Salar del Carmen 400 Cerro Topater 400 1.0 56.1* 70-75 143 25-30 10 1.0 40.5* 70-75 143 20-30 10 270 J Health Popul Nutr Sep 2006 Table 3. Arsenic-removal conditions: Salar Del Carmen, Cerro Topater, and Taltal, Chile Arsenic-removal conditions Arsenic in raw water (µg/L) Chemical dosage Oxidant (mg/L Cl2) Coagulant (mg/L FeCl3) Decantation rate (m3/m2/day) Filtration rate (m3/m2/day) Sludge generation (kg/day) Arsenic in finished water (µg/l) *H2SO4 for adjustment of pH moval, the arsenic-removal process becomes a simple device for removal of suspended material. Arsenic spe- ciation, pH, coagulant doses, and agitation speed are important parameters in this process (7,19). Any prob- lems that may originate in the process of floc formation and separation by sedimentation and/or filtration may limit the efficiency of removal of arsenic from water. Recent studies demonstrated that the presence of hardness in water to be treated could favour removal of arsenic, but that some anions, especially phosphate, carbonate, and silicate, may compete with arsenic for the sorption sites, thus interfering with removal of ar- senic (20,21). Chilean water has both hardness and these competitive anions. The efficiency of the process is, thus, sensitive to the water matrix in this condition. Quick and accurate measurement of concentrations of arsenic in water has also been a fundamental factor in improving the efficiency of the removal process. It has always been important to use the analytical method which has best responded to the requirements of the control process, to the country’s economic situation, and to the abilities of its technicians. Initially, the Gut- zeit method was used (22), but later, in the 1980s, the silver diethyldithiocarbamate colorimetric method was used (23), since the 1990s, hydride generation-atomic absorption spectrometry has been used (24). This has allowed more frequent adjustment of the process and the reduction of the detection limits. Achieving higher efficiency in the removal of arsenic from water has involved greater expenditure for increasing coagulant dosage and dose automation, con- tinuous control of pH, and more frequent monitoring of concentrations of arsenic. The FeCl3 dose may be reduced if pre-treatment of pH is applied (19). Greater investment, operations and maintenance costs also re- quire additional resources to train and maintain teams of highly-qualified technicians to operate the plants. Sancha AM Taltal 70 1.0 8.0 - 150 - 10 Finally, the disposal of arsenical sludge generated dur- ing treatment of water has always been and will always be difficult due to its dangerous characteristics. In the case of Chile, the problem has been solved by carry- ing out this process in the desert. During the first years, this sludge was disposed of without taking any special precautions. In recent years, the process has taken place in specially-engineered sites using a geotextil as com- ponent of a landfill bottom and capping barrier systems (25) The price of drinking-water in Chile is determined, in part, by the cost of infrastructure for treatment of water, the chemical reagents used, and the system’s op- eration and maintenance costs. In Antofagasta, where arsenic is removed from water, the cost for a family that consumes 20 m3 of water per month is currently US$ 46.48, or 2.3¢ per litre. The cost for a family in San- tiago, where arsenic is not removed, amounts to US$ 16.18, or about a third of the cost in Antofagasta, for the same volume of water. These figures reflect partially the relatively high cost of removal of arsenic in North- ern Chile. FUTURE OF ARSENIC REMOVAL Chile is making efforts to find cost-effective solutions to achieve lower levels of residual arsenic concentration in drinking-water from surface and underground water supplies. In this new scenario, another possibility that has been considered, for Northern Chile, is to replace some current sources of surface water with de-salinized sea water (26,27). This option would only be applicable in coastal cities, but not in plants located farther from the ocean, i.e. in Salar del Carmen Plant (Antofagasta), but not in Cerro Topater (Calama). In the central zone of Chile, where some surface waters have concentrations of arsenic in the range of 14 to 16 µg/L, modifications to the current coagulation process used for removing turbidity could meet a 10- µg/L standard. In the case of groundwater with concen-