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DVGW Technology Report No. 2/10
Tap water turbidity may be caused by the mobilisation of sediments in drinking water pipelines. Quality impairments of this kind are also referred to as brown water or rust water and are usually dealt with by purging the drinking water distribution network, a method that does not always sustainably eliminate the sediments, however. So far, neither a tool suitable for determining the accumulation of sediments in drinking water pipelines and devising appropriate purging schemes nor research projects of practical relevance have been available in the field. The DVGW-Technologiezentrum Wasser (DVGW Water Technology Centre, TZW Dresden) launched a research project to investigate the processes that lead to the formation of sediments in drinking water distribution systems and has developed a model that helps to compute optimised purging schemes for drinking water pipelines.
The research work, which went on for several years, included large-scale investigations of both various drinking water networks and small and semi-technical experimental plants. In this context, the researchers purpose-built test facilities consisting of reference pipelines whose inner diameters and pipe roughnesses were equivalent to those of actual pipelines used in the field. The interrelationships found in the course of the pipeline network analyses were then studied in detail on the test facilities, enabling the researchers to draw further conclusions on the formation of sediments and corrosion deposits.
Measurements conducted in a test facility
The test facility was built from 4-metre segments of cast iron pipelines that were removed from a supply area and installed in the experimental plant. These pipeline segments were then hydraulically calibrated and the resulting pipeline diameter determined by volume measurement. Using the observed values, the reference pipelines were modified to reflect field conditions by lining them with cement mortar and mineral aggregates. The first step in determining the formation of sediments in a supply area is to purge a segregated section of the pipeline network with a clear surge of water according to a previously defined systematic purging schedule.
The second step is to conduct continuous turbidity measurements at representative locations in the network. Systematic purging is repeated after a pre-defined period (e.g., one year), recording any sediments found as well as their composition.
A comparison of research findings from different areas has led to the observation that sediments form according to a characteristic pattern, enabling the researchers to identify the source of the sediments by the composition of the purged substances, with the prevailing water temperature influencing the rate of corrosion and the formation of sediments resulting from it.
Sediments from different sources
Next to corrosion, there are other sources that produce suspended solids, the rate of flow determining how much debris prone to mobilisation will accumulate inside the pipelines. The precipitation of dissolved substances may induce particulate formation in the process, or else particulates may agglomerate to form larger or denser particles.
This research project led to the development of a process model from which a computation program was derived that simulates sediment formation over time on the basis of hydraulic data and the results obtained from network analyses.
The next step consists of computing the purging intervals for the pipelines, always taking into account the risk of particle mobilisation.
The program helps to resolve the following problems in the field:
- Computation of optimised purging schedules for drinking water pipelines
- Definition of measures to be taken in the event of network alterations entailing hydraulic changes within the network
- Evaluation of rehabilitation measures entailing a change in sediment formation
Preventive network maintenance
This model is based on the approach of purposefully discharging sediments before they reach a critical level and mobilisation becomes an actual risk. Purging is initiated even before mobilisation occurs, thus preventing the water from turning turbid. The model facilitates economically efficient preventive network maintenance and permits better control of drinking water turbidity in the future.
In addition to elaborating the model, the project also offered major insights that provide a better understanding of the process of sediment formation. Thus the researchers were able not only to identify a number of key parameters to describe the process of sediment formation and particle mobilisation, but also to devise a method for distinguishing between corrosion deposits and water-borne sediments as well as a method helping them to determine the location of sediments prone to mobilisation. Temperature was rated to be a crucial factor in the formation of corrosion deposits in unprotected cast-iron and steel pipelines.
The computation of optimised purging intervals facilitates estimating the impact of altered hydraulic conditions on potentially occurring turbidity. The computer program makes available all project results so that they can be applied in the field.
