Objective To identify the main causes of secondary pollution in the process of deep water purification treatment by analyzing potential factors for color deepening of heated piped direct drinking water in a company.
Methods Totally 33 water samples purified in piped direct drinking water system were collected from basement to the 17th floor of a company in Jiaxing City, and 1 g of suspicious crystal substances in the bottom of the water tank were also collected. Another 16 samples of unpurified water from municipal water supply system were collected as controls. Color, turbidity, odor and taste, nitrite, visible substances, lead, arsenic, copper, cadmium, iron, manganese, zinc, oxygen consumption, pH value, total bacterial count, and total coliform were tested according to the Standard Test Methods for Drinking Water (GB/T 5750-2006).
Results The qualification rate of the 33 samples of cold direct drinking water was 27.3%, and that of the 16 samples of water from municipal water supply was 100.0%. The disqualification rates of total bacteria count and copper in the cold direct drinking water samples were 57.6% and 69.7% respectively. The average content of copper in the cold direct drinking water samples was 1.12 times of the compulsory water quality limit. Other 14 indicators were all 100.0% qualified. The pH values ranged between 5.0 and 6.8, with an average of 6.1. In contrast, all test results of the control samples met the requirements of Standards for Drinking Water Quality (GB 5749-2006). There were differences in disqualification rates of total bacteria count and copper between the water samples from direct drinking device and municipal water supply (P<0.05). The nitrite contents in the outlet water of purification device (direct drinking water) were significantly higher than in the inlet water (municipal water supply water) (P<0.05). The crystal substances in the bottom of water tank were confirmed as crystal of copper hydroxide, and the black substances were copper oxide.
Conclusion Using copper pipes, long-time stay in pipes of purified water, and incomplete circulation of water in branch pipes cause secondary pollution, and they are main contributors for color deepening of heated direct drinking water. The weak acid outlet water in reverse osmosis apparatus tends to corrode fresh copper pipes with absorption of oxygen, which will turn blue copper hydroxide into black copper oxide when heated.
DownLoad: