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.
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.