Leak-Tightness of Pressurized Sewer Pipelines

Pressure sewer lines account for approximately 10% of Bavaria’s sewage, combined, and stormwater sewer systems, making them a significant component of the drainage system.

Compared to gravity-flow sewers, pressurized sewer lines pose a greater risk of environmental damage because they are always fully filled. Due to the pressure inside the pipe, any damage will inevitably result in wastewater seeping into the surrounding soil. Regular testing of the integrity of pressurized sewage pipelines is therefore absolutely essential.

Under the currently valid Self-Monitoring Ordinance, pressure sewer lines—as part of the combined sewer system—must already be inspected for leaks. However, due to the lack of a testing procedure, leak inspections have not been carried out in the past. The forthcoming revised version of the Self-Monitoring Ordinance explicitly requires the inspection of pressurized sewer lines for leaks.

The visual inspection method established for gravity sewers—e.g., using camera inspection—is generally not feasible for pressurized sewer lines due to their sometimes small diameters, tight bends, and long run lengths. Furthermore, even small defects in pressurized wastewater pipes can cause significant amounts of water to leak out; these defects may be easily concealed by a biofilm or deposits and cannot always be reliably detected through visual inspection.

Pressure testing methods are therefore generally used to verify the leak-tightness of pressurized wastewater pipelines. To date, however, a corresponding standardized test procedure has only been established for pressurized drinking water pipelines (DIN EN 805 and DVGW W 400-2). Since these test procedures are intended for acceptance testing of new construction, the test pressures are 15 or 21 bar, with holding times of up to 48 hours. The test procedures therefore cannot be applied to pressurized sewer lines that are currently in operation.

Consequently, the University of Applied Sciences Augsburg was commissioned by the Bavarian State Ministry for the Environment and Consumer Protection and the State Office for the Environment, as part of two research projects, to develop a test procedure specifically adapted to pressurized wastewater pipelines.

Through 263 pressure tests on 47 operational pressurized sewer pipelines, the respective influences of pipeline pressure and leakage, air content, material deformation, pipeline routing, and pipeline diameter were investigated. Based on these results, the “HSA Standard Procedure” was developed and verified.

The HSA Standard Procedure is generally performed with the pipeline fully filled. The pressure pipeline must be vented as thoroughly as possible by flushing it beforehand with industrial wastewater, drinking water, or sewage. The test procedure is also performed using industrial wastewater, drinking water, or sewage and consists of three parts:

• Preliminary Test
• Pressure drop test
• Main test

The preliminary test serves to acclimate a test section to the applied test pressure by means of a pressure-holding phase. To do this, the test pressure plus 0.2 bar is applied to the line. A pressure drop is to be expected, primarily due to material elongation, when a test pressure greater than the normal operating pressure is applied. After the line pressure drops to the test pressure, the initial value of test pressure plus 0.2 bar is restored by replenishing the pressure. This is intended to allow pressure fluctuations—primarily caused by material effects—to subside as much as possible during the preliminary test.

The subsequent pressure drop test is used to verify the absence of air. It is absolutely essential that the system be free of air for the main test to be conducted; otherwise, incorrect results may occur. In the pressure drop test, the pressure in the pressure line is spontaneously reduced by 0.50 bar ±0.05 bar by draining water. The volume of water drained, as well as the pressure before and after the pressure release, must be recorded. An Excel-based tool developed specifically by Augsburg University of Applied Sciences calculates the required air-free condition for the results of the pressure drop test on a line-by-line basis. The main test may only begin if the air content is below the maximum permissible level. If the permissible air content is exceeded, the test must be terminated and the required air-free condition restored.

In the subsequent main test, the pressure loss is recorded over a period of 1 hour. If the pressure loss after one hour is below the maximum permissible pressure loss, the pressure test is passed and the pipe section is deemed leak-tight. The maximum permissible pressure loss is specific to each pipeline, taking into account the air content calculated in the pressure relief test. The calculation tool developed by Augsburg University of Applied Sciences is also used to calculate the maximum permissible pressure loss.

This test method is thus the first pressure test procedure that can be applied to operational pressurized sewer pipelines for all common diameters, pipe materials, and pipeline routes.

Upon completion of the research project, the Bavarian State Office for the Environment plans to introduce the developed testing method for operational pressurized sewer pipelines.

We would like to thank the Bavarian State Office for the Environment and the Bavarian State Ministry for the Environment and Consumer Protection for funding and overseeing the research project.

The research project "New Testing Method for Pressurized Sewer Pipelines in Operation" was recognized as a KUMAS Flagship Project for 2023.