FULL SCALE OPERATION OF A UNIQUE LANDFILL BIOREACTOR: THE CALGARY BIOCELL

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1 FULL SCALE OPERATION OF A UNIQUE LANDFILL BIOREACTOR: THE CALGARY BIOCELL J. Patrick. A. HETTIARATCHI (1), Carlos HUNTE (1), and Sunil KUMAR (2) (1) Center for Environmental Engineering Research and Education (CEERE), University of Calgary, 2500 University Drive NW, Calgary, T2N 1N4, Calgary, Canada (2) NERI, Calcutta, India Corresponding Author: Tel: , jhettiar@ucalgary.ca Abstract The Biocell is a waste cell that promotes sustainable solid waste management and is an extension of the landfill bioreactor concept. A pilot scale field Biocell is constructed in Calgary, Alberta, Canada. After final closure, the Biocell is currently operated as an anaerobic bioreactor for enhanced landfill gas production using leachate recirculation. The gas generated is being used for electricity production. In a second stage, air will be injected into the solid waste matrix to convert the operation to an aerobic bioreactor. In this stage, the Biocell acts as an in-ground composter. In a final stage, the Biocell will be mined for material and space recovery. The City of Calgary has partnered with University of Calgary and local consulting firms to construct and operate the pilot-scale Biocell. Some outstanding features of the Calgary Biocell include the use of thin biocovers (TBC) as intermediate covers placed after every lift to act as methane oxidizing covers. From early results of the Calgary Biocell project, methane production can commence in the early stages of landfilling. Waste settlement and gas generation was monitored both during cell construction and operation of the Biocell. Results indicate that the Biocell is reaching its stated objective of enhanced gas production.. Keywords: Biocell; Landfill gas; Biocover; Leachate; Design 1. INTRODUCTION Waste disposal in Open Dumps was an accepted practice until it was realized that Open Dumps create negative human health and environmental impacts. After that, the conventional dry-tomb type sanitary landfills is designed and constructed to eliminate the problems associated with Open Dumps with features, such as provision of bottom liner with leachate collection and removal systems along with a provision of putting final cover system to minimize infiltration of precipitation With provision of leachate collection system and final cover, groundwater contamination from landfill leachate can be minimized which was a major concern with Open Dumps (Hartz and Ham, 1983). 340

2 The adoption of dry-tomb type sanitary landfills has enabled engineers to solve the problem of groundwater contamination associated with land disposal of waste, but it leaves behind several other problems, including problems with landfill gas, long-term liability with the landfill and the loss of valuable space. Loss of space is a critical issue accentuated by the situations in major cities around the world. Space being used up for conventional landfills force municipalities to look for new landfill space every few years. With increasing public apathy towards landfill disposal of waste and lack of real estate near the big cities, locating new landfills is rapidly becoming a daunting task. Furthermore, the largest anthropogenic source of atmospheric methane (CH 4 ) in many developed countries is from landfill emissions. In Europe, 30% of anthropogenic CH 4 emissions are from landfills whereas in Canada, the estimations for the year 2001 show that 23 Mt of a total of 93 Mt (or about 25%) of anthropogenic CH 4 emissions are from landfills. One popular option is to capture the produced landfill gas and use it as an energy source. However, this is economical only when a sufficient quantity of gas is available. Bioreactor landfills are relatively new concept of waste disposal. The foremost idea of bioreactors is the recirculation of leachate and other liquids to encourage faster microbial waste decomposition (Pohland 1975, Reinhart and Townsend 1998, Warith et al. 2001, McBean et al. 2007). This results in enhanced gas generation and a faster decrease in the waste volume. Conventional bioreactor landfills involve aerobic or anaerobic conditions (US EPA 2000). Considering the pros and cons of aerobic and anaerobic processes, a new concept of landfill waste disposal known as sustainable landfill biocell concept or the Biocell is introduced. The Biocell is a variation of the landfill bioreactor approach and involves the operation of a landfill cell under sequential anaerobic-aerobic conditions with leachate recirculation to take advantage of both forms of biodegradation. The operation of the Biocell as an anaerobic bioreactor enhances CH 4 production, while the aerobic phase enhances waste decomposition to a point where it could be mined in a third stage for resource and space recovery; thus making the landfill operation sustainable. Considering the difficulties in locating new landfills in or near urban centers, using and reusing of space is one of the key benefits of a Biocell. Therefore, Biocell is a holistic approach to waste disposal; with energy recovery, CH 4 emission control, minimization of groundwater contamination, and space recovery as some of the direct benefits. The Biocell technology could potentially eliminate CH 4 emissions from landfills; therefore a Biocell could also be termed the zero methane landfill. A project termed as the Calgary Biocell Project has been implemented in Calgary, Alberta, Canada, which incorporates the above described concept. A full scale landfill cell has been constructed and operated as a Biocell. The design, construction and operation of the Biocell, monitoring of the Biocell during its operation and, using an innovative thin biocover as an intermediate cover, and problems encountered, landfill gas generation and leachate quality along with waste settlement are described in this paper. 2. CALGARY BIOCELL Full-scale bioreactors, both anaerobic and aerobic, are in operation in various parts of the world. Among these, several bioreactors in North America, including one in Quebec, Canada, are being used as pilot scale operations to gather field data on enhancement of decomposition with leachate recirculation. However, these projects are either aerobic or anaerobic 341

3 bioreactors. Full-scale mining of landfills have been practiced in several locations in various parts of the world to recover space as well as potential recyclables. The award-winning Calgary Biocell is a unique facility and the first of its kind where the three processes, anaerobic bioreactor, aerobic bioreactor and mining are applied in one landfill cell (Figure 1). Unlike a traditional landfill or a bioreactor landfill, the Calgary Biocell is not a permanent facility. The contents of the Biocell will be removed once complete degradation of biomass is achieved and a new Biocell constructed, thereby reusing valuable space. Anaerobic Year 1 Anaerobic Year 2 Mining/ Space Recovery Year 6 Anaerobic Year 3 Aerobic Year 5 Aerobic Year 4 Figure 1. The Three Stages of Biocell Operation The Biocell is located within the Shepard Waste Management Facility, and is owned and operated by the City of Calgary, Canada. Construction of the Biocell was started in the summer of Although the Biocell is an attractive alternative to conventional landfilling, a number of technical obstacles could prevent its large-scale adoption. For example; the Biocell design needs to account for increased generation rates of leachate and landfill gas during the anaerobic stage of operation. Furthermore, the existence of layers of different permeability within the Biocell could create side-slope leachate seeps and even slope failure. Opening of garbage bags to expose waste to moisture and achieving uniform moisture and air distribution during aerobic stage of operation, throughout the waste mass are some of the challenges faced by designers. The Biocell is located in Calgary, Alberta, Canada, where the average annual precipitation is low at 400 mm, and temperature during the winter can be as low as 30 0 C. Therefore, this project presented special weather related challenges in addition to those typically associated with bioreactor landfills. 3. DESIGN, CONSTRUCTION AND OPERATION OF BIOCELL 3.1 Design and construction of Biocell Calgary Biocell covers an area of 100 m x 100 m with a waste footprint of 85 m x 85 m and a maximum height of 18 m at the time of placing the final cover. The schematic diagram of the Biocell is presented in Figure 2. The base of the Biocell is about 50m 50m, and it extends approximately 10 m below from the top of upper perimeter berm. Approximate 342

4 ground elevation of the biocell site is 1025 m. The elevation of upper perimeter berm is m. The shape of the Biocell is like a pyramidal square frustum increasing sectional area with side slope of 3H: 1V up to ground level. A side slope of maximum 30% is provided for the portion of landfill above the existing ground. The composite liner system in the Biocell consists of 80 mil HDPE geomembrane and a 1.0 m thick compacted clay liner. To protect HDPE liner from the drainage gravel of leachate collection system a geotextile cushion layer is placed on top of the liner. A leachate collection and recirculation system is provided at the Biocell. Leachate collection and removal system consists of a layer of drainage gravel across the entire base of the cell, and a geocomposite on the perimeter slopes. A 0.6 m deep gravel filled trench sloping towards leachate collection sump is provided for leachate removal. The leachate sump area is doublelined with a studliner and installed with a leak detection system. Collected leachate is recirculated by an automated submersible pump system installed in the leachate sump. Liquid injection system consists of horizontal trenches installed with perforated pipes and four vertical wells. Horizontal trenches are placed in three layers two under each intermediate cover and one underneath the final cover. Vertical wells extend up to and elevation 3 m above the leachate collection system. HDPE pipes are used for wells to provide flexibility, considering the differential settlement in the Biocell. The produced biogas is collected using a combination of vertical and horizontal wells and transferred to the ENMAX grid for energy generation. The horizontal wells are placed in three layers two under each intermediate cover and one under the final cover. The First layer consisting of four independent horizontal wells connected to the South manifold is placed in Phase I of the operation under the first intermediate cover. In Phase II four independent horizontal wells are installed under the second intermediate cover and connected to the West manifold. The third layer consists a ring header placed under the biocap. In addition four vertical wells perforated at the lower portion are placed under the biocap. The various steps of Biocell construction include excavation, laying of geosynthetic, clay and geomembrane over clay liners and leachate and gas collection system..figure 3 shows the Biocell excavaction. 343

5 Commercial recovery CO2 emissions CH 4 & CO 2 emissions Oxidation in landfill bio-cover (Methanotrophs) Final bio-cover Solid waste-3 rd lift (8 m) CH 4 & CO 2 generation GL GL 2 nd Intermediate thin biocover (30 cm) Solid waste-2 nd lift (5 m) CH 4 & CO 2 generation 1 st Intermediate thin biocover (30 cm) Solid waste-1 st lift (5m) CH 4 & CO 2 generation Figure 2: Schematic Diagram of the Calgary Biocell 3.2 Biocell Operation The Biocell received a feedstock consisting about 30,000 tonnes of residential solid waste (high in organics), and 25,000 tonnes of select commercial waste, placed in three lifts of 5-6 m each. Figure 3 shows Biocell where waste are being placed. In addition about 25,000 tonnes of biosolids (digested sludge from the Shepard sludge lagoon) were sprayed on each layer of feedstock to provide necessary moisture. Contrary to the conventional landfills, where rollers are used to compact waste, low ground pressure equipment were used to spread the feedstock. Lower layers were provided lower compaction and the weight of subsequent layers would increase the waste density. Depending upon the availability, storm water was sprayed at a rate of 0.1 m 3 /m 2 of surface area for a 0.5 m thick waste layer. The biosolids were also sprayed as required to provide necessary moisture. The side slope on the exterior of the biocell was maximum 30%. Figure 3: Picture Showing Biocell Excavation 344

6 Figure 4: Biocell Accepting Waste Before starting the filling operation, some sensors such as piezometers, thermocouples, settlement gauges, and load cells were installed on the cell bottom. Leachate collection and removal system was also installed before waste placement. Initial waste layer of 1.5 m was placed keeping in mind the protection of this infrastructure. Piping for liquid injection and LFG collection system and sensors were installed underneath the intermediate covers. To minimize disease vectors, blowing of litter by wind, and to improve site appearance; daily covers were placed on the working face at the end of each day. Considering the problem of side seeps and difficulty of uniform moisture distribution, low permeability material is not recommended for daily covers. A hydromulch daily cover was applied using a hydroseeder. Two intermediate covers were placed on top of first and second lift respectively. The purpose of these covers is to provide a more effective barrier between the waste and the environment during extended period of time (See Figures 5 and 6). These covers also act as fire breaks within the landfill. The un-compacted biocap material was used for intermediate covers to oxidize methane to carbon dioxide. To prevent the potential dust problem water was sprayed during dry and windy weather conditions. Final cover was placed progressively as the areas in the Biocell were completed. Components of the final cover are detailed below: Geocomposite as a LFG collection layer. Bio-cap layer of 500 mm thickness. 40 mil (1 mm) LLDPE membrane as the barrier layer. Subsoil layer spread evenly over the barrier layer 350 mm. Topsoil layer spread evenly over the subsoil layer 150 mm. Seeded to native grass establishment. In the aerobic phase of the operation, when air will be injected in the Biocell top three layers (topsoil, subsoil and LLDPE membrane) will be removed. This will permit venting of the injected air. Although significant methane generation is not anticipated during the aerobic phase, but there would be small anaerobic pockets where injected air could not penetrate. To oxidize any methane generated in those pockets before it escapes to atmosphere, bio-cap layer will be reactivated. 345

7 4. LANDFILL GAS (LFG) COLLECTION/ AIR INJECTION Gas recovery from Biocell started after its closure using a combination of vertical and horizontal wells. Gas collected at Biocell is sent to the Enmax power grid with the other LFG collected from Shepard Landfill, where it is utilized for power generation. The capacity of the gas reciprocating engine is 380 kw, which is fed by the biocell and the other conventional cells at the Shepard Landfill. Biocell provides about 30% of total gas collected at the Shepard landfill. Majority of gas collection/air injection wells are horizontal wells in the Biocell. Horizontal gas collection wells are placed in three layers two layers of four wells under each intermediate cover and one ring header under the bio-cap. These wells consist of perforated pipes surrounded by the course aggregate. In addition there are four vertical gas collection/ air injection wells to provide increased redundancy. Vertical wells are HDPE pipes perforated at the bottom. Top 3-4 m of pipe is solid to avoid air intrusion into the Biocell. Each pipe network has been placed independent of each other to avoid damage due to extensive settlement in Biocell. The gas flow from Biocell depends upon the requirement of the gas reciprocating engine. Gas flow data for the Biocell from October 2006 to April 2007, and January 2007 to January 2008 is shown in Figure 5. Currently gas flow from Biocell is about m 3 /hr, depending upon the requirement of the reciprocating engine. The average gas flow calculated based on cumulative gas collected is shown in Figure 6. Most (60-70%) of the recovered gas is collected in the horizontal wells as many of the vertical wells were broken due to extensive settlement in the Biocell and producing no gas at all. In addition there were problems in gas collection during winter season due to freezing of gas lines. Figure 5: LFG Flow Rate and Methane and Oxygen concentrations From Oct 2006 to Apr 2007 and Jan 2007 to Jan 2008 (Source: Shepard Landfill Biocell Operational and Maintenance Data Prepared by CH2M HILL) 346

8 Figure 6: Average LFG Flow Rate and Cumulative Gas Collected Since October BIOCELL LEACHATE Leachate generated in the Biocell is trapped by the perforated pipes at the bottom and conveyed to the leachate sump. In the first stage of Biocell operation, the collected leachate is recirculated after ensuring the quality is acceptable. This helps in maintaining optimum moisture content in the Biocell and leachate treatment over the time. Target moisture composition for the operation of Biocell has been 50% by weight. Leachate is injected using the horizontal and vertical wells placed under intermediate covers and the biocap. Horizontal liquid injection wells are placed in three layers of three wells each under both intermediate covers and the biocap layer of the final cover. In addition there are four vertical liquid injection wells. Leachate sump is a concrete sump and it is lined with a polyethylene membrane to prevent any leakage. Also a leak detection system is placed under the sump to identify any leakage. 6. WASTE SETTLEMENT IN BIOCELL This accelerated degradation rate also leads to accelerated rate of settlement as compared to traditional dry tombs landfills. The prediction of settlement prediction is therefore important for effective design and operation of a bioreactor landfill. Prediction of settlement would allow estimation of air space, planning of construction sequence, design of both intermediate and final covers as well as planning for expansions. Though the majority of settlement takes place as a result of the decomposition of solid waste over a number of years, considerable amount occurs during the initial construction stage, which usually goes unnoticed. As a result, settlement is typically considered after closure of a landfill with little or no allowance made for settlement during the construction phase. The practice of landfill settlement is still predominantly empirical and thus the common available techniques make no attempt to simulate the real mechanisms of waste settlement. The biocell is being operated in an anaerobic bioreactor mode to enhance biodegradation and collect methane generated in the process to utilize it for energy generation. Because of enhanced waste decomposition and waste settlement is also accelerated by leachate recirculation. The variation of the density profile within the LBC in time and space is one of the primary issues of interest in considering settlement in this project. 347

9 7. CONCLUSIONS The Calgary Biocell serves as a demonstration project of this kind. The project is achieving towards sustainable waste management. One of the problems encountered at Biocell site includes problems due to freezing of gas collection lines in winter season. This may be resolved by providing adequate insulation on these lines in future projects. In addition there were damages to gas collection wells because of extensive settlement in Biocell. Probably drilling of vertical wells after primary settlement has taken place may reduce these damages in future projects. The Calgary Biocell serves as a demonstration project as well as a research facility. The Calgary Biocell is being monitored for a number of parameters using automated sensor systems as well as using manual techniques. The extensive monitoring program is designed to allow collection of data related to leachate redistribution within the Biocell, waste settlement, biochemical reaction kinetics, leachate quality, performance of thin intermediate biocovers and final biocovers, as well as to determine the general waste biodegradation characteristics. The automated sensor system would allow collection of landfill gas quantity and quality data on a time-resolved basis. Some of the monitoring sensors are installed before and during waste placement to enable data gathering from the beginning of Biocell operation. The waste settlement profile has also been monitored on a time-resolved basis during and after completion of filling operations using settlement plates and associated sensor systems. The moisture content and temperature have been monitored real-time throughout the cell during and after cell filling. The collected data is used for day-to-day operation of the Biocell as well as for research purposes. REFERENCES [1] Hartz, K.E. and Ham, R.K., Moisture Level and Movement Effects on methane generation from landfill samples, Waste Mangt. and Res., 1 (1983) [2] McBean, E.A., Syed-Ritchie, S. and Rovers, F.A. Performance results from the Tucuman solid waste bioreactor, Waste Managt., 27(2007) [3] Pohland, F., Accelerated solid waste stabilization and leachate treatment by leachate recycle through sanitary landfills, Progress in Water Tech., 7(3-4) (1975) [4] Reinhart, D.R., and Townsend, T.G., Landfill bioreactor design and operation, (1998), Lewis Publishers, New York, NY, USA. [5] USEPA. State of the practice for bioreactor landfills, Workshop on bioreactor landfills Arlington, Virginia September 6-7, [6] Warith, M., Smolkin, P.A., and Caldwell, J.G., Effect of leachate recirculation on enhancement of biological degradation of solid waste: case study Practice Periodical of Hazardous, Toxic, and Radioactive Waste Managt., 5(1) (2001)