Abstract: A portable or personal water filtration apparatus comprising upstream and downstream removable reservoirs and a back-washable filter interposed therebetween. The back-washable filter comprises an upstream housing and a downstream housing, inside of which are fitted a membrane cartridge and a slidable plunger. The downstream housing is rotatable relative to the upstream housing between two or more positions, including, for example, an open flow position, a closed flow position, and a back-wash position. In the back-wash position, the downstream housing and plunger are axially extendable relative to the upstream housing and membrane cartridge in order to obtain a charge of clean water in the plunger that may then be forced backward through the filter by returning the plunger to its starting position. This dislodges accumulated debris from the membrane and reverses the slowing of flow observed through repeated use of the filter.