The main objective of this study was to identify design and operational differences between the Danish and the German waterworks in order to learn if the groundwater treatment in the Danish and German water supplies is comparable. Furthermore, the aim is to compile existing national designs to create an overview of Danish and German designs, respectively. The work was carried out with an emphasis on iron, manganese, and ammonium removal, and thus the aeration and filtration method were evaluated. Design differences in groundwater treatment between 19 Danish waterworks, 9 waterworks from Berlin and 3 from Hamburg were compiled. The design differences were revealed by questionnaires, review of standards, and an expert interview. The most significant difference was the design of the filters. In Denmark most of the waterworks apply two-stage filtration, where both pre- and afterfilters consist of quartz. A total of 69 % of the Danish filters used in the project had a supporting layer. The average height of the Danish filters was 1.05 meter and 0.66 meter for prefilter and afterfilter, respectively. In Berlin and Hamburg the most applied filtration type was a single filtration through a mono-media (quartz) filter with an average filter height of 2 meters. Half of them (47%) were constructed with a supporting layer. In all filtration types identified in this study the average German filter height exceeded the average Danish. While the grain size distribution in the effective filter layer was found to be relatively alike and within the range of 1 to 3 mm, with a slightly larger grain size in the Danish filters. The grain size found in the Danish supporting layer was much larger than in the German supporting layers. Furthermore, a special design for prefilters, so-called roughing filters, was identified in Copenhagen waterworks, where the grain size distribution was between 24 and 150 mm. Despite the generally smaller grain size, the average filter velocity was found to be slightly larger in the German filters. The application of aerators and the backwashing procedure in Denmark and Germany were found be different. In Denmark the most frequently found aerators are the stairs aerator and the multiple slat tray aerator, whilst in Germany the most applied forms of aerators are spray aerators and waterfall aerators. The backwashing procedure in Denmark was mainly found (37 %) as a combination of first an air flush, and then water, whilst the most frequently found (50 %) backwashing procedure in Germany was a combination with an additional simultaneous air and water flush in between the separate air and water flushes. The more extensive backwashing procedure in the German filter is believed to be connected to the higher height of the filters. Empty-bed contact time was calculated, and was found to be significantly shorter in the Danish filters, where the average empty-bed contact time was 23 minutes, compared to an average of 38 minutes in the German filters. The water chemistry suggested that the groundwater types found in Denmark, Berlin and Hamburg are similar, but this was not investigated further. The conclusion was that the filters in Denmark are not comparable to the filters found in Berlin and Hamburg, based on the fundamental differences in the construction concerning the filter height, grain size, and numbers of filter steps and the resulting diverse EBCT. The difference in filter designs needs to be considered when applying future research to both supplies.