The Bioreactor Landfill: Its Status and Future

The bioreactor landfill: Its status and future

The bioreactor landfill provides control and process Debra R. Reinhart optimisation, primarily through the addition of leachate College of Engineering and Computer Science, University of Central Florida. PO Box 162993, Orlando, FL 32816-2993, USA or other liquid amendments. Sufficient experience now exists to define recommended design and operating prac- Philip T. McCreanor tices. However, technical challenges and research needs School of Engineering, Mercer University, 1400 Coleman Ave. remain related to sustainability, liquid addition, leachate Macon, GA 31207, USA hydrodynamics, leachate quality, the addition of air, and Timothy Townsend cost analysis. 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 of the barriers if the landfill is designed and operated as a Today integrated management of municipal solid waste bioreactor. results in recycling, composting, incineration, or landfilling The bioreactor landfill provides a similar approach and of waste. A landfill is an engineered land method of solid treatment as is utilised in organic solid waste digestion. waste disposal in a manner that protects the environment. The bioreactor landfill provides control and process Within the landfill biological, chemical, and physical optimization, primarily through the addition of leachate processes occur that promote the degradation of wastes or other liquid amendments, if necessary. Beyond that, and result in the production of contaminated leachate and bioreactor landfill operation may involve the addition of gas. Thus, the landfill design and construction must biosolids and other amendments, temperature control, include elements that permit control of landfill leachate and nutrient supplementation. The bioreactor landfill and gas. The inclusion of environmental barriers such as attempts to control, monitor, and optimise the waste landfill liners and caps frequently excludes moisture that is stabilization process rather than contain the wastes as essential to waste biodegradation. Consequently, waste is prescribed by most regulations. contained or entombed in the modern landfill and remains The bioreactor landfill has been defined by a Solid practically intact for long periods of time, possibly in excess Waste Association of North America working group as of the life of the barriers. However, waste stabilisation can (Pacey et al. 1999): be enhanced and accelerated so as to occur within the life “…a sanitary landfill operated for the purpose of

172 Waste Management & Research The bioreactor landfill: Its status and future transforming and stabilizing the readily and moderately 1997 that identified over 130 leachate-recirculating land- decomposable organic waste constituents within five to fills (Gou & Guzzone 1997). The number of recent litera- ten years following closure by purposeful control to ture references has also increased dramatically. In a 1998 enhance microbiological processes. The bioreactor landfill article, a large solid waste engineering consulting firm significantly increases the extent of waste decomposition, reported that over 25% of their clients have experimented conversion rates and process effectiveness over what with leachate recirculation but many chose to discontinue would otherwise occur within the landfill.” this process (Wintheiser 1998). There are four reasons generally cited as justification for These historical facts suggest that attempts to optimise bioreactor technology: (1) to increase potential for waste landfill degradation processes are usually restricted to to energy conversion, (2) to store and/or treat leachate, leachate recirculation. In addition, it appears that the (3) to recover air space, and (4) to ensure sustainability. percentage of bioreactor landfills is still small, perhaps This fourth justification for the bioreactor, sustainabili- 5–10% of landfills, although the number of landfills ty, has the greatest potential for economic benefit due to recirculating leachate is increasing. Reluctance to employ reduced costs associated with avoided long-term monitor- bioreactor technology can be attributed to several factors ing and maintenance and delayed siting of a new landfill. including a perception that the technology is not well A sustainable landfill would meet the following criteria; demonstrated, technical impediments, unclear cost impli- contents of the landfill are managed so that outputs are cations, and regulatory constraints. released to the environment in a controlled and accept- In the US, landfill regulations under Subtitle D of the able way, residues left should not pose unacceptable Resource Conservation and Recovery Act, permit environmental risk, the need for post-closure care is not leachate recirculation at lined landfills, but restrict it to passed on to the next generation, and the future use of the return of liquids that originate in the landfill. A recent groundwater and other resources are not compromised rule interpretation expands moisture input to uncontami- (IWMSLWG, 1999). This paper discusses the current nated water, although liquid wastes are still excluded. The status of the bioreactor landfill as it relates to design and US Environmental Protection Agency has expressed cer- operating concepts. The bioreactor landfill has developed tain concerns associated with bioreactor landfills that over the past three decades from a laboratory concept to include the long-term fate of metals, the lack of data that its present status as a viable waste management tool. The demonstrate the reduction of environmental risk and lia- complete history of its development is beyond the scope of bility, and increased operational requirements during the this paper, but may be found elsewhere (Reinhart & active phase of landfilling (Fuerst 1999). Technical issues Townsend 1998). that must be addressed include landfill gas capture, leachate treatment and storage, landfill space and capacity reuse, greenhouse gas abatement, bioreactor design, Current technology implementation status solid waste density considerations, settlement, waste pre- The benefits of landfill bioreactor operation were well treatment, cover, and management of amendments. proven in the laboratory during the early 1970’s (Pohland In the 1997 SWANA survey (Gou & Guzzone 1997) 1975 and Pohland 1980), with pilot and full-scale demon- only six US states allowed bioreactor landfills, although stration occurring in the 1980’s (Natale & Anderson 1985 most states approved of leachate recirculation. However, & Pacey et al. 1987). By 1988, over 200 US landfills were several states have clearly embraced the technology, for practicing leachate recirculation, although with little engi- example, the New York Code of Regulations (360-2.9) neering input to design and operation. A survey of US states the following: states completed in 1993 found that full-scale leachate “…active landfill management techniques to encourage recirculation was occurring in twelve states. A review of rapid waste mass stabilisation and alternate energy the literature at that time identified less than twenty full- resource production and enhanced landfill gas emission scale leachate-recirculating landfills located in the US, collection systems are encouraged and should be Germany, United Kingdom, and Sweden (Reinhart & addressed in the landfill’s engineering report and in the Townsend 1998). However, the Solid Waste Association of operations and maintenance manual.” North America (SWANA) conducted a US survey in In addition Florida, California, Delaware, and Iowa have

Waste Management & Research 173 D. R. Reinhart, P. T. McCreanor, T. Townsend

Table 1. Description of Recent Full-Scale Bioreactor Landfill Tests

Location Size Start Up Leachate Bioreactor Comments Date Recirculation Cost Technique

Kootenai Co., 2.83 ha 1993 (landfill Surface spray $1 035 000 First lined landfill in Idaho. Idaho operation) (summer only) amortized + (Miller & Emge 1995 (leachate trenches 24.4 m operating costs = 1996) recirculation) spacing Wells $449 600 yr–1

Bluestem SWA, 0.20 ha 1998 Trenches 4.6 m $959 000 Experimenting with bag Linn Co. Iowa 7700 tons waste spacing (cell construction) opening, biosolids addition. (Hall 1998) divided into 2 10 670 l d–1 subcells.

Milwaukee 61 m x 12.2 m 1999 trenches NA No compaction, shredded, (Viste 1997) biosolids added.

Keele Valley LF Pilot 1990 Vertical wells - NA Well water added to adjust Toronto, Canada 1.2 wells ha–1 moisture content not leachate. (Mosher et al. ~ 190 - 400 lpm 1997)

Eau Claire, WI 720 tpd landfill, 1998 Trenches 7.6 m spacing NA Tire chips acceptable in trenches, 7 Mile Creek SL (Phase I at 180 tpd) 73 lpd m–2 gas production increased (Magnuson 1998) by 25% in wells near recirculation.

Yolo County, CA Two 930 m2 cells 1995 14 infiltration $563 000 Enhanced gas production, (Yolo Co. 1998) 4080 kg MSW each trenches at surface. (cell construction) settlement. Shredded tires 12 m deep. successful in LFG collection.

Lower Spen Valley Two cells ~ 860 1991 Trenches NA Biosolids and wastewater LF West Yorkshire, tons waste ~ addition. Low temperature UK (Blakey et al. 890 m2 each prevented maximum gas 1997) ~ 5.5 m deep production.

Crow Wing MSW 5.18 ha 1997 11 trenches, $290 000 No off-site hauling LF, Minn 15m spacing, $72 500 of leachate in 1998, (Doran 1999) 310 l d–1 m–1 savings yr–1 Recirculation operated (1997–8) 3 mos yr–1.

Worcester Co. 6.9 ha, 24 m deep 1990 Vertical wells surrounded $50 000 Net benefit $3.2 million LF, MD by 7.6 m of gravel blanket. per 6.9 ha cell (after mining) (Kilmer & Tustin Avg. 65% of leachate 1999) recirculated.Upper layers did not degrade extensively.

Lyndhurst LF, 1.3 ha 1995 Recharge wells NA Complete instrumentation Melbourne, and trenches for monitoring leachate, Australia temperature, gas, climate, (Yuen et at. 1995) moisture distribution, head on liner.

VAM Waste 7062 m2 1997 Trenches 10 m horizontal, NA Gas collection in wood chips Treatment,Wijster, 3 m vertical spacing at the top liner. Filled with the Netherlands (plus surface infiltration mechanically separated organic (Oonk & Woelders at 5–m spacing) fractions < 45 mm diameter. 1998)

174 Waste Management & Research The bioreactor landfill: Its status and future

Table 1. Continued

Location Size Start Up Leachate Bioreactor Comments Date Recirculation Cost Technique

Baker Rd LF, 3.24 ha, 3 m 1996 20 vertical wells $25 – 30 000 Air injected into LCS system, Columbia County, capital, Operating Settlement increased by 4.5%, Georgia & Maintenance biodegradation rate increased (Hudgins & Marks costs not reported. by > 50%. 1998)

Live Oak LF, 1.01 ha, 9 m 1997 27 vertical wells, NA Air and liquid injection into Atlanta, Georgia, 1.5 – 4.6 m deep, same well improved fluid USA 18 air injection wells. distribution. (Johnson & Baker 1999)

Shin-Kamata LF, NA 1975 Horizontal Pipes NA Semiaerobic process using Fukuoka City, large leachate collection pipes Japan that draw in air. (Fukuoka City Environmental Bureau, 1999)

Trail Road LF, 270 m x 500 m 1992 Infiltration lagoons NA Lagoons were moved around Ontario, Canada ~ 50% of field capacity. .achieved. (Warith et al. 1999)

all invested significantly in bioreactor landfill research. regions, including Australia, Canada, South America, The European Union (EU) Council Directive on South Africa, Japan, and New Zealand. Because of the Landfilling of Waste has identified the need to optimise simplicity of implementation, it is expected that landfill final waste disposal methods and ensure uniform high bioreactors will have a prominent role in waste manage- standards of landfill operation and regulation throughout ment throughout the world, provided the essential ele- the European Union (European Commission, 1999). ments for proper operation are present. These elements These standards require a strategy that limits the quantity include a leachate collection system, liner, gas collection of biodegradable wastes entering the landfill and conse- system, and controlled moisture introduction. quently, the practicality of a bioreactor. Much of this paper therefore addresses recent research occurring outside of Table 2. Objectives of field scale bioreactor operations Europe where landfill bioreactor technology is more applicable. However, researchers in the EU have suggest- • Demonstrate accelerated landfill gas generation and biological stabilisation while maximising landfill gas capture ed that sustainability can be accomplished either through • Monitor biological conditions to optimise bioreactor process extensive waste preprocessing or a concept called the • Landfill life extension through accelerated waste degradation flushing bioreactor. The flushing bioreactor achieves waste • Inform regulatory agencies • Better understand movement of moisture stabilization and contaminant removal within a generation • Evaluate performance of shredded tires in LFG collection through the addition of large volumes of water (IWML- • Achieve a 50% waste diversion goal WG, 1999). Costs for the flushing bioreactor, however, • Reduce usable gas extraction period to three years may be two to four times higher than the conventional • Reduce leachate management costs • Shorten time period required to put the site to a beneficial end use landfill (Karnik & Perry 1997). • Evaluate performance of leachate recirculation techniques While much of the landfill bioreactor research has his- • Investigate the use of bioreactor to treat mechanically separated torically occurred in Europe and the US, there is a clear organic residue • Investigate the use of air injection to increase waste biodegradation rate trend of the application of this technology outside of these

Waste Management & Research 175 D. R. Reinhart, P. T. McCreanor, T. Townsend

Landfill bioreactor technology Table 3. Lessons learned from field-scale bioreactor operations Table 1 provides a summary of recent bioreactor field-scale • Sealed system can result in plastic surface liners ballooning and tearing operating characteristics. In many cases, the operation was • Rapid surface settlement can result in ponding • Short circuiting occurs during leachate recirculation, preventing initiated to gather information required to design and con- achievement of field capacity for much of the landfill struct the bioreactor at full scale. A summary of project • Continuous pumping of leachate at two to three times the generation objectives is provided in Table 2. Table 3 identifies lessons rate is necessary to avoid head on the liner build up • A more permeable intermediate cover may be more efficient in learned from these operations. Provided below is a more rapidly reaching field capacity than leachate recirculation detailed discussion of the state-of-the-art of landfill biore- • Low permeability intermediate cover and heterogeneity of the waste actor technology. leads to side seeps • Accelerated gas production may lead to odors if not accommodated by aggressive LFG collection Design and operation for leachate recirculation • Leachate infiltration and collection piping are vulnerable to irregular Previous experience and research indicates that the con- settling and clogging trol of waste moisture content is the single most important • Waste is less permeable than anticipated • Increased condensate production led to short circuiting of moisture factor in enhancing waste decomposition in landfills into landfill gas collection pipes (Pohland 1975). Leachate recirculation has been found to • Storage must be provided to manage leachate during wet weather be the most practical approach to moisture content periods • Conversely, leachate may not be sufficient in volume to completely control therefore, full-scale bioenhancement efforts tend wet waste, particularly for aerobic bioreactors to focus on this technique. The type of leachate recircula- • Increased internal pore pressure due to high moisture content may tion system utilised and the method of operation are lead to reduced factor of safety against slope stability and must be selected after appropriate consideration of project goals considered during the design process • Channeling leads to immediate leachate production, however long related to moisture distribution, minimising environmen- term recirculation increases uniform wetting and declining leachate tal impact, and regulatory compliance. generation as the waste moisture content approaches field capacity

Table 4. 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 Some chemicals that do not degrade or transform under anaerobic conditions may do so under aerobic conditions. recalcitrant chemicals 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 The addition of air to landfills has long been associated with the potential for landfill fires. In uncontrolled, aerobic Gas mixtures 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. Cost 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.

Unknown gas emissions 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.

176 Waste Management & Research The bioreactor landfill: Its status and future

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.

Fig. 1. Liquid addition requirements to meet 50% field capacity as a While a properly designed and operated landfill will mini- function of incoming waste mass and moisture content (wet basis) mize extreme fluctuations of leachate generation with rainfall events, in wet climates leachate generation will at Considerations for leachate recirculation systems times exceed the amount needed for recirculation. Other To optimise bioreactor operations, the operator must be factors such as construction, maintenance, regulations, able to control waste moisture levels. Waste moisture is etc. may also dictate that leachate not be recirculated from controlled by the rate at which leachate is introduced, time to time. Therefore, it is very important to have which is a function of waste hydraulic conductivity, and contingency plans in place for off-site leachate manage- the efficiency of the leachate introduction technique. ment for times when leachate generation exceeds on-site Leachate introduction techniques include surface applica- storage capacity. tion and injection through vertical wells or horizontal Leachate recirculation should be controlled to minimise trenches. In order to maximise the area impacted, leachate outbreaks and to optimise the biological processes. recirculation operations should be cycled from one area to Grading the cover to direct leachate movement away from another, pumping at relatively intense rate for a short peri- side slopes, providing adequate distance between slopes od of time, then moving to another area. Empirical data and leachate injection, eliminating perforations in recircu- provide some guidance for rates of moisture input of lation piping near slopes, and avoiding cover that has approximately 2 to 4 m3 day–1 linear m–1 of trench and 5 to hydraulic conductivity significantly different from the 10 m3 day–1 well–1, however field experimentation is waste can control seeps. In addition, it may be desirable to required to determine site specific capacity. reduce initial compaction of waste in order to facilitate The quantity of liquid supplied is a function of waste leachate movement through the waste. A routine moni- characteristics such as moisture content and field capaci- toring program designed to detect early evidence of out- ty. In some cases, the infiltration of moisture resulting from breaks should accompany the operation of any leachate rainfall is insufficient to meet the desired waste moisture recirculation system. Alternate design procedures such as content for optimal decomposition. Therefore, the addi- early capping of side slopes and installation of subsurface tion of supplemental liquids (i.e., leachate from other drains may also be considered to minimise problems with areas, water, wastewater, or biosolids) may be required. side seepage. Sufficient liquid supply must be assured to support project The depth of leachate on the liner is a primary regula- goals. For example, the goal of moisture distribution might tion in the US to protect groundwater and is a major be to bring all waste to field capacity. Fig. 1 illustrates the concern for regulators approving bioreactor permits. liquid volume requirements for a landfill to reach a waste Control of head on the liner requires the ability to field capacity of 50% (by weight) as a function of incom- maintain a properly designed leachate collection system, ing waste mass in tonnes d–1 and initial waste moisture monitor head on the liner, store or dispose of leachate content. This figure assumes wetting of 100% of the waste outside of the landfill, and remove leachate at rates two to and a density of 1 g cm–3. However, wetting is frequently three times the rate of normal leachate generation. Several incomplete due to preferential flow paths and recircula- techniques are used to measure head on the liner includ- tion device inefficiencies, therefore less liquid than indi- ing sump measurements, piezometers, bubbler tubes, or cated will actually be required. The most efficient pressure transducers. Measuring the head with currently approach to reach field capacity is to increase moisture available technology provides local information regarding

Waste Management & Research 177 D. R. Reinhart, P. T. McCreanor, T. Townsend leakage potential, however for a more realistic evaluation Equations for both isotropic (Equation 2) and anisotropic a more complete measurement may be required. (Equation 3) conditions were developed. The construction, operation, and monitoring of leachate recirculation systems will impact daily landfill x_ 2π k (2a) = tan x operations. If a leachate recirculation system is to be y ( q ) utilised, it should be viewed as an integral part of landfill operations. Installation of recirculation systems must be q coordinated with waste placement, and should be consid- Y = (2b) max 2πk ered during planning of the fill sequence. An operating plan for leachate recirculation at a landfill should be developed with all of the above considera- q x = tions in mind, including the selection of the type of device max 2k (2c) 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 q x = operations. Until more operational data become available, well 4k (2d) system design (i.e. placement of recirculation devices) will be based on equations derived using traditional groundwater movement laws or mathematical simulation of leachate q x k routing in a waste mass. Examples of such equations and x = tan–1 y (3a) 2πky (y √ k ) modeling results for two of the most commonly used x recirculation methods are presented below, followed by a design example. q Y = (3b) max 2π√k k Horizontal trenches x y Horizontal trenches are constructed by excavating the surface of landfilled compacted solid waste, placing a q perforated pipe in the trench, and backfilling with a xmax = (3c) 2ky permeable material. The trench is then covered, preferably with additional compacted solid waste. Horizontal trenches have the advantage of good moisture distribution q x = within the landfill, but can be difficult to construct for max 4k (3d) some landfill configurations. y Al-Yousfi (1992) developed an equation that can be used to estimate the required horizontal distance between where: trenches. Equation 1 was based on the pipe perforation Ymax = maximum upward impact of line source, L spacing, delivery head, and waste hydraulic conductivity. q = leachate injection rate, L2 T–1 k = average waste hydraulic conductivity, LT–1 ≤ –1 E 2h (1) kx = horizontal waste hydraulic conductivity, LT –1 ky = vertical waste hydraulic conductivity, LT where: x = horizontal distance from the line source, L E = spacing between trenches, L y = vertical distance from the line source, L

h = delivery head of leachate, L xmax = maximum impact of line source, L xwell = impact of line source at y=0, L Townsend (1995) developed equations based on uniform flow theory for saturated conditions to estimate the Equations 2 and 3 represent the outer limit of the flow area influenced by a horizontal infiltration trench. path of liquid discharged from a horizontal line source, or

178 Waste Management & Research The bioreactor landfill: Its status and future

Fig. 2. Saturated flow zone surrounding a horizontal injection well flow system under steady conditions trench, in a saturated flow field, see Fig. 2. However, the Fig. 3. Schematic depicting the calculation method for the lateral and upward movement from a recirculation trench, leachate applied landfill is typically unsaturated. Hydraulic conductivity of continuously at 8 m3 m–1 day–1 for one week, waste permeability a media is a function of moisture content and is at its max- = 1x10–3 cm s–1 imum in saturated conditions and declines with decreasing saturation. Therefore, Equations 2 and 3 may overestimate recirculation rate, waste hydraulic conductivity, the moisture movement due to the variation in hydraulic anisotropies, heterogeneities, and daily cover materials on conductivities encountered in the unsaturated environ- leachate routing. The effects of recirculation rate and ment and heterogeneities in the waste mass. waste hydraulic conductivity are discussed in this paper. McCreanor (1998) used the United States Geological Unlike the Townsend approach, this model does not Survey’s Saturated-Unsaturated Flow and Transport assume saturated conditions and allows the user to more model (SUTRA) to simulate the behaviour of horizontal closely simulate actual landfill conditions. leachate recirculation trenches and vertical leachate recir- Fig. 3 provides a schematic diagram of the simulated culation wells. The modeling effort evaluated the effect of leachate recirculation trench. Fig.s 4 and 5 present the

Fig. 4. Maximum lateral movement versus hydraulic conductivity for Fig. 5. Maximum upward movement versus hydraulic conductivity for intermittent leachate injection (8 hr on/16 hr off) via a horizontal intermittent leachate injection (8 hr on/16 hr off) via a horizontal trench. Application rates represent the total amount of leachate input trench. Application rates represent the total amount of leachate input per day per day

Waste Management & Research 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. 4. 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 Fig. 6. Calculation of lateral and upward movement from a recirculation well, leachate applied intermittently (8 hr on/16 hr off) at operation does not permit horizontal trenches. The mois- 10 m3 m–1 day –1 for 3 weeks, waste permeability = 1x10–3 cm s–1 ture distribution from vertical wells is limited, therefore a large number of wells may be required. well for injection of leachate. Fig. 6 provides a schematic Al-Yousfi (1992) proposed that the radius of influence diagram of the simulated leachate recirculation well. The of a well, defined as the maximum distance of leachate relationship among the lateral movement of leachate, movement from the well, could be estimated based on a leachate application rate and waste hydraulic conductivity mass balance of the leachate. Inflow from the well side for recirculation with a vertical well are presented in Fig. 7 area must be equal to the outflow from the zone of influ- as described by McCreanor (1998). Vertical wells should be ence. Combining this concept with Darcy’s Law resulted spaced at approximately twice the indicated lateral move- in Equation 4. ment and distanced at least the indicated lateral movement from the landfill boundaries. The modeling effort rK R= w (4) also found that the upward movement from the uppermost K r leachate injection point was less than 1 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 –1 Kr = hydraulic conductivity of refuse, LT

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 4 is that it ignores the effect of Fig. 7. Lateral movement versus flow rate for intermittent leachate flowrate on the radius of influence. application (8 hr on/16 hr off) via a vertical well. Application rates McCreanor (1998) also modeled the use of a vertical represent the total amount of leachate input per day

180 Waste Management & Research The bioreactor landfill: Its status and future

Design example tent is to promote rapid waste stabilisation. This rapid sta- To illustrate the use of equations developed by McCreanor bilisation results in the production of large quantities of (1998), horizontal trench and vertical well leachate recir- landfill gas (LFG). An integral part of the design and oper- culation system requirements will be calculated for a 3-ha ation of a bioreactor landfill is the design and operation of landfill which plans to recirculate leachate at a rate of 20 an effective LFG collection system for both regulatory and m3 ha–1 day–1 (a total of 60 m3 day–1). The systems will be environmental reasons. Since gas is often considered the designed using Fig.s 3 through 7. The landfill has an aeri- major source of odors at landfill sites, the accelerated pro- al footprint of 300 m by 100 m and the waste is estimated duction of gas may also result in increased odours. to have a hydraulic conductivity of 10–4 cm s–1. A major component of the design of a bioreactor is the If horizontal trenches are used, they will be run parallel incorporation of an aggressive gas collection system. Thus, to the 100 m side of the landfill and have a perforated well-operated bioreactors that effectively control fugitive section of 60 m, providing a 20 m buffer on each end to limit emissions from the landfill surface, including the working the chance of side seeps. Leachate will be pumped to one face, could actually reduce odors relative to conventional trench each day for 8 hours, for a leachate application rate sites where gas control is less efficient. Techniques of 1 m3 m–1 day–1. The lowest leachate application rate employed as part of a bioreactor landfill are often similar presented in Fig. 4 is 2 m3 m–1 day–1. By extrapolation, we to systems installed at conventional landfills. In can estimate the lateral movement to be 3.2 m for an appli- bioreactors, however, the gas is produced at a much cation rate of 1 m3 m–1 day–1 and a hydraulic conductivity of greater rate earlier in the life of the landfill. Measures may 10–4 cm s–1. Similarly, the upward movement can be esti- need to be implemented to capture this large volume of gas mated to be 1.5 m using Fig. 5. Therefore the trenches earlier than might occur in conventional landfills. Such should be spaced 6.4 m apart, a total of 41 trenches, and dis- measures include collection from the leachate collection tanced at least 1.5 m from the landfill grade. system, from horizontal wells installed within the waste, If 1.2 m diameter vertical wells are used, leachate will and from surface collection systems. Some of the same be applied to four wells per day for 8 hours. The average strategies used to control leachate migration, such as early daily application rate per well is then 15 m3, resulting in a capping of side slopes, fit well into the strategy of landfill lateral movement of 4 m, from Fig. 7. Each well would gas collection. then impact an area of 50.3 m2. A 10 m buffer distance around the landfill perimeter will be used to prevent side- System assurance for bioreactor landfills seeps. The area receiving leachate recirculation is then Because successful operation of a bioreactor requires the 280 m by 80 m (22,400 m2) and requires 445 wells. movement of large quantities of moisture and results in rapid Increasing the loading to 20 m3 day–1 well–1 would increase degradation of waste, the effective performance of leachate the lateral movement to 4.75 m and decrease the number collection and recirculation system components is critical. of wells required to 316. In either case, the large number Small perforations tend to clog and biological growth can of wells required may adversely impact landfill operations. impede drainage of trenches. Consequently, critical compo- These design examples would produce a conservative nents must be oversized and easily maintained through clean- device spacing. The waste will most likely be anisotropic ing and/or component replacement. Many sites operate which can be expected to result in lateral movements pressurised drain fields, rather than relying on gravity drainage greater than those indicated in Fig.s 4 and 7. The exact to maintain desired flow rates. High-density polyethylene effect of heterogeneities within the waste mass is difficult pipes are preferred due to their strength and durability. The to predict. Modeling of heterogeneous waste masses by use of inexpensive recycled materials such as tire chips in drain McCreanor (1998) indicated that leachate will move fields is gaining in popularity. around low hydraulic conductivity materials but did not Rapid settlement resulting from waste decomposition suggest a significant increase in the lateral movement. may also play a role in the integrity of the landfill system. Leachate recirculation and gas collections devices must be Design and operation for waste stabilisation and gas designed in a manner to accommodate settlement over production time, and routine monitoring and inspection of these sys- The desired result of increasing the waste moisture con- tems must be provided.

Waste Management & Research 181 D. R. Reinhart, P. T. McCreanor, T. Townsend

Research and data needs solution to the early elimination of soluble inorganic con- Data are accumulating that support the advantages of the taminants, studies show that the rapid introduction of two bioreactor landfill. However, a significant number of tech- to four liquid bed volumes reduces ammonia dramatically nological challenges remain that can only be met through (IWMLWG, 1999), however application of this technolo- the continued implementation of the technology and gy raises many technical and economical issues. meticulous data gathering. Of great importance is the dissemination of data in a uniform manner so that this Impact of compaction information can be interpreted and universally applied. The present philosophy of landfilling is to purchase Waste input histories, leachate recirculation rates, increasingly heavy compaction equipment and pack the leachate recirculation device description, leachate and gas maximum amount of waste possible into each volume of quantity and quality, and waste characteristics are critical landfill space. This practice is justified by the high cost of parameters that must be clearly described. The discussion landfill construction. Since hydraulic conductivity is below provides some insight into the level of understand- inversely related to specific weight, highly efficient use of ing we presently have regarding bioreactor landfills. airspace effectively reduces the ability to move moisture through the waste. Conversely, since field capacity is also Sustainability inversely related to waste density, increased compaction Waste samples recovered at bioreactor landfills in actually achieves this level of saturation with less moisture Delaware (Germain et al. 1997), Georgia (Johnson & addition. However, this effect may have little consequence Baker 1999), and Maryland (Kilmer & Tustin 1999) if moisture cannot move through the waste. Several biore- revealed high levels of waste degradation in wet areas over actors have been constructed in Iowa, Wisconsin, and the relatively short periods of time (one to three years), as UK with little or no compaction (Hall 1998; Viste 1997; measured by an increase in percentage of fines (50–75% as Blakey et al. 1997). Compaction also contributes to compared with 35–40% in dryer areas). Biological anisotropic conditions within the landfill that magnify lat- Methane Potential tests supported these observation at eral movement of moisture. In fact, leachate seeps landfills in the UK (Blakey et al. 1997) and Florida assumed to be caused by leachate recirculation were (Reinhart & Townsend 1998), where available gas poten- observed 85 m away from recirculation wells at the Keele tial remained at levels twice as high in dry areas as com- Valley Landfill in Toronto, Canada (Mosher et al. 1997). pared to wet areas within bioreactor landfills. Substantial It is expected that settlement will ultimately occur as a and accumulating evidence exists to support the enhanced result of moisture addition, the weight of overlying layers, degradation rates for organic biodegradable waste fractions and waste degradation, therefore airspace lost to initial in wet landfills. placement without compaction may be recovered with However, the long-term fate of remaining waste compo- time. The Keele Valley Landfill (Mosher et al. 1997) nents subject to continued chemical and biological reported settlement rates of 10–12 cm month–1 in wet processes within the landfill is largely unknown. For exam- areas as opposed to 5–7 cm month–1 in dry areas. The two ple, under anaerobic conditions, sulfides and humic sub- areas had similar characteristics (age, 36 m depth, and stances often bind heavy metals. Over time, oxygen and waste characteristics). Experimental cells constructed in water may enter the landfill creating conditions that may Yolo County, CA found wet cell settlement rates to be mobilise metals and flush remaining inorganic contami- more than three times higher than a parallel control cell nants out of the landfill. Evaluation and prediction of the over a 17-month period (Yolo Co 1998). Slightly lower set- fate of these components poses difficult challenges. Can tlement enhancement (~ 5%) was reported at aerobic these wastes be eliminated or more permanently cells in Columbia Co., Georgia, however this was a rela- immobilised before the barrier degrades? Are there tively shallow landfill (Hudgins & March 1998). The Trail circumstances where the environment can assimilate Road Landfill in Ontario, Canada reported a 40% recov- these contaminants with little risk to human health or the ery of airspace following eight years of recirculation environment? Alternatively, can the landfill be used as a (Warith et al. 1999). Although these latter sites practiced reactor to extract resources and energy within a reason- conventional compaction techniques for today, the impact able time span? The flushing bioreactor offers possible of enhanced degradation on settlement is clear. Additional

182 Waste Management & Research The bioreactor landfill: Its status and future settlement studies in sites with and without compaction detected in the leachate collection system 40 minutes are needed, as well as demonstration of successful airspace later. Calculations from the test suggested that only 30% recovery prior to final closure. of the recirculated leachate actually flowed through the waste. From water balance modeling and actual leachate Addition of nonindigenous liquids measurement, it was estimated that less than 50% of field US Federal regulations prohibit the addition of bulk or capacity at the Trail Road Landfill in Ontario, Canada noncontainerised liquid wastes to landfills, but permit was utilised after seven years of recirculation (Warith et recirculation of leachate and gas condensate and may per- al. 1999). At the Yolo County test site, it was observed mit the addition of water. The introduction of biosolids that leachate volumes dramatically declined with time as and fresh water to landfills has been investigated by many repeated recirculation ensured that greater volumes of researchers (Reinhart & Townsend 1998) with conflicting waste were allowed to reach field capacity (Yolo Co. results relative to the impact on pH and enhanced gas pro- 1998). To improve the ability to maximise storage and duction. However, these studies suggest that the addition waste degradation, landfills must be constructed to opti- of inoculum and the increase in moisture content are mise uniformity, minimise preferential flow paths, and effective in enhancing waste degradation rates, provided maintain competent movement of leachate through the leachate buffering is ensured. Field trials adding biosolids landfill. (Blakey et al. 1997; Viste 1997; Hall 1998), contaminated McCreanor (1998) modeled the effects of hydraulic well water (Mosher et al. 1997), wastewater (Blakey et al. conductivity, heterogeneity, and anisotropy on leachate 1997), and fresh water (Yolo Co 1998; Johnson & Baker hydrodynamics using an unsaturated flow mathematical 1999) have reported favorable observations relative to model. Guyonnet & Come (1997) also incorporated increased gas production and leachate quality. leachate hydrodynamics in an optimisation tool for landfill Nonindigenous liquids are added at these sites (1) to bioreactor operation. Mathematical models of this nature supplement nutrients and moisture, (2) to dispose of liquid are challenged by the difficulty in linking Darcian flow and waste products, (3) to compensate for insufficient leachate flow channeling. Another challenge is the difficulty in val- volumes, and/or (4) to avoid concentration of inorganic idating model results. Enhanced time domain reflectome- contaminants in leachate. Liquid addition above normal try, frequency domain reflectometry, neutron density, and leachate generation will be essential to many bioreactor electrical conductivity sensors have been used to monitor operations, particularly in arid regions and in aerobic in situ waste moisture content with varying success. landfills to compensate for evaporative loss. Data must be Without reliable data of this nature, our understanding of compiled that will convincingly demonstrate that the risk leachate movement and the effect of landfill operations of groundwater contamination due to the addition such as recirculation, compaction, waste selection, mixing, of nonindigenous liquids is offset by the enhanced rate and bag opening will be limited. of the degradation of wastes associated with bioreactor technology. Geotechnical properties Increased moisture content leading to waste saturation Leachate hydrodynamics can lead to positive internal pore pressure and reduced Moisture content control is the most critical parameter angle of friction. The impact of waste decomposition on for successful bioreactor operation. Waste characteristics, shear strength is also of critical concern. Many studies most notably large pore volumes and heterogeneity, lead report finding mud-like conditions at the bottom of wet, to rapid vertical flow of leachate along preferential flow deep landfills. Over aggressive leachate recirculation paths and consequent incomplete use of available mois- (compounded by the use of impermeable cover soil) has ture storage. Zeiss & Uguccioni (1997) conducted a labo- been cited as a contributing factor to the catastrophic ratory evaluation of channeled flow effects on effective slope failure of a landfill in Columbia, SA (Maier 1998). storage and field capacity, important input variables to To minimise such risk, research is needed to better define the water balance calculation. Lithium chloride tracer geotechnical properties of stabilised waste and to provide introduced at the Lower Spen Landfill in the UK, which appropriate input to accurately analyse bioreactor landfill has an average depth of 3 m (Blakey et al. 1997) was stability.

Waste Management & Research 183 D. R. Reinhart, P. T. McCreanor, T. Townsend

Leachate quality regarding the control of such systems can be resolved, It has been well documented in laboratory and pilot-scale hybrid systems could offer a number of benefits. For research studies that moisture input improves gross organ- example, short-term air addition could be used to ic leachate quality (measured as chemical oxygen demand increase the landfill temperature, stimulating anaerobic (COD) or total organic carbon (TOC)) as compared to conditions and promoting waste stabilisation. Another dry control cells. Leachate organic quality in field-scale option would be to add air following anaerobic studies also appears to rapidly improve after an initial degradation to remove excess moisture from the peak. Researchers at a landfill in Worcester County, landfill and fully compost the waste. The cycling of Maryland calculated a 12-month half-life for leachate aerobic and anaerobic conditions also offers possibilities COD (Kilmer & Tustin 1999). Leachate COD at the of treatment of some recalcitrant chemicals and Rosedale Landfill in Aukland, New Zealand peaked at chemical byproducts, in the same manner as modern 43,000 mg l–1 after 12 months then decreased to 30% of wastewater treatment (e.g. nitrification and denitrifica- the peak within 18 mos. tion of ammonia). Field studies also find low concentrations of hazardous organics in bioreactor leachate. Bioreactors Cost analysis would tend to optimise removal of hazardous organic Bioreactor cost impacts are difficult to predict, although contaminants by (1) stripping volatile organics by several researchers have attempted to model life cycle increased gas production, (2) optimising conditions for costs with varied results (Karnick & Perry 1997; Vroon biodegradation, and (3) stimulating immobilisation of et al. 1998; Anex 1996). Benefits that may have eco- contaminants through humification. These mechanisms nomic consequences include enhanced and more rapid have been confirmed in the laboratory by Sanin & gas production, recovered landfill space, reduced envi- Barlaz (1998) and Pohland et al. (1992). The conse- ronmental impact, and reduced post-closure care. quences of the reduced leachate organic strength with Offsetting these cost benefits would be the capital and respect to risk of groundwater contamination must be operating costs of implementing bioreactor technology. demonstrated to regulators. The long-term fate of heavy Full-scale operating bioreactors report annual cost sav- metals and other inorganic compounds, however, may be ings varying from $75,000 to $500,000. However, different as discussed above. Gambelin & Cochrane (1999) developed a life cycle analysis that estimated a cost differential of $1.40 to Aerobic bioreactors $2.15 tonne–1 in favor of dry landfills. Anex (1996) The traditional method of landfill bioreactor operation developed an optimal control method that uses cost fac- involves enhancing waste stabilisation by anaerobic tors to specify bioreactor landfill design and operating microorganisms. This paper primarily addresses the parameters. His method showed that under certain cir- traditional mode of operation. Recently, however, cumstances, landfills bioreactors are economically favor- increased interest has been focused on the introduction able. Predictive models are difficult to apply generally of oxygen to the landfill to create an aerobic bioreactor. because of the large number of site specific variables, Air is typically injected into the landfill with the including potential for site reuse, waste composition, same devices as used to extract gas or inject leachate, leachate treatment costs, climate, opportunity for bene- vertical and horizontal wells. Aerobic bioreactors have ficial use of landfill gas, and local regulatory agency been promoted as a method to accelerate waste stabilisa- flexibility. Development of cost analysis procedures that tion and to reduce methane content in landfill gas allow site specific comparison of landfill design and (Johnson & Baker 1999; Hudgins & March 1998). operating criteria and account for and quantify differ- Concerns that remain, which limits use of this technology, ences in environmental impact as well as capital and include landfill fires and added power costs. Potential operating/maintenance expenses are important for the advantages and disadvantages of aerobic bioreactors are widespread application of this technology. presented in Table 4. However, if recovered landfill space can be efficiently An area of further exploration is the use of hybrid, or used and post-closure care issues resolved, bioreactor combined anaerobic and aerobic systems. If issues technology should prove to be cost effective in many

184 Waste Management & Research The bioreactor landfill: Its status and future situations. To obtain regulatory agency acceptance of Conclusion reduced long-term care, analysis is required to substanti- The application of landfill bioreactor technology is logical ate laboratory and pilot-scale observations that extension of liquid treatment processes. Technical chal- bioreactor landfills will minimise long-term environmen- lenges remain that must be addressed by the continued tal risk and liability. In addition, closure requirements funding of large-scale research projects. Results must be may need to be modified to permit time to recover reported in a manner that permits universal application of landfill space made available as a result of enhanced the data. In the not too distant future, this approach to waste degradation and to permit controlled infiltration waste management will be the norm and the sustainable of moisture at the top of the landfill. landfill a reality.

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