Principle
- Multiphase extraction/high vacuum extraction
- Bio-slurping
- Water purification
- Air purification
- NAPL removal
Figure: Diagram of NAPL extraction
In-situ NAPL extraction is a variation of ‘pump & treat’ or ground-water extraction is also referred to as high-vacuum extraction / multi-phase extraction / dual-phase extraction or bio-slurping. An in-situ floating-layer remediation removes un-dissolved organic pollutants (LNAPL floating layers or residual DNAPL) that are present on the ground-water as free-phase.
The theory behind the technique is based on the application of sub-normal pressure (‘vacuum’) to extract and remove NAPLs via the gas-phase on the one hand, to control capillary forces immobilised by NAPLs in the soil on the other hand, and to extract as much NAPL and vapour as possible.
Implementation area and implementation conditions
Removal from soil without excavation taking place is possible using vertical filters which are connected to pumps; connected to the filter’s built-in pump or to an above-ground vacuum pump. The filters that have built-in pumps are connected to a double pump system. A pump ensures a decrease in the level of ground water, which creates a force that drives the floating layer towards the vertical filter. A pump is placed at a higher point in the level indicator, whereby the floating layer can be selectively removed.
There are numerous variations in the implementation of in-situ pump systems, whereby different methods are used for extracting the floating layer. The floating layer can be pumped away via a floating pump or a pump installed at a fixed height. In other systems, a floating filter displaces the floating layer into a reservoir, which is then pumped away using a pump at ground level..
Bio-slurping or multi-phase systems are characterised by one pump system that has been set up at ground level for the extraction of the floating layer, water and air-phase.
A buffer and an oil separation system must be placed behind the pump systems.
Besides active NAPL removal, the NAPL can also be removed via skimming, in vertical filters, on a drain with drainage well or even with an open trench or gravel-filled trench.
The soil must be sufficiently permeable in order to guarantee that the floating layer flows to the extraction systems. The viscosity of the floating layer must be low enough to allow it to flow towards the filter. Heavy oil components, such as fuel oil or motor oil, will only be slowly transported towards the filter system due to their high viscosity.
Costs
The costs for in-situ pump systems amount to circa 400 or 500 euros per week per filter. If the number of filters increases, then the average cost per filter will decrease. The treatment of removed oil products is an additional cost, which can cost up to 150 or 250 euros/ton. In some cases the recuperated oil product is of such quality that it can be re-used (e.g. as fuel).
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 |
|
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, from 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.
The extraction of ground-water provides relatively few problems to the environment. 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.
[1] in combinatie met zandfiltratie