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Activated Sludge Plants: Dimensioning

Eduardo Cleto Pires

2

History

Seminal paper:

• Ardern, E. and Lockett, W.T. (1914)

Experiments on the oxidation of sewage

without the aid of filters, Journal Society of

Chemical Industries, v.33, p.523

• First experiments were performed in batch reactors

keeping the biological sludge from batch to batch

3

History

Continuous flow reactors soon substituted batch reactors using a settling tank for sludge separation and return.

The process was “discovered” instead of “invented” as Arden & Lockett were investigating sewage treatment using published observations on the effect of oxygen on sewage and almost accidentally observed the effects of sludge recycling.

4

Why the denomination of “activated sludge”?

Ardern & Lockett observed that the sludge improved (faster reaction) after a few batches as if “activated” by the recycling and aeration!

• Thus activated sludge

• Actually, a mixture of microorganisms formed macroscopic

flocs and this diverse and concentrated population of

microorganisms is responsible for the higher efficiency.

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Definition

Activated Sludge

• is the name given to the mixture of

microorganisms organized as biological flocs

formed by recirculation of the biomass from a

separation device back into the aeration tank.

6

Pros and Cons

Pros• high treatment efficiency• operation control and flexibility• small foot print

Cons• needs precise control and operation• needs frequent laboratory measurement of

many variables• higher costs

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Activated Sludge Fundamentals

8

Activated sludge process kinetics

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Substrate utilization rate – U

The substrate utilization rate is the ratio of substrate removal velocity and the mass of microorganisms.

av

dSdtU

X

U - substrate utilization rate [T-1]

S - substrate concentration [ML-3]

Xav - volatile suspended solids at the aeration

tank – MLVSS* [ML-3]

t - time [T]

* - mixed liquor volatile suspended solids

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Substrate utilization rate – U

Assuming constant U and solving for an interval

equal to the hydraulic detention time (qH):

00 ee

av H av

Q S SS SU or U

X X V

S0 - influent substrate concentrationSe - effluent substrate concentration Q - sewage flowrateV - aeration tank volumeqH - hydraulic detention time (V/Q)

av

dSdtU

X

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Michaelis-Menten kinetic equation

max

av S

dS U Sdt UX K S

Umax - maximum substrate utilization rateS - substrate concentration at the aeration tankKS - half-velocity constant (substrate concentration when U is equal to Umax

Limiting Substrate Concentration

Su

bst

rate

Util

iza

tion

Ra

te

Ks

Umax

maxU

2

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Properties of the Michaelis-Menten equation

High substrate concentration: equation approaches zero order kinetics.

Low substrate concentration: equation approaches first order kinetics.

S maxS S K U U

max max maxS S

S S S

U S U S UK S K if k then U kS

K S K K

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Removal rate of BOD or COD

Apply the Michaelis-Menten or first order kinetics

equation being the COD or the BOD represented by S.

Assuming first order kinetics (S ≡ BOD or COD):

3 1;av

dS LkX S K S k K

dt MT T

For sanitary sewage: K ranges from 0,017 to 0,030 d-1.

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Mass balance around the aeration tank

Mass balance assuming first order kinetics:

0 e av e

Influent mass of Effluent mass of Removed mass ofsubstrate per substrate per substrate per

unit time unit time unit time

QS QS kX S V

V Se Xav

Aeration TankQ S0 Q Se

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Mass balance around the aeration tank

0 0e e

e av e

S S S S

kS X K S

This equation is used to estimate the needed hydraulic detention time to reach the desired effluent concentration

V=

QSimplifying and remembering that

where K = kXav

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Mass balance considering recirculation

0

0

r e r e av e

removalinfluent effluent

e av e

QS Q S Q Q S kX S V

simplifying

QS QS kX S V

• This is the same expression obtained without

sludge return (recirculation).• Thus the recirculation of the sludge has no effect on

the mass balance (it is an internal process!).

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If recirculation has no effect on mass

balance why is it used?

• To keep the biomass in the aeration tank!

• To uncouple the biomass retention time

from the hydraulic retention time.

18


Then, how could we avoid or reduce

sludge recirculation?

• Using attached biomass (biomass carrier).

• suspended growth x attached growth

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Food to Microorganisms ratio (F/M)

This ratio is also known as food-mass

ratio.

Mass refers to the amount of

microorganisms measured as the

concentration as volatile suspended

solids

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Food / Microorganisms ratio (F/M)

It is the ratio of the available food [ F ]

(substrate) in the aeration tank and the

quantity of microorganisms that will feed on

this substrate (MLVSS) [ M ].

0

av

QSF

M X V

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Food / Microorganisms ratio (F/M)

Usual values (NBR 12209:2011)• high rate

• F/M from 0,70 to 1,10 kgBOD/kgMLVSS

• conventional • F/M from 0,20 to 0,70 kgBOD/kgMLVSS

• extended aeration• F/M 0,15 kgBOD/kgMLVSS

22

Solids retention time – θc

Average time that a particle remains in aeration.

Also known as:• sludge age• mean cell residence time

Numerically it is equal to the mass of

suspended solids at the aeration tank and

the mass of wasted solids (excess sludge).

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effluent biomass

(sludge) - carriedby the effluent wastewater

biomass (sludge ) inthe aeration ta

wasted biomass(

avc

u uv e ev

slud )

nk

ge

X V

Q X Q X

Qe - effluent flowrate (withdraw at the secondary clarifier)Qu - sludge flowrate (withdraw at the secondary clarifier)Xav - volatile suspended concentration solids at the aeration tankXuv - volatile suspended solids concentration in the wasted sludgeXev - volatile suspended solids concentration in the treated wastewater

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Expected values

• Conventional processes

• 4 to 15 days

• θc < 4 d: floc is not dense enough to settle

• θc > 15 d: floc is too small and does not settle

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Controls nitrification:

• high qc favors nitrification

Used as a control parameter:

• estimation of the sludge volume to be wasted

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Synthesis and auto-oxidation

Synthesis• a fraction of the organic matter is synthesized

into new cells• the mass of microorganisms increases

Endogenous respiration or auto-oxidation• a fraction of the cells decays

• the mass of viable microorganisms decreases

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Degradation of the organic matter

Organic Matter

CO2, H2O, N2, P End products

CO2, H2O, NH3, P Non-biodegradable

end products

New cells

Energy Synthesis

Endogenous respiration

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Synthesis fraction• biomass yield Y

a S

substrate utilizationactive microorganismsrategrowth rate

dX dSY

dt dt

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Decay fraction (endogenous respiration)• endogenous decay coefficient- kd

a endogenousd av

concentration ofactive microorganismsactive microorganisms

decay rate

dXk X

dt

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Balance of cell mass at the aeration tank:

aa endogenousav S

Synthesis Decay

avd av

dXdXdX

dt dt dt

dX dSY k X

dt dt

Y - 0,40 to 0,50 mgSSV/mgBODremoved This is the produced sludge!

kd - 0,05 to 0,10 (mgSSV/d)/mgSSV

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Relating θc, Y, kd and U

Mass balance around the WWTP:

0av

Influent volatilesolidsVolatile solids

mass at the WWTP

ee ev u uv d av

Volatile solids mass in thetreated effluent and in the resulting mass balance be

wasted sludge

dXV QX

dt

dSQ X Q X Y k X V

dt

tween the volatile

synthesized and decaied solids

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e e ev u uvd

av av

dS Q X Q XYk

X dt X V

avc

u uv e ev

X V

Q X Q X

Assuming steady state and neglecting the influent active volatile solids:

but

then1 e

dc av

dSYk

X dt

Full equation:

0av

e ev u uv

ed av

dXV QX Q X Q X

dtdS

Y k X Vdt

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0

0 0

1 1

1 1

ed

c c av

c e c eav

d c av d c

Q S SYU k Y

X V

Y S S YQ S SX V

t k X k

one develops other relations:Rearranging 1 ed

c av

dSYk

X dt

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Sludge production

Net sludge yield - ∆X:

0 e d av

decayedproduced

X Y S S Q k X V

The sludge yield is also expressed as:

0obs eX Y S S Q

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Sludge production

The net yielded sludge is the sludge that

needs to be wasted.

• this sludge is digested in anaerobic reactors (large

plants) prior to discharge or, in small plants, mixed

with lime for chemical stabilization and discarded.

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Relation between Y and Yobs

Equating both equations for ∆X:

0

1

avobs d

e

obsd c

X VY Y k

S S Q

YY

k

or

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Sludge recirculation

Keeps a high and constant sludge

concentration at the aeration tank.

Inoculation of the aeration tank speeding

the stabilization of the organic matter.

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Sludge recirculation

Recommended recirculation rates:

• MLVSS < 3500 mg/L 25%

• 3500 < MLVSS < 4500 mg/L 50%

• MLVSS > 4500 mg/L 100%

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Oxygen requirements

Oxygen is consumed• to provide energy for synthesis of new cells• endogenous respiration

Injection of oxygen (air) provides• mixing in the aeration tank, keeping the flocs

suspended• stripping (removal) of volatile compounds,

formed as metabolites or existing in the polluted water

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Oxygen requirements

2 avO S XM a M b M

2

av

O 2 2

S 0 e

X av

2

M = required mass of O [kgO /d]

M = removed BOD [(S -S )Q] [kgBOD/d]

M = mass of volatile solids in the aeration tank [kg]= X Volume

a = fraction of the removed matter used for synthesis [kgO /kgBO removed

2

D ]

b = endogenous respiration oxygen consumption coefficient [kgO /kgMLVSS ]

2 0O e avM a S S Q b X V

Required mass of oxygen:

Thus

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Oxygen requirements – design criteria

Approximate values for a' and b’

• a’ ≈ 0,52

• b’ ≈ 0,12 d-1

Minimum oxygen concentration at the

aeration tank

• 1,5 a 2 mgO2/L

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Choosing an aeration system

Consider

• shape of the aeration tank

• mixing requirements

• cost

• operation

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Aeration systems

Conventional• air is injected into the liquid phase in the

aeration tank and oxygen from the air transfers to the water• diffused air• mechanical mixing• mixed systems (diffused air + mechanical mixing)• conventional systems are useful for biomass

concentration up to 4.500 mg/l

44

Aeration systems

Pure oxygen• oxygen is injected directly into the liquid phase

• oxygen concentrator

• liquid oxygen tank

• pure oxygen requires specific equipment for injection

• high biomass concentration, up to 8.000 mg/l

• low hydraulic retention time: 3 hr!

Surface aeration

45

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Surface aerationMaintenance: many units to keep running

Bubbling aeration

47

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Fine bubble diffusers

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Distribution of diffusers in an aeration tank

Distribution of diffusers in an aeration tank

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Micro holes (micro pore) pipe diffuser

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Dimensioning of the aeration system

Diffused air

• air pressure needs to surpass

•water column (static pressure)

•pipeline pressure drop

•pressure drop at the diffusers

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Dimensioning of the aeration system

Diffused air • blowers (compressors)

• centrifugal blowers• flowrate above 30 m3/min and pressure

from 5 to 7 MWC• positive displacement blowers

• flowrate below 30 m3/min and pressure above 6 MWC

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Power requirement

P - blower power [kW]Mair - required air mass [kg/s]Qair - air flow rate [m3/s]R - gas constant for air [8,314 kJ/kmol.K]8,41 - air constant [kg/kmol] – adjustment of unitsT0 - inlet absolute temperature of the air [K]pe - absolute pressure at the blower inlet [atm]ps - absolute pressure at the blower outlet [atm]h - blower efficiency [0,70 to 0,80]

0,283

0 18,41

air s

e

M RT pP

p

1,2

air air air

3air

M Q

kg/m

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Recommendations

first estimate of pressure drop

• 1.2 to 1.5 water column

blowers should be capable do deliver 1.5

times the required air flow

always have a spare blower

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Dimensioning of surface aerators

Surface aerators

• Required power is estimated using manufacturer’s data

• oxygen transfer rate as a function of power: standard

conditions (sea level at 20°C and tap water)

• the standard condition values are corrected to field

conditions: altitude, temperature, sewage characteristics

• Besides oxygen transfer it is necessary to consider

mixing (suspension of the solid phase)

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Correcting to field conditions

200 1.029.02

TSW LC CN N

N - oxygen mass transferred under field conditionsN0 - oxygen mass transferred under standard conditionsCSW - oxygen saturation concentration in the aeration tank at temperature T

- assumed as 95% of the tap water saturation concentrationCL - oxygen concentration in the saturation tank

- usually the minimum value is 2.0 mgO2/la - correction coefficient to take into account industrial wastewater mixed

with the sanitary sewage- usual values are in the range 0.8 to 0.9

NOTE: some constant values may differ depending on the author or country standards.

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Secondary clarifiers

The quality of the secondary clarifier is fundamental to assure the operation of activated sludge plants.

Dimensioning depends on sludge settleability and standard design procedures and dimensions.

Dimensions used should be the ones that provide the highest safety factors.

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Nitrogen removal with denitrification

Clarifier

Recirculation

Anoxic tankBOD removalDenitrification

Aeration tankBOD removalNitrification

Influent

Effluent

Wastedsludge

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Nitrogen removal with denitrification

Oxidation Ditch

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References

Tchobanoglous, G.; Burton, F.L. and Stensel, H.D. Wastewater Engineering Treatement and Reuse (Metcalf & Eddy). McGraw Hill, 4th. ed., 2003 (Chap. 7 and 8)