Lead - dealing with the legacy
The standard for lead in drinking water changes in December next year. WRc's Mark Smith and Ian Walker consider the technical and compliance issues this raises for those working in the water industry
The value of lead as a malleable, flexible, weather-proof substance with a multitude of uses has been known for at least 5,000 years. It was used by all the early civilisations, with lead linings being used in the Hanging Gardens of Babylon.
In Roman times, lead was used extensively, and lead pipes, looking not much different from modern ones, can be seen in Bath today. In 1845, Sir John Franklin led an expedition to find the North-west passage between North America and the North Pole.
The entire expedition perished and their true fate was only discovered about 10 years later when among deprivations and disease, increased lead levels in bones and strange behaviour reported among crew records led to the suspicion of lead poisoning being among the reasons for the downfall of the expedition. Ironically, an innovative water purification system specially designed for the expedition that incorporated lead pipes may well have been the main cause of the poisoning. In the 1920s, tetraethyl lead (TEL) was developed as an ‘antiknock’ agent for cars and, although there were early deaths and reports of insanity in the refinery, it was introduced into petrol until it was banned in the late 1990s, following which a huge fall in blood lead levels around the world was recorded.
While many of the causes of lead exposure, such as paint and petrol have been dealt with, the exposure through water remains, and there is a need to be continually vigilant and look to reduce the remaining exposure to lead. Consequently, the regulated lead standard in drinking water in the UK will be reduced from 25 to 10μg/l in December 2013.
Water suppliers have adopted a range of strategies to reduce lead, with phosphate dosing underpinning ad-hoc pipe replacement and rehabilitation, but this may not be sustainable even in the short term. Phosphate dosing reduces the plumbosolvency of the lead pipe by creating a less soluble layer at the pipe water interface.
This is a dynamic equilibrium process and phosphate has to be continuously dosed to be effective. In the UK, this strategy has been very successful at achieving the 25μg/l standard and indications are that it will allow the 10 μg/l standard to be achieved in many supplies, but not all.
However, any strategy that needs continuous dosing of chemicals is expensive and in conflict with Water Framework Directive pressures of preventing nutrients getting into water bodies. Phosphate is a finite resource and water companies compete for supply with the agricultural and chemical sector.
Estimates put global resources as being sufficient for 100-200 years (the majority is mined as rock not bird droppings), but supply and demand variations make for a volatile market. In late 2007 and early 2008, the price of phosphate rock jumped dramatically worldwide due to increased agricultural demand and limited supplies. The 2008 Wenchuan earthquake in China interrupted mining for several months and caused a supply shortage resulting in the average US price more than doubling.
Spot prices in exporting regions approached US$500 per tonne – more than five times the average price at that time. So, realistically, phosphate dosing has to be seen as an interim solution and with research for an alternative chemical being unable to identify anything more effective or cheaper, we must apply direct action to the lead pipes in the ground. This means adopting strategies that are based on planned replacement or renovation programmes and the subsequent elimination of phosphate dosing.
Before addressing specific technical solutions, it is worth discussing the issue of ownership and responsibility. In the UK, the pipe that delivers water from the main to the property is called the service pipe. This is divided into the communication pipe that is owned by the supplier and runs from the main to the boundary of the property and the supply pipe, which is owned by the property owner, and runs onwards to the property itself.
Any water company renovation schemes will therefore only address the communication pipe. This approach is fundamentally flawed as a strategy to remove any health risk to the consumer, since the communication pipe is usually the shortest section of the service pipe.
The longer supply pipe may still allow sufficient lead to dissolve into the water to breach standards. This would be especially so if phosphate dosing were stopped at the same time. We are in danger of investing substantial sums of the public’s money to ensure water company assets no longer contribute to the problem, without achieving the new standards. Following any investment, should water quality fail at the tap, guess who will be held responsible?
Alongside the traditional methods, there are some innovative technologies available for dealing with lead pipes in the ground. On the replacement side, open cut is the traditional approach and involves digging a trench and laying a new pipe with the old pipe being removed or left in place.
This can be expensive and disruptive to traffic, but is relatively straight forward and can be undertaken without the need for specialist equipment. Moling and directional drilling methods both leave the old pipe in place and require only two holes to be dug, one at each end of the pipe run.
Moling uses a special impact head, which is forced horizontally through the ground, creating a route for the new pipe to be pulled through whereas directional drilling allows the operator to steer the drilling head around obstacles or other services. Directional drilling, which is used for larger diameter pipes (above 50mm), is substantially more expensive than moling or open cut, but has the advantage of being usable over longer lengths.
Pipe-splitting installs a new pipe along the route of the old pipe and employs an enlarging head with a blade being pulled through the existing lead pipe. The blade cuts the pipe, enlarging the bore as it does so and a new pipe is pulled into place behind the blade.
The route and depth of the old pipe determine whether this approach is applicable. The method is relatively simple and leaves the old lead pipe in the ground but currently, in the UK, this technology is not widely used.
Pipe-pulling removes the old pipe and replaces the new one in the old track. A cable is inserted into the existing pipe and anchored, usually at regular intervals.
The cable is then winched out, gradually removing the old pipe and pulling a new pipe into place along the same route. Since the new pipe follows the route of the old pipe, there is very little disturbance of other services.
There is a possibility of the cable breaking under the force required to remove the old pipe should, for example, repair collars or a concrete encasement exist. Some sophisticated variants calculate the force likely to be needed and no attempt is made to remove the pipe above defined limits. The removed pipe has value as scrap which provides added economic benefit as well as removing lead from the environment.
Lead pipes are generally still very serviceable and are well suited to rehabilitation where the inside of the pipe is lined to isolate the lead surface from the water. Two techniques have been commercialised: slip lining with a plastic tube and epoxy resin coating.
In both techniques the lead pipe needs to be isolated, emptied and cleaned using swabs, brushes or, in one innovative variation, air blown abrasives. For slip lining, a string with a “bullet” attachment is blown through the pipe using short bursts of high pressure compressed air and then the string is attached to the lining tube and used to pull the lining into position.
Once fixed at either end, hot water is circulated and then the internal pressure is raised to expand the liner to ensure a close fit within the pipe. The whole lining process can be completed within an hour, excluding any excavations.
Resin lining has been in use since 2000 and involves a specially developed resin being blown through the pipe with hot air and thus creating a coating approximately 0.1mm thick on the inside of the pipe. As with the plastic liner, this system also has the added advantage of fixing pre-existing leaks.
The best solution from a health risk perspective is to have no lead pipes in the ground. From a technical viewpoint, dosing with phosphate whilst undertaking targeted removal and renovation offers a more pragmatic and perhaps affordable route.
The elephant in the room, from a UK perspective, is that if the water companies only deal with their bit of pipe then a lot of money will be spent with little long term health benefit if the rest of the pipe is not also dealt with.
So who pays for the customer-side intervention? This is a strategic matter for the UK and some political leadership would be
very welcome before a lot of money is potentially wasted.
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