In order to study the applicability of biological ammonia removal from drinking water, field experiments with 10 L/h flow rate were carried out at a case-study area, where raw water contained arsenic, iron, manganese, dissolved gases (methane, aggressive carbon dioxide) and ammonia. The technology applied at the waterworks consisted of potassium permanganate (KMnO4) dosing and rapid sand filtration followed by sodium hypochlorite (NaOCl) disinfection. The treated water fulfilled the requirements for iron, arsenic and usually for manganese as well; however, it contained ammonia and dissolved gases. The aim of the investigation was to find out how the current technology could be modified in order to meet the drinking water requirements. In the first experimental setup, treated water of the waterworks was aerated and directed to the biological reactor, where ammonia was converted into nitrate. In the second experimental setup, the untreated water was first aerated, followed by a biological reactor (sand filter), KMnO4 dosing and a second sand filtration stage. Both experiments were successful; the treated water fulfilled the iron, manganese, arsenic and ammonia limits. Furthermore, the studied technologies were capable of removing dissolved gases. Modification of the current technology according to the second experimental setup is suggested, for two reasons: the KMnO4 dose could be decreased to one-third of the current value, and the explosive methane gas could be removed in the first treatment step.