Advanced treatment process for hot spring wastewater containing arsenic

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Inorganic arsenic compounds are commonly observed in solution as AsO_2^- and AsO_4^<3-> and these oxo-anions are typically water soluble. Arsenic does not form an insoluble hydroxide and can be removed by coprecipitation system. The most important mechanism for the accumulation of arsenic in sludge settling tank during wastewater-sludge interaction is adsorption on and coprecipitation with hydrous iron oxides in contaminated wastewater. Hydrous iron oxides is a suitable coagulant for the removal of arsenic than aluminum compounds. Hydrous iron oxides were a effective coagulant even if the influent were highly contaminated with arsenic (1.5mg/1). The aluminum compounds did not reduce the arsenic concentration of the discharged wastewater to 0.1mg/1 of the limitation level of discharged wastewater when the influent was highly contaminated with arsenic (1.5mg/1). Amount of sludge volumes which were produced during the wastewater treatment process was not effected by the species of coagulants but by the amounts of coagulants. The activity of activated sludge in oxidation ditch was not affected by 20 mg/1 of hydrous iron oxides. The concentration of iron in the discharged wastewater was increased with increasing the amount of coagulant (hydrous iron oxides) added to the modified oxidation ditch process. The concentration of coagulant added to the chemical sedimentation tank needed lower than that added to the oxidation ditch to attain the same arsenic removal efficiency.