The Research On Water Chemical Stabilization And Control Methods In The Urban Water Supply System

The problems of water chemical stability were generally exist in domestic water supply industries at present, and corrosion was more serious in particular. It is meaningful to improve the water supply system chemical stability and control corrosion of disribution, which play the economic and health dual roles for the protection of pipeworks and tap water quality.To comprehensive and objective evaluation of the chemical stability of water, three water stable indexes were used in this paper. For iron pipes without lining, CCPP and LR were used to evaluate water stablization; as to cement pipes and iron pipes with cement mortar lining, using AI and LR to analysis water stablization.On the basis of surveying of water stablization of water supply system in the special economic zone of Shenzhen, it was found that the raw water, treated water,tap water were corrosive and aggressive in the most time of a year.The risk of iron release in distribution by the anion to penetrate turbcles in corroded iron pipes was exist.It was found that the tubercles had similar morphological structure, with almost three layers: the outer layer, the"shell-like"layer and the porous interior. The main chemical composition of the tubercles were iron corrosion byproducts, Fe3O4 andα-FeOOH are the main chemical composition of the outer layer and the"shell-like"layer, Fe3O4,α-FeOOH,γ-FeOOH are the main chemical composition of the the porous interior. The Cement mortar layer had similar morphological structure as tubercles gradually in long time running and iron corrosion products became one of the main chemical composition of the cement mortar layer.While using the same raw water, both of the effulent of conventional treatment and Ozone-BAC treatment were more corrosive than the raw water.The treated water can be close to stabilization by dosing alkaline chemicals to increase the pH bettween 8.0 to 8.5. As to the effect of various pH control methods, NaOH> Ca(OH)2> Na2CO3> NaHCO3. It was better to dose lime 3~5 mg/L in the point after dosing coagulant in one minute or dose caustic soda 2~3 mg/L after filtration. The cost of dosing lime was lower than caustic soda.The project experiences of controlling iron release in the distribution of a large industrial group in southern China indicated that dosing lime or caustic soda to increase the treated water pH value bettween 8.0 to 8.5 were both very effective. As to small-scale water utilities, if the raw water with low turbidity in long term, the technology of direct filtration and dosing caustic soda afeter filtration were worth consider. The cost of equipment and chemical in caustic soda treatment were lower than lime treatment.As to the raw water of Shenzhen, increase the pH of treated water could not achieve water stable, the LR was still in high level.The recarbonation and reminellation could effectively decrease LR of treated water by dosing carbon dioxide and lime water. To reduce the risk of iron release in distribution, the total alkalinity of treated water in Shenzhen should be more than 50 mg/L. Through carrying on the pilot scale experiment for some time, it was found that treated water stabilization could be achieved by dosing carbon dioxide 18mg/L and lime 20mg/L in the raw water.