Plumbosolvency ­ control and minimisationArticle Title

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

caused.

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

15 years.

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.