Impact of urbanism on source separation systems: A life cycle assessment

This study aims to assess the effect of different urban configuration regarding the choice of wastewater management of the district with source separation systems. Understanding this link can guide researchers, and also urban actors, in order to choose the best source separation solution to implement in a specific urban configuration. For this purpose, an integrated modelling approach was used to model the district with different types of urban planning, the water resources recovery facility (WRRF) and create a life cycle inventory to carry out a life cycle assessment (LCA). Six different urban configurations were tested with three different source separation scenarios and compared with an advanced WRRF with high level of nutrients and organic matter recovery. This study concludes that urine source separation is beneficial compared to advanced WWRF for all the urban configurations. Sewer construction was identified as the main contributor to environmental impact for the low-density configuration (pavilions), limiting the benefits of source separation in this urban settlement. Blackwater separation with a decentralised treatment is only beneficial for high densely populated area. Treatment of blackwater and greywater for reuse, has greater impact than reference scenario, in all urban configurations, due to high energy consumption for greywater treatment. Future research should therefore explore technical solutions for limiting the energy consumption.

Methodological framework for Life Cycle Assessment of sustainable aviation (SA) systems

A comprehensive framework is proposed for Life Cycle Assessment (LCA) in the field of commercial aviation (passengers and cargo), capable to ensure transparency and comparability when evaluating the overall environmental performances of four emerging aviation systems, i.e., biofuels, electrofuels, electric, and hydrogen. The projected global revenue passenger kilometer (RPK) and is suggested as the functional unit for two timeframes representing near-term (2035) and long-term (2045), and for two segments, namely domestic and international. To solve the difficulty of comparing liquid fuels and electric aviation, the framework proposes a methodology to translate projected RPK into energy requirements for each of the studied sustainable aviation systems. Generic system boundaries are defined with their key activities for all four systems, with the biofuel system being sub-divided into two categories to distinguish whether it stems from residual or land-dependent biomass. The activities are grouped in seven categories: (i) conventional (fossil-based) kerosene activity, (ii) conversion processes from feedstock supply (to fuel or energy production for aircraft operation), (iii) counterfactual uses of constrained resources and displacement effects associated to co-products management, (iv) aircraft manufacture, (v) aircraft operation, (vi) additional infrastructure needed, and (vii) end-of-life management (aircraft and batteries). Considering applying regulations, the framework also includes a methodology to handle: (i) hybridization (the use of more than one source of energy/propulsion system to power an aircraft), (ii) the mass penalty affecting the number of carried passengers in some of the systems, and (iii) impacts stemming from non-CO₂ tailpipe emissions - aspects that are currently neglected in most LCA studies. The proposed framework builds upon the most recent knowledge in the field; however, some choices are dependent on upcoming scientific advances concerning e.g., tailpipe emissions at high altitude and their environmental impacts, new aircraft configuration, etc., and are subject to significant uncertainties. Overall, this framework provides a guideline for LCA practitioners addressing new energy sources for future aviation.

From hemp grown on carbon-vulnerable lands to long-lasting bio-based products: Uncovering trade-offs between overall environmental impacts, sequestration in soil, and dynamic influences on global temperature

In this study, the potential of carbon storage in soil combined with mitigation via bio-based products is investigated for the case of 100 years of hemp cultivation on carbon-vulnerable land (CV-lands) in France. The originality of this study lies in the coupling of soil organic carbon (SOC) simulations (over 100 years of hemp cultivation) with consequential life cycle assessment (LCA) to investigate the mitigation potential of different environmental impacts, and the coupling with dynamic LCA to investigate the long-term effects on global warming. When hemp stems (straw) are left on the ground, SOC increases of 25.8 t ha^-1 are observed over 100 years. However, the greenhouse gas (GHG) emissions that result from diverting the initial land use to hemp cultivation cannot be compensated for and, therefore, this scenario cannot mitigate global warming or most other impacts. Two long-lasting product scenarios were studied: insulation boards in buildings and car panels, both involving the production of hemp concrete as co-product. Our study shows that, even though no additional long-term carbon sequestration in soil could be achieved, both scenarios ensured a long-term climate benefit well beyond 2100, mostly because of carbon sequestered in the hemp-based products but also as a result of avoided fossil-based products. Uncertainty analyses reveal that the yield is the most influential parameter, inducing significant uncertainties in all scenarios and most impact categories. According to the overall results obtained, the car panel scenario is the most promising pathway with the lowest environmental impacts and the highest potential for long-term global warming mitigation; this is in part due to the reduction of fuel consumption during the use phase.

Circular bioeconomy: Life cycle assessment of scaled-up cascading production from orange peel waste under current and future electricity mixes

Orange peel waste (OPW) is present in large quantities both locally and globally, which makes them feasible input into the circular bioeconomy. However, due to their antimicrobial and anti-nutritional activity, they are problematic biomass, and proper waste management is yet to be determined. This life cycle assessment (LCA) quantifies the environmental performance of biorefinery producing limonene, citric acid, and animal feed from OPW generated from juice factories. Only previously assessed sustainable technologies were considered (cold press, microwave extraction, solid-state fermentation). The life cycle inventories were refined by a scale-up procedure to reflect industrial production at i) 0.5 t, ii) 100 t, and iii) 1000 t of OPW weekly. The data were translated per functional unit of 1 tonne OPW. Three electricity mixes and both attributional (average) and consequential (marginal) inventories were compared. Results showed that the environmental performance, in particular for climate change, was essentially dependent upon the electricity input, with 4388 CO₂ eq for current electricity mix, 2404 CO₂ eq using renewable, and 594 CO₂ eq using electricity from wind. Business-as-usual scenarios for OPW (incineration and animal feed) showed better performance in most scenarios, representing -150 CO₂ eq (animal feed) and -135 CO₂ eq (incineration) in the climate change. Lower impacts are reached due to avoided impacts of fossil fuel use and conventional feed cultivation. Renewable energy had better environmental performance than the current electricity mix, except for eutrophication, due to digestate spreading associated with biogas use, calling for mitigation action. Our results suggest that stopping the biorefinery processes immediately after the recovery of limonene via solvent-free microwave extraction process, with subsequent use of the dried OPW as animal feed, is the most environmentally performant option. This represents a feasible strategy for the circular bioeconomy and is in line with the updated food waste hierarchy.

Process-based LCA of ultrafiltration for drinking water production

Researchers and industrials need decision-making tools to make informed decisions on environmental mitigation strategies and proceed with the overall ecodesign of processes. In this study, a tool that couples membrane filtration process modelling and life cycle analysis has been developed, for which material and energy flows are calculated for variable operating conditions and are the basis for environmental impact assessment. The resulting generic model has been applied to dead-end ultrafiltration of ground and surface waters for drinking water production with cellulose triacetate hollow fibers. Operating strategies have been investigated to mitigate environmental impacts of the two major hotspots (electricity and backwash cleaning chemical consumptions). Adjusting filtration cycle duration and filtration flux has shown to be a promising lever. The developed model is sufficiently flexible and modular for its adaptation to other membrane materials, filtration configurations (i.e. cross-flow) as well as to other applications.

A step closer to circular bioeconomy for citrus peel waste: A review of yields and technologies for sustainable management of essential oils

This study presents a critical overview of reported essential oil (EO) extractions from citrus peel wastes (CPW), including harmonized data on the various citrus species and cultivars. Harmonization is vital to enable sustainable management practices. The review only includes eco-efficient extraction techniques. In total, the review contains 66 quantified examples using i) mechanical cold press ii) thermal extraction with water or steam media iii) thermal microwave-assisted extraction iv) other innovative methods (such as ultrasound). The technologies were assessed for their potential use in cascading production to achieve economies of scope, particularly considering the use of extraction residues for subsequent fermentation to produce various products from energy carriers to enzymes. Two techniques were found insufficient for direct use in fermentation. Cold press extracts an inadequate amount of EO (average yield 2.85% DW) to ensure suitable fermentation, while solvent extraction contaminates the residues for its subsequent use. Extractions using water media, such as hydrodistillation and microwave-assisted hydrodistillation (average EO yield 2.87% DW), are feasible for the liquid-based fermentation processes, such as submerged fermentation. Steam extraction is feasible for any type of fermentation. Our review highlighted solvent-free microwave extraction (average EO yield 5.29% DW) as the most effective method, which provides a high yield in a short extraction time. We also uncovered and discussed several inconsistencies in existing yields and energy consumption published data.

Addressing temporal considerations in life cycle assessment

In life cycle assessment (LCA), temporal considerations are usually lost during the life cycle inventory calculation, resulting in an aggregated "snapshot" of potential impacts. Disregarding such temporal considerations has previously been underlined as an important source of uncertainty, but a growing number of approaches have been developed to tackle this issue. Nevertheless, their adoption by LCA practitioners is still uncommon, which raises concerns about the representativeness of current LCA results. Furthermore, a lack of consistency can be observed in the used terms for discussions on temporal considerations. The purpose of this review is thus to search for common ground and to identify the current implementation challenges while also proposing development pathways. This paper introduces a glossary of the most frequently used terms related to temporal considerations in LCA to build a common understanding of key concepts and to facilitate discussions. A review is also performed on current solutions for temporal considerations in different LCA phases (goal and scope definition, life cycle inventory analysis and life cycle impact assessment), analysing each temporal consideration for its relevant conceptual developments in LCA and its level of operationalisation. We then present a potential stepwise approach and development pathways to address the current challenges of implementation for dynamic LCA (DLCA). Three key focal areas for integrating temporal considerations within the LCA framework are discussed: i) define the temporal scope over which temporal distributions of emissions are occurring, ii) use calendar-specific information to model systems and associated impacts, and iii) select the appropriate level of temporal resolution to describe the variations of flows and characterisation factors. Addressing more temporal considerations within a DLCA framework is expected to reduce uncertainties and increase the representativeness of results, but possible trade-offs between additional data collection efforts and the increased value of results from DLCAs should be kept in mind.

Modeling equations and dataset of model parameters for ultrafiltration membrane fabrication

In the related research article, entitled “A generic process modeling ‒ LCA approach for UF membrane fabrication: Application to cellulose acetate membranes” [1], a generic model is described and used to obtain the list of material and energy flows as a function of operating conditions for ultrafiltration (UF) hollow fibers preparation by non-solvent induced phase separation. In this data article, equations of the model, a dataset of model parameters and modelled data are detailed. modeling equations are developed from material and energy balances for each unit operation (i.e. from polymer solution mixing to module conditioning) based on an industrial membrane fabrication process of UF cellulose acetate modules. These equations may be reused as such or adapted to other membrane materials and industrial practices. The dataset of model parameters relates to industrial on-site measurements and scientific literature for the existing cellulose-based module. The modelled data corresponds to a reference situation for which hollow fibers (inner and outer diameters equal to 0.93 mm and 1.67 mm, respectively) are fabricated from a polymer solution composition of 20 wt.% of cellulose triacetate, 78 wt.% N-methyl-2-pyrrolidone and 2 wt.% lithium chloride.

Sensitivity analysis of temporal parameters in a dynamic LCA framework

Including the temporal dimension in the Life Cycle Assessment (LCA) method is a very recent research subject. A complete framework including dynamic Life Cycle Inventory (LCI) and dynamic Life Cycle Impact Assessment (LCIA) was proposed with the possibility to calculate temporal deployment of climate change and ecotoxicity/toxicity indicators. However, the influence of different temporal parameters involved in the new dynamic method was not still evaluated. In the new framework, LCI and LCIA results are obtained as discrete values in function of time (vectors and matrices). The objective of this study is to evaluate the influence of the temporal profile of the dynamic LCI and calculation time span (or time horizon in conventional LCA) on the final LCA results. Additionally, the influence of the time step used for the impact dynamic model resolution was analysed. The range of variation of the different time steps was from 0.5day to 1year. The graphical representation of the dynamic LCA results shown important features such as the period in time and the intensity of the worst or relevant impact values. The use of a fixed time horizon as in conventional LCA does not allow the proper consideration of essential information especially for time periods encompassing the life time of the studied system. Regarding the different time step sizes used for the dynamic LCI definition, they did not have important influence on the dynamic climate change results. At the contrary, the dynamic ecotoxicity and human toxicity impacts were strongly affected by this parameter. Similarly, the time step for impact dynamic model resolution had no influence on climate change calculation (step size up to 1year was supported), while the toxicity model resolution requires adaptive time step definition with maximum size of 0.5day.

Operational integration of time dependent toxicity impact category in dynamic LCA

Life Cycle Assessment (LCA) is the most widely used method for the environmental evaluation of an anthropogenic system and its capabilities no longer need to be proved. However, several limitations have been pointed out by LCA scholars, including the lack of a temporal dimension. The objective of this study is to develop a dynamic approach for calculating the time dependent impacts of human toxicity and ecotoxicity within LCA. A new framework is proposed, which includes dynamic inventory and dynamic impact assessment. This study focuses on the dynamic fate model for substances in the environment, combined with the USEtox® model for toxicity assessment. The method takes into account the noisy and random nature of substance emissions in function of time, as in the real world, and uses a robust solver for the dynamic fate model resolution. No characterization factors are calculated. Instead, a current toxicity is calculated as a function of time i.e. the damage produced per unit of time, together with a time dependent cumulated toxicity, i.e. the total damage produced from time zero to a given time horizon. The latter can be compared with the results obtained by the conventional USEtox® method: their results converge for a very large time horizon (theoretically at infinity). Organic substances are found to disappear relatively rapidly from the environmental compartments (in the time period in which the emissions occur) while inorganic substances (i.e. metals) tend to persist far beyond the emission period.

Cost versus life cycle assessment-based environmental impact optimization of drinking water production plants

Empowering decision makers with cost-effective solutions for reducing industrial processes environmental burden, at both design and operation stages, is nowadays a major worldwide concern. The paper addresses this issue for the sector of drinking water production plants (DWPPs), seeking for optimal solutions trading-off operation cost and life cycle assessment (LCA)-based environmental impact while satisfying outlet water quality criteria. This leads to a challenging bi-objective constrained optimization problem, which relies on a computationally expensive intricate process-modelling simulator of the DWPP and has to be solved with limited computational budget. Since mathematical programming methods are unusable in this case, the paper examines the performances in tackling these challenges of six off-the-shelf state-of-the-art global meta-heuristic optimization algorithms, suitable for such simulation-based optimization, namely Strength Pareto Evolutionary Algorithm (SPEA2), Non-dominated Sorting Genetic Algorithm (NSGA-II), Indicator-based Evolutionary Algorithm (IBEA), Multi-Objective Evolutionary Algorithm based on Decomposition (MOEA/D), Differential Evolution (DE), and Particle Swarm Optimization (PSO). The results of optimization reveal that good reduction in both operating cost and environmental impact of the DWPP can be obtained. Furthermore, NSGA-II outperforms the other competing algorithms while MOEA/D and DE perform unexpectedly poorly.

Evaluation of new alternatives in wastewater treatment plants based on dynamic modelling and life cycle assessment (DM-LCA)

With a view to quantifying the energy and environmental advantages of Urine Source-Separation (USS) combined with different treatment processes, five wastewater treatment plant (WWTP) scenarios were compared to a reference scenario using Dynamic Modelling (DM) and Life Cycle Assessment (LCA), and an integrated DM-LCA framework was thus developed. Dynamic simulations were carried out in BioWin(®) in order to obtain a realistic evaluation of the dynamic behaviour and performance of plants under perturbation. LCA calculations were performed within Umberto(®) using the Ecoinvent database. A Python™ interface was used to integrate and convert simulation data and to introduce them into Umberto(®) to achieve a complete LCA evaluation comprising foreground and background processes. Comparisons between steady-state and dynamic simulations revealed the importance of considering dynamic aspects such as nutrient and flow peaks. The results of the evaluation highlighted the potential of the USS scenario for nutrient recovery whereas the Enhanced Primary Clarification (EPC) scenario gave increased biogas production and also notably decreased aeration consumption, leading to a positive energy balance. Both USS and EPC scenarios also showed increased stability of plant operation, with smaller daily averages of total nitrogen and phosphorus. In this context, USS and EPC results demonstrated that the coupled USS + EPC scenario and its combinations with agricultural spreading of N-rich effluent and nitritation/anaerobic deammonification could present an energy-positive balance with respectively 27% and 33% lower energy requirements and an increase in biogas production of 23%, compared to the reference scenario. The coupled scenarios also presented lesser environmental impacts (reduction of 31% and 39% in total endpoint impacts) along with effluent quality well within the specified limits. The marked environmental performance (reduction of global warming) when nitrogen is used in agriculture shows the importance of future research on sustainable solutions for nitrogen recovery. The contribution analysis of midpoint impacts also showed hotspots that it will be important to optimize further, such as plant infrastructure and direct N2O emissions.

Integrated earth system dynamic modeling for life cycle impact assessment of ecosystem services

Despite the increasing awareness of our dependence on Ecosystem Services (ES), Life Cycle Impact Assessment (LCIA) does not explicitly and fully assess the damages caused by human activities on ES generation. Recent improvements in LCIA focus on specific cause-effect chains, mainly related to land use changes, leading to Characterization Factors (CFs) at the midpoint assessment level. However, despite the complexity and temporal dynamics of ES, current LCIA approaches consider the environmental mechanisms underneath ES to be independent from each other and devoid of dynamic character, leading to constant CFs whose representativeness is debatable. This paper takes a step forward and is aimed at demonstrating the feasibility of using an integrated earth system dynamic modeling perspective to retrieve time- and scenario-dependent CFs that consider the complex interlinkages between natural processes delivering ES. The GUMBO (Global Unified Metamodel of the Biosphere) model is used to quantify changes in ES production in physical terms - leading to midpoint CFs - and changes in human welfare indicators, which are considered here as endpoint CFs. The interpretation of the obtained results highlights the key methodological challenges to be solved to consider this approach as a robust alternative to the mainstream rationale currently adopted in LCIA. Further research should focus on increasing the granularity of environmental interventions in the modeling tools to match current standards in LCA and on adapting the conceptual approach to a spatially-explicit integrated model.

Emergy evaluation using the calculation software SCALE: case study, added value and potential improvements

This paper reports the emergy-based evaluation (EME) of the ecological performance of four water treatment plants (WTPs) using three different approaches. The results obtained using the emergy calculation software SCALE (EMESCALE) are compared with those achieved through a conventional emergy evaluation procedure (EMECONV), as well as through the application of the Solar Energy Demand (SED) method. SCALE's results are based on a detailed representation of the chain of technological processes provided by the lifecycle inventory database ecoinvent®. They benefit from a higher level of details in the description of the technological network as compared to the ones calculated with a conventional EME and, unlike the SED results, are computed according to the emergy algebra rules. The analysis delves into the quantitative comparison of unit emergy values (UEVs) for individual technospheric inputs provided by each method, demonstrating the added value of SCALE to enhance reproducibility, accurateness and completeness of an EME. However, SCALE cannot presently include non-technospheric inputs in emergy accounting, like e.g. human labor and ecosystem services. Moreover, SCALE is limited by the approach used to build the dataset of UEVs for natural resources. Recommendations on the scope and accuracy of SCALE-based emergy accounting are suggested for further steps in software development, as well as preliminary quantitative methods to account for ecosystem services and human labor.

Leaching of hazardous substances from a composite construction product--an experimental and modelling approach for fibre-cement sheets

The leaching behaviour of a commercial fibre-cement sheet (FCS) product has been investigated. A static pH dependency test and a dynamic surface leaching test have been performed at lab scale. These tests allowed the development of a chemical-transport model capable to predict the release of major and trace elements over the entire pH range, in function of time. FCS exhibits a cement-type leaching behaviour with respect to the mineral species. Potentially hazardous species are released in significant quantities when compared to their total content. These are mainly heavy metals commonly encountered in cement matrixes and boron (probably added as biocide). Organic compounds considered as global dissolved carbon are released in significant concentrations, originating probably from the partial degradation of the organic fibres. The pesticide terbutryn (probably added during the preservative treatment of the organic fibres) was systematically identified in the leachates. The simulation of an upscaled runoff scenario allowed the evaluation of the cumulative release over long periods and the distribution of the released quantities in time, in function of the local exposure conditions. After 10 years of exposure the release reaches significant fractions of the species' total content - going from 4% for Cu to near 100% for B.

Modelling inorganic and organic biocide leaching from CBA-amine (Copper-Boron-Azole) treated wood based on characterisation leaching tests

Numerical simulation of the leaching behaviour of treated wood is the most pertinent and less expensive method for the prediction of biocides' release in water. Few studies based on mechanistic leaching models have been carried out so far. In this work, a coupled chemistry-mass transport model is developed for simulating the leaching behaviour of inorganic (Cu, B) and organic (Tebuconazole) biocides from CBA-amine treated wood. The model is based on experimental investigations (lab-scale leaching tests coupled with chemical and structural analysis). It considers biocides' interactions with wood solid components and with extractives (literature confirmed reactions), as well as transport mechanisms (diffusion, convection) in different compartments. Simulation results helped at identifying the main fixation mechanisms, like (i) direct complexation of Cu by wood-phenolic and -carboxylic sites (and not via monoethanolamine; complex) on lignin and hemicellulose and strong dependence on extractives' nature, (ii) pH dependent binding of tebuconazole on polarized OH moieties on wood. The role of monoethanolamine is to provide a pore-solution pH of about 7.5, when copper solubility is found to be weakest. The capability of the developed model to simulate the chemical and transport behaviour is the main result of this study. Moreover, it proved that characterization leaching tests (pH dependency and dynamic tests), combined with appropriate analytical methods are useful experimental tools. Due to its flexibility for representing and simulating various leaching conditions, chemical-transport model developed could be used to further simulate the leaching behaviour of CBA treated wood at larger scales.

Biocide leaching from CBA treated wood - a mechanistic interpretation

Treated wood is frequently used for construction. However, there is a need to ensure that biocides used for the treatment are not a threat for people or environment. The paper focused on Pinus sylvestris treated with copper-boron-azole (CBA), containing tebuconazole as organic biocide and monoethanolamine (Mea). This study investigates chemical mechanisms of fixation and mobilisation involved in the leaching process of the used inorganic and organic biocides in CBA. A pH dependent leaching test was performed, followed by a set of complementary analysis methods in order to identify and quantify the species released from wood. The main findings of this study are: - Organic compounds are released from untreated and treated wood; the quantity of released total organic carbon, carboxylic and phenolic functions increasing with the pH. - Nitrogen containing compounds, i.e. mainly Mea and its reaction products with extractives, are released in important quantities from CBA treated wood, especially at low pH. - The release of copper is the result of competitive reactions: fixation via complexation reactions and complexation with extractives in the liquid phase. The specific pH dependency of Cu leaching is explained by the competition of ligands for protonation and complexation. - Tebuconazole is released to a lesser extent relative to its initial content. Its fixation on solid wood structure seems to be influenced by pH, suggesting interactions with \OH groups on wood. Boron release appears to be pH independent and very high. This confirms its weak fixation on wood and also no or weak interaction with the extractives.

Cost-performance indicator for comparative environmental assessment of water treatment plants

To compare potable water production plants on the basis of the environmental impacts generated by the treatment, including water resource depletion, Life Cycle Assessment (LCA) methodology is often used as referential. A comparison based only on the environmental impacts can however be misleading. Criteria for drinkability are usually defined as thresholds and the actual water quality gain achieved by different treatment chains shall be considered in the assessment for a fair comparison. Otherwise, chains treating low quality water resources could be disadvantaged as compared to alternatives using higher quality water resource, also when the depletion of the raw resource is included in the impact assessment. In this study, a novel Cost-Performance (CP) indicator has been developed and tested for the case of two existing water treatment plants located in the Paris Region. CP is the ratio between the total environmental impact generated by the treatment (i.e. the LCA score, eventually monetarised) and the total quality gain from raw to treated water. For the test case, three life cycle impact assessment methods, ReCiPe, Stepwise and Eco-costs (the latter two including monetarisation) have been considered. The water quality gain is based on 8 relevant parameters measured before and after treatment. The parameters are further aggregated using the French water quality valuation system SEQ-Eau. Paired t-test is then used to calculate the confidence interval for the average quality gain which then determines the confidence interval of the CP. Independent t-test on the CPs of the two alternative plants allows checking if their performances can be distinguished. Although in the specific test case the comparison is not conclusive, due to the similarity between the water quality gains, realistic breakthrough values have been obtained, especially using ReCiPe. The meaningfulness of the monetarisation of the LCA results has been highlighted as well.

Modelling inorganic biocide emission from treated wood in water

The objective of this work is to develop a chemical model for explaining the leaching behaviour of inorganic biocides from treated wood. The standard leaching test XP CEN/TS14429 was applied to a commercial construction material made of treated Pinus sylvestris (Copper Boron Azole preservative). The experimental results were used for developing a chemical model under PHREEQC(®) (a geochemical software, with LLNL, MINTEQ data bases) by considering the released species detected in the eluates: main biocides Cu and B, other trace biocides (Cr and Zn), other elements like Ca, K, Cl, SO(4)(-2), dissolved organic matter (DOC). The model is based on chemical phenomena at liquid/solid interfaces (complexation, ion exchange and hydrolysis) and is satisfactory for the leaching behaviour representation. The simulation results confronted with the experiments confirmed the hypotheses of: (1) biocide fixation by surface complexation reactions with wood specific sites (carboxyl and phenol for Cu, Zn, Cr(III), aliphatic hydroxyl for B, ion exchange to a lesser extent) and (2) biocide mobilisation by extractives (DOC) coming from the wood. The maximum of Cu, Cr(III) and Zn fixation occurred at neutral pH (including the natural pH of wood), while B fixation was favoured at alkaline pH.

Quantitative geochemical modelling using leaching tests: application for coal ashes produced by two South African thermal processes

The present work focuses on the reactivity of coal fly ash in aqueous solutions studied through geochemical modelling. The studied coal fly ashes originate from South African industrial sites. The adopted methodology is based on mineralogical analysis, laboratory leaching tests and geochemical modelling. A quantitative modelling approach is developed here in order to determine the quantities of different solid phases composing the coal fly ash. It employs a geochemical code (PHREEQC) and a numerical optimisation tool developed under MATLAB, by the intermediate of a coupling program. The experimental conditions are those of the laboratory leaching test, i.e. liquid/solid ratio of 10 L/kg and 48 h contact time. The simulation results compared with the experimental data demonstrate the feasibility of such approach, which is the scope of the present work. The perspective of the quantitative geochemical modelling is the waste reactivity prediction in different leaching conditions and time frames. This work is part of a largest research project initiated by Sasol and Eskom companies, the largest South African coal consumers, aiming to address the issue of waste management of coal combustion residues and the environmental impact assessment of coal ash disposal on land.

Modelling and simulation of concrete leaching under outdoor exposure conditions

Recently, a demand regarding the assessment of release of dangerous substances from construction products was raised by European Commission which has issued the Mandate M/366 addressed to CEN. This action is in relation with the Essential Requirement No. 3 "Hygiene, Health and Environment" of the Construction Products Directive (89/106/EC). The potential hazard for environment and health may arise in different life cycle stages of a construction product. During the service life stage, the release of substances due to contact with the rain water is the main potential hazard source, as a consequence of the leaching phenomenon. The objective of this paper is to present the development of a coupled chemical-transport model for the case of a concrete based construction product, i.e. concrete paving slabs, exposed to rain water under outdoor exposure conditions. The development of the model is based on an iterative process of comparing the experimental results with the simulated results up to an acceptable fit. The experiments were conducted at laboratory scale (equilibrium and dynamic leaching tests) and field scale. The product was exposed for one year in two types of leaching scenarios under outdoor conditions, "runoff" and "stagnation", and the element release was monitored. The model was calibrated using the experimental data obtained at laboratory scale and validated against measured field data, by taking into account the specific rain water balance and the atmospheric CO2 uptake as input parameters. The numerical tool used in order to model and simulate the leaching behaviour was PHREEQC, coupled with the Lawrence Livermore National Laboratory (LLNL) thermodynamic data base. The simulation results are satisfying and the paper demonstrates the feasibility of the modelling approach for the leaching behaviour assessment of concrete type construction materials.

Leaching behaviour of low level organic pollutants contained in cement-based materials: experimental methodology and modelling approach

The aim of this paper is the investigation of the leaching behaviour of different porous materials containing organic pollutants (PAH: naphthalene and phenanthrene). The assessment methodology of long term leaching behaviour of inorganic materials was extended to cement solidified organic pollutants. Based on a scenario-approach considering environmental factors, matrix and pollutants specificities, the applied methodology is composed of adapted equilibrium and dynamic leaching tests. The contributions of different physical and chemical mechanisms were identified and the leaching behaviour was modelled. The physical parameters of the analysed reference and polluted materials are similar. A difference in the pore size distribution appears for higher naphthalene content. The solubility of the PAH contained in the material is affected by the ionic strength and by the presence of a co-solvent; the solution pH does not influence PAH solubility. The solubility of the major mineral species is not influenced by the presence of the two PAH nor by the presence of the methanol as co-solvent in the range of the tested material compositions. In the case of the leaching of a monolith material the main transport mechanism is the diffusion in the porous system. For both mineral and organic species we observed at least two dynamic domains. At the beginning of the leaching process the released flux is due to the surface dissolution and to the diffusion of the main quantity dissolved in the initial pore solution. The second period is governed by a stationary regime between dissolution in pore water and diffusion. The model, coupling transport and chemical phenomena in the pore solution, at the monolith surface and in the leachate simulates satisfactory the release for both mineral and organic species.

Assessment of the multi-scale leaching behaviour of compacted coal fly ash

Peer experimental-modelling tools were developed and applied in the case of coal fly ashes with the aim to assess the leaching behaviour of ash compacted layers in a use scenario. Laboratory-scale (dissolution kinetics, ANC test, column percolation) and field pilot experimental studies (release monitoring during 18 month, hydrodynamic study, ANC on 44 month leached waste) were performed in order to identify and quantify the main transport phenomena and chemical processes. A quantitative geochemical model was developed taking into account equilibrium chemical reactions as well as kinetic processes for silicate phases like albite, K-feldspar and Ca-olivine. Phases like BaHAsO(4) and a solid solution Ba(x)Sr(1-x)(SO(4))(y)(CrO(4))(1-y) were proposed to explain the complex leaching behaviour of As, Cr, Ba, S; the soluble CaMoO(4) seems to control the Mo concentration. At neutral and acid pH, the model of surface complexation on ferric hydroxides was added for describing the behaviour of As, Cr, and Mo. At each scale the dynamic processes were identified and quantified by modelling. During the first contact with water an equilibration time of about 10 days was identified and then considered in all other laboratory experiments (ANC, column percolation). The hydrodynamic properties of compacted fly ashes were identified: a high water retention capacity (97% of the pores are still filled after draining under normal pressure), a flow regime close to plug type, a low fraction of stagnant zones (<0.03%). The scenario factors like carbonation and rainfall play an important role on the leaching behaviour at field scale. The carbonation diminishes the leachate pH from 11 to 8.5. The alternation of rain periods determines an apparent batch behaviour which slows down the outflow of the initial soluble fraction in pore water, if compared with the laboratory percolation column. The coupled geochemical-transport model was validated by comparison of the simulation results on ANC data obtained on the waste after 44 months of leaching under natural conditions.

Assessment of the multi-compound non-equilibrium dissolution behaviour of a coal tar containing PAHs and phenols into water

Herein, an experimental study coupled with a model in order to assess the non-equilibrium and multi-compound dissolution behaviour of a coal tar containing PAHs and phenols into water, is presented. For this aim, two experimental studies has been carried out: (1) coal tar-water partition equilibrium and (2) dissolution dynamics of coal tar under controlled hydrodynamic conditions in percolation columns packed with glass beads. The dissolution amount of the three target constituents (i.e. phenol, naphthalene and phenanthrene) was monitored by UV detection. The dissolution behaviour was modelled using a predictive fraction approach. The partition coefficients have been estimated from experimental data and the obtained results show that the partition coefficient of each constituent between the aqueous phase and the tar depends on the activities of the constituent in both phases and cannot be estimated only from the solubility of the pure compound in water. The non-equilibrium dissolution model was established, applied for the experimental conditions and validated for three target compounds adjusting the effective interfacial area between tar and water. This parameter is specific of the experimental set-up. The global behaviour of coal tar has been modelled taking into account four categories of compounds according to their water solubility and volatilities. The mass transfer parameters have been estimated using available correlations. The results of this paper indicate that a model based on component fractions can be used to assess the non-equilibrium dissolution behaviour of a coal tar.

Release dynamic process identification for a cement based material in various leaching conditions. Part I. Influence of leaching conditions on the release amount

In this paper we investigate at laboratory scale the influence of the liquid/solid leaching conditions on the release of different chemical species from a reference porous material obtained by solidification of PbO and CdO with Portland cement. The pH influence on the dissolution of pollutants and the initial pore solution composition (target elements: Na(+), K(+), Ca(2+), Pb(2+), Cd(2+), SO(4)(2-)) were assessed by applying a methodology consisting of two equilibrium leaching tests, the Acid Neutralization Capacity (ANC) and the Pore Water (PW) tests and geochemical modelling. Samples of the same material were submitted in parallel to four different dynamic leaching tests in order to determine the influence of the sample shape (monolithic or granular) and eluate hydrodynamics (instantaneous L/S ratio, eluate renewal) on the leaching of the target elements. The comparison criteria were the eluate saturation state, the cumulative release and the released flux. Generally, the eluates obtained in the tests applied on granular material were more concentrated, even saturated for the eluate pH value with respect to Ca(2+), Pb(2+) and SO(4)(2-). The consequence of the eluate saturation is the slowing down effect on the dynamic release. The highest released flux was observed for the Monolith Leaching Test (MLT) involving the highest instantaneous L/S ratio and the lowest solid/liquid exchange surface and for which no saturation was observed, except Pb(2+) and SO(4)(2-) in some eluates. The maximum cumulative released-mass was obtained for the Column Leaching Test (CLT) applied on granular material having the highest exchange surface, the lowest instantaneous L/S and a continuous input flow of the leachant. The experimental results demonstrate the significance of the liquid/solid contact type which is also a scenario specific parameter.

Release dynamic process identification for a cement based material in various leaching conditions. Part II. Modelling the release dynamics for different leaching conditions

This paper deals with process identification and model development for the case of a porous reference material leaching under certain hydrodynamic conditions. Four different dynamic leaching tests have been applied in order to take into account different types of solid/liquid contact conditions corresponding to various real leaching scenarios: monolithic and granular material with sequential eluate renewal, and granular material and continuously renewed eluate with different hydrodynamic conditions (dispersion, residence time). A coupled chemical-mass transfer model has been developed to describe the leaching behaviour under all experimental conditions. Diffusion has been considered as the mass transport mechanism inside the saturated porous material and dispersive convection as that in the leachate. Two specific phenomena have been identified and considered in the model: (i) the early surface dissolution of the material which results in high Ca concentration and (ii) the late weak dissolution of Na and K giving rise to a long-term residual release. The intrinsic material parameters such as the initial concentrations in the pore water and solid phases were determined by applying equilibrium leaching tests and geochemical modelling. Diffusion coefficients for different elements and the late solubility of alkalines have been found to reach the same values in the four tests. The estimated values of the surface dissolution kinetic constant have shown a dependence on leachate hydrodynamics when the thickness of the degraded layer is nearly the same in the four tests (intrinsic parameter of the material). The competition between the four main dynamic processes, i.e. diffusion, convection, late dissolution, and surface dissolution, has been emphasized and compared in the four leaching tests: the hydrodynamic dispersion and the residence time had no effect on the leaching behaviour of alkalines, which is controlled by diffusion, whereas the behaviour of calcium (a major element of the material) was strongly influenced. This has significant effects on eluate pH values and on the concentration of Pb (the monitored pollutant). The model was then applied to simulate a landfill scenario in the case of a stabilized/solidified incinerator residue containing heavy metals and chloride. A high rain infiltration level and the use of small blocs are favourable conditions for enhanced pollutant release.