Plumbosolvency control and minimisationArticle Title
Although water leaving a treatment works is almost entirely free of lead, by the time it reaches the taps of about nine million homes it has picked up lead from the pipes and fittings. Andrew Elphinston, senior chemical engineer for Binnie Black & Veatch, examines the issues arising from plumbosolvency and what can be done to comply with legislation and reduce the risk to health.
For many years lead has been recognised as being toxic to the human body. It
is a cumulative poison and once absorbed into the body, is stored in varying
proportions in the blood, soft tissue and, principally, in the bones and
teeth. At low levels it is difficult to distinguish the symptoms of lead
poisoning from a host of other ailments. A mild form of toxicity may feel
similar to a Sunday morning hangover but is unlikely to be caused by the
lead cap on the wine bottle. At higher concentrations there is evidence that
lead can affect the development of the nervous system which is of particular
concern in infants and young children when irreversible damage may be
Water leaving a treatment works is almost entirely free of lead but as it
passes through the distribution network to the tap it is picked up from
contact with some of the pipework and fittings. Water companies have not
surprisingly invested heavily in ensuring that the majority of lead-run
joints have been removed from the main distribution network, so by far the
greatest source is now from the pipework that connects the main in the
street to the house. Lead pipe was still in common use until the 1970s and
it is estimated that nine million homes are still using it. Once in the
house there are other sources as well, including fashionable antique brass
fittings, not to mention the lead solder used in copper pipework.
Lead can be present in two forms in water particulate and dissolved.
Particulate lead is generally associated with lead flaking from old pipes or
galvanic corrosion products and is best removed with a domestic filter at
the point of use. Dissolved lead results primarily from dissolution of lead
corrosion products, in the form of lead carbonates, formed on the pipewall.
A range of factors including alkalinity, temperature and contact time
affects the dissolution of lead and there are a number of ways to reduce it.
In general, low alkalinity water with a low pH has a greater tendency to
dissolve lead than waters with a high alkalinity and high pH. Solubility
increases with temperature and higher concentrations are witnessed in summer
than in winter in affected supply zones.
Extensive work has been carried out by the water industry examining the way
in which lead is introduced into the drinking water supply and on measures
to suppress and control it. As well as the control of temperature and pH a
number of chemical treatments have been tested at pilot scale and by far the
most effective is phosphate dosing.
Phosphate can be dosed in two main forms, either as orthophosphoric acid or
as one of the sodium phosphates, usually a solution of monosodium phosphate.
Addition of phosphate at the treatment works is particularly effective in
reducing lead pick-up, the basic mechanism being the formation of an
Œinsoluble’ layer of lead phosphate over the soluble carbonate deposited on
the pipewall. The phosphate layer takes some time to develop on the pipewall
depending upon the phosphate dose. Typically it would take a year for a full
protective scale to build up, based on an initial dose of 1mg/l of
phosphorous, although temperature and the condition, age and history of the
pipework being coated would affect this. Once the layer has been established
it can normally be maintained by a continuous lower dose of around 0.5mg/l.
Lead take-up in a system receiving phosphate dosing remains temperature
dependent to a certain degree but the effects are much less dramatic than
with undosed water.
One of the figures shows the effect of 1mg/l phosphate dose on lead
concentrations for samples taken from a typical supply zone. Dosing
commenced at the start of 1996 and the chart demonstrates clearly the effect
of the build up of the phosphate layer on lead concentration over the three
year sample period.
Current World Health Organisation (WHO) guidelines recommend a maximum
allowable concentration (MAC) of lead of 50µg/l. Implementation of phosphate
dosing, sometimes in combination with pH control, has generally resulted in
these levels being met. However, following a review of epidemiological data
WHO has recommended a reduced MAC for lead of 10µg/l be set. It is expected
that new regulations will be enacted in the UK in the next year, imposing a
new interim standard of 25µg/l to be met within five years and 10µg/l within
Evidence suggests that most water companies will be able to achieve the
interim level of 25µg/l by optimising phosphate dosing equipment enhanced by
suitable pH control. It is unlikely, however, that concentrations as low as
10µg/l can be consistently achieved by the use of phosphate dosing alone if
the existing compliance sampling regime is retained. Failures are likely to
be the result of high concentrations caused by particulate lead, for which
the only remedy available at present is pipe lining or replacement.
Compliance with the 10µg/l standard is likely to require extensive pipe
replacement or lining programmes with the not insignificant problem of
identifying which pipes require replacement. Added to this is the question
of who pays. The water supplier is responsible for the supply system up to
the stop cock before entering the house. Beyond the stop cock it is the
responsibility of the house owner. Clearly it will be necessary to replace
or line the pipework but some consumers may not be open to sharing the costs
involved. An alternative solution may be the fitting of filters to all
domestic drinking water appliances but this introduces additional problems
beyond the control of the water company. Either way the solution promises to
be costly and ultimately someone has to foot the bill.
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