Effect Of Phosphate Inhibitors On Iron Release From Cast Iron Pipes In Desalinated Water

It has been a global hot issue for the growing shortage of freshwater resources. With the continuous development of desalination technology and the enlargement of the scale of production, desalinated water will gradually be supplied in municipal pipe network as unconventional water to meet the growing demand. When transported in corroded iron pipe, desalinated water could destroy the original balance between pipe scale and water, dissolve pipe scale, lead to substandard effluent in pipe ends such as high turbidity, chromaticity or iron content, result in phenomenon of "red water", seriously affect the normal use of water. In order to effectively control iron release, desalinated water quality should be adjusted before supplied into municipal network. In this paper, experiments are conducted using the cast iron pipe widely used in actual pipe network, to investigate the iron release caused by desalinated water into corroded cast iron pipes, to compare the iron release inhibition effect of adding different phosphate, and to find out the best type, concentration and water quality conditions for dosing phosphate. This project aims to help provide advice for waterworks in transport desalinated water in cast iron pipes, thus ensuring the chemical stability of desalinated water and minimize the "red water" phenomenon.The paper first analyzes physical and chemical characteristics of pipe scales. Studies have shown that pipe scales has a layered structure, the surface layer that contact with water is of loose attachments, the middle layer is a layer of hard shell, while inside layer is loose structure. The main elements of the scale are Fe and O, and main compounds are Fe3O4and Fe2O3.Secondly, bench-scale pipe reactor are made of ductile cast iron and grey cast iron pipe collected in actual municipal pipe network, experiments are conducted using the pipe reactor to simulate iron release when desalinated water entering into cast iron pipes, to compare the effect of adding different phosphate inhibition on iron release in cast iron pipe, and to find out the main factors influencing the iron release. Results are as follows: (1) In both ductile iron pipes and grey cast iron pipe, when test water switch from tap water to desalinated water, iron release has a quantity increase. mixing the two souces of water could help decrease iron release, but cannot completely solve the problem of high iron release caused by water change.(2) In ductile iron pipes, adding disodium hydrogen phosphate and sodium tripolyphosphate could inhibit iron release, while in grey cast iron pipe, disodium hydrogen phosphate, sodium dihydrogen phosphate and sodium phosphate have positive impact on iron release.(3) Among the factors affecting phosphates controlling iron release, mixed blending ratio of desalinated water and tap water has the largest effect, the second is the hardness of water, and the last is the adding concentration of phosphate.