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How to Correctly Sample Oily Water

It sounds simple doesn’t it! However, there are many basic sampling mistakes that can lead to big errors. When done correctly a good lab can measure your oily water with precision and accuracy. This is a rough guide to help you avoid the common pitfalls – for more information, give us a call and we can provide you with in-depth instructions. Our recommendations come from years of experience taking water samples, we follow the guidance of the 22nd Edition of the Standard Methods for the Examination of Water and Wastewater, prepared and published by the American Public Health Association (APHA), American Water Works Association (AWWA) and the Water Environment Federation (WEF).

Stay Safe

Don’t take a sample unless you know it’s safe to do so. There could be all sorts of nasty chemicals mixed in with the water you’re sampling so make sure to check with the relevant personnel that it’s safe and you have any necessary Personal Protective Equipment (PPE).

Sample Jar

The Standards recommend a 1L labelled, wide-mouthed clear glass bottle with a Teflon coated lid. Each of these features makes sampling and analysis more accurate.

  • Volume
    Having a larger volume (1L) reduces sampling errors. The water leaving the sampling point is rarely perfectly mixed. The larger volume allows better averaging
  • Jar Material
    Clear glass bottles are recommended because they let you see what’s going on inside, the glass is inert and won’t react with your sample plus it’s easier to remove the oil stuck to the walls of the jar to include in the analysis
  • Lid material
    The lid of the jar should have a Teflon (PTFE) lining that prevents the oil absorbing to it

Once you’ve selected an appropriate bottle it must be cleaned with hot water and detergent, rinsed with water and finally rinsed with an appropriate solvent and acidified.

Ultraspin strainer basket

Oily Water Sample Jar

1L labelled, wide-mouthed clear glass bottle with a Teflon coated lid

Collection Technique

These steps need to be followed to obtain an accurate, representative sample.

Step 1:
Open the sample valve completely (full bore) and allow it to flush for 2 minutes – this will clear out the sludge that accumulates over time behind the deadleg of the valve

Step 2:
Close the valve to a level you are comfortable with for the sampling. Once you’ve selected this valve position do not adjust it until the sampling is complete as this can dislodge debris that you don’t want in the sample

Step 3:
Place jar under the flow from the sample point and fill jar to around 90% full. DO NOT OVERFLOW THE JAR – oil floats and this is what will be lost. Remove the jar from the water stream BEFORE you close the sample point

Step 4:
Close the lid tightly and refrigerate as soon as possible if you can’t analyse the sample immediately

What if something goes wrong? If the sample overflows, you spill some or you get a slug when sampling. You CAN’T start again using the same bottle. You need to discard the first bottle as it will now be coated with oil and have no preservative in it. Get a whole new bottle and start again from Step 1.

Sample Photos

When taking photos of samples, 3 sets are required – each consisting of 3 views (side, top and bottom as shown below)

1st set – Take photos immediately after collecting and preparing the sample.

2nd set – Allow sample to rest on flat surface for 5 minutes.

3rd set – Allow sample to rest on flat surface for a further 15 minutes.

Side ViewTop ViewBottom View
Horizontal to the sample bottle

45° to the oily water surface

45° to the bottom of the bottle

Preservation is Key!

Preservation of the sample is important and we recommend two strategies to achieve this. First, the sample should be acidified to a pH of two or below. Second, the sample should be kept below 4°C. You can use an Esky/insulated cool box with cool blocks if you need to transport the samples, avoid putting loose ice in with the sample as this will melt.

If these conditions aren’t achieved, bacteria and other bugs can grow exponentially and chew up the oil. Depending on how much time passes before the lab can test the sample, more and more oil will disappear from the sample – meaning the lab results will be very inaccurate, reporting a much lower oil content than is truly the case onsite.

These tips are a good place to start, however before you dive in and start taking samples we do recommend you drop us a line so we can talk you through the finer details or if you want to know more about oily water, check out our Oily Water Tutorials in our Learning Centre

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