Oil in Water and VOC Monitoring for Environmental Protection

A wastewater treatment plant


Now, more than ever, the environment is one of the key issues concerning everyone globally, from politicians and scientists, to the general public. Responsible businesses must acknowledge the importance of protecting our ecosystems, and prosecutions for polluting the environment can not only lead to expensive fines, but can also result in irreparable damage to an organisation’s public image.

Correspondingly, both prosecutions by environmental agencies, and the media interest in these cases, are on the rise. Water utility companies are also recognising the importance of the issue and are investing in detection and monitoring equipment, not just for clean water intake but also for waste water discharge from storm drains. This has led to some high profile companies being taken to court, large fines issued and expensive clean-up operations undertaken.

In some of these cases involving hydrocarbon pollutants, early warning VOC detection technology could have prevented or significantly reduced both the impact on the environment and the clean-up costs. Available VOC detection methods can be simply installed, very easily maintained and calibrated, and have a very low cost of ownership.


Volatile Organic Compounds (VOCs) are naturally occurring in the atmosphere. Humans, plants and animals naturally breathe them out, however they can be damaging to breathe in above certain levels.

A few of the more common VOCs and their industrial sources are shown below. The term “VOCs” actually covers a very wide range of organic compounds. In the table in the Appendix you can find a list of VOCs and their threshold values.

Most Common VOCs and Industrial Sources
VOC Industrial Source
BTEX (Benzene, Toluene, Ethyl benzene, Xylene), Hexane, Cyclohexane and Trimethylbenzene. Petrol, diesel, fuel oil, paint thinners, oil based stains and paints, insecticides, mineral spirits and furniture polishes
Acetone, Ethyl Alcohol, Isopropyl Alcohol, Methacrylates , Ethyl Acetate Nail polish and remover, colognes, perfumes, rubbing alcohol, hair spray
Tetrachlorethene(PERC) and Trichloroethene (TCE) Dry cleaning liquid, spot removers, fabric/leather cleaners
d-limonene (citrus odour), a-pinene (pine odour), isoprene Citrus (orange) oil or pine oil cleaners, solvents and some odour masking products
Tetrahydrofuran, Cyclohexane, Methyl ethyl ketone (MEK), Toluene, Acetone, Hexane, 1,1,1-Trichloroethane, Methyl-iso-butyl ketone (MIBK) PVC cement and primer, various adhesives, contact cement, model cement
Methylene chloride, Toluene, older products may contain Carbon Tetrachloride Paint stripper, adhesive (glue) removers
Methylene chloride, PERC, TCE, Toluene, Xylenes, Methyl ethyl ketone, 1,1,1-Trichloroethane Degreasers, aerosol penetrating oils, brake cleaner, carburettor cleaner, commercial solvents, electronics cleaners, spray lubricants
1,4-Dichlorobenzene, Naphthalene Moth balls, moth flakes, deodorisers, air fresheners
Freon (Trichlorofluoromethane, Dichlorodifluoromethane) Refrigerant from air conditioners, freezers, refrigerators, dehumidifiers
Heptane, Butane, Pentane Aerosol spray products for some paints, cosmetics, automotive products, leather treatments, pesticides
Formaldehyde Upholstered furniture, carpets, plywood, pressed wood products

Cost of the clean-up

Equipment failure, tank leaks, vandalism, accidental spillage or illegal dumping are just a few ways in which VOCs can contaminate the environment.

As the Environment Agency is becoming more effective at bringing cases to court, environmental issues are also under increasing public and political scrutiny, leading to harsher penalties, fines and negative publicity for those who cause damage to the environment.

Below are a few examples where spills and leaks have occurred, and adequate procedures were not in place, to contain or detect these spills. This has led to the Environment Agency bringing legal action / court cases against the businesses involved, and the issuing of fines and orders to clean up the contamination.

The cost of a leak

Over a period of about 4 weeks in July 2005 some 653 tonnes of kerosene leaked from a small hole in the base of a tank at a storage facility at Waterston, Milford Haven.

The resultant pollution led to the destruction of habitat of the nearby Hazelbeach stream and the closure of the beach during August 2005.

The company was subsequently prosecuted by the Environment Agency Wales and pleaded guilty, were fined £29,900 and ordered to pay costs of £39,801. In addition, the company has estimated that the clean-up operation cost them around £3 million

The cost of equipment failure

A fuel distribution company admitted polluting a tributary of the River Clyst with 22,000 litres of red diesel.

The leak was traced to the company after significant quantities of oil was spotted in the watercourse, however initial checks showed no loss of stock on their computer system. Pressure tests and excavation revealed a hole in a supply pipe, leading to a fuel dispensing island.

Magistrates recognised the work, undertaken by the company, to remediate the environment, and fined them £5,000 with costs of £3,700.

More recently (2019) the Northern Ireland Environment Agency found red diesel discharging to the Ballyclare river from a culvert and this has led to actions being taken,

The cost of human error

A haulage firm was ordered to pay £6,867 in fines and costs after an oil interceptor overflowed, polluting the River Thames with thick black waste engine oil and causing significant damage to local wildlife.

The company admitted in court that they had failed to empty the interceptor, preventing it from operating correctly. Around 40 swans were affected and some had to be rescued by the Swan Sanctuary in Shepperton.

The cost of vandalism

Oil from a forklift repair site ended up in a drain near Spalding resulting in one swan having to be cleaned up and the death of invertebrates.

The business was fined £8,000 and ordered to pay full Environment Agency costs of £4,000 by Spalding Magistrates’ Court after pleading guilty to polluting a tributary of Hammond Beck.

Environment Agency officers were told that an oil tank had been vandalised on the site and a pipe had been ripped off.

The business hadn’t realised there was a surface water drain underneath the area where the tank had stood and so hadn’t notified the Agency of the spill. Correspondingly, the environmental impact may have been reduced, if it had been realised sooner that the oil had entered the surface water drainage for the site.

Detection Approaches and issues

There are various methods of measurement and detection of VOCs; the most accurate, but slowest and most expensive, are the laboratory based methods of Gas Chromatography, Mass Spectrometry and Flame Ionisation Detection (FID) analysers, whose costs can run to $100K and more.

Laser based methods of oil in water detection are available, but these require the oil to be in an emulsion form or have formed an oil slick on the surface (oil on water). These methods can only detect relatively high concentrations of oil - typically 1ppm for oil in water and much higher for oil on water.

The oil on water detector measures the reflectance change of the surface - oil can reflect light better than water. The system requires a motionless surface, free from dust or dead leaves and away from direct sunlight.
The oil in water detectors use either light scatter techniques or fluorescence and can measure down to 1ppm concentrations. This technique offers 24/7 online monitoring, but is susceptible to false results if the water has a high turbidity level. The detectors need regular maintenance and cleaning of the sample chamber, as particles can clog the system up.

There are fortunately more cost effective, faster and more portable methods of VOC detection, for example photoionisation detectors (PIDs) and electronic Nose Technology (E-NOSE).

A photoionisation detector (PID) uses an ultraviolet (UV) lamp to irradiate incoming gas. The UV energy ionises the molecules, producing an ionic current which is then measured. PID’s are broadband detectors and are not selective, as they ionise all molecules passing through the detector, which have similar ionisation energy as the UV lamp used. The advantage of PIDs is that they can give a quick on the spot measurement of VOC concentrations.

Some of the disadvantages of PID’s are:

  • water vapour, condensation, temperature and quenching can limit their performance to at best 0.1ppm (under controlled condition) but 1ppm typically.
  • PIDs need regular maintenance and calibration of the UV lamp, driver and detection circuit.
  • The cell also needs regular cleaning as dust and microfibres can increase condensation.
  • The calibration procedure is expensive and complicated and uses 10 ppm compressed isobutylene gas. (See Appendix IV)

E-NOSE technology uses a semiconducting material (metal oxide) which is applied to a non-conducting substance (substrate) between two electrodes. The substrate is heated to a temperature (around 400oC) at which the presence of the gas can cause a reversible change in the conductivity of the semi-conducting material

  • When no gas is present, oxygen is ionised onto the surface and the sensor becomes semi-conductive.
  • When molecules of the gas of interest are present, they replace the oxygen ions, decreasing the resistance between the electrodes.
  • This change is measured electrically and is proportional to the concentration of the gas being measured

This makes the E-NOSE a broadband VOC detector technology. An example of this type of sensor technology is the MS1200 VOC monitors.

This method has the advantages of:

  1. High sensitivity i.e. the monitor can detect VOC concentration levels to 1 ppb.
  2. Sensors have a long life and do not require cleaning.
  3. The sensors auto zero before every sample measurement to take into account sensor drift and aging effects. This is done by passing filtered clean dry air over the sensors at every sample cycle.
  4. There are 2 filter materials used in the monitor, a dust filter, activated carbon . These are the only consumables used in the systems and only need replacing every 6 months.
  5. Due to the robustness of the sensor technology the monitors can be deployed as an online monitoring system giving 24/7 detection and measurement.

A validation check procedure has been developed; this method introduces a 50ppb toluene in water solution, the monitor then samples the air/toluene vapour mixture to check the response

In Summary

PIDs provide quick on the spot VOC detection and measurement technology, but have lower sensitivity and are more expensive compared to E-NOSE. They also require increased maintenance, more frequent calibration and are sensitive to changes in humidity.

E-NOSE is a cost effective, robust, high sensitivity solution with low maintenance costs and 24/7 monitoring coverage. The only consumable used is filtering materials, which need replacing only once every 6 months, together with a simple validation check.

The MS1200 monitor has now been used extensively on clean water intake monitoring. There are also now a number of sites which are using the monitor for industrial emissions monitoring and VOC detection.


Several technologies are available for the purpose of detecting VOCs in industrial discharges. It has been argued that the most robust, sensitive and effective method, uses gas sensing semiconductor E-NOSE devices, due to their long life, infrequent calibration requirements and simple maintenance. The example referred to is the Multisensor MS1200.

Case studies have been presented to show how easily such an instrument can be deployed and integrated into the existing emergency/security control system, within a modern industrial complex. Thus providing both re assurance to the company using it and/or to agencies protecting the environment, into which discharges are made.

Table: Threshold Limit Value

Here you can see some of the Threshold Limit Value (TLV) for some of the more common VOCs. The TLV of a chemical substance is a level to which it is believed a worker can be exposed, day after day for a working lifetime, without adverse health effects.

VOC TLV Boiling Point °C Vapour Pressure at 20 °C
Benzene 0.5 ppm 80 10kPa
Toluene 50 ppm 111 2.3kPa
Ethyl benzene 100 ppm 136 0.9kPa
Xylene 100 ppm 144 0.91kPa
Cyclohexane 100 ppm 81 10.3kPa
Petrol 300 ppm 20-200 220kPa(Butane)
Acetone 500 ppm 56 24kPa
Isopropyl Alcohol 200 ppm 83 4.4kPa
Trichloroethene 50 ppm 87 7.8kPa
Tetrahydrofuran 50 ppm 66 19.3kPa
Methyl ethyl ketone 200 ppm 80 10.5kPa
Methylene chloride 50 ppm 40 47.3kPa
1,1,1-trichloroethane 350 ppm 74 13.3kPa
Formaldehyde 0.3 ppm -20 3.46kPa at 25
Water n/a 100 2.3kPa