Plumbosolvency determinants

Assuming that these recommendations are adopted by the Member States, there will be a clear requirement for problems with lead in drinking water to be quantified and for improvement measures to be taken to protect public health, where necessary. This will be a very significant step forward, bearing in mind the extent of current difficulties in the implementation of standards for lead in drinking water. The incorporation of risk management strategies is particularly important as it will require Member States and their water suppliers to proactively determine the extent of plumbosolvency in their areas and to take appropriate improvement action. 

Step 2 Determine the plumbosolvency of the treated water input(s) to the water supply zone and a representative number of locations within the zone this will typically require five samples to be tested using the established laboratory procedure of Colling et al. (1987). Test results can be obtained within one month. 

For these reasons, a more accurate risk assessment will be obtained if pipe-work characteristics and water consumption have been determined by inspection and if the plumbosolvency of the water in the supply zone has been determined by testing. 

Ortho-phosphate suppresses plumbosolvency by converting some of the lead carbonate in the corrosion film to lead phosphate, which is less soluble. Within the three-dimensional corrosion film there is an equilibrium between lead phosphate and lead carbonate as determined by the relative concentrations of ortho-phosphate, carbonate and bicarbonate ions in the water within the lead pipes. 

Laboratory plumbosolvency testing to directly determine the corrosivity of the water supply to lead pipes (Section 10.1). 

The plumbosolvency of a water supply is determined by the quality of the source water(s) and by water temperature. There are exceptions, such as lead leaching from brass and galvanic corrosion effects (see Chapter 1), but case studies indicate that generally the worst lead in drinking water problems relate to the presence of lead pipes. We can therefore focus on the interaction of water with lead pipes. 

The models, which are described in more detail elsewhere (Van der Leer et al, 2002), enable the most relevant features of a water supply zone to be incorporated in the prediction of zonal compliance with lead standards, as a function of boA plumbosolvency (corrosivity of the water to lead) and the zone s physical characteristics. A zonal model simulates the emissions of lead at individual simulated houses, through time, across an entire water supply zone or area of supply. It uses a single pipe model to determine the lead emission profile at each simulated house, the characteristics of each simulated house being the outcome of the random ascription of variables, which follows the Monte Carlo method for establishing a probabilistic firame-work. 

As M (the initial mass transfer rate which determines the initial slope of the dissolution curve) and E (the equilibrium concentration) reduce, the water is less plumbosolvent (less lead dissolves curves A to C) and these factors can be determined by stagnation sampling at appropriate reference houses or by laboratory plumbosolvency testing. Curves A1 and A2 differ in shape as a consequence of the relationship between the 30 minutes stagnation and equilibrium concentrations, which vary for individual waters (Hayes, 2008). The exponential curve and the assumption of plug flow are both approximations, but they enable the computational demands of the model to be greatly reduced. Extensive research (Hayes, 2002 and Van der Leer et al, 2002) has demonstrated that these approximations are adequate when compared to the more scientifically exact diffusion model and the three dimensional simulation of turbulent flow. 

Laboratory testing using sections of new lead pipe is reproducible and can determine the plumbosolvency of drinking water. 

Laboratory plumbosolvency testing coupled with computational comphance modeUing to quickly determine the likely optimum dose, which should then be confirmed (and adjusted if necessary) by routine monitoring of in-situ lead pipes at consumers houses this approach can minimise the number of iterative changes to water treatment conditions and can save both time and money. 

The volumetric profiles shown in Table 3.1 are not influenced significantly by temperature over the range exhibited by seasonal variation in water supplies. The profiles apply to any plumbosolvency condition and can therefore be used in conjunction with predicted lead concentrations. The flow condition that applies can be determined by calculating the Reynolds number that relates to the pipe diameter and flow involved. The skewing effect is discussed further in Chapter 8. 

The aim of building a computational model is to be able to use it for investigating complex processes or structures, to an extent far more than could ever be achieved experimentally. In the context of this project, the aim was to mimic the numerous permutations of the variables that determine lead emissions to drinking water across an entire water supply system, so that variables of interest could be investigated. The variables of greatest interest are those that affect the plumbosolvency of the... 

Analysis of corrosion deposits from 6 lead pipes exhumed from City A indicated a dominance of Pb(II) compounds, which are more soluble than Pb(IV) compounds. This is consistent with the moderately high lead concentrations found in both LCR surveys and sequential sampling exercises. Despite pH elevation to 10.0 at the treatment works (which does not normally fall below 9.5 in the distribution network), 7 of the 9 surveys failed the LCR for lead over the period 2007 to 2011. The dominance of Pb(II) compounds also means that rapid laboratory plumbosolvency testing can be used in the quantification of M and E, and that the supply system should be amenable to orthophosphate treatment (albeit the transition characteristics of lead corrosion deposits under high pH conditions would have to be determined). 

With this in mind, the three orthophosphate dosing scenarios were modelled. The values of M (0.02) and E (30) are readily achievable, based on UK experience, and demonstrated a significant further reduction in lead concentrations that would (if achieved in practice) comply with the LCR. To determine the likely dose of orthophosphate, it would be a fairly simple matter to undertake laboratory plumbosolvency testing with results obtainable within a month. Any organic influences should be captured by such testing. 

However, if the values of M and E for City B were found to be higher, as might be determined by laboratory plumbosolvency testing, it would cast into doubt the assumptions about pipe lengths and diameters, which are key drivers in determining... 

To utilise these computational modelling tools further will require better calibration data. Better indicative results could quickly be obtained using assumptions about pipe-work circumstances if the level of plumbosolvency could be determined by laboratory testing. It would also be necessary to firm up on an estimate of the percentage occurrence of lead service lines in the City. This would enable a better assessment of the extent of plumbosolvency problems in City C to be determined, together with a better understanding of the effectiveness of orthophosphate dosing. 

The length and diameter of the lead pipe were held constant at 15 m and 18 mm, respectively. The length of the copper pipe was varied although the diameter was held constant at 18 mm. In all cases the plumbosolvency factors were set at M = 0.05 and E = 75. The results for 30 minutes stagnation and plug flow are shown in Figure 8.6 from which it can be determined that ... 

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