DP19A6

ExxonMobil ProprietaryWATER POLLUTION CONTROL Section Page

AERATION SYSTEMS FOR BIOLOGICAL XIX-A6 1 of 31

DESIGN PRACTICES TREATMENT OF WASTEWATER December, 2001

ExxonMobil Research and Engineering Company – Fairfax, VA

CONTENTSSection Page

1.0 SCOPE ......................................................................................................................................................3

2.0 REFERENCES...........................................................................................................................................3

3.0 DEFINITIONS ............................................................................................................................................3

4.0 BACKGROUND AND SELECTION CRITERIA.........................................................................................54.1 BACKGROUND.................................................................................................................................54.2 SELECTION CRITERIA.....................................................................................................................5

5.0 MECHANICAL AERATION SYSTEMS .....................................................................................................55.1 DESCRIPTION..................................................................................................................................5

Mechanical Aerators-Vertical Axis ........................................................................................................5Mechanical Aerators-Horizontal Axis ....................................................................................................5Performance .........................................................................................................................................6

5.2 DESIGN CONSIDERATIONS............................................................................................................65.3 DESIGN PROCEDURE FOR MECHANICAL SURFACE AERATORS .............................................65.4 SAMPLE DESIGN PROBLEMS ......................................................................................................105.5 OPERATING STRATEGIES AND ENHANCEMENTS ....................................................................11

6.0 DIFFUSED AERATION SYSTEMS .........................................................................................................116.1 DESCRIPTION................................................................................................................................116.2 DESIGN CONSIDERATIONS..........................................................................................................126.3 DESIGN PROCEDURE...................................................................................................................126.4 SAMPLE DESIGN PROBLEMS ......................................................................................................156.5 OPERATING STRATEGIES AND ENHANCEMENTS ....................................................................17

7.0 ASPIRATING AERATORS SYSTEMS ....................................................................................................17

8.0 JET AERATION SYSTEMS.....................................................................................................................17

9.0 DEEP TANK AERATION.........................................................................................................................18

10.0 AERATION SYSTEM PERFORMANCE TESTING ...............................................................................18

11.0 NOMENCLATURE.................................................................................................................................19

Changes shown by ➧

ExxonMobil ProprietarySection Page WATER POLLUTION CONTROL

XIX-A6 2 of 31 AERATION SYSTEMS FOR BIOLOGICALDecember, 2001 TREATMENT OF WASTEWATER DESIGN PRACTICES


CONTENTS (Cont)Section Page

TABLESTable 4.2-1 ExxonMobil Facilities Aeration Systems.....................................................................20Table 5.2-1 Sample Duty Specification Sheet to be Provided to the Aeration Vendor ..................21Table 5.3-1 Solubility of Oxygen in Water (mg/L) Exposed To Water-Saturated Air at

Atmospheric Pressure (101.3 kPa) ............................................................................22Table 6.1-1 Typical Diffused Aeration Diffusers ............................................................................23Table 6.3-1 Surface Do Saturation Values (CT,Pd), mg/L...............................................................26Table 6.3-2 Diffuser Submergence Factors ..................................................................................27

FIGURESFigure 4.2-1 Aeration Selection Criteria Decision Tree ..................................................................27Figure 5.1-1 Mechanical Aerators for Biological Wastewater Treatment Applications ...................28Figure 5.3-1 Solubility of Oxygen in Pure Water (Water in Contact with Air)..................................29Figure 6.3-1 Typical Coarse Bubble Diffused Aeration Performance Curves.................................30Figure 9.0-1 Degassing Trough for Deep Tank Aeration Effluent...................................................31

Revision Memo

12/01 Original Issue of Design Practice XIX-A6. This Design Practice was formerlySection 6 of DP XIX-A5. Expanded Section 6.3. Corrections made to Table6.3-1. Added Section 9.0 on Deep Tank Aeration and Section 10.0 on AerationSystem Performance Testing.





1.0 SCOPEThis section covers selection of aeration equipment for activated sludge aeration basins and lagoons, and design proceduresfor calculating actual oxygen demand and for estimating standard airflow rate and horsepower requirements. Aerationequipment discussed includes mechanical surface aerators, fine and coarse bubble diffused aerators, aspirating aerators andjet aerators. Aeration equipment is duty specified to a qualified aeration vendor. The vendor calculates the standard air flowrates and horsepower requirements based on their equipment. Calculations in this section can be used to verify the vendor'scalculations.

2.0 REFERENCES2.1 DESIGN PRACTICEDP XIX-A5 Biological Treatment of Wastewater

2.2 EMRE WATER AND WASTEWATER DESIGN GUIDE (TMEE 080)DG 11-9-1 Aeration Systems

2.3 OTHER REFERENCES1. Metcalf & Eddy Inc., Wastewater Engineering Treatment, Disposal, and Reuse, 2nd Edition, McGraw-Hill Inc., New York

(1979)2. Grady Jr., C. P. L., Lim, H. C., Biological Wastewater Treatment, Theory, and Applications, Marcel Dekker, Inc., New York

(1980)3. Eckenfelder Jr., W. W. and Grau, P., Activated Sludge Process Design and Control: Theory and Practice, Water Quality

Management Library Volume 1, Technomic Publishing Co, Inc., Lancaster, PA, (1992)4. Metcalf & Eddy Inc., Wastewater Engineering Treatment, Disposal, and Reuse, 3rd Edition, McGraw-Hill Inc., New York

(1991)5. Eckenfelder Jr., W. W., Industrial Water Pollution Control, 2nd Edition, McGraw-Hill Inc., New York (1989)6. Hayes, B. E. and Cancellaire, M. C., Industrial Water Pollution Control Technology Course - Eckenfelder's Method

Compared to the Design Practices, 91 ECS2 191 (December 1991)7. Great Lakes-Upper Mississippi River Board of State Public Health and Environmental Managers, Recommended

Standards for Wastewater Facilities, Health Education Services, Albany, NY (1990)8. Givens, S. W. and Grady, C. P. L., et al, Biological Process Design and Pilot Testing for a Carbon Oxidation, Nitrification,

and Denitrification System, Environmental Progress Vol. 10, No. 2 (May 1991).9. Water Pollution Control Research Series, No. 12020, 2/70, Petrochemical Effluents Treatment Practices Detailed, U.S.

Department of the Interior, Federal Water Pollution Control Administration (February 1970).10. Water Environment Federation, Manual of Practice No. 8 - Design of Municipal Wastewater Plants, WEF and ASCE, Book

Press, Brattleboro, VT (1992)11. Water Environment Federation, Aeration - Manual of Practice FD-13, Alexandria, Virginia (1988).12. Clesceri, L., et al, Standard Methods for the Examination of Wastewater, 17th Edition, American Publish Health

Association, Washington, DC, 1989.13. Eckenfelder Jr., W. W., Patoczka, J., Watkin, A. T., Wastewater Treatment, Chem. Eng. (September 2, 1985)14. Rich, L. G., Designing Aerated Lagoons to Improve Effluent Quality, Chemical Engineering, May 30, 1983, pp. 67 - 7015. American Society of Civil Engineers, Measurement of Oxygen Transfer in Clean Water, ASCE 2-91 Second Edition, (1993)16. American Society of Civil Engineers, Standard Guidelines for In-Process Oxygen Transfer Testing, ASCE-18-96, (1997)

3.0 DEFINITIONSActive Solids / Biomass - The portion of the solids in a biological system composed of microorganisms that are activelymetabolizing the substrate (removing the organic or inorganic contamination). Non-biodegradable solids accumulate in theactivated sludge system and reduce the percentage of active solids (biomass / organisms) in the system.Aerated Lagoon - An oxidation pond with aeration devices. Mixing energy supplied to an aerated lagoon is usually insufficientto completely mix the system.Aerobic - A system or process which is active in the presence of dissolved oxygen. In biological waste treatment, aerobicrefers to a microbiological system in which microorganisms use dissolved oxygen in the metabolism of the substrate (removecontaminants).




3.0 DEFINITIONS (Cont)Anaerobic - A system or process which is active in the absence of dissolved oxygen. In biological waste treatment, anaerobicrefers to a microbiological system in which microorganisms metabolize (oxidize) the substrate in the absence of dissolvedoxygen.Anoxic - A term frequently used to describe a system or process which is active in the absence of dissolved oxygen but in thepresence of nitrate. In these systems, nitrate, not dissolved oxygen, acts as the terminal electron acceptor for the metabolismof the substrate.Biochemical Oxygen Demand (BOD, BOD5, BODULT) - A general measure of organic material in wastewater samples thatcan be biologically degraded. It is the quantity of oxygen consumed during the biological decomposition (oxidation) of materialin water. Certain inorganic compounds that exert an immediate oxygen demand (e.g., sulfite) will be detected in the BOD test.BOD is usually measured over a specific time period; a five-day period is commonly used, with the result expressed as BOD5.If the biological decomposition is allowed to proceed to completion, the quantity of oxygen consumed is termed the ultimateBOD, often designated BODULT and is normally measured over 20 days. In this case, some nitrogen compounds can beoxidized, a process called nitrification. BOD is normally expressed mg/L (ppm). BOD5 is typically 60 percent of BODULT.BIOX - Abbreviation for BIological OXidation and commonly used to describe an activated sludge system but can be used inreference to other biological oxidation processes used to treat wastewater.Carbonaceous Biological Oxygen Demand (CBOD) - A general measure of organic material in wastewater samples that canbe biologically degraded; similar to BOD, but a nitrification inhibitor is used to eliminate the interference of nitrifying bacteria.Chemical Oxygen Demand (COD) - A measure of the amount of organic or reduced inorganic compounds in a sample thatcan be oxidized by a strong oxidizer, usually potassium dichromate and sometimes potassium permanganate. COD of awastewater is generally greater than the BOD since the wastewater may contain oxidizable material that cannot be biologicallydegraded. The COD test is simpler and faster than the BOD test. Caution must be used when analyzing the COD in high salt(chloride) wastewater streams since the salts will interfere with the test results. COD is expressed as ppm or mg/L.Dissolved Oxygen (DO) - Dissolved oxygen level, measured in mg/L (ppm), is an important monitoring parameter forbiological systems and receiving water bodies. It indicates whether a biological treatment unit can sustain a healthy microbialpopulation or whether the receiving water body can sustain microbial, aquatic fish, or plant life. A minimum DO value forhealthy biological treatment systems is between 1 to 2 ppm (mg/L). The maximum concentration soluble in water under normalconditions (saturation concentration) is between 8 to 10 ppm (mg/L) and is a function of salinity and temperature.Endogenous Respiration - The energy required for cell maintenance. Other factors such as cell death and predation areusually combined with endogenous respiration in a term called endogenous decay.Heterotrophs - Organisms that obtain carbon for the formation of cell tissue from dissolved organic substrates.Hydraulic Retention Time (HRT) - The length of time the influent wastewater is retained in the aeration basin (not includingthe effect of sludge recycle; i.e., aeration volume divided by influent volumetric flowrate).Mixed Liquor Suspended Solids (MLSS) - The concentration of total suspended solids in the aerated section of a biologicaltreatment unit or lagoon. MLSS is normally expressed in units of ppm or mg/L.Mixed Liquor Volatile Suspended Solids (MLVSS) - The portion of the MLSS which volatilizes at 1022°F (550°C). Biologicalsolids (microorganisms) are the main contributors to MLVSS. For systems which add powdered activated carbon, a separateacid digestion step can be done to distinguish between MLVSS due to carbon and MLVSS due to biomass. MLVSS is normallyexpressed in units of ppm or mg/L.pH - A measurement of the acidic or basic character of a solution at a given temperature. It is defined as the negativelogarithm (to the base 10) of the hydrogen ion concentration (-log[H+]). Pure water is slightly ionized with a pH of 7, and atequilibrium the ion product, Kw , is [H+][OH–] = 1.01 x 10–14 at 25°C. Generally, biological treatment systems operate best atpH ranges between 6.5 to 8.5. Wastewater outside the 5.5 to 9.5 pH range (before being commingled in the mixing zone of thewastewater effluent and the receiving water body) can potentially cause harm to the receiving water aquatic life as well as thebiological treatment microorganisms.Total Dissolved Solids (TDS) - A measure of all dissolved material in a solution, including inorganic salts (e.g., NaCl, MgCl,etc.) that typically make up the bulk of the TDS measured in the standard lab test. TDS is used to determine the salt levels ofwastewater. Measurement for TDS consists of passing a sample through a standard glass fiber filter, and the filtrate isevaporated to dryness in a weighed dish and dried to a constant weight at 356°F (180°C). The material remaining on the filterpaper is the total dissolved solids and is reported in units of ppm or mg/L. Conductivity can be used for a quick substitutemeasurement for TDS (reported in units of micromhos or microsiemens). As a rule-of-thumb, for wastewater streams at pH 7,the TDS of that stream in ppm (mg/L) can be approximated by multiplying the conductivity in units of micromhos ormicrosiemens by 0.7.Theoretical Oxygen Demand (ThOD) - The oxygen required to oxidize all organic or reduced inorganic compounds to CO2,SO4, NO3, etc. In practice, the TOD and COD measure by analytical procedures will approach the calculated ThOD. ThOD isnormally expressed as ppm or mg/L.





3.0 DEFINITIONS (Cont)Total Organic Carbon (TOC) - The quantity of organically bound carbon in a sample. TOC is commonly used as areplacement for BOD since the test for TOC is significantly faster than the 5-day test for BOD, and the BOD test can sometimesgive erroneous results. TOC is normally expressed as ppm or mg/L.Total Oxygen Demand (TOD) - The amount of oxygen required to oxidize all oxidizable substances in a sample, including thebiodegradable organic matter. It is measured using a special analytical instrument. TOD is normally expressed as ppm ormg/L.

4.0 BACKGROUND AND SELECTION CRITERIA4.1 BACKGROUNDAeration provides dissolved oxygen in the wastewater and necessary mixing for sludge suspension in the suspended growth orcombination systems, intimate sludge-waste contact, and dispersion. There are two basic methods to supply oxygen andmixing to the biological treatment system: agitate the wastewater mechanically so the wastewater contacts the air from theatmosphere, or introduce air or pure oxygen into the wastewater with submerged diffusers or devices. (11)

4.2 SELECTION CRITERIAThe selection criteria are based mainly on the amount of oxygen transfer needed and the volume (particularly depth) of theaeration basin. Figure 4.2-1 contains a decision tree for rough guidelines used to choose which type of aeration system isrecommended for a particular application. Since temperature / heat loss from mechanical aerators is higher than diffused, inthe case of cold climates, diffused aeration should be considered. In terms of minimum depth, it is difficult to select either type(mechanical or diffused) for tanks / basins less than 6 ft (2 m). Aeration systems used at various ExxonMobil faciities are listedin Table 4.2-1.

5.0 MECHANICAL AERATION SYSTEMS5.1 DESCRIPTIONMechanical aeration systems agitate the wastewater to serve two functions: supply oxygen to the wastewater by contacting itwith the air from the atmosphere and provide the necessary mixing for sludge suspension. There are two groups of mechanicalaerators: aerators with vertical axis and aerators with horizontal axis. Within these two groups aerators are classified assurface or submerged. Figure 5.1-1 contains examples of various types of mechanical aerators that are available.

Mechanical Aerators-Vertical Axis

Surface aerators with vertical axis are designed to induce either updraft or downdraft flows below the aerator. Impellers areeither totally or partially submerged and are attached to the motors. The aerators can be mounted on floats (pontoon-mounted)or fixed (mounted on a bridge or on columns). Updraft impeller types and speed vary including centrifugal (high-speed), radial-axial (medium speed) and axial (low speed). Low speed aerators are driven through a reduction gear by an electric motor whilehigh-speed aerator impellers are coupled directly to the electric motor rotating shaft. Low speed aerators are usually mountedon fixed supports because they tend to be heavier and require more maintenance than high speed. The updraft aeratorsusually are positioned at, or slightly below, the surface, where they pump large amounts of water and propel it into the air.Down draft impeller types also vary including open-turbine, closed turbine, and forced air-propeller aerators. The down draftaerators force liquid from the top through a vertical tube to the basin bottom while entraining air. The aerated water is expelledfrom the bottom of the tube. Down draft aerators are usually less efficient than updraft aerators but are used when turbulenceand splashing by updraft aerators would throw water out of the basin.Submerged surface aerators with vertical axis are classified into three groups: turbine, impeller (suction), and porous-rotatingdiscs. Turbine aerators either have mechanical means for drafting air in the liquid or use spargers that are fed air by blowers.Impeller aerators are operated with impeller created suction or air from a blower. Porous rotating discs using forced air have acored impeller with a porous top plate secured to a hollow shaft. Air is blown through the hollow shaft in the impeller core andis passed through the porous plate.

Mechanical Aerators-Horizontal Axis

Mechanical aerators with horizontal axis are either surface aerators or submerged aerators. The surface aerators have ahorizontal cylinder rotor with bristles / angle steel / plastic bars submerged in the bulk liquid of the aeration basin / tank. As thecylinder rotor rotates, the bars drive air into the water and throw water jets and droplets into the air while the liquid is propelledforward to provide mixing.




5.0 MECHANICAL AERATION SYSTEMS (Cont)An example of a submerged aerator with horizontal axis is a disc aerator. Thin discs are mounted on a horizontal shaft thatrotates within the wastewater. As the shaft rotates, recesses and nodules on the disc face propel and transfer oxygen into thewastewater.

Performance

Mechanical aerators are rated in terms of their oxygen-transfer rate under standard conditions, expressed in pounds of oxygenper horsepower-hour (kilograms of oxygen per kilowatt-hour). Standard conditions include temperature at 20°C, dissolvedoxygen of 0.0 mg/L, and clean tap water. Standard transfer rates for mechanical aerators range from 1.5 to 5 lb O2/hp hr.

5.2 DESIGN CONSIDERATIONSIn the design of a surface aeration system, considerations must be given to selecting which type of surface aerator will providesufficient amount of oxygen and mixing and still be most economical for the aeration basin volume and depth. Once the type ofsurface aerator has been identified, aerator layout is critical. Another important consideration to keep in mind if surfaceaerators are chosen is that the temperature / heat loss for mechanical surface is higher than diffused. Like all other rotatingequipment, mechanical surface aerators require maintenance and therefore consideration should be made to their accessibility.Also, certain surface aerators have better control techniques to vary the oxygen concentration in the basin / tank. For example,surface high speed aerators usually have two options: on or off, while surface low speed aerators can be controlled bysubmergence, speed, and the option of being on or off. Table 5.2-1 shows a sample duty specification sheet that is normallysupplied to the aeration vendor.

5.3 DESIGN PROCEDURE FOR MECHANICAL SURFACE AERATORSBefore the surface aerators can be designed, the aeration basin / depth must be assumed, since many of the parameters usedto size the surface aerators are depth dependent. The aeration basin / tank volume is set to provide sufficient hydraulicretention time for degradation and the basin water depth is set to provide effective and economical aeration and mixing. Depthselection is governed by aerator size, impeller speed and design, civil engineering considerations, construction costs, theavailable plot area, and the water table. Once the basin water depth is decided, 2 to 3 ft of freeboard above the water level isadded to obtain the total basin depth. When using mechanical surface aerators the basin / tank depth normally ranges from6 - 12 ft. Surface aerators used in depths greater than 15 ft require draft tubes or double impellers.Once the total basin depth is assumed, two equations are used to determine the Total BHP requirement. One equationcalculates the BHP necessary to fulfill the oxygen requirement while the other determines the BHP necessary to keep the solidsin suspension. The greater of the two requirements is used for the design purposes. Once the total BHP has been determined,the total number of aerators and layout configuration of these aerators can be can be estimated.

A. Calculation of BHP needed for O2 requirement

Quick, Rough Sizing Basis

Use 2 lb O2/bhp hr as the actual oxygen transfer rate, N, of the surface aerators under operating conditions when temperatureis around 25°C and TDS levels are 0 - 1000 mg/L to determine the total surface aerator HP required for the oxygenrequirements using Eq. 5.3-1. If the operating conditions are not 25°C and 0 to 1000 mg/L TDS, use the standard procedures:

Total BHP for O2 requirement = ))hr/bhpO(lb(224

)/dOlb(AOR2

2⋅⋅

Eq. (5.3-1)

where: AOR= Actual Oxygen Required, calculated from Step 10 and 11 in Section 6.3 of DP XIX-A5

Note: 1W = 1.341 x 10-3hp1kg = 2.20462 lb





5.0 MECHANICAL AERATION SYSTEMS (Cont)Standard Procedures

Step 1. Determine the actual oxygen transfer rate of the surface aerators under operating conditions, N (lb O2/bhp hr) usingthe following equation:

( ) ( ) α⋅⋅−⋅= − )20T(Lswo 024.1

17.9CCNN Eq. (5.3-2)

Csw = Css fp β Eq. (5.3-3)

( )( )p760

pfp −−= π Eq. (5.3-4)

where: N = Actual oxygen transfer rate of the surface aerators under operating conditions, lb O2/bhp hr

No = Standard oxygen transfer rate of the surface aerators under standard conditions, clean water, 20°C, zero dissolved oxygen and 1 atm pressure, usually supplied by the aeration vendor, estimated to be at 3.4 lb O2/bhp hr for large horsepower surface aerators (100 hp). The standard oxygen transfer rate decreases with horsepower size.

Csw = Saturation oxygen solubility in the mixed liquor at operating temperature and design altitude, mg/L

Css = Oxygen saturation value in pure water at the temperature of the mixed liquor, mg/L(see Figure 5.3-1)

fp = Pressure factor for oxygen solubility, dimensionlessπ = Total pressure, mm Hgp = Saturation water vapor pressure at the temperature of the mixed liquor, mm Hg

β =etemperatursameatwatercleaninoxygenofionconcentratSaturation

liquormixedtheinoxygenofionconcentratSaturation , dimensionless Eq. (5.3-5)

The Beta factor usually ranges from approximately 0.8 to 1.0. It is difficult to measure the saturation concentration of theoxygen in the mixed liquor accurately with a dissolved oxygen membrane probe because many wastewaters containsubstances that interfere with the dissolved oxygen determination. The value of beta, therefore is usually based on thedissolved solids content of the process water and should be calculated as the ratio of the dissolved surface saturationconcentration in water, with dissolved solids equal to that of the process water, to the dissolved oxygen surface saturationconcentration in clean water. Therefore, to determine the value of Beta, obtain a TDS value of the mixed liquor, assume that allthe TDS is NaCl and estimate the total Chloride (g/kg of solution) ≈ Chlorinity content in the mixed liquor. Using Table 5.3-1 atthe mixed liquor temperature and Chlorinity, determine the oxygen solubility (saturation concentration of oxygen in the mixedliquor). The saturation concentration of oxygen in clean water at the same temperature can be determined from this samechart assuming the Chlorinity = 0. The ratio of these two oxygen solubilities = the Beta factor. (Many factors influence the Betafactor including temperature, pressure, and wastewater characteristics.)

Chlorinity0andTemp.liquormixedatwatercleanofsolubilityOxygenChlorinityandTemp.liquormixedtheatsolubilityOxygen=β , dimensionless Eq. (5.3-6)

where: CL = Concentration of dissolved oxygen to be maintained in the aeration basin, mg/L, usually = 2 mg/L,

9.17 = Saturation oxygen concentration in clean water at 20°C and 1 atm pressure, mg/L,T = Mixed liquor temperature in aeration basin, °C,

C20atwatercleanofa)(KoxygenoftransferofRateC20atliquormixedtheofa)(KoxygenoftransferofRateFactor,Alpha

L

L�

�

=α , dimensionless Eq. (5.3-7)




5.0 MECHANICAL AERATION SYSTEMS (Cont)The alpha factor ranges from approximately 0.2 to greater than 1.0. For surface aerators in refinery wastewater, 0.8 istraditionally used. (Many factors influence the alpha value including process conditions such as surfactants, turbulence, powerinput per unit of volume, tank geometry, geometric scales between aeration tank and aeration device, bubble size, degree oftreatment and wastewater characteristics. Vendors often supply alpha factors for their equipment. Use of the vendor-suppliedvalues is only recommended if the vendor can supply data to support their use in the similar-type wastewater.)

Step 2. Determine the total BHP required to fulfill the oxygen requirements:

Total BHP for O2 requirement = hr))/bhpO(lb(N24

/d)O(lbAOR2

2⋅⋅

Eq. (5.3-8)

B. Calculation of BHP needed for solids suspension in an activated sludge unit

Step 1. Estimate the brake horsepower per thousand gallons:

��

�

��

��

�

�=

gal1000kgal1)gal(V

trequiremenOforBHPgal1000

BHP 2 Eq. (5.3-9)

Step 2. If BHP/1000 gal is greater than or equal to 0.14, then the total brake horsepower needed for the oxygen requirement isalso sufficient to keep the solids suspended in an activated sludge unit. (Aerated lagoons have different requirementsdiscussed in DP XIX-A5, Section 8.0.)If BHP/1000 gal is less than 0.14 for an activated sludge unit, then the total brake horsepower needed for both the oxygenrequirement and solids suspension is calculated by:

Total BHP required for O2 and solids suspension = (1.4 x 10-4) V (gal) Eq. (5.3-10)

If BHP/1000 gal is greater than 0.35, the total brake horsepower may be too high for mechanical aeration (check with vendor);consider diffused aeration.Note: Aerated lagoons have different requirements, refer to DP XIX-A5, Section 8.0.

C. Determine the aerator horsepower size and number of surface aerators needed to supply the total BHP.Step 1. The aerator horsepower size is determined by taking into account the horsepower sizes which are offered by theaeration vendors, the representative circles of influences (diameters for complete mixing, DCM, and impingement patterndiameters, DIP) and the depth of the aeration basin. The aeration basin depth is usually the limiting parameter, as largehorsepower sized surface aerators can not be used in shallow aeration basin depths because the aerator hydraulic energytends to scour the bottom of the basin. The table below describes the common horsepower sizes available and their circles ofinfluence and minimum basin water depth recommended.





5.0 MECHANICAL AERATION SYSTEMS (Cont)

AERATOR HORSEPOWER SIZE,BHP

CIRCLE OF INFLUENCE FORCOMPLETE MIXING, DCM, ft

CIRCLE OF INFLUENCE FORIMPINGEMENT PATTERN

DIAMETERS, DIP, ft

MINIMUM BASIN WATERDEPTH RECOMMENDED FOR

THIS AERATOR SIZE, ft*

1 18 6.5 62 23 7 63 42 14 65 45 15 6

7.5 50 18 810 51 18 1015 62 20 1020 72 20 1025 80 24 1030 88 24 1040 102 26 1050 105 26 1260 115 27 1275 130 30 12100 150 38 12125 165 40 12150 185 40 12

* Basin with a depth greater than 15 ft that use surface aerators require draft tubes on the aerators, or double impellers foroxygen transfer and mixing.

Step 2. Using the aeration basin depth as the deciding parameter, choose an aerator horsepower size and estimate thenumber of surface aerators that would be required to supply the Total BHP required for O2 and solids suspension, rounding upto the next higher whole number:

# of aerators = BHPSize,HorsepowerAerator

BHP,suspensionsolidsandOforrequiredBHPTotal 2 Eq. (5.3-11)

Step 3. After determining the number of aerators necessary for the Total BHP requirement, lay out the aerators within thebasin area using the following guidelines:• The aerators should be laid out such that the circles of influence are within the minimum-maximum range and are as far

apart as possible, in other words the complete mixing zones are just about touching. If placement requires that the mixingzones partially overlap, the aerators should be laid out such that the impingement pattern diameters do not overlap.

• It is important to maintain complete aeration coverage throughout the aeration basin without creating any dead zones.• Ensure that each aerator has an equal area of influence, and the interaction of the aerators does not cause a vibration

problem.

Step 4. Repeat steps 2 and 3 until the optimum number of aerators and aerator horsepower size is established to supplysufficient oxygen at a minimum cost.

Step 5. If layout considerations permit, and if adjustment of basin geometry will permit a more economical use of aerators,adjust basin geometry (length, width, and depth) and repeat steps as necessary to determine the most economical unit.




5.0 MECHANICAL AERATION SYSTEMS (Cont)

5.4 SAMPLE DESIGN PROBLEMSFrom the data given in the sample design problem of the activated sludge section:

V = 5.8 Mgal (21,800 m3)AOR, O2 = 27,800 lb/d (12,700 kg/d)Min. temperature = 60°F (15°C)T = 68°F ( 20°C)Max. dissolved solids = 3000 mg/LTotal pressure = 760 mm Hg

Note: The following is an example for an aeration system in an activated sludge process.Aerated lagoons have different criteria which are covered in DP XIX-A5, Section 5.1.3.

A. Calculation of BHP needed for O2 requirement

Step 1: Determine the actual oxygen transfer rate of the surface aerators under operating conditions, N (lb O2/bhp hr)using Eq. 5.3-2.a. Assumptions:

Assume No = 3.4 lb O2/bhp hr (this value is normally supplied by the aeration vendor)Assume α = 0.8 because this is surface aerators and refinery wastewaterAssume CL = 2 mg/L recommended concentration of dissolved oxygen maintained in the aeration basin / tank

b. Use Figure 5.3-1 to determine Css at 68°F (20°C), Css = 9.4 mg/Lc. From Steam Tables determine, p, the saturation water vapor pressure at 20°C, p = 17.55 mm Hgd. Determine the value of fp from Eq. 5.3-4.

1)55.17760()55.17760(fp =

−−=

e. Determine the value of β by assuming the maximum dissolved solids is all NaCl and determining the chlorine content. UseTable 5.3-1 to determine the saturation concentration of oxygen in the mixed liquor at 20°C with this chlorinity and in cleanwater at 20°C

chlorinity8.1solutionkg

g8.1ClL

mg1821Clmmole1Clmg5.35

NaClmmole1Clmmole1

NaClmg5.58NaClmmole1NaCl

Lmg3000 ≈==⋅⋅⋅

From Table 5.3-1, at 20°C and 1.8 Chlorinity (interpolation), the saturation concentration in the mixed liquor = 8.92 mg/LFrom Table 5.3-1, at 20°C and 0.0 Chlorinity the saturation concentration in pure water = 9.09 mg/LTherefore from Eq. 5.3-6:

98.009.992.8

C20atwatercleaninoxygenofionconcentratsaturationC20aliquormixedtheinoxygenofionconcentratsaturation ===β

�

�

f. Calculate Csw from Eq. 5.3-3:

212.998.014.9fCC psssw =⋅⋅== β

g. Calculate N from Eq. 5.3-2:

hrbhpOlb14.28.0)024.1(

17.9)2212.9(4.3)024.1(

17.9)CC(NoN 2)2020()20T(Lsw

⋅=⋅⋅−⋅=⋅⋅−⋅= −− α

Step 2. Determine the total BHP required to fulfill the actual oxygen requirements:Use Eq. 5.3-8 to calculate the total BHP required for oxygen:

Total BHP for O2 requirement = BHP54114.224

800,27)]hrBHP/Olb(N[24

)d/Olb(AOTR2

2 −⋅

−⋅⋅





5.0 MECHANICAL AERATION SYSTEMS (Cont)B. Calculate the BHP needed for solids suspension:

Step 1. Estimate the brake horsepower per thousand gallons (Eq. 5.3-9):

gal1000BHP09.0

kgal5800BHP541

gal1000kgal1)gal(V

trequiremenOforBHPgal1000

BHP 2 ==

��

�

��

��

�

�=

Step 2. Compare to 0.14, since 0.09 < 0.14, the total brake horsepower needed for both oxygen and solids suspension iscalculated by Eq. 5.3-10.

Total BHP required for O2 and solids suspension = (1.4 x 10-4) V (gal) = 812 bhp ≈ 800 hp

C. Estimate the aerator horsepower size and number of surface aerators needed.Assuming the basin / tank water depth is around 12 ft, a 100 bhp surface aerator would be an appropriate aerator horsepowersize. The total number of aerators is calculated by the following equation, rounded up to the next whole number (Eq. 5.3-11).

# of aerators = aeratorssurface8100800

BHPSize,HorsepowerAeratorBHP,ssuspensionsolidsandOforrequiredBHPTotal 2 ==

Select the aeration-basin size. If a rectangular basin is desired, select the length, L (ft), and the width, W (ft), to provide thenecessary basin area determined by dividing the volume calculated in Eq. 6.3-3 of DP XIX-A5 (converted to ft3) by theassumed basin / tank water depth above. If a circular basin is desired, select the diameter, such that:

A (ft2) = 4

)diameter( 2π

Volume = 5.8 Mgal = 775,460 ft3 (21,961 m3)

Area = 775,460 ft3 / 12 ft = 60,165 ft2 (21,961 m3 / 3.7 m = 6,003 m2)

Assume 200 ft (61 m) width by 300 ft (91 m) length and space the aerators such that the complete mixing zones are touchingbut the impingement pattern diameters do not overlap. Two rows of four aerators will be sufficient.

5.5 OPERATING STRATEGIES AND ENHANCEMENTSAeration is critical for biodegradation of organics. A dissolved oxygen level of 2 mg/L should be maintained in the basin / tankto ensure enough oxygen is supplied for the microorganisms. The mixing intensity of the aerators should be high enough tokeep the solids suspended. For surface aerators most of the control is done by turning the aerators on or off. Some othermechanical aerators have different control mechanisms that allow the operator to change the dissolved oxygen level in thebasin by changing the motor speed. In the case where forced air is used in addition to the mechanical aerator, the amount offorced air can be regulated.

6.0 DIFFUSED AERATION SYSTEMS6.1 DESCRIPTIONDiffused aeration systems supply oxygen to the microorganisms in the wastewater by injecting air or pure oxygen underpressure into the wastewater below the liquid surface. A diffused aeration system consists of diffusers submerged in thewastewater connected to header pipes and air mains. Blowers force air through the headers and air mains and eventuallythrough the diffusers. There are many types of diffuser devices which used to be categorized as fine-bubble or coarse-bubble,but the definition of these categories is not always clear, so diffuser devices are now categorized by the physical characteristicsof the equipment: porous, non-porous, other. Diffused aeration systems can also be installed for total floor coverage in a gridpattern or arranged to produce a spiral roll pattern within the basin / tank.




6.0 DIFFUSED AERATION SYSTEMS (Cont)Porous diffuser devices come in many shapes including the plate, dome, disc and tube. Categorically, these diffusers usuallyproduce a fine bubble. Table 6.1-1 contains a description and picture of each. Either rigid ceramic and plastic materials orflexible plastic or cloth sheaths are used to manufacture these diffusers. Plate diffusers are installed in the bottom of the basin /tank. A chamber underneath the plates acts as an air plenum. Several plates can be fixed over a common plenum. One of theproblems associated with plate diffusers is that air distribution among several plates attached to the same plenum is notuniform. Plates are also difficult to remove for repair as they are grouted in place. Expansion of a diffused plate system is alsodifficult. The dome diffuser is a circular disc with a downward-turn, peripheral edge. The disc diffuser is a flat version of thedome diffuser. The tube diffusers are simply tubes extended off the air distribution system.Non-porous diffuser devices also come in many shapes and sizes as shown in Table 6.1-1. Bubble size can be both coarseand fine. Some have fixed orifices: perforated piping, spargers, and slotted tube. One has a valved orifice that contains acheck valve to prevent backflow when air is turned off. Another type is the static tube diffuser which is a stationary vertical tubethat functions like an air lift pump. Perforated hose is also used as a non-porous diffuser.Other devices include jet aerators and aspirating aerators, also shown in Table 6.1-1. Jet aerators are unique in that they useboth a liquid recirculation pump and compressed air together. The mixture is discharged through a nozzle to promote intimatecontacting between the liquid and the air. Aspirating aerators may be inclined or vertical, and consist of a hollow tube with anelectric motor and propeller or impeller. The subsurface propeller or impeller draws air from the atmosphere and injects itunder the water surface. Individual aspirating aerators are mounted on fixed supports or booms or floats much like mechanicalsurface aerators. The vertical aspirating aerators sit on the bottom of the basin or tank.

6.2 DESIGN CONSIDERATIONSWith diffused aeration systems, consideration must be taken in choosing the aeration basin / depth and diffuser type. Oxygentransfer efficiency increases with basin water depth. Oxygen transfer efficiency also increases with decreasing bubble size.Although fine bubble diffusers may have a higher oxygen transfer efficiencies; their performance can decline as diffusersbecome clogged. Cleaning mechanisms to unclog the perforations have been developed, including removing the diffusers, in-situ methods that interrupt the process, and in-situ methods that do not interrupt the process. Some fine bubble diffusers use aflexible membrane sheath to create the fine bubble. Care must be taken to ensure that the membrane material is compatiblewith the wastewater. For these reasons, coarse bubble diffusers are normally recommended for refinery activated sludgesystems greater than 12 ft deep.Table 5.2-1 shows a sample specification sheet that is normally supplied to the aeration vendor. There tends to be someconfusion between actual and standard oxygen demand. The actual oxygen demand is governed by the amount of organicremoval and biomass population in the system under operating conditions. The standard oxygen demand is usually about twotimes that of the actual demand when the temperature and pressure are similar to standard conditions. The reason for theincrease is that all the aeration equipment parameters are measured under standard conditions (20°C, O mg/L dissolvedoxygen, 1 atm, clean water) and in order to predict the oxygen transfer under operating conditions, much more oxygen isrequired. In other words the amount of oxygen needed under operating conditions is usually about two times that needed inclean water at standard conditions when the temperature and pressure under operating conditions is close to the standardvalues.

6.3 DESIGN PROCEDUREBefore the diffused aeration systems can be designed, the aeration basin / depth must be assumed, since many of theparameters used to size the system are depth dependent. The aeration basin / tank volume is set to provide sufficienthydraulic retention time for degradation and the basin water depth is set forth to provide effective and economical aeration andmixing. For diffused aeration systems, oxygen transfer efficiency increases with basin water depth, simply because theaeration bubbles have a longer contact time with the wastewater in deeper systems.Several steps are involved in calculating the aeration and mixing requirements when using diffused aeration systems. The firststep involves standardizing the actual oxygen transfer rate requirement. The next step is an iterative process to determine thestandard oxygen transfer efficiency and the standard airflow. Once the standard airflow has been calculated, the total blowerBHP requirement can be estimated. Diffuser arrangement is aeration system dependent and is usually supplied by the aerationvendor who optimizes the spacing for the most economical layout. For diffused aeration, typically full floor coverage is desired.The vendor should provide adequate reference to support designs recommending less then full floor coverage.

A. Standardizing the Actual Oxygen RequirementThe oxygen requirement as calculated in Step 10 of the Activated Sludge Design Procedure is termed the Actual OxygenRequirement (AOR, lbs O2/d) under operating conditions and is used to calculate the Standard Oxygen Transfer RateRequirement under standard conditions. Standard conditions are in clean water at 20°C and 1 atm pressure. Since thediffused aeration systems are tested for aeration parameters at standard conditions, it is necessary to create a StandardOxygen Transfer Rate Requirement based on the Actual Oxygen Requirement under operating conditions.





6.0 DIFFUSED AERATION SYSTEMS (Cont)Quick, Rough Sizing Basis

1. Use the equation below to convert the Actual Oxygen Requirement to the Standard Oxygen Transfer Rate Requirementwhen the temperature is around 25°C, TDS levels are 0 - 1000 mg/L TDS, and location is at about sea level:

SOTR (lb O2/hr) = 224

)/dayOlb(AOR 2 ⋅ Eq. (6.3-1)

2. Calculate the standard airflow, Q, in scfm

Q = scfm,SOTE

SOTR62.96 ⋅ Eq. (6.3-2)

where: SOTE =Standard Oxygen Transfer EfficiencyUse 1% per ft of depth for coarse bubble diffused airUse 1.5% per ft of depth for fine bubble diffused air

3. Calculate the Blower Power Requirement, Pe

Pe = ��

�

�

��

�

�−�

�

�

� ⋅+⋅⋅ 15.14

d433.02.16355.0Q283.0

, BHP Eq. (6.3-3)

where: d = diffuser submergence, ft

Standard Procedures

1. Estimate the SOTR using the following equation:

SOTR =

)20T(

20

Lwalt

2

*C

C*C

24/d)Olb(AOR

−θ∞

βα ��

�� −⋅

��

��

�

Eq. (6.3-4)

where: SOTR (lb O2/hr) = Standard Oxygen Transfer Rate, lb O2/hrAOR (lb O2/day) = Actual Oxygen Required, lb O2/d

C20atwatercleanof)aK(oxygenoftransferofRateC20atliquormixedtheof)aK(oxygenoftransferofRate,FactorAlpha

L

L�

�

=α , dimensionless from Eq. (5.3-7)

The alpha factor ranges from approximately 0.2 to greater than 1.0. In refinery water, alpha is usually 0.7 for coarse bubblediffusers, 0.6 for fine bubble diffusers, and 0.9 for jet aeration. (Many factors influence the alpha value including processconditions such as surfactants, turbulence, power input per unit of volume, tank geometry, geometric scales between aerationtank and aeration device, bubble size, degree of treatment and wastewater characteristics.)

etemperatursameatwatercleanin*CoxygenofionconcentratSaturationliquormixedthein*CoxygenofionconcentratSaturation

∞∞β = , dimensionless from Eq. (5.3-5)




6.0 DIFFUSED AERATION SYSTEMS (Cont)The Beta factor usually ranges from approximately 0.8 to 1.0. It is difficult to measure the saturation concentration of theoxygen in the mixed liquor accurately with a dissolved oxygen membrane probe because many wastewaters containsubstances that interfere with the dissolved oxygen determination. The value of beta, therefore is usually based on thedissolved solids content of the process water and should be calculated as the ratio of the dissolved surface saturationconcentration in water, with dissolved solids equal to that of the process water, to the dissolved oxygen surface saturationconcentration in clean water. Therefore, to determine the value of Beta, obtain a TDS value of the mixed liquor, assume that allthe TDS is NaCl and estimate the total Chloride (g/kg) ≈ Chlorinity content in the mixed liquor. Using Table 5.3-1 at the mixedliquor temperature and chlorinity, determine the oxygen solubility (saturation concentration of oxygen in the mixed liquor). Thesaturation concentration of oxygen in clean water at the same temperature can be determined from this same chart assumingthe chlorinity = 0. The ratio of these two oxygen solubilities = the Beta factor. (Many factors influence the Beta factorincluding temperature, pressure, and wastewater characteristics)

Chlorinity0andeTemperaturliquormixedatwatercleanofsolubilityOxygenChlorinityandeTemperaturliquormixedtheatsolubilityOxygen=β , dimensionless from Eq. (5.3-6)

C *walt = Aeration system dissolved oxygen saturation at operating conditions, mg/L; CT,Pd x fd Eq. (6.3-5)

where: CT,Pd = Dissolved oxygen saturation at operating temperature and pressure, mg/L (see Table 6.3-1),

fd = Super saturation depth correction factor, varies with diffuser (see Table 6.3-2)

CL = Concentration of dissolved oxygen to be maintained in the aeration basin mixed liquor, mg/L,usually = 2 mg/L,

*C 20∞ = Aeration system equilibrium dissolved oxygen saturation concentration at Standard Conditions, mg/L.This value is aerator diffuser dependent. The equations shown below are based on a simpleperforated pipeline, total floor coverage grid system.

d2020 fxC*C =∞ Eq. (6.3-6)

where: C20 = Surface dissolved oxygen saturation concentration at Standard Conditions, C20 = 9.09 mg/L

fd = Super saturation depth correction factor (see Table 6.3-2)

or

*C 20∞ = 0.075d + 9.1 when d = aeration system submergence for 0 < d ≤ 20 ft Eq. (6.3-7)

*C 20∞ = 0.12 (d – 20) + 10.6 when d = aeration system submergence for d > 20 ft Eq. (6.3-8)

θ = theta value (1.024), this value incorporates temperature influence on the oxygen transfer coefficient,

T = mixed liquor temperature, °C,

2. Estimate the Standard Oxygen Transfer Efficiency (SOTE) and Standard Airflow RateUse an aeration system performance curve supplied by the aeration vendor as shown in Figure 6.3-1 with the StandardOxygen Transfer Efficiency, % versus Airflow, SCFM/1000 ft3 of liquid for a specific liquid depth, iteratively solve thefollowing equation to find both the airflow, Q and SOTE. The aeration system performance curve will vary depending uponthe aeration system diffuser type. The curve shown in Figure 6.3-1 is for a simple perforated pipe total floor coverage gridsystem and can be used to get a rough idea of the efficiency and airflow. Diffuser spacing and mixing requirements havebeen factored into the performance curve.





6.0 DIFFUSED AERATION SYSTEMS (Cont)

SOTESOTR62.96Q ⋅= Eq. (6.3-9)

where: Q = Standard airflow, scfm,SOTR = Standard Oxygen Transfer Rate Requirement, lb O2/hr,SOTE = Standard Oxygen Transfer Efficiency, Percent

3. Determine the Total Blower (BHP) Power Requirement

Pe = ��

�

�

��

�

�−�

��

�

�1

PP

en550TRw

n

1

2a Eq. (6.3-10)

w = (Q x 0.075) / 60 Eq. (6.3-11)

Ta = T + 460 Eq. (6.3-12)

P1 = Pb – Pi Eq. (6.3-13)

P2 = Pb + Pw + Ph Eq. (6.3-14)

Pw = 0.433 times the orifice submergence in water (depth in ft) Eq. (6.3-15)

where: e = Specified efficiency for delivered, brake, wire, or some other efficiency reference, decimal

n = (K - 1)/K; n = 0.283 for air; K = 1.395 for airPb = Barometric pressure at plant site, psiaPe = Power input at a specified efficiency, hpPh = Air piping system friction losses, (assume 1.5 psi)Pi = Inlet losses, (assume 0.2 psi)Pw = Water pressure, psiP1 = Compressor inlet absolute pressure, psiaP2 = Compressor outlet absolute pressure, psiaQ = Standard airflow, scfmR = 53.5 ft/°R, universal gas constant for airT = Ambient air temperature, °FTa = Compressor inlet absolute temperature, °Rw = Weight flow of air, lb/s

6.4 SAMPLE DESIGN PROBLEMSFrom the data given in the sample design problem of the activated sludge section:

V = 5.8 Mgal (21,800 m3)AOR,O2 = 27,800 lb/d (12,700 kg/d)Min. temperature = 60°F (15°C)T = 68°F (20°C)Max. dissolved solids = 3000 mg/LTotal pressure = 760 mg Hg

Note: The following is an example for an aeration system in an activated sludge process.Aerated lagoons have different criteria which are covered in DP XIX-A5, Section 8.0.




6.0 DIFFUSED AERATION SYSTEMS (Cont)A. Standardizing the Actual Oxygen Requirement

Step 1. Estimate the SOTR using Eq. 6.3-4:a. Assumptions:

Assume α = 0.7 because this is coarse bubble diffused aerators and refinery wastewaterAssume CL = 2 mg/L recommended concentration of dissolved oxygen maintained in the aeration basin / tankAssume the water depth = 18 ft

b. Determine the value of ββββ by assuming the maximum dissolved solids is all NaCl and determining the chlorinecontent. Use Table 5.3-1 to determine the saturation concentration of oxygen in the mixed liquor at 20°C with thischlorinity and in clean water at 20°C

Chlorinity8.1solutionkg

g8.1ClL

mg1821Clmmole1Clmg5.35

NaClmmole1CLmmole1

NaClmg5.58NaClmmole1NaCl

Lmg3000 ≈==⋅⋅⋅

From Table 5.3-1, at 20°C and 1.8 Chlorinity (interpolation), the saturation concentration in the mixed liquor= 8.92 mg/LFrom Table 5.3-1, at 20°C and 0.0 Chlorinity the saturation concentration in pure water = 9.09 mg/LTherefore from Eq. 5.3-6:

98.009.992.8

C20atwatercleaninoxygenofionconcentratSaturationC20atliquormixedtheinoxygenofionconcentratSaturation ===β

�

�

c. Estimate C*walt by using Eq. 6.3-5 and Tables 6.3-1 and 6.3-2 for the values of CT,Pd and fdFrom Table 6.3-1, at 20°C,and 760 mm Hg (14.73 psia), CT,Pd = 9.09 mg/LFrom Table 6.3-2, at 18 ft deep, fd = 1.14. Therefore:

4.1014.109.9fC*C ddP,Twalt =⋅=⋅=

d. Estimate *C 20∞∞∞∞ by using Eq. 6.3-6.

4.1014.109.9fC*C d2020 =⋅=⋅=∞

Note *Cwalt and *C 20∞ are the same in this case because the location is at standard conditions.

e. Solve for SOTR using Eq. 6.3-4.

)20T(

20

Lwalt

2

*CC*C

24)d/Olb(AOR

SOTR−θ

∞

βα��

�

�

��

�

� −⋅

��

��

�

= = )2020(024.1

4.1024.1098.07.0

24800,27

−��

��

� −⋅

��

��

�

= 2098 lb O2/hr

Step 2. Estimate the SOTE and Standard Airflow RateFrom Figure 6.3-1 at a depth of 18 ft, assume the SOTE = 16.25%, calculate the Standard Airflow as follows fromEq. 6.3-9:

scfm454,1225.16209862.96

SOTESOTR62.96Q =⋅=⋅=

Estimate the Airflow, scfm/1000 ft3 of liquid and check that these values intersect on the performance curve inFigure 6.3-1.





6.0 DIFFUSED AERATION SYSTEMS (Cont)At a Standard Oxygen Transfer Efficiency of 16.25%, traveling horizontally until hitting the performance curve at an 18 ft liquiddepth, and then traveling vertically until to intersect the x axis the value of 17 scfm/1000 ft3 is found. This corresponds with12,474 scfm/721,980 ft3. Diffuser spacing and mixing requirements have been factored into the performance curve.

Step 3. Determine the Total Blower (BHP) Power requirements from Eq. 6.3-10:

w = (Q x 0.075) / 60 = (12,474 scfm x 0.075) / 60 = 15.6 lb/s from Eq. (6.3-11)

Ta = T (°F) + 460 = 68 + 460 = 528 °R from Eq. (6.3-12)

Pb = 14.7 psiaPi = 0.2 psi (assumed)

P1 = Pb – Pi = 14.7 – 0.2 = 14.5 psia from Eq. (6.3-13)

Pw = 0.433 x 18 ft = 7.794 psi from Eq. (6.3-15)

Ph = 1.5 psi (assumed)

P2 = Pb + Pw + Ph = 14.7 + 7.794 + 1.5 = 23.99 psia from Eq. (6.3-14)

n = (K - 1)/K; n = 0.283 for air: K = 1.395 for aire = 0.65 (assumed)R = 53.5 ft/°R universal gas constant for air

Solving for Pe:

��

�

�

��

�

�−�

�

�

�

⋅⋅⋅⋅=

��

�

�

��

�

�−�

��

�

�= 1

5.1499.23

65.0283.05505285.536.151

PP

en550TRwP

283.0n

1

2ae = 667 BHP from Eq. (6.3-10)

6.5 OPERATING STRATEGIES AND ENHANCEMENTSAeration is critical for biodegradation of organics. A dissolved oxygen level of 2 mg/L should be maintained in the basin/tank toensure enough oxygen is supplied for the microorganisms. The mixing intensity of the aerators should be high enough to keepthe solids suspended. For diffused aerators most of the control is done by controlling the amount of air from a variable speedblower.

7.0 ASPIRATING AERATORS SYSTEMSAspirating aerators are discussed in the diffused aeration section. They consist of a motor driven aspirator pump(propeller/impeller). The pump draws air in through the hollow shaft and disperses under the liquid surface where the propelleror impeller mixes it to create turbulence and diffused air bubbles. Aspirating aerators are normally recommended forapplications where oxygen transfer is not the main objective. For example, aspirating aerators have been used to supplyoxygen to ponds to prevent them from going septic. Aspirating aerators are sometimes also used for temporary additionaloxygen supply.

8.0 JET AERATION SYSTEMSJet aerators are discussed in the diffused aeration section because they are a type of diffuser, but are unique in that they alsouse a liquid recirculating pump to immediately contact the liquid and compressed air and eject it through a nozzle into thebasin/tank. This system is usually used for deep (25 ft or greater) applications.




9.0 DEEP TANK AERATIONDeep tank aeration generally refers to systems that are 30 ft (9 m) or greater in depth. Some non-ExxonMobil systems havebeen built as deep as 100 ft (30 m). The advantage of deep tank aeration is a smaller footprint and higher oxygen transferefficiency. The disadvantage is a compressor may be needed instead of a blower, which significantly increases the capitalcost. Aeration systems that have been used in deep tank service include coarse bubble diffused air, fine bubble diffused air, jetaeration, and static tubes. ExxonMobil has deep tank aeration systems operating at 30 ft (9 m) depth and at 41 ft (12.5 m)depth (Table 4.2-1). Both of these are coarse bubble diffused air systems. Although there has not been a lot of testing byaeration system vendors above 30 ft depth, it is generally believed that the increase in oxygen transfer efficiency per ft ofincreased depth starts to fall off above 30 ft. For this reason, and because greater depths will require a compressor instead ofa blower, it is not likely that depths significantly above 30 ft will be cost effective, unless it is a retrofit application or plot space islimited.Another issue with deep tank aeration is that there is an increased tendency for floating solids in the clarifier due to gas bubbleson the biosolids. A flocculating clarifier or a degassing trough may be needed to degass the solids and promote flocculation. Adegassing trough was specified for the 41 ft ExxonMobil application, but not for the 30 ft application. The degassing troughshown in Figure 9.0-1 is a proprietary design by The Advent Group, which was built for Ingolstadt Refinery.

10.0 AERATION SYSTEM PERFORMANCE TESTINGAeration system oxygen transfer performance can be measured either in clean water or in-process. Aeration system vendorstypically guarantee performance of their equipment in clean water only. The alpha factor is used to convert clean waterperformance to in-process performance at equivalent conditions of temperature, geometry and mixing. The alpha factor canvary from 0.2 to 1. As discussed in Sections 5.0 and 6.0, alpha factors of 0.8 for surface aerators, 0.7 for coarse bubblediffusers, 0.6 for fine bubble diffusers and 0.9 for jet aeration are traditionally used for refinery wastewater. Vendors shouldhave documentation of oxygen transfer efficiency tests of their aeration system at various depths under clean water conditionsto substantiate their quoted efficiencies. There should be third party involvement/certification of these tests. If this data is notavailable at conditions similar to the project design, then a clean water pilot test may be warranted. If this data is available, andvendor design calculations can be checked according to the procedures in Sections 5.0 and 6.0, then pre- or post-startupperformance testing is generally not warranted.The method for clean water oxygen transfer testing is documented in ASCE Standard 2-91, Second Edition. The methodessentially consists of removal of dissolved oxygen (DO) from the water by sodium sulfite addition followed by reoxygenation ofthe water to near saturation level. The DO inventory of the water volume is monitored during re-aeration and a simplified masstransfer model is used to estimate the oxygen transfer coefficient, KLa. If a full scale, on-site performance test is desired, thismethod can be used prior to startup.Guidelines for in-process oxygen transfer testing are documented in ASCE Standard 18-96. Several methods (non-steadystate, offgas, and inert gas tracer) are detailed and compared. These methods are considered well developed and providesatisfactory precision, however due to the wide range of process variables and wastewater characteristics which impact theprecision of these methods, they are not recommended by ASCE for compliance testing of aeration equipment. Due to theeffort involved in applying these methods, we anticipate that they would have limited application; for example to evaluate anoperating system which is not meeting expected performance. Two other methods, (steady state and mass balance method)are also discussed in the ASCE Standard, but are significantly less accurate than the first three.





11.0 NOMENCLATUREAOR Actual Oxygen Required, lbs O2/dayC Actual oxygen concentration, mg/LC20 Surface dissolved oxygen saturation concentration at standard conditionsCL Concentration of dissolved oxygen to be maintained in the aeration basin

20*C∞ Aeration system equilibrium dissolved oxygen saturation concentration at standard conditions

Cs Saturation oxygen concentration, mg/LCss Oxygen saturation value in pure water at the temperature of the mixed liquor, mg/LCsw Saturation oxygen solubility in the mixed liquor at operating temperature and design altitude, mg/LCT,Pd Dissolved oxygen saturation at operating temperature and pressure, mg/L

*Cwalt Aeration system dissolved oxygen saturation at operating conditions

d Diffuser submergence, ftDCM Circle of influence - diameter for complete mixingDIP Circle of influence - diameter for impingement patternDo Initial dissolved oxygen concentration, mg/Le Efficiencyfd Supersaturation depth correction factorfp Pressure factor for oxygen solubilityKLa Overall oxygen transfer coefficient, d-1

MLSS Mixed liquor suspended solids, mg/LMLVSS Mixed liquor volatile suspended solids, mg/LN Actual oxygen transfer rate of surface aerators under operating conditions, lb O2/bhp hrNo Standard oxygen transfer rate of the surface aerators under standard condition, clean water, 20°C, 0 mg/L

DO, 1 atm pressure, usually supplied by aeration vendor, lb O2/bhp hrp Saturation water vapor pressure at temperature of the mixed liquor, mm HgPb Barometric pressurePe Power input for blower at specified efficiencyP1 Compressor inlet absolute pressureP2 Compressor outlet absolute pressureQ Standard air flow, scfmSOTE Standard oxygen transfer efficiency, %SOTR Standard oxygen transfer rate requirement, lbs O2/hrT Temperature, °CTa Compressor inlet absolute temperature, °Ru Specific microbial growth rate, d-1

V Aeration tank reactor volume, Mgal (m3)W Weight of air, lb/sα Alpha factor for aeration

β Beta factor for aerationπ Total pressure, mm Hgθ Theta Value for aeration




TABLE 4.2-1EXXONMOBIL FACILITIES AERATION SYSTEMS

BASIN WATER DEPTHLOCATION

ft (m)AERATION SYSTEM TYPE AERATION SYSTEM

MANUFACTURER

Activated Sludge UnitsAntwerp N/A Mechanical Surface N/ABenicia 16 (5) Coarse bubble diffused air Sanitaire Stainless Steel Fixed

HeaderBaton Rouge 14 (4.3) Mechanical SurfaceBaton Rouge AggressiveBiological Treatment

21 (6.4) Fine bubble diffused air Norton Ceramic Dome

Beaumont N/A Mechanical Surface N/ACerro Negro 18 (5.5) Fine bubble diffused air Flexline Fine Bubble Membrane

Diffuser(US Filter, Envirex)

Chalmette 10 (3) Mechanical Surface N/AFlorham Park Sanitary N/A EDI Membrane fine bubble diffused

airN/A

Fos N/A Mechanical SurfaceIOCO N/A Static Tube Coarse Bubble Diffused

AirAertec Kenic

Ingolstadt(Process, high TDS)

41 (12.5) Coarse bubble diffused air Aertec

Ingolstadt(sanitary and low TDS)

6.6 (2) Surface Mechanical /Submerged aspirating

N/A /ABS Frings Tauchbeluefter

Karlsruhe N/A Mechanical Surface N/APort Jerome 23 (7) Coarse bubble Diffused/Mechanical Vibrair by DegremontRotterdam 30 (9.2) Diffused N/ATrecate 7.5 (2.3) Mechanical Surface N/ASanta Ynez N/A Coarse bubble diffused air Sanitaire Stainless Steel Fixed

HeaderSlagen 19.7 (6) Fine / Coarse bubble diffused air Nopol Elastomer Membrane Disc

LagoonsSriracha 6.6 (2) Mechanical Surface N/ASingapore 7 (2.1) Mechanical and coarse bubble

diffused airAertec

Baytown 20 (6) Fine bubble diffused air Norton Ceramic DomeBillings N/A Mechanical Surface and subsurface N/A





TABLE 5.2-1SAMPLE DUTY SPECIFICATION SHEET TO BE PROVIDED TO THE AERATION VENDOR

BASINS / TANK GEOMETRY:NUMBER OF BASINS =DIMENSIONS =SLOPE =(include a drawing of the layout)WORKING CAPACITY =

BIOX SYSTEM DESIGN:INLET TOTAL BOD5, lb/day =AMMONIA, lb N/day =BOD REMOVED, lb/day =(95% reduction)

ADDITIONAL PARAMETERS AND WATER QUALITYSite Elevation above sea level, ft =TEMPERATURE, °F =MLVSS, mg/L =MLSS, mg/L =Maximum TDS, mg/L =SRT, days =

ACTUAL OXYGEN REQUIREMENTS, lb O2/day =TOTAL FLOW: AVERAGE, gpm =

MAXIMUM, gpm =PREDICTED PERFORMANCE

OUTLET BOD DISCHARGE, lb/day =OUTLET BOD CONCENTRATION, mg/L =

ADDITIONAL INFORMATION FOR AERATION SYSTEM DESIGN• ALPHA VALUE OF ______ IS RECOMMENDED FOR COARSE BUBBLE (Refinery wastewater alpha = 0.7)• ALPHA VALUE OF ______ IS RECOMMENDED FOR FINE BUBBLE (Refinery wastewater alpha = 0.6)• ALPHA VALUE OF ______ IS RECOMMENDED FOR MECHANICAL SURFACE AERATORS (Refinery wastewater

alpha = 0.8)• FOR DIFFUSED AERATION: AIR TO BE SUPPLIED BY 3 x 50% BLOWERS WITH VARIABLE CONTROL ON 1

BLOWER

UTILITIESElectrical Power: Voltage, V =Frequency, Hz =Phase, # =




TABLE 5.3-1SOLUBILITY OF OXYGEN IN WATER (mg/L) EXPOSED TO

WATER-SATURATED AIR AT ATMOSPHERIC PRESSURE (101.3 kPa)

TEMPERATURE CHLORINITY

°C °F 0 5.0 10.0 15.0 20.0 25.0

0.0 32.0 14.62 13.72 12.88 12.09 11.35 10.65

5.0 41.0 12.77 12.02 11.32 10.65 10.03 9.44

10.0 50.0 11.28 10.65 10.05 9.49 8.95 8.45

15.0 59.0 10.08 9.54 9.02 8.54 8.07 7.64

20.0 68.0 9.10 8.62 8.17 7.74 7.34 6.96

21.0 69.8 8.91 8.45 8.02 7.60 7.21 6.84

22.0 71.6 8.74 8.29 7.87 7.47 7.08 6.72

23.0 73.4 8.57 8.14 7.73 7.33 6.96 6.60

24.0 75.2 8.41 7.99 7.59 7.20 6.84 6.49

25.0 77.0 8.26 7.85 7.45 7.08 6.72 6.39

26.0 78.8 8.11 7.71 7.32 6.96 6.61 6.28

27.0 80.6 7.96 7.57 7.20 6.84 6.50 6.18

28.0 82.4 7.82 7.44 7.07 6.73 6.40 6.08

29.0 84.2 7.69 7.31 6.96 6.62 6.29 5.99

30.0 86.0 7.55 7.19 6.84 6.51 6.19 5.89

31.0 87.8 7.43 7.07 6.73 6.40 6.10 5.80

32.0 89.6 7.30 6.95 6.62 6.30 6.00 5.71

33.0 91.4 7.18 6.84 6.51 6.20 5.91 5.63

34.0 93.2 7.06 6.73 6.41 6.11 5.82 5.54

35.0 95.0 6.95 6.62 6.31 6.01 5.73 5.46

40.0 104.0 6.41 6.12 5.84 5.57 5.32 5.07

45.0 113.0 5.92 5.66 5.41 5.17 4.94 4.72

50.0 122.0 5.47 5.24 5.01 4.79 4.59 4.39

Reference 12





TABLE 6.1-1TYPICAL DIFFUSED AERATION DIFFUSERS

CATEGORY TYPE DESCRIPTION COMMENTS FIGURE

Porous Plate Square ceramic platesinstalled in fixed holderson the basin/tank floor.

Uniform distribution withnumerous platesattached to the sameplenum is difficult.Removing / Adding platesis difficult.Plates are typically12 square in. (30 cmsquare) and 1 to1.5 in. thick(2.5 to 3.8 cm).

—

Porous Dome Dome-shaped ceramicdiffusers mounted on airdistribution pipes locatedat the basin / tank floor.

Generally installed in atotal floor coverage ortightly space rows alongthe side or middle tocreate a single-spiral rolleffect.Dome diameters areabout - in. (18 cm) and1.5 in. high (3.8 cm). Diffuser

Header

Dome Diffuser

19A6f0a

Porous Disc Ceramic discs or flexibleporous membranemounted on airdistribution pipes locatedat the basin / tank floor.

Assembly includesindividual control orifices.Disc diameters rangefrom 7 - 9.5 in.(18 - 24 cm).

DiffuserHeaderPipe

Disc Diffuser

ControlOrifice

19A6f0b

Porous Tube Tubular-shaped diffuserthat uses rigid ceramicmedia or a flexible plasticor synthetic rubbersheath mounted on an airdistribution pipes.

Tube diffuser length is20 - 24 in.(50 to 60 cm)The material used for thesynthetic rubber sheathsmust be compatible withthe wastewater.

Sheath

Diffuser Header Pipe

Nipple Clamp

19A6f0c

Non Porous Fixed orifice:Perforated Piping

Piping with small holesdrilled along the length ofthe pipe.

The advantage of thistype of diffuser is itssimplicity.

DiffuserHeaderPipe

Orifice

BlowoffLeg19A6f0d

Non Porous Fixed orifice:Spargers

Molded plastic devicesmounted on airdistribution piping.

— —

Non Porous Fixed orifices:Slotted Tube

Perforated / Slottedstainless steel tubingattached to airdistribution piping

— —




TABLE 6.1-1 (Cont)TYPICAL DIFFUSED AERATION DIFFUSERS


Non Porous Valved Orifice

Vibrating Cap

Device which contains acheck valve to preventbackflow when air isturned off.

Concern with life of movingparts

Cone

DiffuserHeader

Pipe

RetainerAssembly Plastic Disc

19A6f0e

Non Porous Static Tube Vertical stationary tubesmounted on thebasin / tank floor andfunctions like an air liftpump.

—

DiffuserHeader Pipe

DiffuserMembrane

Air

AnchorAir Orifice

Static Tube

19A6f0f

Non Porous Perforated hose Anchored perforatedhose that runs along thebasin / tank floor.

— —

Other Devices Jet Nozzle that discharges amixture of pumped liquidand compressed air.

Liquid recirculation pumpand air blower required.Vendor claims intimatemixing of liquid / air innozzle creates highoxygen transfer.

Water In(Behind) Air In

Air and Water Out19A6f0g

Other Devices Aspirating Inclined propeller pumpassembly mounted at thebasin surface. Air isdrawn through the hollowshaft and an air / watermixture is dischargedbelow the liquid surface.

Oxygen Transferefficiency under standardconditions is relativelylow compared to otherdevices. Air Intake

Air InjectedInto the Water

Propeller

19A6f0h





TABLE 6.1-1 (Cont)TYPICAL DIFFUSED AERATION DIFFUSERS


Other Devices Vertical Aspirating Vertical impeller assemblysits on the bottom of thebasin. Air is drawn throughthe hollow shaft and anair/water mixture isdischarged below the liquidsurface.

—

DP19A6f0i




TABLE 6.3-1SURFACE Do SATURATION VALUES (CT,Pd), mg/L

ALTITUDE [ft (m)] OR BAROMETRIC PRESSURE [psia (bar)]TEMPERATURE 0 ft

(0.0 m)1,000 ft

(304.8 m)2,000 ft

(609.6 m)3,000 ft

(914.6 m)4,000 ft

(1,219.2 m)5,000 ft

(1,524 m)

°C °F 14.73 psia(1.02 bar)

14.21 psia(0.98 bar)

13.70 psia(0.94 bar)

13.20 psia(0.91 bar)

12.73 psia(0.87 bar)

12.26 psia(0.85 bar)

0 32.0 14.62 14.11 13.59 13.10 12.63 12.171 33.8 12.44 13.72 13.22 12.74 12.29 11.832 35.6 13.83 13.34 12.86 12.39 11.95 11.513 37.4 13.46 12.99 12.52 12.06 11.63 11.204 39.2 13.11 12.65 12.19 11.75 11.33 10.915 41.0 12.77 12.32 11.87 11.44 11.03 10.636 42.8 12.45 12.01 11.58 11.16 10.76 10.367 44.6 12.14 11.71 11.29 10.88 10.49 10.108 46.4 11.84 11.42 11.01 10.61 10.23 9.859 48.2 11.56 11.15 10.75 10.36 9.99 9.62

10 50.0 11.29 10.89 10.50 10.12 9.75 9.4011 51.8 11.03 10.64 10.26 9.88 9.53 9.1812 53.6 10.78 10.40 10.02 9.66 9.31 8.9713 55.4 10.54 10.17 9.80 9.44 9.11 8.7714 57.2 10.31 9.95 9.59 9.24 8.91 8.5815 59.0 10.08 9.73 9.37 9.03 8.71 8.3916 60.8 9.87 9.52 9.18 8.84 8.53 8.2117 62.6 9.67 9.33 8.99 8.66 8.35 8.0518 64.4 9.47 9.14 8.81 8.49 8.18 7.8819 66.2 9.28 8.95 8.63 8.32 8.02 7.7220 68.0 9.09 8.77 8.45 8.15 7.85 7.5621 69.8 8.91 8.60 8.28 7.98 7.70 7.4222 71.6 8.74 8.43 8.13 7.83 7.55 7.2723 73.4 8.58 8.28 7.98 7.69 7.41 7.1424 75.2 8.42 8.12 7.83 7.54 7.27 7.0125 77.0 8.26 7.97 7.68 7.40 7.14 6.8726 78.8 8.11 7.83 7.54 7.27 7.01 6.7527 80.6 7.97 7.69 7.41 7.14 6.89 6.6328 82.4 7.83 7.56 7.28 7.02 6.76 6.5229 84.2 7.69 7.42 7.15 6.89 6.64 6.4030 86.0 7.56 7.29 7.03 6.77 6.53 6.2931 87.8 7.43 7.17 6.91 6.66 6.42 6.1832 89.6 7.31 7.05 6.80 6.55 6.32 6.0833 91.4 7.18 6.93 6.68 6.43 6.20 5.9834 93.2 7.07 6.82 6.57 6.34 6.11 5.8835 95.0 6.95 6.71 6.46 6.23 6.00 5.7836 96.8 6.84 6.60 6.36 6.13 5.91 5.6937 98.6 6.73 6.49 6.26 6.03 5.81 5.6038 100.4 6.62 6.39 6.16 5.93 5.72 5.5139 102.2 6.52 6.29 6.06 5.84 5.63 5.4340 104.0 6.41 6.19 5.96 5.74 5.54 5.33





TABLE 6.3-2DIFFUSER SUBMERGENCE FACTORS

DIFFUSER SUBMERGENCE

ft mSUPER SATURATION DEPTH

CORRECTION FACTOR, fd

0 0.0 1.005 1.52 1.046 1.83 1.04

10 3.05 1.0815 4.57 1.1218 5.49 1.1420 6.10 1.1625 7.62 1.23

FIGURE 4.2-1AERATION SELECTION CRITERIA DECISION TREE (1)

DP19A6f42-1

Determine O2 RequirementAnd Estimate Aeration Volume

And Depth

ConsiderDiffusedAeration

ConsiderMechanical

Aeration

WillAerators

Be MountedOn FixedStructure

ConsiderJet

Aeration

ConsiderHigh

Speed

IsDepth>12 Ft

NoYes

YesIsDepth>25 Ft

No

ConsiderCoarseBubble

Yes

No

No

Yes

ConsiderLow

SpeedWill

Solids BeA Clogging

Issue?

ConsiderFine

Bubble

Note:(1) Yes/No decisions that are provided are not absolute. They are

meant to provide a rough guideline and it is expected that therewill be exceptions.




FIGURE 5.1-1MECHANICAL AERATORS FOR BIOLOGICAL WASTEWATER TREATMENT APPLICATIONS

Axial Aerator with Open Blades

Submerged Low-Speed Aerator

High-Speed Aerator

Air Inlet

Full Length Baffle

Draft Tube and Cone

Anti-Swirl Baffles

Anti-Swirl BafflesDraft Tube Intake

Air Sparge Pipe (typ)

Axial Flow Impeller

Draft Tube Submerged Turbine Aerator

Motor

Gear Reducer

Open Rotor Downdraft Aerator

Cone-Aerator

Air EntrainmentBubbles

Motor/GearHelical Vortices

Flow Direction

Cone Aerator with Ribs

DP19A6f51-1





FIGURE 5.3-1SOLUBILITY OF OXYGEN IN PURE WATER (WATER IN CONTACT WITH AIR)

60504030201005

6

7

8

9

10

11

12

Temperature, ° C DP19A6f53-1

Oxy

gen

Solu

bilit

y, m

g / L




FIGURE 6.3-1TYPICAL COARSE BUBBLE DIFFUSED AERATION PERFORMANCE CURVES

100908070605040302010

25 ft

24 ft

20 ft

18 ft

10 ft

15 ft

Airflow, scfm

1,000 ft3 liquid

10

5

0

15

20

25

30

35

Stan

dard

Oxy

gen

Tran

sfer

Effic

ienc

y (S

OTE

),%

DP19A6f63-1





FIGURE 9.0-1DEGASSING TROUGH FOR DEEP TANK AERATION EFFLUENT

Notch for scum removal

Antivortex Plates

Inlet

Elevation View

DP19A6f90-1

(Courtesy of The Advent Corporation)

DiffuserPlate

Inlet

PipeBaffles

BaffleSupports

Outlet

Valve

Plan View

Valve

DP19A6

Documents

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