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BIOREACTOR LANDFILL By

ADELEKE Olukunle Francis (奥陆克) Shanghai University. January 2003.

1.0 INTRODUCTION Background Currently, more than 150 million tons of municipal solid waste (MSW) is produced annually from 688 cities in China, with an annual growth of 9% [3]. Each city dweller now produces 440 kg of wastes a year. Solid waste in the country has piled up to a total of 10 billion tons, occupying nearly 800 million square meters of land. There has also been sharp increase on industrial waste, with 650 million tons produced each year [3]. Landfill (70%), incineration (10%) and compost (20%) are the three main ways of disposing MSW in China. Most cities use centralized stacking and simple landfill treating methods [6]. More than 85 per cent of the country's MSW are buried in rubbish plants after disposal. Less than 50% of the wastes are disposed of harmlessly, with 1% of the wastes being utilized [3]. The solid waste management system that is in place in most parts of China do not adequately address the volume needs of the nation, particularly those of the over-populated urban centers [6]. In the USA, sanitary landfill is also the most predominant MSW disposal option. The major environmental concerns regarding MSW landfills are related to gas migration and leachate discharges. In the US the current federal regulations governing MSW landfills under Subtitle D of the Resource Conservation and Recovery Act emphasize minimizing infiltration, collecting leachate generated by the landfill, and mandate maintenance of the landfill integrity for a minimum period of 30 years. These regulations create conditions that delay, rather than eliminate, the eventual degradation of MSW, leading to the creation of a "dry tomb." [1]. Bioreactor Landfill Option

The bioreactor landfill was defined by SWANA as: “…..a sanitary landfill operated for the purpose of transforming and stabilizing the readily and moderately decomposable organic waste constituents within five to ten years following closure by purposeful control to enhance microbial processes [9].

In the present sanitary landfill, the inclusion of environmental barriers such as 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 a long time, and continue to generate leachate and gas for decades following waste placement possibly in excess of the life of the barriers. However, waste stabilization can be enhanced and accelerated so as to occur within the life of the barriers

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if the landfill is designed and operated as a bioreactor. The bioreactor landfill provides a similar approach and treatment as is utilized in organic solid waste digestion [9].

The bioreactor landfill is a sanitary landfill in which liquid, temperature, and air - in the case of the aerobic process - are managed in a controlled manner to achieve rapid stabilization of the food, greenwaste, and paper-waste constituents. To optimize the rapid waste stabilization of these elements, moisture conditioning of the waste must be established and maintained at a relatively uniform level, and gas composition, flow, and temperature must be carefully maintained and monitored [7].

The bioreactor landfill requires specific management activities and operational modifications to enhance microbial decomposition processes. The single most important and cost-effective method is liquid addition and management. Leachate and LFG condensate may not be available in sufficient quantity to sustain the bioreactor process. Water or other nontoxic or nonhazardous liquids and semi liquids are suitable amendments to supplement leachate (depending on climatic conditions and regulatory approval). Other strategies, including waste shredding, pH adjustment, nutrient addition, waste predisposal and postdisposal conditioning, and temperature management may also optimize the bioreactor process [8].

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 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.

Researchers in the EU have also suggested that sustainability can be accomplished either through extensive waste preprocessing or the concept called flushing bioreactor. The flushing bioreactor achieves waste stabilization and contaminant removal within a generation through the addition of large volumes of water. The costs however are much higher than the conventional landfill [9]. Regulations

In the US, present regulations generally encourage landfills to remain relatively dry. In most cases, the final moisture content remains close to that of the entering waste. The bioreactor and leachate recirculating landfills differ from the dry Subtitle D landfills in that they each receive managed liquid additions to augment waste stabilization. Subtitle D already allows the recirculation of leachate and condensate, but it does not allow the introduction of other free liquids. In most cases, existing landfills do not produce sufficient leachate and condensate to optimize the composting benefit and therefore they attain only a portion of potential stabilization benefits.

Since 1990, SEPA has promulgated regulations and legislative documents on solid wastes management. On April 1, 1996, China issued the Law on the Prevention and Control of Environmental Pollution by Solid Wastes. Standard for Pollution Control on

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the Landfill Site for Domestic Waste GB16889 was also issued in 1997. Some local governments have also enacted regulations and standards for the prevention and control of pollution by solid waste [6].

The standards promulgated by the European Union require a strategy that limits the quantity of biodegradable wastes entering the landfill and consequently, the practicality of a bioreactor [9]. Waste characterizations, moisture content and its implications on Bioreactor Landfills

Since the main purpose of the bioreactor landfill is to enhance the decomposition of the biodegradable components of the waste (food, greenwaste and paper), thus, the higher the putrescibles in a waste mass, the more suitable it is for bioreactor landfill treatment. Analysis of waste components from several countries shows that the percentage of biodegradable organics (mainly food wastes) increases from high income countries to low-income countries. Also, the initial moisture content of the waste will determine how much additional liquid will need to be added to achieve field capacity (FC) of 60-80% wet weight. It has been reported that the average landfilled MSW in USA and possibly other developed countries has a moisture content of 25% wet weight. Table 1 shows the MSW composition from OECD countries while Table 2 shows that of low, middle and high-income countries in Asia. Table 3 also shows the waste composition of Qingdao and Guangzhou in China.

Municipal solid waste composition in China is significantly different than in North America. According to the Qingdao Municipal Government, organic food waste makes up approximately 60% of municipal solid waste and inorganics make up approximately 30%. An interesting comparison is that, according to UNICEF for the years 1980-1985, 61% of a Chinese family's budget was spent on food compared to 11% for a North American family. Also, the density of municipal solid waste in Qingdao is approximately 400 kg/m3 (~670 lb.cu.yd.), and the moisture content of the waste is approximately 40% on a wet basis [4]. Putrescibles and plastics are the two main components of the MSW from Guangzhou, with putrescibles accounting for over 58% both in 1994 and 1999. The moisture content was also found to be high (47.4%).

Table 4 shows solid waste moisture contents and densities as reported by specific cities. Usually the higher the percentage of organic matter, the higher the moisture content and the density of the waste stream. The waste density of low income countries such as China, India, and Mongolia is further influenced by significant quantities of discarded coal ash residue. Low income countries have a wet waste density typically between 350 to 550 kg/m3, middle income countries range from 200 to 350 kg/m3, and high income countries from 150 to 300 kg/m3 [11].

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Table 1. Estimated MSW Composition from OECD Countries

Source: Reference 10

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Source: reference 10

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Table 3: Waste components from Qingdao and Guangzhou

Components Qingdaoa

Guangzhoub

1994 1999 Food and putrescibles 60 59.6 58.1 Paper 3.1 NA 6.3 Plastic 4.5 15.9 14.5 Glass 0.8 2.9 2.0 Metal 0.3 0.6 0.6 Others 31.3 21.0 18.5

a Reference 4 b Reference 2

Table 4: Solid Waste Moisture content and Densities

Source: Reference 11

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2.0 BIOREACTOR LANDFILL TECHNOLOGY Leachate Recirculation and Bioreactor landfills A bioreactor landfill is more than one that simply recirculates leachate. Subtitle D already allows the recirculation of leachate and condensate, but it does not allow the introduction of other free liquids. Existing regulations do not facilitate current leachate recirculation projects’ achievement of FC and thereby attain the optimum waste-stabilization performance of a true bioreactor landfill. Nonetheless, compared to the conventional Subtitle D landfill, leachate recirculation projects generally reduce long-term environmental risk because they significantly reduce LFG generation and produce less contaminant loading in the leachate.

Leachate recirculation is usually done to defray annual offsite leachate and condensate transport and disposal costs, pretreat leachate before onsite disposal, or achieve a higher effective waste density (less annual use of landfill airspace). It is also useful in accelerating waste degradation, resulting in increased landfill gas (LFG) generation, which in turn could increase energy sales and revenues. It has been reported that there were more than 130 landfill leachate recirculation projects in the United States by 1997. Most of these did not have the Subtitle D composite lining, and yet the environmental results have generally been satisfactory [7].

In China, although many researches have been reported on possibility of using leachate recycling for landfill leachate treatment and trials conducted at some landfill, but there is no landfill yet that fully where leachate recirculation is used as the main treatment method. Leachates from landfills are still treated by chemical and biochemical means [13]. For example, the collected leachate from Asuwei landfill in Beijing is pumped to a treatment plant that holds 1000 m3 leachate per day and had a series of aeration channels followed by settlement tanks. At the Laogang landfill in Shanghai, collected leachate is pumped into one of the two treatment plants on site. Both include anaerobic and aerobic processes before they discharged leachate for final polishing into the weed bed zone, located between the landfill and the sea [5]. Aerobic and Anaerobic Bioreactors

Bioreactor landfills can be categorized broadly as aerobic or anaerobic. However, there are also ongoing studies of aerobic/anaerobic and semiaerobic bioreactors, which combine elements of both aerobic and anaerobic systems [1].

Anaerobic Bioreactors. Anaerobic bioreactor landfills seek to stabilize landfilled

waste rapidly by the addition of moisture to uniformly wet the waste mass. Landfill degradation of MSW frequently is rate-limited by insufficient moisture. The maximum methane production in landfills occurred at moisture content of 60-80% wet weight. This suggests that most landfills are well below the optimum moisture content for methane production. Also, the liquid absorptive capacity is about 16-29% or 30-60 gal/yd3 of

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waste which represents a large potential capacity for leachate storage. Liquid can be injected into the waste via horizontal trenches, vertical wells, surface

infiltration ponds, spraying, and prewetting of waste. Anaerobic bioreactor landfills initially should be carefully monitored. If the waste is wetted too rapidly, a buildup of volatile organic acids might lower the leachate pH, inhibiting the methane-producing bacteria population and reducing the rate of biodegradation. Leachate parameters (such as pH, volatile organic acids, and alkalinity) and LFG parameters (such as methane content) are direct indicators of an established methane-producing bacteria population. Optimal conditions for methane-producing bacteria are a pH of greater than 6.5. A high volatile organic acids-to-alkalinity ratio (>0.25) indicates that the leachate might have a low buffering capacity and conditions could soon inhibit methane generation.

The gas content of anaerobic bioreactors is similar to that of conventional landfills, with methane and carbon dioxide each making up approximately 50% of the total LFG volume. When the methane content of the LFG exceeds approximately 40%, the methane-producing bacteria population can be considered established. A decrease in the methane gas content below 40% is a possible indication that the waste is becoming too wet or dry. Once the methane-producing bacteria population has become established, the rate of leachate recirculation may be increased [1].

Aerobic Bioreactors. Aerobic bioreactors operate by the controlled injection of

moisture and air into the waste mass through a network of horizontal and/or vertical pipes. Aerobic landfill processes are analogous to wet composting operations in which biodegradable materials are rapidly biodegraded using air, moisture, and increased temperatures created by biodegradation. Prior to air injection, liquid is pumped under pressure into the waste mass through injection wells in order to wet the waste mass to moisture content between 50% and 70% by weight. Once optimal moisture conditions have been reached, air injection commences. Blowers typically are used to force air into the waste mass through a network of perforated wells that have been installed in the landfill. The rates of injection of air and leachate into the landfill are similar to the air and moisture application rates used in many composting systems. The aerobic process continues until most of the easily and moderately degradable compounds have been degraded and the compost temperature gradually decreases during the final phase of "curing" or maturation of the remaining organic matter.

Optimum temperatures for waste degradation within an aerobic bioreactor landfill are between 140º and 160ºF. Due to the substantial amounts of heat generated, large quantities of leachate can be evaporated. Waste temperatures are controlled by changing the rate of air and liquid injection. The potential for waste combustion typically is managed by ensuring that the waste mass is wetted adequately and air injection is uniform throughout the waste mass to minimize methane generation. Waste temperatures are maintained in the optimal range, and only enough air is injected into waste to support aerobic biodegradation.

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Aerobic bioreactor landfills are much more operationally intense than anaerobic bioreactor landfills; however, post closure costs should be reduced substantially due to reductions in LFG generation and cover settlement.

Because of higher reaction rates, aerobic biodegradation is a more rapid process than anaerobic biodegradation. Consequently, aerobic landfills offer the potential to achieve the same waste stabilization in two or four years that conventional landfills require decades or longer to reach [1]. A comparison of the conventional landfills with anaerobic and aerobic bioreactor landfills is given in Table 5.

Table 5. Comparison of Bioreactor Landfills

Conventional Landfill

Anaerobic Bioreactor

Aerobic Bioreactor

Typical Settlement After: 2 years 10 years

2-5% 15%

10-15% 20-25%

20-25% 20-25%

Anticipated Waste-Stabilization Time Frame

30-100 years 10-15 years 2-4 years

Methane Generation Rate

Base case Two times base case

10-50% base case

Liquid Storage Capacity Utilized in Waste Mass

None 30-60 gal./yd.3 30-60 gal./yd.3

Liquid Evaporation Negligible Negligible 50-80%*

Average Capital Cost

Low Medium High

Average O&M Cost

Low Medium High

Average Closure/ Postclosure Cost

High Medium Low

* Liquid evaporation rate is highly dependent on site-specific characteristics. Source: Reference 1.

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Examples of Bioreactor Landfill Activities Some examples of existing full-scale bioreactors around the world with a description

of the leachate recirculation technique employed are given in Table 6 below: Table 6. Description of Recent Full-Scale Bioreactor Landfill Tests Location Size Start up

Date a Leachate Recirculation Technique

Comments

Idaho 2.83 ha 1993 Surface spray (summer only) trenches 24.4m spacing wells.

First lined landfill in Idaho

Iowa 0.20 ha 7700 tons waste divided into 2 subcells.

1998 Trenches 4.6 m spacing. 10670 ld-1

Experimenting with bag opening, biosolids addition.

Milwaukee 61mx12.2m 1999 trenches No compaction, shredded, biosolids added.

Toronto, Canada

Pilot 1990 Vertical wells 1.2 wells ha-1 190-400 lpm

Well water added to adjust moisture content not leachate.

7 Mile Creek SL

720 tpd landfill,

1998 Trenches 7.6 m spacing. 73 lpd m-2

Tire chips acceptable in trenches, gas production increased by 25% in wells near recirculation.

Yolo County, CA

Two 930 m2 cells 4080kg MSW each 12m deep.

1995 14 infiltration trenches at surface.

Enhanced gas production, settlement. Shredded tires successful in LFG collection.

Yorkshire, UK

Two cells 860 tons waste 890m2 each 5.5m deep

1991 Trenches Biosolids and wastewater addition. Low temperature prevented maximum gas production.

Crow Wing MSW LF, Minn.

5.18 ha 1997 11 trenches, 15m spacing, 310 l d-1 m-1

No off-site hauling of leachate in 1998, Recirculation operated 3 mos yr-1.

Worcester Co. LF, MD

6.9 ha, 24 m deep

1990 Vertical wells surrounded by 7.6 m of gravel blanket.

Net benefit $3.2 million per 6.9 ha cell (after mining) Avg. 65% of leachate recirculated. Upper layers did not degrade

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extensively. Lyndhurst LF, Melbourne, Australia

1.3 ha 1995 Recharge wells and trenches

Complete instrumentation for monitoring leachate, temperature, gas, climate, moisture distribution, head on liner.

VAM Waste Treatmant. Wijster, The Netherlands

7062 m2 1997 Trenches 10m horizontal, 3 m vertical spacing (plus surface infiltration at 5m spacing)

Gas collection in wood chips at the top liner. Filled with mechanically separated organic fractions <45 mm diameter.

Baker Rd LF, Columbia County, Georgia

3.24 ha, 3m 1996 20 vertical wells Air injected into LCS system, Settlement increased by 4.5%, biodegradation rate increased by>50%.

Live Oak LF, Atlanta, Georgia, USA

1.01 ha, 9m 1997 27 vertical wells, 1.5-4.6m deep, 18 air injection wells

Air and liquid injection into same well improved fluid distribution.

Shin-Kamata LF, Fukuoka City, Japan

NA 1975 Horizontal Pipes Semiaerobic process using large leachate collection pipes that draw in air.

Trail Road LF, Ontario, Canada.

270m x 500m

1992 Infiltration lagoons Lagoons were moved around 50% of field capacity.

a start-up date for leachate recirculation Source: Reference 9. Potential Advantages and Disadvantages of Bioreactor Landfill Anaerobic Bioreactor Advantages

• leachate storage within the waste mass, • landfill airspace savings (increased rate of landfill settlement), • more rapid waste stabilization than conventional landfills, • increased methane generation rates (200-250% increase typical) and thus suitable

for waste to energy programs, • potential for limited landfill mining, and • lower postclosure costs.

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Disadvantages

• Increased cost of basic piping, pumps and electricity for leachate recirculation • The generation of methane, a green house gas is increased significantly increased

and can lead to more environmental problems if not properly handled.

Aerobic Bioreactor

Advantages

• More rapid waste and leachate stabilization than anaerobic bioreactor • Landfill airspace savings (increased rate of landfill settlement) • Reduction of methane, a GHG generation by 50-90% • Capability of reducing leachate volumes by up to 100% due to evaporation • Potential for landfill mining and sustainability • Degradation of some recalcitrant chemicals and ammonia

Disadvantages

• Risk of fire and explosive gas mixtures through addition of air to landfill • Additional cost will be incurred supplying power required to add air to the landfill

over that required for the anaerobic bioreactor. • Although methane emission may decrease, but other hazardous and noxious

chemicals e.g. nitrous oxide may be emitted.

Potential Benefits of Bioreactor Landfill

The potential benefits that can be derived from the bioreactor landfill can be in the form of environmental, regulatory, monetary and social benefits. Some of the key benefits are:

Greenhouse Gas Abatement. The substantial reduction of the quantity of methane generated by an aerobic bioreactor landfill is a benefit to the environment since methane is significantly more effective as a GHG than is carbon dioxide.

Conversely, in the anaerobic bioreactor landfill, the increased generation rate and recovery of methane can lead to economic benefits because the gas can be sold or used for energy on-site if it is produced in high-enough concentrations. Also, lessening the amount of gas generated from the landfill after capping also can decrease long-term postclosure care costs.

Leachate Strength Reduction. Low-cost partial or complete treatment, significant biological and chemical transformation of both organic and inorganic constituents, although mostly relevant to the organic constituents can be achieved in bioreactor

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landfills. Environmental Protection. Bioreactor landfills reduce the biodegradable fraction of

the waste mass within a relatively short period of time. This stabilization of the waste mass will occur during the active period of landfilling and well within the postclosure care period. In the alternative dry-tomb scenario, stabilization of the waste mass may not occur until such time as the landfill cap and/or liner system fails. Therefore, bioreactor landfills create a stabilized waste mass and lessen the risk of leachate polluting the groundwater in the future.

Rapid Settlement. Accelerating MSW degradation can reduce the need for new landfills by recapturing landfill airspace. Increased rates of settlement before closure will permit additional MSW to be placed in the landfill before a cap is put in place. Waste settlement varies greatly and is dependent on type of waste, amount of cover, and compaction.

Postclosure Maintenance and Risk. Rapid waste stabilization would reduce the postclosure care, maintenance and operation activities and costs of bioreactor landfills since waste stabilization would have been achieved before final closure. Even in the event of partial liner failure, there should be no risk of increased gas generation, worsening leachate quality, or increased settlement rate or magnitude.

Sustainability. Approximately 50-70% of MSW is composed of biodegradable waste. The rapidly stabilized waste in a bioreactor landfill would have a minimized potential to generate LFG and further degrade if exposed to air or water. If the waste mass can be stabilized sufficiently in a bioreactor landfill, then it can be more safely excavated and recycled. After the waste has been stabilized, it then can be excavated and trommeled. Approximately half of the stabilized waste is a soil/compost mixture that may be utilized as landfill cover material. A fraction of the stabilized waste (approximately 10%) consists of metals, which could be recovered from the stabilized waste and recycled, thereby diverting it from the wastestream. The remaining fraction of stabilized waste consists of dirty plastic material and other miscellaneous inert materials. The dirty plastic material could be used as feedstock for low-grade plastic wood products or as a fuel source. The remaining miscellaneous inert material then would be landfilled. Bioreactor Landfill Issues

Some of the lessons gathered from the existing field-scale bioreactor activities according to Reinhart D. R. et al. (2002) are given below:

• 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

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• 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 3.0 DESIGN AND OPERATIONAL CONSIDERATIONS

In designing the conventional landfill, the design components include the liner, leachate collection facilities, gas collection and management facilities, and final cap. These same components must be adapted during the operational period of the bioreactor landfill to manage leachate, including liquid introduction, and to handle enhanced gas generation. The following issues will however have to be taken care of in the case of bioreactor landfills:

Leachate recirculation systems

The principal design element that is added to a bioreactor landfill is the leachate recirculation system [12]. In a conventional landfill, leachate is directed to a treatment facility after being pumped from the landfill. 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 [9].

There are several methods of reintroducing leachate into the landfill environment, depending on specific site conditions. Spray irrigation, surface application, vertical well injection and horizontal well injection are used. Factors such as ease and cost of

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installation, waste quantity, moisture distribution, climate and regulatory compliance will affect which method is chosen. In order to maximize 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 2 to 4 m3 day-1 linear m-1 of trench and 5 to l0 m3 day-1 well-1; however field experimentation is required to determine site specific capacity [10]. Auxiliary Moisture Addition. 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. 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. Allowable leachate head on bottom liner. 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 require 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

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regarding leakage potential; however for a more realistic evaluation a more complete measurement may be required. Leachate Seeps. Leachate recirculation should be controlled to minimize outbreaks and to optimize 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 minimize problems with side seepage. Operational issues. The construction, operation, and monitoring of leachate recirculation systems will impact daily landfill operations. If a leachate recirculation system is to be utilized, 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 [9]. Liners Systems One argument in favor of bioreactors is that the accelerated decomposition phase should occur during the early years of the landfill, when the liner system is most effective. Presently, there are liner systems are much more effective than the liner systems that existed several years ago. However, additional liner protection may be required for bioreactor landfills where leachate recirculation is to be practiced over that of conventional landfills. For example, in New York, the double composite liner is standard. Having two liner systems, one above the other, allows the effectiveness of the upper liner to be monitored. Any leachate that leaks through the first liner can be collected from the lower liner. Similar liner systems may be required in other states. For bioreactor landfills, the leachate collection system must be designed to accommodate the higher volumes of water that will be moving through the landfill. This may mean increasing the pipe size at

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some locations, adding pumping capacity and specifying a more permeable drainage layer above the landfill liner [12]. Landfill Gas Recovery One of the objectives for building a bioreactor is to recover landfill gas quickly and to extract the resulting energy value. Some modifications to the conventional landfill gas recovery system will be necessary when building a bioreactor. The most challenging aspect is installing the gas recovery system so that it is operational when gas generation begins. To efficiently control gas and avoid odor problems, the bioreactor LFG extraction system may require installation of larger pipes, blowers, and related equipment early in its operational life. Horizontal trenches, vertical wells, near-surface collectors, or hybrid systems may be used for gas extraction. Greater gas flows are readily accommodated by increased pipe diameter as capacity increases as the square of pipe diameter. Liquid addition systems should be separate from gas extraction systems to avoid flow impedance.

Enhanced gas production can negatively impact sideslopes and cover if an efficient collection system is not installed during active landfill phases. Uplift pressure on geomembrane covers during installation can cause ballooning of the membrane and may lead to some local instability and soil loss. Temporary venting or aggressive extraction of gas during cover installation might facilitate cover placement. Once the final cover is in place, venting should be adequate to resist the uplift force created by LFG pressure buildup. The designer should consider the pressure buildup condition on slope stability when the collection system is shut down for any significant amount of time.

Cell Size For economic and regulatory reasons, an emerging trend in traditional landfill design is to build deep cells (or phases) that are completed within two to five years. This trend bodes well for bioreactor landfill evolution. Phased cell construction can more easily take advantage of emerging technological developments, rather than committing long term to a design that might prove to be inefficient. Once closed, methanogenic conditions within the cell (phase) are optimized and gas generation and extraction are facilitated. However, extremely deep landfills might be so dense in the lower portions that refuse permeability will inhibit leachate flow. In these instances, it might be necessary to limit addition and/or recirculation to the upper levels or develop adequate internal drainage management capability. Solid Waste Density

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Adding liquid to solid waste will increase its density, which can be of critical importance in the design of load-bearing structural members in the landfill. Most notably, the leachate and LFG collection system must be designed to accommodate the increased load, which may be as much as 30% heavier because of expected moisture uptake and settlement.

Landfill Stability

Addition of liquid into the refuse to increase biological activity will increase the total weight of the refuse mass and may cause an increase in internal pore pressure. This stability issue can be readily assessed and resolved with standard geotechnical analyses. Seismic effects should also be considered during geotechnical analysis, when appropriate.

Settlement

A bioreactor landfill will experience more rapid, total, and complete settlement than will a drier landfill. Accelerated settlement results from both an increased rate of solid waste decomposition and increased compression through higher specific weights. Settlement during the landfilling operations will impact the performance of the final surface grade, surface drainage, roads, gas-collection piping system, and leachate-distribution piping system. Because of the significant increase in settlement magnitude and rate, it could be very beneficial to overfill the refuse above design grade before placement of the final cover. Alternatively, a significant benefit may accrue if final cover and final site-improvement installations are postponed and the rapid settlement is used to recapture airspace. Settlement impacts can be readily accommodated by the project design. Since settlement will be largely complete soon after landfill closure, long-term maintenance costs and the potential for fugitive emissions will be avoided.

Solid Waste Pretreatment or Segregation

Bioreactor operations are most efficient and effective where the refuse has high organic content and large exposed specific surface area. For this reason, bioreactor operations should be concentrated on waste segregated to maximize its organic content and shredded, flailed, or otherwise manipulated to increase its exposed surface area. Waste segregation could include separation of construction and demolition wastes from MSW. Limited shredding can be obtained by spreading refuse in thin lifts and using landfill equipment to break open plastic bags and break down containers. Mechanical shredding can be efficient and effective in reducing particle size and opening bags; however, it is an intensive, high-maintenance, and high-cost activity that might not be cost-effective. Moreover, shredded wastes may become exceedingly dense after placement, thereby limiting moisture penetration.

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Daily and Intermediate Cover

The use of soil cover in a bioreactor landfill requires special attention. A cover more permeable than the waste can direct leachate to the sides, where the leachate must be properly collected and drained. Low-permeability daily cover can create barriers to the effective percolation of leachate and water. It can also impede leachate distribution and LFG flow to collection and distribution systems; its ability to serve as a barrier should be reduced through scarifying, or partial removal, prior to placing solid waste over it. When placed within 50 ft. of the slopes, it should be graded to drain back into the landfill to preclude leachate from reaching the slope and emerging as a seep. Use of alternative covers that do not create such barriers can mitigate these effects.

Odor Control Odor control can be more challenging when waste is wet. Consequently, the operator must be prepared to take appropriate action if problems arise. This could include quickly covering an area with earth or introducing a fresh waste layer over a bioreactor cell. The operator also must be prepared to discontinue leachate recirculation if any of these issues emerges. Regulatory Controls and Variances Specific regulations require attention when implementing the bioreactor. Current regulations prohibit the addition of liquids into a landfill. Consequently, to add additional water besides leachate, a variance is necessary. Existing regulations specifying allowable height of leachate on bottom liner may be modified in favor of bioreactor systems. Landfill operators may find that state regulatory agencies require more extensive and frequent groundwater monitoring. This is because bioreactors contain more water than conventional landfills, and the pressure from the leachate is higher. Landfill operators may receive special approval from state regulatory agencies to refill the space that becomes available as the waste consolidates. Regulatory authorities may consider reducing the landfill’s long-term care requirements, given that bioreactors stabilize waste much faster than conventional landfills. Public Perception Bioreactor developers must take into account the public’s concerns, given their limited amount of bioreactor experience. Developers should carefully plan and organize the bioreactor’s operation, but also be forthright regarding plans for refilling the consolidated bioreactor cells. Facility neighbors will be particularly concerned about potential groundwater and odor problems. The long-term disposition of the bioreactor site also

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should be specified. In the long run, a bioreactor landfill may create benefits not available with a conventional landfill. The bioreactor will reach a stabilized state much more quickly than a conventional facility. Consequently, the landfill is less likely to contaminate the environment over the long-term. 4.0 CONCLUSION With China’s huge population and high waste-generation rate and limited land area, the bioreactor landfill technology might provide an environmentally friendly, economical and sustainable option for solving part of these problems. Moreover, food and other putrescible organic waste makes up large part (over 50%) of the wastes from most Chinese cities; which makes it suitable for the technology. Also, the initial high moisture content of the wastes (over 40% which is much higher than that from the US – about 25%) also means that less moisture will need to be added to achieve field capacity. With anaerobic systems, the rapid rate of methane generation can also favor waste to energy projects. However, pilot-scale experiments will be needed to complement the existing researches on leachate-recirculating systems and subsequent adaptation to bioreactor landfills in order to be able to fully understand the workability of the technology in China. REFERENCES 1. Campman, C. and Yates, A. 2002. “Bioreactor Landfills: An Idea whose time has

come.” MSW Management. September/October 2002. 2. Chung, S. S. and Poon, C. S. 2001. “Characterization of municipal solid waste and its

recyclable contents of Guangzhou”. Waste Management & Research. Volume 19, December 2001.

3. Grand Exhibition Services Co., Ltd. 2003. “Waste Disposal Shanghai 2003”. http://www.grandexh.com/waste2003/Exhibiting/Market%20Info.htm 4. Henderson, J. P. and Chang, T. J. “Solid Waste Management in China”. Ed. H. L.

Hickman, Jr. http://www.ecowaste.com/swanabc/papers/hend01.htm 5. Johannessen, L. M. and Boyer, G. 1999. “Observations of Solid Waste Landfills in

Developing Countries: Africa, Asia, and Latin America”. Urban Development Division. The World Bank. June 1999. Pp 16 – 23

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6. Liu, G., Harris, J. and Adams, C. 2000. “ China Solid Waste Management

Technologies”. U. S. Department of Commerce - National Trade Data Bank, November 3, 2000.

7. Pacey, J. 2001. “Bioreactor Landfill: an Overview Perspective”. MSW Management.

Elements 2001. http://www.forester.net/msw_0011_bioreactor.html 8. Pacey, J. et. al. “The Bioreactive Landfill”. MSW Management. http://www.forester.net/msw_9909_bioreactive_landfill.html 9. Reinhart, D. R., McCreanor. P. T. and Townsend, T. 2002. “The Bioreactor Landfill: Its

status and Future”. Waste Management & Research. 2002.20. 172-186. 10. The World Bank. “Waste Generation Rates.” http://www.worldbank.org/html/fpd/urban/publicat/annex2.pdf 11. The World Bank. “What a waste: Solid Waste Management in Asia. http://www.worldbank.org/html/fpd/urban/publicat/annex1.pdf 12. Walsh, P. and O’Leary, P. 2002. “Bioreactor Landfill Design and Operation”. Waste

Age. June 2002. www.wasteage.com. 13. Yueqing, X. and Guojian, L. 1999. “Recirculation Treatment Method of Leachate

from Municipal Solid Waste Landfill Site”. (in Chinese). 环境卫生工程处 1999 年

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