Example 2.1
Selection of remediation method on the basis of comparative analysis
Establishment of various industries on the banks of a hypothetical river resulted in contamination of its waters and sediments. Analysis of sediments revealed that the concentration of heavy metals (lead, zinc, cooper) has exceeded by far the threshold concentrations indicating harmful effects. Samples retrieved from the riverbank indicate that the average thickness of the contamination sediment is 250 mm, and the total contaminated sediment volume is about 150,000 m3.
Select the most effective remediation method, considering (permanent and temporary) environmental, technical and economic criteria.
Answer:
Five (5) remediation alternatives are tentatively selected for the comparative analysis (Mohamed and Antia 1998). These consist of (in order of increasing cost):
- Storage of sediments on land
Dredge sediments, reduce their volume (via dehydration), transport them and store them in a waste depository.
Estimated Cost: $5 mil
2. On-site storage of sediments on the riverbed
Cover the sediments with a permeable geotextile, and compact them by overlaying a bed of crushed stones.
Estimated Cost: $6 mil
3. Encapsulation of sediments within the river area
Dredge sediments by means of suction flow-reversing, transport them via pipelines, and store them in watertight basins constructed within the river. This option will reduce the width of the river, but to acceptable levels.
Estimated Cost: $10 mil
4. Off-site stabilization of sediments
Dredge sediments and treat them with chemical agents to form a cement-like product. Use the material in landfills or for general embankment construction.
Estimated Cost: $6 mil to $12 mil
5. On-site stabilization of sediments
Mix the sediments directly with chemical agents and cement to create a highly resistant material of low permeability. Treated material can be left in place.
Estimated Cost: $12 mil to $24 mil
Each family of criteria (permanent environmental, temporary environmental and technical/economical) is divided into specific criteria/aims, and each remediation option is rated according to that criterion (1st, 2nd, 3rd etc.), in the order of most desirable to least desirable solution.
At the same time, each one of the criteria is assigned to an importance class IC, according to the project specifications, stakeholders feedback, risks etc. Three (3) importance classes are used in this example. In this way, ratings are weighted according to Table 2.2 in a five-level mark scale. Higher marks correspond to the most desirable option: A is the highest mark and E is the lowest. Tables 2.3 to 2.5 present the criteria, divided into three families, their ratings, and the importance class they are assigned to.
| Importance Class, IC | Weight of criterion | ||
|---|---|---|---|
| Rated 1st | Rated 2nd | Rated 3rd | |
| 1 | A | B | C |
| 2 | B | C | D |
| 3 | C | D | E |
| Criterion | Options | Rating | IC |
|---|---|---|---|
| Eliminate the contamination of the river sediment | Complete treatment and elimination of pollutants | 1st | 2 |
| Confinement of pollutants | 2nd | ||
| Sediment management in the affected area | Sediments remain in the affected area | 1st | 1 |
| Part or all sediments transported outside affected area | 2nd | ||
| Preserve site conditions | No changes in the river layout | 1st | 3 |
| Changes in the river layout | 2nd | ||
| Minimise risk of polluting groundwater and surface water after treatment | No risk; contaminants transported outside affected area | 1st | 2 |
| Low risk; contaminants fully confined in the affected area | 2nd | ||
| Moderate risk; contaminants partially confined in the affected area | 3rd | ||
| Riverbed characteristics following the completion of work | Riverbed similar to natural state | 1st | 3 |
| Riverbed composed of small stones (added material) | 2nd | ||
| Cemented material covering the riverbed | 3rd |
Twelve (12) criteria in total are considered in this example. Other, project-specific criteria may apply e.g. impact of works on industrial activity or local economy, potential for area redevelopment, timetable issues etc.
The ratings of the criteria are also project-dependent. For example in this case emphasis is put on criteria related to inconvenience of the local residents during works (IC1), compared to temporary environmental impact (IC3, Table 2.4), to minimise community reaction to the project.
Results are summarised in Table 2.6, where the marks scored by each remediation solution are reported, according to its technical details in the light of the offered options. The solution that scores more A- marks (encapsulation is this case) is the preferred one. If two or more alternatives have the same number of A- marks, we count the B- marks etc. Therefore the second most preferable option in this case is storage of sediments on land, followed by on-site stabilisation.
| Criterion | Options | Rating | IC |
|---|---|---|---|
| Inconvenience caused by construction site | Majority of works taking place outside affected area; minimal installations; works completed within 12 months | 1st | 1 |
| Majority of works taking place in affected area; installations and storage on site; works completed within 12 months | 2nd | ||
| Majority of works taking place in affected area; installations and storage on site; works completed in more than 12 months | 3rd | ||
| Inconvenience due to trucking activity | Minimal trucking activity | 1st | 1 |
| Heavy trucking activity | 2nd | ||
| Very heavy trucking activity | 3rd | ||
| Environmental impact of works | Little or no drop in river water; minimal dredging | 1st | 3 |
| Little or no drop in river water; dredging | 2nd | ||
| Major drop in river water | 3rd |
| Criterion | Options | Rating | IC |
|---|---|---|---|
| Effect of works on river functions | River section similar to original state | 1st | 2 |
| River section similar to original state, alteration of banks outline | 2nd | ||
| Characteristics of river section altered | 3rd | ||
| Past experience and feasibility | Proven technology for similar treatment | 1st | 1 |
| Proven technology, but for a different treatment | 2nd | ||
| Technology not proven in practice (pilot or pre-industrial stage) | 3rd | ||
| Necessity for monitoring | No follow up required; pollutants removed from the area | 1st | 2 |
| Small-scale monitoring required | 2nd | ||
| Large-scale monitoring required | 3rd | ||
| Total cost | $10mil or less | 1st | 1 |
| $10 to 30mil | 2nd | ||
| over $30mil | 3rd |
| Mark | Storage on land | On-site storage | Encapsulation | Off-site stabilization | On-site stabilization |
|---|---|---|---|---|---|
| A | 2 | 2 | 3 | 2 | 2 |
| B | 5 | 2 | 1 | 3 | 4 |
| C | 4 | 4 | 6 | 3 | 5 |
| D | 1 | 3 | 1 | 3 | 1 |
| E | 0 | 1 | 1 | 1 | 0 |
| Ranking | 2nd | 5th | 1st | 4th | 3rd |
