a conventional activated sludge treatment conventional aeration

8/3/2019 A Conventional Activated Sludge Treatment Conventional Aeration

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ITT Water & Wastewater AB

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Design recommendation

Wastewatertreatment plantfunction

Conventional activatedsludge treatmentaeration

Table of contents

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

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

3 Complete mix activated sludge aeration design considerations .......................4

3.1 Fine bubble aeration estimates ..........................................................................4

3.2 Fine bubble considerations ................................................................................4

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

3.3.1 Sizing and density of diffusers ............................................................................................... 4

3.3.2 Non-tapered aeration............................................................................................................. 5

3.3.3 Tapered aeration ................................................................................................................... 5

3.4 Pressure monitoring and gas cleaning...............................................................5

3.4.1 Flow monitoring...................................................................................................................... 5

3.4.2 Pressure monitoring............................................................................................................... 5


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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 odour 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

aw as 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 Complete mix activated sludge aeration design considerations

Suitable aeration for a conventional (complete mix) activated sludge system for thesupply of oxygen for biological reactions and to keep solids in suspension is finebubble aeration.

3.1 Fine bubble aeration estimates

For an activated sludge process, fine bubble aeration is recommended. Rule ofthumb estimates generally apply to fine bubble aeration:

AOR/SOR ratio 0.47

Oxygen transfer efficiency (OTE) 6.5% per meter of diffuser submergence inclear water

Air contains 23% oxygen by weight under the standard conditions

Csat20 is generally lower for coarse bubble aeration compared to fine bubbleaeration after 1.2 m of submergence.

0.6 for domestic wastewater Alpha gradients can exist in coarse bubble aeration

95-99% in wastewater compared to pure water. Accepted range 0.95-0.99.

= 1.024 is generally used.

3.2 Fine bubble considerations

Increased diffuser intensity and lower air rates per diffuser increase OTE

Increased OTE lower air and power requirements


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Basins with 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.

3.3 Aeration tank sizing and equipment configurations

3.3.1 Sizing and density of diffusers

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

aeration tank and the equipment configurations. A number of steps need to be taken:

Calculate AOR from equation 1.

Size the aeration tank. Determine the BOD loading if applicable for 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].The tank floor is covered with diffusers and 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.

3.3.2 Non-tapered aeration

In an activated sludge system, an oxygen gradient may exist along the tank with highoxygen demands at the head of the basin. Non-tapered systems have a uniform

distribution of diffusers on the basin floor. Non-tapered systems are suitable for complete mix reactors. For SBR, the

valves must be throttled to match the oxygen gradients and prevent overaeration at the basin tail.

Overall blower output requires the re-throttling of the aeration valves.

System operates at the least efficiency at the head of the basin.

3.3.3 Tapered aeration

In tapered systems, the distribution of the equipment is proportional to the oxygen

demand gradient. Consistent OTE means better overall efficiency


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No diffusers operate at excessively low or high rates

Substantial reduction in power consumption.

3.4 Pressure monitoring and gas cleaning

3.4.1 Flow monitoring

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 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 grid

per tank.

3.4.2 Pressure monitoring

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 conventional activated sludge treatment conventional aeration

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