DESIGN OF PRESSURIZED LIQUID DISTRIBUTION SYSTEM FOR LANDFILL LIQUIDS ADDITION AND AUGMENTATION Xianda Zhao, Ph.D., P.E. CTI and Associates, Inc., Brighton, Michigan USA

Morgan Subbarayan, P.E. CTI and Associates, Inc., Brighton, Michigan USA

Te-Yang Soong, Ph.D., P.E. CTI and Associates, Inc., Brighton, Michigan USA

ABSTRACT

Whether a landfill facility is conducting a bioreactor operation with large-scale liquid injection or simply recirculating site-generate leachate, achieving uniform liquid distribution in the waste mass is always a critical operational goal. Several methods of liquid introduction have been adopted by the industry. Of these methods, subsurface lateral injection lines (including perforated plastic pipes) have become “standard” design for many landfill engineers. The subsurface lateral injection lines not only provide for safe liquid injection, they also allow for the introduction of a large volume of liquid – even after the waste mass has reached its permitted grade.

Unfortunately, improperly-designed lateral injection lines may result in uneven liquid distribution. Primary concerns associated with uneven distribution include: leachate outbreaks, differential settlements, unstable working surfaces, and sometimes even slope instability.

This paper provides methodology for the design of subsurface lateral injection lines,

specifically the design of perforated pipes (pipe sizing, perforation sizing and the selection of spacing between perforations). Essential design equations, design principle and criteria will be presented. A design example will also be used to illustrate the step-by-step design procedures.

INTRODUCTION

During the lifespan of a landfill, moisture in the incoming waste as well as liquid entering the waste mass (in forms of precipitation, snowmelt, surface runoff, and other liquid addition) generates leachate. Leachate carries the characteristics of the waste constituents and needs to be properly contained, collected, removed, treated, and ultimately disposed of safely, in order to protect human health and minimize adverse effects to the environment. Due to the high cost of leachate treatment and disposal, much research has been performed to find alternative uses for leachate that can reduce amounts that must be removed from the landfill.

Since as-received waste typically still possesses additional moisture absorptive capacity, re-

introducing leachate back into the waste mass (commonly referred as “leachate recirculation”) offers an effective way of reducing leachate treatment costs. The actual moisture absorptive capacity remains in the waste mass (sometimes referred “moisture deficit”) varies greatly depending on the

geographic location, climate, type of waste and other pertinent factors. For landfill sites that are located in arid or semi-arid areas and for landfills that receive large amount of incoming waste volume, the remaining moisture absorptive capacity can be very significant. Such large amount of absorptive capacity represents an immense cost-saving potential for landfill owners and operators due to the circumvention of leachate disposal and treatment. In fact, reintroducing collected leachate is widely practiced in the municipal solid waste (MSW) landfills in the United States nowadays.

In addition to cost savings, re-introducing leachate offers additional advantages in the operation of MSW landfills. For example, greater moisture content will increase waste compaction therefore increasing the filling capacity and consequently, service life of the facility. Furthermore, increased moisture promotes and accelerates biological decomposition of organic wastes, which will yield more reusable volume. Ultimately, decomposed wastes are biologically-stabilized which greatly reduces the long-term adverse impacts to human health and environment.

Recently, bioreactor landfills have been designed, constructed, and operated at a number of

commercial and municipal facilities throughout the United States. In bioreactor landfills, moisture content in the waste material is quickly increased to an elevated level to allow for the initiation of biological decomposition processes at a relatively early stage of waste filling. To achieve this goal, a large amount of liquid is generally required and in some cases, addition of supplementary liquid is necessary. Possible sources of supplementary liquids include leachate from other sites, storm water, wastewater (including biosolid and septage), commercial liquids, animal manure, and others.

Whether a landfill is conducting a bioreactor operation with large-scale liquid injection or simply recirculating site-generate leachate, achieving uniform liquid distribution in the waste mass is always a critical operational goal. Several methods of liquid introduction have been adopted by the industry: surface spraying, infiltration ponds, subsurface injection via vertical wells, and subsurface injection via lateral injection lines. Due to concerns such as nuisance, safety, and volume restriction associated with some of the methods, subsurface lateral injection lines have become “standard” approach for many landfill engineers. The subsurface lateral injection lines not only allow for safe liquid injection, they also allow for introduction of large volume of liquid – even after the waste mass has reached its permitted grade.

Unfortunately, improperly-designed lateral injection lines can result in uneven liquid

distribution, which will eventually lead to issues such leachate outbreaks, differential settlements, unstable working surface, or even slope instability.

This paper provides design methodology for the design of subsurface lateral injection lines, including pipe sizing, perforation sizing and the perforation interval determination. Essential design equations will be presented first, followed by the design principle and criteria and the recommended design procedures. A design example will also be presented to illustrate the step-by-step design procedures. TYPICAL DESIGN AND COMMONLY SEEN ISSUES

Typical design and construction of subsurface lateral injection lines include perforated plastic pipes surrounded by porous media. The porous media allows for storage and rapid spreading of liquids. Both trench- and mound-designs have been used in the industry (Figure 1). These lateral distribution lines are typically horizontally spaced at 50 to 200 ft intervals and staggered vertically every 10 to 50 ft (Figure 2).

Porous Media

Perforated Pipe

MSWWaste

Porous Media

“Mound”design

“Trench”design

Figure 1 – Typical Subsurface Lateral Liquid Injection Lines

10 – 50 ft

50 – 200 ft

Figure 2 - Typical Layout of Subsurface Lateral Liquid Injection Lines

Adequately designed lateral injection lines should carry the injected liquid to the end of the

perforated pipe and evenly discharge liquid along the entire line. Without proper engineering design, un-even distribution, prolonged percolation time and excessive pressure buildup can be expected. It is very common to see perforated injection pipe with relatively large perforations (e.g., ½ inch or greater in diameter) drilled at a densely-spaced pattern (e.g., 4 perforations for every 6 inches). Such design minimizes the entrance pressure head hence results in a quick pressure drop along the pipe. Consequently, vast majority of the injected liquid is discharged near the entrance of the pipe. As illustrated in an example shown in Figure 3, ninety percent of the injected liquid is discharged within the first 30 ft of the pipe and the discharge rate rapidly diminish beyond that point.

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Distance from End of Pipe (ft)

Uni

t Dis

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ate

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)

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rate

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ipe

(gpm

)

Unit Discharge Rate Flow rate in Pipe

Total flowrate = 200 gpmEntrance Pressure = 0.5 ft W.C.Pipe ID = 3 inchesPerforation: 4 holes every 6 inchesHole size: ½ inch

Figure 3 - Unit Discharge Rate and Flow Rate: the “Typical” Practice

In order to uniformly distribute the injected liquid along the entire pipe length, a “pressurized”

perforated pipe design is necessary, of which both the sizing and number of the perforations need to be reduced. In an example illustrated in Figure 4, one ¼ inch perforation is drilled for every linear foot of the pipe. As seen in the results, the perforated pipe is pressurized (entrance pressure head is 9 ft) and a relative uniform distribution of liquid along the entire length is achieved (between 2.2 and 1.8 gpm for any given perforation). The following sections will focus on the design of the pressurized liquid injection pipes.

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owra

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e (g

pm)

Unit Discharge Rate Flow rate in Pipe

Total flowrate = 200 gpmEntrance Pressure = 9 ft W.C.Pipe ID = 3 inchesPerforation: 1 holes every 1 ftHole size: ¼ inch

Figure 4 - Unit Discharge Rate and Flow Rate: the “Pressurized” Design

DESIGN METHOLOGY

Design Equations

The unit discharge rate (q) from each of the perforations is governed by the size of the perforation and the static pressure at its respective location along the pipe:

2/1279.112 PdgPBAq == (1)

Where q = flow rate per perforation (gpm) B = orifice coefficient, assumed as 0.60 A = area of orifice (in2) g = gravitational acceleration (32.2 ft/s2) P = pressure head over orifice (water column in ft.) d = diameter of perforation (inch) According to Bernoulli’s equation, total head at any given point in liquid under motion is the sum of pressure, velocity and elevation heads:

Zg

VPh ++=2

2

(2)

Where h = total head (feet) P = pressure head (feet) V = velocity (ft/sec) g = gravitational acceleration (32.2 ft/s2) Z = elevation head (feet)

Change of total head in pipes is primarily due to friction and other minor losses. Since perforated pipes are typically constructed with straight sections with limited number of joints, minor losses are generally considered negligible. Therefore, the friction loss along the pipe will determine the change in total head. Friction loss in pipes can be calculation using Hazen-Williams equation as:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛= 8655.4

85.185.1100002082.0DQ

CLh f (3)

Where hf = friction loss head (feet) L = length of pipe (feet)

C = pipe friction factor (150 for HDPE pipes) Q = flow rate in pipe (gpm) D = nominal pipe size (inch)

Due to discharge at perforations, flow in perforated pipes varies along the pipe length (Figure 5). Flow in perforated pipes can be obtained by summing discharges from all of the downstream holes:

∑=

=i

jji qQ

1 (4)

Where Qi = flow in the pipe before perforation “i” (gpm) qj = discharge rate at perforation “j” (downstream of “i”, gpm)

n n-1 n-2 n-3 123

q1q2q3qn-3qn-2qn-1qn

∆LL

Q= Qn

d

D

Q1Q2Q3Qn-3Qn-2Qn-1

Figure 5 – Flow Rate along Perforated Pipe

Friction loss in each section between two perforations can be determined as:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛∆= 8655.4

85.185.1100002082.0DQ

CLh i

fi (5)

Where “∆L” is the spacing between two adjacent perforations. “∆L” can be calculated based on total number of perforations (“n”) as:

1−

=∆n

LL (6)

Based on the conservation of energy, the total head can be calculated as:

ifi

iii

ii hz

gVPz

gVPh +++=++= +

++ 1

21

1

2

22 (7)

For low velocity flow (less than 5 ft/sec), the kinetic head is generally very low (less than 0.4 ft) and is typically neglected. For horizontal-placed pipes, pressure at an upstream perforation can be determined as:

ifii hPP +=+1 (8)

The unit discharge rate at an upstream perforation can be calculated as:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛∆+=+=+ 8655.4

85.185.122

1100002082.079.1179.11

DQ

CLPdhPdq i

ifii i (9)

The unit discharge rate at the end of the pipe (q1) can be calculated as:

12

1 79.11 Pdq = (10)

where P1 is the pressure at end of the pipe.

Once the far-end pressure value (P1) and the far-end discharge rate (q1) are determined, unit discharge rate for all perforations can be obtained using Equations (4) and (9). The entrance pressure and the total flow rate will be utilized in the pumping system calculations. The above-listed procedures can be readily incorporated in spreadsheet programs. However, a trial-and-error process may be required to match the pumping system requirements. Design Principles

The ratio of unit discharge rates between the first and the last perforations can be used to quantify the uniformity of liquid distribution. In other words, if the ratio for a given perforated pipe design is closer to unity, the liquid is more evenly distributed. As the examples illustrated in Figures 3 and 4, a satisfactory ratio of 1.2 can be found in the “pressurized” pipe design whereas a ratio greater than 10,000 (which is clearly inadequate) can be seen in the low pressure design.

Note that the variation in the unit discharge rates is caused by the pressure change in the pipe and the relative change of the discharge rate can be determined as:

n

n

n

n

PPP

qqq 11 −

=−

(11)

Deriving from Equation (11), a correlation between the change in discharge rate and the change in pressure can be developed as:

2

2 ⎟⎟⎠

⎞⎜⎜⎝

⎛ ∆−

∆=

∆

nnn qq

qq

PP (12)

where ∆q = qn - q1 ∆P = Pn - P1

The correlation between the change in pipe pressure and the change in unit discharge rates can be established using Equation (12), see Figure 6.

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0% 5% 10% 15% 20% 25% 30%Change in Unit Discharge Rate

Cha

nge

in E

ntra

nce

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ssur

e

Figure 6 – Correlation between Change in Unit Discharge Rate and Change in Entrance Pressure

As previously discussed, pressure change in pipe is primarily due to friction loss:

∑−

=

=∆1

1

n

ifi

hP (13)

The total friction loss in perforated pipes can be estimated using Equation (14):

FPP *∆=∆ (14)

Note that “F” is a correction factor and “∆P*” is the friction loss calculated for a solid wall pipe having same diameter, length, and total flow rate. Equations (15) and (16) depict the determinations for “F” and “∆P*”, respectively. Note that the correlation shown in Equation (15) is established based on an assumption that the change of unit discharge rate is less than 20%.

( ) 85.1

1

1

85.1

1 nn

iF

n

i

−=∑−

= (15)

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛=∆ 8655.4

85.185.1* 100002082.0

DQ

CLP n (16)

As shown in Equation (15), the correction factor “F” is a function of the number of perforations along the pipe. As the number of perforations increases, the correction factor decreases and ultimately levels off at a value of 0.36 (Figure 7). For most design with more than 100 holes along the pipe, a correction factor “F” of 0.36 can be used.

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0.35

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0.45

0.50

0.55

0.60

0.65

0 50 100 150 200Number of Hole on Perforated Pipe (n)

F=hf

/hf*

Figure 7 - Friction Loss Correction Factor for Perforated Pipe

Finally, by combining and rearranging Equations (14) and (16), the required pipe diameter can be determined as:

3802.02055.0

6193.1 ⎟⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛∆

=CQ

PLFD n (17)

Design Procedures

Uniform liquid distribution along the perforated liquid injection pipes can be achieved by proper selection of pipe diameter, size of perforations, and spacing between perforations. A systematic procedure can be presented in a flowchart format as shown in Figure 8. Individual steps will be discussed in detail in the following subsections.

CalculationTotal flow rate (Q)

SelectPerforation diameter (d) and unit discharge rate (q)

CalculationPressure difference between first and last perforations

Eq. 17 and Fig. 7

CalculationDetermine pipe diameter

InputPipe length (L) and linear discharge rate (q*)

InputAllowed variation

for unite discharge rate

DesignPumping system

and entrance pressure

CalculateNumber of perforations (n) and spacing (∆L)

Satisfied?YES

No

Satisfied?

OutputPerforation diameter (d), spacing (∆L),

pipe diameter (D) and entrance pressure (Pn)

YESNo

Cha

nge

linea

r dis

char

ge ra

te (q

*)

Figure 8 – Design Procedure Flowchart

1. Selecting input parameters

Length of the perforated pipe (“L”) is generally determined by the dimensions of the waste lift where the injection line is to be installed. To avoid leachate outbreaks on refuse slopes, the injection lines should not be located within 50 ft of the exterior waste slope. The injection lines are typically spaced horizontally 50- to 150 ft with a vertical interval of 20 ft. The linear discharge rate (q*) should also be pre-selected. The actual value is controlled by the infiltration capacity of the waste. Reinhart and Townsend (1998) suggested that injection rates between 25- to 50 gpd/ft are adequate. To further promote lateral distribution and minimize biological clogging, intermittent liquid injection should also be considered (Reinhart and Townsend, 1998). Generally speaking, selecting linear discharge rate between 0.2- to 0.4 gpm/ft seems appropriate.

2. Determining total flow rate

The total flow rate (Q) can be calculated as:

*LqQ = (18)

Note that this total flow rate is identical to the entrance flow rate (Qn). 3. Determining the entrance pressure

Based on the required flow rate, the pumping and forcemain analyses can be conducted and subsequently, the entrance pressure can be determined. Since the procedure is a common practice for hydraulic engineers, no detailed discussed will be provided herein. Note that, however, the entrance pressure should be less than 5 psi (11.5 ft of water column) to avoid excessive increase in pore pressure which may adversely impact the slope stability (Bachus, et al., 2004).

4. Selecting size of perforations and calculating the unit discharge rate

As long as the clogging potential is avoided, smaller perforation sizes are preferred for better liquid distribution. Unit discharge rate can be determined based on the entrance pressure and the perforation size using Equation (1). See Figure 9 for typical correlations.

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0 2 4 6 8 10 12Unit Discharge Rate (gpm)

Pre

ssur

e (ft

)

d=1/8 in

d=3/16 in

d=1/4 in

d=5/16 in

d=3/8 in

d=7/16 in

d=1/2 in

Maximum Recommended Entrance Pressure 11.5 ft

Figure 9 – Unit Discharge Rate vs. Entrance Pressure for Different Sizes of Perforation

5. Calculating number and spacing of perforations

Number of perforations (n) can be calculated as:

qQn = (19)

Spacing between perforations (∆L) can be calculated as:

1−

=∆n

LL (20)

In most cases, the number of perforations (n) should be greater than 50 (i.e., q/Q < 2%) and spacing between perforations (∆L) should be less than 2% of the length of perforated section (L). If these requirements are met, the design procedure can continue. Otherwise, a new perforation size shall be selected and Step 4 shall be repeated until all design requirements are met.

6. Selecting the allowable variation for unit discharge rate and calculating the corresponding

allowable pressure difference

Friction loss along the perforated pipe can be minimized but can not be completely eliminated. In other words, some differences in the unit discharge rate will always exist. A tolerable variation should be pre-determined for each project. To maintain a reasonable pipe size, the tolerance (∆q/qn) can be set between 10% and 20%. The corresponding variance in pressure between the two extreme ends of the perforated pipe can be calculated using Equation (12) or Figure 6. Subsequently, the allowable pressure drop (∆P) in the perforated pipe can be calculated based on the entrance pressure.

7. Determining pipe size

Size of the perforated pipe can be determined using Equation (17), or Figure 10, based on the unit friction loss and total flow rate. Unit friction loss can be calculated by dividing the allowable pressure drop (∆P) by the length of perforated pipe (L). Note that the correction factor (F) can be obtained from Figure 7. If the number of perforation is greater than 100, the value of “F” can be assumed as 0.36. The diameter of the perforated pipe should be rounded to higher standard size. If the result is not satisfactory, a new linear discharge rate can be selected and the entire design procedure can be repeated. The following example illustrates the use of the above-mentioned design procedures.

DESIGN EXAMPLE

A landfill plans to install several leachate recirculation lines on the active surface. Based on the geometry of the lift boundary, three subsurface leachate injection lines will be installed (Figure 11). Leachate will be pumped from the storage facility, through a forcemain, into a control vault located at the base of the northeastern slope. Designated transmission lines will direct leachate from the vault into the perforated pipes. Only one line will be used during each injection event.

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tion

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D=2 in

D=3 inD=4 in

D=5 in

D=6 inD=2 in

D=3 in

D=4 in

Figure 10 - Unit Friction Loss at Different Flow Rate

100 ft100 ft100 ft50 ft

600

ft

300

ft

150

ft

Lift Boundary

Leachate RecirculationLines

ControlVault

LeachateStorage Facility

Forcemain

Leachate TransmissionLines

N

Figure 11 – Layout of the Proposed Leachate Recirculation System (Example Problem)

Using the recommended procedures discussed earlier, the following design can be formulated:

1. Selecting input parameters

Line Length (ft)

Linear Discharge Rate (gpm/ft)

1 600 0.15 2 300 0.30 3 150 0.60

Note that the selected linear discharge rates will result in similar total discharge rate for each of the 3 injection lines.

2. Determining total flow rate using Equation (18)

Line Linear Discharge Rate (gpm/ft)

Total Flow Rate (gpm)

1 0.15 90 2 0.30 90 3 0.60 90

3. Determining the entrance pressure

To avoid excessive velocity and friction the loss, flow rate will be controlled below 100 gpm. Note that the entrance (immediately before the first perforation) pressures listed below were determined via separate forcemain analyses. Differences in the calculated entrance pressures are results of the different lengths in the transmission pipes between the control vault and the perforated pipes.

Line Entrance Pressure (Water column in ft.)

1 2.5 2 3.4 3 4.3

4. Selecting size of perforations and calculating the unit discharge rate

A perforation size is pre-selected as 3/16 inches in diameter. With that, the unit discharge rates can be calculated using Equation (1). Results of the unit discharge rates are listed below:

Line Entrance Pressure (Water column in ft.)

Diameter of Perforation (inch)

Unit Discharge Rate (gpm)

1 2.5 3/16 0.66 2 3.4 3/16 0.76 3 4.3 3/16 0.86

5. Calculating number and spacing of perforations

For Line 1, the number of perforations can be calculated as:

13766.0

90≈==

qQn

Subsequently, the spacing between perforations (∆L) can be determined as:

ftn

LL 40.41137

6001

≈−

=−

=∆

For ease of construction, the spacing is set to 4.5 ft, which will result in a total of 134 perforations. Same procedures can be repeated for Lines 2 and 3. Results for all three lines are listed below. Line Length

(ft) Total Flow Rate

(gpm) Unit Discharge Rate

(gpm) No. of

Perforations Spacing

(ft) 1 600 90 0.66 135* 4.5 2 300 90 0.76 121 2.5 3 150 90 0.86 101 1.5

Note: *Spacing for the last two perforations at end of the pipe is 1.5 ft.

6. Selecting the allowable variation for unit discharge rate and calculating the corresponding allowable pressure difference

The allowable variation in the unit discharge rate is pre-selected as 20%. With that, the allowable pressure drop can be calculated using Equation (12).

Line Variation in Unit Discharge Rate

Entrance Pressure (Water column in ft.)

Allowed Pressure Drop (ft)

1 20% 2.5 0.90 2 20% 3.4 1.22 3 20% 4.3 1.55

7. Determine pipe size

Diameter of the pipe can be calculated using Equation (17). The correction factor “F” can be selected using Figure 6. Since the numbers of perforations are greater than 100 for all three lines, an “F-value” of 0.36 will be used for the pipe sizing calculations for all lines.

Line Length

(ft) Total Flow

Rate (gpm)

Allowed Pressure Drop

(ft)

No. of Perforations

Pipe Diameter - Calculated

(inch)

Pipe Diameter - Selected

(inch) 1 600 90 0.90 135 4.11 4 2 300 90 1.22 121 3.35 4 3 150 90 1.55 101 2.77 3

Note that the pipe diameter can also be selected based on the unit friction loss and the total flow rate using Figure 10.

8. Results verification

Based on output of the design procedures (i.e., pipe size, perforation size and spacing,

entrance pressure, etc.), three simulations were executed using a spreadsheet program that incorporates Equations (4), (9) and (10). Results of the simulation illustrate the predicted discharge rate at each of the perforations along the entire perforated sections, see Figure 12. Further examining the results shown in Figure 12 reveals that the actual variations of unit discharge flow rates (changes between the first and the last perforations) are 18%, 7% and 10% for Lines 1, 2 and 3, respectively. All of which are less than the pre-selected maximum allowable variation (20%, see Step 6 in the previous section). Therefore the design is verified as appropriate. Otherwise different pipe sizing may be considered and the procedures can be repeated until the result is successfully verified.

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per

Hol

e (g

pm)

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Distance from the end of pipe (ft)

Uni

t Dis

char

ge F

low

Rat

e (g

pm)

Figure 12 – Predicted Discharge Flow Rate along the Perforated Pipes (Example Problem)

MAXIMUM PERFORATION LENGTH

There is a theoretical length limitation to the perforated section of any liquid injection pipe. In other words, one set of design parameters (i.e., pipe size, perforation size and spacing, allowable variation in unit discharge rate and entrance pressure, etc.) will not offer same performance when different perforated lengths are used.

To demonstrate this fact, two design charts were developed and shown in Figures 13 and 14. Both charts assumed a 5-ft spacing between perforations and an entrance pressure of 5-ft of water column. Additionally, both charts correlate the perforation size with the maximum pipe length and the corresponding discharge flow rates for different pipe sizes. The only difference between Figures 13 and 14 is the allowable variation among the unit discharge rates – a maximum variation of 10% and 20% were assigned in Figures 13 and 14, respectively.

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1/8 3/16 1/4 5/16 3/8 7/16 1/2

Orifice Diameter (in)

Max

Pip

e Le

ngth

(ft)

D=6 inD=5 inD=4 inD=3 inD=2 in

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1/8 3/16 1/4 5/16 3/8 7/16 1/2

Orifice Diameter (in)

Tota

l Flo

wra

te (g

pm)

D=6 inD=5 inD=4 inD=3 inD=2 in

Figure 13 – Correlation between Size of Perforation, Maximum Pipe Length and Discharge Flow Rates for Different Pipe Sizes (maximum allowable difference on unit discharge rate = 10%)

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1/8 3/16 1/4 5/16 3/8 7/16 1/2

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Max

Pip

e Le

ngth

(ft)

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1/8 3/16 1/4 5/16 3/8 7/16 1/2

Orifice Diameter (in)

Tota

l Flo

wra

te (g

pm)

D=6 inD=5 inD=4 inD=3 inD=2 in

Figure 14 – Correlation between Size of Perforation, Maximum Pipe Length and Discharge Flow Rates for Different Pipe Sizes (maximum allowable difference on unit discharge rate = 20%)

As clearly indicated in Figures 13 and 14, all of the design parameters are interrelated and no “typical” perforated pipe design is universally applicable. Design engineers should have a thorough understanding of both the project requirements and the design mechanism in order to provide an effective design of liquid injection system.

PARAMETRIC ANALYSES AND OBSERVATIONS

This section presents the results of a series of parametric analyses. The parametric analyses were designed to demonstrate the sensitivity embedded in various design parameters in the design of perforated liquid injection pipes. Four critical design parameters including size of perforation, size of pipe, spacing between perforations, and the entrance pressure were evaluated. Table 1 summarizes the results of the parametric analyses and the authors’ observations and comments.

Table 1 – Results of the Parametric Analyses with Observations

Design Parameter

Range of Variation

Graphical Results Observations/Comments

Perforation size 1/8- to 1/2” in diameter

Figure 15 • Smaller perforations allow for longer liquid injection distances.

• However, total flow rate will decrease when smaller perforations are used, which implies a longer injection period during each injection event.

• Maximum injection distance varies between 80- to 540 ft within the range of analyses.

Pipe size 2- to 6 inch in diameter

Figure 16 • Larger pipes allow for longer injection distances. • Maximum injection distance varies between 100- to

710 ft within the range of analyses.

Perforation spacing

1- to 10 ft. Figure 17 • Maximum injection length can be increased by using larger perforation spacing.

• Due to potential clogging of perforations, the authors recommend a maximum of 10-ft spacing between perforations.

Entrance pressure

1 to 10 ft of water

column

Figure 18 • Entrance pressure has only slightly influence on the maximum injection length.

• However, flow rate does increase with higher pressure, which implies a shorter injection period during each injection event.

CONCLUSIONS

One of the essential goals when designing liquid injection lines is to uniformly distribute liquid into the waste mass. According to the information documented in literature and the authors’ past project experiences, the most effective injection method is the use of lateral injection lines (trenches or mounds). However, an improperly-designed lateral injection line can still result in an uneven liquid distribution, which may eventually lead to issues such leachate outbreaks, differential settlements, unstable working surface, or even slope instability.

A systematic design procedure is recommended and presented in this paper, following which

will allow the engineers to properly select system parameters (e.g., pipe size, perforation size, perforation spacing, and entrance pressure) and meet their project-specific requirements.

0

100

200

300

400

500

600

1/8 3/16 1/4 5/16 3/8 7/16 1/2Hole Size (inch)

Max

imum

Len

gth

(ft)

0

20

40

60

80

100

120

Tota

l Flo

wra

te (g

pm)

Max Pipe Length Flowrate

Entrance Pressure = 5 ft W.C.Pipe ID = 3 inchesPerforation: 1 hole every 5 ft

Figure 15 - Effect of Perforation Sizes (∆q/q =10%)

0

100

200

300

400

500

600

700

800

2 3 4 5 6Pipe ID (inch)

Max

imum

Len

gth

(ft)

0

30

60

90

120

150

180

210

240

Tota

l Flo

wra

te (g

pm)

Max Pipe Length Flowrate

Entrance Pressure = 5 ft W.C.Perforation: 1 hole every 5 ftHole size = ¼ inch

Figure 16 - Effect of Pipe Sizes (∆q/q =10%)

050

100150200250300350400450

1 2 3 4 5 6 7 8 9 10

Spacing between Holes (ft)

Max

imum

Pip

e Le

ngth

(ft)

dq/q=10% dq/q=20%

Entrance Pressure = 5 ft W.C.Hole size = ¼ inchPipe ID = 3 inch

0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9 10

Spacing between Holes (ft)

Tota

l Flo

wra

te (g

pm)

dq/q=10% dq/q=20%

Figure 17 - Effect of Perforation Spacing

050

100150200250300350400450

1 2 3 4 5 6 7 8 9 10

Entrance Pressure (ft W.C.)

Max

imum

Pip

e Le

ngth

(ft)

dq/q=10% dq/q=20%

Hole size = ¼ inchSpacing = 5 ftPipe ID = 3 inch

0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9 10

Entrance Pressure (ft W.C.)

Tota

l Flo

wra

te (g

pm)

dq/q=10% dq/q=20%

Figure 18 - Effect of Entrance Pressure

REFERNCES

Reinhart, D.R. and T.G. Townsend (1998). “Landfill Bioreactor Design and Operation”, published by Lewis Publishers, New York Bachus, R.C., M.F. Houlihan, E. Kavazanjian, R. Isenberg, and J.F. Beech (2004). “Bioreactor landfill stability: key considerations”, in MSW Management, September/October, http://www.mswmanagement.com/mw_0409_biostability.html