A random sample in water and wastewater technology is a representative sample that is taken from a water, wastewater or sludge stream in order to analyze its chemical, physical or biological properties. It serves as a basis for assessing water quality, monitoring processes or ensuring compliance with legal requirements.

Correct sampling is crucial to obtain reliable and meaningful analytical results that serve as a basis for operational decisions and regulatory evidence.

Types of samples

In water and wastewater technology, a distinction is made between different types of sampling depending on the objective and process requirements:

  1. Single sample:

    • A one-off withdrawal of a defined quantity of water at a specific time.
    • Application:
      • Short-term monitoring of a process.
      • Analysis of acute events (e.g. incidents, exceeding of limit values).

  2. Mixed sample (time or flow proportional):

    • Composition of several individual samples taken over a defined period of time.
    • Proportional to time:
      • Samples are taken and mixed at fixed intervals.
    • Proportional flow:
      • Sample quantity is dosed depending on the flow rate.
    • Application:
      • Long-term monitoring to determine average values.
      • Evaluation of wastewater discharges or peak loads.

  3. Continuous sampling:

    • Automated sampling of a sample over a longer period of time using sampling devices.
    • Application:
      • Monitoring of processes with high fluctuations.
      • Used for industrial wastewater with variable composition.

Importance of sampling in practice

Monitoring water quality
  • Purpose:
    • Determination of parameters such as pH value, conductivity, nutrient concentrations, heavy metal content or organic loads (BOD, COD).
  • Regulatory requirements:
    • Compliance with legal limits in accordance with the EU Water Framework Directive, Waste Water Ordinance or Drinking Water Ordinance.
    • Proof of compliance with approval requirements.
Process control in water and wastewater systems
  • Monitoring and optimization of treatment processes.
  • Identification of faults, such as toxic influences, biofouling or scaling.
Evaluation of discharges and residues
  • Analysis of industrial wastewater or wastewater treatment plant effluents:
    • Ensure that no pollutants enter water bodies.
    • Evaluation of the concentration of nutrients and pollutants in sewage sludge for agricultural use.
Research and development
  • Preparation of material balances or development of new treatment processes.
  • Long-term observation of trends, e.g. through continuous monitoring.

Technical background: Sampling methods

Manual sampling
  • Carried out by trained personnel who take samples from specified points.
  • Advantages:
    • Flexibility in the selection of the sampling point.
    • Suitable for one-off or short-term examinations.
  • Challenges:
    • Risk of sample falsification due to human error.
    • Time-consuming with frequent sampling.
Automatic sampling
  • Use of sampling devices that take samples in proportion to time or flow rate.
  • Examples:
    • Suction siphon: Removal of liquids via a hose line.
    • Submersible pumps: Pumping water samples from deep or hard-to-reach areas.
  • Advantages:
    • High accuracy and reproducibility.
    • Possibility of long-term monitoring and mixed sampling.
  • Challenges:
    • Cost-intensive purchase and maintenance.
    • Risk of contamination in the event of improper handling.

Factors for a representative sample

A representative sample requires the consideration of various technical and process-related aspects:

  1. Sampling point:

    • Selection of a location that reflects the actual composition of the medium.
    • Example: At the inlet or outlet of a system, in the middle of a pipe cross-section or at the bottom of a sedimentation basin.

  2. Sample quantity and time:

    • Depending on the flow velocity, the volume flow and the parameters to be analyzed.
    • Regular sampling to record fluctuations in the composition.

  3. Sample preservation:

    • Prevention of chemical or biological changes between sampling and analysis.
    • Measures:
      • Cooling to 4 °C.
      • Addition of preservatives (e.g. sulphuric acid to fix ammonium).

  4. Transportation and storage:

    • Use sterile containers made of glass or plastic.
    • Avoid UV light and mechanical influences.

Challenges and optimization of sampling

Challenges
  1. Falsification due to sedimentation:
    • Particles may settle during sampling, resulting in an inaccurate representation.
  2. Short-term fluctuations:
    • High variability in the composition of wastewater requires timed sampling.
  3. Contamination:
    • Improper handling or contaminated containers can affect the analysis results.
Optimization options
  • Use of automated sampling devices for high reproducibility.
  • Regular calibration and maintenance of devices to avoid technical faults.
  • Staff training in sampling and conservation techniques.

Conclusion

Sampling is an indispensable tool in industrial water and wastewater technology. It provides the basis for precise analyses required for process control, compliance with environmental regulations and quality assurance. Representative and accurate sampling is crucial to obtain reliable results and make informed decisions. By using modern technologies and adhering to proven methods, fluctuations can be minimized and high accuracy of analysis results can be guaranteed.

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