeration basin (mixed with oxygen and bacteria).

This is the most common method used today.Since the wastewater is completely mixed with bacteria

and oxygen, the volatile suspended solids concentration and oxygen demand are the same throughout the tank.



Contact StabilizationMicroorganisms consume organics in the contact

tank.Raw wastewater flows into the contact tank where it

is aerated and mixed with bacteria.Soluble materials pass through bacterial cell walls,

while insoluble materials stick to the outside.Solids settle out later and are wasted from the system

or returned to a stabilization tank.Microbes digest organics in the stabilization tank, and

are then recycled back to the contact tank, because they need more food.



Detention time is minimized, so the size of the contact tank can be smaller.

Volume requirements for the stabilization tank are also smaller because the basin receives only concentrated return sludge, there is no incoming raw wastewater.

Often no primary clarifier before the contact tank due to the rapid uptake of soluble and insoluble food.



Extended Aeration

Used to treat industrial wastewater containing soluble organics that need longer detention times.

This is the same as complete mix, with just a longer aeration.

Advantage - long detention time in the aeration tank; provides equalization to absorb sudden/temporary shock loads.

Less sludge is generally produced because some of the bacteria are digested in the aeration tank.

One of the simpler modifications to operate.



Design ConsiderationThe quality or characteristics of raw waste

water to be treated.

The desired quality or characteristics of effluent or treated waste water.

The type of reactor that will be used.

Volumetric and organic loading that will be applied to the reactor.



Amount of O2 required and the aeration system will provide to supply O2 and to support mixing.

The quantity of sludge that will be generated and wasted for its further management.

Besides these nutrient requirements of microbes, environmental conditions under which plant operated.



Design steps

The design computations require the determination of:

Volume or dimensions of the aeration tankAmount of O2 required and power needed for aerationQuantity of sludge that will produced for particular

waste and treatment conditions Volume and dimensions of sec. settling tank



Design criteriaNo of aeration tanks, N= min. 2 (small plants) = 4 or more (large plants)Depth of waste water in tank= 3-4.5 m (usually) = 4.5-7.5 m (diffuse

aeration) = 1-6 m (surface

aeration)Freeboard= 0.3-6 m (diffuse aeration) = 1-1.5 m (surface aeration)Rectangular aeration tank L:B= 5:1 and B:D=3:1 to 4:1

(depends on the aeration system)



Air requirement:I. 20-55 m3 of air/Kg of BOD removed for

diffuse aeration when F/M => 0.3II. 70-115 m3 air/Kg of BOD removed for

diffuse aeration when F/M <= 0.3 Power required for complete mixing : 10-

14 kW/1000 m3 of tank volume for surface aeration system

THANK YOU!!!



NIAZ AHMEDSOCES

PCSIR LAB COMPLEXCELL NO.

03333259613

Activated Sludge Process

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