Oil in water monitors are used to detect the presence of hydrocarbons in water.

General Information

Terminology

The meaning of the phrase "oil in water" is highly contextual with reference to both oil and water. Within the oil and gas industry, produced water—that which is brought to the surface of a well mixed with residual oil and gas—is the type frequently tested and monitored. Additionally, the term "oil" may also refer to different substances divided by oil type or hydrocarbon type.

There are two broad types of oil:

• Dispersed oil exists in water as small droplets of varying sizes. It contains aliphatic and aromatic hydrocarbons (described in the section immediately following).
• Dissolved oil is primarily composed of aromatic hydrocarbons and is highly soluble in water.

Basics of Hydrocarbons

Hydrocarbons, or compounds which contain only carbon and hydrogen, make up a large portion of petroleum compounds. The group itself can be further divided into several different compound types.

• Aliphatic molecules do not contain a benzene ring. Aliphatics with single carbon-carbon bonds are referred to as alkanes, while alkenes and alkynes have double and triple bonds, respectively.
• Aromatic hydrocarbons contain benzene rings with six carbon atoms each bound to a single hydrogen atom.

Petroleum (crude oil) typically contains approximately 30% alkanes, 50% naphthenes (a type of cycloalkane), and 15% aromatics, with the remaining composition comprised of asphaltics.

Oil in Water Applications

Environmental regulations specify composition limits for effluent water; for this reason, accurate monitoring of effluent water in wastewater, oil and gas, and manufacturing industries is a requirement. In addition to environmental concerns, high oil content in process water is often a telltale indicator of equipment failure (particularly in the case of a leaking heat exchanger), or that the wastewater under measurement is unfit for reuse.

In general, oil in water monitors are useful in any application which requires oil monitoring within a treated or untreated water system, including wastewater, groundwater, produced water, process water, and natural run-off.

Methods

The detection of oil in water is method-dependent, meaning that there must be some standardization of test methods used to establish baseline measurements for which to compare data from disparate sources and formulate discharge limits and other legislation. By rule, reference methods are more time-consuming and involved than field methods, but are absolutely essential for the accuracy of subsequent monitoring. Three types of reference methods and nine types of field measurements are discussed below.

All processes are described in greater detail, with formulas and standard recommendations, in the Measurement of Oil in Produced Water text listed in the Resources section below. 

Reference Processes

Infrared Absorption

This method begins by extracting an oily water sample using a solvent. The sample is then purified, dried, and placed into an infrared instrument, which transmits infrared radiation through the sample. By comparing the absorbance and transmittance of this radiation to known standards, the oil content can be determined.

Like other spectroscopic methods of water quality testing, infrared absorption is heavily reliant on the Beer-Lambert Law, described in detail here.

Infrared absorption may involve the transmission of one or three separate wavelengths. ASTM D 7066, the most common active single-wavelength standard, specifies emission at 2930 cm^-1, and that the sample be extracted using S-316 solvent. The standard also recommends that instruments be calibrated using n-hexadecane with isooctane or the oil within the test sample.

While single-wavelength testing is capable of measuring only the total hydrocarbons within a sample, the three-wavelength method allows the calculation of aromatic and aliphatic compounds. It typically involves emission at 3030, 2960, and 2930 cm^-1 (the vibrating wavelengths of C-H [aromatic], CH-H [methylene], and CH₂-H [methyl] respectively).

A three-wavelength method used to detect hydrocarbons of various sizes to determine areas of thick spilled oil.

Image credit: USGS

Despite the fact that both infrared methods are well-established and fairly simple to employ using portable instruments, their lack of compositional detail and frequent use of ozone-depleting Freon solvents in unregulated testing has rendered them less-used.

Gravimetric

Gravimetric methods rely on the weight difference between oils, water, and solvents. A sample is first removed using a solvent, and the solvent/oil mixture is separated from the water. The solvent is then evaporated, leaving only the residual oil, which is dried and weighed. While older standard methods recommended the use of Freon solvents, current standards, including EPA Method 1664 A, specify n-hexane solvents.

While gravimetric methods are simple and inexpensive to perform, they lack compositional analysis and can fall victim to volatile compound loss through evaporation.

GC-FID

GC-FID involves sophisticated gas chromatography (GC) instrumentation. In a GC-FID process, the sample is extracted by a solvent as in the two other methods. The sample is dried and purified before injection into a GC instrument. A carrier gas is used to move the sample through a column, while different hydrocarbons leave the column at different times, at which point they are measured and burned off by a flame ionization detector (FID). The most prominent GC-FID standard method is described in ISO 9377.

Simplified GC-FID process.

Image credit: ETS Laboratories

GC-FID methods are fast becoming the preferred reference method due to their lack of CFC solvents and volatile emission, as well as their detailed compositional analysis. Their drawback, however, is that their sophisticated instrumentation is expensive and typically requires skilled operators.

Field Measurement Processes

While reference methods are effective (and essential) for continuous monitoring, comparing results, and developing legislation and guidelines, they may be too involved or complicated for processes such as process optimization, which requires repeated, quick results. For this reason, inexpensive benchtop or online devices are available for taking field measurements.

Benchtop units may be correlated to a specific reference method and typically use one of the following processes for measurement.

• Colorimetric measurement using a spectrophotometer. Samples tested by this method must show color, as colorless oil is undetectable using the process.
• Fiber optic sensing using a directly-injected sensor. The sensor's polymer coating absorbs hydrocarbons from the sample; the light transmitted through the fiber is measured before and after absorption to determine the hydrocarbon content.
• Various means of infrared testing, which are similar to the reference method. These processes may or may not use solvents and are particularly useful for measuring aliphatic compounds. Depending upon the specific infrared method used, this testing may or may not release volatile compounds.
• Ultraviolet methods are typically used to determine concentration of aromatic compounds, which absorb UV light. UV fluorescence is one of the most common field measurement techniques, behind only infrared testing.

Hydrocarbon fluorescence. Ultraviolet testing is dependent on the spike seen between 300 and 400 nm (part of the UV spectrum).

Image credit: SEOS

Online instruments are used to continuously test a process stream and may use one of the methods described below.

• Focused ultrasonic acoustics involve a directly-inserted transducer which detects the acoustic echoes of solid particles and oil droplets. The acoustic signals are used to calculate particle size and oil concentration.
• Fiber optic sensing, which is the same process used in benchtop units.
• Image analysis uses video microscopes to capture images of a sample stream. Particles on each image are counted and analyzed according to the sample volume. Particles are typically distinguished by shape, as oil droplets are nearly spherical.
• Light scattering methods involve passing light through an oily sample. Particles scatter light and reduce the transmitted light; by measuring both quantities of light at different angles, oil particles can be detected and measured.
• Photoacoustic sensors use pulsed lasers to cause sudden local heating around oil particles. This heating causes high frequency pressure waves, which are measured and correlated to determine oil concentration.
• UV fluorescence, which is the same as the benchtop method. 

Instrument Selection Criteria

The selection of one of the methods listed above is highly dependent on the type of measurements to be performed. For example:

• Process optimization applications require an instrument with highly accurate and repeatable measurements.
• Colorless samples may not be compatible with colorimetric instruments.
• Solvents used must be acceptable to the testing environment.
• Some applications, especially offshore, require a portable or compact meter for space savings. For online meters, the space available for fitting the instrument must be carefully considered.
• For both benchtop and online instruments, it is helpful to investigate whether the instrument supplier has experience using or testing the device for measurements similar to those made in the intended application. 

Standards

Additional standards related to oil in water monitors are listed below.

• PREN 16691 - Determination of PAH in whole water samples using [...] gas chromatography mass spectrometry
• ISO 15680 - GC determination of [hydrocarbons] using purge-and-trap and thermal desorption
• ASTM 7678 - Standard test method for TPH in water and wastewater with solvent extraction using mid-IR laser spectroscopy 

Resources

EPA Analytical Method 1664 (gravimetric method)

Yang, M. Measurement of Oil in Produced Water (pdf)

Image Credits:

Georgia State University | Arjay Engineering | Turner Designs Hydrocarbon Instruments