Aeration system design in integrated fixed-film activated sludge (IFAS) very good.pdf

8/9/2019 Aeration system design in integrated fixed-film activated sludge (IFAS) very good.pdf

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Without institutional design criteria (and supporting design tools) and knowledge about the

proper application of IFAS and MBBRs, the consulting engineer will rely on manufacturers (orvendors) to develop and propose a process and mechanical design. Comparison of a

manufacturer contrived IFAS or MBBR process and mechanical design with process and

mechanical designs for alternative wastewater treatment technologies that were designed by a

consultant will not allow for a fair comparison based on equivalent standards despite the fact that both designs were developed for the same wastewater treatment plant (WWTP) improvement or

expansion project. Due to unknowns by both parties, namely the manufacturer and consultant, it

is typical for consultancy designers to incorporate conservatism (in this case to the IFAS processand mechanical designs) that can result in the technology (i.e., IFAS) being non-competitive with

other alternatives being evaluated (primarily from a cost perspective). Thus, IFAS may be

injudiciously eliminated from consideration although the technology may warrant the mostsignificant monetary and operational benefits had the technology been properly evaluated and

compared with competing environmental biotechnologies.

Boltz et al. (2009a; 2009b) and others have created process models that accurately simulate IFAS

and MBBR process performance, and McQuarrie et al. (2010) documented practical IFAS process design guidelines. Boltz et al. (2010a) and McQuarrie and Boltz (2011) have (1)

presented design criteria for sizing free-moving plastic biofilm carrier retention screens, (2)established practical limitations for the amount of free-moving plastic biofilm carriers that may

be placed in a bioreactor, and (3) defined system hydraulics with threshold parameters including

the basin approach velocity and carrier retention screen hydraulic loading rate. Therefore, process model(s), free-moving plastic biofilm carrier retention screen sizing criteria, IFAS zone

or MBBR basin configuration standards, and criteria describing the amount of free-moving

plastic biofilm carriers (or biofilm surface area) that are required to meet a wastewater treatmentobjective exist, have been documented and referenced in this manuscript, and have been

demonstrated accurate in multiple full-scale, operating aerobic IFAS zones and MBBRs(including secondary processes, post-denitrification, and tertiary nitrification). However, the

description of a comprehensive methodology for state-of-the-art aerobic IFAS zone and MBBR

aeration system design, to the knowledge of the authors, has not been presented. In this paper,the aeration system includes stainless-steel pipe diffusers, manifold (or submerged air header),

down (or drop) pipes, and manually operated air-flow control valves. This manuscript will

present a method for the design of an aeration system in aerobic IFAS zones (that contain free-

moving plastic biofilm carriers) and MBBRs.

GENERAL PROCESS MECHANICAL DESIGN CRITERIA

Free-moving plastic biofilm carrier based aerobic IFAS zones and MBBRs use a piping networkand air diffusers that (1) are capable of passing adequate air flow to meet process oxygen

requirements, (2) have characteristics that do not require excessive additional blower capacity

beyond that required to pass air through a fine-bubble diffuser-based aeration system, (3) has a

sufficient number of drop pipes equipped with manually operated flow control vales to promote arolling water circulation pattern (i.e., for the uniform distribution of free-moving plastic biofilm

carriers, (4) can structurally withstand the weight imparted by biofilm covered plastic carriers

when the tank is drained, (5) does not have a propensity for orifice clogging, and (6) requiresinfrequent maintenance. These objectives have been met in aerobic IFAS zones and MBBRs


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with an engineered system that consists of stainless steel components including manually

controlled air-flow control valves, drop pipes, manifolds (or submerged air headers), and perforated pipe distributors (or diffusers). Figure 1 is a photograph that depicts the aeration

system components from the stainless steel drop pipe to the manifold and diffusers.

Figure 1. (left) Photograph depicting the air flow control valve, stainless steel drop pipe,

manifold, and pipe diffusers. (right) Pictured on top (a) is the stainless steel diffuser with 4-mmdiameter orifices situated along the underside of the pipe, and (b) the butterfly valve that is

attached to each drop pipe.

The first aeration system of this kind was installed in 1992 at the MBBR-based Eidsfoss WWTP

(Eidsfoss is a village in Hof municipality, Vestfold County, Norway). The Lillehammer WWTP,

a larger MBBR, was installed in 1994 in Lillehammer, Norway, to handle municipal wastewatergenerated by attendees of the 1994 Winter Olympic Games. These MBBRs included K1-type

free-moving plastic biofilm carriers (see McQuarrie and Boltz 2011 for a description of thecarrier characteristics) and aeration systems consisting of the components described in this

manuscript. Neither of these WWTPs experienced plastic biofilm carrier attrition or structuraldeterioration requiring carrier replacement. In addition, aeration equipment installed in these

WWTPs has not experienced malfunction or performance deterioration, and aeration system

maintenance was not required during the 23 and 21 year existence at the Eidsfoss andLillehammer WWTPs, respectively (Ødegaard 2013). Construction of the Broomfield Water


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Reclamation Facility (WRF), Broomfield, Colorado, U.S.A., began in 2002 with an aeration

system that is consistent with the units described in this manuscript. The Broomfield WRF hasnot required aeration system maintenance since it began operation in 2003.

The air diffusers used in aerobic IFAS zones and MBBRs are known as medium-bubble

diffusers. Their oxygen transfer characteristics are in between conventional coarse-bubble and

fine-bubble diffusers. The medium-bubble diffusers are less affected by fouling or scaling because the large discharge (4-mm diameter) orifices are difficult to clog (Stenstrom and Rosso

2008). However, a benefit inherent to medium-bubble diffusers is that they require less

maintenance than fine-bubble diffusers. In fact, operational experience with wastewatertreatment facilities such as the Eidsfoss WWTP, Lillehammer WWTP, and Broomfield WRF has

yet to establish precedence for maintenance of the aeration system described in this paper

because these aeration systems have not yet required maintenance during the 23, 21, and 10-

years of operation since start-up, respectively.

The medium-bubble diffusers are characterized by lower oxygen transfer efficiency than fine-

bubble diffusers because the larger bubbles they expel travel through the water column rapidly,

and have a lower surface-to-volume ratio. However, medium-bubble diffusers have moreefficient oxygen transfer efficiency than coarse-bubble diffusers. The presence of plastic biofilm

carriers has a positive impact on oxygen transfer efficiency in aerobic IFAS zones (Pham et al.2008). It can be inferred that although the diffuser itself releases larger bubbles than fine-bubble

diffusers, the transit of bubbles through a water column containing evenly distributed free

moving plastic biofilm carriers that shear the bubbles into smaller bubbles burst at the tank

surface in the “medium” range (i.e., bubble size distribution is typically in the range 5 and 50mm). Pham et al. (2008) demonstrated improved standard oxygen transfer efficiency (SOTE) in

a 2.1-meter (7-foot) deep test tank (1.2-meter x 1.2-meter, or 4-foot x 4-foot, plan) having a 2.0-

meter (6.5-foot) SWD and the coarse bubble diffuser mounted 0.15-meter (0.5-feet) above thetank bottom. The SOTE was 2.35 percent per meter (0.72 percent per foot) of diffuser

submergence when the plastic biofilm carrier fill was zero. Alternatively, the SOTE was 3.20 percent per meter (0.98 percent per foot) of diffuser submergence when the plastic biofilm

carrier fill was 25 percent. A 50 percent plastic carrier fill resulted in a SOTE of 2.60 percent per

meter (0.79 percent per foot) of diffuser submergence. The reduced SOTE at 50 percent fill wasattributed to a poorly configured diffuser grid lay out and poor mixing of the plastic biofilm

carrier fill. Unfortunately, the impact of biofilm covered plastic carriers has not been

investigated; therefore, the impact of biofilm covered plastic carriers on oxygen transferefficiency in aerobic IFAS and MBBR zones is poorly understood. Operational experience has

proven that medium-bubble diffusers require less maintenance than fine-bubble diffusers - a trait

that is attractive to operation and maintenance staff.

DETAILED DESIGN METHODOLOGY

Process mechanical equipment included in aerobic IFAS zones and MBBRs include: (1) free-

moving plastic biofilm carriers; (2) plastic biofilm carrier retention screens, wall mountingdevices, structural support assemblies (if needed), and auxiliary portal screens (e.g., screens to

cover floor drain openings, scum over flow, draining, and equalization portals); and (3) the

aeration system which includes stainless steel pipe diffusers, manifold, flexible couplings (which


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is an alternative fixture), drop pipes, and flow control valves. Design criteria for process

mechanical features listed as items (1) and (2) has been documented by Boltz et al. (2010a) andMcQuarrie and Boltz (2011). The detailed design method presented in this section is dedicated to

the aeration system.

Low-pressure air enters an aerobic IFAS zone or MBBR through one or more drop pipes thatconnect to a manifold (or air header). The air then exits the aeration system through orifices

situated along the underside of stainless steel pipe diffusers that are attached to the basin bottom.

Multiple drop pipes are typically incorporated into the system. When the drop pipes are equippedwith flow control valves they aid in air flow reduction which helps to control dissolved oxygen

concentration, and promotes the rolling water circulation pattern that uniformly distributes free-

moving plastic biofilm carriers throughout the basin. Historically, process oxygen requirementsand the distribution of free-moving plastic biofilm carriers in aerobic IFAS zones have been

achieved with medium-bubble diffusers that are made of stainless-steel pipes having circular

orifices situated along the bottom of the pipe.

General Aeration System Design Criteria Drop pipes are typically provided for every 1 to 3 manifold (or submerged air headers) and will

depend on the basin configuration. Drop pipes are typically equipped with manually modulatedair flow control valves. There may be 2 or more drop pipes per aerobic IFAS zone or MBBR.

The number of drop pipes is determined by the need to meet air flow rate turn down

requirements which are typically 50 percent of the maximum design air flow rate. Diffusers

typically used in IFAS and MBBRs are 25-mm (1 inch) diameter stainless-steel pipes with 4-mm(5/32 inch) diameter orifices spaced (Lorifice spacing) 38 to 102-mm (1.5 to 4.0 inches) along the

underside of the diffuser pipe. Placing the orifices too close together will lead to uneven air flow

distribution inside the pipe diffuser, and placing the orifices too far apart leads to cost prohibitiveapplication. The air diffuser is generally anchored 0.30 meter (1.0 foot) above the tank bottom,

and is spaced 0.30, 0.45, 0.60, or 0.90 meters (1.0, 1.5, 2.0, or 3.0 feet) apart. The medium- bubble diffuser orifice must be smaller than the plastic biofilm carrier to eliminate the potential

for air-pipe and orifice plugging. The maximum distance between grids should not exceed 1.83

meters (6 feet). The minimum distance of any grid from the basin wall is 0.90 m (although 0.45m is preferred). Installed aeration grids must be leveled to within 6.5 mm (0.25 inches).

As derived later in this paper, the maximum air flow rate per orifice (Qorifice, maximum) is 1.75m

3/hr, with 50 percent turn down possible. The minimum air flow rate per orifice (Q orifice, minimum)

is 1.60 m3/hr, with 50 percent turn down possible. Design air velocity in the manifold is 13 m/s

in 0.1 to 0.2-meter diameter pipes, and 20 m/s in pipes with a diameter larger than 0.2 meters.

While designing the aeration system one must follow the steps listed in Table 1.

Calculating the Actual Oxygen Transfer Rate

The basis for designing aeration systems in aerobic IFAS zones and MBBRs is the actual oxygentransfer rate (AOTR), and air flow rate that is required to either meet demands imposed by the

biological wastewater treatment process or evenly distributed free moving plastic biofilm carriers

throughout the IFAS zone or MBBR. Depending on the design approach, this value may begiven as output from a whole-plant wastewater treatment plant model, or simulator. In any event,

this paper will offer the design engineer methodology for establishing the AOTR.


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Table 1. Abbreviated aeration system design methodology and associated criteria for aerobicIFAS zone(s) and MBBR(s) aeration systems

Step in Designing Aeration System Parameter values

A.

Air Flow rate

Calculate required actual air flow rates in the

aerobic IFAS zone or MBBR (Qair, total, zone)

Use a process model (see Boltz et al. 2009a,b; 2010b) oralternative method (see Grady et al. 2011)

B. Orifices

Calculate the total number of required orifices

(Norifices) per aerobic IFAS zone or MBBR

Maximum (Qorifice, maximum) - 1.75 m3/hr per orifice

Minimum (Qorifice, minimum) - 1.60 m3/hr per orifice

Maximum diffuser density is 55 m3/hr/m2 of floor

C. Laterals and Diffusers

Calculate:

a. number of laterals/manifold (Nlaterals, manifold)

b. number of diffusers/manifold (Ndiffusers, manifold)

based on an assigned diffuser spacing (Ldiffuser spacing)

Ldiffuser spacing is in the range 0.30 to 102 mm apart (1.5 to4.0 inches apart; on center of the diffuser pipe).

D. Manifold, Number of

Calculate:

a. number of manifold/basin length (Nmanifold, length)

b. number of manifold/basin width (Nmanifold, width)

based on an assigned grid width (Wgrid)

Wgrid which is generally 2.1 or 2.4 meters (7 to 8 feet)

for grids with diffusers on both sides of the manifold .

Note: Wgrid is fixed at either 2.1 or 2.4 meters (7 or 8

feet), but the diffuser length will vary depending on if

the manifold diameter (Dmanifold) is 76, 102, 152, 203,254, or 305-mm (3, 4, 6, 8, 10, or 12-inch).

E. Orifices (Calculated)

Calculate:

a. number of orifices per lateral (Norifices per laterals)

b.

total number of orifices per zone (Norifices, total, zone)c. total number of orifices (Norifices, total)

based on an assigned orifice spacing (Lorifice spacing)

Lorifice spacing is in the range 38 to 102-mm (1.5 to 4.0 in)

Keep the orifice spacing as close together as possible to

reduce the number of diffusers and minimize the cost of

the aeration system. Lorifice spacing may be 38, 41, 44, 48,51, 54, 57, 60, or 102-mm apart.

F. Air Flow Rate per Orifice (Calculated)

Calculate:

a. air flow rate per orifice (Q’orifice)

b. ensure 'orificesQ is within tolerance

Calculated air flow rate per orifice (Q’orifice) must be

within tolerance:

'orificesQ ≥ Q orifice, minimum= 1.60 m

3/hr/orifice

'orificesQ ≤ Q orifice, maximum= 1.75 m

3/hr/orifice

G. Manifold Diameter (Dmanifold)

Calculate:

a. air flow velocity in manifold (Vmanifold

)

b. ensure Vmanifold is within tolerance

based on an assigned manifold diameter (Dmanifold)

Vmanifold must be less than 13 m/s when D manifold is 0.1 to

0.2-m, and less than 20 m/s when Dmanifold is greater than

0.2 m. If Vmanifold is greater than the maximum allowable

velocity, then the manifold diameter (Dmanifold) must beincreased. Manifold are typically available as 0.076,

0.102, 0.152, 0.203, and 0.254-m (3, 4, 6, 8, and 10-

inch) diameter stainless steel pipes.

Criteria for determining Dmanifold when N manifold, width isgreater than 1 is discussed in a later section.


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The actual amount of oxygen required by an aerobic IFAS zone or MBBR must be obtained by

applying factors to a standard oxygen requirement that accounts for the effects of salinity-surface

tension (beta factor, ), temperature, elevation, diffuser depth, the desired dissolved oxygenoperating level, and the effects of mixing intensity and basin configuration. The interrelationshipof these factors is given by the following Eq. (1).

F024.1C

CCSOTR AOTR 20T

20,S

H,T,s

(1)

Where,

AOTR = actual oxygen transfer rate under field conditions (kg O2/hr)SOTR = standard oxygen transfer rate in tap water at 20°C, and zero D.O. (kg O2/hr)

= salinity-surface tension correction factor (typically 0.95)

H,T,s

C = average DO sat. conc. in clean water at temperature T and altitude H (mg/L)

21

O

P

P

2

1CC t

H,atm

dH,T,s

H,T,s

When multiplied by 0.5 the term in brackets above represents the average

pressure at mid-depth and accounts for the loss of oxygen to biological uptake. Ifthe biological uptake rate is not considered, then the following expression can be

used:

H,atm

md,wH,atm

H,T,sH,T,s P

PPCC

Cs,T,H = oxygen sat. conc. in clean water at temperature T and altitude H (mg/L)

Pd = pressure at the depth of air release (kPa)Patm,H = atmospheric pressure at altitude H (kPa)

Pw,md = pressure at mid-depth above point of air release due to water column (kPa)

Ot = percent oxygen concentration leaving tank, typically (18 to 20 percent)

C = dissolved oxygen concentration in the bulk of the water (g/m3)

Cs,20 = dissolved oxygen sat. conc. in clean water at 20°C and 1 atm (mg/L)

T = operating temperature (°C)

= oxygen transfer correction factor for wastewaterF = fouling factor (typically 1.0 for medium-bubble diffusers)

If the diffuser’s characteristic SOTE is known, then the SOTR can be converted into an air flow

rate using Eq. (2).

100SOTE

SOTR Qair (2)

Where,


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Qair = air flow rate (Nm3/hr)

SOTE = standard oxygen transfer efficiency;(percent; typical value defined as side water depth x 3.45 percent per meter of

submergence)

Once an AOTR and the total air flow rate requirement has been established for the aerobic IFASzone or MBBR, the next step in this aeration system design is to determine the air flow rate that

will be distributed through each orifice that is distributed along that bottom of the stainless steel

pipe diffuser.

Diffusers: Air Distribution through a Perforated Pipe

Uniform air distribution is essential for efficient diffuser operation. To ensure optimum air

distribution proper consideration must be given to flow behavior in the distributor, flow

conditions upstream and downstream of the diffuser, and the aeration system distribution

requirements (Perry and Green 1997). Aeration systems in aerobic IFAS zones and MBBRstypically use 25-mm (1-inch) diameter stainless steel pipes with 4-mm diameter orifices. Figure

2 illustrates a conceptual stainless steel pipe diffuser with orifices situated along the underside ofthe pipe.

Figure 2. Medium-bubble diffuser (also known as a perforated pipe distributer). The diffusers

typically have a 25-mm diameter and 4-mm diameter orifices situated along the underside of the pipe.

Methodology for obtaining uniform air flow distribution in the stainless-steel pipe diffusers

typically used in aerobic free-moving plastic biofilm carrier-based IFAS zones and MBBR

aeration systems is to make the average pressure drop across the diffuser orifices ( p0) largewhen compared to the change in pressure (P) over the length of the pipe. Consequently, therelative variation in pressure drop across the orifices that are evenly spaced along the underside

of the pipe diffuser will be small, and so will be the variation in air flow. When the area of anindividual orifice is small when compared to the cross-sectional area of the pipe (i.e., diffuser),

pressure drop through the orifice may be expressed in terms of the discharge coefficient (CDC,0)and the air flow velocity across the orifice (Eq. 3).


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2

V

C

1 p

2

0

2

0,DC

0

(3)

Where,

0 p = average pressure drop across the diffuser orifices (mm H 2O)CDC, 0 = discharge coefficient (= 0.65, a typical value for this system)

= density (kg/m 3)V0 = gas velocity (m/hr)

The application of Eq. (3) is illustrated in Figure 3 to determine various orifice pressure drops

( p0) as a function of air flow rate through an individual orifice. A grey band highlights theminimum air flow rate per orifice (Qorifice, minimum) and maximum air flow rate per orifice (Qorifice,

maximum) of 1.60 to 1.75 m3/hr/orifice, respectively, corresponding to a pressure drop of 180 to

230 mm H2O (7 to 9 inches). It is desirable to maintain a 4-mm diameter orifice with a maximum

spacing of 102 mm (or 4.0 inches) and minimum spacing of 38 mm (or 1.5 inches).

A flat pipe has a propensity for orifice clogging. Therefore, it is standard practice to bend the end

of the diffuser pipe downward as pictured in Figure 4. The height of each bend is determined by

the maximum acceptable air flow per orifice. For example, according to Figure 3 a maximumallowable air flow rate per orifice (i.e., Qorifice, maximum) of 1.75 m

3/hr/orifice would require a 230

mm H2O (or 9 inch) depth of bend at the pipe end.

If the bend at the pipe end has a length that is less than the flow correlated length value, the

majority of air flow will discharge through the end of the pipe (which is not completely sealed)

because it’s length is not long enough to maintain the pressure required to sustain a Qorifice,

maximum equal to 1.75 m

3

/hr/orifice (as illustrated graphically in Figure 3). The end of the pipediffusers are not completely sealed to allow an outlet (in addition to the orifices) for any water

and solids that may have accumulated in the aeration pipes.

The maximum air flow rate per orifice (Qorifice, maximum) is assigned based on practical rational. As

mentioned, a bend at the pipe end must be 230 mm H2O (or 9 inch H2O) to maintain pressure

that is required to sustain Qorifice, maximum equal to 1.75 m3/hr/orifice. The diffuser grid is anchored

0.3 m (1 foot) above the basin floor. Allowing for a 76 mm (3-inch) freeboard to accommodate

fixtures and the pipe diffuser itself makes 230 mm (9-inches) for the pipe end bend the longest

practically constructible. The design engineer must recall that trying to push an air flow rate perorifice (Qorifice, maximum) greater than that associated with the flow correlated length of the bend at

the end of the pipe will result in a majority of air flowing out of the (unsealed) end of the pipe,

not the orifices.


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Figure 3. Pressure loss ( p0) through individual orifice due to friction induced by air flowthrough the 4-mm diameter orifices that are situated along the underside of the 25-mm diameterstainless steel pipe diffusers.

Figure 4. Photos depicting stainless steel pipe diffusers and the downward bend at the pipe end.

Grid Sizing: Number and Spacing of Manifold, Diffusers, and Orifices

Prior to initiating what is an iterative design process, one must establish design parameters anddetermine their values. The first design parameter is air flow (Qair, total, zone [=] m

3/hr) and AOTR

([=] kg O2/hr) for each of the individual aerobic IFAS zones or MBBR basins. An array of total

design air flow rates may be developed using a wastewater treatment plant simulator that


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incorporates a mathematical model describing a one-dimensional biofilm (see Boltz et al. 2010b

for a list of models and simulators that are capable of describing IFAS and MBBR processes).This information is coupled with the maximum allowable air flow rate per orifice (Qorifice, maximum)

as determined using Eq. (3). Second, the maximum site temperature (Tmax,site), site elevation

(Esite), and blower discharge temperature (T blower discharge) must be established. These parameter

values are used to establish the parameters applied to Eq. (1) when defining the AOTR if it is not provided by a simulator. Next, the diffuser depth is typically assigned as the SWD minus 0.3

meters (1 foot). Then, one must make an initial assignment of the diffuser grid width (Wgrid)

which is generally 2.1 or 2.4-meters (7 or 8-feet) wide. The manifold length (Lmanifold) may be aslong as practically allowable by constraints imposed by the physical dimensions of the basin and

velocity constraints in the manifold. Next, one must assign diffuser spacing (Ldiffuser spacing) which

is generally 0.30, 0.45, 0.60, 0.90-meters (1.0, 1.5, 2.0, or 3.0-feet) apart (on center of thediffuser pipe).

The maximum air flow rate per orifice (Qorifice, maximum = 1.75 m3/hr/orifice) and the actual air

flow rate required for biological wastewater treatment or evenly distributing free moving plastic

biofilm carriers (Qair, total, zone [=] m3/hr) are applied as described by Eq. (4) to establish the total

number of orifices that are required in the aerobic IFAS zone or MBBR.

imummax,orifice

zone,total,air

orificesQ

Q N (4)

As a matter of terminology, a lateral is defined here as stainless steel pipe diffusers tip-to-tip, andcaptures the entire width of the diffuser grid. The width of the grid (Wgrid) is fixed at either 2.1 or

2.4 meters (7 or 8 feet), but the diffuser length will vary depending on the manifold diameter

(Dmanifold) which may be 76, 102, 152, 203, or 254-mm (3, 4, 6, 8, or 10-inch). The number of

laterals per manifold (Nlaterals per manifold) can be calculated using Eq. (5):

1

L

Lceiling N

spacingdiffuser

manifoldmanifold per laterals (5)

It should be noted that one (1) additional lateral is added to the count because a diffuser grid willalways require one additional diffuser per length. For example, if the manifold length (Lmanifold)

is equal to 0.6-m, or 2-ft, and the diffuser spacing (Ldiffuser spacing) is equal to 0.3-meter, or 1-foot,

then dividing the grid length by the diffuser spacing (i.e., Lmanifold ÷ L diffuser spacing =) two (2), butthe manifold will in fact require three laterals: one at the beginning, middle, and end).

Once the number of laterals per manifold has been established, the design engineer must confirmthat the manifold length (Lmanifold) is adequate. Recall, diffuser spacing (Ldiffuser spacing) may be

0.30, 0.45, 0.60, or 0.90-meters (or 1.0, 1.5, 2.0, or 3.0-feet) apart (on center of the stainless steel

diffuser pipe). Therefore, diffuser spacing (Ldiffuser spacing) of 0.3, 0.6, and 0.9 meters (or 1.0, 2.0,

or 3.0-feet) will always results in the manifold length (Lmanifold) being an integer. Only the 0.45-meter (or 1.5-foot) diffuser spacing (Ldiffuser spacing) will not result in an integer because of

manifold lengths potentially ending in 0.15-m (0.5-foot) increments.


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In the event that Eq. (5) is not an integer then the function must be round up to the nearest

integer. Essentially, Eq. (5) is a ceiling function whereby ceiling( x) = [x] is the smallest integerthat is not less than x (e.g., N laterals per manifold = [(L manifold ÷ L diffuser spacing) + 1] = [(10 m ÷ 0.5 m) +

1] = 23.22; ceiling(23.22) = 24; Nlaterals per manifold = 24). Having defined a lateral here as the

stainless steel pipe diffusers tip-to-tip plus the manifold diameter - which captures the entire

width of two diffusers and the manifold diameter - the number of diffusers per manifold can becalculated using Eq. (6).

2 N N manifold per lateralsmanifold per diffusers (6)

Given the aerobic IFAS zone or MBBR dimensions which include the SWD, basin length (L basinlength) and width (L basin width) the dimensionless number of manifold (Nmanifold, length) required acrossthe basin length can be calculated using Eq. (7).

spacinggridgrid

length,spacingwalllengthsin ba

length,manifoldLW

LLintn N (7)

Here, Lwall spacing, length (= L wall spacing, length 1 + L wall spacing, length 2) is total desired distance between the

end of the diffuser grid and the basin wall. Typically, Lwall spacing, length = 1.8 meters and represents

the total space between the end of the grid and the wall on both ends of the manifold. The desiredspace at each end of the diffuser grid is 0.9 meters (i.e., Lwall spacing, length 1 = L wall spacing, length 2 = 0.9

meters). The grid spacing is the distance required between grids to allow for manifold

connections or reducers. A typical grid spacing is 0.9 meters (or 3.0 feet) (i.e., Lgrid spacing = 0.9meters). Eq. (7) may not produce an integer. In the event that Eq. (7) is not an integer then the

function must be round to the nearest integer, or nint (i.e., round down or round up).

Again, given the aerobic IFAS zone or MBBR dimensions which include the SWD, basin length

(L basin length) and width (L basin width), the dimensionless number of manifold (Nmanifold, width) requiredacross the basin width can be calculated using Eq. (8).

manifold

width,spacingwallwidthsin ba

width,manifoldL

LLintn N (8)

Similar to Eq. (7), the result produced by Eq. (8) may not be an integer. In the event that Eq. (8)

is not an integer then the function must be round to the nearest integer (i.e., round down or round

up). Should Nmanifold, width be greater than one (1) (i.e., more than one manifold is required across

the basin width) then the manifold (or submerged air header) will have a sequentially reducing

manifold diameter (Dmanifold) for each manifold across the basin width. Figure 5 depicts aninstallation in which Nmanifold, width is equal to 2. The manifold is connected to the end of the

subsequent manifold. Each manifold has a different diameter (i.e., Dmanifold, 1 ≠ D manifold, 2).Therefore, they are connected at each end with an eccentric reducer coupling. However, the

manifold length (Lmanifold) should be modified until Nmanifold, width is as close to 1 or 2 as possible.


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Figure 5. (top) Picture of an aerobic IFAS zone at the Fields Point Wastewater Treatment

Facility, Providence, Rhode Island, and the diffuser grid layout. Nmanifold, width = 2; therefore,

extending from the drop pipe the first manifold (Nmanifold = 1) has a diameter (D manifold, 1) that is

greater than the second manifold (Nmanifold = 2) diameter (D manifold, 2) (i.e., Dmanifold, 1 > D manifold, 2).(bottom) Schematic of aerobic IFAS zone depicted above.

Orifice spacing (Lorifice spacing) may be 38 and 102 mm (1.5 and 4.0 inches) apart. It is desirable tokeep the orifice spacing as close together as possible (i.e., 38 mm or 1.5 inches) to reduce the

number of diffusers (Ndiffusers) and minimize aeration system cost. Equipped with this

information, one may calculate the number of orifices per lateral (Norifices per lateral) using Eq. (9).


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spacingorifice

spacingendgrid

lateral per orficesL

LWintn N (9)

The new parameter Lend spacing (= L end spacing 1 + L end spacing 2) is the total desired distance betweenthe last orifice and the end of the diffuser, which is 0.66 m (or 26 inches) (i.e., Lend spacing 1 = L end

spacing 2 = 0.33 m). The final equation, Eq. (10), describes the total number of orifices provided in

the aerobic IFAS zone or MBBR. The total number of orifices can be calculated using Eq. (10).

length,manifoldwidth,manifoldmanifold per lateralslateral per orificescalculated,total,orfices N N N N N (10)

The total number of orifices required in an aerobic IFAS zone or MBBR may be expanded toaccount for the number of orifices in a system of multiple trains (Norifices, total, system) (or parallel

reactors with identical conditions) by applying Eq. (11) which multiplies Norifices, total, calculated (or

Eq. (10)) by the total number of trains (Ntrains).

trainscalculated,total,orificessystem,total,orfices N N N (11)

If there is only one train, then Ntrains = 1. If there are multiple reactors in series, then this design

procedure is repeated using the air flow rate that is required to sustain biological transformation processes or evenly distribute free moving plastic biofilm carriers throughout the second aerobic

IFAS zone or MBBR in the series (typically, Qair, total, zone, 1 > Q air, total, zone, 2). The procedure is

repeated for n-reactors in series. Finally, the air flow rate (Qair, total, zone) is divided by the totalnumber of orifices (Norifices, total, system) to ensure that the calculated air flow rate per orifice

(Q’orifice) according to Eq. (12) is within the tolerable design values (i.e., Qorifice, maximum and

Qorifice, minimum).

system,total,orifices

zone,total,air '

orifices N

QQ (12)

The design orifice air flow rates (i.e., Qorifice, maximum and Q orifice, minimum) must be compared withthe calculated orifice air flow rate (Q’orifice). The calculated air flow rate per orifice (Q’orifice)

must be less than Qorifice, maximum and greater than Q orifice, minimum. A manual optimization procedure

is executed by modifying parameters until the percent error, or Eq. (13), is less than an

acceptable value, typically 5 percent.

100Q

QQabsoluteError Percent

orifice

'orificeorifice

(13)


15/28

Manifold Sizing and Selecting the Number of Drop Pipes

Manifold, or submerged air headers, are typically available in 0.076, 0.102, 0.152, 0.203, and

0.254-m (3, 4, 6, 8, and 10-inch) diameter stainless steel pipes. Design air flow velocity in the

manifold (Vmanifold) must be less than 13 m/s in 0.1 to 0.2-m diameter manifold (Dmanifold) pipes,

and less than 20 m/s in manifold pipes with a diameter greater than 0.2 m. If Eq. (5) results in Nmanifold, width being one (1) then the air flow velocity in the manifold (V manifold, 1) can be

calculated using Eq. (14).

pipedrop per manifold pipesdrop1,manifold

zone,total,air

1,manifold N NA

QV

(14)

Here, Amanifold, 1 is the cross-sectional area of the first manifold (m2) across the basin width. The

calculated manifold velocity (Vmanifold, 1) can be compared with acceptable criteria. In the event

that Eq. (5) results in Nmanifold, width being greater than one (1) the second manifold will have a

smaller diameter than the upstream manifold (i.e., Dmanifold, 1 > D manifold, 2). The velocity in thesecond manifold (Vmanifold, 2) may be calculated using Eq. (15).

2,manifold

1,grid,total,air

pipedrop per manifold pipesdrop

zone,total,air

2,manifoldA

Q N N

Q

V

(15)

Here, Amanifold, 2 is the cross-sectional area of the second manifold (m2) across the basin width.

The aeration system will have maximum flexibility with an increased number of drop pipes.

Typically, there is 1 - 3 manifold (or air headers) per drop pipe. The incorporation of multipledrop pipes allows the operator flexibility to ensure that there is a proper rolling water circulation

pattern. In addition, the greater number of drop pipes allow the operator some crude turn-down

control by isolating a section of the diffuser grid.

The next section will step the reader through an example application of this design methodology.

EXAMPLE: APPLICATION OF THE DETAILED DESIGN METHODOLOGY

This section will illustrate an example application of the aerobic IFAS zone and MBBR aeration

system design methodology presented in the previous section. The example aerobic system (withfree moving plastic biofilm carriers) has the following characteristics:

Known parameters and their values:

Design air flow rate (Qair, total, zone) = 14,357 m3/hr (8,450 cfm)

Min. design air flow rate/orifice (Qorifice,min.) = 1.60 m3/hr/orifice (0.942 cfm/orifice)


16/28

Max. design air flow rate/orifice (Qorifice,max.) = 1.75 m3/hr/orifice (1.030 cfm/orifice)

Basin (or tank) length (L basin length) = 17.4 meters (57.0 feet) Basin (or tank) width (L basin width) = 11.9 meters (39.0 feet) Number of identical parallel trains = 2

Initial estimates of key design parameter values:

Manifold length (Lmanifold) = 10.0 meters (33.0 feet); see note Step 2 Grid spacing (Lgrid spacing) = 1.5 meters (5.0 feet) Grid width (Wgrid) = 2.44 meters (8.0 feet) Diffuser spacing (Ldiffuser spacing) = 0.3 meters (1.0 foot) Wall spacing (Lwall spacing, length) = 1.8 meters (6.0 feet) Wall spacing (Lwall spacing, width) = 1.2 meters (4.0 feet) End spacing (Lend spacing) = 0.66 meters (26 inches) Manifold diameter (Dmanifold) = 0.3 meters (1.0 foot) Number of drop pipes per zone (Ndrop pipes) = 2

Orifice spacing (Lorifice spacing) = 0.060 meters (2.375 inches)

Step 1: Calculate the total number of required orifices (N orifices) using Eq. (4):

orifices204,8

orificehr

m75.1

hr

m357,14

Q

Q N

3

3

imummax,orifice

zone,total,air

orifices

Step 2: Calculate the number of required laterals per manifold (N laterals per manifold) using Eq. (5):

manifold

laterals34

manifold

laterals3.34intn N

1m3.0

m10intn1

L

Lintn N

manifold per laterals

spacingdiffuser

manifoldmanifold per laterals

Objective is to reduce the number of diffusers as a cost saving measure.

Ldiffuser spacing typically starts at the largest interval and is systematically reduced if needed. Lorifice spacing is typically minimized (i.e., 38 mm) and L diffuser spacing is adjusted as required

to meet the designated percent error, Eq. (13).

Lmanifold must be confirmed adequate based on the calculated number of laterals permanifold. Using Ldiffuser spacing equal to 0.3-m (or 1-ft), L manifold will be an integer. As

illustrated in Figure 6 below, a manifold length (Lmanifold) equal to 10 m (or 33 ft) is

required to accommodate Nlaterals equal to 34.


17/28

Figure 6. Illustration of method to confirm that the designated manifold length (Lmanifold) is

adequate.

Step 3: Calculate the number of required diffusers per manifold (N diffusers per manifold) using Eq.(6):

manifold

diffusers682342 N N manifold per lateralsmanifold per diffusers

Step 4: Calculate number of manifold (N manifold, length) required across basin length via Eq. (7):

495.3intnm5.1m44.2

m8.1m4.17intn

LW

LLintn N

spacinggridgrid

length,spacingwalllengthsin ba

length,manifold

Step 5: Calculate number of manifold (N manifold, width) required across basin width via Eq. (8):

11.1intnm0.10

m2.1m9.11intn

L

LLintn N

manifold

width,spacingwallwidthsin ba

width,manifold

Nmanifold, width > 1 will result in having multiple manifold pipe diameters.

The manifold pipe diameter (Dmanifold) will decrease as the distance from the drop pipeincreases (i.e., Dmanifold, 1 > D manifold, 2).

Manifold with different diameters (i.e., Dmanifold, 1 ≠ D manifold, 2) placed in series will beconnected with eccentric stainless steel pipe reducers to maintain an equivalent distance between the bottom of the manifold and the basin floor (or slab).


18/28

Step 6: Calculate the number of orifices (N orifices per lateral) per lateral using Eq. (9):

lateral

orifices30

m06.0

m66.0m44.2intn

L

LWintn N

spacingorifice

spacingendgrid

lateral per orfices

Step 7: Calculate the total number of orifices (N orifices, total) per grid using Eq. (10):

length,manifoldwidth,manifoldmanifold per lateralslateral per orificescalculated,total,orfices N N N N N

grid

orifices080,4

grid

manifold41

manifold

laterals34

lateral

orifices30 N calculated,total,orfices

Step 8: Calculate the total number of orifices (N orifices, total, system) per system using Eq. (11):

system

orifices160,8

system

grid2

grid

orifices080,4 N

N N N

system,total,orfices

trainscalculated,total,orificessystem,total,orfices

Step 9: Determine the calculated air flow rate per orifice (Q’ orifice) according to Eq. (12):

orificehr

m75.1

system

orifices160,8

systemhr

m357,14

N

QQ

3

3

system,total,orifices

zone,total,air '

orifices

'orificesQ ≥ Q orifice, minimum= 1.60 m3/hr/orifice (0.942 cfm/orifice)

'orificesQ ≤ Q orifice, maximum= 1.75 m3/hr/orifice (1.030 cfm/orifice)

Step 10: Access the calculated air flow rate per orifice (Q’ orifice) according to Eq. (13):

%0.010075.1

75.175.1absolute100

Q

QQabsoluteError Percent

imummax,orifice

'

orificeimummax,orifice

Calculated percent error of 0.0% < 5% threshold previously determined to be acceptable


19/28

Step 11: Access the calculated air flow velocity (V manifold) in the manifold according to Eq. (14):

s

m1.14

min

s60

hr

min60

hr

m777,50V

hr

m777,50

412

3.0

hr

m357,14

N NA

QV

manifold

2

3

length,gridswidth,gridsmanifold

flowair

manifold

Manifold, or submerged air headers, are typically available in 0.076, 0.102, 0.152, 0.203, 0.254,

and 0.305-m (3, 4, 6, 8, 10, and 12-inch) diameter stainless steel pipes. Design air velocity in themanifold can be no greater than 13 m/s in 0.1 to 0.2-m diameter pipes, and no greater than 20

m/s in larger diameter pipes. The air velocity in the 0.3-m diameter manifold is 14.1 m/s, which

is less than the 20 m/s velocity limit for manifold diameters greater than 0.2 m.

Nmanifold, length is four (4); therefore, four (4) manifold pipes will [hypothetically] exist across thetank length. Drop pipes are typically provided for every 1 to 3 manifold, and N drop pipes for this

example has been assigned as two (2) (i.e., Ndrop pipes = 2). Drop pipes are equipped with

manually modulated flow control valves. Figure 7 is a diagram that illustrates the example

aerobic IFAS zone aeration system. The aerobic zone with free-moving plastic biofilm carriershas two drop pipes (i.e., example Ndrop pipes = 2), and each drop is equipped with two manifold

pipes. Each manifold has four, 2.44-m (8-foot) wide diffuser segments.

Figure 7. Configuration of the example aeration system.


20/28

This section has effectively demonstrated application of the aeration design methodology presented in this manuscript. It should be noted that orifice spacing (Lorifice spacing) can be

manipulated to meet the previously determined acceptable percent error once a reasonable grid

layout has been established.

SYSTEM OXYGEN TRANSFER EFFICIENCY TEST METHODOLOGY

Typical SOTE values applied to the design of full-scale aeration systems in aerobic IFAS zones

and MBBRs with medium-bubble diffusers is typically 3.45 percent per meter of water

submergence. In addition, full-scale free moving plastic biofilm carrier based aerobic IFASzones and MBBRs with medium-bubble diffusers have been designed with 1.0 fouling (F) and

0.95 beta () factors. The minimum air flow rate that is required to uniformly distribute the freemoving plastic biofilm carriers throughout the basin is in the range 5 to 10 m

3/hr/m

2 of basin

floor, with a typical design value of 8 m3/hr/m

2 of basin bottom (McQuarrie and Boltz 2011).

The design engineer must specify and over see the implementation of an essential component of

aeration systems using stainless steel pipe diffusers, manifold, and down pipes - testing the clean

water oxygen transfer rate (OTR) in a full-scale aerobic IFAS zone or MBBR. Methodology

exists to quantify the OTR in aeration systems used for wastewater treatment. A standard methodwas developed by the American Society of Civil Engineers (ASCE 2007) to measure oxygen

transfer in clean water. The test method is based upon removal of dissolved oxygen from the

water volume by sodium sulfite followed by reoxygenation to near saturation. The methodspecifies a minimum number, distribution, and range of dissolved oxygen concentration

measurements at each determination point. Data obtained at each determination point are then

analyzed by a simplified mass transfer model to estimate that apparent mass transfer coefficient

under the test environmental conditions TLaK (1/hr) and the steady-state dissolved oxygensaturation concentration as time approaches infinity * C (g/m 3).

The methodology, namely ASCE Method 2-06, described by ASCE (2007) has been successfully

applied to evaluate aeration systems - such as those whose design is similar to what is describedin this manuscript - in full-scale wastewater treatment plants that were designed to incorporate

free-moving plastic biofilm carrier based reactors and associated aeration system. The success of

operating aeration systems like those described in this manuscript have been establishedfollowing strict adherence to performance testing guidelines documented in the respective

Projects’ Technical Specifications. Example projects with fully functional aeration systems (in

the free-moving plastic biofilm carrier zone) similar to those described in this manuscript (i.e.,

including stainless steel pipe diffusers, manifold, and down pipes) that perform as designed andwere successfully tested using ASCE Method 2-06 as reviewed in this section include:

Fields Point Wastewater Treatment Facility aerobic IFAS zone(s), James River Wastewater Treatment Plant aerobic IFAS zone(s), and Grand Chute-Menasha West Wastewater Treatment Facility aerobic IFAS zone(s).


21/28

Due to the importance of demonstrating the effectiveness of a well-designed aeration system for

any wastewater treatment plant owner, the authors review and discuss application of ASCEMethod 2-06 for the aforementioned parameter estimation, namely the apparent mass transfer

coefficient TL

aK and the steady-state dissolved oxygen saturation concentration as time

approaches infinity * C (g/m 3). Indeed, the OTR of an aeration system that has been designed by the methodology presented in this paper may be tested using ASCE Method 2-06 as follows.

First, the design engineer must obtainTL

aK - the clean-water volumetric oxygen transfer

coefficient (1/hr) at temperature T (°C) - through a nonlinear regression analysis fitting Eq. (16)

to measured dissolved oxygen concentration profiles obtained in a full-scale aerobic IFAS zoneor MBBR (e.g., Figure 8). Eq. (16) is the simplified mass transfer model described in ASCE

Method 2-06.

0TL ttaK 0 expCCCC

(16)

Here, C is the dissolved oxygen concentration in the bulk of the water (g/m3), * C is the steady-

state dissolved oxygen saturation concentration as time approaches infinity (g/m3), C0 is the

initial dissolved oxygen concentration at t = t0 (g/m3), and t is time (hr). A non-linear regression

analysis that fits Eq. (16) to the dissolved oxygen concentration profiles obtained by collectingdissolved oxygen measurements at measured time intervals until the water is nearly saturated

(e.g., using data such as that listed in Table 2 and the measured dissolved oxygen concentration

profiles plotted in Figure 8) will result inTL

aK , * C , and C0 values for each measurement

location. Eq. (16) is derived from the “two-film” theory (Lewis and Whitman 1924) which states

that the transfer rate can be expressed in terms of an overall mass transfer coefficient and

resistances on either side of the interface. Sparingly soluble gases such as oxygen result in the

resistance primarily being in the liquid film; therefore, the gas film can be ignored (Stenstrom etal. 2006). The general equation describing resistance to mass transfer is expressed as Eq. (17).

CCaK dt

dCTL

(17)

The integrated and re-arranged form of Eq. (17) is presented as Eq. (18), which is also utilized in

the log-deficit parameter estimation method (Annex F of ASCE Method 2-06) whereby the

parametersTL

aK and * C are estimated with a linear regression analysis.

12

0

TL tt

CCCCln

aK

(18)

Eq. (16) and Eq. (18) will facilitate estimation of the parameters * C and TLaK when the proper

statistical analyses (based on the average of measured dissolved oxygen concentration profiles)


22/28

are applied. TheTL

aK value for each test and at each dissolved oxygen sample location must be

adjusted to a standard temperature (i.e., 20C). The temperature corrected mass transfercoefficient

C20LaK

may be calculated using Eq. (19).

C20T

TLC20L aK aK

(19)

Here,C20L

aK

is the mass transfer coefficient corrected to temperature 20°C (1/hr), T is the water

temperature during dissolved oxygen concentration measurement (C), and is an empiricaltemperature correction factor commonly regarded as 1.024. The dissolved oxygen field

saturation concentration as time approaches infinity, * C , can be adjusted to standard

temperature (20°C) and the site specific elevation using Eq. (20).

1CC

TC20 (20)

Here,

C20S

TS

C

C= sat. conc. temperature correction factor

TSC = surface oxygen sat. conc. at system temperature T (g/m

3)

C20SC = surface oxygen sat. conc. at the standard temperature 20°C (g/m

3)

S

b

P

P= pressure correction factor

P b = barometric pressure at site elevation (E site) (mm Hg)

PS = standard pressure (760 mm Hg or 29.9 inches Hg)

The temperature corrected mass transfer coefficient determined using Eq. (19) is based on data

collected according to protocol outlined by ASCE Method 2-06. Then, the SOTR (kg O2/hr) may

be calculated using Eq. (21).

C20LW aK CVSOTR

(21)

Here, VW is the tank water volume (m3). The average mass transfer coefficient may replace

C20LaK

if multiple measurements (i.e., n-measurements) were collected and statistical analyses (i.e., curve-fitting) efforts executed. The average mass transfer coefficient may be calculated using Eq. (22).

n

aK

aK

n

1ii,C20L

average,C20L

(22)


23/28

Here,average,C20L

aK

is the average mass transfer coefficient (1/hr), and n is the number of

observations. Once the SOTR has been determined, Eq. (21) can be used to calculate the SOTE. Table

2 presents example raw data obtained (consistent with ASCE Method 2-06) from a clean wateroxygen transfer test conducted in a full-scale basin. The aeration system in this basin was designed for

an aerobic MBBR that contains an aeration system designed consistent with the methodologydescribed in this paper. The following example calculation (which uses data from Table 2 that isaffiliated with Test Number 2 and Probe 1) illustrates how the raw data listed in Table 2 may be used

in Eq. (21) to determine the SOTR.

hr

lb273

hr

kg124

hr

178.20

g000,1

kg1

m

g66.10m560SOTR

aK CVSOTR

3

3

C20LC20W

Table 2 does not list the air flow rate (Qair ) that was applied when each test was performed. However,

if the air flow rate was available the design engineer could use Eq. (2) 100SOTESOTR Qair todetermine the system’s SOTE.

Table 2. Example clean water oxygen transfer test results obtained from an aerobic MBBRhaving an aeration system designed by methodology consistent with that described in this paper.

Test No. Water

Temp

(ºC)

Probe K La (1/hr)

C*∞

(g/m3)

K La20

(1/hr)

C*∞20

(g/m3)

SOTR

(lb/hr)

Avg.

SOTR

(lb/hr)

Avg.

SOTE

(%)

1 13.113.1

13.113.113.1

12

345

19.5718.57

17.2017.7316.93

10.6510.29

10.0510.3010.35

23.0521.87

20.2520.8819.94

10.7610.44

10.2410.4610.50

305.5281.4

255.6269.0257.9

273.88 20.97

2 13.1

13.2

13.2

13.2

13.2

1

2

3

4

5

17.64

18.19

19.32

17.90

19.43

10.53

10.20

9.92

10.27

10.22

20.78

21.37

22.70

21.03

22.83

10.66

10.39

10.14

10.45

10.41

272.9

273.6

283.8

270.9

292.9

278.82 21.34

3 13.2

13.213.2

13.213.2

1

23

45

18.25

18.2819.36

17.7017.67

10.38

10.199.93

10.2610.36

21.44

21.4822.75

20.7920.76

10.55

10.3810.16

10.4410.53

278.7

274.9284.7

267.6269.3

275.04 21.14

Figure 8 illustrates example clean water dissolved oxygen profiles (affiliated with the data listed in

Table 2) established for oxygen transfer efficiency tests conducted on the aerobic free-moving plastic

biofilm carrier based reactor having an aeration system designed by methodology that is consistentwith the one described in this manuscript.


24/28

0

2

4

6

8

10

12

0 0 . 5 1

1 . 5 2

2 . 5 3

3 . 5 4

4 . 5 5

5 . 5 6

6 . 5 7

7 . 5 8

8 . 5 9

9 . 5 1 0

1 0 . 5

1 1 . 3

1 1 . 7

1 2 . 2

1 2 . 7

1 3 . 2

1 3 . 8

1 4 . 2

Time (min)

D . O .

( m g / L )

Probe 1

Probe 2

Probe 3

Probe 4Probe 5

Figure 8. Example dissolved oxygen concentration profiles obtained during clean water oxygentransfer testing conducted on an aerobic free-moving plastic biofilm carrier based reactor whose

aeration system design is consistent with the methodology described in this paper.

CONCLUSIONS

State-of-the-art IFAS utilizes free-moving plastic biofilm carriers, and IFAS represents a future

evolution of the activated sludge process that allows for a greater degree of nitrification in

smaller systems (i.e., bioreactors and clarifiers). However, an imperfect understanding of IFASand MBBR process and mechanical design has hindered the widespread application of this

environmental biotechnology. Methodology for the design of aeration systems in aerobic IFAS

zones using free moving plastic biofilm carriers and MBBRs is the least documented processmechanical design approach. Therefore, the average process engineer has a poor understanding

of the aeration system design methodology. Aerobic IFAS zones and MBBRs use an engineered

aeration system consisting of stainless-steel pipe diffusers, manifold (or submerged air header),down pipes, and air-flow control valves. The so-called medium-bubble diffusers have relatively

large circular orifices (i.e., when compared to membrane diffusers) situated along the undersidewhich are less susceptible to scaling and fouling. These diffusers have slightly more efficient

oxygen transfer efficiency than coarse-bubble diffusers, which is further enhanced by the

presence of free moving plastic biofilm carriers. Operational experience has proven thatmedium-bubble diffusers require less maintenance than fine-bubble diffusers. In fact, correctly

designed modern aeration systems in MBBRs have boasted installations, for example, the


25/28

Lillihammer WWTP, Lillihammer, Norway, that has been in operation since 1994 without

maintenance, breakage, or declining system performance. In addition, correctly designedmodern aeration systems in aerobic IFAS zones have boasted installations, for example, the

Broomfield WRF, Broomfield, Colorado, U.S.A., that has been in operation since 2003 without

maintenance, breakage, or declining system performance.

The absence of generally accepted design criteria and evaluation protocol for aerobic IFAS zone

and MBBR aerations systems has hindered engineers when designing, evaluating, and estimatingthe cost of these systems. Full-scale WWTPs exist that have fully functional aeration systems (in

the free-moving plastic biofilm carrier zone) designed using a method similar to the

methodology described in this paper, and tested using ASCE Method 2-06 (reviewed in this

paper). Specifically, the design methodology that has been presented in this manuscript has proven sufficient for (but not limited to) the following full-scale, operational wastewater

treatment plants:

Fields Point Wastewater Treatment Facility aerobic IFAS zone(s),

James River Wastewater Treatment Plant aerobic IFAS zone(s), and Grand Chute-Menasha West Wastewater Treatment Facility aerobic IFAS zone(s).

Criteria and methodology for the design of stainless steel circular pipe diffuser-based aeration

systems that are commonly used in aerobic IFAS zones and MBBRs has been summarized,

theoretically justified where applicable, practical aspects fully vetted, and an example aerationsystem has been fully developed in this manuscript. The presentation of a standard design

method for engineered aeration systems consisting of stainless-steel pipe diffusers, manifold (or

submerged air header), down pipes, and air-flow control valves typical of aerobic IFAS zonesand MBBRs will fill a significant gap in the information that is required to design and estimate

the cost of aerobic IFAS zones and MBBRs. The application of ASCE Method 2-06 has been

reviewed and selected examples were also presented and discussed in this manuscript. It can beconcluded then that an accurate methodology for the design of aeration systems in aerobic IFAS

zones using free moving plastic biofilm carriers and MBBRs has been presented in thismanuscript. Further, this paper has presented evidence that ASCE Method 2-06 may be applied

to demonstrate the adequacy of an aeration system designed according to the methodology

described in this manuscript.


26/28

NOMENCLATURE

Amanifold = manifold cross-sectional area (m2 or ft

2)

C = dissolved oxygen concentration in the bulk of the water (g/m3)

Cs,20 = dissolved oxygen sat. conc. in clean water at 20°C and 1 atm (mg/L)C

* = clean-water (site-specific) dissolved oxygen concentration (g/m

3)

C0 = initial dissolved oxygen concentration at t = t 0 (g/m3)

CDC,0 = discharge coefficient (= 0.65, a typical value for this system)

Cs,T,H = oxygen sat. conc. in clean water at temperature T and altitude H (g/m3)

H,T,s

C = average oxygen sat. conc. in clean water at temp. T and alt. H (mg/L)

TSC = surface oxygen sat. conc. at system temperature T (g/m

3)

Dmanifold = manifold diameter (meters or feet)

TLaK = oxygen mass transfer coefficient under the test temperature, T (1/hr)

L basin length = basin (or tank) length (meters or feet)

L basin width = basin (or tank) width (meters or feet)

Lmanifold = manifold length (meters or feet)

Lgrid spacing = grid spacing (meters or feet)

Lorifice spacing = orifice spacing (meters or feet)

Ldiffuser spacing = diffuser spacing (meters or feet)

Lwall spacing, length = wall spacing from the end of the tank length (meters or feet)

Lwall spacing, width = wall spacing from the end of the tank width (meters or feet)Lend spacing = end spacing (meters or feet)

Lorifice spacing = orifice spacing (meters or feet)

Norifices = total number of orifices required in aerobic IFAS or MBBR zone

Nlaterals per manifold = number of laterals that are dedicated to one manifold

Ndiffusers per grid = number of diffusers per lateral that is dedicated to one grid

Nmanifold, length = total number of manifold distributed over the basin length

Nmanifold, width = total number of manifold distributed over the basin width

Norifices, total, calculated = calculated total number of orifices per aerobic IFAS zone or MBBR

Ntrains = total number of identical trains if basins (or reactors) are in parallel

Norifices, total, system = calculated total number of orifices per parallel aerobic IFAS/MBBR

Ndrop pipes = total number of drop pipes per aerobic IFAS zone or MBBR

Nmanifold per drop pipe = total number of manifold per drop pipe, may be 1 - 3 but 2 is typical

Ot = percent oxygen concentration leaving tank, typically (18 to 20 percent)


27/28

P b = barometric pressure at site elevation (E site) (kPa or mm Hg)

Pd = pressure at the depth of air release, or from the orifice in this case (kPa)

Patm,H = atmospheric pressure at altitude H (kPa)

Pw,md = pressure due to water column, mid-depth above point of air release (kPa)

PS = standard pressure (kPa or mm Hg; 760 mm Hg or 29.9 inches Hg) p0 = average pressure drop across the diffuser orifices (mm H 2O)

Qair, total, zone = design air flow rate (m3/hr or cfm)

Qair, total, grid = air flow rate directed to the designated manifold (m3/hr or cfm)

Qorifice, minimum = minimum design air flow rate/orifice (m3/hr/orifice or cfm/orifice)

Qorifice, maximum = maximum design air flow rate/orifice (m3/hr/orifice or cfm/orifice)

Q’orifice = calculated air flow rate/orifice (m

3/hr/orifice or cfm/orifice)

T = temperature (°C)

t = time (hr)

Vmanifold = gas velocity in manifold (m/s or ft/s)

V0 = gas velocity (m/hr)

VW = volume of water in tank (m3)

Wgrid = grid width (meters or feet)

= pressure correction factor

= oxygen transfer correction factor for wastewater

= salinity-surface tension correction factor (typically 0.95 for this system)

F = fouling factor (typically 1.0 for medium-bubble diffusers)

= empirical temperature correction factor commonly regarded as 1.024 = density (kg/m 3)

= calculated saturation concentration temperature correction factor


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REFERENCES

1. ASCE, American Society of Civil Engineers (2007). Measurement of Oxygen Transfer in

Clean Water. Standard 2-06. Edited by: Michael Stenstrom. ISBN 13: 978-0-7844-0848-3.

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Boltz, J.P., Johnson, B.R., Daigger, G.T., and Sandino, J. (2009a). Modeling integratedfixed film activated sludge (IFAS) and moving bed biofilm reactor (MBBR) systems I:

mathematical treatment and model development. Water Environment Research. 81(6), 555-

575.

3. Boltz, J.P., Johnson, B.R., Daigger, G.T., Sandino, J., and Elenter, D. (2009b). Modeling

integrated fixed film activated sludge (IFAS) and moving bed biofilm reactor (MBBR)

systems II: evaluation. Water Environment Research. 81(6), 576-586.

4. Boltz, J.P., Morgenroth, E., deBarbadillo, C., Dempsey, M., McQuarrie, J., Ghylin, T.,Harrison, J., and Nerenberg, R. (2010a). Biofilm Reactor Technology and Design (Chapter

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biofilm reactors for engineering design. Water Science and Technology. 62(8). 1821-1836.

6. Grady, Jr., L., Daigger, G.T., Love, N.G., and Filipe, C.D.M. (2011). Biological Wastewater

Treatment, 3rd

Ed. IWA Publishing, London.

7. Lewis, W.K., and Whitman, W.G. (1924). Principles of gas absorption. Industrial and

Engineering Chemistry. 16. 1215-1220.

8. McQuarrie, J.P. and Boltz, J.P. (2011). Moving bed biofilm reactor technology: process

applications, design, and performance. Water Environment Research. 83(6). 560-575.

9. McQuarrie, J.P., Boltz, J.P., McQuarrie, J.P., and Daigger, G.T. (2010). Interactions

between suspended biomass and biofilm in the integrated fixed-film activated sludge (IFAS)

process: implications on the practical design of IFAS systems. Proceedings of the WEF/IWA

Biofilm Reactor Technology Conference, Portland, Oregon.

10. Ødegaard, H. (2013). Personal correspondence. Email having Subject: Question regarding your MBBR/IFAS media and aeration system. Message sent August 8, 2013 at 10:56 am

EDT.

11. Perry, R.H., and Green, D.W. (1997). Perry’s Chemical Engineers’ Handbook, 7th

Edition.

McGraw-Hill. New York.

12. Pham, H., Viswananthan, S., Kelly, R.F. (2008). Evaluation of plastic carrier media impacton oxygen transfer efficiency with coarse and fine-bubble diffusers. Proceedings of the

Water Environment Federation Technical Exhibition and Conference, Chicago, IL.13. Stenstrom, M.K., Leu, S.-Y., and Jiang, J. (2006). Theory to practice: oxygen transfer and

the new ASCE standard. Proceedings of the Water Environment Federation Technical

Exhibition and Conference, Washington, D.C.

14. Stenstrom, M., and Rosso, D. (2008). Aeration and Mixing. In: Biological Wastewater

Treatment: Principles, Modelling, and Design. pp. 245-272. IWA Publishing. London.

Documents

Aeration system design in integrated fixed-film activated sludge (IFAS) very good.pdf