Techniques
- Placement of a chemical reactive zone: Zero-value iron, pillared clays
- Placement of an adsorptive zone: Granular active carbon, alumino-silicates, zeolites, pillared clays, peat barrier, wooden bank/wooden barrier.
- Placement of a biological reactive zone - aerobic: iSOC, ORC, Peroxide
- Placement of a biological reactive zone - anaerobic: HRC infiltration, molasses, protamylase, milk whey, milk, lactate, ethanol... potentially in combination with nutrients and/or bacteria.
Diagram of reactive screens
The prevention of pollutant dispersion via ground-water can be implemented by placing an in-situ screen (reactive screen). The screen is placed in the flow direction of the ground-water, in the plume of the ground-water pollution. In order to direct the ground-water flow into an opening in the screen, a ‘funnel and gate’ system can be placed, which consists of a water-inhibiting or water-resistant screen wall (the funnel) which leads the water to an opening (gate) in the wall. A reactor or a reactive zone can be implemented at the opening. Pollutants are volatilised, broken down and established in the in-situ screen or the in-situ reactor.
A screen that biologically removes pollutants is also referred to as a bio-screen.
To encourage ground-water flow through the screen, an additional water extraction system can be placed down-stream from the screen.
There are many ways to implement in-situ screens. A number of systems (namely compressed-air injection screens and screens consisting of iron pellets) are also used in practice. However, literature still contains very few descriptions about experiences on this front.
For further technical information, we refer to a report at http://www.clu-in.org/download/remed/tmt_wall.pdf.
Implementation area and implementation conditions
The screens must be able to, over long periods (years), restrict the dispersion of ground-water pollutants without any or limited maintenance. The screens are not suitable to prevent the dispersion of floating layers or sinking layers.
As expected, the screens are very good for removing biologically degradable pollutants, with a high yield (> 90%).
Costs
The costs of the investigation, including costs for field characterisation, potency determination, design, training and discussion, are estimated at €12.500 to €50.000 per location (OVB, 2004). The costs are determined by the complexity of the case and the presence of existing and usable level indicators.
http://www.rtdf.org/public/permbarr/ highlights a firm example of a reactive iron screen placed at the former site, polluted with VOCI, of dry-cleaning business in Germany. The wall was ca. 35m long, 1 m wide and 11 m deep, filled with granular iron/gravel at a ratio of 1:2. The design amounts €30.000, installation including reactive material to €93.000 (approximately 500 euro/ton iron) and monitoring amounts to €13.000 (plus €24.000 for installation of gas-measuring equipment).
The costs for installation of a biologic reactive screen will be proportionate to the size; it is also greatly determined by the method of implementation and the type of reactive material.
For funnel & gate configuration the costs lie between 50.000 and 300.000 euros per gate (OVB, 2004). However, the costs for the reactor/reactive zone (gate) continue to be strongly determined by the implementation and the type and quantity of used materials.
Further, costs related to long-term follow-up (monitoring) must also be taken into consideration.
Environmental burden and measures to be implemented
When in-situ screens are placed using a filling material, the existing soil is replaced. If the extracted ground is polluted, this must be taken away for processing.
In the most intensive form of screening, no or very little energy will be used. If the ground-water flow or the quantity of oxygen at the location must be altered, a certain amount of energy will be required. The use of energy is determined by the type of system that is used. For example, an extraction system uses more energy in comparison to a compressed-air injection system.