Monitoring
and mitigating the risk of rainwater/wastewater pollution has become a public priority.
Systems dealing with chronic pollution, such as oil-skimmers and oil-separators, are
pretty efficient in retaining most of the hydrocarbons pollution. However, in case of
accidental hydrocarbons pollution, a solution providing the treatment by local separation
will only be efficient if a detection system is used as an alarm.
To detect this type of pollution as soon and as fast as possible the IFP (French Institute
of Petroleum) and NEREIDES have designed a hydrocarbons detector with a sensitive
polymeric membrane.
This membrane offers very short reaction times and detects close to all hydrocarbons and
chloric solutions.
A hydrocarbon pollution can, at times, escape
a decantation tank or an interception system.
The oil spill detector, if linked to an automation system, can activate a valve that will
shut down the outflow and preserve the environment.
The detection system, “Oilspy”, allows a continuous monitoring of wastewater or
rainwater before it returns to the environment. Also, in case of accidental pollution, the
operator can rapidly locate the problem.The European
council directive of the 21st
of May 1991 concerning urban
waste water treatment (91/271/EEC), defines the legal framework on dealing with the
run-off rain water and the waste water discharge. Also, the directive indicates that
agglomerations have the responsibility over run-off rain water and the urban waste water
discharge within their perimeter.
The separation of run-off rain water and waste
water enacted since the early 90’s allows local governments across Europe to comply with the European directives on the matter without
engaging gigantic financial means and technical solutions.
However, separation is not equivalent to treatment.
Many studies conducted in the USA and Europe show
that the mean annual quantity of pollution from run-off water origin discharged within the
environment is similar to the levels of urban waste water discharge after treatment.
Therefore, risk mitigation of run-off water pollution has become a public priority.
Local governments have put in place run-off water management strategies as of the early 90’s.
For example, 120 storm retention basin have been installed in the French administrative
region of “Bas-Rhin”, those basins
represent 30,000 m³ of storage capacity.
To deal with
pollution that could occur in run-off rain water discharge a simple typology has been put
in place which reminds the adequate solution to each problem. Two types of pollution are
to distinguish within run-off rain water:
>“occasional” pollution,
result of the normal wash-off of industrial and urban grounds by rain water.
>“accidental” pollution,
result of unexpected spills or leakages from industrial sites, by nature this type of
pollution is unpredictable and difficult to characterize.
Hydrocarbons
pollution represent 30% of the overall “occasional” pollution. S
everal companies have designed and manufacture an array of solutions (decantation tanks,
separators, skimmers etc.) to capture those hydrocarbons particles.
For hydrocarbons particles inferior to 50 microns, those systems are effective between 35
and 90% depending on the technical solution.
Since it is difficult to predict the
frequency or the extent of a typical accidental pollutions, systematic treatment solutions
are not easy to put in place. However an alarm system allows an operator to be informed as
soon as possible of this type of pollution. Whenever an alarm goes off, the procedure would be to
shut down the water flow, identify the polluter and activate the appropriate pollution
treatment response.
Also, in case of heavy rains,
having a storm retention basin integrated within the urban waste water network allows to
differ run-off discharge to the environment. Placing an alarm system within those basin
would allow monitoring of the run-off before discharge, an obstruction of the outflow
could be activated automatically before contamination.
To summarize the
several situation, a detection and alarm system for accidental pollution should be
considered when :
-
A mechanical hydrocarbons pollution treatment system which will,
eventually, from time to time, let small particles of pollutants go through.
-
Complementing a separator or skimmer to function as an indicator
for maintenance
-
As a watchdog for industrial waste water discharge to offer the
ability to shut down the water flow when a pollution is detected.
In practice pollution
detection should be considered at two levels, as close of the potential pollution source
as possible (upstream of a retention basin within a industrial complex); and between a
urban waste water treatment plant and the environment.
Detection by
polymeric membrane
The alarm system presented is
a polymeric membrane detection device which reacts to hydrocarbons. The membrane was
conjointly developed by NEREIDES and the IFP (French Institute of Petroleum). The membrane
is maintained in tension by a spring mechanism, and it is stable in air and when immersed
in water. Whenever the membrane enters in contact with hydrocarbons, it dissolves and the
rupture activates the mechanism and transmits the alarm. According to its polymeric
formula and its thickness, it is possible to vary the sensitivity of the detecting device.
As of now, only one type of membrane is sold (thickness of 0.1 mm),
others are currently in development.
The laboratory test results
show conclusive results on a large array of hydrocarbons (aliphatic and aromatic) (refer
to table 1). Independent test on the membrane have been conducted by the BRGM (French
Research Bureau of Geology and Mining) and
the CEDRE (French Center for
Documentation, Research and Experimentation on Accidental Oil-Related Pollution in Water),
they both confirm the high sensitivity of the membrane for a pollution as thick as 0.1mm.
The pollutants tested include: super grade petrol, kerosene, benzene, toluene and xylene
(for futher details refer to table 1).
The membrane is a detection
system that is sensitive, reliable and user friendly. As compared to other methods of
detection, it allows detection of a large array of hydrocarbons (aliphatic in particular)
and the reaction time is very short (5 seconds for benzene, 5-10 seconds for chloric
solvents).
The system does not drift,
does not need any calibration nor complementary analytic tools to operate as opposed to
the usual sampling systems (fluometric systems for example). The membrane appears to be
very sensitive to aromatic hydrocarbons but does not offer the possibility to detect the
hydrocarbons attached to solid particles. However the cumulative properties of the
membrane will, at times, cause an activation of alarm due to regular absorption of
particles that are below the level of detection. Therefore, to ensure an optimum
functionning, the membrane has to be replaced every 6 months.
After laboratory and in
situ experiences, the product has evolved over the years. Specially regarding the
stabilization of the membrane over time and the capacity of the device to operate within
an explosive context (Atex certification). As of today, no equivalent detection device by
polymeric membrane exists with similar reactivity and capacity.
The Oilspy Detection
Device
The fuse membrane is placed
between a lower fixed axis and a moving upper axis. The moving upper axis is linked to a
dry-contact. The dry-contact activates a printed circuit which is placed within a
protective aluminium. The membrane is usually maintained at a nominal tension of 150g,
however it is possible to increase the tension in order to accelerate the reaction time
for a given context. For example the device will detect a crude oil leak in less than 4
minutes if the membrane is at a 700g tension. The device can be installed on buoyant
structure or a slide to follow the water level variations and does not necessitate any
calibration. The option for communication of the signal include cable, radio and GSM.
Those three solutions allow the system to communicate from a distance of a few meters to
several kilometres.
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Detecting
accidental hydrocarbons pollution in rainwater
