Oil in Water Analysis Method Match Regulatory Method - Case study

The analysis of oil in water is a worldwide problem that crosses many industries, analytical methods and regulatory agencies. For those in the petroleum industry, the offshore platform oil and grease limits for produced water can be quite different from the effluent coming out of the refinery. For the expanding hydraulic fracturing industry, oil in water limits are driven by whether the frac or produced water will be treated for reuse, disposed of down an inactive well, or sent off-site for treatment.

Those individuals tasked with having to determine the amount of oil in water know it can be a challenging measurement. It is complicated by the fact that otl comes in many forms and is not a unique chemical entity Added to the chemical non-uniformity is a number of different analytical methods that could be used, ie` solvent extraction/gravimetric, gas chromatography, solvent extraction/infrared, and solvent extraction/UV to name a few.

The measurement is, therefore, defined by the particular regulatory method used to measure it. When EPA 1664 is the regulatory method, the "oil" is anything that is extracted into hexane and remains after the hexane has been evaporated and shows up as weight. In regions where infrared analysis is the defining method, the "oil" is whatever is extracted into the solvent and has carbon-hydrogen bonds that absorb infrared light at a specific freduency. Edch method is looking at different properties of oil and can potentially give different results.
The answer to the commonly asked question of how one type of oil in water measurement compares with different regulatory methods is not always simple and straightforward. Listed below are four factors that need to be considered:

1. Precision and bias for each method
There are acceptable errors for each method typically expressed in the precision and bias statement for the method. For example, EPA Method 1664 states in their "Ongoing precision & recovery" (section I7.0)thatfora 100 ppm sample the acceptable range is 78 -114 pom. If the test includes the silica gel treatment (SGT) to remove the polar organics. the acceptable range is 64 - 132 ppm. Therefore, if the result from a laboratory for a silica gel treated sample is 65 ppm and the alternate method result is 130 ppm, they are within the acceptable range.

2. Operator errors
As suggested previously, even the same method can give significantly different results. With any method where there is sample preparation, the human factor is added in. If a solvent/sample mix is only shaken for one minute rather than the required two minutes, the amount of oil extracted into the solvent will be significantly less. In some cases, it has been half the reading. The chart below shows a comparison of a five way sample split that was analysed on two infraCal TOG/TPH Analysers (see photo) using infrared analysis and at three laboratories using the EPA 1664 hexane/gravimetnc method. The results disprove the common misconception that the lab is always right.

3.  Sampling
Sample collection and handling can cause differences for comparative testing. In order lo have an objective comparison, the samples should be identical. If there is variability in the waste _ stream, this can be a difficult task. The oJd adage that oil and water do not mix holds true for wastewater as well.

Oil also likes to stick to surfaces. If sample collection containers are being reused, they should get a final solvent rinse after cleaning to remove any residual oil. For samp* analysis, the solvent should either be Wended in the sample container or if the sample is to be transferred to another container, the sample container should be rinsed with a portion of the solvent that will be used for the extraction. If not, any oil on the container surface will be excluded from the reading.

4. Sample Disparities
Not all oil in water samples are alike which makes it difficult when comparing analytical methods that look at different properties of the oil m order to make the measurement. Oil is a mix of chemical components that changes from one location to another-it can even change at the same location.

For example, samples often contain a mix of aromatic and aliphatic hydrocarbons. UV fluorescence only detects aiomatic hydrocarbons while infrared will detect both aromatic and aliphatic hydrocarbons. The two analytical methods could give different results if the aromatic/aliphatic ratio changes.

Samples that contain volatile hydrocarbons could also show different results between EPA 1664