a digestion aerobic stabilization

7/29/2019 A Digestion Aerobic Stabilization

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ITT Water & Wastewater AB

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Design recommendation

Wastewatertreatment plantfunction

Aerobic stabilization

Table of contents

1 General recommendations guideline............................................................... 2

2 AOR-to-SOR conversion ................................................................................... 3

3 Aerobic digester design considerations ......................................................... 4

3.1 Fine bubble considerations ............................................................................ 4

3.2 Aeration tank sizing and equipment configurations........................................ 4

3.2.1 Sizing and density of diffusers ....................................................................................... 4

3.3 Mechanical aeration....................................................................................... 5

3.3.1 Product data for design.................................................................................................. 5

3.4 Pressure monitoring and gas cleaning........................................................... 6

3.4.1 Flow monitoring.............................................................................................................. 6

3.4.2 Pressure monitoring....................................................................................................... 6


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Aeration design recommendations

1 General recommendations guideline

Illustrated in this section is the design aid and general considerations in evaluatingsizing and the possible oxygen demand in biological systems. First, a number ofbasic points need to be considered:

What type of wastewater function is available?

Is aeration required for the biological processing of nitrogen andcarbonaceous material?

Is aeration required for odor control or cleaning?

Is aeration required for separating particles?

Is aeration required for mixing purposes?

The field conditions are needed to estimate the actual oxygen requirements (AOR):

What is the sites elevation?

Site temperature and climate, the varying temperatures on an annual basis

What are the BOD variations: peak, average, and minimum loadings

What are the working dissolved oxygen (DO) levels?

Diffuser submergence

Alpha and Beta factors

AOR and standard oxygen requirement (SOR) specifications recommendations:

If possible, perform an AOR-to-SOR conversion to avoid confusion

If that is not possible, provide the oxygen requirements as AOR and provideas much information that is available to assist the supplier in this conversion.

SOR determines the sizing and design of aeration equipment.

It is important to consider the stream loadings. These are BOD loadings that arereturned to the aeration process. If they are transported back to the aeration process,they may elevate the oxygen requirements significantly, for example downstream the

sludge treatment. Possible sources of loadings are: Septage receiving stations


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Centrifuge concentrate

Effluent from dissolved air flotation thickeners

Supernatant from aerobic or anaerobic digestion

Filter press or vacuum filter filtrate

2 AOR-to-SOR conversion

SOR calculations are needed to estimate the size of the aeration equipment. TheAOR demand is the sum of the following sources:

BOD. For solids retention times of 5-10 days, the kg of oxygen per kg of BODusually varies from 0.92-1.07. A conservative value of 1.1 kg O2/kg BOD isused on occasion. Higher values are valid for long detention times with loworganic loadings and additional sludge oxidation.

Ammonia. Usually one kg of ammonia requires 4.3-4.6 kg of oxygen.

Oxygen requirements are reduced if denitrification occurs in the treatment stepas oxygen is released when nitrite is reduced to nitrogen gas. This could beused as a safety factor in the overall design.

Sidestream loading must be considered

Generally, the following equation (1) can be used to convert AOR to SOR forequipment sizing:

20

T

field

msi

field

T

20

DOP

PCsat

CsatAOR=SOR (1)

Where AOR = actual oxygen requirements (field conditions)SOR = standard oxygen requirements (standard conditions)

=KL

awa s t e w a t e r

KLa

t a p w a t e r

= saturation factorPfield = barometric pressure at the treatment sitePmsi = barometric pressure at mean sea levelT = operating temperature of wastewater (C)Csat20 = surface DO saturation concentration at 20C and standard conditionsfor the particular aeration equipment at the design submergenceCsatT = Surface DO saturation concentration at design temperature T and14.7 PSIA for the particular equipment at the submergence

DOfield = dissolved oxygen in wastewater = temperature correction factor


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Alpha () is generally the most difficult parameter to estimate. It is often affected by:

BOD loading

Type of aeration

Process (nitrifying or non-nitrifying)

Type of mixing

Location in the tank

Waste type

Submergence of device

3 Aerobic digester design considerations

Suitable aeration for aerobic digestion for the supply of oxygen for biologicalreactions and to keep solids in suspension is fine bubble diffusers.

3.1 Fine bubble considerations

Increased diffuser intensity and lower air rates per diffuser increase OTE

Increased OTE lower air and power requirements

Basins with a high length to width ratio or waste with low organic strength, theair required to mix liquors could exceed the oxygen demand at some points,which is defined as mixing limited. Use valve throttling.

3.2 Aeration tank sizing and equipment configurations

3.2.1 Sizing and density of dif fusers

The following guideline can be used to approximately determine the sizing of the

digester (aeration tank) and the equipment configurations. A number of steps need tobe taken:

Calculate AOR from equation 1.

Size the digester. Determine the BOD loading if applicable for an aerationfunction [mass per volume, M/V]. Determine the loading of BOD on a mass perday basis. Determine the tank volume. With this information, the depth andwidth of the tank, calculate the tank dimensions.

Air requirements. Estimate SOR from AOR/SOR ratio estimates. Withsubmergence of aerator, determine OTE. From the oxygen weight of air, theactual weight of air and oxygen demand, calculate the air rate requirement[V/T].


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The tank floor is covered with diffusers that can be expressed as an AT/AD (areatank/area diffusers) ratio. A large number means less density.

Practical range: 4.5-20

Values larger than 20 may not keep solids suspended or mix liquors.

3.3 Mechanical aeration

3.3.1 Product data for design

For aerating the digester mechanically, jet aerators have the following capacities fordesigning the system:

1-70 kW (input power)

SOTR 1 - 80 kg O2/h (0.5 -180 lb/h)

Thrust up to 1,700 N

SAE around 1 up to 1.4 kg O2/kWh

Possible to install without emptying the tank

Non-clogging N-pump technique

Self-aspirating to 7 m (possible to pressurise)

Bulk flow approach for mixing

One unit supplying

- Air

- Water

- Air/water mixture & distribution

Installations modes include:

S-install mode self-standing

- Most common and recommended

- Suitable for an empty or a filled tank

P-install mode guided

- Basically only if jet aerator is installed in very strong flows

- Tank has to be empty.


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3.4 Pressure moni toring and gas cleaning

3.4.1 Flow moni tor ing

If fouling is an issue for monitoring the ageing of a diffuser, install a pressuremonitoring system. Consider the following:

Indicate the need for cleaning or a replacement

Significant power savings can be made

Process tool to balance the air flows between the aeration grids

Applicable to both porous ceramic and flexible membrane diffuser materials

Specify the pressure monitoring on a portion or all the fine bubble aerationgrids to provide the system headloss to the operator

Without a gas cleaning system, use pressure monitoring on one aerating gridper tank.

3.4.2 Pressure moni tor ing

Porous ceramics may require regular maintenance

For gas cleaning purposes, there is no real minimum for the number ofdiffusers per aeration grid

Preferred design maximum of diffusers per aeration grid is 1,000 to 1,500.

Absolute maximum number of diffusers per grid is 2,000.

a digestion aerobic stabilization

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