When the arsenic content in raw water exceeds 0.05 mg / L, the process parameters should be determined by experiments. The fluoride content in raw water should be less than 10mg / L, and the suspended solids should not be more than 5mg / L.

The particle size of activated alumina should not be more than 2.5mm, generally 0.4-1.5mm. Activated alumina should have enough mechanical strength. Before the raw water comes into contact with the filter material, the pH value should be reduced, which should be determined by technical and economic comparison, and generally should be adjusted to between 6.0 and 7.0. Acid solutions such as sulfuric acid, hydrochloric acid and acetic acid can be added to the raw water or carbon dioxide gas can be added to reduce the pH value. The dosage shall be calculated according to the alkalinity and pH value of the raw water or determined by test. The filtration rate of the filter can be adopted in the following two ways: (1) when the pH value of the inlet water of the filter is greater than 7.0, the intermittent operation mode should be adopted. The design filtration rate is 2 ~ 3m / h, the continuous operation time is 4 ~ 6h, and the intermittent operation time is 4 ~ 6x. (2) When the pH value of influent water is less than 7.0, continuous operation mode can be adopted, and the filtration rate is 6 ~ 10m / h. The flow direction of raw water through the filter layer can be from bottom to top or from top to bottom. When pH value is adjusted by sulfuric acid solution, top-down method should be adopted. When carbon dioxide is used to adjust pH value, bottom-up method should be adopted. The fluorine content of the effluent at the end of the single filter defluorination cycle can be slightly higher than 1 mg / L, and the end-point fluorine content should be determined according to the mixing regulation capacity, but the fluorine content of the mixed post-treatment water should not be greater than 1.0 mg / L. The periodic adsorption capacity of filter material is mainly selected according to the fluorine content of raw water, pH value, filtration rate, thickness of filter layer, end-point fluorine content and performance of filter material. (1) When the pH value is adjusted to 6.0-6.5 by sulfuric acid solution, it can be 4-5g (f) / kg (Al2O3). (2) When the pH value is adjusted to 6.5-7.0 by carbon dioxide, it is generally 3-4g (f) / kg (Al2O3).

The thickness of filter material for a single filter shall be as follows: (1) when the fluorine content of raw water is less than 4 mg / L, the thickness of filter material shall be greater than 1.5 m; (2) when the fluorine content of raw water is 4-10 mg / L, the thickness of filter material shall be greater than 1.8 M. Note: when the pH value is adjusted by sulfuric acid, the filter layer thickness can be reduced to 0.8-1.2m when the scale is small and the filtration rate is low.