An apparatus provides a water bottle mouthpiece attachment which can filter microplastics and other contaminant substances from bottles, particularly disposable plastic water bottles. The filter of the apparatus includes a filter housing that is attached to a cap that fits inside a common plastic bottle. Soft rubber rings, tapering from to radially larger on the top to radially smaller on the bottom in a generally conical fashion, ensure a watertight fit to the inside opening of any drinking bottle. A friction closing/opening spout at the top of the filter enables selective opening of the bottle and prevents accidental spillage of bottle contents. The filter housing is also offset from the center, giving space to an air valve that allows air to enter the bottle without disturbing the flow of water from the bottle, thus providing optimal lamellar flow without allowing fluid to escape through the air valve during consumption.