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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
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
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
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
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