10 1 1 854 196 bioreactor landfill status and future

Waste Manage Res 2002: 20: 172–186 Printed in UK – all rights reserved Copyright © ISWA 2002 Waste Management & Research ISSN 0734–242X The bioreactor landfill: Its status and future The bioreactor landfill provides control and process optimisation, primarily through the addition of leachate or other liquid amendments Sufficient experience now exists to define recommended design and operating practices However, technical challenges and research needs remain related to sustainability, liquid addition, leachate hydrodynamics, leachate quality, the addition of air, and cost analysis Debra R Reinhart College of Engineering and Computer Science, University of Central Florida PO Box 162993, Orlando, FL 32816-2993, USA Philip T McCreanor School of Engineering, Mercer University, 1400 Coleman Ave Macon, GA 31207, USA Timothy Townsend Assistant Professor, Department of Environmental Engineering and Science, PO Box 116450, University of Florida, Gainesville, Florida 32611, USA Keywords – Landfill, bioreactor, leachate, recirculation, sustainability, wmr 341–2 Corresponding author: Debra R Reinhart, College of Engineering and Computer Science, University of Central Florida PO Box 162993, Orlando, FL 32816-2993, USA Received 29 September 1999, accepted in revised form 21 February 2002 Introduction Today integrated management of municipal solid waste results in recycling, composting, incineration, or landfilling of waste A landfill is an engineered land method of solid waste disposal in a manner that protects the environment Within the landfill biological, chemical, and physical processes occur that promote the degradation of wastes and result in the production of contaminated leachate and gas Thus, the landfill design and construction must include elements that permit control of landfill leachate and gas The inclusion of environmental barriers such as landfill liners and caps frequently excludes moisture that is essential to waste biodegradation Consequently, waste is contained or entombed in the modern landfill and remains practically intact for long periods of time, possibly in excess of the life of the barriers However, waste stabilisation can be enhanced and accelerated so as to occur within the life 172 of the barriers if the landfill is designed and operated as a bioreactor The bioreactor landfill provides a similar approach and treatment as is utilised in organic solid waste digestion The bioreactor landfill provides control and process optimization, primarily through the addition of leachate or other liquid amendments, if necessary Beyond that, bioreactor landfill operation may involve the addition of biosolids and other amendments, temperature control, and nutrient supplementation The bioreactor landfill attempts to control, monitor, and optimise the waste stabilization process rather than contain the wastes as prescribed by most regulations The bioreactor landfill has been defined by a Solid Waste Association of North America working group as (Pacey et al 1999): “…a sanitary landfill operated for the purpose of Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 The bioreactor landfill: Its status and future transforming and stabilizing the readily and moderately decomposable organic waste constituents within five to ten years following closure by purposeful control to enhance microbiological processes The bioreactor landfill significantly increases the extent of waste decomposition, conversion rates and process effectiveness over what would otherwise occur within the landfill.” There are four reasons generally cited as justification for bioreactor technology: (1) to increase potential for waste to energy conversion, (2) to store and/or treat leachate, (3) to recover air space, and (4) to ensure sustainability This fourth justification for the bioreactor, sustainability, has the greatest potential for economic benefit due to reduced costs associated with avoided long-term monitoring and maintenance and delayed siting of a new landfill A sustainable landfill would meet the following criteria; contents of the landfill are managed so that outputs are released to the environment in a controlled and acceptable way, residues left should not pose unacceptable environmental risk, the need for post-closure care is not passed on to the next generation, and the future use of groundwater and other resources are not compromised (IWMSLWG, 1999) This paper discusses the current status of the bioreactor landfill as it relates to design and operating concepts The bioreactor landfill has developed over the past three decades from a laboratory concept to its present status as a viable waste management tool The complete history of its development is beyond the scope of this paper, but may be found elsewhere (Reinhart & Townsend 1998) Current technology implementation status The benefits of landfill bioreactor operation were well proven in the laboratory during the early 1970’s (Pohland 1975 and Pohland 1980), with pilot and full-scale demonstration occurring in the 1980’s (Natale & Anderson 1985 & Pacey et al 1987) By 1988, over 200 US landfills were practicing leachate recirculation, although with little engineering input to design and operation A survey of US states completed in 1993 found that full-scale leachate recirculation was occurring in twelve states A review of the literature at that time identified less than twenty fullscale leachate-recirculating landfills located in the US, Germany, United Kingdom, and Sweden (Reinhart & Townsend 1998) However, the Solid Waste Association of North America (SWANA) conducted a US survey in 1997 that identified over 130 leachate-recirculating landfills (Gou & Guzzone 1997) The number of recent literature references has also increased dramatically In a 1998 article, a large solid waste engineering consulting firm reported that over 25% of their clients have experimented with leachate recirculation but many chose to discontinue this process (Wintheiser 1998) These historical facts suggest that attempts to optimise landfill degradation processes are usually restricted to leachate recirculation In addition, it appears that the percentage of bioreactor landfills is still small, perhaps 5–10% of landfills, although the number of landfills recirculating leachate is increasing Reluctance to employ bioreactor technology can be attributed to several factors including a perception that the technology is not well demonstrated, technical impediments, unclear cost implications, and regulatory constraints In the US, landfill regulations under Subtitle D of the Resource Conservation and Recovery Act, permit leachate recirculation at lined landfills, but restrict it to the return of liquids that originate in the landfill A recent rule interpretation expands moisture input to uncontaminated water, although liquid wastes are still excluded The US Environmental Protection Agency has expressed certain concerns associated with bioreactor landfills that include the long-term fate of metals, the lack of data that demonstrate the reduction of environmental risk and liability, and increased operational requirements during the active phase of landfilling (Fuerst 1999) Technical issues that must be addressed include landfill gas capture, leachate treatment and storage, landfill space and capacity reuse, greenhouse gas abatement, bioreactor design, solid waste density considerations, settlement, waste pretreatment, cover, and management of amendments In the 1997 SWANA survey (Gou & Guzzone 1997) only six US states allowed bioreactor landfills, although most states approved of leachate recirculation However, several states have clearly embraced the technology, for example, the New York Code of Regulations (360-2.9) states the following: “…active landfill management techniques to encourage rapid waste mass stabilisation and alternate energy resource production and enhanced landfill gas emission collection systems are encouraged and should be addressed in the landfill’s engineering report and in the operations and maintenance manual.” In addition Florida, California, Delaware, and Iowa have Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 173 D R Reinhart, P T McCreanor, T Townsend Table Description of Recent Full-Scale Bioreactor Landfill Tests Location Size Start Up Date Leachate Recirculation Technique Bioreactor Cost Comments Kootenai Co., Idaho (Miller & Emge 1996) 2.83 1993 (landfill operation) 1995 (leachate recirculation) Surface spray (summer only) trenches 24.4 m spacing Wells $1 035 000 amortized + operating costs = $449 600 yr–1 First lined landfill in Idaho Bluestem SWA, Linn Co Iowa (Hall 1998) 0.20 7700 tons waste divided into subcells 1998 Trenches 4.6 m spacing 10 670 l d–1 $959 000 (cell construction) Experimenting with bag opening, biosolids addition Milwaukee (Viste 1997) 61 m x 12.2 m 1999 trenches NA No compaction, shredded, biosolids added Keele Valley LF Toronto, Canada (Mosher et al 1997) Pilot 1990 Vertical wells 1.2 wells ha–1 ~ 190 - 400 lpm NA Well water added to adjust moisture content not leachate Eau Claire, WI Mile Creek SL (Magnuson 1998) 720 tpd landfill, (Phase I at 180 tpd) 1998 Trenches 7.6 m spacing 73 lpd m–2 NA Tire chips acceptable in trenches, gas production increased by 25% in wells near recirculation Yolo County, CA (Yolo Co 1998) Two 930 m2 cells 4080 kg MSW each 12 m deep 1995 14 infiltration trenches at surface $563 000 (cell construction) Enhanced gas production, settlement Shredded tires successful in LFG collection Lower Spen Valley LF West Yorkshire, UK (Blakey et al 1997) Two cells ~ 860 tons waste ~ 890 m2 each ~ 5.5 m deep 1991 Trenches NA Biosolids and wastewater addition Low temperature prevented maximum gas production Crow Wing MSW LF, Minn (Doran 1999) 5.18 1997 11 trenches, 15m spacing, 310 l d–1 m–1 $290 000 $72 500 savings yr–1 (1997–8) No off-site hauling of leachate in 1998, Recirculation operated mos yr–1 Worcester Co LF, MD (Kilmer & Tustin 1999) 6.9 ha, 24 m deep 1990 Vertical wells surrounded $50 000 by 7.6 m of gravel blanket Net benefit $3.2 million per 6.9 cell (after mining) Avg 65% of leachate recirculated.Upper layers did not degrade extensively Lyndhurst LF, Melbourne, Australia (Yuen et at 1995) 1.3 1995 Recharge wells and trenches NA Complete instrumentation for monitoring leachate, temperature, gas, climate, moisture distribution, head on liner VAM Waste Treatment,Wijster, the Netherlands (Oonk & Woelders 1998) 7062 m2 1997 Trenches 10 m horizontal, m vertical spacing (plus surface infiltration at 5–m spacing) NA Gas collection in wood chips at the top liner Filled with mechanically separated organic fractions < 45 mm diameter 174 Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 The bioreactor landfill: Its status and future Table Continued Location Size Start Up Date Leachate Recirculation Technique Bioreactor Cost Comments Baker Rd LF, Columbia County, Georgia (Hudgins & Marks 1998) 3.24 ha, m 1996 20 vertical wells $25 – 30 000 capital, Operating & Maintenance costs not reported Air injected into LCS system, Settlement increased by 4.5%, biodegradation rate increased by > 50% Live Oak LF, Atlanta, Georgia, USA (Johnson & Baker 1999) 1.01 ha, m 1997 27 vertical wells, 1.5 – 4.6 m deep, 18 air injection wells NA Air and liquid injection into same well improved fluid distribution Shin-Kamata LF, Fukuoka City, Japan (Fukuoka City Environmental Bureau, 1999) NA 1975 Horizontal Pipes NA Semiaerobic process using large leachate collection pipes that draw in air Trail Road LF, Ontario, Canada achieved (Warith et al 1999) 270 m x 500 m 1992 Infiltration lagoons NA Lagoons were moved around ~ 50% of field capacity all invested significantly in bioreactor landfill research The European Union (EU) Council Directive on Landfilling of Waste has identified the need to optimise final waste disposal methods and ensure uniform high standards of landfill operation and regulation throughout the European Union (European Commission, 1999) These standards require a strategy that limits the quantity of biodegradable wastes entering the landfill and consequently, the practicality of a bioreactor Much of this paper therefore addresses recent research occurring outside of Europe where landfill bioreactor technology is more applicable However, researchers in the EU have suggested that sustainability can be accomplished either through extensive waste preprocessing or a concept called the flushing bioreactor The flushing bioreactor achieves waste stabilization and contaminant removal within a generation through the addition of large volumes of water (IWMLWG, 1999) Costs for the flushing bioreactor, however, may be two to four times higher than the conventional landfill (Karnik & Perry 1997) While much of the landfill bioreactor research has historically occurred in Europe and the US, there is a clear trend of the application of this technology outside of these regions, including Australia, Canada, South America, South Africa, Japan, and New Zealand Because of the simplicity of implementation, it is expected that landfill bioreactors will have a prominent role in waste management throughout the world, provided the essential elements for proper operation are present These elements include a leachate collection system, liner, gas collection system, and controlled moisture introduction Table Objectives of field scale bioreactor operations • Demonstrate accelerated landfill gas generation and biological stabilisation while maximising landfill gas capture • Monitor biological conditions to optimise bioreactor process • Landfill life extension through accelerated waste degradation • Inform regulatory agencies • Better understand movement of moisture • Evaluate performance of shredded tires in LFG collection • Achieve a 50% waste diversion goal • Reduce usable gas extraction period to three years • Reduce leachate management costs • Shorten time period required to put the site to a beneficial end use • Evaluate performance of leachate recirculation techniques • Investigate the use of bioreactor to treat mechanically separated organic residue • Investigate the use of air injection to increase waste biodegradation rate Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 175 D R Reinhart, P T McCreanor, T Townsend Landfill bioreactor technology Table Lessons learned from field-scale bioreactor operations Table provides a summary of recent bioreactor field-scale operating characteristics In many cases, the operation was initiated to gather information required to design and construct the bioreactor at full scale A summary of project objectives is provided in Table Table identifies lessons learned from these operations Provided below is a more detailed discussion of the state-of-the-art of landfill bioreactor technology • Sealed system can result in plastic surface liners ballooning and tearing • Rapid surface settlement can result in ponding • Short circuiting occurs during leachate recirculation, preventing achievement of field capacity for much of the landfill • Continuous pumping of leachate at two to three times the generation rate is necessary to avoid head on the liner build up • A more permeable intermediate cover may be more efficient in rapidly reaching field capacity than leachate recirculation • Low permeability intermediate cover and heterogeneity of the waste leads to side seeps • Accelerated gas production may lead to odors if not accommodated by aggressive LFG collection • Leachate infiltration and collection piping are vulnerable to irregular settling and clogging • Waste is less permeable than anticipated • Increased condensate production led to short circuiting of moisture into landfill gas collection pipes • Storage must be provided to manage leachate during wet weather periods • Conversely, leachate may not be sufficient in volume to completely wet waste, particularly for aerobic bioreactors • Increased internal pore pressure due to high moisture content may lead to reduced factor of safety against slope stability and must be considered during the design process • Channeling leads to immediate leachate production, however long term recirculation increases uniform wetting and declining leachate generation as the waste moisture content approaches field capacity Design and operation for leachate recirculation Previous experience and research indicates that the control of waste moisture content is the single most important factor in enhancing waste decomposition in landfills (Pohland 1975) Leachate recirculation has been found to be the most practical approach to moisture content control therefore, full-scale bioenhancement efforts tend to focus on this technique The type of leachate recirculation system utilised and the method of operation are selected after appropriate consideration of project goals related to moisture distribution, minimising environmental impact, and regulatory compliance Table Advantages and disadvantages of aerobic bioreactor Potential advantages Rapid waste stabilisation Aerobic waste decomposition has been cited as a more rapid means of waste stabilisation Thus, aerobic bioreactors can recover volume and become stable more rapidly than anaerobic systems Improved gas emissions Methane is not a byproduct of aerobic decomposition, and thus methane emissions are reduced in aerobic bioreactors Other chemicals in landfill gas associated with anaerobic conditions, many of which cause odours are also reduced Degradation of recalcitrant chemicals Some chemicals that not degrade or transform under anaerobic conditions may so under aerobic conditions Thus aerobic bioreactors may offer greater treated for some organic wastes and ammonia Removal of moisture The addition of air acts to strip moisture from the landfill This provides advantages for drying out wet landfills and minimising leachate production Potential disadvantages Risk of fire and explosive Gas mixtures Cost Unknown gas emissions 176 The addition of air to landfills has long been associated with the potential for landfill fires In uncontrolled, aerobic respiration can increase waste temperatures to levels where waste combustion may be a concern Uncontrolled air addition could also result in creating gas mixtures with explosive characteristics Proper control of the process remains a major issue Additional costs will be incurred supplying power required to add air to the landfill Unlike mechanical blowers used to extract landfill gas, blowers for aerobic landfills will have to handle an extra volume of gas not involved with the decomposition reaction and will require greater pressures to force air through the waste The ability of adding air to deep, well-compacted landfills is an unknown Emissions of methane and other compounds produced under anaerobic conditions (e.g volatile acids, hydrogen sulfide) may decrease, but other hazardous and noxious chemicals may still be released Nitrous oxide, a more potent greenhouse gas than methane may be emitted Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 The bioreactor landfill: Its status and future Fig Liquid addition requirements to meet 50% field capacity as a function of incoming waste mass and moisture content (wet basis) Considerations for leachate recirculation systems To optimise bioreactor operations, the operator must be able to control waste moisture levels Waste moisture is controlled by the rate at which leachate is introduced, which is a function of waste hydraulic conductivity, and the efficiency of the leachate introduction technique Leachate introduction techniques include surface application and injection through vertical wells or horizontal trenches In order to maximise the area impacted, leachate recirculation operations should be cycled from one area to another, pumping at relatively intense rate for a short period of time, then moving to another area Empirical data provide some guidance for rates of moisture input of approximately to m3 day–1 linear m–1 of trench and to 10 m3 day–1 well–1, however field experimentation is required to determine site specific capacity The quantity of liquid supplied is a function of waste characteristics such as moisture content and field capacity In some cases, the infiltration of moisture resulting from rainfall is insufficient to meet the desired waste moisture content for optimal decomposition Therefore, the addition of supplemental liquids (i.e., leachate from other areas, water, wastewater, or biosolids) may be required Sufficient liquid supply must be assured to support project goals For example, the goal of moisture distribution might be to bring all waste to field capacity Fig illustrates the liquid volume requirements for a landfill to reach a waste field capacity of 50% (by weight) as a function of incoming waste mass in tonnes d–1 and initial waste moisture content This figure assumes wetting of 100% of the waste and a density of g cm–3 However, wetting is frequently incomplete due to preferential flow paths and recirculation device inefficiencies, therefore less liquid than indicated will actually be required The most efficient approach to reach field capacity is to increase moisture content through wetting of the waste at the working face and then uniformly reach field capacity through liquid surface application or injection The addition of supplemental liquids increases the base flow of leachate from the landfill This additional flow must be considered during design, especially following rain events when large amounts of leachate may be generated Sufficient leachate storage must be provided to ensure that peak leachate generation events can be accommodated While a properly designed and operated landfill will minimize extreme fluctuations of leachate generation with rainfall events, in wet climates leachate generation will at times exceed the amount needed for recirculation Other factors such as construction, maintenance, regulations, etc may also dictate that leachate not be recirculated from time to time Therefore, it is very important to have contingency plans in place for off-site leachate management for times when leachate generation exceeds on-site storage capacity Leachate recirculation should be controlled to minimise outbreaks and to optimise the biological processes Grading the cover to direct leachate movement away from side slopes, providing adequate distance between slopes and leachate injection, eliminating perforations in recirculation piping near slopes, and avoiding cover that has hydraulic conductivity significantly different from the waste can control seeps In addition, it may be desirable to reduce initial compaction of waste in order to facilitate leachate movement through the waste A routine monitoring program designed to detect early evidence of outbreaks should accompany the operation of any leachate recirculation system Alternate design procedures such as early capping of side slopes and installation of subsurface drains may also be considered to minimise problems with side seepage The depth of leachate on the liner is a primary regulation in the US to protect groundwater and is a major concern for regulators approving bioreactor permits Control of head on the liner requires the ability to maintain a properly designed leachate collection system, monitor head on the liner, store or dispose of leachate outside of the landfill, and remove leachate at rates two to three times the rate of normal leachate generation Several techniques are used to measure head on the liner including sump measurements, piezometers, bubbler tubes, or pressure transducers Measuring the head with currently available technology provides local information regarding Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 177 D R Reinhart, P T McCreanor, T Townsend leakage potential, however for a more realistic evaluation a more complete measurement may be required The construction, operation, and monitoring of leachate recirculation systems will impact daily landfill operations If a leachate recirculation system is to be utilised, it should be viewed as an integral part of landfill operations Installation of recirculation systems must be coordinated with waste placement, and should be considered during planning of the fill sequence An operating plan for leachate recirculation at a landfill should be developed with all of the above considerations in mind, including the selection of the type of device used to introduce liquid and its placement in the landfill While these devices have been used in the field, little data have been collected from full-scale leachate recirculation operations Until more operational data become available, system design (i.e placement of recirculation devices) will be based on equations derived using traditional groundwater movement laws or mathematical simulation of leachate routing in a waste mass Examples of such equations and modeling results for two of the most commonly used recirculation methods are presented below, followed by a design example Equations for both isotropic (Equation 2) and anisotropic (Equation 3) conditions were developed x_ = tan 2π kx y q ( E ≤ 2h (1) where: E = spacing between trenches, L h = delivery head of leachate, L Townsend (1995) developed equations based on uniform flow theory for saturated conditions to estimate the area influenced by a horizontal infiltration trench 178 (2a) Ymax = q 2πk (2b) xmax = q 2k (2c) xwell = q 4k (2d) ( ) x = q tan–1 x ky 2πky y √ kx (3a) Ymax = q 2π √ kx ky (3b) xmax = q 2ky (3c) xmax = q 4ky (3d) Horizontal trenches Horizontal trenches are constructed by excavating the surface of landfilled compacted solid waste, placing a perforated pipe in the trench, and backfilling with a permeable material The trench is then covered, preferably with additional compacted solid waste Horizontal trenches have the advantage of good moisture distribution within the landfill, but can be difficult to construct for some landfill configurations Al-Yousfi (1992) developed an equation that can be used to estimate the required horizontal distance between trenches Equation was based on the pipe perforation spacing, delivery head, and waste hydraulic conductivity ) where: Ymax = maximum upward impact of line source, L q = leachate injection rate, L2 T–1 k = average waste hydraulic conductivity, LT–1 kx = horizontal waste hydraulic conductivity, LT–1 ky = vertical waste hydraulic conductivity, LT–1 x = horizontal distance from the line source, L y = vertical distance from the line source, L xmax = maximum impact of line source, L xwell = impact of line source at y=0, L Equations and represent the outer limit of the flow path of liquid discharged from a horizontal line source, or Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 The bioreactor landfill: Its status and future Fig Saturated flow zone surrounding a horizontal injection well flow system under steady conditions Fig Schematic depicting the calculation method for the lateral and upward movement from a recirculation trench, leachate applied continuously at m3 m–1 day–1 for one week, waste permeability = 1x10–3 cm s–1 trench, in a saturated flow field, see Fig However, the landfill is typically unsaturated Hydraulic conductivity of a media is a function of moisture content and is at its maximum in saturated conditions and declines with decreasing saturation Therefore, Equations and may overestimate the moisture movement due to the variation in hydraulic conductivities encountered in the unsaturated environment and heterogeneities in the waste mass McCreanor (1998) used the United States Geological Survey’s Saturated-Unsaturated Flow and Transport model (SUTRA) to simulate the behaviour of horizontal leachate recirculation trenches and vertical leachate recirculation wells The modeling effort evaluated the effect of recirculation rate, waste hydraulic conductivity, anisotropies, heterogeneities, and daily cover materials on leachate routing The effects of recirculation rate and waste hydraulic conductivity are discussed in this paper Unlike the Townsend approach, this model does not assume saturated conditions and allows the user to more closely simulate actual landfill conditions Fig provides a schematic diagram of the simulated leachate recirculation trench Fig.s and present the Fig Maximum lateral movement versus hydraulic conductivity for intermittent leachate injection (8 hr on/16 hr off) via a horizontal trench Application rates represent the total amount of leachate input per day Fig Maximum upward movement versus hydraulic conductivity for intermittent leachate injection (8 hr on/16 hr off) via a horizontal trench Application rates represent the total amount of leachate input per day Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 179 D R Reinhart, P T McCreanor, T Townsend effect of leachate application rate and the hydraulic conductivity of the waste mass on the lateral and vertical movement of leachate from the horizontal trench The lateral movement is one-half of the area wetted by the trench A conservative design would space the trenches at twice the lateral movement indicated in Fig The indicated lateral and vertical leachate movement should be considered the minimum distance required between the landfill boundaries and the trenches Vertical wells Vertical wells for leachate recirculation are constructed in the same manner as vertical wells for gas extraction, generally requiring drilling into the waste mass and installation of piping In some cases, wells are constructed as waste is placed, by installing pipe sections at each waste lift Vertical wells are advantageous for landfills where waste is already in place or where landfill configuration or operation does not permit horizontal trenches The moisture distribution from vertical wells is limited, therefore a large number of wells may be required Al-Yousfi (1992) proposed that the radius of influence of a well, defined as the maximum distance of leachate movement from the well, could be estimated based on a mass balance of the leachate Inflow from the well side area must be equal to the outflow from the zone of influence Combining this concept with Darcy’s Law resulted in Equation R = rKw Kr (4) Fig Calculation of lateral and upward movement from a recirculation well, leachate applied intermittently (8 hr on/16 hr off) at 10 m3 m–1 day –1 for weeks, waste permeability = 1x10–3 cm s–1 well for injection of leachate Fig provides a schematic diagram of the simulated leachate recirculation well The relationship among the lateral movement of leachate, leachate application rate and waste hydraulic conductivity for recirculation with a vertical well are presented in Fig as described by McCreanor (1998) Vertical wells should be spaced at approximately twice the indicated lateral movement and distanced at least the indicated lateral movement from the landfill boundaries The modeling effort also found that the upward movement from the uppermost leachate injection point was less than m in all cases where: R = radius of influence zone, L r = radius of recharge well, L Kw = hydraulic conductivity of media surrounding well, LT–1 Kr = hydraulic conductivity of refuse, LT–1 Al-Yousfi estimated that the ratio of Kw/Kr ranges from 30 to 50 Considering a well diameter of 60 cm, the influence radius would range from 18 to 30 m It was then concluded that wells should be spaced no more than 60 m apart to ensure efficient wetting of the waste mass A shortcoming of Equation is that it ignores the effect of flowrate on the radius of influence McCreanor (1998) also modeled the use of a vertical 180 Fig Lateral movement versus flow rate for intermittent leachate application (8 hr on/16 hr off) via a vertical well Application rates represent the total amount of leachate input per day Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 The bioreactor landfill: Its status and future Design example To illustrate the use of equations developed by McCreanor (1998), horizontal trench and vertical well leachate recirculation system requirements will be calculated for a 3-ha landfill which plans to recirculate leachate at a rate of 20 m3 ha–1 day–1 (a total of 60 m3 day–1) The systems will be designed using Fig.s through The landfill has an aerial footprint of 300 m by 100 m and the waste is estimated to have a hydraulic conductivity of 10–4 cm s–1 If horizontal trenches are used, they will be run parallel to the 100 m side of the landfill and have a perforated section of 60 m, providing a 20 m buffer on each end to limit the chance of side seeps Leachate will be pumped to one trench each day for hours, for a leachate application rate of m3 m–1 day–1 The lowest leachate application rate presented in Fig is m3 m–1 day–1 By extrapolation, we can estimate the lateral movement to be 3.2 m for an application rate of m3 m–1 day–1 and a hydraulic conductivity of 10–4 cm s–1 Similarly, the upward movement can be estimated to be 1.5 m using Fig Therefore the trenches should be spaced 6.4 m apart, a total of 41 trenches, and distanced at least 1.5 m from the landfill grade If 1.2 m diameter vertical wells are used, leachate will be applied to four wells per day for hours The average daily application rate per well is then 15 m3, resulting in a lateral movement of m, from Fig Each well would then impact an area of 50.3 m2 A 10 m buffer distance around the landfill perimeter will be used to prevent sideseeps The area receiving leachate recirculation is then 280 m by 80 m (22,400 m2) and requires 445 wells Increasing the loading to 20 m3 day–1 well–1 would increase the lateral movement to 4.75 m and decrease the number of wells required to 316 In either case, the large number of wells required may adversely impact landfill operations These design examples would produce a conservative device spacing The waste will most likely be anisotropic which can be expected to result in lateral movements greater than those indicated in Fig.s and The exact effect of heterogeneities within the waste mass is difficult to predict Modeling of heterogeneous waste masses by McCreanor (1998) indicated that leachate will move around low hydraulic conductivity materials but did not suggest a significant increase in the lateral movement Design and operation for waste stabilisation and gas production The desired result of increasing the waste moisture con- tent is to promote rapid waste stabilisation This rapid stabilisation results in the production of large quantities of landfill gas (LFG) An integral part of the design and operation of a bioreactor landfill is the design and operation of an effective LFG collection system for both regulatory and environmental reasons Since gas is often considered the major source of odors at landfill sites, the accelerated production of gas may also result in increased odours A major component of the design of a bioreactor is the incorporation of an aggressive gas collection system Thus, well-operated bioreactors that effectively control fugitive emissions from the landfill surface, including the working face, could actually reduce odors relative to conventional sites where gas control is less efficient Techniques employed as part of a bioreactor landfill are often similar to systems installed at conventional landfills In bioreactors, however, the gas is produced at a much greater rate earlier in the life of the landfill Measures may need to be implemented to capture this large volume of gas earlier than might occur in conventional landfills Such measures include collection from the leachate collection system, from horizontal wells installed within the waste, and from surface collection systems Some of the same strategies used to control leachate migration, such as early capping of side slopes, fit well into the strategy of landfill gas collection System assurance for bioreactor landfills Because successful operation of a bioreactor requires the movement of large quantities of moisture and results in rapid degradation of waste, the effective performance of leachate collection and recirculation system components is critical Small perforations tend to clog and biological growth can impede drainage of trenches Consequently, critical components must be oversized and easily maintained through cleaning and/or component replacement Many sites operate pressurised drain fields, rather than relying on gravity drainage to maintain desired flow rates High-density polyethylene pipes are preferred due to their strength and durability The use of inexpensive recycled materials such as tire chips in drain fields is gaining in popularity Rapid settlement resulting from waste decomposition may also play a role in the integrity of the landfill system Leachate recirculation and gas collections devices must be designed in a manner to accommodate settlement over time, and routine monitoring and inspection of these systems must be provided Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 181 D R Reinhart, P T McCreanor, T Townsend Research and data needs Data are accumulating that support the advantages of the bioreactor landfill However, a significant number of technological challenges remain that can only be met through the continued implementation of the technology and meticulous data gathering Of great importance is the dissemination of data in a uniform manner so that this information can be interpreted and universally applied Waste input histories, leachate recirculation rates, leachate recirculation device description, leachate and gas quantity and quality, and waste characteristics are critical parameters that must be clearly described The discussion below provides some insight into the level of understanding we presently have regarding bioreactor landfills Sustainability Waste samples recovered at bioreactor landfills in Delaware (Germain et al 1997), Georgia (Johnson & Baker 1999), and Maryland (Kilmer & Tustin 1999) revealed high levels of waste degradation in wet areas over relatively short periods of time (one to three years), as measured by an increase in percentage of fines (50–75% as compared with 35–40% in dryer areas) Biological Methane Potential tests supported these observation at landfills in the UK (Blakey et al 1997) and Florida (Reinhart & Townsend 1998), where available gas potential remained at levels twice as high in dry areas as compared to wet areas within bioreactor landfills Substantial and accumulating evidence exists to support the enhanced degradation rates for organic biodegradable waste fractions in wet landfills However, the long-term fate of remaining waste components subject to continued chemical and biological processes within the landfill is largely unknown For example, under anaerobic conditions, sulfides and humic substances often bind heavy metals Over time, oxygen and water may enter the landfill creating conditions that may mobilise metals and flush remaining inorganic contaminants out of the landfill Evaluation and prediction of the fate of these components poses difficult challenges Can these wastes be eliminated or more permanently immobilised before the barrier degrades? Are there circumstances where the environment can assimilate these contaminants with little risk to human health or the environment? Alternatively, can the landfill be used as a reactor to extract resources and energy within a reasonable time span? The flushing bioreactor offers possible 182 solution to the early elimination of soluble inorganic contaminants, studies show that the rapid introduction of two to four liquid bed volumes reduces ammonia dramatically (IWMLWG, 1999), however application of this technology raises many technical and economical issues Impact of compaction The present philosophy of landfilling is to purchase increasingly heavy compaction equipment and pack the maximum amount of waste possible into each volume of landfill space This practice is justified by the high cost of landfill construction Since hydraulic conductivity is inversely related to specific weight, highly efficient use of airspace effectively reduces the ability to move moisture through the waste Conversely, since field capacity is also inversely related to waste density, increased compaction actually achieves this level of saturation with less moisture addition However, this effect may have little consequence if moisture cannot move through the waste Several bioreactors have been constructed in Iowa, Wisconsin, and the UK with little or no compaction (Hall 1998; Viste 1997; Blakey et al 1997) Compaction also contributes to anisotropic conditions within the landfill that magnify lateral movement of moisture In fact, leachate seeps assumed to be caused by leachate recirculation were observed 85 m away from recirculation wells at the Keele Valley Landfill in Toronto, Canada (Mosher et al 1997) It is expected that settlement will ultimately occur as a result of moisture addition, the weight of overlying layers, and waste degradation, therefore airspace lost to initial placement without compaction may be recovered with time The Keele Valley Landfill (Mosher et al 1997) reported settlement rates of 10–12 cm month–1 in wet areas as opposed to 5–7 cm month–1 in dry areas The two areas had similar characteristics (age, 36 m depth, and waste characteristics) Experimental cells constructed in Yolo County, CA found wet cell settlement rates to be more than three times higher than a parallel control cell over a 17-month period (Yolo Co 1998) Slightly lower settlement enhancement (~ 5%) was reported at aerobic cells in Columbia Co., Georgia, however this was a relatively shallow landfill (Hudgins & March 1998) The Trail Road Landfill in Ontario, Canada reported a 40% recovery of airspace following eight years of recirculation (Warith et al 1999) Although these latter sites practiced conventional compaction techniques for today, the impact of enhanced degradation on settlement is clear Additional Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 The bioreactor landfill: Its status and future settlement studies in sites with and without compaction are needed, as well as demonstration of successful airspace recovery prior to final closure Addition of nonindigenous liquids US Federal regulations prohibit the addition of bulk or noncontainerised liquid wastes to landfills, but permit recirculation of leachate and gas condensate and may permit the addition of water The introduction of biosolids and fresh water to landfills has been investigated by many researchers (Reinhart & Townsend 1998) with conflicting results relative to the impact on pH and enhanced gas production However, these studies suggest that the addition of inoculum and the increase in moisture content are effective in enhancing waste degradation rates, provided leachate buffering is ensured Field trials adding biosolids (Blakey et al 1997; Viste 1997; Hall 1998), contaminated well water (Mosher et al 1997), wastewater (Blakey et al 1997), and fresh water (Yolo Co 1998; Johnson & Baker 1999) have reported favorable observations relative to increased gas production and leachate quality Nonindigenous liquids are added at these sites (1) to supplement nutrients and moisture, (2) to dispose of liquid waste products, (3) to compensate for insufficient leachate volumes, and/or (4) to avoid concentration of inorganic contaminants in leachate Liquid addition above normal leachate generation will be essential to many bioreactor operations, particularly in arid regions and in aerobic landfills to compensate for evaporative loss Data must be compiled that will convincingly demonstrate that the risk of groundwater contamination due to the addition of nonindigenous liquids is offset by the enhanced rate of the degradation of wastes associated with bioreactor technology Leachate hydrodynamics Moisture content control is the most critical parameter for successful bioreactor operation Waste characteristics, most notably large pore volumes and heterogeneity, lead to rapid vertical flow of leachate along preferential flow paths and consequent incomplete use of available moisture storage Zeiss & Uguccioni (1997) conducted a laboratory evaluation of channeled flow effects on effective storage and field capacity, important input variables to the water balance calculation Lithium chloride tracer introduced at the Lower Spen Landfill in the UK, which has an average depth of m (Blakey et al 1997) was detected in the leachate collection system 40 minutes later Calculations from the test suggested that only 30% of the recirculated leachate actually flowed through the waste From water balance modeling and actual leachate measurement, it was estimated that less than 50% of field capacity at the Trail Road Landfill in Ontario, Canada was utilised after seven years of recirculation (Warith et al 1999) At the Yolo County test site, it was observed that leachate volumes dramatically declined with time as repeated recirculation ensured that greater volumes of waste were allowed to reach field capacity (Yolo Co 1998) To improve the ability to maximise storage and waste degradation, landfills must be constructed to optimise uniformity, minimise preferential flow paths, and maintain competent movement of leachate through the landfill McCreanor (1998) modeled the effects of hydraulic conductivity, heterogeneity, and anisotropy on leachate hydrodynamics using an unsaturated flow mathematical model Guyonnet & Come (1997) also incorporated leachate hydrodynamics in an optimisation tool for landfill bioreactor operation Mathematical models of this nature are challenged by the difficulty in linking Darcian flow and flow channeling Another challenge is the difficulty in validating model results Enhanced time domain reflectometry, frequency domain reflectometry, neutron density, and electrical conductivity sensors have been used to monitor in situ waste moisture content with varying success Without reliable data of this nature, our understanding of leachate movement and the effect of landfill operations such as recirculation, compaction, waste selection, mixing, and bag opening will be limited Geotechnical properties Increased moisture content leading to waste saturation can lead to positive internal pore pressure and reduced angle of friction The impact of waste decomposition on shear strength is also of critical concern Many studies report finding mud-like conditions at the bottom of wet, deep landfills Over aggressive leachate recirculation (compounded by the use of impermeable cover soil) has been cited as a contributing factor to the catastrophic slope failure of a landfill in Columbia, SA (Maier 1998) To minimise such risk, research is needed to better define geotechnical properties of stabilised waste and to provide appropriate input to accurately analyse bioreactor landfill stability Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 183 D R Reinhart, P T McCreanor, T Townsend Leachate quality It has been well documented in laboratory and pilot-scale research studies that moisture input improves gross organic leachate quality (measured as chemical oxygen demand (COD) or total organic carbon (TOC)) as compared to dry control cells Leachate organic quality in field-scale studies also appears to rapidly improve after an initial peak Researchers at a landfill in Worcester County, Maryland calculated a 12-month half-life for leachate COD (Kilmer & Tustin 1999) Leachate COD at the Rosedale Landfill in Aukland, New Zealand peaked at 43,000 mg l–1 after 12 months then decreased to 30% of the peak within 18 mos Field studies also find low concentrations of hazardous organics in bioreactor leachate Bioreactors would tend to optimise removal of hazardous organic contaminants by (1) stripping volatile organics by increased gas production, (2) optimising conditions for biodegradation, and (3) stimulating immobilisation of contaminants through humification These mechanisms have been confirmed in the laboratory by Sanin & Barlaz (1998) and Pohland et al (1992) The consequences of the reduced leachate organic strength with respect to risk of groundwater contamination must be demonstrated to regulators The long-term fate of heavy metals and other inorganic compounds, however, may be different as discussed above Aerobic bioreactors The traditional method of landfill bioreactor operation involves enhancing waste stabilisation by anaerobic microorganisms This paper primarily addresses the traditional mode of operation Recently, however, increased interest has been focused on the introduction of oxygen to the landfill to create an aerobic bioreactor Air is typically injected into the landfill with the same devices as used to extract gas or inject leachate, vertical and horizontal wells Aerobic bioreactors have been promoted as a method to accelerate waste stabilisation and to reduce methane content in landfill gas (Johnson & Baker 1999; Hudgins & March 1998) Concerns that remain, which limits use of this technology, include landfill fires and added power costs Potential advantages and disadvantages of aerobic bioreactors are presented in Table An area of further exploration is the use of hybrid, or combined anaerobic and aerobic systems If issues 184 regarding the control of such systems can be resolved, hybrid systems could offer a number of benefits For example, short-term air addition could be used to increase the landfill temperature, stimulating anaerobic conditions and promoting waste stabilisation Another option would be to add air following anaerobic degradation to remove excess moisture from the landfill and fully compost the waste The cycling of aerobic and anaerobic conditions also offers possibilities of treatment of some recalcitrant chemicals and chemical byproducts, in the same manner as modern wastewater treatment (e.g nitrification and denitrification of ammonia) Cost analysis Bioreactor cost impacts are difficult to predict, although several researchers have attempted to model life cycle costs with varied results (Karnick & Perry 1997; Vroon et al 1998; Anex 1996) Benefits that may have economic consequences include enhanced and more rapid gas production, recovered landfill space, reduced environmental impact, and reduced post-closure care Offsetting these cost benefits would be the capital and operating costs of implementing bioreactor technology Full-scale operating bioreactors report annual cost savings varying from $75,000 to $500,000 However, Gambelin & Cochrane (1999) developed a life cycle analysis that estimated a cost differential of $1.40 to $2.15 tonne–1 in favor of dry landfills Anex (1996) developed an optimal control method that uses cost factors to specify bioreactor landfill design and operating parameters His method showed that under certain circumstances, landfills bioreactors are economically favorable Predictive models are difficult to apply generally because of the large number of site specific variables, including potential for site reuse, waste composition, leachate treatment costs, climate, opportunity for beneficial use of landfill gas, and local regulatory agency flexibility Development of cost analysis procedures that allow site specific comparison of landfill design and operating criteria and account for and quantify differences in environmental impact as well as capital and operating/maintenance expenses are important for the widespread application of this technology However, if recovered landfill space can be efficiently used and post-closure care issues resolved, bioreactor technology should prove to be cost effective in many Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 The bioreactor landfill: Its status and future situations To obtain regulatory agency acceptance of reduced long-term care, analysis is required to substantiate laboratory and pilot-scale observations that bioreactor landfills will minimise long-term environmental risk and liability In addition, closure requirements may need to be modified to permit time to recover landfill space made available as a result of enhanced waste degradation and to permit controlled infiltration of moisture at the top of the landfill Conclusion The application of landfill bioreactor technology is logical extension of liquid treatment processes Technical challenges remain that must be addressed by the continued funding of large-scale research projects Results must be reported in a manner that permits universal application of the data In the not too distant future, this approach to waste management will be the norm and the sustainable landfill a reality References Al-Yousfi, A B (1992) Modeling of Leachate and Gas Production and Composition at Sanitary Landfills PhD Dissertation, University of Pittsburgh, Pittsburgh, PA Anex, R.P (1996) Optimal Waste Decomposition- Landfill as Treatment Process Journal of Environmental Engineering, ASCE, 122(11) 964–974 Blakey, N.C., K Bradshaw, P Reynolds, & K Knox (1997) Bio-Reactor Landfill-A Field Trial of Accelerated Waste Stabilization Proceedings from Sardinia 97, Sixth International Landfill Symposium, Volume I, S Margherita di Pula, Cagliari, Italy, 13-17 October, 1997, 375–386 Clarke, W P (2000) Cost-Benefit Analysis of Introducing Technology to Rapidly Degrade Municipal Solid Waste Waste Management & Research, 18(12), 510–524 European Commission (1999) Handbook for the Implementation of EC Environmental Legislation, 99/31/EC Fuerst, S (1999) EPA – Bioreactor Considerations Presented at the TNRCC Environmental Trade Fair, Austin, TX, May Gambelin, D J., D Cochrane & B Clister (1998) Life Cycle Analysis of a Bioreactor landfill in California Proceedings from SWANA’s 3rd Annual Landfill Symposium, Palm Beach Gardens, FL, June 22–24, 1998, 7–19 Germain, A., W Gregory Vogt, & Daniel A Fluman (1997) Waste Characterization Study for Excavated Test Cells Central Solid Waste Management Center Proceedings from SWANA’s 2nd Annual Landfill Symposium, Sacramento, California, August 4–6, 1997, 45–114 Gou, B & B Guzzone, (1997) State Survey on Leachate Recirculation and landfill Bioreactors Solid Waste Association of North America, Silver Springs, Maryland, USA Griffin, G.C Logholm, S.T., Kilmer, K., & G.L Dix (1997) Practical Aspects of Leachate Recirculation in Landfills Proceedings from SWANA’s 2nd Annual Landfill Symposium, Sacramento, California, August 4–6, 1997, 105–114 Guyonnet, D., & B Come (1997) Bioreactor Landfill Optimization Proceedings from Sardinia 97, Sixth International Landfill Symposium, Volume I, S Margherita di Pula, Cagliari, Italy, 13–17 October, 1997, 351–358 Hall, T J (1998) Implementing Low-Cost Bioreactor Cell Technology in Iowa Proceedings from SWANA’s 3rd Annual Landfill Symposium, Palm beach Gardens, FL, June 22–24, 1–5 Hudgins, M & J March (1998) In-Situ Municipal Solid Waste Composting Using an Aerobic Landfill System American Technologies, Inc., Aiken, SC, USA Institute of Wastes Management Sustainable Landfill Working Group (1999) The Role and Operation of the Flushing Bioreactor Johnson, W H and J Baker (1999) Operational Characteristics and Enhanced Bioreduction of Municipal Waste Landfill Mass by a Controlled Aerobic Process Proceedings from SWANA’s 4th Annual Landfill Symposium, Denver, CO., June 28–30 Karnik, M & C Perry (1997) Cost Implications of Operating Landfills as Flushing Bioreactors Proceedings from Sardinia 97, Sixth International Landfill Symposium, Volume I, S Margherita di Pula, Cagliari, Italy, 13–17 October, 1997, Volume I, 419–426 Kilmer, K S & J Tustin (1999) Rapid Landfill Stabilization and Improvements in Leachate Quality by Leachate Recirculation Proceedings from SWANA’s 4th Annual Landfill Symposium, Denver, CO., June 28–30 Magnuson, A (1998) Leachate Recirculation MSW Management, pp 24–31, March/April Maier, T B (1998) Analysis Procedures for Design of Leachate Recirculation Systems Proceedings from SWANA’s 3rd Annual Landfill Symposium, Palm Beach Gardens, FL, June 22–24, 49–58 McCreanor, P T (1998) Landfill Leachate Recirculation Systems: Mathematical Modeling and Validation PhD Dissertation, University of Central Florida, Orlando, FL Miller, D.E & Stephen M Emge (1996) Unique Design and Operations Approaches Enhance Leachate Recirculation System Performance at the Kootenai County (Idaho) Landfill Proceedings from SWANA’s 1st Annual Landfill Symposium, Wilmington, Delaware, November 4–6, 1996, 157–175 Mosher, F., Edward A McBean, Anthony J Crutcher, & Neil MacDonald (1997) Leachate Recirculation to Achieve Rapid Stabilization of Landfills Theory and Practice Proceedings from SWANA’s 2nd Annual Landfill Symposium, Sacramento, California, August 4–6, 1997, 121–134 Natale, B.R and W.C Anderson (1985) Evaluation of a Landfill with Leachate Recycle Draft report to US EPA Office of Solid Waste Novella, P.H., G.A Ekama, & G.E Blight (1997) Effects of Liquid Replacement Strategies on Waste Stabilization at Pilot Scale Proceedings from Sardinia 97, Sixth International Landfill Symposium, Volume I, S Margherita di Pula, Cagliari, Italy, 13–17 October, 1997, Volume I, 387–396 Pacey, J P., Ramin Yazdani, D Reinhart, R Morck, D Augenstein (1999) The Bioreactor Landfill: An Innovation in Solid Waste Management SWANA, Silver Springs, Maryland Pohland, F.G (1975) Sanitary Landfill Stabilization with Leachate Recycle and Residual Treatment U.S Environmental Protection Agency, Cincinnati, Ohio, EPA-600/2-75-043 Pohland, F.G (1980) Leachate recycle as landfill management option Journal of Environmental Engineering 106(EE6):1057–1069 Pohland, F G., W H Cross, J P Gould & D R Reinhart (1992) The Behavior and Assimilation of Organic Priority Pollutants Codisposed with Municipal Refuse US EPA, EPA Coop Agreement Cr-812158 Reinhart, D R & T G Townsend (1998) Landfill Bioreactor Design & Operation Lewis Publishers, Boca Raton, FL Sanin F D & M A Barlaz (1998) Natural Attenuation of Hazardous Organics During Refuse Decomposition in a Municipal Landfill SWANA’s Wastecon 1998, 36th Annual International Solid Waste Exhibition, Charlotte, NC, October 26–29, 1998, 35–39 Townsend, T G (1995) Leachate Recycle at Solid Waste Landfills using Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 185 D R Reinhart, P T McCreanor, T Townsend Horizontal Injection PhD Dissertation, University of Florida, Gainesville, FL Viste, D.R (1997) Waste Processing and Biosolids Incorporation to Enhance Landfill Gas Proceedings from Sardinia 97, Sixth International Landfill Symposium, Volume I, S Margherita di Pula, Cagliari, Italy, 13–17 October, 1997, 369–374 Vroon, R., Hans Oonk, & W van marwijk (1998) A Lab-Scale Exploration of the Long-Term Behaviour of mechanically Separated Organic Residue in a Flushing Bioreactor Presented at the 3rd Swedish Landfill Research symposia, Lulea, Sweden, October 6–9 Warith, J A., P A Smoklin, & J G Caldwell (1999) Effect of Leachate Recirculation on the Enhancement of Biological Degradation of Solid Waste: Case Study Proceedings from SWANA’s 4th Annual Landfill Symposium, Denver, CO., June 28–30 186 Wintheiser, P (1998) Leachate Recirculation: A Review of Operating Experience at Municipal Solid Waste Landfills Proceedings from SWANA’s 3rd Annual Landfill Symposium, Palm Beach Gardens, FL, June 22–24, 59–62 Yolo County Division of Integrated Waste Management (1998) Trash to Cash – Controlled Landfill Bioreactor Project Public Technology, Inc Energy Program, Order No 98/97-317 Yuen, S T S., J R Styles & T A McMahon (1995) an Active landfill Management by leachate Recirculation: A Review and an Outline of a Full-Scale Project Proceedings Sardinia 95, 5th International Landfill Symposium, S Margheria de Pula, Cagliari, Italy, October 2–6, 1995, 403–418 Zeiss, C, & Mark Uguccioni (1997) Modified Flow Parameters for Leachate Generation Water Environment Research, 69(3), 276–285 Waste Management & Research Downloaded from wmr.sagepub.com at PENNSYLVANIA STATE UNIV on March 6, 2016 ... of Landfills Theory and Practice Proceedings from SWANA’s 2nd Annual Landfill Symposium, Sacramento, California, August 4–6, 19 97, 12 1 13 4 Natale, B.R and W.C Anderson (19 85) Evaluation of a Landfill. .. Minn (Doran 19 99) 5 .18 19 97 11 trenches, 15 m spacing, 310 l d 1 m 1 $290 000 $72 500 savings yr 1 (19 97–8) No off-site hauling of leachate in 19 98, Recirculation operated mos yr 1 Worcester Co LF,... 300 m by 10 0 m and the waste is estimated to have a hydraulic conductivity of 10 4 cm s 1 If horizontal trenches are used, they will be run parallel to the 10 0 m side of the landfill and have