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