Dwr Cymru Welsh Water supplies over three million people with drinking water throughout most of Wales (UK). Ortho-phosphate has increasingly been dosed at around 1 mg/L (P) to further reduce the corrosivity of supplies to the lead pipes which connect approximately 30% of houses to water mains in the company's area, additional to long-establish pH adjustment measures. The installation of new ortho-phosphate dosing schemes and the optimisation of these and existing dosing schemes, 29 schemes in total, were subject to a regulatory programme of work, agreed with the Drinking Water Inspectorate (DWI). Optimisation comprised (i) selection of appropriate ortho-phosphate doses by a procedure involving laboratory based plumbosolvency testing linked to zonal lead emission (compliance) modelling, (ii) tight dose control and (iii) extensive monitoring of lead in supply by random daytime (RDT) sampling and by the use of lead pipe test rigs. The successful outcome was confirmed by 99% of over 5,000 RDT samples complying with the future standard of 10 μg/L for lead in drinking water.