Assessment of Materials for Sewer Systems

The results of research into the life cycle of sewer pipes in the UK has been published late in 2001. The research was sponsored by the Department of Trade and Industry (Dti) as part of their commitment to helping the construction industry to tackle sustainability issues. The need for an effective sewerage system was recognised in the UK during the 19th Century, when Victorian engineers set high standards of efficiency and durability. The 20th Century saw the introduction of new and innovative materials, including concrete and plastics. So, what is the best way forward for the 21st Century?

Aims
The aim of the recent research was to establish the most appropriate materials for sewerage systems in the 21st Century in terms of health standards, while also embodying economic and environmental principles. That said, the research was instigated by the Concrete Pipe Association (CPA) and supported by the British Cement Association (CPA). The stated objectives for this life cycle assessment were ‘to build on the work already undertaken in Europe and to position concrete as the most environmentally friendly material for drainage and sewerage pipeline systems’. A leading Dutch company, INTRON, was commissioned to undertake the research in the UK, collecting data from 1999 to 2000. A full life cycle was included, from mining the raw materials, production of pipes and manholes, to decommissioning at the end of the system’s useful life. As well as concrete, other materials investigated were clay and five types of plastic.
Although the aim was to place concrete at the top of the list, the research had to be undertaken with a high degree of quality, openness and transparency. The Building Research Establishment (BRE) was invited to perform an independent critical review of the research, according to the ISO 14040 standards. The BRE agreed that the methodology of the research was scientifically valid, data collection and sources were reasonable and appropriate to the goal of the study, and the study was transparent and consistent.

Scope
A functional unit was defined for the research as 1km of gravity main sewerage system under a road in a non-aggressive soil and groundwater environment, used for the removal of mixed household water. Pipe diameters, number of junctions and number of manholes were also standardised. This functional unit was used as the basis to compare seven different materials as follows:
Concrete
Clay
Solid wall PVC
Twin wall PVC
Ultrarib PVC
Twin wall PP
Spirally wound HDPE

The manholes were the same for all of the pipe materials, being concrete with iron covers. Pipe seals were considered equal for all systems.
The sewer systems were modelled on a cradle-to-grave basis. The life cycle stages included:
Manufacturing sequence of all parts of the sewer system
Transport of these parts to the site
Construction of the sewer system
Use and maintenance over 50 years
Demolition or abandonment of the system
Disposal of waste

Validity
For all the life cycle processes, data on environmental inputs and outputs were collected, including input of raw materials, energy consumption, emissions to air, water and soil, and any solid wastes produced. All data was checked for robustness by sensitivity analysis, where any major assumptions were examined in order to determine their effect on the results. For example, concrete pipes are manufactured at a greater number of locations than clay or plastic pipes, so transport emissions were assumed to be lower for concrete. Also, the effects of transportation of bedding materials and excavated earth were looked at, as was the effect of abandoning the various types of sewer at the end of its service life. As previously stated, all work was ratified by the BRE.

Conclusions
The environmental performance of products can be measured using a number of parameters, e.g. energy consumption, emissions and waste. The environmental effects during the life cycles of the sewer systems have been compared making it possible to make decisions about the relative environmental ranking of the various materials investigated. As you can probably imagine, concrete came out top in most parameters, otherwise the publication from which the information in this article is taken would probably not have been printed. This fact does not invalidate the overall conclusions. Concrete performed best in the following 7 categories:
Depletion of raw materials
Ecotoxic effects on sediment
Ecotoxic effects on soil
Ecotoxic effects on water
Energy requirements
Toxicity to humans
Waste chemicals

There were two categories where the performance of concrete was at the bottom of the environmental heap, those being non-chemical waste and depletion of the ozone layer, both of which are comparatively high for concrete pipes.
The following claims are made based on the overall results:
Concrete and clay sewer systems can be regarded as comparable
Concrete can be regarded as more environmentally sound than PVC or twinwall HDPE/PP
Concrete can be regarded as moderately better than spirally wound HDPE

So, from all this we can conclude that concrete or clay are the best environmental options for main sewerage systems. If for some reason it must be a plastic system, then spirally wound HDPE is the one to go for.

Acknowledgements
This article is based on ‘Environmental Assessment of UK Sewer Systems’, a 20 page booklet available from the following sources:
Department of Trade and Industry (Dti), 1 Victoria Street, London SW1H 0ET
Concrete Pipe Association (CPA), 0116 253 6161
British Cement Association (BCA), Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS