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.
In Germany, the design of treatment plants is based on the specifications of DWA-A 102-2 and DWA-M 179-1, which define criteria for effectiveness, hydraulic design and the reduction of the annual pollutant load.
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
EU Regulatory Framework
EU Water Framework Directive
The aim of the EU Water Framework Directive (WRRL) is to maintain or achieve good ecological and chemical status in all bodies of water. All EU Member States are responsible for ensuring this.
Among other things, the WRRL requires programmes of measures to reduce the discharge of pollutants.
German Regulatory Framework
Water Resources Act (WHG)
The Water Resources Act defines road runoff as runoff that, in accordance with the state of the art, must be treated in order to adequately comply with the prohibition of deterioration under the Water Framework Directive. According to Section 54(1) of the WHG, wastewater is ‘water (rainwater) collected from precipitation and flowing off built-up or paved areas’.
DWA-A 138-1
The DWA-A 138-1 standard, ‘Systems for the infiltration of rainwater – Part 1: Planning, Construction and Operation’, the requirements for rainwater treatment via infiltration are described.
For decentralised treatment systems, in addition to the required removal efficiencies for fine filterable substances (AFS63) and dissolved substances (heavy metals zinc and copper), the design criteria for hydraulic capacity are specified. Larger inflows must be regulated, for example by backflow to adjacent areas, an upstream storage tank or an emergency overflow into the sewerage system. In principle, rainwater from traffic or operational areas must not be allowed to infiltrate untreated in accordance with the regulations of DWA-A 138-1.
Areas of application
Rainwater treatment using the partial-flow method is used, for example, for discharge into surface waters. According to the DWA-A 138-1 regulations, it is not permitted for discharge into groundwater.
Other typical applications include:
- large paved areas with varying rainfall intensities
- decentralised rainwater treatment systems with inflow limitation
German Regulatory Framework
DWA-A 102-2
The DWA-A 102-2 standard, “Principles for the management and treatment of stormwater runoff for discharge into surface waters – Part 2: Emission-based assessments and regulations”, describes treatment measures in terms of their effectiveness and application area, as well as design approaches for treatment plants.
The December 2020 version mentions partial-flow treatment for the first time and states that it is, in principle, possible. However, since 2004 there have been isolated regulatory requirements for the treatment of stormwater intended for discharge into surface waters, which are similar to the current partial-flow method. Such treatment plants with inflow limitation are hydraulically designed according to the critical rainfall intensity rkrit. The critical rainfall intensity describes the intensity of rainfall up to which the entire discharge is routed through the treatment plant. A detailed explanation can be found in the FAQ section of this page.
For centralised treatment systems, 15 l/(s*ha) has proven to be a suitable critical rainfall intensity. For decentralised systems (such as the DRAINFIX CLEAN BASIC filter channel), lower values for rkrit may also be applied where appropriate, provided these systems are highly efficient and achieve the required reduction in the annual load discharged into the watercourse.
When performing hydraulic design calculations for rainwater treatment systems with inflow limitation and open overflows, we recommend applying a safety margin of 30%. This corresponds to a design value of 20 l/(s·ha).
DWA-M 179-1 GD
“Decentralised systems for rainwater treatment – Part 1: General information and discharge into surface water bodies”
This guidance note contains recommendations on the design and operation of systems prior to discharge into surface water bodies. It also describes the creditable performance of such systems.
Regulations:
- When treating partial flows (and full flows), the connection area must not be less than 1%, even if the hydraulic efficiency would be sufficient in theory. Smaller connection area ratios should be avoided for operational reasons (e.g. increased maintenance requirements).
- When designing a partial flow treatment plant to ensure the required hydraulic efficiency, the following applies:
An efficiency of 95% is achieved if
- the principle of surface filtration is applied
- the filtration velocity does not exceed 2.5 m/h,
- the average filter depth is at least 20 cm, and
- a surface area ratio of at least 1% is maintained.
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.
FAQ
Frequently asked questions and answers regarding full-flow and partial-flow treatment
