A PRESENTATION ON ACTIVATED SLUDGE PROCESS AND KINETICS OF ASP

Activated Sludge Process is the suspended-growth biological treatment process, based on providing intimate contact between the sewage and activated sludge.

The Activated Sludge is the sludge obtained by settling sewage in presence of abundant O2 so as to enrich with aerobic micro-organisms.

Activated Sludge Process

Activated sludge in sewage in presenceof oxygen results into:

Oxidation of organic matter present. Coagulation and flocculation of suspended and colloidal matter

Under the activated sludge Process basically three processes occurs:-

The organisms oxidize a portion of organicMatter present in sewage into CO2,water and energy.

Organic matter + O2+ Bacteria CO2+ H2O+ NH3 +Energy + other end products

Oxidation

In second phase the other microbes get converted into new bacterial cells usinga part of energy released in oxidation

Organic Matter+O2+Bacteria+Energy New microbial cells

Synthesis

Finally the new microbial cells begin to consume their own cell tissue to obtain energy for cell maintenance.

New microbial cell tissue + 5O2 5CO2+NH3+2H2O+Energy

Endogenous Respiration

Conventional plug flow:- Settled water and recycled activated sludge enter the head end of the aeration tank and are mixed by diffused air or mechanical aeration. Duringthe aeration period adsorption, flocculation and oxidation of organic matter occurs.

Description of Activated sludge processes:-

Modified aeration:- It is similar to conven- -tional plug flow except that shorter aeration time and higher F/M ratio are used.

Tapered aeration:- Varying aeration rates are applied over the tank length depending on the oxygen demand. Greater amounts of air are supplied to the head end of the aeration tank, and the amount diminish as the mixed liquor approaches the effluent end.

SecondarySecondaryclarifierclarifier

EffluentEffluent

SLUDGE RETURNSLUDGE RETURNSLUDGE SLUDGE WASTE WASTE

ReactorReactor

CompressedCompressed

airair

Primary Primary

effluenteffluent

Tapered aerationTapered aeration

Step feed aeration

Generally three or more parallel channelsare used. The settled waste water is introduced at several point in the aeration tank to equalize the F/M ratio, thus lowering peak oxygen demand.

SecondarySecondaryclarifierclarifier

EffluentEffluent

SLUDGE RETURNSLUDGE RETURNSLUDGE SLUDGE WASTE WASTE

ReactorReactor

CompressedCompressed

airair

Primary Primary

effluenteffluent STEP FEED AREATIONSTEP FEED AREATION

It operate in the endogenous respirationphase of the growth curve, which requiresa low organic loading and long aerationtime.

Extended aeration :-

SecondarySecondaryclarifierclarifier

EffluentEffluent

SLUDGE RETURNSLUDGE RETURNSLUDGE SLUDGE WASTE WASTE

ReactorReactor

Primary Primary

effluenteffluent

Extended AerationExtended Aeration

screens Grit chamber

PrimarySettling

tankaeration chlorination

Raw water

SecondarySettling

tank

Effluentto disposal

Reareation

Activated Sludge Pump

Activated Sludge return

SludgeThickener

Sludge Digestion To Sludge Drying

Flow Diagram of ASP

1)Assimilation:- The microorganisms must first become assimilated to their surroundings and food provided also called Lag Phase.

2)Log Growth Phase:- In this phase the growth is rapid. Here bacterial cells reproduce binary fission.

Growth And Food Utilization

3)Stationary Phase:- In this phase the production of new cells is roughly offset by death and endogenous respiration.

4)Endogenous Phase:- In final phase, the cells are unable to obtain food from external sources and result into decrease in biomass concentrations.

Cell Growth

The new cell produced undergoes an auto oxidation which involves the breakdown of organics in cell itself to produce energy for the maintenance of remaining cell mass. Thus, there is continuously a fractional decrease in cell mass with time.

dX/dt = μ XdX/dt = the growth rate of biomass, mg/L t X = the concentration of biomass, mg/L μ = the growth rate constant,1/t

Factors affecting the rate of biomass production and food utilization:-

1) Temperature2) pH3) Toxicants

Complete mixing in aeration tank. Influent substrate concentration is constant. No microbial solids in raw water. All reactions occur in reactor. No microbial activity in clarifier. No Sludge accumulation. Steady state conditions prevails.

Kinetics of activated Sludge process

Assumptions:

A,T,

Va, X,Se

SecondarySecondary

ClarifierClarifier

Q, So

Q(1+R)

X, Se

(Q-Qw),

Xe, Se

Qw, Xr, SeRQ, Xr, Se

ACTIVATED SLUDGE KINETIC MODEL

Microorganism and Substrate Mass Balance

Rate of accumulation = Biomass in + Biomass of biomass in system growth - biomass out

(dX/dt)V = QXo + V [rg- kdX ] –QX rg = μ mXS /(Ks+S)(dX/dt)V = QXo + V[μ mXS/(Ks+S) - kdX ] - QX

rg = Growth rate of biomass

Kd= Endogenous decay coefficient.

(dX/dt)V = ZeroQX = VX [μmS/(Ks+S) - kd ]

Q/V = μ mS/(Ks+S) - kd =1/θ

θ = Hydraulic retention time

Hydraulic retention time

Mean Cell Retention Time

The time for which the cells remain in the system. It is given as-

θc = Mass of solids Mass of solids in system leaving system/day

θc = VX/(QwXw+ QeXe)

Substrate Balance Equation

(dS/dt)V = QSo – QS – V[μ mXS/ Y(Ks+S)]

For Steady State

Y=Decimal fraction of Food mass converted to biomass

dS/dt = Zero(So – S) = θ[μ mXS/Y(Ks+S)]

X = μm(So - S)/K(1+kdθ)

X = Y(So - S)/(1+kdθ)

Effluent concentration

Effluent substrate concentration

S = Ks(1+θkd)/θ(Yk - kd)-1