Ortho-phosphate dosing

After ortho-phosphate dosing commenced in 1995, a marked improvement was observed over the next three years (1996-8), although dosing was not fully optimised. Thereafter, the full benefit of dosing was demonstrated over the period 1999-03, albeit a single sample exceeded all three lead standards in 2001. Despite this single atypical result, the comparison of the bulked results for 1990-4, 1996-8 and 1999-03 clearly demonstrate the success of ortho-phosphate dosing. 

The observed lag in responding to ortho-phosphate dosing is particularly important. It is likely explained by the longer time taken for the corrosion deposits within the lead pipes to reach equilibrium with the applied ortho-phosphate dose, due to organic and iron contents. In contrast, the observed lag in responding to ortho-phosphate was between 6 to 12 months in chalk ground-water fed supply systems in Eastern England. 

The consequence of these observed lags is that zonal compliance monitoring by random daytime sampling needs to extend to several years to be able to demonstrate a particular ortho-phosphate dose, making dose optimisation very difficult by zonal compliance monitoring alone. 

Arising from the preliminary assessment of sampling methods (COST Action 637, 2008) the data for lead that has so far emerged, based on at least 100 samples in each case, is summarised in Table 5.1. The term emerging evidence is used as either the data has only recently been obtained or the data has not been previously published in a collated EU-wide format. In Table 5.1, with the exception of the data from Wales, all data presented was obtained between 2002 and 2008. The data from Wales derived from 1990 to 1995, prior to the commencement of ortho-phosphate dosing to reduce plumbosolvency it is typical of the UK more generally (prior to ortho-phosphate dosing) based on other case studies (for example Hayes et al., 2006) and has been included for comparison. 

Following the commencement of ortho-phosphate dosing in Wales, less than 1% of RDT samples now exceed 10 pg/l (Hayes et al., 2008). 

In the UK, one of the criteria for optimising ortho-phosphate dosing was that no more then 2% of RDT samples should exceed 10 p l in practice, in England and Wales as a whole, a failure rate as low as 1% has been achieved. 

Ortho-phosphate dosed 0.02 30 Must be correctly dosed. Required doses tend to range from 0.5 to 2.0 mg/l (P) and must be applied consistently. 

The phosphate dosed condition that was modelled (M = 0.02, E = 30) was predicted to achieve or only slightly exceed one of the UK s optimisation criteria (Drinking Water Inspectorate, 2001) for ortho-phosphate dosing, that no more than 2% of RDT samples should exceed 10 pg/1. This dosed condition has readily been achieved in practice and, where necessary, lower values of M and E have been achieved by shghtly higher phosphate doses in order to meet the 2% RDT target (Hayes et at, 2006, 2008). 

Optimal ortho-phosphate dosing will reduce risk by up to 500 times. 

In the late 1970s, trials in Eastern England indicated that pH elevation of high alkalinity waters could at best achieve a 30% reduction in lead solubility. There were concerns about calcite precipitation and the approach was abandoned in favour of ortho-phosphate dosing. However, few high alkaUnity suppUes were ortho-phosphate dosed until the mid-1990s, when compUance with the former 50 p.g/1 standard was enforced. 

With the advent of the current (25 p.g/1 from 2003) and future (10 pg/1 from 2013) lead standards in the EU, 95% of the UK s public water supphes are now ortho-phosphate dosed, clearly accepting the limitation of pH elevation alone to achieve these modem day standards. 

Ortho-phosphate dose mg/l as P Zero poly-phosphate Poly-phosphate 0.2 mg/l as P Poly-phosphate 1.6 mg/l as P... 

Engineering design of ortho-phosphate dosing piants... 

Figure 8.1 General diagram of an ortho-phosphate dosing plant...

There is no simple control loop available to use in the adjustment of ortho-phosphate doses, particularly when in-situ lead pipes can take two to three years to equilibrate with any given orthophosphate dose. 

A typical ortho-phosphate dose can be applied and its success (or otherwise) reviewed after a sufficient period of sampling. With groundwater supplies, a minimum period of sampling of one year may well be adequate. However, with surface derived waters, particularly those with a total organic carbon content greater than 3 mg/1, sampling for at least two years will likely be necessary. 

After the sampling period, the ortho-phosphate dose can be adjusted (up or down) depending on the optimisation criteria that have been set. One of the UK s optimisation criteria was that no more than 2% of random daytime samples from consumers taps should exceed a lead concentration of 10 pg/1. 

As described in Section 10.2, lead pipe test rigs can be installed at strategic locations within a water supply system. A typical deployment would be (i) treated water at the treatment works, before orthophosphate dosing, (ii) treated water at the treatment works, after ortho-phosphate dosing, (iii) within the near distribution system, close to the treatment works, and (iv) within the far distribution system, distant from the treatment works. 

If new lead pipes are installed in the rigs, they equilibrate quickly with their feed water. This indicates the ultimate performance potential of the applied ortho-phosphate dose but does not provide any indication of Ae time that in-situ lead pipes will take to reach equilibrium. If old exhumed lead pipes are installed in the rigs, there is a likelihood of erratic results caused by the physical disturbance of the lead corrosion deposits when exhumed and in subsequent handling. 

Reference houses provide a direct measure of the response of in-situ lead pipes to ortho-phosphate dosing. They also provide evidence of the time being taken by in-situ lead pipes to achieve equilibrium (see Table 3.7). Monitoring at reference houses does not provide a measure of the extent of zonal lead emissions. 

The best and quickest way to achieve optimisation of ortho-phosphate dosing is to use several techniques in combination ... 

Zonal lead emission modelling to investigate corrective options and to determine ortho-phosphate dosing requirements (Section 10.3). 

The ortho-phosphate doses needed to achieve the UK criterion, that no more than 2% of random daytime samples exceed 10 pg/1, varied from 0.5 to 2.0 mg/1 (P) in the great majority of cases. Exceptionally, with poorly treated, highly coloured, low alkalinity waters, a dose of 2.8 mg/1 (P) was found to be necessary. Table 8.3 illustrates the dose requirements for low and high alkalinity waters, based on extensive laboratory plumbosolvency testing. Such doses of ortho-phosphate are considered to be entirely safe, given that ortho-phosphate concentrations are many thousands of times higher in many carbonated soft drinks. 

Table 8.3. Summary (based on a simple classification) of ortho-phosphate doses required to achieve a iead emission of < 10 pg/i after 30 minutes contact at 25 C in laboratory tests (from Hayes, 2008)...

Ortho-phosphate dose range (mg/i P) Percentage of high alkalinity ground waters Percentage of high alkalinity surface waters Percentage of low alkalinity surface waters... 

In practice, the ortho-phosphate dose required is also a function of the percentage reduction in plumbosolvency needed to achieve the optimisation criterion, which is strongly influenced by the extent of occurrence of lead pipes within the water supply system. In the Wales (UK) case smdy (Hayes et al., 2008), the required percentage reduction in plumbosolvency and the required average ortho-phosphate dose was established for a total of 29 dosing schemes that were subject to regulatory control, plus 9 others not subject to regulatory control. 

Ortho-phosphate dosing of water supplies is extensive in the UK and commonplace in the US, but elsewhere it is very limited. This is due to an under-estimation of the extent of problems with lead in... 

It is clear, in general terms, that the environmental impact of dosing ortho-phosphate to a water supply is small. Even so, a local environmental impact assessment should be undertaken as circumstances vary. Generally, there is a need for more research to establish specific ecological objectives and, determine any necessary constraints on phosphate discharges and the need for phosphate elimination in sewage treatment systems. The positive environmental impact of ortho-phosphate dosing will be the avoidance of major disruption, particularly in urban and sub-urban locations. 

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