Timebomb in UK sewerage
The current rate of sewer replacement means the industry is facing a potential calamity, argues Stuart Crisp of the Concrete Pipeline Systems Association
The Defra white paper, Water for Life, published in December 2011, noted that only 1% of public sewers were replaced each year between 2000 and 2008. Our own research, looking at the figures submitted to the industry regulator, Ofwat by the water and sewerage companies between 2006 and 2010 showed that, on average, just 0.2% of sewers were renovated or replaced on an annual basis during that time.
This may be a great testament to the condition of our sewerage network now, or it may be a disastrously shortsighted policy storing up huge problems for the future. Either way, if this rate of replacement continues, then any new wastewater pipeline installed today would need to last for over 500 years before it is replaced.
Given this background, it would be reasonable to expect water companies to consistently be demanding products that can approach this extreme durability requirement. What we in fact find is that water utilities’ specifications for gravity sewers are not consistent, and typically define a design life requirement of between 50 and 125 years.
Of course this does not mean that the products will disintegrate at the end of this time – the critical term here is ‘design life’ as opposed to ‘service life’. In consultation with industry stakeholders, the CPSA has developed a definition of design life as the length of time an element or system can be expected to perform satisfactorily before its anticipated performance falls below the original requirements.
From an economic perspective, this is usually the time period over which an asset is depreciated. However, it is never anticipated that a product will immediately cease to function at the end of its design life, and the CPSA also considers the service life.
Service life is defined by the CPSA as the period of time during which the product will perform its original function without major repairs. Some loss of performance can be accepted.
Any pipeline installed today will need to have a service life of several hundred years, so it is worth considering what products can credibly claim to achieve the longest service life at the lowest cost. The Highways Agency recognises that concrete pipes and manholes manufactured in the UK to BS EN 1916 / 1917 and BS 5911 series will comply with their design life requirement of 120 years for structures and the BRE, in Special digest 1: Concrete in Aggressive Ground concludes that the concrete mixes used for these pipes should ensure that they have a design life exceeding 100 years.
Of course, there are other pipe material options for wastewater pipes, with plastic being a relatively common choice. Plastic pipes cannot empirically demonstrate a lifespan of 100 years and the BBA certificates for a number of plastic sewer pipe products manufactured from PVC-U, HDPE and polypropylene indicate a design life of only 50 years.
The BBA observes that it does not expect an immediate deterioration of product performance at the end of this time, but the huge discrepancy between this certified design life and the long-term service life requirement makes it difficult to understand the industry’s decision to use plastic. Clearly it is difficult to make decisions based on maintenance requirements that may not arise for 100 years.
However, if the service life of pipelines is not a key consideration then we are storing up a monumental problem for further generations. It is normal practice for manufacturers to design products to meet the specification criteria.
In our case, it is perfectly possible to re-engineer concrete pipes to achieve a reduced design life to a level equivalent to plastic pipes and with a consequent reduction in durability. As an organisation representing manufacturers, the CPSA would be very reluctant to support a move that reduced the quality of the manufactured product, but consistent and realistic specifications that are upheld by the client, would support our rigour on this issue.
The CPSA has produced an information sheet summarising this issue which can be downloaded at concretepipes.co.uk
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