uk.jpg (7062 byte) Detecting accidental hydrocarbons pollution in rainwater

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 m 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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