Abstract
Materials and energy used for construction of a hill-type landfill of 4 million m3 were quantified in detail. The landfill is engineered with a liner and leachate collections system, as well as a gas collection and control system. Gravel and clay were the most common materials used, amounting to approximately 260 kg per tonne of waste landfilled. The environmental burdens from the extraction and manufacturing of the materials used in the landfill, as well as from the construction of the landfill, were modelled as potential environmental impacts. For example, the potential impact on global warming was 2.5 kg carbon dioxide (CO2) equivalents or 0.32 milli person equivalents per tonne of waste. The potential impacts from the use of materials and construction of the landfill are low-to-insignificant compared with data reported in the literature on impact potentials of landfills in operation. The construction of the landfill is only a significant contributor to the impact of resource depletion owing to the high use of gravel and steel.
Introduction
The environmental impacts from waste management systems have been assessed often during the last two decades by life cycle assessment (LCA). The assessments assume a ‘zero burden’ upper boundary for the waste, as the materials constituting the waste were produced and used for a purpose, not for becoming waste. The assessments focus on the operation of the waste management technologies, while capital goods and infrastructure are seldom included. Capital goods is a common term for the materials and energy used to establish the facility prior to the operation. Our knowledge about the importance of capital goods in waste management is rudimentary. Cleary (2009) showed in a review of 20 peer-reviewed articles on LCAs on waste management systems that only 2 stated they included capital goods, 7 specifically excluded the emissions from the production of capital goods and infrastructure, and the remaining 11 did not mention whether capital goods were included or not.
Landfills are included in most waste management systems and are large constructions (Bagchi, 1994; Christensen, 2011; Ecobalance 1999), but only a few studies have addressed the environmental impacts from the materials and energy used (capital goods) in the construction of landfills (Ecobalance, 1999; Frischknecht et al., 2007; Menard et al., 2004).
Menard et al. (2004) compared the capital goods and operational performance of two landfill technologies: an engineered landfill and a bioreactor landfill. Menard et al. (2004) describe the engineered landfill with no utilisation of landfill gas and two scenarios for the bioreactor landfill: one with utilisation of landfill gas to electricity and another with utilisation for heat. They concluded that the two bioreactor scenarios had lower environmental impacts than the engineered landfill owing to the utilisation of the landfill gas and the fact that the engineered landfill had lower efficiency of the gas extraction, and thereby higher emissions to the atmosphere. The amount of materials and energy used in construction of the landfill were not significantly different for the two technologies (Menard et al. 2004). The quantification of materials for constructing the landfills determined by Menard et al. (2004) are shown later in this article for comparison with the current study.
The study by Frischknecht et al. (2007) was based on data from Ecoinvent (Ecoinvent, 2011) and did not present the amounts of materials needed for construction of landfills, but showed the impact assessment. For the capital goods related to waste management they found that the potential impacts depend on the type of waste included in the assessment. For the sanitary landfill and residual materials landfill the main impacts (>90%) were found to be on ‘Resources/Mineral Extraction’. For the categories ‘Land Use’, ‘Non-Renewable Cumulative Energy Demand’, ‘Climate Change’, ‘Acidification’ and ‘Terrestrial Ecotoxicity’ the impacts categories were significant (10–90%). The general conclusion was that ‘Land Use’ and ‘Resources/Mineral Extraction’ were the main factors related to the capital goods. The data concerning capital goods used by Frischknecht et al. (2007) are not presented within this article as they were not presented in the article by Frischknecht et al. (2007).
The report from Ecobalance (1999) is based on data from 100 landfills reporting their use of materials, landfill capacity, type of liner system and gas collection systems. The report presents ave-rage numbers for the landfills and constitutes a life cycle inventory (LCI) of a modern municipal solid waste landfill. Some of the results from the LCA of the full lifetime of the landfill are summarised by Camobreco et al. (1999). The data are presented later in this article.
Methods
The goal of this study was to quantify the use of materials and energy for constructing a modern landfill. Only the extraction and production of materials and the construction of the landfill are assessed in this study and therefore not the use and disposal phase of the landfill (see Figure 1). Maintenance and spare parts during the life time of the landfill are included, but the worn out parts from maintenance or substitution are not considered disposed until the disposal phase of the capital goods (above ground) of the landfill, which is not included in this article. The inventory data were furthermore used to model the environmental profile of the capital goods in order to assess their significance.
System boundary for the environmental impact assessment of capital goods. All energy and transportation are included.
Inventory data
The data on material use and landfill construction were defined for a hypothetical hill-type landfill with a capacity of approximately 4 million m3 constructed from scratch with an appropriate liner and leachate collection system, as well as a gas collection and utilisation system. All information is based on design reports for a range of landfills made by COWI or LFG Consult-two Danish companies that have, for decades, been involved in landfill design and construction.
The inventory is built in such a way that the data can be extrapolated easily to present a landfill with slightly different features or a different capacity. Adjustments to different landfill depths and wastes density can easily be made. Similarly, the inventory can also be modified to cover landfills located in a pit or canyon. The hill-type landfill is assumed to be constructed on a bare field with some soil excavated (see details later), while for a pit or canyon landfill excavation of soil before construction is not needed. The use of materials for the liner system was estimated to be 5% higher for a pit or canyon landfill than for the hill-type landfill with the same filling capacity due to the topographical difference. The energy consumption for constructing a landfill in a pit or canyon may be higher owing to the need for more energy for machinery operating on the slopes. The extra amount of energy needed in comparison to the hill-type landfill is, however, not specified.
Environmental profile
The landfill inventory data were subject to an environmental assessment in order to estimate the significance of the capital goods.
Data on the environmental profile of the materials and energy used in the construction of the landfill were, in most cases, obtained from the Swiss database Ecoinvent 2.2 (Ecoinvent, 2011). Some data were from environmental product declarations from companies producing the used products e.g. pumps (Flygt, 2012). All information about energy consumption by machi-nery during construction were from Caterpillar (2009) and Stripple (2001). Data were taken from published reports and documents where possible. However, in a few cases, data were obtained by personal communication with people from companies producing materials and components for landfills.
All concrete was modelled as not being reinforced because of limited LCI data. The steel used for reinforcement was included in all cases, but only as the production of steel owing to lack of data for reinforced concrete. The steel used for reinforcement is different from the steel used for pumps, containers, fence and gates. These two types of steel are also different with respect to their environmental profile.
The gravel roads were modelled as excavation of gravel at pit, transportation of gravel from pit to the site and distribution of the gravel on site by a dumper. Asphalt roads were included using LCI data from the Ecoinvent database (Ecoinvent, 2011).
Diesel consumption at the site was modelled as combusted in a EURO3-type truck as it was not possible to model it as used in the specific types of machinery (e.g. dumper) used at the landfill site. The actual emissions are likely to be larger.
The environmental impact assessment was made by means of Simapro 7.2 developed by PRé consultants (PRé, 2011). This software is user-friendly and was chosen because it contains comprehensive databases from, among others, the Ecoinvent 2.2 (Ecoinvent, 2011).
Emissions from the production of materials and energy were aggregated, characterised and normalised into the impact categories presented in Table 1. From the Environmental Design of Industrial Products (EDIP) methodology (Wenzel et al., 1997) the non-toxic categories chosen were ‘Global Warming’, ‘Ozone Depletion’, ‘Acidification’, ‘Terrestrial Eutrophication’, ‘Aquatic Eutrophication’ (nitrogen and phosphorous equivalents), ‘Pho-tochemical Ozone Formation’ (related to the impacts on vegetation and human health) and ‘Resource Depletion’. The latest updated normalisation references for 2004 were used (Laurent et al., 2011a).The toxicity related categories (‘Human Toxicity’, ‘Cancer’ and ‘Non-Cancer Related and Ecotoxicity’) were defined by the USEtox methodology (USEtox, 2009). The normalisation references for the USEtox methodology were determined by Laurent et al. (2011b) (see Table 1).
Environmental impact categories and normalisation references used for the assessment (Source: Laurent et al. 2011a, 2011b).
EDIP: Environmental Design of Industrial Products; CO2: carbon dioxide; CFC: chlorofluorocarbon; SO2: sulphur dioxide; UES: unprotected eco-system. CTU: comparative toxic unit; h: human; e: ecotoxicity.
Presentation of a hill-type landfill
The landfill described in this article is a constructed example of a hill-type landfill. The landfill covers 300,000 m2 and contains 30 cells with a total capacity of approximately 4 million m3 (4,075,776 m3).
The layout of the landfill area will always depend on the available space provided for constructing a landfill. In this article the landfill is assumed to be rectangular to ease the calculation of materials needed. The collected data can be applied easily to other shapes of landfills. The quantification of materials and energy are presented in Table 2 and described in the next section. The concept of the landfill construction is shown in Figure 2.
Materials and energy used for the construction of a hill-type landfill of 4 million m3.
Al: aluminium; reinf. steel: reinforcement steel; Cu: copper; HDPE: high density polyethylene; PP: polypropylene; PVC: polyvinylchloride; SCADA: Screen capture and data analysis.
Design of landfill, showing the membrane system, leachate collection and gas collection. HDPE: high density polyethylene.
Landfill elements
The construction of the landfill was divided into several construction elements in order to ease the presentation and organise the inventory data.
Transportation
All transportation at the landfill site was quantified as listed in Table 3. This was based on the amounts of materials needed and the machinery used for handling the materials. Capacities of the machinery (e.g. movement of m3/h) were used together with the fuel consumption per hour to quantify the amount of diesel consumed for the whole construction of the landfill. An efficiency of 50 minutes per hour was used for all machinery at the landfill, but full efficiency was assumed for the truck. See Table 3 for details about the machinery. Materials needed for producing the machinery were not quantified as only the landfill itself is considered in this article. The machinery could be used elsewhere before and after the use at the landfill, and is therefore not quantified here.
Machinery used for the construction of a landfill of 4 million m3. The capacity shown is at 100% efficiency of the machinery (60 minutes per hour).
Transportation of materials was assumed to take place by a single type of truck with a consumption of diesel of 0.03 litres per tonne and km. This number was calculated from the study by Stripple (2001). However, the diesel consumption for the transport by truck depends on the type, size and driving of the truck, and also on the load.
The distance driven with the materials are shown in Table 4. It is assumed that all materials are accessible within a distance of 15–50 km. This may not always be the case for all materials, but as the diesel consumption is linear with distance, adjustments can easily be made to fit specific needs. Transportation of diesel and asphalt was not included. The transportation of soil was only at site (<1 km).
Transport distances from production of materials and goods to site.
HDPE: high density polyethylene; PP: polyproylene; PVC: polyvinylchloride.
Preparation of the area
All plants and trees were cut down during preparation of the area. The energy consumption for cutting and removing the vegetation was quantified to be 0.08 l of diesel per m2 (ROAD-RES, 2005). The top soil (0–0.2 m) was removed and stored at the landfill site for later use. Also, the soil below (0.2–1 m) was removed and stored for later use. Lowering of the ground water table in the area of the landfill was not considered.
About 60,000 m3 (58,448 m3) of soil from the site was compacted for the construction of embankments. A bulldozer, a compactor and a dumper were used for preparation of the site. The details about diesel consumptions, etc. can be found in Table 3.
Bottom liner
The bottom liner consists of two layers: a clay layer of 0.5 m and on top a plastic membrane of high density polyethylene (HDPE) with a thickness of 0.0015 m (see Figure 2). The clay was excavated at a mine, transported by truck 15 km to the site, spread with a bulldozer and compacted by a compactor. Watering of the clay liner was not included.
The HDPE membrane was 300,000 m2 with a total weight of 423 tonnes. It was transported to the site as rolls by truck (25 km). The HDPE liner was rolled out and welded with a device using electricity from a diesel generator. The energy consumption for rolling out the HDPE liner and welding it was considered insignificant and no quantification was attempted.
Leachate drainage and management
Gravel for use as a drain layer (0.5 m) on top of the liner was transported 15 km to the site by truck and spread on site by a dumper. About 150,000 m3 of gravel was used for the drainage layer of the whole landfill. In total, 37,000 m of HDPE pipes of 0.110 m diameter suitable for pressure below 10 bar were used for draining the leachate. The pipes were transported to the site by truck and distributed on site with a front loader. Only half of the capacity of the front-loader was assumed to be used for distributing the pipes at site as the pipes have a high volume compared with the weight. Coarse gravel was used around the drainage pipes constituting a trapezium cross-sectional area of ~0.218 m2 being 3 times higher and 3 times wider than the pipe diameter of 0.110 m. Below the coarse gravel a 2-m-wide HDPE geo-textile was used (0.0015 m thickness). This geotextile was transported to the site by truck for 25 km. At the discharge leachate drainage points from each landfill cell a concrete plate was installed of 2 m × 3 m × 0.1 m to protect the outlet. In total, 30 discharge points were used in the 300,000 m2 large landfill. The concrete plates contained about 5% of reinforcement steel.
Two leachate collection and inspection wells of concrete were installed in each cell amounting to 60 wells in total for the landfill. The wells contained 5% steel reinforcement. The lifetime of these wells was estimated to be 30 years, so maintenance will be necessary. Measuring equipment for flow, electricity and screen capture and data analysis (SCADA) for the collection and inspection wells were not included.
Transportation pipes (in total: 8205 m) were installed for the transport of the leachate from the cells to the leachate tank. These pipes were of HDPE with a diameter 0.25 m and made for a pressure of 10 bar. The diesel consumption used for excavating the soil, laying the pipes, backfilling and compaction of the soil was included in the quantification. The wastewater treatment plant was outwith of the scope of this study.
Wells were installed for the cleaning of the transport pipes. These wells were 0.30 m in diameter and installed for each 100 m of pipe in the landfill. In total, 82 wells were included–each containing a polyvinylchloride pipe, HDPE bottom and polypropylene cap (Wavin, 2009). The excavation and soil work for the wells were included in the estimated soil works and thus the diesel consumption is accounted for in the inventory.
Leachate tank
The leachate tank was constructed in reinforced concrete and had a diameter of 30 m and a height of 3 m. The electronic installations included a breaker panel and estimated 100 m of cables and cable trays.
Pumping station
The station for pumping leachate from the landfill to the municipal wastewater treatment system was constructed in concrete with 5% reinforcement of steel (depth = 6 m, width = 5 m and length = 6 m ). Materials for connecting pipes, fittings, valves and an aluminium ladder of 6 m were quantified. The pumping station was assumed to use two pumps of 74 kg and a hydraulic power of 1.29 kW. Data about the pumps were provided by Flygt (Flygt, 2012). The SCADA system for the pumping station was estimated to contain 28 m of cable tray and cables, and 1 breaker panel. The breaker panel was not quantified owing to lack of data about the size.
Surface water collection/systems
Ditches were established for the collection of surface water. The excavated soil from this was stored for other purposes at the landfill. Ponds for the storing of collected surface water were not included. This kind of storage can be constructed in concrete and the water can be used for dust control during filling of the landfill.
Groundwater monitoring wells
Six groundwater monito-ring wells were established: three upstream and three downstream of the landfill. Data for these wells were obtained from a study from the Danish utility company Copenhagen Energy (B. Godskesen, personal communication).
Fence and gates
A fence and gates were installed in order to avoid illegal dumping and other unwanted activities at the landfill site. Data about gates and fence were obtained from the Danish company Ishøj Hegn (L. Visti, personal communication). A fence 2 m high surrounded the site, placed at a distance of 65 m from the landfill. The fence also surrounded the receiving area at the entrance of the site, which was estimated to be 40,000 m2. The two combined gates were each 3 m wide and 2 m high. The total length of the fence was approximately 3100 m.
Receiving area
Two weighbridges were installed in order to weigh all arriving and departing trucks. Data for the material composition of this were obtained from the Danish company Scanvægt (Scanvægt, 2012). Weighbridges of 18 m × 3 m were used. The concrete parts were estimated to have a weight of 49 tonnes and the compact ring-torsion load cells were assumed to consist only of steel and have a total weight of 0.21 tonnes.
Two buildings were placed in this area: one containing offices (200 m2 with 2 floors), the other building containing garages and a truck wash (total: 300 m2). The detailed data for the buildings were obtained from the Swiss database Ecoinvent (Ecoinvent, 2011).
Asphalt roads were used in the receiving area and to the landfill cells. The asphalt road was estimated as twice the length of the sides of the landfill (total: 1094 m). Gravel roads were used on the landfill body during the filling of the landfill. These were moved and changed as the cells were filled. A total of 1500 m of gravel roads were estimated.
An estimated number of 32 steel light stands were used to lighten the trafficked area. The electrical wiring was estimated to amount to 2800 m of electric cables. Cables for signals and signs were estimated to amount to 1000 m.
No arrangements, such as grates, bumps and washing facilities for wheels, as suggested by Bagchi (1994), were included. The washing hall was the only facility for cleaning the trucks.
Top cover and gas collection
When landfill cells are completed, the waste is levelled by a bulldozer and the gas collection system is installed. One vertical well (diameter: 0.600 m) was drilled and equipped in the centre with a perforated HDPE gas extraction pipe (diameter: 0.160 m), surrounded with gravel. One well was used per 2000 m2 of surface (or 5 wells per cell) for collecting the gas (see Figure 2). A HDPE membrane with a diameter of 10 m was installed around each well to limit atmospheric air to the well as the pumping of landfill gas creates a low pressure in the landfill. All wells were connected by HDPE pipes (diameter of 0.075 m) to a pumping system, and the landfill gas was collected for use in a gas motor at the site. The pumping system was placed in four 40-ft containers, each of 6000 kg and with 9897 kg of equipment. Gas motors and generators were placed in 4 other containers, each with a total weight of 25,000 kg. The top cover consisted of a layer of gravel (0.5 m) on top of the waste and a layer of soil (0.8 m), and, finally, the original top soil (0.2 m). A small flare of 2.3 tonnes of steel was installed at the site. Details on the flare were from Uniflare (Uniflare, 2012).
In some countries synthetic top liners are prescribed. The material use for this can be estimated from the data on the synthetic liner specified for the bottom liner. This is also applicable for additional layers of soil or clay for the top cover.
Soil balance
Soil was excavated to prepare the area and to provide materials for soil embankments and top cover. Table 5 reveals that more soil was stored than later used. The surplus soil was assumed to stay on the site for additional landscaping. Two types of soil were excavated during the preparation of the site; top soil (0–0.2 m) and subsoil (0.2–1 m); these were kept separate and used for different purposes. The soil excavated from ditches was used as back-fill.
Soil balance of the construction of a hill type landfill (4 million m3 and 300,000 m2). Positive numbers are tonnes of soil excavated and transported for temporary storage at the landfill site. Negative numbers are tonnes of soil taken from the storage.
Embankments were constructed from soil excavated from the site. The amount is shown in Table 2, but does not contribute to the soil balance in Table 5, as the soils was used directly and not stored before use.
Daily cover of the waste with soil was not included in the quantification as this is a part of the daily landfill operation and therefore not a part of the construction of the landfill.
Life time of materials and components
The estimated lifetimes of the components above ground in the landfill are shown in Table 6. These are used to calculate the need for maintenance and replacement of components. The time for operating the landfill is estimated to be 10 years and the aftercare period 30 years.
Lifetime and thereby need for maintenance or change of components for a landfill of 4 million m3.
Results and discussion
The results are presented in two parts: quantification of capital goods in terms of materials and energy used for the construction of the landfill, and the environmental profile of the capital goods as modelled by environmental impact assessment.
Capital goods (material and energy use) for construction of landfill
Aggregated data per tonne of waste for the materials used to build a hill-type landfill of 4 million m3 is presented in Table 7 and listed together with the results from Menard et al. (2004) and Ecobalance (1999). Some materials were reported only in this study: cables and reinforcement steel, as well as the small amounts of aluminium, copper (for pumps) and polypropylene.
Aggregated quantified amounts of materials used per ton of waste landfilled: this study and data reported by Menard et al. (2004) and Ecobalance (1999).
HDPE: high density polyethylene; PP: polypropylene; PVC: polyvinylchloride.
In this study infiltration of rain to the landfill was allowed to shorten the period of aftercare, while Ecobalance (1999) and Menard et al. (2004) used a full top cover to avoid infiltration of rain. Ecobalance (1999) reports a lower consumption of HDPE despite a top cover containing a HDPE liner and pipes. This could be due to the reported data being average numbers. Menard et al. (2004) included both a geo-net and geo-membrane, leading to a higher consumption of HDPE.
The use of concrete per tonne of waste was higher in this study than reported by Ecobalance (1999). Data from Ecobalance (1999) represented the average of 13 sites, where 8 had no consumption of concrete and 1 had a high consumption owing to a concrete road (Ecobalance, 1999). This explains the low average value reported by Ecobalance (1999). In the present study the total diesel consumption per tonne of waste was 0.24 l of diesel; Ecobalance (1999) reports 0.29 l fuel per tonne of waste, while Menard et al. (2004) report 0.52 l per tonne of waste. All studies included transportation to site and at site. Gravel and clay used in drainage layers, and as a geological barrier, respectively, were the materials used in the largest amounts. Ecobalance (1999) reports the use of gravel, and both Ecobalance (1999) and Menard et al. (2004) report the use of sand. Limited access to clay could be the reason for Menard et al. (2004) reporting use of a geosynthetic clay liner. Different design and use of material for the liner and leachate drainage system are the likely causes of these differences.
Parts of the landfill will be the same no matter the capacity and amount of waste landfilled. These parts constitute the ‘Basis’ and include receiving area with weighbridge, office buildings, monitoring wells, lighting, flare, roads and gates. The variable parts depend on the size of the landfill and these are top cover, liner system, gas and leachate collection system, fence and all wells, pipes and tanks. The ‘Basis’ contributes 16–40 % of the total impacts on ‘Aquatic Eutrophication’, ‘Resources’ and the toxicity categories (see Figure 3). For most of the non-toxicity impact categories the ‘Basis’ contributes approximately 10% of the total impacts (see Figure 3).
Contribution to EDIP impact categories (non-toxicity impact categories) and USEtox impact categories on ‘Human Toxicity’ related to cancer and non-cancer, and Ecotoxicity from the construction of the basis and the variable parts of a hill-type landfill of 4 million m3.
The use of materials is clearly defined by the design of the landfill. The design depends on regulations for protecting the ground water resource, for the length of the aftercare period and the final use of the site. A different liner system or top cover, thicker gravel/clay layers or different materials will change the total amounts of materials per tonne of waste.
Environmental impact assessment of capital goods use for construction of landfill
The impacts related to the capital goods used for construction of a hill-type landfill of 4 million m3 are presented in Figure 4. The potential impacts are given in milli person equivalents (mPE) per tonne of waste as ‘normalised impact potentials’. The results are presented in this section for each of the impact categories. The impact on ‘Ozone Depletion’ was not significant and is not shown.
Contribution to EDIP impact categories (non-toxicity impact categories) and USEtox impact categories on ‘Human Toxicity’ related to cancer and non-cancer, and Ecotoxicity from the construction of the assessed hill-type landfill of 4 million m3. All units are in milli person equivalents per tonne of waste. EP(N): emission potential expressed as nitrogen equivalents and EP(P) in phosphorous equivalents.
‘Global Warming’
It is mainly the production of plastic for liner and pipes and gravel excavation that cause the impacts on ‘Global Warming’. Other contributors to impacts on ‘Global Warming’ are the transportation of materials to the site and the use of machinery during construction.
‘Ozone Formation’ (impact on vegetation and humans)
The activities contributing the most to the ‘Ozone Formation’ was the excavation and production of gravel and clay. The impacts from these materials do not include the transportation to site as this is counted separately.
‘Acidification’
Use of machinery and thereby use of fuels for the excavation and preparation of clay and gravel gave the highest contribution to ‘Acidification’. Also, the use of fuel at site contributes to the impact on ‘Acidification’.
‘Terrestrial Eutrophication’
Transportation at the landfill site and the use of gravel gave the main impacts on ‘Terrestrial Eutrophication’. A lot of energy is used for excavating the large amounts of gravel used in the construction of the landfill.
‘Aquatic Eutrophication’ (nitrogen and phosphorous equivalents)
The major impact on ‘Aquatic Eutrophication’ was caused by the excavation and processing of gravel, and the use of machinery at the landfill site. One of the main contributors to the impact category ‘Aquatic Eutrophication’ expressed as phosphorous equivalents was the office and garage/carwash buildings constructed at the site. The buildings contribute due to the resources used for the building materials, but also the energy used during construction of the buildings. Another main contributor is the use of steel for, for example, gas management containers.
‘Resource Depletion’
The main impact on ‘Resource Depletion’ was caused by the use of steel, aluminium and copper, but also by the excavation and preparation of gravel. The gravel has lower impacts per kg material compared with the metals, but was used in a large quantity resulting in major contribution to the potential impact on ‘Resource Depletion’.
‘Human Toxicity’ (cancer/non-cancer)
The impacts on ‘Human Toxicity’ were caused mainly by the production of steel. For the cancer-related ‘Human Toxicity’ it was the steel used for equipment for gas collection and gas motors, fence and gates, and the reinforcement steel used in concrete constructions. For the non-cancer-related ‘Human Toxicity’ it was also the use of steel, but also the office and garage/carwash buildings at the site. For both impact categories the use of gravel caused a high impact because of energy use for excavation and processing.
Ecotoxicity
The impacts on ‘Ecotoxicity’ was caused by the use of steel for equipment used for gas management, fence and gates, but also reinforcing steel used for concrete structures. The use of gravel also caused major impacts on this category owing to energy used for excavation of the gravel at the mine.
Waste types
The capital goods used in construction depends on the waste landfilled. Materials for the gas collection system are only re-levant when organic waste is part of the waste landfilled. Gas and gas collection will not be an issue in landfills with inert and non-organic waste. Materials used for construction of a leachate collection and drainage system will usually be relevant as handling of leachate is needed for most kinds of modern landfills.
The material consumption per tonne of waste depends highly on the height of the waste landfilled and the bulk density of the waste in situ. Municipal solid waste can be compacted to 0.7–1 tonne per m3 (Christensen, 2011), although the compaction of the waste may depend on the scale of the landfill, as small sites often do not use heavy compactors, but prefer lighter and more versatile machinery on site. Less waste in the landfill will mean higher impacts from the capital goods per tonne of waste. The data presented in Figure 4 are calculated for an average height of 10 m, a maximum height of 20 m and with an average density of 0.85 tonnes per m3 corresponding to a total capacity of 3.5 million tons of waste (3,464,410 t). The data can be adjusted easily to represent other depths or densities.
Impacts per tonne of waste
The total impacts per tonne of waste are presented in Figure 4. The materials used in large quantities contribute the most to the potential impacts. Gravel and clay, as well as steel, contribute to all the impact categories. The impacts caused by transportation of materials and use of machinery at site are also significant, particularly for the ‘Ozone Formation’ impacts on vegetation and human health, ‘Acidification’ and ‘Terrestrial Eutrophication’. The potential impact on ‘Global Warming’ is 0.32 mPE per tonne of waste (see Table 8). Manfredi et al. (2010) reported a potential impact of 4 mPE per tonne of waste from the operation of a landfill with low organic waste. For a conventional landfill with energy recovery a contribution to ‘Global Warming’ was reported of −21 mPE/t wet waste (Manfredi and Christensen, 2009). This indicates that, with respect to ‘Global Warming’, the contribution of capital goods to the overall environmental profile of a landfill is small-to-insignificant (1.5–8%). The impact on ‘Resource Depletion’ was not assessed by Manfredi et al. (2010) or Manfredi and Christensen (2009), but is considered to be significant for the capital goods.
Potential impact on ‘Global Warming’ from the capital goods and references from literature.
Sensitivity of results
Transportation distances used for the quantification of diesel are estimated to be relatively short (15–50 km). The impact on ‘Global Warming’ constitutes 18% from the transportation of goods to the landfill. Changing the distance of the transporting the HDPE membrane from 25 km to 1000 km increases the total impact on ‘Global Warming’ by only 0.5%. Changing the transportation distance of gravel from 15 km to 100 km increases the impact on ‘Global Warming’ by 39%.
The potential environmental impacts depend on choice of materials and the design of the landfill. As an example a geo-membrane of HDPE covering the whole landfill to avoid infiltration of water would increase the environmental impact potentials by 40%.
The aftercare period of landfills is specified to be of minimum 30 years in both the European and the North American landfill regulations. Therefore, an aftercare period of 30 years was chosen for this study. Kjeldsen et al. (2002) described a need for longer aftercare period due to non-stabilised conditions in the landfills. The potential environmental impacts would be higher in this study, if a longer aftercare period was considered. The higher impacts would be caused by the use of steel in the capital goods (fence, gates and gas motor) needed for the longer period of time.
The result of the environmental impact assessment depends on the processes chosen from the available database. If processes with higher or lower impacts were used, the total impacts would differ. The processes used for the modelling are shown in Table 9. In this way the reader will be able to either do the same modelling or to change some of the processes if more relevant data are available.
Material processes used in modelling of the environmental profile of the capital goods.
HDPE: high density polyethylene; PP: polypropylene; PVC: polyvinylchloride; UCTE: Union for the Co-ordination of Transmission of Electricity.
Conclusions
Capital goods in terms of materials and energy used for constructing an engineered landfill of 4 million m3 were quantified and presented. The largest amounts of materials used were clay and gravel followed by concrete and plastic for liners and pipes. Expressed per tonne of waste the amounts of materials and energy used were of the same order as data provided by two earlier studies. Differences were apparent with respect to the amount of clay and gravel used, but this can likely be ascribed to the different design of the bottom liner and leachate drainage system. The data provided in the current study are very detailed and allow the reader to make inventories for other landfill designs. Capital goods as materials and energy used per tonne of waste are likely to depend on the requirements for liners, the depth of the landfill, the bulk density of the waste after landfilling and transportation. These factors are believed to be of much more importance than the uncertainty of the individual numbers provided in the inventory table based on a hypothetical hill-type landfill. Except for the receiving area with buildings and weighbridge, most use of materials and energy for construction of a landfill are linear with the size of the landfill.
The materials used in the largest amounts (clay and gravel) were also the materials contributing most to the environmental profile of the capital goods expressed as impact categories (‘Global Warming’, ‘Ozone Formation (vegetation and human)’, ‘Acidification’, ‘Terrestrial and Aquatic Eutrophication’, ‘Resource Depletion’ and ‘Human Toxicity (cancer)’). Plastics (liner, pipes) and metals (reinforcement, pumps, fence and gates) were used in smaller amounts, but have higher environmental impacts from their production, and thus also showed significant contribution to all impact categories. Plastic contributed mostly to ‘Global Warming’, ‘Ozone Formation (vegetation and human)’, ‘Acidification’, ‘Human Toxicity (cancer)’ and ‘Ecotoxicity’). The steel contributed mostly to ‘Aquatic Eutrophication’ (phosphorous equivalents), ‘Resource Depletion’, ‘Human Toxicity (cancer- and non-cancer-related)’ and ‘Ecotoxicity’.
The transport of materials to the landfill, and their transport and handling at the landfill contributed significantly to many impact categories in terms of diesel combustion [‘Global Warming’, ‘Ozone Formation (human)’, ‘Acidification’, ‘Terrestrial and Aquatic Eutrophication’ (nitrogen equivalents)].
The potential impact of the capital goods on ‘Global Warming’ was 0.32 mPE/t of waste, which is low-to-insignificant compared with the impact potential of 4 mPE/t waste from the operation of a landfill with low organic waste reported by Manfredi et al. (2010) and to –21 mPE/t of wet waste for a conventional landfill with energy recovery (Manfredi and Christensen, 2009). The impact of the capital goods on ‘Resource Depletion’, however, is significant owing to the use of gravel and steel.
Footnotes
Acknowledgements
The authors would like to thank the graduate school Residual Resource Research (3R) at the Technical University of Denmark for partly financing the scholarship for this PhD research.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.