A pressure vessel having an outer shell and a flexible inner liner. The outer shell and inner liner are configured so that the water passing through the water treatment system engages the inner liner, and not the outer shell. The inner liner is blow molded from a material that does not leach undesirable substances into the water flowing through the water treatment systems. The inner liner may include a receptacle portion to house one or more water treatment components. The receptacle portion may be flexible under the anticipated range of internal pressures. The receptacle portion may be disposed in the outer shell with the outer shell concealing expansion/contraction of the inner liner and providing the pressure vessel dimensional stability. The inner liner may include a rigid neck portion mounted to the outer shell. The outer shell and/or inner liner may include internal ribs configured to maintain spacing therebetween.