In recent years, TZW has developed the methodology of condition-based network flushing as a result of research and consultancy projects. Positive practical experience has led to this approach being taken into account in the revision of DVGW rule W 291. Condition-based flushing is already being implemented in practice at German water supply companies in Chemnitz, Leipzig and at RWW.
With condition-based flushing, pipes are flushed when the defined deposit level is exceeded. The defined deposit level describes the amount of loose deposits in a pipe during normal fluctuations in flow velocity that does not lead to visible turbidity of the water and does not favour the growth of hygienically relevant bacteria and invertebrates. The condition-based flushing strategy is already used by a number of water supply companies in Germany, whereby TZW is involved in the development of the flushing plans and the evaluation of the flushing data, and in some cases also in the execution of the flushing. The decision to implement a condition-based flushing strategy had the following background for the companies:
- Brown water occurred during operational changes in the drinking water pipe network.
- The previous flushing strategy did not lead to sustainable control of brown water.
- Customer complaints should generally be minimised. Findings from systematic pipe network maintenance should be utilised for asset management.
The structure of a condition-orientated flushing strategy consists of the following steps:
- Development of the flushing plan: A specific flushing plan is developed for the drinking water network, in which the sequence of pipes to be flushed and the required valve positions for each flushing section are defined.
- Basic flushing: Based on the flushing plan, the deposits in the pipework are removed. During flushing, a flushing device is used to record the turbidity, temperature, flow rate and pressure of the flushing water online. The data is used for further analyses.
- Repeat flushing: To determine the rate of deposit formation, flushing is repeated after a previously defined operating interval (e.g. after one year) using the flushing device.
- Development of the condition-based flushing strategy: A specific flushing interval is calculated for each flushing section based on the data obtained. Practical flushing districts are then defined, taking the flushing intervals into account. In future, these districts will be systematically flushed according to the defined operating intervals based on the flushing plan.
Overall, the experience of the practice partners shows that the condition-based flushing strategy can ensure sustainable water quality in the distribution network.
The text is an extract from an article that appeared in DVGW energie wasser praxis 11/2023. You can read the full text in German here (pdf-file). In the PRAXIS: RELEVANT series, TZW presents examples of successful projects that have been implemented together with water supply companies.