Lecture 21a Ppt
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Transcript of Lecture 21a Ppt
Two types of aerators are commonly used:
Bubble Aerators
From AirCompressor
From AirCompressor
Liquid OutLiquid In
Liquid InLiquid Out
Liquid Out
Liquid In
Liquid In Liquid Out
Turbine Aerators
Turbine Aerator: Design Procedure
Aerator Rating: 1, 2, 5 KW ; Area of Influence: 5m x 5m x 3m (depth)Aerator Rating: 10, 25, 50 KW; Area of Influence: 6m x 6m x 4m (depth)
Standard O2 transfer efficiency (SOT) is given by the manufacturer as,kg O2 transferred per hour per KW under standard conditions
Standard Conditions: T = 20oC; [O2]l = 0.0; In tap water
SOT calculation: (Generally specified by the manufacturer)Fill up a 5x5x3 m or 6x6x4 m tank (depending on aerator size) with tap waterAerate overnight. Measure DO in the morning. This is . Measure temp.Add sodium sulfite to de-aerate water, i.e., [O2]l = 0.0Aerate from t = 0 to t = t. Measure [O2]l at various times during aeration.
sl2 ]O[
2 l
s2 lL 2 l 2 l
0 o2 l
Ls[O ] t2 l 2 l
s2 l
Ls2 l 2 l
d[O ]K .a [O ] [O ]
dtIntegrating,
d[O ]K .a.dt
[O ] [O ]
[O ]ln K .a.t
[O ] [O ]
l2
sl2
sl2
]O[]O[
]O[
Time
Semi-Log Scale
Slope = (KL.a)T
celciusin T
)02.1(
)a.K()a.K(
20TTL
20L
)V.(]O.[)a.K(SOT 20sl220L
Given SOT how to find the Actual Oxygen Transfer Efficiency (AOT)??
both depend on temperature
sl2L ][O and a.K
9.0]O[
][O ;8.0
)a.K(
)a.K(sl
F2
sl
W2
FTL
WTL
celcius in T ;)02.1(
)a.K()a.K(
20TTL
20L
tablesfrom obtained re, tempeartu withdecreases ]O[ sl2
ion)nitrificat of case (in mg/L 3or mg/L 1]O[ l2
)V.(]O.[)a.K(SOT 20sl
F2
F20L Generally 1.5 – 2.0 kg O2 / h/KW
)V.(]O[]O[.)a.K(AOT l2sl
W2
WTL
T
)V.(]O[]O.[]02.1.[.)a.K(AOT l2sl
F2
20TF20L
T
Impact of wastewater
Energy requirements for maintaining completely mixed conditions in the aeration tank
For an aeration tank of volume V,The P/V ratio for maintaining completely mixed conditions is 15 – 20 W/m3
Hence in actual design power requirement in an aeration tank must be calculated both from O 2 requirement and mixing perspectives, and the larger value adopted.
In many cases, the mixing requirement becomes the controlling factor for provision of power to the aeration tank
Consequences of Oxygen DepletionIn Rivers/Lakes
Dead algae causesOxygen depletion
Consequences of NutrientAddition in Lakes
High BOD demand in rivers/lakes causes dissolved oxygen depletion and suffocation / death of fish.
High nutrient loading in lakes and slow flowing rivers causes eutrophication, i.e., excessive growth of aquatic algae/plants.
Consequences of Pollutant Loading to Natural Water Bodies
Algae: Present as a suspension in water; Increases dissolved oxygen concentration of waterImportant fish foodDead Algae may cause DO depletion
Aquatic Plants: The leaves are outside the waterCauses DO depletionDead aquatic plants in increase DO depletion
Nitrification
In wastewater, nitrogen is present as TKN, i.e., a mixture of organic-N and ammonia-N
During BOD removal in ASP, a part of the TKN is incorporated into the cell biomassAmount Incorporated = 0.124. (X) kg/d
The remaining amount remains in wastewater as ammonia – N
Presence of high concentrations of ammonia-N in water gives it a disagreeable smell
pK = 9.4
Presence of high concentrations of ammonia-N in water is also toxic to fish in surface water
It is thus desirable that the ammonia present in wastewater is converted to nitrate before releaseThe biological process for converting ammonia to nitrate is known as Nitrification.
Biological conversion of ammonia to nitrate is a two step process,ammonia to nitrite conversion by Nitrosomonas species of bacteria Nitrite to nitrate conversion by Nitrobacter species of bacteria
The first step of the process, i.e., conversion of ammonia to nitrite is rate limiting.
Both Nitrosomonas and Nitrobacter are aerobic chemo-autotrophic microorganisms.
HNH NH 34
Classification of Microorganisms
All microorganisms needs three things to survive, 1) Energy source, 2) Carbon / food source3) Terminal electron acceptor
Aerobic microorganisms use oxygen as the terminal electron acceptorAnaerobic microorganisms use chemicals other than oxygen as terminal electron acceptor
Heterotrophic microorganisms use organic carbon as energy source. Hence they need organic carbon to survive. They also use organic carbon as food source.
Chemo-autotrophic microorganisms use chemical compounds other than organic carbon as energy source. They do not need reduced carbon to survive. They use inorganic carbon as food source.
Photo-autotrophic microorganisms use light (photons) as energy source and inorganic carbon as food source.
Nitrosomonas / Nitrobactor: Energy source: Ammonia / NitriteFood Source: Inorganic CarbonElectron acceptor: Oxygen
Aerobic, chemo-autotrophic microorganisms
Two Stage NitrificationStage 1:
Low c
Only BOD removal and no nitrification Stage 2:
High c
Removal of residual BODNitrification
Stage 1
Stage 2
Single Stage Nitrification
• Nitrification along with the ASP process itself• Low c system; extended aeration• Oxygen provision must be made for nitrifying microorganisms• BOD5 is removed and residual ammonia is converted to nitrate in the aeration tank