The Establishment Of Water Chemical Stability Evaluation Frame For The Low Hardness And Low Alkalinity Water

The water in Shenzhen city have the characteristics of low hardness andalkalinity, and the traditional water purification process on the water chemicalstability is poorer, which lead to pipe network corrosion seriously, and sometimeseven the phenomenon of "yellow water". An effective way to improve the chemicalstability of water quality is in addition of remineralization process in the waterworks.At present, the traditional water chemical stability index basically has saturationindex, stability index, Riddick corrosion index, calcium carbonate precipitationpotential and larson ratio. These indices are applicable to different water qualitycharacteristics. This paper takes shenzhen source water as the research object, andthrough corrosion experiments focused on the low hardness and low alkalinity waterbefore and after remineralization corrode A3steel, galvanized steel, ductile iron andgray cast iron, whereby the establishment of water chemical stability evaluationsystem from the low hardness and low alkalinity raw water conditions. The resultsshow that:1) The outlet before the mineralization has calcium hardness35.0⁵0.1mg/L asCaCO₃, alkalinity31.4³5.0mg/L as CaCO₃and pH7.1⁷.3. Four pipe corrosionrate in the summer were:0.6,0.12,0.97and0.87mm/a; water in the winter for fourpipe corrosion rate were0.24,0.07,0.56and0.6mm/a, all belong to serious corrosion.2) After dosing lime alone, water quality chemical stability have a slight degreerise, and the corrosion of metal pipes have certain control. When water pH valuereached the cap8.5of drinking water sanitary standard, according to the traditionalstability index is identified just reach the steady state. In this conditions the four metalpipe corrosion rate were0.48,0.076,0.24and0.77mm/a, which corrosion situationremains very serious.3) The remineralization process of CO₂associated with lime can obvious improvethe water chemical stability and control pipeline corrosion. When water alkalinityreach80mg/L as CaCO₃, the traditional stability index judge it of scaling state. Thewater on the four metal pipe corrosion rate were0.15,0.25,0.14and0.27mm/a,belonging to an acceptable level, and have a protective effect on the pipeline network.4) When water alkalinity reach80mg/L as CaCO₃, a certain degree rang changechloride and sulphate, the influence of the pipeline corrosion is not big. This shows that Larson Ratio and Riddick corrosion index should not use to evaluate waterchemical stability.5) To evaluate the low hardness and low alkalinity water chemical stability inShenzhen City, alkalinity, calcium hardness and the CCPP are recommended. Toachieve water quality chemical stability, it is recommended that alkalinity and calciumhardness both reach80¹20mg/L as CaCO₃, and CCPP in3¹0mg/L as CaCO₃.