Metal Residuals in Contact Filtration of Humic Drinking Water

摘要

The concentration of Al and Fe residuals in treated drinking water is regulated in the EU directive to be < 0.2 mg/l. This is also the limit in the new Norwegian standard, with a proposed guideline value of 0.15 mg Me/l in cases where metal coagulant is used in water treatment. Residual metal in treated water is controlled by the solubility of hydroxide precipitates, and the capability of the treatment process to remove precipitated metal. In general, the solubility of metals in water is dependent on pH, temperature and content of organic and inorganic substances. In a pilot plant with two parallel filter columns receiving identical water the effects of different filtration parameters on residual metal were studied. One filter was a conventional dual media anthracite/sand, and the other was a more coarsely grained dual media clay aggregate (Filtralite) filter. When the traditional metal salts aluminium sulphate (AS) or iron chloride sulphate were used to coagulate low turbidity humic water, residual metal problems were experienced for both filters. A high residual turbidity caused by insufficient filterability of the humic-Al-precipitates was observed with the coarser Filtralite filter, especially when AS was used for coagulation. For raw water with higher turbidity the particulate metal residuals were generally lower. The conflicting pH ranges for optimum turbidity and organic matter removal, and minimum solubility of precipitate were narrowed down when PAC was used for coagulation.