An apparatus for mixing two liquids is provided. The apparatus includes a static mixer, and a first and second feed module for feeding the two liquids to the mixer. The feed modules include pressurizable chambers for accommodating flexible containers which hold the liquids to be mixed. The liquids are forced through the static mixer when the chambers including the flexible containers are pressurized. Pressurization is achieved by pressurized gas stored in pressure reservoir chambers that are connectable to the chambers holding the flexible containers. Related methods of mixing two liquids are also provided.