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Rainwater treatment: full-flow or partial-flow treatment?

Rainwater from sealed surfaces such as roads or car parks can contain pollutants such as oils, heavy metals, tyre abrasion or fine particles. Therefore, before it seeps away or is discharged into water bodies, it often needs to be treated.

The type of rainwater treatment required depends, among other things, on the discharge route and the level of contamination of the surface. Relevant regulations specify whether the entire flow must be treated via a full-flow treatment system or only a portion of it (partial-flow treatment).

On this page When to use which method? Full-flow treatment Partial-flow treatment Product Finder FAQ

Full-flow or partial-flow treatment – when to use which method?

Full-flow and partial-flow treatment form the central basis for the design of rainwater treatment systems. The way in which they differ is in terms of how much of the collected rainwater is passed through a treatment system.

The choice of the appropriate method depends in particular on:

  • where the treated water is discharged (groundwater or surface water),
  • which water law requirements apply and
  • what hydraulic loads occur.

Full-flow treatment: Principles and applications

Full-flow treatment refers to the complete purification of the entire volume of stormwater runoff. In this process, the entire inflow is routed through the treatment plant without any hydraulic diversion or bypass. The treated water is then – for example – infiltrated, discharged into a watercourse or discharged into the sewerage system.

This process ensures that the entire flow is treated regardless of the intensity of rainfall. This allows pollutants to be reliably retained before the water enters the natural water cycle.

Areas of application

Full-flow treatment is particularly necessary where strict water quality requirements apply.

Typical applications include:

  • Discharge into groundwater (infiltration)
  • Areas with high levels of contamination
  • Situations where overflow is not permitted

Partial flow treatment: Principles and applications

Partial flow treatment describes a process in which only a defined portion of the rainwater is treated by the treatment system. The excess volume is discharged via an overflow device.

According to this principle, during normal rainfall events, the runoff flows through the filter channel (‘first flush’). Only during prolonged or heavy rainfall does the water accumulate in the channel and enter the overflow. Due to dilution, the overflowing water has only a low concentration of pollutants, whilst the more heavily contaminated portion continues to be treated by the treatment plant.

Areas of application

Rainwater treatment using the partial-flow method is used, for example, for discharge into surface waters.

Other typical applications include:

  • large paved areas with varying rainfall intensities
  • decentralised rainwater treatment systems with inflow limitation

HAURATON Rainwater Treatment Solutions

A range of technical solutions is available for implementing rainwater treatment. Systems such as the DRAINFIX CLEAN filter channel enable decentralised, compliant treatment of rainwater in both full-flow and partial-flow operation.

The HAURATON Product Finder

Find the right product for your project, information for your planning, tender texts, product illustrations, technical drawings and CAD data etc.

FAQ

Frequently asked questions and answers regarding full-flow and partial-flow treatment

  • In full-flow treatment, the entire stormwater runoff is passed through the treatment plant. In partial-flow treatment, only a defined proportion of the runoff is treated, whilst excess water is discharged via an overflow during periods of heavy rainfall.

  • Full-flow treatment is particularly necessary where there are high water quality requirements, such as when discharging into groundwater. In such cases, the entire volume of stormwater runoff must be treated via a treatment plant.

  • In the case of infiltration, the water enters the groundwater directly. To prevent the introduction of pollutants, the entire discharge must be treated.

  • The reason lies in the so-called first-flush principle. During rainfall events, the initial portion of the runoff contains the highest concentration of pollutants. This more heavily contaminated runoff is specifically routed through the treatment plant.

    During prolonged or heavy rainfall, the pollutants become diluted, so that subsequent runoff has significantly lower concentrations. In this case, the excess volume is discharged via an overflow, whilst the more heavily polluted portion continues to be treated.

  • The critical rainfall intensity (rkrit) refers to the rainfall intensity up to which the resulting rainwater runoff is completely routed through and treated by a rainwater treatment plant. It serves as the key design parameter for the hydraulic design of such plants. If this rainfall intensity is exceeded, the additional water may be discharged untreated into the watercourse via an overflow.

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