How to meet EU lead standards by 2003
Neil Tarbet from WRc takes a look at the problem of lead pipes and how to choose an appropriate technique, such as trenchless pipe pulling or lining.The revised EU drinking water directive (1998) includes a requirement to reduce the amount of lead present in supplies.
A stepped approach is allowed in recognition of the enormity of the task - there are an estimated 7-10M properties currently receiving water via lead pipes in the UK alone.
At the same time the directive recognises that water utilities do not have full control over pipes within customers' properties and it could be difficult for a government to introduce legislation forcing customers to remove their lead pipes.
A get-out clause has therefore been introduced which says that the requirements of the directive have been met, provided the water supply complies with its requirements at the final point of the distribution system over which the utility has control; the boundary stop-tap.
However this get-out clause does not apply to Œpremises and establishments where water is supplied to the public, such as schools, hospitals and restaurants'.
The problem is therefore how the requirements of the directive can be met, given these restrictions. In the majority of cases it will be possible to meet the interim 25µg/l limit by water treatment and in favoured areas it may be possible to meet the final 10µg/l value. However it is likely that for most waters, pipe replacement or rehabilitation will be necessary.
Various systems for the renovation or removal of lead pipes have been developed, in addition to traditional open-cut trenching. Open cut trenching does not require special skills or equipment but is very disruptive and costly. Other than for the replacement of the short-side supply pipe, it is generally considered as as a last resort.
For long-side supply pipes, installation of a new pipe by Œpneumatic impact moling¹ is the most common method. However, impact moling cannot be used in all situations, particularly where the ground is non-compressible, rocky or congested by other services. If the length to be installed is greater than 10m, intermediate digs may be required.
The more innovative pipe replacement techniques fall into two types; pipe pulling and pipe splitting. The basic principle common to pipe pulling is simple the lead pipe is gripped and pulled from the ground by a mechanical device, whilst at the same time a new service pipe is pulled in behind the lead pipe.
The mechanical device used to pull out the lead pipe may be a winch, hydraulic jack or even a backhoe mechanical digger. The key difference between the methods is the system used to grip the pipe.
The simplest systems use a bullet-shaped tool. The pulling cable is fed from the pulling device, through the lead pipe and connected to the nose of the bullet, which engages into the end of the lead pipe. The new service pipe is attached to the tail of the bullet. As the cable is tensioned, the bullet pushes the lead pipe out of the ground and the new pipe is pulled into the space vacated by the lead pipe.
The bullet technique was originally developed to remove galvanised steel pipe. However, lead is much more flexible and malleable than steel and often laid with bends and loops rather than in a straight line. Applying pulling force at the end of the pipe can therefore cause it to distort and buckle, making it difficult or impossible to remove.
Methods have been devised to overcome this (see left). Feeding a cable through a heavily encrusted or convoluted pipe can prove difficult and the thicker the cable, the more difficult it is to feed through but the greater the pulling force that can be applied.
Hard, dry ground conditions also make it more difficult to pull out lead pipe. The main benefits of pipe pulling are that it works relatively quickly and it can be used in congested service areas as the new pipe is installed along the line of the old.
For pipe splitting, the tool is pulled through the pipe by a cable. The split pipe is forced apart because there is insufficient space for the replacement pipe, which is connected to the tail end of the splitting tool. There is also a risk that adjacent services, e.g electricity cables, may be damaged if the cutter drifts out of the lead pipe at bends.
An alternative to removal is the renovation of the lead pipe by thin wall lining. A thin wall tube is inserted to provide a physical barrier between lead and water. This approach has several advantages. It is rapid, is not affected by ground conditions, suitable for lining through to the consumer's stop tap and helps to reduce leakage.
Unfortunately it may not be possible to insert the liner into a convoluted or distorted pipe.
The diameter of the pipe is reduced by the liner, although the improved hydraulic performance of the lined pipe generally compensates for the reduction in diameter.
The long-term performance of the lining depends to some extent upon the condition of the pipe. There are two processes available in the UK, the Neofit PET lining technique (£4-5/m) and the cheaper folded Polyline Microliner (£1.50/m) which is pulled through and then inflated using compressed air.
Choice of the most appropriate method will depend upon a range of technical
and economic factors.