The groundwater used as drinking water often contains unacceptable amounts of iron and manganese compounds and ammonium ions. These substances are efficiently removed using conventional technologies (aeration and filtration through granular media filters). However, the efficiency is drastically reduced for water containing considerable amounts of organic matter, which usually enters the water in wellfields linked with surface water. This paper presents a technology development for treating groundwater with such complex problems. A combination of open and closed aeration and filtering through three filters containing sand, zeolite and sand with oxidizing bacteria are used. Polyaluminium chloride acting as coagulant is the only chemical substance used in the technology. The developed water treatment technology was tested for eight months. The technology ensured the reduction of iron, manganese, ammonium, and organic compounds by 98%, 87%, 99%, and 80%, respectively. The efficiencies are sufficient to meet the requirements of the Council Directive 98/83/EC on the quality of water intended for human consumption for most wellfields fed by both underground and surface water.