Srtm2Osm tool uses Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) to generate elevation contours (isohypses) of a selected terrain. The tool writes contours as OSM ways into an OSM file. This then enables rendering of the terrain using Osmarender XSLT transformations or other OSM renderers.
Don't extract and delete the .hgt.zip files, but when loading these files extract them in memory. This will save some space on disk.