Activated Sludge and SBR PDF

8/10/2019 Activated Sludge and SBR PDF

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Activated Sludge Treatment andSBRs ATUs

Pre-class questions Name 4 components necessary to have

an activated sludge system forwastewater treatment.

Aeration tank Sludge recycle Clarification tank Oxygen supply Sludge wasting


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Aeration Tank

Waste sludge

EffluentFlow, Q

BODTKNTSS

Sludge recycle

Oxygen suppliedclarifier

Clarifiers for small system should be designed at hydraulic loadsof 200 gal/d-ft 2 compared to 600 gal/d-ft 2 for larger systems

Why is that?


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Mean cell residence time

average time bacteria and solids are in the aeration tank

Also more commonly called solids retention time (SRT)

MCRT or SRT =lbs of solids in reactor or aeration tank divided by the lbs wasted per dayThe lbs wasted per day on average equals the lbs produced per day.

If a system has shorter SRT, a larger fraction of its solids are removed per day

Thus less solids in system, less bacteria to consume foodand lower efficiency

But the removal efficiency is in general only a problemwhen the SRT gets below some critical level

What is the definition of MCRT?


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Why do you suppose nitrification system requires a

longer MCRT, per Table 7-10.

The ammonia oxidizing bacteria haveslower kinetics

grow slower than heterotrophic bacteria need more time to achieve the same

efficiency of substrate removal


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Define SVI and what does it indicate aboutan activated sludge system?

SVI is the volume in mL that 1.0 gr (dryweight) of sludge occupies after 30

minutes of settling. Higher SVI means poorer settling Example 2 liter cylinder

sludge conc. = 3000 mg/Lsettled volume at 30 min = 600 mLSVI = mL/g


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High Growth of FilamentsSludge Volume Index > 300 mL/gPrefer SVI =


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What does SBR stand for? Sequencing Batch Reactor One tank used for aeration and settling Steps

Feed or fill React Settle Decant effluent withdrawal Idle


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Batch-Fed Activated SludgeProcess:

TIME

React 30 -40%

Cycle %

Mix Aerate Settle

15 - 20%

IdleWithdraw

15 - 20 %

Fill

20 - 30%


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Rules to remember about activatedsludge treatment

They all produce excess sludge that has to be

removed and disposed Oxygen is required at sufficient rate

2.0 to 2.5 lb O 2/lb BOD including nitrification

Provided with proper aeration design Conservative designs needed for small systems High peak flows (3-4 times average daily Most of the flow can be in a few hours each day 18 to 24 hour aeration time good baed on avg flow Clarifies at 200 gpd/ft 2 based on average flow


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Design Guidelines

Temperature, 0C Min. SRT, days10 2015 1520 10

Nitrification


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Nitrification

Temperature, 0C Min. SRT, days10 2015 1520 10

SRT, days lb O2/lb BODr 5 1.03

10 1.1715 1.26

18 1.3020 1.3225 1.3630 1.39

40 1.4360 1.48

y = 0.183Ln(x) + 0.7559X=SRT

Oxygen Required


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Oxygen RequiredSRT, days lb O2/lb BODr

5 1.03

10 1.1715 1.2618 1.3020 1.32

25 1.3630 1.3940 1.4360 1.48

y = 0.183Ln(x) + 0.7559X=SRT


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0.820.3360

0.820.3840

0.820.4230

0.820.45250.820.4820

0.870.5315

0.910.5712

0.940.6010

1.000.667

1.030.696

1.090.754

1.180.822

1.230.871g TSS/g BODg TSS/g BODSRT, days

Net YieldNet Yield,

settlingsettling

No

primaryWith primary

y

=

0.9967x-0.24

53

y

=

0.9967x-0.24

53

y = 0.9967x-0.2453 y = 1.2463x-0.1184

Net sludgeYield Values


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Simple activated sludge design

Important to check vendor unit supplyIs the tank volume sufficient?Is the oxygen supply sufficient at peak loads?Is the clarification area sufficient?Did they estimate sludge production accurately?Is sludge handling design adequate?


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Package Units for small flows Modules installed in ground for


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Treats flow forabout 40 homes


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Small FlowWWT

Plant Owner


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SBR System components Tank (depth can vary from 6 ft to 25 ft)

Decant system and effluent pump Aeration and mixing equipment Feed pump

Effluent collection chamber Level sensors Computer controller for cycle times and on/off

controls Influent valves for multiple tanks


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Effluent Quality from well-designedwell operated SBRs

BOD < 8.0 mg/L TSS < 8.0 mg/L NH4-N < 1.0 mg/L NO3-N


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Advantages Single tank design provides simple

installation Quiescent settling provides low effluent

TSS Can provide good nitrogen removal if

properly designed and enough carbon and

alkalinity Can be used to provide flow equalization


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Disadvantages?


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Key design elements SRT gets total volume

Cycle Times Fill Volume/decant volume

Effluent volume = fill volume

Vs

Vs = settled volume

VF = fill volumeVF+VS= total tank liquidvolumeVF/VT= critical design parameter

VF

What is a good value for VF/V

T?


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Given the following for a 2 tank SBR system, what is the time per cycle of fill,

aerate, settle, decant and idle and what is the aeration time per cycle?

Average Flow = 10,000 gal/d = 416.6 gal/hr Based on SRTand load, volume/tank = 10,000 gallons

Assume: VF/VT = 0.20

Settle time = 1.0 hr Decant time = 0.5 hrsIdle time = 0.3 hrs


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Typical Design (2 tanks) Average Flow = 10,000 gal/d = 416.6 gal/hr Based on SRTand load, volume/tank = 10,000 gallons

Assume: V F/VT = 0.20, VF=0.2(10000) = 2000 gallons# of cycles per day/tank =10,000 gal/day divided by 2 tanks

divided by 2000 gal/cycle= (10000)/(2*2000gal/fill) = 2.5 per day per tank

Cycle time = 24/2.5 = 9.6 hrsTotal cycle time = Tfill+Taerate+Tsettle+Tdecant+Tidle = 9.6 hrs

Fill time, = 2000 gal/416.6 gal/hr = 4.8 hrs

Settle time = 1.0 hr Decant time = 0.5 hrsIdle time = 0.3 hrs

Aeration/react time = 9.6-4.8-1.0-0.5-0.3 =3.0 hrs

Fraction of aeration time =3/9.6 = 0.31

Activated Sludge and SBR PDF

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