Oil is one of the most common types and highly visible forms of water pollution. As it can easily spread, even small quantities can potentially cause harm to the aquatic environment.
Oil is also a risk to sewage treatment works, where accidental discharges can be difficult and costly to clean up, and just a small quantity of oil can have a disproportionate impact by tainting drinking water at an extremely low concentration.
Trade effluent is a liquid waste produced in the course of any trade or industry, which is discharged to the waste water system, and must be controlled because of the potential harm it can cause. As such, the discharge of trade effluent requires a consent from representative authorities.
Typical consent parameters and example figures are given below:
• Oil (and grease) – typically 100 mg/l
• Suspended solids – limit typically 1000 mg/l
• COD (Chemical Oxygen Demand) – limit typically 2000 mg/l
• BOD (Biochemical Oxygen Demand) – limited typically 1000 mg/l
• pH – limit typically in the range: pH 6-10; pH 5-11
Oil is therefore one of the key parameters that must be monitored and controlled.
Sources of oil contamination to water
There are many potential sources that could lead to water contamination by oil. According to SEPA, the Scottish Environmental Protection Agency, the most common types of oil pollutants are diesel, central heating oil, waste oil and to a lesser extent, petrol.
The most frequent causes of oil pollution are spillage during delivery, leakage from poorly maintained or damaged tanks, lack of oil separators, waste oil disposal into drainage systems, waste oil dumped onto or into the ground, and in some cases waste oil burnt in the open. In the majority of these incidents there is often no emergency oil spill response plan and/or equipment available. Other sources of pollution include corroding pipelines (below and above ground) and illegal oil discharges at sea.
For large power plant operations, a significant amount of cooling water is used to cool equipment such as pump motors, compressors and transformers. Oil leaks sometimes occur which can result in oil pollution of the cooling water. Oily wastewater can also come from boiler feed, leaks from lubrication systems, and from drip pans such as those below transformers.
For oil refineries, a huge amount of water is used for operations such as de-salting and cooling. It was reported by CONCAWE (Conservation of Clear Air and Water in Europe) that the total aqueous effluent from the EU refineries in 2010 reached 1,583 million tonnes. Process water in particular that resulted from desalting, can contain a significant amount of crude oil.
Legal aspects of oil in water discharges
In many countries, regulations related to aqueous discharges in surface waters set maximum values to the quantity of a limited number of contaminants which can be released in the surface waters.
As for discharge in the European Union (EU), mineral oil and hydrocarbons are List 1 substances under the original Dangerous Substances Directive 76/464/EEC. The Directive covered discharges to inland surface waters, territorial waters, inland coastal waters and ground water. Therefore discharge of water contaminated with oil and hydrocarbons is strictly controlled and regulated.
The Directive 76/464/EEC was integrated into the Water Framework Directive (2000/60/EC) which was adopted in September 2000, and the Directive 76/464/EEC was subsequently repealed in 2013. Directive 76/464/EEC has since been codified as 2006/11/EC.
The Water Framework Directive 2000/60/EC is less prescriptive, it better integrates the various components, but is very complex, as it takes into account the characteristics of the water in which the discharge takes place, the environment, the local species, human activities, and their interactions.
For surface water discharges, which refer to those that are discharged to groundwater, sea, river or lake, in terms of mineral oil and hydrocarbons contained in the discharge water, the discharge limit depends on the potential local environmental impact. There are generally two types of consents – numeric and descriptive. In general, for wastewater discharge into the control water, in numeric terms, up to 5 or 10 mg/l of oil and hydrocarbons may be consented from governmental agencies depending upon the quantity of the discharge and also the receiving environment. In descriptive terms “no visible oil” has been also used.
Effluent from industrial processes is normally discharged to a sewer, subject to the approval of water authorities. Concentration of oil in these discharged waters can vary significantly from a few mg/l to hundreds of mg/l. For refineries in Europe, an annual average of a maximum of 5 mg/l in the effluents was stipulated in PARCOM Recommendation 89/5.
Overall, mineral oil and hydrocarbons are List 1 substances and therefore discharge of these to the control water is strictly regulated. The control water covers virtually all fresh and saline natural waters up to the offshore territorial limit, including rivers, streams, lakes, estuaries, coastal water and ground water.
Oil in water treatment technologies
Oil in water can be present in both dispersed and dissolved forms. Dispersed oils are those found in water in the form of small droplets. Dissolved oils are those present in a soluble form. Examples include BTEX (Benzene Toluene, Ethyl-benzene and Xylene).
There are many treatment technologies that can be used to treat oil-contaminated water. They range from mechanical, physical/chemical to biological. These include:
- Gravity and enhanced gravity
- Gas flotation
- Filtration (sand, walnut shell, membrane etc)
- Absorption and adsorption
- Chemical / oxidation
For dispersed oils, gravity, gas flotation and filtration-based technologies are commonly used. Separation efficiency will depend on many factors, e.g. oil droplet size, density of the oil, viscosity and density of the water phase, bubble size, flow rate, temperature. For dissolved oils, gravity or filtration-based methods are ineffective. Other technologies such as absorption, adsorption, chemical / oxidation and biological based are often required.
A European standard (BS EN 858-1:2002 and BS EN 858-2:2003) for the design and use of prefabricated oil separators is available. There are two classes referred to in the standard. Class 1 separators, which are designed to achieve a concentration of less than 5 mg/l of oil under standard test conditions, should be used when it is required to remove very small droplets. Class 2 separators are designed to achieve a concentration of less than 100 mg/l under standard test conditions and are suitable for dealing with discharges where a lower quality requirement applies (e.g. where the effluent passes to foul sewer).
For oil refineries, a typical wastewater treatment plant consists of primary and secondary oil/water separation to remove most of the oil, followed by biological treatment, and tertiary treatment (if necessary) to remove the remaining oil and other contaminations. For the primary and secondary oil / water separation, this is usually achieved by using an API gravity separator followed by a Dissolved Air Flotation (DAF) or Induced Gas Flotation (IAF) unit. Water discharging from the flotation units is then routed to an aeration tank / clarifier, which constitutes the biological system. A tertiary treatment may be added prior to the discharge of the treated water. According to CONCAWE, the concentration of oil in the discharged effluents from refineries in Europe have been consistently less than 2 mg/l, since the year 2000.
Oil in water analysis methods
Oil in water is a method-defined parameter. As mentioned previously, oil in water can be present in different forms: free oil, dispersed oil and dissolved oil.
Free oil usually refers to oil floating on the surface of water or those very large oil droplets that would settle to the surface very quickly.
Dispersed oil refers to oil in water in the form of small droplets, which may range from sub-microns to hundreds of microns. It may contain aliphatic, aromatic hydrocarbons and other organics, e.g. acids and phenols.
Dissolved oil refers to oil in water in a soluble form. Aliphatic hydrocarbons in general have very low solubility in water. It is the aromatic hydrocarbons, in particular the single ring BTEXs and two ring NPDs (Naphthalene, Phenanthrene and Dibenzothiophene), together with those organic acids (e.g. fatty acids and naphthenic acids) and phenols that form the bulk of dissolved oil.
It is worth mentioning that the amount of dissolved and dispersed oil in water can increase or decrease depending on the processing conditions, such as temperature, pressure, flow rate and what treatment technologies are used. Also some of these constituents may be present but might not contribute to the measured oil in water. The relative contribution that these components make to the oil in water content depends on the method used for analysis.
There are three main types of reference oil in water measurement methods. These are briefly described below.
In a typical infrared absorption based method, an oily water sample is first acidified, then extracted typically by a chlorofluorocarbon (CFC) solvent. Once the extract is separated from the water sample, it is dried and purified by the removal of polar compounds. A portion of the extract is placed into an infrared instrument, where the absorbance is measured. By comparing the absorbance obtained from a sample extract to those that are prepared with known oil concentrations, the oil concentration in the original sample can be calculated. A well- used example of Infrared based reference method is the IP 426/98.
Gravimetric-based methods measure anything extractable by a solvent, which is not removed during a solvent evaporation process and is capable of being weighed. In a typical gravimetric-based method, an oily water sample is acidified and then extracted by a solvent. After separating the solvent (now containing oil) from the water sample, it is placed into a flask, which has been weighed beforehand. The flask is placed into a temperature controlled water bath, and the solvent is evaporated at a specific temperature, condensed and collected. After the solvent is evaporated, the flask now containing the residual oil, is dried and weighed. Knowing the weight of the empty flask, the amount of residual oil can be calculated. A good example of gravimetric-based reference methods is the USA EPA Method 1664.
GC-FID (Gas Chromatography and Flame Ionisation Detection)
Unlike infrared and gravimetric methods, the use of GC-FID offers the potential for obtaining details of the different types of hydrocarbons in the oil fraction.
In a typical GC-FID method, an oily water sample is acidified and extracted by a solvent. The extract is then dried and purified before a small amount of the extract is injected into a GC instrument. With the help of a carrier gas and the chromatographic column, different groups of hydrocarbons will then leave the column at different times and be detected. An example of a GC-FID-based reference method is the ISO 9377-2.
Which reference method to use?
It should be emphasised that different methods will produce different results. It should also be said that each separate method will require different instruments and procedures, which affects costs (capital and operational), training, health and safety.
Until recently, infrared-based methods were commonly used with portable fixed wavelength instruments available. However, due to the use of CFC, and the lack of compositional details from the measurement method, they are becoming less popular.
Gravimetric methods are simple and relatively cheap, but again they do not provide details of the composition. Due to the evaporation procedures involved in gravimetric methods, there is also some loss of volatile components.
GC-FID methods do not require the use of CFCs, have no issues with the loss of volatiles, and have the potential to provide detailed information on composition, but they necessitate sophisticated instruments which require skilled operators.
Sampling and sample handling
While measurement methods are very important for obtaining good results, it is important to understand that a measurement method can only give a result as good as the sample provided. Therefore, if the samples used for the analysis are not representative of the flow stream, the results obtained will be of little use.
To obtain a representative sample, there are a number of aspects that one has to consider:
• Location of taking a sample
• Selection of the appropriate sampling devices
• Iso-kinetic sampling if samples are taken from a pipe. Iso-kinetic sampling means that samples are taken such that the velocity of fluid in the sampling pipe is the same as that in the main flow pipe
• Sample bottles, which must be scrupulously clean
For regulatory compliance monitoring, samples may have to be taken at specified locations, while for process control and optimisation, this is not an issue.
Once a representative sample is obtained, the sample must be properly handled, which will depend on when, where and how the sample is to be analysed and also on whether the samples are for regulatory compliance or process optimisation. In general, sample handling may include the following aspects:
Acidification serves two main purposes: firstly, to preserve the samples by killing bacteria which can degrade oil; secondly to dissolve precipitates such as iron oxide, calcium carbonate, which can stabilise an emulsion and therefore prevent a complete separation between solvent extract and water after the extraction process. In general, the pH value needs to be lowered to less than two. Usually this is achieved by adding a small volume of diluted HCl solution.
If a water sample is to be transported, in addition to acidification, the sample should be stored and transported in a suitable sealed container to prevent the ingress of light. Exposure to light may degrade hydrocarbons in the water sample and change the oil concentration.
Similarly, if oil in water samples are to be stored for whatever reason, according to the ISO 5667-3, in an ideal situation, they should be stored in a refrigerator with a temperature kept between 1 °C to 5 °C. The ISO standard also states that the maximum recommended preservation time before analysis for an oily water is one month.
Future trends and needs
With an increasing environmental awareness, a tighter regulation with a reduced oil discharge limit (currently 5 mg/l is widely accepted in the EU) may be introduced. A similar change has already taken place for the offshore oil and gas industry in the North Sea for which the oil in produced water performance standard of 40 mg/l was reduced to 30 mg/l in January 2007.
Treatment technology continues to evolve and improve, in particular, the likes of membrane technologies. This enables oil contaminated water to be treated and increasingly made available for re-use (for agriculture, livestock feeding, refill of aquifers and even drinking water). With a world population growing fast, water re-use will become increasingly important. Alongside the treatment technology, portable oil in water analysis methods as well as sophisticated online oil in and on water monitors have also been developed, which helps the monitoring and optimisation of treatment processes.
In recent years, substantial progress has also been made in the exploration and production of unconventional resources around the globe, in particular in the USA. This is a new frontier for Europe and UK. However, there are signs, certainly in the UK, that unconventional resources will be explored. One of the key issues related to unconventional gas exploration and production, that must be addressed, is the flowback water, and subsequent produced water. Both of them will contain oil as well as other components that require treatment.
Oil is one of the most common types of water pollution in the UK. It can result in harm to the aquatic environment, water treatment plants and also generate bad publicity. Pollution of oil can come from many different sources. Whilst treatment technologies are available, they are not always there at the right place and right time. Oil in water can be present in different forms. It usually forms part of trade effluent consent. Measurement and monitoring plays a key role in terms of process control and reporting for regulatory compliance. Oil in water is a method defined parameter, thus different methods will produce different results.
Common oil in water analysis reference methods include infrared, gravimetric and GC-FID based, each has its own advantages and disadvantages. To get a good oil in water result, in addition to having a good analysis method, sampling and sample handling is equally important, as measurement can only produce a result as good as the sample can provide.
Published: 06th Sep 2016 in AWE International