Activated Sludge Process

Activated sludge: it is a sludge Containing a large concentration of highly active aerobic

micro organisms.

Principle:

Activated sludge is mixed with raw sewage along with large quantity of air for about 4 to 8 hours in a aeration tank, The settled sludge in SST is called activated sludge. It is again

recycled to the head of aeration tank to be

mixed with sewage being treated.

Flow diagram:

Q = Rate of flow of sewage.

Qr = Rate of returned sludge

QE = Rate of sewage effluent

Qw = Rate of wasted sludge. .

Xt = mixed liquor suspended solids (MISS) in mg/l in aeration tank .

XR = Concentration of solids in the returned sludge

XE = Concentration of solids in the effluent

Xw = Concentration of solids in the wasted effluent.

yi =  BOD5 of the effluent sewage

ye = BOD5 of the effluent sewage

V = Aeration tank volume

Components of ASP:

• Primary settling tanks:Lesser Detention lime, i.e., 1.5 to 2 hours
• Aeration tank:Detention time :4 to 8 hours.
• Secondary sedimentation tank : detention time, 1.5 to 2 hours
• Sludge thickener and sludge digestion tank: Sludge from a secondary settling tank contain too much of m.c. i.e., 98 to 99%, therefore bulky, the m.c. is first reduced by sending it to ‘sludge thickener’.

1. m.c. will be reduced from 98 to 93 %.
2. helps in reducing the capacity of digestion tank.
3. similar to circular settling tanks with Dt of 12 to 24 hours.

Bulking of sludge and Its control: Under sick conditions, the settled sludge may contain more moisture and thus resulting in swelling of sludge volume. Due to sludge hulking.it remains in suspensiom and carried in the effluent of secondary clarifiers.

Remedial Measures:

• Elimination of industrial wastage
• Chlorination of the sewage
• Increased aeration
•  Raising the pi-I of sewage to 8 by adding lime.

Activated Sludge process variables:

Hydraulic Retention Time (HRT) = Volume of the tank/ Rate of Sewage flow into the tank

HRT = V/Q

V = Volume of aeration tank,

Q = Sewage inflow rate excluding sludge returned.

HRT is defined as an average time for which the sewage flowing into the aeration tank remains in the aeration tank.

Volumetric BOD loading or (Volumetric Organic loading) (u):

It is defined as BOD load applied per unit volume of aeration tank.

u = Q.y1/V

Food (F) to Micro Organism (M) Ratio : ( F/ M ) ratio:

It is also called Organic Loading.

It is defined as the ratio of kg of BOL) applied per day (representing microbial feed) to kg of MLSS in aeration tank (micro organism).

(F/M) = (Q.y)/(V.x1)

F/M ratio is main factor controlling the design.

Lower is the F/M ratio; higher is the BOD removal in the plant.

Sludge age or Mean cell residence time (θc) :

It is defined as Average Time for which particals of suspended solids remain under aeration.

Sludge Volume Index (SVI): It is the volume occupied in ml by one gm of

solids in the mixed liquor after settling for 30 minutes

SVI = Volume of sludge settle in ml/MLSS in gm Usually an SVI of 50 to 150 mI/gm is adopted

Usual values adopted In the conventional ASP:

F/M =0.4 to 0.3

Mean cell residence time = 5 to 15 days

QR/Q =0.25 to 0.5

Efficiency = 85 to 95 %