Waste LCA and the future.

Life cycle assessment (LCA) models quantifying the environmental aspects of waste management have become an integral part of waste management decision-making over the last two decades and have provided ample knowledge on both environmental benefits and drawbacks in the way we handle waste. Waste management and LCA modelling of waste management systems will soon be challenged by profound changes necessary in our societies and sectors to meet sustainable development goals. Foreseen changes in energy, material, and nutrient provision will directly and indirectly affect waste management in terms of its operation and goals. This study reflects on anticipated changes in society and industrial sectors and how these changes may affect waste management and LCA modelling of waste management systems in terms of waste input, the modelling of technologies and systems and exchanges of energy, materials, and nutrients, as well as how it may affect impact assessment and the interpretation of results. The study provides practical recommendations for LCA modelling of future waste management systems, which will hopefully lead to robust assessments that can support decision-making in an evolving society subject to great changes.

Application of LCA modelling in integrated waste management.

Life cycle assessment (LCA) has been used in waste management for the last two decades and hundreds of journal papers have been published. The use of LCA in waste management has provided a much-improved holistic view of waste management including waste flows and potential environmental impacts. Although much knowledge has been obtained from LCA studies, there is still a need to use LCA models in integrated waste management. This paper describes six areas where LCA is expected to play a role in waste management in the future: 1) understanding an existing waste management system; 2) improving existing waste management systems; 3) comparing alternative technologies/ technology performance; 4) technology development/prospective technologies; 5) policy development/strategic development; and 6) reporting. Illustrative examples are provided for each application area.

A comprehensive substance flow analysis of a municipal wastewater and sludge treatment plant.

The fate of total organic carbon, 32 elements (Al, Ag, As, Ba, Be, Br, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, N, Na, Ni, P, Pb, S, Sb, Se, Sn, Sr, Ti, V, and Zn) and 4 groups of organic pollutants (linear alkylbenzene sulfonates, bis(2-ethylhexyl)phthalate, polychlorinated biphenyl and polycyclic aromatic hydrocarbons) in a conventional wastewater treatment plant were assessed. Mass balances showed reasonable closures for most of the elements. However, gaseous emissions were accompanied by large uncertainties and show the limitation of mass balance based substance flow analysis. Based on the assessment, it is evident that both inorganic and organic elements accumulated in the sewage sludge, with the exception of elements that are highly soluble or degradable by wastewater and sludge treatment processes. The majority of metals and metalloids were further accumulated in the incineration ash, while the organic pollutants were effectively destroyed by both biological and thermal processes. Side streams from the sludge treatment process (dewatering and incineration) back to the wastewater treatment represented less than 1% of the total volume entering the wastewater treatment processes, but represented significant substance flows. In contrast, the contribution by spent water from the flue gas treatment process was almost negligible. Screening of human and eco-toxicity by applying the consensus-based environmental impact assessment method USEtox addressing 15 inorganic constituents showed that removal of inorganic constituents by the wastewater treatment plant reduced the toxic impact potential by 87-92%.

Quantifying capital goods for waste incineration.

Materials and energy used for the construction of modern waste incineration plants were quantified. The data was collected from five incineration plants (72,000-240,000 tonnes per year) built in Scandinavia (Norway, Finland and Denmark) between 2006 and 2012. Concrete for the buildings was the main material used amounting to 19,000-26,000 tonnes per plant. The quantification further included six main materials, electronic systems, cables and all transportation. The energy used for the actual on-site construction of the incinerators was in the range 4000-5000 MW h. In terms of the environmental burden of producing the materials used in the construction, steel for the building and the machinery contributed the most. The material and energy used for the construction corresponded to the emission of 7-14 kg CO2 per tonne of waste combusted throughout the lifetime of the incineration plant. The assessment showed that, compared to data reported in the literature on direct emissions from the operation of incinerators, the environmental impacts caused by the construction of buildings and machinery (capital goods) could amount to 2-3% with respect to kg CO2 per tonne of waste combusted.

Home composting as an alternative treatment option for organic household waste in Denmark: An environmental assessment using life cycle assessment-modelling.

An environmental assessment of the management of organic household waste (OHW) was performed from a life cycle perspective by means of the waste-life cycle assessment (LCA) model EASEWASTE. The focus was on home composting of OHW in Denmark and six different home composting units (with different input and different mixing frequencies) were modelled. In addition, incineration and landfilling was modelled as alternatives to home composting. The most important processes contributing to the environmental impact of home composting were identified as greenhouse gas (GHG) emissions (load) and the avoided emissions in relation to the substitution of fertiliser and peat when compost was used in hobby gardening (saving). The replacement of fertiliser and peat was also identified as one of the most sensible parameters, which could potentially have a significant environmental benefit. Many of the impact categories (especially human toxicity via water (HTw) and soil (HTs)) were affected by the heavy metal contents of the incoming OHW. The concentrations of heavy metals in the compost were below the threshold values for compost used on land and were thus not considered to constitute a problem. The GHG emissions were, on the other hand, dependent on the management of the composting units. The frequently mixed composting units had the highest GHG emissions. The environmental profiles of the home composting scenarios were in the order of -2 to 16 milli person equivalents (mPE) Mg(-1) wet waste (ww) for the non-toxic categories and -0.9 to 28mPEMg(-1) ww for the toxic categories. Home composting performed better than or as good as incineration and landfilling in several of the potential impact categories. One exception was the global warming (GW) category, in which incineration performed better due to the substitution of heat and electricity based on fossil fuels.

Mass balances and life cycle inventory of home composting of organic waste.

A comprehensive experimental setup with six single-family home composting units was monitored during 1 year. The composting units were fed with 2.6-3.5 kg organic household waste (OHW) per unit per week. All relevant consumptions and emissions of environmental relevance were addressed and a full life-cycle inventory (LCI) was established for the six home composting units. No water, electricity or fuel was used during composting, so the major environmental burdens were gaseous emissions to air and emissions via leachate. The loss of carbon (C) during composting was 63-77% in the six composting units. The carbon dioxide (CO(2)) and methane (CH(4)) emissions made up 51-95% and 0.3-3.9% respectively of the lost C. The total loss of nitrogen (N) during composting was 51-68% and the nitrous oxide (N(2)O) made up 2.8-6.3% of this loss. The NH(3) losses were very uncertain but small. The amount of leachate was 130 L Mg(-1) wet waste (ww) and the composition was similar to other leachate compositions from home composting (and centralised composting) reported in literature. The loss of heavy metals via leachate was negligible and the loss of C and N via leachate was very low (0.3-0.6% of the total loss of C and 1.3-3.0% of the total emitted N). Also the compost composition was within the typical ranges reported previously for home composting. The level of heavy metals in the compost produced was below all threshold values and the compost was thus suitable for use in private gardens.

Greenhouse gas emissions from home composting of organic household waste.

The emission of greenhouse gases (GHGs) is a potential environmental disadvantage of home composting. Because of a lack of reliable GHG emission data, a comprehensive experimental home composting system was set up. The system consisted of six composting units, and a static flux chamber method was used to measure and quantify the GHG emissions for one year composting of organic household waste (OHW). The average OHW input in the six composting units was 2.6-3.5 kg week(-1) and the temperature inside the composting units was in all cases only a few degrees (2-10 °C) higher than the ambient temperature. The emissions of methane (CH(4)) and nitrous oxide (N(2)O) were quantified as 0.4-4.2 kg CH(4)Mg(-1) input wet waste (ww) and 0.30-0.55 kg N(2)OMg(-1)ww, depending on the mixing frequency. This corresponds to emission factors (EFs) (including only CH(4) and N(2)O emissions) of 100-239 kg CO(2)-eq.Mg(-1)ww. Composting units exposed to weekly mixing had the highest EFs, whereas the units with no mixing during the entire year had the lowest emissions. In addition to the higher emission from the frequently mixed units, there was also an instant release of CH(4) during mixing which was estimated to 8-12% of the total CH(4) emissions. Experiments with higher loads of OHW (up to 20 kg every fortnight) entailed a higher emission and significantly increased overall EFs (in kg substance per Mg(-1)ww). However, the temperature development did not change significantly. The GHG emissions (in kg CO(2)-eq.Mg(-1)ww) from home composting of OHW were found to be in the same order of magnitude as for centralised composting plants.

Energy recovery from waste incineration: assessing the importance of district heating networks.

Municipal solid waste incineration contributes with 20% of the heat supplied to the more than 400 district heating networks in Denmark. In evaluation of the environmental consequences of this heat production, the typical approach has been to assume that other (fossil) fuels could be saved on a 1:1 basis (e.g. 1GJ of waste heat delivered substitutes for 1GJ of coal-based heat). This paper investigates consequences of waste-based heat substitution in two specific Danish district heating networks and the energy-associated interactions between the plants connected to these networks. Despite almost equal electricity and heat efficiencies at the waste incinerators connected to the two district heating networks, the energy and CO(2) accounts showed significantly different results: waste incineration in one network caused a CO(2) saving of 48 kg CO(2)/GJ energy input while in the other network a load of 43 kg CO(2)/GJ. This was caused mainly by differences in operation mode and fuel types of the other heat producing plants attached to the networks. The paper clearly indicates that simple evaluations of waste-to-energy efficiencies at the incinerator are insufficient for assessing the consequences of heat substitution in district heating network systems. The paper also shows that using national averages for heat substitution will not provide a correct answer: local conditions need to be addressed thoroughly otherwise we may fail to assess correctly the heat recovery from waste incineration.

Waste collection systems for recyclables: an environmental and economic assessment for the municipality of Aarhus (Denmark).

Recycling of paper and glass from household waste is an integrated part of waste management in Denmark, however, increased recycling is a legislative target. The questions are: how much more can the recycling rate be increased through improvements of collection schemes when organisational and technical limitations are respected, and what will the environmental and economic consequences be? This was investigated in a case study of a municipal waste management system. Five scenarios with alternative collection systems for recyclables (paper, glass, metal and plastic packaging) were assessed by means of a life cycle assessment and an assessment of the municipality's costs. Kerbside collection would provide the highest recycling rate, 31% compared to 25% in the baseline scenario, but bring schemes with drop-off containers would also be a reasonable solution. Collection of recyclables at recycling centres was not recommendable because the recycling rate would decrease to 20%. In general, the results showed that enhancing recycling and avoiding incineration was recommendable because the environmental performance was improved in several impact categories. The municipal costs for collection and treatment of waste were reduced with increasing recycling, mainly because the high cost for incineration was avoided. However, solutions for mitigation of air pollution caused by increased collection and transport should be sought.

Life cycle assessment of disposal of residues from municipal solid waste incineration: recycling of bottom ash in road construction or landfilling in Denmark evaluated in the ROAD-RES model.

Two disposal methods for MSWI bottom ash were assessed in a new life cycle assessment (LCA) model for road construction and disposal of residues. The two scenarios evaluated in the model were: (i) landfilling of bottom ash in a coastal landfill in Denmark and (ii) recycling of bottom ash as subbase layer in an asphalted secondary road. The LCA included resource and energy consumption, and emissions associated with upgrading of bottom ash, transport, landfilling processes, incorporation of bottom ash in road, substitution of natural gravel as road construction material and leaching of heavy metals and salts from bottom ash in road as well as in landfill. Environmental impacts associated with emissions to air, fresh surface water, marine surface water, groundwater and soil were aggregated into 12 environmental impact categories: Global Warming, Photochemical Ozone Formation, Nutrient Enrichment, Acidification, Stratospheric Ozone Depletion, Human Toxicity via air/water/soil, Ecotoxicity in water/soil, and a new impact category, Stored Ecotoxicity to water/soil that accounts for the presence of heavy metals and very persistent organic compounds that in the long-term might leach. Leaching of heavy metals and salts from bottom ash was estimated from a series of laboratory leaching tests. For both scenarios, Ecotoxicity(water) was, when evaluated for the first 100 yr, the most important among the twelve impact categories involved in the assessment. Human Toxicity(soil) was also important, especially for the Road scenario. When the long-term leaching of heavy metals from bottom ash was evaluated, based on the total content of heavy metals in bottom ash, all impact categories became negligible compared to the potential Stored Ecotoxicity, which was two orders of magnitudes greater than Ecotoxicity(water). Copper was the constituent that gave the strongest contributions to the ecotoxicities. The most important resources consumed were clay as liner in landfill and the groundwater resource which was potentially spoiled due to leaching of salts from bottom ash in road. The difference in environmental impacts between landfilling and utilization of bottom ash in road was marginal when these alternatives were assessed in a life cycle perspective.

Assessment of long-term leaching from waste incineration air-pollution-control residues.

Assessment of long-term leaching from MSWI air-pollution-control (APC) residues is discussed with respect to use in environmental impact assessment, such as life-cycle assessment (LCA). A method was proposed for estimating leaching as a function of the liquid-to-solid (L/S) ratio in a long-term perspective (L/S 5000l/kg). Data for changes in residue pH as a function of L/S was used in combination with pH dependent leaching data to predict leachate concentrations of Al, Ca, Cd, Ba, Mg, Ni, Pb, S, Pb, V and Zn as a function of L/S. Mass balance calculations were used to determine the element fractions leached with respect to L/S. The estimated long-term leaching from a semi-dry residue and a fly ash was compared with short-term leaching determined by batch tests at L/S 10l/kg, both carbonated and non-carbonated versions of the residues were investigated. Generally, very high L/S ratios above 2000l/kg were required to leach 20-30% of the solid contents. However, Ca and S were depleted at L/S 200-900l/kg. The long-term leachate concentrations were found to either remain at the same level as the initial leaching determined by the L/S 10 batch test, or to significantly decrease compared with the initial leaching. Only Al and Zn were found to show higher leachate concentrations at L/S ratios above 3000-5000l/kg. Carbonation generally prolonged the time needed for depletion from the solid residues; however, Ca and S were depleted faster than in the case of non-carbonated residues. This study shows that uncritical use of batch leaching data for assessing the potential leaching is highly problematic, and evaluations of residue disposal should include scenario specific quantification of the long-term leaching.

Chromium release from waste incineration air-pollution-control residues.

Cr release overtime was investigated in batch experiments for eleven air-pollution-control residues from eight different municipal solid waste incinerators covering all majorflue gas cleaning technologies. Cr released during 168 h of contact with water showed significant variations among the residues studied. Also for the individual residue, large variations were observed depending on the liquid-to-solid ratio used in the leaching test and the degree of carbonation. It is argued that Al(0) present in the residues can control Cr leaching by reducing Cr(VI) released from the solid phase by dissolution and that exposure to oxygen-either prior to or during the leaching test-depletes the reduction capacity of Al(0) leading to increased Cr leaching. A dynamic model is shown to describe Cr release from all investigated residues by accounting for Al(0) oxidation with Cr(VI), O2, and water as well as Cr(VI) dissolution. The paper reveals that Al-O2-Cr(VI) interactions must be considered very carefully when interpreting Cr leaching data.

Xenobiotic organic compounds in leachates from ten Danish MSW landfills--chemical analysis and toxicity tests.

A monitoring program comprising chemical analysis and biological toxicity testing of leachate samples from 10 Danish landfills (six engineered and four uncontrolled) revealed the presence of 55 different xenobiotic organic compounds (XOCs) and 10 degradation products of XOCs. The compounds belong to the following groups: BTEX, C3-benzenes, bicyclo compounds, napthalenes, chlorinated aliphatics, phenols (chloro-, methyl-, dimethyl, nonyl-), pesticides, and phthalates. Concentrations of single XOCs ranged from <0.1 to 2220 microg/L. A pesticide screening including 101 different compounds resulted in detection of 18 pesticides and three degradation products. The findings of degradation products of toluene, phenols, phthalates, pesticides, and nonylphenol ethoxylates show that degradation occurred inside the landfills. In biotests with bacteria and algae it was found that the non-volatile organic compounds were toxic as the samples only needed to be pre-concentrated from 1.3 to 9.4 times to give 50% inhibition of the test organisms. One of the ten samples proved to be genotoxic in the umuC test after 141 times pre-concentration. A major part of the organic chemicals causing toxicity remains unknown and it is recommended to combine chemical analyses and biotests in future monitoring programs.

Leaching from solid waste incineration ashes used in cement-treated base layers for pavements.

Waste incineration bottom ash and treated flue gas cleaning products mixed with 2.5% of cement (50 kg/m3) were tested in the laboratory in terms of compressive strength and tank leaching tests over a 64-day period. Although the material displayed lower mechanical strength than a reference concrete, the strength still was sufficient for use as a base layer for roads. The metal content in the incineration-residue-based specimens was up to 100 times higher than in the reference concrete, suggesting that the mixed waste incineration residue should be used only for dedicated purposes. The leaching of Cl and Na was increased by a factor of 20-100 from the incineration-residue-based specimens as compared to the reference, while the leaching of K, Ca and SO4 was increased by a factor of 2-10. The leaching of heavy metals was also higher from the incineration-residue-based specimens than from the reference with respect to Cu (50 times), Cd, Pb and Zn (5 times), but not with respect to Cr and Ni. The leaching curves did only allow for a closer evaluation of the leaching process in a few cases. The physical retention of the constituents seemed to be the same in the reference as in the incineration-residue-based specimens. Heavy metal leaching was limited by enhanced chemical retention in the incineration-residue-specimens as compared to the reference. Since no quality criteria in terms of leaching from a monolithic material are currently available, the leaching issue must be evaluated case by case.

Chemical characteristics and methane potentials of source-separated and pre-treated organic municipal solid waste.

A research project has investigated the biogas potential of pre-screened source-separated organic waste. Wastes from five Danish cities have been pre-treated by three methods: screw press; disc screen; and shredder and magnet. This paper outlines the sampling procedure used, the chemical composition of the wastes and the estimated methane potentials.

On-site treatment and landfilling of MSWI air pollution control residues.

Air pollution control (APC) residues from municipal solid waste incineration (MSWI) are difficult to landfill due to substantial leaching of trace metals. An on-site pretreatment prior to landfilling of APC-residues was investigated in terms of bench-scale experiments with a semidry APC-residue and a fly ash. The treatment involved mixing of the residues with a ferrous sulphate solution and subsequent oxidation of the suspension. Afterwards, the suspension was spread on a dedicated landfill section and allowed to drain by gravity through the drainage system of the landfill. The wastewater from the process, collected through the drainage system, contained large concentrations of salts (Cl: 14-30 g/l, Na: 4-9 g/l, K: 5-11 g/l, Ca: 2-12 g/l) but low concentrations of trace metals (e.g. Pb: 14-100 microg/l, Cd: <2-7 microg/l). The treated residues left in the landfills were later subject to leaching by simulated rainfall. The leachate contained low concentrations of trace metals (Pb: <120 microg/l, Cd: <2 microg/l, Cr: <485 microg/l). The leachate concentrations from the treated APC-residues were substantially reduced compared to concentrations in leachate from untreated APC-residues. Particularly in the early stages of the leaching, concentrations of trace metals were reduced by up to four orders of magnitude.

Treatment of waste incinerator air-pollution-control residues with FeSO4: concept and product characterisation.

This paper describes a new concept for treatment of air pollution-control (APC) residues from waste incineration and characterises the wastewater and stabilised residues generated by the process. The process involves mixing of APC-residues with a ferrous sulphate solution and subsequent oxidation of the suspension (Ferrox-process 1996). The process results in a significant reduction in the leaching of salts and heavy metals from the residue, by washing out most of the salts and by binding the heavy metals in the iron oxides formed. In the laboratory, a semidry gas-cleaning residue and a fly ash were treated by the process. The generated wastewater contained low concentrations of heavy metals (e.g. Pb: 27-39 microg l(-1) and Cd: 2.6-4.6 microg l(-1)), but high concentrations of salts (e.g. Cl, Na, K, and Ca). The treatment process redUced the leaching of Pb from the residues by more than two orders of magnitude at fixed pH as determined by pH-static leaching tests. Likewise, the leaching of Cd, Zn and Cu was significantly reduced. The effect on elements that form oxyanions (e.g. Cr) is marginal and in the current process there is no reduction in the release of Hg.

Treatment of waste incinerator air-pollution-control residues with FeSO4: laboratory investigation of design parameters.

The key design parameters of a new process for treatment of air-pollution-control (APC) residues (the Ferrox-process) were investigated in the laboratory. The optimisation involved two different APC-residues from actual incinerator plants. The design parameters considered were: amount of iron oxide supplied, the liquid-to-solid ratio of the process, the separation of solids and wastewater, the sequence of material mixing, the possibilities of reuse of water, the feasibility of using secondary (brackish) water, and simple means to improve the wastewater quality. The investigation showed that an optimum process configuration could be obtained yielding a stabilised solid product with low leaching of heavy metals and a dischargable wastewater with high contents of salts (in order to remove salts from the solid product) and low concentrations of heavy metals. The amount of iron added to the APC-residues must be optimised for each residue. The overall water use can be limited to a L/S-ratio of 3 l kg(-1) including water used for washing of the treated products.

A simple model for the distribution and fate of organic chemicals in a landfill: MOCLA.

A simple mathematical model (MOCLA: Model for Organic Chemicals in Landfills) is presented, describing the distribution of organic chemicals between leachate, gas and solid waste. The model also predicts the fate of the chemicals in terms of emissions with leachate and landfill gas and in terms of degradation and transformation in the landfill. Local equilibrium is assumed for the distribution of the chemicals in the landfill as expressed by Henry's Law for the leachate-gas interface, and by the linear partition coefficient based on the waste solid organic carbon content for the waste-leachate interface. Degradation of the chemicals is expressed as a first order reaction. Annual specific leachate and gas generation data in combination with data on landfill area and volume allow for prediction of main emission routes. Model simulations involving two landfill scenarios for a number of chemicals with different physico-chemical characteristics indicate that volatilisation is a likely route for some chemicals (e.g. vinyl chloride, and some of the freons), while other chemicals (e.g. phenol, lower chlorinated aliphatic compounds) more likely will appear as dissolved in the leachate. However, many chemicals will be strongly associated with the solid waste (e.g. dichlorobenzene, naphthalene, and higher Polycyclic Aromatic Hydrocarbons (PAHs), and the pool available will be able to supply the landfill gas and leachate with organic trace components for decades unless these are degraded in the landfill. The model suggests that, in addition to the physico-chemical characteristics of the organic chemical, the presence of free phases of organic chemicals and the degradability of the chemicals are the main factors controlling the fate and emissions of organic chemicals in the landfill environment.

High temperature co-treatment of bottom ash and stabilized fly ashes from waste incineration.

Bottom ashes from two Danish municipal solid waste incineration plants were heated at 900 degrees C with iron oxide stabilized air pollution control residues at actual mass flow ratios (9:1), simulating a treating method for the residues. The two residues were co-treated, producing one combined stream that may be utilized as a secondary road construction material. Scanning electron microscope analysis and grain size distribution analysis indicated that sintering of the particles did not occur. Batch leaching tests at liquid/solid 10 l/kg at a range of pH-values (6-10) quantified with respect to Cd, Cr and Pb revealed significant positive effects of co-heating the ashes, although Pb showed slightly increased leaching. At a liquid/solid ratio of 10 l/kg the leachate concentrations were still low at pH 7-10 and the release of Pb was, thus, not expected to limit the utilization of the mixed ashes. The process, thus, fixates the metals in the solid residues without altering the leaching properties of the bottom ash too significantly.