Principle
The following techniques can be used:
Figure: Diagram of infiltration in relation to in-situ clean up
The biological break-down of pollutants (in the saturated and unsaturated zone) can be stimulated by infiltrating water with ‘additives’. The aerobic as well as the anaerobic break-down can be stimulated by adding specific electron acceptors and substances that induce aerobic and anaerobic break-down. The literature list contains an overview report of the most suitable electron acceptors for various types of pollutants (OVAM 2003).
The most commonly used electron acceptor is ambient air, though this can only dissolve in water in limited quantities (up to ca. 10 mg/l), and will thus be quickly consumed by higher concentrations of organic pollutants. By using (relatively expensive) pure oxygen, the maximum solubility of oxygen in ground-water can be attained (50 mg/l). A new way of diffusely introducing pure oxygen into the ground-water is by using the ISOC® system. This is a probe with a membrane system, that is hung into ground-water (level indicator), through which pure oxygen can be dosed. Prevailing studies regarding the amount of necessary oxygen need to be done.
Possibly, by adding specific chemicals, the co-metabolic break-down of various chlorine-based compounds can be stimulated (e.g. volatile chlorinated compounds).
By using a peroxide (also in ‘slow-release’ form, e.g. ORC®), even higher levels of oxygen can be attained. If a too high dose of peroxide is introduced, this may form toxic conditions for bacteria. An alternative to oxygen as an electron acceptor is nitrate, which stimulates the anaerobic break-down of aromatics. However, only a limited number of compounds can be broken down with nitrate as an electron acceptor (for example, benzene and mineral oil are not broken down, though they are in the presence of a very low volume of oxygen (micro-aerofil).
In some cases there is a lack of nutrients or bacteria. These can then be infiltrated via drains, deep wells and vertical filters. Mostly ammonium nitrate is used as nitrogen source, and Na-or Ka-phophate as phophor source.
If extraction is required, the technique must be combined with water extraction, which is normally followed by water purification.
Implementation area and implementation conditions
Because water is only able to slowly disperse into the soil, the permeability of the soil must be at least 0.5 to 1 m/day.
The success of bio-restoration via infiltration is mainly determined by the following factors:
Geochemical conversion (inc. iron precipitation, gas-forming)
Iron may precipitate if oxygen-free iron-rich water comes into contact with oxygen.
Hydrological problems (inc. heterogeneity of soil);
The permeability of the soil in a vertical and horizontal direction is mostly difficult to estimate due to the shortage of information relating to geo-hydrologic soil structure);
Physical problems (inc. Colloidal blockages);
Due to the in-flow of fine particles, the infiltration means (and/or outlet) can become blocked;
biology (inc. increase of bio-mass);
Due to the flow of nutrients or oxygen, biological activity may be increased whereby the infiltration means can be blocked by microbial growth.
Technological problems (inc. placement of too few extraction wells). The infiltration capacity of an infiltration well is mostly 50 to 80% less than that of an extraction well.
Costs
The costs of a full-scale in-situ bio-restoration via rinsing with water can vary between €37 per m³ to €223 per m³. The costs for the provision of infiltration means are similar to those for re-infiltration of ground-water. The overall cost will, to a great extent, be determined by the cost of chemicals - which depend greatly on the type and prevention of pollution .
Environmental burden and measures to be implemented
Initially, the extracted ground-water can be regarded as harmful to the environment. The discharged waste water stream must be purified. There are number of techniques available, see table below for the Best Available Techniques for ground water purification:
|
Pollutant |
Technique |
|---|---|
|
VOCl |
|
|
BTEX |
|
|
Mineral oil |
|
|
PAK |
|
|
Naphthalene |
|
|
Heavy metals |
|
|
MTBE |
|
|
BOD/COD/suspended solids |
|
If the pumped water is cleaned and re-infiltrated, then there is barely any waste water flow from polluted ground-water. Another possibility is the pumping of polluted ground-water and the infiltration of clean (tap) water. This also depends on the selected purification installation which will lead to emissions into surface water, the sewage system and air. Low concentrations are attainable with the available purification techniques.
During the extraction of ground-water, there is a risk that pollutants may be released into the air (an air outlet is only present in vacuum pumps). Undesirable emissions into the air may occur in pump containers, buffer tanks, biological purification (bio-rotor) or during air stripping of extracted ground-water.
A waste stream, of drilled or excavated polluted ground, could possibly be released when extraction and infiltration filters are placed. In addition, a waste stream of activated carbon or polluted silt, may be released during the purification of ground-water. In most cases, this waste stream is disposed of or incinerated. If a regeneratable activated carbon filter is used, the carbon can be re-used.
Extraction and infiltration filters are mostly placed underground, so that they present no problems. Only compressors and pumps may lead to noise-related problems. Buildings can remain intact and roads do not have to be dug up. In placing this installation, and possibly the clean up installation, short-term problems may arise from lorry transport.