10.1.1.854.196 bioreactor landfill status and future

Technical issues that must be addressed include landfill gas capture, leachate treatment and storage, landfill space and capaci-ty reuse, greenhouse gas abatement, bioreactor design, sol

Waste Management & Research

ISSN 0734–242X

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

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

The bioreactor landfill: Its status and future

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, sustain-ability, 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

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

prac-tices However, technical challenges and research needs

remain related to sustainability, liquid addition, leachate

hydrodynamics, leachate quality, the addition of air, and

cost analysis

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,

sustainabili-ty, has the greatest potential for economic benefit due to

reduced costs associated with avoided long-term

monitor-ing and maintenance and delayed sitmonitor-ing 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

accept-able way, residues left should not pose unacceptaccept-able

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

demon-stration 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

engi-neering 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

full-scale 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 land-fills (Gou & Guzzone 1997) The number of recent litera-ture 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 impli-cations, 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 uncontami-nated water, although liquid wastes are still excluded The

US Environmental Protection Agency has expressed cer-tain 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 lia-bility, 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

capaci-ty reuse, greenhouse gas abatement, bioreactor design, solid waste density considerations, settlement, waste pre-treatment, 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

Table 1 Description of Recent Full-Scale Bioreactor Landfill Tests

Technique Kootenai Co., 2.83 ha 1993 (landfill Surface spray $1 035 000 First lined landfill in Idaho.

(Miller & Emge 1995 (leachate trenches 24.4 m operating costs =

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.

Keele Valley LF Pilot 1990 Vertical wells - NA Well water added to adjust

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

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

Lower Spen Valley Two cells ~ 860 1991 Trenches NA Biosolids and wastewater

Crow Wing MSW 5.18 ha 1997 11 trenches, $290 000 No off-site hauling

(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

did not degrade extensively Lyndhurst LF, 1.3 ha 1995 Recharge wells NA Complete instrumentation

liner.

VAM Waste 7062 m 2 1997 Trenches 10 m horizontal, NA Gas collection in wood chips

the Netherlands (plus surface infiltration mechanically separated organic

1998)

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

conse-quently, 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

suggest-ed that sustainability can be accomplishsuggest-ed 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

(IWML-WG, 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

his-torically 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 manage-ment throughout the world, provided the essential ele-ments for proper operation are present These eleele-ments include a leachate collection system, liner, gas collection system, and controlled moisture introduction

Table 1 Continued

Technique Baker Rd LF, 3.24 ha, 3 m 1996 20 vertical wells $25 – 30 000 Air injected into LCS system,

1998)

Live Oak LF, 1.01 ha, 9 m 1997 27 vertical wells, NA Air and liquid injection into

(Johnson &

Baker 1999)

Shin-Kamata LF, NA 1975 Horizontal Pipes NA Semiaerobic process using

(Fukuoka City

Environmental

Bureau, 1999)

Trail Road LF, 270 m x 500 m 1992 Infiltration lagoons NA Lagoons were moved around

.achieved.

(Warith et al 1999)

Table 2 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

Landfill bioreactor technology

Table 1 provides a summary of recent bioreactor field-scale

operating characteristics In many cases, the operation was

initiated to gather information required to design and

con-struct the bioreactor at full scale A summary of project

objectives is provided in Table 2 Table 3 identifies lessons

learned from these operations Provided below is a more

detailed discussion of the state-of-the-art of landfill

biore-actor technology

Design and operation for leachate recirculation

Previous experience and research indicates that the

con-trol 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

recircula-tion system utilised and the method of operarecircula-tion are

selected after appropriate consideration of project goals

related to moisture distribution, minimising

environmen-tal impact, and regulatory compliance

Table 3 Lessons learned from field-scale bioreactor operations

• 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

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.

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

applica-tion and injecapplica-tion 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

peri-od of time, then moving to another area Empirical data

provide some guidance for rates of moisture input of

approximately 2 to 4 m3day–1linear m–1of trench and 5 to

required to determine site specific capacity

The quantity of liquid supplied is a function of waste

characteristics such as moisture content and field

capaci-ty In some cases, the infiltration of moisture resulting from

rainfall is insufficient to meet the desired waste moisture

content for optimal decomposition Therefore, the

addi-tion 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 1 illustrates the

liquid volume requirements for a landfill to reach a waste

field capacity of 50% (by weight) as a function of

content This figure assumes wetting of 100% of the waste

and a density of 1 g cm–3 However, wetting is frequently

incomplete due to preferential flow paths and

recircula-tion device inefficiencies, therefore less liquid than

indi-cated 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 sur-face 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 mini-mize 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 manage-ment 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 recircu-lation 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 moni-toring program designed to detect early evidence of out-breaks 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 regula-tion 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 includ-ing sump measurements, piezometers, bubbler tubes, or pressure transducers Measuring the head with currently available technology provides local information regarding

Fig 1 Liquid addition requirements to meet 50% field capacity as a

function of incoming waste mass and moisture content (wet basis)

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

consid-ered during planning of the fill sequence

An operating plan for leachate recirculation at a

land-fill should be developed with all of the above

considera-tions 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

groundwa-ter 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

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

trench-es 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 1 was based on the pipe perforation

spacing, delivery head, and waste hydraulic conductivity

where:

E = spacing between trenches, L

h = delivery head of leachate, L

Townsend (1995) developed equations based on

uni-form flow theory for saturated conditions to estimate the

area influenced by a horizontal infiltration trench

Equations for both isotropic (Equation 2) and anisotropic (Equation 3) conditions were developed

(2a)

(2b)

(2c)

(2d)

(3a)

(3b)

(3c)

(3d)

where:

xwell = impact of line source at y=0, L Equations 2 and 3 represent the outer limit of the flow path of liquid discharged from a horizontal line source, or

(2πkx

)

y q

2πk

2k

4k

2πk tan

–1(x

√ky)

2π√kxky

2ky

4ky

trench, in a saturated flow field, see Fig 2 However, the

landfill is typically unsaturated Hydraulic conductivity of

a media is a function of moisture content and is at its

max-imum in saturated conditions and declines with decreasing

saturation Therefore, Equations 2 and 3 may overestimate

the moisture movement due to the variation in hydraulic

conductivities encountered in the unsaturated

environ-ment 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

recir-culation 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 3 provides a schematic diagram of the simulated leachate recirculation trench Fig.s 4 and 5 present the

Fig 4 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 5 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

Fig 2 Saturated flow zone surrounding a horizontal injection well

flow system under steady conditions

Fig 3 Schematic depicting the calculation method for the lateral and upward movement from a recirculation trench, leachate applied continuously at 8 m 3 m –1 day –1 for one week, waste permeability

= 1x10 –3 cm s –1

effect of leachate application rate and the hydraulic

con-ductivity of the waste mass on the lateral and vertical

movement of leachate from the horizontal trench The

lat-eral 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

indi-cated 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,

gen-erally requiring drilling into the waste mass and

installa-tion 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

mois-ture 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

influ-ence Combining this concept with Darcy’s Law resulted

in Equation 4

(4)

where:

R = radius of influence zone, L

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

influ-ence 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

flowrate on the radius of influence

McCreanor (1998) also modeled the use of a vertical

well for injection of leachate Fig 6 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 7

as described by McCreanor (1998) Vertical wells should be spaced at approximately twice the indicated lateral ment and distanced at least the indicated lateral move-ment from the landfill boundaries The modeling effort also found that the upward movement from the uppermost leachate injection point was less than 1 m in all cases

Fig 6 Calculation of lateral and upward movement from a recirculation well, leachate applied intermittently (8 hr on/16 hr off) at

10 m 3 m –1 day –1 for 3 weeks, waste permeability = 1x10 –3 cm s –1

Fig 7 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

Kr

Design example

To illustrate the use of equations developed by McCreanor

(1998), horizontal trench and vertical well leachate

recir-culation system requirements will be calculated for a 3-ha

landfill which plans to recirculate leachate at a rate of 20

m3ha–1day–1(a total of 60 m3 day–1) The systems will be

designed using Fig.s 3 through 7 The landfill has an

aeri-al footprint of 300 m by 100 m and the waste is estimated

to have a hydraulic conductivity of 10–4cm 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 8 hours, for a leachate application rate

of 1 m3 m–1 day–1 The lowest leachate application rate

presented in Fig 4 is 2 m3 m–1 day–1 By extrapolation, we

can estimate the lateral movement to be 3.2 m for an

appli-cation rate of 1 m3 m–1day–1and a hydraulic conductivity of

10–4 cm s–1 Similarly, the upward movement can be

esti-mated to be 1.5 m using Fig 5 Therefore the trenches

should be spaced 6.4 m apart, a total of 41 trenches, and

dis-tanced 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 8 hours The average

daily application rate per well is then 15 m3, resulting in a

lateral movement of 4 m, from Fig 7 Each well would

around the landfill perimeter will be used to prevent

side-seeps The area receiving leachate recirculation is then

Increasing the loading to 20 m3 day–1 well–1would 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 4 and 7 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 sta-bilisation results in the production of large quantities of landfill gas (LFG) An integral part of the design and oper-ation of a bioreactor landfill is the design and operoper-ation 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 pro-duction 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 compo-nents must be oversized and easily maintained through clean-ing 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 sys-tems must be provided