Ortho-phosphate dose optimisation

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 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. 

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. 

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). 

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. 

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

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. 

Hayes, C.R., Incledion, S. and Balch, M. (2008). Experience in Wales (UK) of the optimisation of ortho-phosphate dosing for controUing lead in drinking water. Journal of Water and Health, 6(2), 177 185. 

Ortho-phosphate dosing was further optimised over the period 2000 to 2003, resulting in the application of an increased average dose of 1.2 mg/1 (P). This average dose has been applied using a winter dose of 1.0 mg/1 (P) and a summer dose of 1.5 mg/1 (P). 

Optimisation of ortho-phosphate dosing, at slightly higher doses, achieved > 99% compliance with a lead standard of 10 pg/1, based on RDT sampling. 

Optimised ortho-phosphate has achieved a very high level of compliance with the future lead standard of 10 pg/1. The integrated approach to optimisation enabled the appropriate doses of ortho-phosphate to be achieved within a relatively short period. 

---