DESIGN OF PRESSURIZED LIQUID DISTRIBUTION SYSTEM FOR LANDFILL LIQUIDS ADDITION AND AUGMENTATION

Transcription

1 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, reintroducing 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

2 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

3 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 5 to 2 ft intervals and staggered vertically every 1 to 5 ft (Figure 2). Trench design Mound design Porous Media Perforated Pipe MSW Waste Porous Media Figure 1 Typical Subsurface Lateral Liquid Injection Lines 5 2 ft 1 5 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 3 ft of the pipe and the discharge rate rapidly diminish beyond that point.

4 Unit Discharge Rate (gpm) Total flowrate = 2 gpm Entrance Pressure =.5 ft W.C. Pipe ID = 3 inches Perforation: 4 holes every 6 inches Hole size: ½ inch Flowrate in Pipe (gpm) Distance from End of Pipe (ft) Unit Discharge Rate Flow rate in Pipe 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 Unit Discharge Rate (gpm) Total flowrate = 2 gpm.5 Entrance Pressure = 9 ft W.C. Pipe ID = 3 inches 5 Perforation: 1 holes every 1 ft Hole size: ¼ inch Distance from End of Pipe (ft) 15 1 Flowrate in Pipe (gpm) Unit Discharge Rate Flow rate in Pipe Figure 4 - Unit Discharge Rate and Flow Rate: the Pressurized Design

5 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: q = 2 1/ 2 = BA 2gP 11.79d P (1) Where q = flow rate per perforation (gpm) B = orifice coefficient, assumed as.6 A = area of orifice (in 2 ) g = gravitational acceleration (32.2 ft/s 2 ) 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: 2 V h = P + + Z (2) 2g Where h = total head (feet) P = pressure head (feet) V = velocity (ft/sec) g = gravitational acceleration (32.2 ft/s 2 ) 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: h f Q =.282L C D (3) Where h f = friction loss head (feet) L = length of pipe (feet) C = pipe friction factor (15 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: Q i = q j i j = 1 Where Q i = flow in the pipe before perforation i (gpm) q j = discharge rate at perforation j (downstream of i, gpm) 1.85 (4)

6 D L L Q= Q n Q n-1 Q n-2 Q n-3 n n-1 n-2 n Q 3 Q 2 Q 1 q n d q n-1 q n-2 q n-3 q 3 q 2 q 1 Figure 5 Flow Rate along Perforated Pipe Friction loss in each section between two perforations can be determined as: Q i h = f i.282 L C (5) D Where L is the spacing between two adjacent perforations. L can be calculated based on total number of perforations ( n ) as: L L = (6) n 1 Based on the conservation of energy, the total head can be calculated as: 2 2 Vi Vi+ 1 = Pi + + zi = Pi zi+ 1 h + h 2g 2g f i (7) For low velocity flow (less than 5 ft/sec), the kinetic head is generally very low (less than.4 ft) and is typically neglected. For horizontal-placed pipes, pressure at an upstream perforation can be determined as: P i+1 = Pi + h (8) The unit discharge rate at an upstream perforation can be calculated as: Q i q = + = + i d Pi h f 11.79d Pi.282 L C (9) i D The unit discharge rate at the end of the pipe (q 1 ) can be calculated as: where P 1 is the pressure at end of the pipe. f i 2 q 1 = d P1 (1) Once the far-end pressure value (P 1 ) and the far-end discharge rate (q 1 ) 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

7 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 1, (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: qn q1 n 1 q n = P P n P (11) Deriving from Equation (11), a correlation between the change in discharge rate and the change in pressure can be developed as: 2 P q q = 2 Pn qn qn (12) where q = q n - q 1 P = P n - P 1 The correlation between the change in pipe pressure and the change in unit discharge rates can be established using Equation (12), see Figure 6. 6% Change in Entrance Pressure 5% 4% 3% 2% 1% % % 5% 1% 15% 2% 25% 3% Change in Unit Discharge Rate 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:

8 = P h fi (13) i= 1 The total friction loss in perforated pipes can be estimated using Equation (14): n 1 * P = P F (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 2%. F = n i i= ( n 1) n (15) * 1 Q n P =.282L C (16) D 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.36 (Figure 7). For most design with more than 1 holes along the pipe, a correction factor F of.36 can be used F=hf/hf* Number of Hole on Perforated Pipe (n) 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: Design Procedures.255 LF Qn D = P C.382 (17)

9 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. Input Pipe length (L) and linear discharge rate (q*) Calculation Total flow rate (Q) Change linear discharge rate (q*) Design Pumping system and entrance pressure Select Perforation diameter (d) and unit discharge rate (q) Calculate Number of perforations (n) and spacing ( L) Satisfied? No YES Calculation Pressure difference between first and last perforations Eq. 17 and Fig. 7 Input Allowed variation for unite discharge rate No Calculation Determine pipe diameter Satisfied? YES Output Perforation diameter (d), spacing ( L), pipe diameter (D) and entrance pressure (Pn) 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 5 ft of the exterior waste slope. The injection lines are typically spaced horizontally 5- to 15 ft with a vertical interval of 2 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 5 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.2- to.4 gpm/ft seems appropriate. 2. Determining total flow rate

10 The total flow rate (Q) can be calculated as: * Q = Lq (18) Note that this total flow rate is identical to the entrance flow rate (Q n ). 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., 24). 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. Pressure (ft) d=1/8 in d=3/16 in Maximum Recommended Entrance Pressure 11.5 ft d=1/4 in d=5/16 in d=3/8 in d=7/16 in d=1/2 in Unit Discharge Rate (gpm) Figure 9 Unit Discharge Rate vs. Entrance Pressure for Different Sizes of Perforation

11 5. Calculating number and spacing of perforations Number of perforations (n) can be calculated as: Q n = (19) q Spacing between perforations ( L) can be calculated as: L L = (2) n 1 In most cases, the number of perforations (n) should be greater than 5 (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/q n ) can be set between 1% and 2%. 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 1, 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 1, the value of F can be assumed as.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.

12 Unit Friction Loss D=2 in D=3 in Unit Friction Loss D=2 in D=3 in D=4 in D=5 in D=6 in.1 D=4 in Flowrate in Pipe (gpm) Flowrate in Pipe (gpm) Figure 1 - Unit Friction Loss at Different Flow Rate N Leachate Storage Facility Control Vault Leachate Transmission Lines Lift Boundary Forcemain 5 ft 15 ft 1 ft 3 ft 1 ft 6 ft 1 ft Leachate Recirculation Lines Figure 11 Layout of the Proposed Leachate Recirculation System (Example Problem)

13 Using the recommended procedures discussed earlier, the following design can be formulated: 1. Selecting input parameters Line Length (ft) Linear Discharge Rate (gpm/ft) 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) Determining the entrance pressure To avoid excessive velocity and friction the loss, flow rate will be controlled below 1 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.) 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) / / / Calculating number and spacing of perforations For Line 1, the number of perforations can be calculated as:

14 Q n = q 9 = Subsequently, the spacing between perforations ( L) can be determined as: L 6 L = = 4. 4 ft n 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) * 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 2%. 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 2% % % 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 1 for all three lines, an F-value of.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) Note that the pipe diameter can also be selected based on the unit friction loss and the total flow rate using Figure Results verification Based on output of the design procedures (i.e., pipe size, perforation size and spacing,

15 entrance pressure, etc.), three simulations were executed using a spreadsheet program that incorporates Equations (4), (9) and (1). 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 1% for Lines 1, 2 and 3, respectively. All of which are less than the pre-selected maximum allowable variation (2%, 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. Unit Discharge Flow per Flow Hole Rate (gpm) (gpm) Distance Length from from the end distal of (ft) pipe (ft) Line 1 Line 2 Line 3 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 1% and 2% were assigned in Figures 13 and 14, respectively.

16 Max Pipe Length (ft) D=6 in D=5 in D=4 in D=3 in D=2 in 4 2 1/8 3/16 1/4 5/16 3/8 7/16 1/2 Orifice Diameter (in) 4 35 Total Flowrate (gpm) D=6 in D=5 in D=4 in D=3 in D=2 in 5 1/8 3/16 1/4 5/16 3/8 7/16 1/2 Orifice Diameter (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 = 1%)

17 25 2 Max Pipe Length (ft) 15 1 D=6 in D=5 in D=4 in D=3 in D=2 in 5 1/8 3/16 1/4 5/16 3/8 7/16 1/2 5 Orifice Diameter (in) 45 4 Total Flowrate (gpm) D=6 in D=5 in D=4 in D=3 in D=2 in 1/8 3/16 1/4 5/16 3/8 7/16 1/2 Orifice Diameter (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 = 2%) 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.

18 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. Design Parameter Table 1 Results of the Parametric Analyses with Observations Range of Variation Perforation size 1/8- to 1/2 in diameter Pipe size Perforation spacing Entrance pressure 2- to 6 inch in diameter Graphical Results Figure 15 Figure 16 Observations/Comments 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 8- to 54 ft within the range of analyses. Larger pipes allow for longer injection distances. Maximum injection distance varies between 1- to 71 ft within the range of analyses. 1- to 1 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 1-ft spacing between perforations. 1 to 1 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.

19 Maximum Length (ft) Entrance Pressure = 5 ft W.C. Pipe ID = 3 inches Perforation: 1 hole every 5 ft Total Flowrate (gpm) 1/8 3/16 1/4 5/16 3/8 7/16 1/2 Hole Size (inch) Max Pipe Length Flowrate Figure 15 - Effect of Perforation Sizes ( q/q =1%) Maximum Length (ft) Entrance Pressure = 5 ft W.C. Perforation: 1 hole every 5 ft Hole size = ¼ inch Pipe ID (inch) Total Flowrate (gpm) Max Pipe Length Flowrate Figure 16 - Effect of Pipe Sizes ( q/q =1%)

20 Maximum Pipe Length (ft) Total Flowrate (gpm) Entrance Pressure = 5 ft W.C. 1 Hole size = ¼ inch 5 Pipe ID = 3 inch Spacing between Holes (ft) dq/q=1% dq/q=2% Spacing between Holes (ft) dq/q=1% dq/q=2% Maximum Pipe Length (ft) Total Flowrate (gpm) Figure 17 - Effect of Perforation Spacing 15 Hole size = ¼ inch 1 Spacing = 5 ft 5 Pipe ID = 3 inch Entrance Pressure (ft W.C.) dq/q=1% dq/q=2% Entrance Pressure (ft W.C.) dq/q=1% dq/q=2% Figure 18 - Effect of Entrance Pressure

21 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 (24). Bioreactor landfill stability: key considerations, in MSW Management, September/October,