Proficiency tests for waste water treatment plants

Guideline for quality-assured self-monitoring - proficiency test - evaluation and assessment

Guideline for quality-assured self-monitoring to support official monitoring at municipal wastewater treatment plants

In 2003, the (former) Ministry of the Environment and Transport of Baden-Württemberg presented a system in the above-mentioned guideline, the application of which enables operators of wastewater treatment plants to upgrade the self-monitoring of their plant in such a way that official monitoring can be reduced to a minimum.
This requires quality assurance of the self-monitoring analysis in accordance with ATV A704, which is verified by independent third parties during an on-site inspection. In addition, proof of successful participation in the relevant proficiency tests of AQS Baden-Württemberg at ISWA must be provided.
Further details can be found in the recommendation for action, which is available from the Ministry of the Environment, Climate Protection and the Energy Sector Baden-Württemberg (see link above).

Proficiency test

Once a year, AQS Baden-Württemberg at ISWA carries out a proficiency test for operational analysis at wastewater treatment plants. The following parameters are to be determined:

  • COD
  • Total nitrogen
  • Nitrate-nitrogen
  • Ammonium-nitrogen
  • Total phosphorus
  • TOC (optional)

As a rule, the analyses are to be carried out using sealed tube test kits.

Evaluation and assessment

The design and evaluation of these proficiency tests are based on the specifications of DIN 38402 - A45 and LAWA- Merkblatt A-3 ("Ringversuche zur externen Qualitätsprüfung von Laboratorien" aus LAWA: AQS-Merkblätter zur Wasser-, Abwasser- und Schlammuntersuchung).
As far as possible, the assigned value is determined from the weights taken when the samples are prepared, otherwise on the basis of the robust mean value of the measurement results of all participants. In the first case, the content of the analyte in the matrix used must be known with high accuracy. This value can be determined from the results of the laboratories and the weights in analogy to the method of standard addition. The standard deviation necessary for the calculation of the zU scores is also obtained from the measurement results of the participating laboratories. Whenever possible, the variance function described in DIN 38402 - A45 is used in setting the standard deviation for proficiency assessment. The PT manager sets upper and lower limits for the standard deviation before the proficiency test is performed, which are intended to prevent the tolerance limits and thus the quality requirements for the laboratories from becoming too sharp or too lax.
All participating laboratories receive a result evaluation sheet after completion of the evaluation, which shows the evaluation of this individual value for all measured values by marking with "s" (successful), "q" (questionable) or "n" (not satisfactory). In addition, it is documented for each parameter whether it was determined "successfully" or "unsuccessfully". Parameters are classified as "successfully" determined if at least 50% of the measured values are within the tolerance limits. Parameters for which no measured values were provided are marked as "not participated".
Of course, the participants also receive a detailed evaluation brochure containing the results of all participating laboratories in anonymized form and presented in tabular and graphical form. These brochures are also available here on the AQS Baden-Württemberg website.


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