Sampling and Testing

Most companies and private individuals who have a tank on their premises, be it a Sewage Treatment Plant, an Oil Water Separator (also widely known as an interceptor in the UK), Grease Trap or Silt Trap, set up an agreement with a tankering company to attend site and empty the contents of their tank on a regular basis. It may a 3,4 or 6 monthly interval for the visit and in many cases the servicing of the tank may not be necessary. This can result in costs that may be avoidable.

Oil Water Separators or Interceptors

The Environment Agency’s pollution prevention guidelines – Use and design of oil water separators in surface water drainage systems: PPG3, states that the depth of accumulated oil and silt be inspected, and that sites should empty their separator as soon as a significant quantity of oil and/or silt had built up.

Under normal operating conditions, the level of oil and silt in many separators does not build up sufficiently to require a complete cleanse. There are, of course, many factors that will effect the build-up of oil and silt, for instance, there will be a seasonal variance, also if a site has a silt trap as a part of the drainage solution.

Separator Sampling and Testing

Sampling of a separator consists of three elements.

  • Assessing the depth and amount of oil and other hydrocarbons accumulated in the separator
  • Assessing the depth and amount of silt accumulated in the separator
  • Sampling of the final discharge of the tank

The depth of oil and silt is checked to ensure that it is within the tolerance as set down by the manufacturer. The sample taken from the final discharge is sent to an independant laboratory and tested for the level of hydrocarbons. The hydrocarbon level will then compared to the permitted allowance as set down in the consent to discharge for the site. The testing of the sample may take up to a week to be completed by the laboratory. A report detailing the findings will then be supplied to the site owner.

Why use sampling and testing rather than periodic emptying?

Depending on the size of an interceptor, tankering companies can charge up to £700 per cleanse, normally this will be at 6 monthly intervals. An interceptor cleanse will involve removing the contents of the tank and refilling with water. What this process will not do is:

  • Determine the amount of accumulated oil and other hydrocarbons
  • Determine the amount of accumulated silt
  • Show the trend of oil and silt build up
  • Indicate whether the cleanse was or was not actually required
  • Confirm whether the tank is discharging within consent levels

Regular sampling can give all of these benefits. Each time a sample is taken, the report will detail what state the contents and discharge are in, and you can make the judgement whether a cleanse is required. If no oil and silt build up has occurred, then a cleanse may be deferred until the results from the next sampling cycle has been completed. Analysis of samples is carried out by a UKAS accredited lab.