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How to Use the Unidirectional Flushing Method

By Krystal Nanan
Published: February 11, 2019 | Last updated: January 31, 2022 03:43:31
Key Takeaways

Unidirectional flushing (UDF) produces higher flushing velocities than conventional methods, making it the preferred method for removing tough tuberculation and biomass deposits in older pipelines.

The cleanliness of water distribution systems is of utmost importance. Unfortunately, because most of the network is buried underground, it is often a neglected element of water utility management processes. As we enter an era of aging infrastructure, customers are beginning to experience customers are beginning to experience the effects of corroded metal and copper piping installed in the past decades. Significant pressure is now being placed on the relevant authorities to take effective measures to properly maintain and operate pipeline distributions systems.


Tuberculation is one of the most common problems associated with aging metal pipes (mainly iron). In addition to causing unwanted taste, odors and discoloration, tuberculation can also be harmful to the distribution infrastructure and household equipment and appliances.

In an attempt to improve operational efficiency and overall water quality standards, several municipalities are turning to the unidirectional flushing method (UDF) to clean their distribution systems.


What is Tuberculation?

Tuberculation is a condition, typically found in relatively older iron pipes, characterized by the formation of mounds of reddish-brown corrosion products on a pipe’s interior surface. These mounds are the result of the unabated growth of iron bacteria and manganese bacteria. The activity of these bacteria results in the deposit and accumulation of iron/manganese oxides, sedimentation and particulate matter on the inner wall of the pipe. (How to assess biologically induced corrosion is examined in the article Testing For Microbiologically Influenced Corrosion in Pipelines.)

Tuberculation typically starts as a biomass slime that coats the pipe’s interior. This biofilm may also show signs of metal oxide precipitation. The thousands of iron or manganese bacteria contained in the biomass metabolize ferrous ions in the water, precipitating ferrous oxide as a byproduct. The ferrous oxide becomes trapped in the film and accumulates with continued activity from the bacterial organisms.

Excessive tuberculation can eventually affect the quality of the water reaching the homes of customers. Reddish-brown discoloration, staining of plumbing fixtures and laundry, and taste and odor anomalies are just some of the effects of tuberculation. This type of corrosion, if left untreated, can significantly reduce the internal diameter of the pipeline, which can lead to several hydraulic issues including decreased output pressure, low flow rate, and even damaged pumps, pipes and other distribution equipment.

Tuberculation is normally associated with pipes constructed prior to the 1940s. The advent of cement mortar linings and the issue of American Water Works Association (AWWA) standards for cement-mortar linings (AWWA C104 and C151) has minimized this problem in modern piping systems. (However, some newer piping materials are even more susceptible to corrosion, as discussed in Decline in Quality of Piping Making Corrosion Inevitable.)


Why Unidirectional Flushing (UDF) is Better than Conventional Flushing

Conventional flushing involves flushing multiple pipes in the system simultaneously. Flushing does not target a single pipeline, and flow comes to the hydrant from random, haphazard directions.

Unidirectional flushing (UDF), on the other hand, is a pipeline cleaning technique that involves directing a high-velocity water flow in only one direction through a single pipeline. UDF is an effective method for clearing debris and other unwanted material from pipe mains; thus, enhancing water quality, reducing turbidity and improving chlorine residual.

UDF is performed by isolating the flow of water in the distribution network to a particular pipe by closing specific valves. Hydrants at relevant locations are then opened to allow water to exit the system, thereby flushing the pipeline under consideration. One of the major advantages of UDF is its improved cleaning efficiency due to the forceful concentration of high-velocity water in a single pipeline. (Another new method is discussed in Ice Pigging: New European Pipe Cleaning Innovation.)

UDF is a controlled, proactive approach to pipeline network maintenance and is typically used by utility companies as part of systematic and routine flushing and maintenance programs. In contrast, conventional flushing is usually done as a reaction to customer complaints.

Using Unidirectional Flushing to Remove Tuberculation

The cleaning efficiency of UDF makes it ideal for removing tuberculation from metal piping. However, considerable planning is required to ensure that cleaning operations produce the desired results. The basic steps to performing UDF on a given pipe include:

  • Pre-planning – This phase involves carefully planning the flushing sequence by using maps, charts, hydraulic models, etc. With assistance from an engineer, a unidirectional flushing plan can be developed to map out the flushing pattern as well as the relevant valve and hydrant locations. Planning minimizes errors and ensures controlled, systematic and successful flushing of the system. Haphazard flushing is worse than no flushing at all!
  • Isolate the pipe network into sections – After the flushing pattern is resolved, the pipe network can now be divided into segments and flushing sequences. Pipe sections should be manageable enough to facilitate flushing runs that can be performed during regular working shifts.
  • Set the target flushing velocities – Determining the appropriate flushing flow velocity is of utmost importance. The velocity should be high enough to remove tough tuberculation, biofilm and other debris while ensuring that the integrity of the pipe or its lining material is not compromised. The appropriate target velocity varies according to pipe conditions; however, 2-5 ft/s is considered to be adequate for most applications.
  • Test and exercise the valves and hydrants – Valves and hydrants are crucial to flushing operations; therefore it is essential that these components are in working order and capable of accommodating flushing procedures. Damaged or non-functioning equipment may need to be repaired prior to the commencement of flushing operations.
  • Flush – Once the previous steps have been completed, the pipe is ready to be flushed. The required hydrant should be opened long enough (usually 5-10 minutes) to scour the pipe until the water is clear and free of debris. During this process, parameters such as pressure, flow and velocity should be carefully monitored using the appropriate tools and equipment. Samples should also be taken at regular intervals during flushing to test whether the turbidity, iron content and bacterial content are within acceptable limits.


Unidirectional flushing is one of the most effective methods for removing tuberculation in pipeline distribution systems. Its efficiency is due mainly to its ability to target single pipelines with controlled high-velocity flows. This attribute makes UDF a significantly more effective cleaning method than conventional flushing. Caution should be exercised to ensure that excessive pressures and flow velocities do not permanently damage the pipeline or its liner.


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Written by Krystal Nanan | Civil Engineer

Profile Picture of Krystal Nanan
Krystal is a civil engineer and project manager with an MSc in Construction Engineering and Management. Her experience includes the project management of major infrastructure projects, construction supervision, and the design of various infrastructure elements including roadway, pavement, traffic safety elements and drainage. Krystal is also a published author with the Transportation Research Board in Washington, D.C.

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