Secondary Treatment (Biological)

8/13/2019 Secondary Treatment (Biological)

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Seminar on Activated Sludge Pro

Submitted to:

Dr. V.K.Rattan

Dr. SSB UICET Panjab University

Submitted by:

Girish Gupta

ME Chemical 2ndyear

Roll No. 1202


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Index

Introduction

Types of biological treatment plant

Activated Sludge Process

Principle of activated sludge process

Activated sludge Process Design equation

Reference


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Secondary Treatment (Biologica

Removal of remaining dissolved or colloidal organic matter

Biodegradation of pollutants is allowed to take place in loca

To promote formation of less offensive oxidized product

What engineers do in water treatment plants?

Ans.

To design capacity of treatment units


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Types of Biological Treatments P

1. Attached Growth Process

2. Suspend Growth Process

1. Attached growth process : microorganisms grow on the surfarock or plastics

Example : Open Trickling Filters

Where water is distributed over rocks and trickles down to drain wthrough vent pipes enclosed bio towers which similar but shapedmedia instead of rocks called R.B.C


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R.B.C

R.B.C. is Rotating Biological Contactors

Consists of Large Submerged Disc

Rotate continuously to provide microorganisms grows on the


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Suspended Growth Process

Biological Treatment Process based on the growth and retensuspension of microorganisms

These microorganisms convert biodegradable, organic wasteconstituents and certain inorganic fractions into new cell mabyproducts, both of which then can be removed by settling, stripping, and other physical means.

Example Activated Sludge Process


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Activated Sludge Process

Activated sludge is a process for treating sewage and industrialwastewaters using air and a biological floc composed of bacterprotozoa

Purposes

1. oxidizing carbonaceous biological matter.

2. oxidizing nitrogeneous matter: mainly ammonium and nitrogenmatter.

3. removing phosphates.

4. driving off entrained gases such as carbon dioxide, ammonia,

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

6. generating a liquor that is low in dissolved or suspended mater


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Activated sludge process

Activated sludge process is consists of

1. Aeration Tank

2. Settling Tank or Clarifiers

Aeration tank contain sludge which can be described as microcontain

Mostly bacteria , Protozoa , Fungi, Algae etc.


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Oxygen Supply

Diffused compressed air

Mechanical surface aeration

Pure oxygen

Purpose of aeration

1. Provides oxygen required for aerobic bio-oxidation

2. Provides sufficient mixing for adequate contact between actand organic substances


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Submersible Aerator/Mixer


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Sludge is continuously mixed and aerated by

1. Compressed Air Bubblers along bottom

2. Mechanical Aerators on surface


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Process

1. Waste water to be treated enters tanks and mixed with cultorganic compound for growth producing over microorganirespirators which mostly in formation of CO2 and water

2. To provide biological removal of nutrients like nitrogen and

3. After sufficient aeration time to reach the required level of tsludge is carried by the flow into the settling tank, or clarifie

often of the circular design.


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Note:

1. the success of this process is the formation of a type of culturflocculate naturally, producing a settling sludge and a reasoupper, or supernatant layer.

2. If the sludge does not behave this way, a lot of solids will be water leaving the clarifier, and the quality of the effluent waspoor.


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The type of activated sludge system that has just been descrcontinuous flow process.

There is a variation in which the entire activated sludge procein a single tank, but at different times.

Steps include filling, aerating, settling, drawing off supernatan

A system like this, called a sequencing batch reactor, can provi

flexibility and control over the treatment, including nutrient remoamenable to computer control.)


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To design of AS, the following must bdetermined:

Volume of reactor1. Number of basins

2. Dimensions of each basin

Volume of reactor is determined from:

1. Kinetic relationships

2. Space loading relationships

3. Empirical relationships Sludge production per day (Xw), kg/day

Oxygen required per day (Or), kg/day

Final clarifier

o Number of basins


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Schematic of activated sludge


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Calculate (R / Q) Ratio

In order to maintain the desired MLSS(Mixed liquid solids) in the aeration tank, R/Q ratio must be calcula

Calculate the Sludge Density Index (SDI)1. Sample MLSS from downstream of aeration tank

2. Determine SS in MLSS

3. Place 1 liter of the MLSS in 1-liter graduate cylinder

4. Settle the sludge for 30 minutes

5. Measure volume occupied by settled sludge

6. Compute SS in settled sludge in mg/l

7. SS represents SDI

8. The test approximates the settling that occurs in final clarifier

If SDI = 10,000 mg/l and MLSS must be 2,500 mg/l

Then, Q(0) + R(10,000) = (Q+R)(2500)

R/Q = (2500)/(7500) = (1/3) = 33.3 %

So, R is 33.3% of feed wastewater (Q)


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Sludge Volume Index (SVI) = 1/ SDI

Is the volume in ml occupied by 1 gram of settled activated sludg

It is a measure of settling characteristics of sludge

Is between 50 and 150 ml/gm, if process is operated properly

Why Qw?Microbes utilize organic substances for respiration and synthesis

The net cell production (Qw) must be removed from the system t

constant MLSS

Qw is usually 1 to 6 % of feed wastewater flowrate (Q)


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Kinetics of Microbial Growth

Biochemical reaction

Biomass concentration.

The concentration of biomass, X (mg/L),increases as a function of time due toconversion of food to biomass:

Where m is the specific growth rate constant

(d-1). This represents the mass of cellsproduced/mass of cells per unit of time.


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Monod Kinetics

oGrowth rate

Growth rate constant, , is a function of the substrate concentr

o Two constants are used to describe the growth rate

m(mg/L) is the maximum growth rate constant (the which the susbtrate concentration is not limiting)

Ksis the half-saturation constant (mg/L) (i.e., concent

S when = m/2


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Substrate utilization

Where Y is the yield factor (mg of biomass produced/mg of foodconsumed)

Y range:

Aerobic: 0.4 - 0.8 mg/mg


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Since the hydraulic retention time, q= V/Qo, then


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Typical range of F/M ratio in activated sludge units

Treatment Proc ess F/MKg BOD

5/Kg MLSS/day

Extended aerat ion 0.03 - 0.8

Convent ional 0.8 - 2.0

High rate > 2.0


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Mean Cell Residence Time (qc)

It is defined as:

X = active biological solids in the reactor

X = active biological solids in the waste activated sludge flow

Units of qcis daysMean cell residence time is sometimes referred to as slu


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F/M Ratio and qc

Both parameters are used characterize the performance of the activprocess

A high F/M ratio and a low qcproduce filamentous growth that have poocharacteristics

A low F/M ratio and a high qccan cause the biological solids to undergoendogenous degradation and cell dispersion

For municipal wastewater

qcshould be at least 3 to 4 days

If nitrification is required, qcshould be at least 10 days


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Divide by X

qc is the average time a cell remains in the system, thus

ekX

tSY

X

tX

//

tX

Xc

/q


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The F/M ratio is the rate of substrate removal per unit weight of the cells,

Thus

X

tS

M

F

/

e

c

kM

FY

q

1


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Since F/M was also expressed as:

Then,

tX

S

M

F

e

c

ktX

SY

q

1


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Types of Reactors

Plug-flow reactors

Dispersed plug-flow reactors

Completely-mixed reactors


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Plug-flow and Dispersed-flow Reactor

In plug-flow reactors, there is negligible diffusion apath through the reactor

In dispersed-flow reactors, there is significant diffus

flow path through the reactor

Both types of reactors are used in conventional and tap

activated sludge


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Conventional Activated Sludge

Rectangular aeration tank

F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day)

Space loading = 0.3 to 0.6 (kg BOD5/day-m3)

qc= 5 to 15 (days)

Retention time (aeration tank) = 4 to 8 (hours)

MLSS = 1500 to 3000 (mg/l)

Recycle ratio (R/Q) = 0.25 to 1.0

Plug-flow and Dispersed-flow

BOD removal = 85 to 95 (%)


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Tapered Aeration

It is a modification of the conventional process

F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day)

Space loading = 0.3 to 0.6 (kg BOD5/day-m3)

qc= 5 to 15 (days)

Retention time (aeration tank) = 4 to 8 (hours)

MLSS = 1500 to 3000 (mg/l)

Recycle ratio (R/Q) = 0.25 to 1.0

Plug-flow and Dispersed-flow

BOD removal = 85 to 95 (%)


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Performance

qis the detention time for the plug-flow reactor

The volume of the plug-flow or dispersed-flow

given by:

qXKt

eS

S

0

q )( RQV


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Design parameters for activated sludge proces


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Process q c d) q h) F/M Qr/Q X (mConventional 5-15 4-8 0.2-0.4 0.25-5 1,500-

Complete-mix 5-15 3-5 0.2-0.6 0.25-1 3,000-

Step-aeration 5-15 3-5 0.2-0.4 0.25-0.75 2,000-

Modif ied-

aeration

0.2-0.5 1.5-3 1.5-5.0 0.05-0.15 200

Contact-

stabi l ization

5-15 0.5-1

3-6

0.2-0.6 0.25-1 1,000-

4,00

10,0Extended-

aeration

20-30 18-36 0.05-0.15 0.75-1.5 3,000-

High-rate

aeration

5-10 0.5-2 0.4-1.5 1-5 4,00

10,0

Pure-oxygen 8-20 1-3 0.25-1.0 0.25-0.5 6,000-


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Design Parameters

The retention time and reactor volume for completely mreactors can be determined by:

q

q

QV

SXK

SS

t

ti

Activated Sludge Design Equat


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Activated Sludge Design Equat

Influent biomass + biomass production = effluent

biomass + sludge wasted

Substitute biomass production equation

Assume that influent and effluent biomassconcentrations are negligible and solve

Mass balance of biomass production

Mass balance of food substrate


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Influent substrate + substrate consumed = effluentsusbtrate + sludge wasted substrate

Substitute substrate removal equation

Assume that no biochemical action takes place in

clarifier. Therefore the substrate concentration in thaeration basin is equal to the substrateconcentrations in the effluent and the wasteactivated sludge. Solve:


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Thank Yo

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