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Trade effluent monitoring

We routinely inspect and sample trade effluent discharges to check compliance with consent conditions and for determining the effluent strength for charging purposes.


Trade effluent sampling is required in order to:

  • assess trade effluent volumes, strengths and loads
  • provide representative data to enable accurate assessment of trade effluent compliance with consents
  • obtain representative trade effluent charging data
  • assess and control the impact of trade effluent discharges on Wessex Water’s wastewater processes.

The sampling programme is drawn up by the commercial waste water advisor and subject to a quarterly review.

Two types of samples will normally be scheduled – charging samples and audit samples:

Charging samples

Charging samples are used to determine the average discharge strength in order to calculate appropriate trade effluent charges. The appropriate sampling frequency for charging samples is based on:

  • income/chargeable volume
  • variability of quality
  • robustness of sampling method.

24-hour flow-related composite samples are preferred but if this is not possible or practicable a 24 hour time average composite sample is acceptable. If 24 hour composites are unobtainable or effluent quality is consistent spot samples are acceptable.

Audit samples

Audit samples are used for monitoring compliance and detecting trends.
Sampling frequency for audit samples is based on:

  • potential impact an (and risk to) staff, wastewater systems, receiving watercourses and water supply routes
  • variability of quality and/or flow rate
  • robustness of sampling method.

Samples scheduled will be a mixture of spot samples for absolute consent conditions and/or composite samples to assess consent load limits.

Sampling point

The sampling point and access arrangements must be approved by Wessex Water. An inspection chamber or manhole point is needed so that samples of trade effluent discharge can be obtained for control and charging purposes.

Positioning of and access to sampling points

  • Drainage from toilets, sinks, hand-wash basins or showers must not discharge through the point.
  • The point must be easily accessed and should not normally be sited in an area with limited access.
  • The inspection point must allow samples to be taken without personnel entering a manhole chamber, and without exposing them to any other hazard.
  • There should be no obstructions to the inspection point, temporary or permanent. This includes obstruction caused by equipment, materials, deliveries or vehicles. The point should not be locked, and vehicles must not park over it.
  • Wessex Water staff should have access at all reasonable times. This is usually within working hours, but in certain circumstances it may be necessary to sample at other times.
  • You will be advised by us if space for an automatic sampling device is needed in the manhole. Some companies may wish to make this provision for their own sampling purposes.

Manhole covers for sampling points

To comply with legislation, the cover at a sampling point must be easy to remove. Note that:

  • a lightweight cover may be required (Light covers suitable for heavy traffic are available)
  • if the manhole cover is large, it should be split. Square manhole covers split into two triangles bolted together should be separated by unbolting
  • the keyholes for lifting irons should be positioned to balance the centre of gravity of the manhole. If they are not, the manhole cover can jam on lifting.

Sampling protocols

It is recommended (especially for high risk discharges) that a trade effluent sampling protocol is agreed. Sampling protocols are designed to ensure that best practice is adopted for trade effluent sampling and monitoring arrangements. This enables Wessex Water to obtain representative data and samples for the accurate assessment of charges and consent compliance. It also enables the discharger to be assured that robust and clearly understood arrangements exist.

Health and safety

Both the occupier of the premises and Wessex Water have a duty under the Health and Safety at Work etc. Act 1974 to ensure that employees can safely carry out their duties. Wessex Water employees shall act with due care for their own health & safety and observe any site specific health and safety arrangements.


Trade effluent customers are encouraged to set up self-monitoring programmes. This assists with the early detection of problems and is good practice.

Self monitoring requirements may be incorporated into trade effluent consents.

Flow measurement

Consent conditions may require flow measuring equipment to be provided. The cost of installing and maintaining such equipment will be borne by the discharger.

Assessment of flow by other means e.g. water meters, process meters or pumping rates may also be considered in some circumstances.

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