Design of Pressurized Liquid Distribution System for Landfill Liquids Addition and Augmentation

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 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 trenchand 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 MSW Waste 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., 1⁄2 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. 0.00 0.50 1.00 1.50 2.00 2.50 0 10 20 30 40 50 60 70 80 90 100 Distance from End of Pipe (ft) U ni t D is ch ar ge R at e (g pm ) 0 50 100 150 200 250 Fl ow ra te in P ip e (g pm ) Unit Discharge Rate Flow rate in Pipe Total flowrate = 200 gpm Entrance Pressure = 0.5 ft W.C. Pipe ID = 3 inches Perforation: 4 holes every 6 inches Hole size: 1⁄2 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 1⁄4 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. 0.00 0.50 1.00 1.50 2.00 2.50 0 10 20 30 40 50 60 70 80 90 100 Distance from End of Pipe (ft) U ni t D is ch ar ge R at e (g pm ) 0 50 100 150 200 250 Fl ow ra te in P ip e (g pm ) Unit Discharge Rate Flow rate in Pipe Total flowrate = 200 gpm Entrance Pressure = 9 ft W.C. Pipe ID = 3 inches Perforation: 1 holes every 1 ft Hole size: 1⁄4 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 / 1 2 79 . 11 2 P d gP BA q = = (1) Where q = flow rate per perforation (gpm) B = orifice coefficient, assumed as 0.60 A = area of orifice (in) g = gravitational acceleration (32.2 ft/s) 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: Z g V P h + + = 2 2 (2) Where h = total head (feet) P = pressure head (feet) V = velocity (ft/sec) g = gravitational acceleration (32.2 ft/s) 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 . 1 85 . 1 100 002082 . 0 D Q C L h 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