Selection of remediation technologies
To select the remediation technologies, 2 subsequent selection filters were applied to the input database of technologies provided by the FRTR matrix (Figure 3). The 1st filter was based on 2 basic parameters, commercial availability of the considered technology and the technology target contaminants overlapping those found in the site under study. The 2nd filter was concerned with site-specific parameters (i.e., hydrogeological and physicochemical characteristics of the investigated environmental matrix) affecting the feasibility of remediation technologies.
At the end of the selection, a pool of remediation technologies was chosen, including all technologies applicable simultaneously or subsequently to the study site.
This selection procedure was implemented into the DESYRE software as a hypertext database document developed in Microsoft® Word through Visual Basic Macro. It included 3 interactive tables (A, B, and C) and a characterization database, which drive the potential expert in the aforementioned selection pathway.
The A table (Figure 4) includes the input database of technologies and groups the cleanup technologies according to the treated contaminated matrix (i.e., soil, surface water, or groundwater, emitted off-gas).
Each technology is characterized by target contaminants, commercial availability, general site characteristics required for applicability, main benefits and disadvantages or drawbacks, and the target pollutants found at the site. Moreover, the last column of the table contains a synthetic judgment or evaluation made by the expert (FA = the technology is fully applicable and the remediation action does not imply any impediment; AR = applicable with reserve [i.e., the cleanup actions have notable troubles in 1 of the listed parameters]; NA = not applicable [i.e., the technology shows a specific impediment]).
Figure Figure 4.. Structures of the A, B, and C tables used sequentially in the selection of remediation technologies.
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The B table (Figure 4) includes only the technologies selected by the expert in the previous table A after application of the 1st filter (i.e., the technologies marked FA or AR). Table B is only descriptive and provides the characterization of the selected technologies according to the additional criteria of remediation strategy (i.e., extraction, removal and retrieval, biodegradation, immobilization, destruction, and chemical transformation), specific cleanup effectiveness for the considered contaminant classes, and capability to be included in train technology treatments.
The C table (Figure 4) lists site-specific parameters that affect the applicability of the selected technologies: either related to the treated matrix (e.g., pH, total organic carbon, hydraulic conductivity, and soil cation exchange capacity) or related to the target contaminants (e.g., vapor pressure, solubility). The applicability range values of these parameters for each technology were obtained by published technical documents (Los Alamos National Laboratory 1996; NATO/CCMS 2001; Vik and Bardos 2003).
Finally, the characterization database (not reported here) includes all the information concerning the geotechnical (e.g., soil granulometry) and physicochemical (e.g., soil organic carbon content) characteristics of investigated environmental media obtained during the characterization process.
With the use of the characterization database and table C, the expert can apply the 2nd site-specific filter by identifying the fit between hydrogeological and physicochemical characteristics of the site, summarized in the characterization database, with the aforementioned applicability range values summarized in the 2nd column of table C (labeled “Site-specific criteria that affect the applicability”). The software then prompts the expert for a final applicability judgment or assessment for all technologies, which is inserted in the 3rd column of table C (labeled “Expert applicability judgment/assessment”). Only the technology marked “Applicable” will be selected, which leads to a pool of remediation technologies actually applicable to the case study.
Setting of comparative criteria set: Macrocriteria and evaluation matrix
To compare the pool of selected remediation technologies, the following 6 comparative macrocriteria were defined: reliability, course of action (i.e., intervention condition), “hazardousness,” community acceptability/impacts, effectiveness, and cost (Figure 5). Each macrocriterion is able to describe a specific aspect of a cleanup action.
The reliability macrocriterion considers the maintenance aspects and results obtained by the technology application to other case studies. The course of action (i.e., intervention condition) macrocriterion identifies the logistic and technical aspects related to a remediation action by differentiating between in situ, ex situ, and off-site technologies and by considering the possibility of creating a train technology. The macrocriterion measuring hazard allows assessment of the potential effects for human health resulting from the technology application (e.g., effects related to the use of hazardous reagents or the emission of dust and volatile substances). With the use of the community acceptability/impacts macrocriteria, the negative effects on the environment, as well as the main factors on which depend public judgment of a particular remediation technology, are evaluated. The effectiveness macrocriterion helps the expert to assess technology performance, which depends on cleanup time and removal rates. Finally, the cost macrocriterion points out the parameters on which depend the actual or real costs of a remediation action (e.g., time, installation and maintenance cost, need of waste disposal).
Figure Figure 5.. Macrocriteria and associated evaluative criteria used for the comparison of remediation technologies.
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Table Table 1.. Qualitative and quantitative rating of the evaluation criteria selected for comparing the remediation technologies
Figure 5 reports the evaluation criteria associated with each macrocriterion. These evaluation criteria were identified from the review of international approaches (UN 1997; UKEA 1999; FRTR 2002), and each can contribute to the definition of more than 1 macrocriterion (e.g., cleanup time influences both effectiveness and cost), as shown in Figure 5. Some of the selected evaluation criteria are strictly correlated with technical aspects (i.e., costs, cleanup time, performance, reliability and maintenance, technology development status, cleanup operation locations, train technology, hazardous reagents use, contaminated matrix removal, and residuals production), whereas other criteria refer to the potential effects on human health and the environment (i.e., dust and volatile substances emission, effects on water, and consequences to soil and community acceptability).
Performance, cost, and cleanup time are the most important criteria in the description of a remediation technology according to cost–benefit analysis. Performance is the removal ratio, expressed as the ratio between the residual (i.e., after treatment) pollutant concentration in a given matrix and the initial concentration in the same matrix. Cost gives information on the overall cost per treated matrix unit (US$/t of soil matrix wet weight and US$/1,000 L of water matrix), and it is strictly related to the removal rates. The costs applied to the study case were obtained from reviewing case studies with features similar to those of the analyzed site, so that site-specific criteria could be introduced (Los Alamos National Laboratory 1996; NATO/CCMS 2001; OCETA 2001; USEPA 2001a, 2001b; Vik and Bardos 2003). Finally, cleanup time is the average time required to clean a site with a specific technology. According to the USEPA (2001a) approach, all cleanup times were estimated by referring to standard conditions, namely 20,000 t of soil and 3,785,000 L of water.
For each evaluation criterion, a qualitative or quantitative rating was defined as shown in Table 1. This rating scheme allowed the so-called evaluation matrix to be obtained—1 for each contaminant class found in the site analyzed (see Table 2, in which the evaluation matrix for inorganics is presented). In this matrix, evaluation criteria were reported in the columns, and the selected technologies suitable to treat specific pollutants were in the rows. The evaluation matrix provides the input data for the technologies ranking step, described in the following paragraph.
Comparative procedure and ranking of the selected remediation technologies
The 3rd step of the proposed methodology, the comparative procedure, aims at obtaining a ranking of remediation technologies according to the macrocriteria previously presented. The final classification will allow the experts to create remediation technologies sets that act as possible remediation solutions.
The comparative process is based on a ranking algorithm that was developed by single-person MCDA tools. Specifically, the selected MCDA method uses a decision matrix as the database (Table 7) that summarizes the available raw data for the decision maker. Each row of the matrix corresponds to a specific alternative (in the current case, a specific remediation technology), whereas each column corresponds to a proposed macrocriterion. Accordingly, each element ij of the matrix represents the judgment of the alternative i with respect to the macrocriterion j (i.e., the jth attribute value for the alternative i) and can be expressed in different ways (i.e., symbolic, numerical, Boolean).
Table Table 2.. Evaluation matrix for inorganic contaminants and the remediation technologies resulting from the selection step. For legend, see Table 1a
| || || || ||Performance (% removal)|| || || || || || || || || || || |
|Soil treatment||Technology||Cleanup time||Overall cost ($/t or $/1,000 L)||As||Cr||Cu||Cd||Hg||Ni||Pb||Zn||Cleanup operation location||Train technology||Effects on waters||Effects on soils||Dust and volatile substances emissions||Contaminated matrix removal||Residuals production||Community acceptability||Hazardous reagents use||Reliability/maintenance||Technology development statusb|
|In situ||Phytoremediation||Average||25||NAV||38||NAV||NAV||42||NAV||NAV||98||In situ||Yes||Yes||No||No||No||Yes||High||No||Worse||I|
| ||Electrokinetic separation||Average||62||88||90||85||93||NAV||90||89||73||In situ||Yes||No||Yes||Yes||No||Yes||High||No||Average||C|
| ||Solidification/stabilization||Better||90||NAV||NAV||NAV||60||NAV||NAV||74||60||In situ||No||Yes||Yes||Yes||No||No||Average||No||Better||C|
|Ex situ||Separation||Better||NaV||NA||NA||NA||NA||NA||NA||NA||NA||Ex situ||Yes||No||Yes||Yes||Yes||Yes||Average||No||Better||C|
| ||Soil washing||Better||200||96||93||87||NAV||NAV||92||91||98||Ex situ||Yes||No||Yes||Yes||Yes||Yes||Low||Yes||Better||C|
|Other||Landfill cap||Worse||NA||NA||NA||NA||NA||NA||NA||NA||NA||In situ||No||Yes||Yes||Yes||No||Yes||Low||No||Better||C|
| ||Landfill cap enhancement||Worse||NA||NA||NA||NA||NA||NA||NA||NA||NA||In situ||No||Yes||Yes||Yes||No||Yes||Low||No||Better||C|
| ||Excavation, retrieval and off-site disposal||Better||NA||NA||NA||NA||NA||NA||NA||NA||NA||Off-site||No||Yes||Yes||Yes||Yes||NA||Low||No||Better||C|
Moreover, according to the discussion in the Background section, the selected MCDA method is based on the weighted averaging operator associated with the absolute AHP to structure the problem into a suitable hierarchy and to determine the criterion weights. This choice was motivated from the aforementioned characteristics, including easy interpretability of its linear form, user-friendly capability, probability of low or null interaction among the criteria, and limited amount of information required.
According to the standard AHP, the absolute AHP is based on 3 fundamental steps: 1) Structuring the problem with hierarchy, which allows a complex problem to be divided into a series of levels of analysis so that each attribute is a member of a small set of attributes on the same level, all attributes are related to a single attribute on the level immediately above them, and the last level is formed by the available alternatives; 2) comparison of judgments, which allows the relative importance of the variables (attributes or alternatives) belonging to the same level and relative to each of the associated variables belonging to the upper level to be calculated with the use of a pairwise comparison method (i.e., for each pair of attributes, the expert specifies a judgment of “how much more important” 1 attribute is compared with another) with a predefined (and limited) scale, usually the natural scale of 1, 2, …, 9 points; 3) analysis of priorities, which leads, through suitable aggregation tools, to a final ranking of the alternatives (Saaty 1980; Norris and Marshall 1995).
Moreover, for specific application in the remediation technologies comparison, in the absolute AHP mode, the relative importance (i.e., the weight) was calculated with the pairwise comparison only for the hierarchic level in which the macrocriteria were included, not for all the levels, as the relative AHP provides for (i.e., overall goal concerning the environmental requalification, macrocriteria, evaluation criteria, and technological alternatives). Therefore, for the overall goal, the relative importance is intended to be the importance of each macrocriterion relative to the 1st hierarchic level (i.e., obtaining a ranking of the selected remediation technologies that facilitates the definition of a remediation plan for the study case).
In accordance with the general considerations for AHP reported in the Background section, the absolute AHP was adopted to avoid both the great number of required comparisons of the alternatives and the undesired rank reversal phenomenon. Thus, the values of the macroattributes for each alternative were directly assigned by the expert. This process allows the expert knowledge to be captured and implemented in a numeric form. In this way, the proposed process can be considered, at the same time, a support and check tool for the experts who have to assess the several selected technologies to obtain different technologies sets.
The overall defined comparative process finally comprised 1) structuring the problem with hierarchy, 2) computing macrocriterion weights, 3) assessing the technologies with the use of a judgment matrix, and 4) computing the final score for each technology.
Structuring the problem in hierarchic levels—The proposed structure include 4 hierarchic levels, as shown in Figure 6. The 1st level consists of the final goal (i.e., obtaining a ranking of the selected remediation technologies to define a remediation plan for the study case); the 2nd level consists of the proposed macrocriteria, the 3rd level concerns the selected evaluation criteria, and the 4th level concerns the different available alternatives (i.e., the selected remediation technologies).
Table Table 3.. Stepwise selection of remedial technologies for inorganic contaminants
|Technologies for inorganics from technologies' database (Table A)||Technologies resulting from the 1st selection||Pool of technologies applicable to the study case for inorganics category|
|Treatments for soil sediments and sludge in situ|
|2||Electrokinetic separation||2||Electrokinetic separation||2||Electrokinetic separation|
|3||Fracturing||3||Fracturing|| || |
|4||Soil flushing||4||Soil flushing|| || |
|6||Vitrification|| || || || |
|Treatments for soil sediments and sludge ex situ|
|7||Chemical extraction|| || || || |
|8||Oxidation/reduction||6||Oxidation/reduction|| || |
|10||Soil washing||8||Soil washing||5||Soil washing|
|Containment and other treatments|
|12||Landfill cap||10||Landfill cap||7||Landfill cap|
|13||Landfill cap alternatives||11||Landfill cap alternatives||8||Landfill cap alternatives|
|14||Excavation, retrieval, and off-site disposal||12||Excavation, retrieval, and off-site disposal||9||Excavation, retrieval, and off-site disposal|
Table Table 4.. Saaty's (1980) numerical scale applied in the matrix of pairwise comparison (reported in Table 5)
|Intensity of importance||Definition|
|1||Equally as important|
|3||Moderately more important|
|5||Strongly more important|
|7||Very strongly more important|
|9||Extremely more important|
|2, 4, 6, 8||Intermediate values between defined rankings|
Computing macrocriteria weights—The 2nd step of the comparison process is the computing of macrocriteria weights. According to the AHP method (Saaty 1980), this is obtained with a pairwise comparison matrix (i.e., Table 5), in which rows and columns list the proposed macrocriteria. For each pair of macrocriteria, the expert specifies a judgment of “how much more important” 1 macrocriterion is than another. To specify the pairwise comparison judgments, a numerical approach was adopted (i.e., the expert answers each question with a number, as in “Attribute A is 3 times as important as Attribute B”) on the basis of the Saaty numeric scale (Saaty 1980) reported in Table 4. With the use of this scale, the macrocriteria weights were successively estimated by applying the eigenvector method (Saaty 1980). Moreover, the developed procedure allows both ordinal (maximum–minimum transitivity) and cardinal consistency analysis to be applied, with the aim to avoid intransitive cycles (Kwiesielewicz and Van Uden 2004) and too-low cardinal consistency (Saaty 1980, 2000).
Evaluating remediation technologies with the judgment matrix—This step evaluates the selected remediation technologies (i.e., the available alternatives) through the so-called judgment matrix (i.e., the aforementioned decision matrix). A specific judgment matrix has to be developed for each contaminant class. In this matrix (Table 7), the rows correspond to the selected remediation technologies that are able to treat a particular contaminant class, whereas the columns correspond to the proposed macrocriteria, whose weights were previously calculated. Each element ij of the matrix represents the score (i.e., expert judgment) of the technology i with respect to the macrocriterion j. The expert assigns this score by assessing the rating that the evaluation criteria, correlated to a specific macrocriteria, assumes for a selected remediation technology in the aforementioned evaluation matrix (see Table 2 for inorganics). These judgments (i.e., scores) are expressed according to a numerical scale (1 = sufficient, 2 = rather good, 3 = satisfactory, 4 = good, and 5 = excellent) that allows several judgment levels to be explained, while avoiding too-heavy computational efforts. Also in this step, a consistency check is performed: if an alternative is dominated by any others, the criteria judgment has to be coherent, otherwise an alarm is sent to the user.
Table Table 6.. Macrocriteria weights calculated by the eigenvector method (Saaty 1980)
|Effectiveness||β1 = 0.4750|
|Community acceptability/impacts||β2 = 0.0288|
|Reliability||β3 = 0.0644|
|Intervention conditions||β4 = 0.0434|
|Hazardousness||β5 = 0.1472|
|Cost||β6 = 0.2412|
Ranking algorithm—The last step of the comparative procedure concerns the definition of the ranking algorithm, which estimates the final score for each selected remediation technology. The proposed algorithm is the sum of the products of the numerical judgment (expressed for each macrocriterion through the judgment matrix) with the corresponding macrocriterion weight according to the Equation 1,
where Pi is the total scoring associated with the ith remediation technology, β′j is the normalized weight associated with the jth macrocriterion, vij is the numerical judgment assigned to the ith alternative (technology) and correlated with the jth macrocriterion, and L is the number of macrocriteria considered.
Table Table 5.. Matrix of pairwise comparison applied to the calculation of macrocriteria weights
| ||Effectiveness||Community acceptability/impacts||Reliability||Intervention conditions||Hazardousness||Cost|
Table Table 7.. Judgment matrix for inorganic contaminants and the remediation technologies selected in the stepwise procedurea
| ||Effectiveness||Community acceptability/impacts||Reliability||Intervention conditions||Hazardousness||Cost|
|Solidification/stabilization in situ||4||1||5||4||3||3|
|Solidification/stabilization ex situ||4||1||5||1||2||2|
|Landfill cap alternatives||1||1||4||3||4||2|
|Excavation, retrieval and off-site disposal||1||1||4||1||1||2|