TEPPFA - Municipal Sewer Systems in Civils

Most European countries are facing the shared challenge of aging sewer lines, and in the years to come, network operators across the continent will have to deal with rather extensive rehabilitation and replacement projects. It is highly important that this task is handled responsibly.

Professor Stein from Bochum, Germany says:

"The maintenance of sewer systems should follow an unwritten agreement between the generations as, due to the longevity of such systems, one generation passes the systems on to the next. But sewer systems deteriorate and the rehabilitation action taken is not always sufficient to stop or reverse the process."

Plastic sewer pipes in operational service have proved themselves time and again. But how do they compare with traditional pipe materials? What are the environmental impacts of sewer defects and leakage?

The main risks are those associated with infiltration of ground water into the sewer and exfiltration of effluent from the sewer.

In 2005, the Sustainable Municipal Pipes Project (SMP) compared thousands of CCTV footage of damaged sewer pipes. It was the first independent study of its kind and its conclusions were very interesting.

The study involved the examination of 1,800 km of existing public sewers. Core samples were taken from routine surveys in Germany and comparative studies in the Netherlands and Sweden. The average age of the pipes inspected was 11.5 years for rigid and 6.8 years for flexible.

Return to Top

Defect Analysis

All visual defects were recorded using the European Standard visual inspection coding system for drains and sewers (EN 13508-2). Furthermore, the data was combined into two grouped categories comprising:

Rigid Pipes (clay and concrete)
Flexible Pipes (PVC, PP, PE)

Statistical analysis techniques were then used to determine the likely environmental consequences of observed defects.

From the analysis of CCTV video footage, and associated sewer pipeline condition reports, the following general statements could then be made:

For both rigid and flexible pipes, many defects can be attributed to defective installation, due to poor techniques or workmanship, and/or lack of supervision
Flexible pipes are significantly less likely to have defects than rigid pipes
A high proportion of the defects observed in rigid pipes are those likely to cause infiltration and exfiltration

These statements clearly favoured plastic pipes as opposed to traditional pipe materials. But there was more…

The authors of the report noted that the environmental impact of the average section caused by infiltration or exfiltration for flexible pipe systems is 15% (less than one-sixth) of that for rigid pipe systems.

Considering the number of defects in reference to the installed length of all sewers of the particular material groups analysed in this study, flexible pipes have, on average, just 20% (one fifth) of the defect rates of rigid pipe systems.

And even when considering defective sections with defect types that are the main causes of infiltration and exfiltration (*), defect rates of flexible pipe systems are, on average, 25% (one quarter) of the defect rates of rigid systems.

It was clear that local conditions surrounding the pipelines are a major influence on the level of environmental impact whatever the type of pipe material. Operators of sewer networks see infiltration as more critical due to the impact on the cost and quality of treatment.

A comparison on a scale of severity was therefore made between the environmental risks associated with different pipe materials:

Very low risk

Medium risk

High risk

Figures could therefore be compiled from statistical modelling of defect frequencies and the expert’s views on the likely impact to the environment. In terms of performance, flexible plastic pipes only represent a very low risk and are clearly the more sustainable option.

Whereas flexible pipes perform better than rigid pipes, good installation practices are a key factor for good performance of sewer pipelines. Installation of flexible pipeline systems significantly reduces risk to the environment so it offers better sustainability and good long service

The detailed results of the study are of high interest for the local municipalities; they can now rank the sewer defects in their system with regard to environment.

Return to Top

Key Conclusions

In service, flexible pipes perform better than rigid pipes
Buried pipelines need flexibility
Fittings for flexible pipeline systems have tighter dimensional tolerances and therefore ensure more reliable connections
Installation of flexible pipeline systems significantly reduces risk to the environment
When choosing which type - rigid or flexible - latter offers better sustainability, flexible sewer pipelines are the sound, long-service option