L-24 Activated Sludge Process

Part-I

Environmental Engineering-II

ASP Flow sheet

• In a sewage (or industrial wastewater) treatment plant, the activated sludge process can be used for one or several of the following purposes:

1. oxidizing carbonaceous matter: biological matter.matter.

2. oxidizing nitrogenous matter: mainly ammonium and nitrogen in biological materials.

3. removing phosphate.

4. driving off entrained gases carbon dioxide, ammonia, nitrogen, etc.

5. generating a biological floc that is easy to settle.

6. generating a liquor(wastewater) that is low in dissolved or suspended material.low in dissolved or suspended material.

• 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, and 4. wasting of excess biomass, and 5. return of remaining biomass to the

aeration tank.

• In activated sludge process wastewater containing organic matter is aerated in an aeration basin in which micro-organisms metabolize the suspended and soluble organic matter. matter.

• Part of organic matter is synthesized into new cells and part is oxidized to CO2 and water to derive energy.

• In activated sludge systems the new cells formed in the reaction are removed from the liquid stream in the form of a flocculent sludge in settling tanks.

• A part of this settled biomass, • A part of this settled biomass, described as activated sludge is returned to the aeration tank and the remaining forms waste or excess sludge.

• The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items:

a) Aeration tank where air (or oxygen) is injected in the mixed liquor.injected in the mixed liquor.

b) Settling tank (usually referred to as "final clarifier" or "secondary settling tank") to allow the biological flocs to settle, thus separating the biological sludge from the clear treated water.

Primary treated waste

• There are a number of factors that affect the performance of an activated sludge treatment system. These include:

1. temperature2. return rates3. amount of oxygen available4. amount of organic matter available4. amount of organic matter available5. pH6. waste flow rates7. aeration time8. wastewater toxicity

Design Criteria

1. HRT(Hydraulic Retention Time):- The ratio volume of aeration basin to the flow rate.

HRT in hrs = [V/Q] x 24HRT in hrs = [V/Q] x 24V= volume in m3Q= flow rate in m3/day

2. Food to microorganisms ratio (F/M)

• The “food” in the ratio is the BOD entering the process (BOD load).

• The “microorganisms” are the activated sludge solids in the aeration activated sludge solids in the aeration tanks, which are measured as ppm or mg/L of MLSS (Xt).

• F/M = (Qx BOD5)/ (V x MLSS)• = Q.La/V.Xt

3. Volumetric BOD loading

It is ratio of the BOD5 applied per unit volume of aeration tank.

Volumetric Load = [Q x La ] / VLa = Influent BOD5 to aeration tankLa = Influent BOD5 to aeration tank

4. Sludge age or Sludge retention time (SRT)

• SRT is denoted by θc and is defined by the equation

θc = X / (∆ X/ ∆ t)X= total microbial mass in the reactor(∆ X/ ∆ t) = total quantity of solids withdrawn (∆ X/ ∆ t) = total quantity of solids withdrawn

daily, including solids deliberately wasted and those in the effluent

Derrivation

• Refer Class note book

L-25

PART-II

L-25

Sludge Volume Index• Sludge Volume Index (SVI) is a very

important indicator that determines your control or rate of desludging on how much sludge is to be returned to the aeration basin and how much to take it out from the system.

• It actually serves as a very important empirical measurement that can be used as a guide to maintain sufficient concentration of activated sludge in the aeration basin.

• SVI can actually be determined through use of standard laboratory test methods to come up with the results.

• Basically the procedure involves measuring the Mixed Liquor Suspended Solids (MLSS) value and also the sludge Solids (MLSS) value and also the sludge settling rate.

• A simple explanation on how it is carried out can be summarized below with accompanying images for easy reference and better understanding:

1. Obtain sample of mixed liquor from the pond discharge pipeline and fill it to a 1 liter graduated measuring cylinder until the 1.0 liter marking.

2. Allow it to settle for 30 minutes3. After the time period, read the marking to determine

the volume occupied by the settled sludge and the reading is expressed in terms of mL/L and this is figure is known as the Vs value.figure is known as the Vs value.

4. Next, for MLSS, there are actually two approaches to get the value. A conventional standard approach is by filtering the sludge, drying it and then weigh the second portion of the mixed liquid.

• Value of Sludge Volume Index can then be calculated from the formula given here. Whereby,

• SVI = Sludge Volume Index, mL/g• SVI = Sludge Volume Index, mL/gSV = Volume of settled solids in one-liter graduated transparent measuring cylinder after 30 minutes settling period, mL/LMLSS = Mixed liquor Suspended Solids, ppm

• Typically a healthy sludge aeration pond basin should have the value registered within 80 to 150 mL/g.

• Sludge volume index is a quality indicator. It reflects the settling quality of the sludge. As the SVI increases, the sludge settles As the SVI increases, the sludge settles slower, does not compact as well, and is likely to result in more effluent suspended solids.

Relation between SVI and

Recirculation rate

• Refer note book