Activated Sludge Treatment and SBRs – “ATUs”

• Pre-class questions• Name 4 components necessary to have

an “activated sludge” system for wastewater treatment.

– Aeration tank– Sludge recycle– Clarification tank– Oxygen supply– Sludge wasting

Aeration Tank

Waste sludge

EffluentFlow, Q

BODTKNTSS

Sludge recycle

Oxygen suppliedclarifier

Clarifiers for small system should be designed at hydraulic loadsof 200 gal/d-ft2 compared to 600 gal/d-ft2 for larger systems

Why is that?

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 dayThus less solids in system, less bacteria to consume food and lower efficiency

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

What is the definition of MCRT? –

Why do you suppose nitrification system requires a longer MCRT, per Table 7-10.

• The ammonia oxidizing bacteria have slower kinetics – grow slower than heterotrophic bacteria – need more time to achieve the same

efficiency of substrate removal

Define SVI and what does it indicate about an activated sludge system?

• SVI is the volume in mL that 1.0 gr (dry weight) of sludge occupies after 30 minutes of settling. – Higher SVI means poorer settling

• Example – 2 liter cylindersludge conc. = 3000 mg/Lsettled volume at 30 min = 600 mLSVI = mL/g

High Growth of FilamentsSludge Volume Index > 300 mL/gPrefer SVI = <150 mL/g

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

Batch-Fed Activated Sludge Process:

TIME

React 30 -40%

Cycle %

Mix Aerate Settle

15 - 20%

IdleWithdraw

15 - 20 %

Fill

20 - 30 %

Rules to remember about activated sludge 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 O2/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/ft2 based on average flow

Design Guidelines

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

Nitrification

NitrificationTemperature, 0C Min. SRT, days

10 2015 1520 10

SRT, days lb O2/lb BODr5 1.03

10 1.1715 1.2618 1.3020 1.3225 1.3630 1.3940 1.4360 1.48

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

Oxygen Required

Oxygen RequiredSRT, days lb O2/lb BODr

5 1.0310 1.1715 1.2618 1.3020 1.3225 1.3630 1.3940 1.4360 1.48

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

0.820.3360

0.820.3840

0.820.4230

0.820.4525

0.820.4820

0.870.5315

0.910.5712

0.940.6010

1.000.667

1.030.696

1.090.754

1.180.822

1.230.871

g TSS/g BODg TSS/g BODSRT, days

Net YieldNet Yield,

settlingsettling

No primaryWith primary

y

=

0.9967x-0.2453

y

=

0.9967x-0.2453

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

Net sludgeYield Values

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?

Package Units for small flows

• Modules installed in ground for <~30,000 gal/day

• Field assembly for <200,000 gal/day• Large usually engineered and concrete or

steel tank construction• Modules contain clarifier, sometimes filter• Include aeration blowers and pumps etc.• Electrical controls

Treats flow for about 40 homes –SBR system

Small FlowWWT

Plant Owner

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

Effluent Quality from well-designed well operated SBRs

• BOD < 8.0 mg/L• TSS < 8.0 mg/L• NH4-N < 1.0 mg/L• NO3-N <10.0 mg/L

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

Disadvantages?

Key design elements

• SRT – gets total volume• Cycle Times• Fill Volume/decant volume

Effluent volume = fill volume

Vs

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

VF

What is a good value for VF/VT?

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/hrBased on SRTand load, volume/tank = 10,000 gallons

Assume: VF/VT = 0.20

Settle time = 1.0 hrDecant time = 0.5 hrsIdle time = 0.3 hrs

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

Assume: VF/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 hrsSettle time = 1.0 hrDecant time = 0.5 hrsIdle time = 0.3 hrsAeration/react time = 9.6-4.8-1.0-0.5-0.3 =3.0 hrs

Fraction of aeration time =3/9.6 = 0.31