Abstract: A method and a device for purifying water from contaminant trace species, especially arsenic, by co-precipitating trace species during oxidation and precipitation of iron compounds with subsequent separation. The co-precipitation is improved by contacting the water with an iron-containing material prior to oxidation to increase the iron content of the water. Arsenic and other trace species harmful to health may be removed from water effectively and in a simple manner, thus being able to comply with the more restrictive limits of arsenic in drinking water.