AWWA WQTC64122

The response of distribution systems to changes in water treatment, e.g. replacement ofchlorine with chloramines or changes in the pH of the treated water) is strongly dependent on themineralogy of the corrosion scales attached to various parts of the system. These determinationsare particularly critical for lead compounds, of which there have been many reported.Knowledge of the crystal form of these compounds is a key contribution to models of leadbehavior. Therefore, it is necessary to understand both the mineralogy and the chemistry of thescales built up on the interior of lead containing materials in the distribution system and tocarefully quantify the scale compositions over their thickness in order to understand distributionsystem lead level responses to various water quality conditions and potential water qualitychanges.Metallic lead (Pb) itself is quite soluble in water and it is only the formation of protectivescales that brings the concentration of Pb down into reasonable ranges. For Pb, the dominantminerals found in distribution systems are simple oxides and carbonates. Phosphates are alsofound, and are increasingly relevant as systems turn to orthophosphate dosing to stabilize Pbscales. This array of minerals differs widely in solubility in water depending on the presence ofH⁺, HCO₃^-, PO₄^3-, SO₄^2-, and the total charge in the solution. Because Pb can occupy threeoxidation states, Pb⁰, Pb²⁺, and Pb⁴⁺, the oxidation level of the water is also critical. Thisparameter is usually expressed as Eh, or the potential relative to a standard hydrogen electrode.As part of AwwaRF Project #3018, "Contribution of Service Line and Plumbing Fixturesto Lead and Copper Rule Compliance Issues," several lead service line pipes, meters and faucetswere obtained from participating utilities and sent to the University of Cincinnati for evaluation.Evaluation of the specimens consisted of visual and photographic observations and analysis ofsamples of scale removed from the interior surfaces of these specimens, as follows:digital macroscopic and microscopic images;collecting a representative scale sample(s) from the specimen for analytical analyses;X-ray diffraction (XRD);scanning electron microscopy (SEM);Laser Raman spectroscopy; and,X-ray fluorescence (XRF).The scale analyses were conducted in two parts. The first stage was bulk characterizationby X-ray diffraction (XRD) for mineralogy, and X-ray fluorescence (XRF) for chemistry. Thesewere followed by more detailed investigations of selected samples by micro-Ramanspectroscopy and scanning electron microscopy (SEM). For each of the specimens, the interiorscales were scraped off twice, the second time more aggressively, to give an upper (L1) and alower (L2) layer. Some samples yielded a third layer (L3)Results from the characterization of scales from these various lead bearing distributionsystem pipes and components were evaluated with respect to the water quality and treatmentpractices of the utilities involved. Two project sites have been completed in detail: a set of pipeloop experiments by the Washington Aqueduct; and, a set of field samples from Madison, Wisconsin.The bulk chemistry for the Washington Aqueduct pipe loop specimens shows that Layer1 is richer in Fe, Mn, Si, Al, and especially P compared to Layer 2, whereas Layer 2 is richer inPb. Thus Layer 2 is the pipe-derived part of the scale, and Layer 1 incorporates substances fromthe water. From round 1 to round 2, there are significant increases in Ca and in P, presumably inresponse to P dosing. Mn and Fe did not show decreases, which would be a concern for scaledisruption when chloramine is substituted for chlorine disinfection.Mineralogically, both layers on

Product Details

Edition:

Vol. - No.

Published:

11/01/2006

Number of Pages:

2

File Size:

1 file , 310 KB