Clean-up techniques for sediments
Sand
screening (datasheet)
(sand)
Final sand screening differs slightly from phase 1 screening
as, although the same screeners can be used, they are equipped
with a finer mesh screen. Furthermore, a larger role is
often attributed to mini sand screeners (5 to 2 mm mesh),
allowing even finer screening. However this technique is
often overused, meaning that limitations and recommendations
should be made by technical advisors and closely followed
by responders.
Draining (datasheet)
(sand)
In the presence of fluid oil, water with a high oil content
may infiltrate into the ground. The technique of draining
can be used to channel this water to contain and recover
it. This involves digging oblique, parallel or radial furrows
on the foreshore using shovels or ploughs, leading to a
collection point at the far end (simple bund, transversal
trench, system of planks, shore-sealing boom or floating
boom on the water).
The oil is collected by skimming, pumping or sorption, depending on the volume of concentrated oil. This technique is easy to implement but can be relatively slow, and may need to be repeated. Hoses or jets can be used on the upper foreshore to improve the process by facilitating the circulation of water and freeing of oil, in particular in the case of coarse sediments.
Underwater
agitation (datasheet)
(sand and stones)
This technique is designed to remove pollutant buried in
sediments. It is carried out under at least 5 cm of seawater
and consists of washing away the sediments using hoses to
free the pollutant, in such a way as to allow its recovery
at the water surface, by skimming or sorption according
to the case in hand.
This solution is particularly suited to fluid substances. It can however, as was the case during the Erika, be useful to break and bring to the surface sunken patches of heavy fuel oil. A fire hose or, better still, a special PVC hose known as an “impact hose”, is sufficient for restricted areas, where several responders operate standing in the water. A mechanical adaptation of this technique could also be envisaged.
Tilling (datasheet)
(sand)
Tilling is carried out in the presence of water and fulfils
the same purpose as underwater agitation. Here however,
an agricultural harrow or a rototiller can be employed to
work the sand. Hoses can also be used simultaneously to
tilling.
Tilling
can also be used for another purpose, to promote natural
break down of oil by improving both aeration and exposure
to UV rays. This technique can be applied on beaches contaminated
by a light pollutant, which no longer appears as mobile
accumulations but rather as a homogeneous and more or less
pronounced coloration of the sand. Where deemed appropriate,
it can be coupled with a bioremediation operation.
Surfwashing (datasheet)
(datasheet)
(sand and stones)
During phase 2, this technique, which involves moving sediments
down the beach in order to subject them to the natural cleaning
action of the sea, is used for various purposes:
For stones:
- whatever the type of pollutant, to complete prior washing
- or when they are lightly oiled
- as an alternative to screening (in the case of the presence of micro tar balls of heavy fuel oil)
- or to accelerate the mixing of the sand in the case of contamination by a light crude.
In
the first case, it is often not necessary to provide a pollutant
recovery system (manual, mop nets or sorption). On the contrary,
in the second case the pollutant is often brought out of
the sediments in the form of clusters of varying sizes or
considerable sheen.
This technique presents geomorphological risks whenever
it involves significant volumes of sediments or concerns
a site where the shingle bar plays a locally important role
in defence against erosion of the shoreline by the sea.
In this case, its implementation necessarily requires prior
approval by a geomorphological expert, as well as regular
monitoring and strict control before worksite closure.
Washing
stones
(stones)
Different techniques can be considered for washing stones,
according to the volume needing treated, the accessibility
and the size of the site. Recourse to a washing agent is
not always necessary, such products being costly and largely
impeding the recovery of effluents.
•
Washing on "cages" (datasheet)
This technique, developed during the Erika pollution, is
an improved version of the on site washing of stones using
a pressure washer in wire mesh tanks or on a raised perforated
sheet of metal. The “cage” is a light wire structure
comprising a perforated metal base, on which the stones
are washed, and three side walls, covered with geotextile,
to contain projections of effluents and oil. All the effluents
pass through the base and into a container installed prior
to operations below the cage.
•
Washing in a concrete mixer (datasheet)
Stones can also be washed using a small concrete mixer set
up on the beach, over a recovery system. The method has
certain variants, which can be used according to the nature
of the pollutant:
- Dry, with the possibility of adding sand as an abrasive
material
- With water, possibly mixed beforehand with solvent, followed
by rinsing.
•
Washing in a concrete mixer drum (datasheet)
This offsite process using a standard concrete mixing drum
allows a reasonable level of washing, with limited material,
human and financial means. This concept was tested in the
UK a few years ago by the Warren Spring Laboratory and has
since been improved and widely used during oil spill response
overseas, notably on Cedre’s recommendation.
The stones are removed, washed and returned to their original location as soon as washing is complete. Such a worksite cannot be improvised, as it must only be set up if the temporary removal of stones is certain not to harm the sedimentary balance of the local environment and if there is a suitable area close by. As for concrete mixers, this washing method has certain variations, however use of a washing agent is near systematic and imposes greater constraints. The contaminated stones turn for a few minutes with a neat washing agent, then with water which is subsequently settled, skimmed and reused. The floating pollutant is removed by an overflow into a specific tank and the stones are rinsed on a wire mesh tank using a thermal pressure washer.
•
Washing at a Polmar plant (datasheet)
A specific stone and sand washing plant was developed after
the Tanio incident in the early 1980s by the Institut Français
du Pétrole, the Laboratoire Central des Ponts-et-Chaussées
and Cedre. This plant works using hot water in a closed
circuit, and the water and products of the process are recycled.
This installation is imposing due to its size and the space
required for its implementation. It has only been used once
for a real spill (Amazzone, 1988, Baie d’Audierne,
Briattany, France). It was thereafter entirely rehabilitated
and returned to the Polmar stockpile in Brest. This plant
ensures a high washing rate (15 to 20 m3/h), while
offering the necessary comfort and safety conditions for
a long-lasting worksite.