Taking a composite sample is a key method in water and wastewater treatment to carry out representative analyses for chemical, physical and biological assessment. Composite samples enable a reliable assessment of the average water quality over a defined period of time or at a specific location. This is crucial for process optimization, compliance with limit values and the planning of efficient water treatment measures.

In this article, the mixed sample is explained in detail, from the technical basics to practical implementation and typical applications in water treatment.

Definition and purpose of the composite sample

A composite sample (also known as an aggregate sample) consists of several individual samples that are combined to produce a representative overall result. These samples cover temporal fluctuations or spatial differences in the water or wastewater composition and provide an average picture of the relevant parameters.

Purpose of the composite sample:

• Representative results: Average values for parameters such as pH value, conductivity, COD (chemical oxygen demand), heavy metals or nutrients (e.g. phosphorus, nitrogen).
• Compliance with regulations: Proof of compliance with legal limits according to environmental regulations or industrial standards.
• Process optimization: Support in the planning and control of treatment systems, e.g. for dosing chemicals or adjusting cleaning cycles.
• Long-term monitoring: detection of trends in water quality through regular analysis of composite samples.

Types of composite sample

There are different types of composite sample, depending on the type of sampling:

1. time-proportional composite sample

This method collects samples at fixed intervals, e.g. every hour, over a defined period of time.

• Area of application: Wastewater systems with uniform loads or for monitoring trends over time.
• Advantage: Easy to implement and well suited for continuous processes.

2. flow-proportional mixed sample

Samples are taken according to the flow rate of the water or wastewater, e.g. larger sample quantities with increasing volume flows.

• Area of application: Systems with highly fluctuating flow rates, e.g. rainwater treatment systems.
• Advantage: More representative for variable volume flows.

3. spatial-proportional mixed sample

Samples are taken at different points in a water or wastewater system and mixed to record the spatial variability.

• Area of application: Wastewater treatment plants with several inlets or in large storage systems.
• Advantage: Provides a comprehensive picture of the spatial water quality.

Carrying out the composite sampling

Taking a composite sample requires careful planning and execution to ensure reliable and reproducible results.

1. planning the sampling

• Objective: Which parameters should be analyzed (e.g. heavy metals, COD, TOC)?
• Place and time: Selection of the sampling points and determination of the time period.
• Norms and standards: Compliance with legal requirements, e.g. in accordance with DIN EN ISO 5667 (water quality - sampling).

2. technical equipment

• Sampling devices: Automatic samplers for time- or flow-proportional samples, manual sampling for smaller systems.
• Container: Sample bottles made of inert material (e.g. glass or HDPE) to avoid contamination.
• Preservation: Chemical preservation or cooling to prevent changes in the water composition.

3. homogenization

The individual samples are combined in a mixing container and thoroughly mixed to obtain a homogeneous total volume.