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.

Characterization of a mineral waste resulting from the melting treatment of air pollution control residues.

Air pollution control (APC) residues which are generated by municipal solid waste (MSW) incineration show a high-level of pollution potential. In order to stabilize such APC residues, the French power supply company (EDF) is developing a thermal treatment process which leads to the production of a vitrified material. A structural characterization of the vitrified product was carried out by applying complementary investigation methods: XRD, SEM, Raman spectroscopy, EPMA, and data interpretation methods such as mineralogical analysis and principal component analysis (PCA). The major phase of the material was a solid solution of melilite type composed of five end-members: gehlenite (44%), åkermanite (25%), ferri-gehlenite (5%), sodamelilite (14%) and hardystonite (11%). The minor phases identified were spinels and pyroxenes. An ANC leaching test was performed in order to observe the treatment effect on pollutant release. The natural pH was close to 10, and the major element release was less than in the case of untreated APC. This was a consequence of melilite formation. The effect of pH was fundamental for heavy metals release: lower solubilization occurs at pH 10 than at APC's natural pH (11-12).

Using PHREEQC for modelling and simulation of dynamic leaching tests and scenarios.

This paper presents an extension of the application field of PHREEQC geochemical software for modelling the dynamic leaching tests (and scenarios) by taking into account the leachant compartment as complex reactive/transport system and the coupling of many reactive compartments linked by substance fluxes. This study focuses on the specific case of dynamic leaching of monolithic porous materials (particularly the case of continuous monolithic leaching test, CMLT) where reaction/diffusion occurs in the porous matrix and where the leachant is a complex reactor in which chemistry coupled with inter-phase mass transfer and convection processes take place. It is demonstrated here that the modelling of open reactors (convection) is possible with PHREEQC by using RATES and KINETICS keyword data blocks. The PHREEQC model was validated by results comparison with analytical solutions of the system equations. Coupling a diffusion compartment with an open reactor (complex boundary conditions for the diffusion equation) requires the introduction of a stagnant cell on the first grid cell of the diffusion compartment in TRANSPORT data block and the use of MIX function for model the monolith/leachant interface transfer. The proposed model was validated by comparison with numerical solutions obtained with MATLAB and by a numerical sensibility study. Finally, the model equations are given for a complex dynamic leaching process of a porous monolith involving beside reaction/diffusion in the monolith, reactions, interface mass transfers, gas absorption and convection in the leachant. Examples of PHREEQC modelling are presented: (1) the case of continuous leaching of a cement based material using carbonated water and (2) a field scale water storage pool constructed with a solidified/stabilised material. The comparison with the experimental leaching data shows the simulation results are very satisfying.

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.

Modeling of solid/liquid/gas mass transfer for environmental evaluation of cement-based solidified waste.

A physicochemical and transport model has been developed for the long term prediction of environmental leaching behavior of porous materials containing inorganic waste solidified with hydraulic binders and placed in a reuse scenario. The reuse scenario considered in the paper is a storage tank open to the atmosphere including material leaching with water and carbonation through the leachate contact with air. The model includes three levels: (i) the physicochemical pollution source term (chemical equilibria in the pore water and diffusion in the porous system); (ii) chemical equilibria and mass transfer in the tank; and (iii) gas/liquid transfer of carbon dioxide. The model was applied to the case of a material obtained through solidification of Air Pollution Control (APC) residues from Municipal Solid Waste Incinerator (MSWI). The simulation results are in good agreement with two scale experimental data: laboratory and field tests. Experimental data and simulations show the main trends for release of elements contained in the material: (i) the release of alkaline metals and chloride is not significantly influenced by carbonation and (ii) the release of Ca and Pb is governed by chemical equilibria in pore water and diffusion, while their speciation in the leachate is determined by pH and the presence of carbonate ions.