Parameter PH Total disolved solids Arsenic Sulphate Chloride Alkalinity Hardness Silica Boron Disolved organic carbon Unit mg/L µg/L mg/L mg/L mg/L CaCO3 mg/L CaCO3 mg/L SiO2 mg/L mg/L Coagulation technology for arsenic removal 269 Table 2. Quality of water in the northern zone of Chile. The process only removes arsenic. Other parameters are essentially the same in the effluent Surface water (range) Groundwater (range) 8.0-8.4 700-800 400-600 80-100 120-140 100-120 130-150 20-30 3-4 Negligible 7.0-8.0 730-790 60-80 - - 50-60 350-400 20-30 2-5 Negligible human consumption should not contain more than 10 µg/L (18). Currently, 99.98% of the Chilean population have access to potable water with arsenic <50 µg/L, but only 52.69% have access to potable water with arsenic <10 µg/L. To meet the new Chilean standard and the WHO guidelines, Chile will need to treat significantly more water for removal of arsenic in other zones of the country. REMOVAL OF ARSENIC The coagulation process consists of the addition of metal-based coagulant, such as ferric chloride (FeCl3), to arsenic-contaminated water. FeCl3 hydrolyzes in water to form positively-charged ferric hydroxide [Fe(OH)3]. Arsenic must be in oxidized form [As(V)] for effective removal. Thus, if any arsenite [As(III)] is present, it may be necessary to oxidize it to As(V) using chlorine as a pre-treatment process. Arsenate [As(V)] is a negatively-charged anion and sorbs to the positively- charged Fe(OH)3 particles or flocs. The sedimentation and filtration processes then remove arsenic particulate. A general schematic diagram of the arsenic-removal treatment process is given in Fig. 2. The arsenic-removal system by means of coagulation in the 1970s delivered water with a residual arsenic con- centration of 120 µg/L to the population of the north- ern Chile. The delivery concentration decreased to 50 µg/L in the 1980s and to as low as 10 µg/L in the 2000s. This increase in efficiency in the removal of arsenic has been achieved by improving the treatment-system follow-up, including control of pH and adjustment of reagent doses (19). Table 3 lists some principal arsenic- removal conditions at Salar del Carmen, Cerro Topater, and Taltal. The coagulation processes are typically used for removing turbidity. When the same processes are used for removing arsenic from surface water, the design of the treatment system should maximize the formation of a floc with characteristics of size, cohesion, and sedi- mentation speed that favour stable arsenic adsorption onto it. In this way, arsenic changes from a dissolved species into a particulate species that can be separated or removed from water by means of sedimentation and filtration. In the case of groundwater, the removal process often includes only oxidation, coagulation, ad- sorption, and filtration. Regardless of the method of re- Fig 2. General schematic Chillean arsenic-removal treatment process: (a) Surface water and (b) Groundwater (a) Raw water Arsenic 400 µg/L (b) Raw water Arsenic 70 µg/L H2SO4 Cl2 Pre-oxidation H2SO4 Cl2 Pre-oxidation FeCl3 Coagulation absorption FeCl3 Coagulation absorption Sedimentation Filtration Filtration Cl2 Post-oxidation disinfection Finished water Arsenic 10 µg/L Cl2 Post-oxidation disinfection Finished water Arsenic 10 µg/L