Horizontal-flow Roughing Filtration (HRF) is presented as an alternative pretreatment method prior to Slow Sand Filtration (SSF). The prefilters remove efficiently solid matter from turbid water without the use of chemicals or mechanical equipment. Developing Countries are therefore most interested in the self-reliant process which recently has been introduced successfully. The following article describes the structures and mechanisms of HRF, gives a brief summary of the laboratory tests conducted in Switzerland and the thereby developed analytical model. Finally, some practical informations for the filter design are given.