Design sewer & physical & biological waste water treatment

7/23/2019 Design sewer & physical & biological waste water treatment

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EAT 356 WATER & WASTE

WATER ENGINEERINGMAHYUN AB WAHAB

SCHOOL OF ENVIRONMENTALENGINEERING

UNIVERSITI MALAYSIA PERLIS


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TODAYS OBJECTIVE

CO3:

Ability to DEFINE, DESCRIBE,DESIGN and EXPLAIN basicstructure involved in physical

and biological unit processapplied in wastewatertreatment in Malaysia


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30 minutes ASSIGNMENT

One sewerage with 225mm

diameter put at 1:200 slope.

Calculate high and sewage

velocity if its flowrate is

432m3/day. Assume n = 0.013


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Please comment


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Introduction

Why we treat sewage or wastewater?

cityWastewaterTreatment

Plant

river

BOD5= 280mg/l

SS = 360 mg/l

Standard A


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Degree of discharge

Discharge by settlement

EXAMPLE for normal composition of sewage

Organic and inorganic

Organic

solid

(1)

Inorganic

(mineral)

(2)

Total

Organic

(1+2)

BOD5

Settlement

suspended solid

39 15 54 19

Non settlement

Suspended solid

26 10 36 23

TOTAL 90 42

Dissolved solid 80 80 160 12

Table 1 :Example of one sewage sample (all unit in mg/l), Bache, 1989


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What are the conclusion??

54/90 = 60% from total suspended solid can settle

19/42 = 45% from BOD5can discharge through

settlement Settlement (physical operation) ONLYcannot

remove all BOD5 In other word, if we just use sedimentation tankto

treat sewage, its still have 55% BOD5 in our finaleffluent


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Determination of unit operation

In treating sewage, a few different unitoperation involved, either physical,

biological or chemical unit operation. This unit operation also can classified to

primary (physical), secondary (biological) or

tertiary treatment. Every unit has different discharge efficiency


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Example

Primary

treatment

Secondary

treatment

BOD 280SS 360

*BOD 150*SS 144

BOD 20SS 50

o ASSUME IN THIS EXAMPLE % PRIMARY TREATMENT

DISCHARGE FOR BOD IS 40% AND SS IS 60%


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Example

From previous figure, if we have to complystandard A, we need biological unitoperation that can discharge

((150-20)/150) (100) BOD5 =% BOD5 ((144-50)/144) (100) SS = ..% SS

So, we have to choose suitable unitoperation

o ASSUME IN THIS EXAMPLE % PRIMARY TREATMENT

DISCHARGE FOR BOD IS 40% AND SS IS 60%


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TYPE OF TREATMENT

Optional treatmentPhysical Chemical Biology

screensing Precipitation Activated sludge

Mixing Disinfection Rotating Biological

Contractors (RBC)Flocculation Absorption Aerated lagoon

Sedimentation Neutralization Trickling FilterFloatation Oxidation

Filtration Settling

General

A few technique to treat sewerage depend on type ofsewerage.


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Cont

Conventional Normally we just apply physical and biological

treatment for municipal sewerage

When we use chemical treatment?

type of treatment can classified as belowPre-treatment

First stage in treat sewerage

Physical treatment and one of the primary treatment

Objective : protect incoming treatment from course objectsuch as wood, rock, metal and etc.

Example : screens, communitor, grit chamber,sedimentation tank


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Cont

Primary treatment Refer to pre-treatment process and primary

sedimentation tank

Objective : remove suspended solid fromwastewater, so that biological plant will not overloaded.

Main function : remove big portion of suspendedsolid from wastewater and a few of BOD

5 Wastewater volume in primary treatment have tocontrol. Not too slow and too fast. WHY?

Effluent from primary sedimentation tank are calledsludge


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Cont

Secondary treatment

Refer to biological plant

Objective : remove organic biodegradable and suspended

solid that cannot remove in primary treatment

Sludge treatment

Beside primary sedimentation tank, sludge also can find insecondary sedimentation tank (after biological plant)

Its mean, output from biological plant have to settle in othertank, commonly called as secondary sedimentation tank.

Sludge also can perform in other process such as septic tank,Imhoff tank and oxidation pond.


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Cont

Tertiary treatment

Advanced treatment

Done when high standard of wastewaterrequired

Its also done when we want to remove specificparameter such as nitrogen and phosphorus

Normally include absorption, reverse osmosisand etc.


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Cont

Practically, not all phase or type oftreatment are use

Its engineer responsibility to decide whichunit suitable for the wastewater they haveto treat.

For example,

influent screens PrimarySedimentation

tank

Aerated lagoonActivated

sludge

SecondarySedimentation

tank

effluent


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Table : Sewage Treatment Method

Sewage

inflow

Preliminary

Treatment

Primary

Treatment

Secondary

Treatment Tertiary Treatment

effluent

discharge

screensing sedimentationActivatedsludge filtration

grit removal floatation biofiltration disinfection

grease tank sedimentation tertiary ponds

pre-aeration

flowmeasurem

ent

flow balancing

removal ofrags,

rubbish,grit, oil,grease

removal ofsettleable and

Floatablematerials

Biologicaltreatment to

Removeorganic andSuspended

solids

biological andchemical treatmentto remove nutrients

and pathogens


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Primary treatment

Screens

Objective : remove course object to avoid

problem in the next unit treatment First unit in wastewater treatment unit series

2 type of screens

Mechanical screensManual screens

2 type of screens

Course screens

Fine screens


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Cont

Mechanical screens

Manual screens

Manual screens


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Cont

Parameter Design criteria

Manually raked Mechanical raked

Flowrate, Q Qpeak= ? Qpeak= ?

Maximum clear spacing 25mm 25mmSlope to vertical 0-45 0-45

Max. approach velocity 1.0m/s 1.0m/s

Max. flow through velocity 1.0m/s 1.0m/s

Min. freeboard 150mm 150mm

Estimated volume of screens pervolume of sewage

30m3/106 m3 See figure

Storage period of screens 7 days 7 days

Screens thickness 25mm N/A

Washing and dewatering of screens No Yes


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Cont

After we know a few design criteria, so, width ofscreens chambercan determine by this formula

Where,

W = chamber width (m)

B = screensing thickness (mm)

S = maximum clear spacing (mm)

F = Qpeak (m3/s)

V = maximum flow through velocity (m/s)

D = depth (m)

( )B S FW XS VD


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Cont

Screens output quantity based on sewageage and clear spacing.

Sewage volume is between 1.3x10-6 to3.67x10-5per m3 flowrate with average valueis 1.5x10-5m3/m3flowrate (Mc Ghee, 1991)


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SKETCH ON SCREENS DESIGN


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Example

One wastewater treatment plan are planned tocater 50,000 people. Determine width of screenschamber and appropriate number of screens

thickness, based on these design criteria: People, P = 50000

Maximum clear spacing = 25mm

Screens thickness = 10mm

q = standard water consumption for Malaysia

Qpeak= 4.7 p-0.11DWF

Volume = 0.9m/s

Depth = 0.85m

FROM QUESTION WE KNOW THAT


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FROM QUESTION WE KNOW THAT,

Screens thickness B = 10 mm 0.01 m

Maximum clear spacing S = 25 mm 0.025 m

Volume V = 0.9 m/s

Depth D = 0.85 m

QPEAK

= 4.7 p-0.11 DWF

DWF = qP

11250 m3/day

Q peak = 34384.577 m3/day

0.39797 m3/s

W = (B+S) x F

S VD

W = 1.4 x 0.5202

W = 0.73 m

W is taken as 0.8 m

Number of screensing thickness

No. of B = 22.8571

No. of B is taken as 23 pieces

so, new width for this chamber is

W = (B+S)*(no. of B)

0.805 m


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Comminutor


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Cont

Another primary unit operation like screens

However, its not functionas a screens but as a

grinder. Grind course thing to smaller size anddischarge

Comminutorwill not effect the rest of operation

with their output But, its still have their disadvantage.Comminutor will increase load for nextoperation(especially biological operation)


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Grit Removal

Grit is a inorganic material such as.and..and..

etcGrit will damage pumps by abrasion andcause serious operation difficulties insedimentation tanks and sludge

digesters by accumulation around andplugging of outlets and pump suctions


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Normally, grit remover is designed toremove inorganic material especially sandwith 0.2mm diameter or bigger (Hammer

and Hammer, 1996) In this research finding also shows 0.2mm

grit with specific gravity 2.65, will have1.2m/min velocity, it is higher thansuspended velocity of organic solid inwastewater (Tebbutt, 1991)


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Cont

Grit remover is one of the most important part incombined sewerage system.

Can u think what are the principle using in gritchamber?

A lots of different types of grit chamber, such as :

Grit chamber

Gravity Channel

Aerated Grit Channels

Vortex Grit Traps

Detritor


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Cont

Every grit removal have different designapproach

It is common practice to remove thismaterial by grit chambers .

So that, we will only learn how to design grit

chamber.


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Design Grit Chamber

Grit chambers are basin to remove theinorganic particles to prevent damage to the

pumps, and to prevent their accumulation insludge digestors.

Grit chamber is a shallow tank, commonlyrectangle, and designed to settle grit

Grit chambers are usually located after the barscreens and before the primary sedimentationtanks.


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Grit chambers are generally designed as longchannels

Grit chambers are designed to be cleanedmanually or by mechanically operateddevices


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Cont

Parameter Design characteristics

Flowrate, Q Qpeak = ?

Minimum retention time, t 1 minutes

Surface Load Rate, SLR (m3/m2.day)


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Design principle

Flowrate, Q

Using Qpeak= .

Retention Time, t t = volume / flowrate

Surface Load Rate (SLR)

SLR = Q / As = flowrate / area of tank

= m3/m2.day


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Cont

Horizontal velocity, Vh

Vhis a horizontal velocity enter

the tank You must know how to

differentiate between vertical

velocityand horizontal velocity Vertical velocity occur because

of gravity Vh


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Cont

We know that,

(3.1)

(3.2)

V

t Q

s

QSLR

A


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Cont

Its 2 option when we want to designgrit chamber

Starting from retention time, t

Starting from Surface Load Rate,

(SLR)


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Starting from retention time, t

Starting from Equation 3.1

We can determine retention time (t) according to

value given in previous table. With knowing t andQ value, we will get V.

With knowing depth of tank, we will get As (how?)

We have Q and As, so that we will get SLR

Compare value of SLR we obtained from thiscalculation and reference value in table given. Ifstill in range, its acceptable.


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Starting from SLR

Starting from equation 3.2

Determine SLR value from table

From Q and SLR value, we can obtain As With knowing depth of tank, we will get V

(how?)

From V, we will get retention time, t


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Example

One grit chamber is designed for a domesticwastewater treatment plant. This plant

receives waste from 8000 people. If Qpeakisused in this design, calculate length, width

and depth of this chamber. Given, SLR is1500 m3/m2 and horizontal velocity is 25

cm/s.


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SOLUTION

Qpeak= 4.7 p-011 DWF

SLR = 1,500 m3 /m2 .day

Vh = 25 cm/s Width (W) _ ?

Length (L) _ ?

Depth (d) _ ?


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DWF = q*P

= (0.225 m3/cap.day) (8000)

= .m3/day

Qpeak = 4.7 (8)-0.11 DWF, p = 8

= 3.739 (1800)

= 6730 m3/day


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Given, SLR = Q/As

= 1,500 m3/m2.hari

= 6730/As = 1,500 m3/m. hari As = 4.487m

2

Assume depth, d = 1 m; So,

Volume, V = As(d)

= 4.487 (1)

= 4.487 m3


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Retention Time, t

= V/Q

= (4.487 m3)/(6,730 m3/day) = ..day

= (6.67 x 10-4day) (24 x 3600 second)

= .. second,


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Given Vh = 25 sm/s

Vh = L/t

(Horizontal velocity = Length/time taken) 25 = L/57.6

L = (25 sm/s) * (57.6 s)

= 1,440 sm

= 14.4 m

= .m


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As = BL

4.186 m2 = B (14.5)

B = 0.288 m Take B as = m

We have assume depth, d =1.0m

Conclusion

L = .m; B = . m ; d =.m

L = .. m

B = .. m

d =.. m


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Grease tank

Grease tank is one of the pre-treatment unitdesigned to remove greasy material and lighterthan water.

Some of wastewater consist high composition offat, grease and oil. Example, wastewater from.

So that, we have to remove this greasy materialbefore its enter next unit operation.

But, grease tank is very rare in domesticwastewater treatment


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Equalization tank

Wastewater is held in the equalization tank toallow solids to begin settling.

Activated sludge from the leveling ponds is mixedwith the wastewater in the equalization tanks tobegin biological digestion of organic contaminants.

The equalization tank also helps to maintain a more

constant flow rate through the treatment plant.


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Sedimentation tank

Sedimentation tank function to settle suspendedsolid in wastewater or in easy word, sedimentationtank separate solids from the liquid stream

Sedimentation tank also known as clarifier

Theoretically, purpose of sedimentation tank is todivide two component, which is :

Sludge (settled suspended solid) Effluent


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Cont

The purpose of the scraper mechanism mountedinside the tank, is to collect the settled solids forremoval from the tank by pumping

In circular sedimentation tanks the clarifiermechanism has sludge scrapers attached to arotating arm scraping the sludge towards a centralhopper.

In rectangular clarifiers scrapers are carried alongthe tank bottom collecting the sludge into a troughor hopper at the influent end of the tank.


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Separation in sedimentation tank


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Design primary sedimentation tank

Primary sedimentation tank is one of the physicalunit operation

It just after grit chamber (if needed)

Function of this tank is ..

What will happen if this solid did not remove well?

Primary sedimentation tank usually design toremove 25-40% BOD and 50-70% SS


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Cont

A few types of primary sedimentation tank

Rectangular Tank

Circular Tank Upward flow tank

All this three types has their own advantages

and disadvantages


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PARAMETER UNIT DESIGN CRITERIA

Fl t 3/d Q k ?


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Flowrate m3/day Q peak = ?

Minimum retention time at Qpeak

Hour 1.5-2.0

SLR at Q peakCircular (


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Cont

Using Retention time

t = V/Q

Example

Volume = 1000 m3

Flowrate = 50000m3/ day

Retention time = ??


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Cont

Using SLR = Q / As

SLR = flowrate / area tank surface

= flowrate / b.l Unit ???

Q

l

b


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Horizontal velocity, Vh

Horizontal velocity is a even velocity enter the tank


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Cont

Differentiate between Vhand Vo(Settlementvelocity)

Settlement velocity, Vois a vertical velocity cause

by gravity Settlement theory stated design sedimentation

tank (Rectangular or Circular Tank) based onreference particle moving from top of the tank to

base of the tank (from point A to point B) Design have to be done with Vhand Voin almost

same magnitude, so that the particle can settleideally.


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Cont

REMEMBER : depth of tank NOTinfluencing particle settlement

Vh= Q / Ah Where,

Ah

= area of cross section

= b x d

d

b

Ah


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Cont

3 settlement cases

If settlement velocity > horizontal velocity

= particle will settle in front of tank If horizontal velocity > settlement velocity

= particle will settle at the end of tank

If horizontal velocity ~ settlement velocity

= ideal settlement will occur

Why we have to avoid case 1 and 2 ?


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Weir Overflow Rate, WOR

Settlement tanks must therefore be designed deepenough to allow all particles to settle, and also tohave flow such that settled solids are not disturbed

and carried over the weir at the outlet of thesettlement tank. In designing sedimentation tank,

Another important parameter in settlement tanks

is the rate at which water flows over the weir,known as the Weir Overflow Rate (WOR)


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Cont

WOR also have its own important function.

WOR control settled sludge.

WOR is a barrier at tank perimeter controllingdischarge of effluent from this tank to the nextunit operation

Length of WOR have to design properly so that,

settled sludge will not suspend again and flowwith effluent


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Cont

Weir overflow rate

Production of sludge

Settled sludge in primary sedimentation tank can be

estimate using this equation

Dry weight =

(kg.day)

(1DWF)suspended

solid

% removal ofsuspended

solid

x x


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Cont

Sludge is produced by settled suspended solid

Sludge production can be estimated from outputper capita

Rough estimation by Fuaad, 1990 is

0.0014m3/people.day or 50gram/capita.day

In designing sedimentation tank, volume of sludge

storage also needed. 2-5% of sedimentation tankvolume is using as a rough estimation.


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Cont

In calculating sludge storage volume,dislodging frequency has to determine.

If dislodging is done every day, so thatstorage volume needed is just one day.

Design approach of sedimentation tank is

similar with design of grit chamber. Its canbe started from SLR or from retention time.


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Design step by step

1. List out all the data given

2. What are the equation we have?

1. t =

2. SLR = ..3. Find As

4. If we got the depth of tank (From reference table) , we will getvolume of tank, V

5. From V, we will know retention time, t6. Check your design

1. SLR

2. Horizontal velocity (for rectangular sedimentation tank)


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EXAMPLE

One primary sedimentation tank are receiving influencefrom one housing area with 100,000 people. Waterconsumption is 200 liter/capita.day. If SLR is 30m3/m2.day,

design: One rectangular primary sedimentation tank

One circular primary tank with slope 7.5. Determinesludge produced.

Using this design data: Use Qpeak as design flowrate

SS in influence is 400mg/l

Efficiency of SS removal is 70%

Assume no slope


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EXAMPLE

One small city consist 15000 person. Designone upward flow tank to cater this city using

given design criteria: q = standard

Retention time at maximum flowrate is 2 hours

SLR = 35m3/m2.day

Sludge production = 0.0016m3/person.day

Dislodging frequency = every day

Slope to horizontal 60


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Secondary sedimentation tank, SST

Secondary sedimentation tank is a tank AFTER biologicaltank

Its purposely to settle sludge that produce in biological

tank. This sludge is pumped back into the inlet end of theprimary sedimentation tanks and settles with the rawsludge

We know in biological tank, a lots of sludge produced from

synthesis and microorganism oxidation process This tank has to design properly to make sure effluent

discharged comply to standard


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Cont

Compare to primary sedimentation tank that has 3option of tank, in SST, normally we designcirculartank

This SST is very important to specific unitoperation, especially ASP and aerated lagoon.

Without this tank, that two process cannot operate

because sludge produce cannot settle. As a result,effluent will consist high number of SS


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Cont

The sludge that collects in the SST is calledaerated sludge or activated sludge because

it is fully aerated.


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Design parameter for secondary sedimentation tank


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Parameter Value for PE 5000

Retentiontime

< 2 hours

SLR 30 m3/m2.day

WOR 150-180 m3/m.day

Minimum

depth

3 m

Solid LoadRate @ Qpeak

< 150 kg/m2.day

Flowrate Qpeak

B i A ti t d Sl d P


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Basic Activated Sludge Process

Primarysedimentation

tank

Aerated tank Secondarysedimentation

tank

Return sludge

effluent


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Documents

Design sewer & physical & biological waste water treatment