**6. Long-term environmental safety evaluations using multiphysics numerical simulations**

Transport of aqueous and gaseous Hg in controlled landfills.

**Figure 16.** *Adsorption isotherms of four samples against gaseous Hg [59].*

In environmental engineering, numerical simulation is a method of predicting the fate and transport of chemicals. It is especially effective for evaluating long-term environmental safety, which cannot be predicted in experiments under limited conditions, such as testing duration, the scale of the domain targeted, and its heterogeneity. Numerical simulation can be conducted by solving governing equations that mathematically express the phenomenon targeted by prediction. Governing equations are always formulated to satisfy the law of mass conservation for targeted substances. Optionally, governing equations are also formulated to satisfy the law of momentum or energy conservation. Here, examples of numerical simulations for the transport of aqueous and gaseous mercury in landfills after disposal of mercury-containing waste are introduced, and the effectiveness of those simulations for long-term safety evaluation are discussed.

Ishimori et al. [63] investigated the environmental safety of landfill sites in which Hg-consisting waste is disposed of. Serial batch tests are conducted to evaluate the long-term leaching and volatilization of Hg stabilized in its sulfide form and solidified using either a sulfur polymer or low-alkaline cement. Using measured Hg leaching and volatilization rates, numerical simulations are conducted to investigate the long-term behavior of Hg after its disposal in landfill sites.

**Figure 17(a)** shows the analysis domain and conditions. A landfill site disposing of Hg-containing waste is modeled in the cross-sectional domain, and the waste is treated through stabilization and solidification techniques as shown in **Figures 3** and **4**. After treatment, the waste takes the form of a 1 m<sup>3</sup> cube. The entire array of solidified cubes is covered with a soil sorption layer, designed to retard the transport of emitted Hg. A drainage pipe is placed at the bottom of the analysis domain to accumulate the leachate, and a final cover or cutoff layer to reduce rainfall permeation is placed on top. **Figure 17(b)** and **(c)** show the initial and boundary conditions of the analysis—the boundary condition at the top of the domain indicates the rainfall

*Engineering Measures for Isolation and Sequestration of Heavy Metals in Waste as Safe… DOI: http://dx.doi.org/10.5772/intechopen.102872*

**Figure 17.**

*Analysis domain and conditions: (a) two-dimensional cross section of the landfill, (b) initial and boundary conditions for seepage analysis, (c) initial and boundary conditions for advection–dispersion analysis [60].*

intensity. For the first 10 years of the analysis, the rainfall intensity for the top boundary condition is considered to be 0 mm/y, as landfilling of the solidified piece would be carried out under a roof; at the end of the landfilling process, the roof would be removed, so in subsequent years, rainfall at the top boundary condition is considered to permeate the final cover with an intensity of either 600 mm/y or 60 mm/y. These different rainfall intensity values are used to evaluate the effects of using a cutoff layer covering the waste site, which would decrease the overall ingress of water to the landfill site. The measured leaching and volatilization rates of mercury are applied to the surface boundaries of the stabilized, solidified Hg-consisting waste.

This numerical simulation investigates the effects of the soil sorption and cutoff layers on the concentrations of dissolved Hg in the leachate at the bottom of the drainage pile and of gaseous Hg emitted from the final cover—the analytical conditions for these models are listed in **Table 3**. These parameters are given to the governing equations. In this study, they are formulated based on the law of mass conservation regarding water, air, dissolved Hg, and gaseous Hg, where the governing equations for water and air are called two-phase flow models, and the governing equations for dissolved and gaseous Hg are called advection–dispersion equations in general. Their equations have been widely used in numerical simulations for fluid flow and chemical substance transport in porous media. A notable point is that the phase transfer rate between dissolved and gaseous Hg is modeled using the Henry constant of Hg. The equation systems mentioned above can be numerically solved using many commercial simulation software programs or open-source codes. The following results are obtained from numerical solutions by COMSOL Multiphysics ver 5.0 (COMSOL, Inc).

A drastic difference between solidification by sulfur polymer and that by lowalkaline cement appears in the total Hg emissions from the landfill (**Figure 18**). The total Hg emission depends significantly on the presence of a soil sorption layer and


#### **Table 3.**

*Analytical conditions for predicting mercury behavior in landfill.*

#### **Figure 18.**

*Total amount of mercury emitted from a landfill with Hg-consisting waste solidified by (a) sulfur polymer or (b) low-alkaline cement.*

cutoff layer as well as on types of binders to solidify the Hg-consisting waste. The most effective measure to reduce Hg emission is considered to be sandwiching sulfur polymer-solidified pieces between sorption layers and covering the landfill surface with a cutoff wall. Numerical simulations will help us design the required geometry and material quality of the soil sorption layer, the cutoff wall, and the stabilized solidified Hg-consisting waste.

*Engineering Measures for Isolation and Sequestration of Heavy Metals in Waste as Safe… DOI: http://dx.doi.org/10.5772/intechopen.102872*

Hazardous waste containment performance depends on the aging of RC materials or their failure due to chemical attack.

Another practice of numerical simulation is the multiphysics of seismic analysis and reactive chemical transport analysis to evaluate the long-term environmental safety of isolation-type landfills that are designed for hazardous waste and that have a reinforced concrete structure [64]. Hazardous wastes possess a hazard to human health and the environment when improperly managed. They have extremely high leaching concentrations, so they cannot be disposed of directly into regular landfill sites. In general, such hazardous wastes are strictly controlled in waste containment facilities whose function is to prevent penetration and thus to avoid the wastes from leaching due to rainfall [2, 3]. As case studies, this containment facility so-called isolation-type landfills have been built using waterproof reinforced concrete with a thickness > 350 mm and compressive strength >25 MPa, based on the regulations in Japan. Ishimori et al. [65] used numerical simulations to show the importance of a multi-barrier system consisting of stabilization/solidification techniques and artificial/ natural soil sorption layers to minimize the negative impacts of a hazardous waste landfill. These numerical simulations consist of seismic analysis to evaluate the stability of the structure in the event of huge earthquakes and reactive chemical transport analysis to predict the long-term leaching concentration profiles from landfills damaged due to deterioration over time or sudden huge earthquakes.

**Figure 19** overviews those numerical studies. The analysis domain consists of a waste containment facility and its surrounding grounds. The environmental safety of hazardous waste in the facility, which is built with reinforced concrete, is evaluated by predicting the concentrations of heavy metals at monitoring well located in the lower reaches of the groundwater. Four numerical studies are performed. In Case 1, no earthquake occurs for 100 years. In Cases 2 and 3, a small and a large earthquake, respectively, occur after 5 years. Each earthquake is equivalent to a magnitude of a Level 1 (L1)

**Figure 19.** *Analysis domain and conditions for case studies.*

or Level 2 (L2) earthquake as defined in Japan, respectively. Case 4 additionally has a 50-cm sorption layer underlying the waste containment facility. Hazardous waste is assumed to be APC residue, and cadmium is targeted as a contaminant.

**Figures 20**–**22** overview the whole analytical procedure. **Table 4** shows the main analytical conditions used. The governing equations are formulated from the law of

**Figure 20.** *Multiphysics.*

**Figure 21.**

*Concept of how to reduce RC strength; from structure analysis given reduced strength, axial force, 2 and bending moment of RC beams are calculated.*

*Engineering Measures for Isolation and Sequestration of Heavy Metals in Waste as Safe… DOI: http://dx.doi.org/10.5772/intechopen.102872*

#### **Figure 22.**

*Concentration profiles in the containment facility; (a) bottom RC, (b) top RC.*


**Table 4.**

*Main analytical conditions.*

momentum balance to perform ground motion analysis and structure analysis. In the ground motion analysis, a governing equation having an unknown variable of horizontal displacement is solved using a given seismic wave as a boundary condition on the base layer. Then the calculated horizontal displacement and shear stress are applied as external forces acting on the landfill. Finally, a structural analysis for RCbased beams simulating the landfill geometry is conducted. From the calculated axial forces and bending moments in the beams, the crack width is estimated according to previous experimental studies [66–69]. The estimated crack width is used to


#### **Table 5.**

*Results of seismic analysis.*

determine the leakage rate from inside to outside the landfill according to the theory of Poiseuille flow. The governing equations mentioned above are solved using Moleman-i plus (Mizuho Information & Research Institute) for seismic analysis and COMSOL Multiphysics ver 5.0 for reactive chemical transport analysis.

**Table 5** shows the results of the seismic analysis. In any top cover with reinforced concretes, a crack is generated, resulting in the infiltration of rainfall into the landfill compartment. Whereas a crack in the bottom-reinforced concrete is not generated in Cases 1 and 2, deterioration due to carbonation and salt damage is considered. The crack width in Cases 3 and 4 is estimated to be 0.11 mm, which is smaller than that in the concrete reinforced with a top cover consisting of reinforced concrete. **Figure 23** shows the results of groundwater flow and transport analysis for Cd leached from the waste containment facility. No Cd appears in Cases 1 and 2 because the bottomreinforced concrete does not have a crack. However, Cd does leak from the crack in Cases 3 and 4; it is transported by groundwater flow and observed in the monitoring well. The soil sorption layer is effective for preventing groundwater contamination, and it is an important factor in controlling the long-term environmental safety of isolation-type landfills.

**Figure 23.** *Cd concentration profiles in groundwater.*

*Engineering Measures for Isolation and Sequestration of Heavy Metals in Waste as Safe… DOI: http://dx.doi.org/10.5772/intechopen.102872*

This section presents a numerical simulation model to evaluate the environmental safety of hazardous waste landfills having a reinforced concrete structure. A multibarrier system, in which a sorption layer is additionally installed under a reinforcedconcrete hazardous waste landfill, can retard the transport of contaminants and will be effective for improving their environmental safety.
