A water filtration device, which comprises a filtration unit, being based on reverse osmosis membrane separation and operable by a displacement pump and comprising an inlet valve for water to be filtered, an outlet valve for filtered permeate water and an outlet valve for retentate waste water, and a chain gear, being mounted to a bicycle frame and operable for driving said displacement pump via a human-powered chainring set comprising a chain and pedals, characterised in that: i) said chain gear, said chain, said chainring set including said pedals, and said bicycle frame are components of a complete, functioning bicycle; ii) said chain gear is only a chain gear or a combination of a chain gear and a belt drive and connected to the displacement pump via a further chain gear or belt drive; iii) said chain gear comprises at least one derailleur mechanism by which the bicycle chain is switchable between at least two sprocket wheels (6a-6c, 7b-7d), of which at least one sprocket wheel (6a-6c), optionally being part of a sprocket cluster, is firmly connected to a hub of the bicycle's rear wheel and intended to drive the bicycle's rear wheel, and at least one sprocket wheel (7b-7d) which is part of an, optionally second, sprocket cluster that rests on a bush that freely rotates on the hub and comprises at least one further sprocket wheel or a pulley that is connected to a sprocket or a pulley of said chain gear or belt drive via a further chain or a belt and intended to drive the displacement pump; and iv) said filtration unit is firmly, but preferably releasably, connected to said bicycle and movable therewith, which renders the entire water filtration device mobile. i) said chain gear, said chain, said chainring set including said pedals, and said bicycle frame are components of a complete, functioning bicycle; ii) said chain gear is only a chain gear or a combination of a chain gear and a belt drive and connected to the displacement pump via a further chain gear or belt drive; iii) said chain gear comprises at least one derailleur mechanism by which the bicycle chain is switchable between at least two sprocket wheels (6a-6c, 7b-7d), of which at least one sprocket wheel (6a-6c), optionally being part of a sprocket cluster, is firmly connected to a hub of the bicycle's rear wheel and intended to drive the bicycle's rear wheel, and at least one sprocket wheel (7b-7d) which is part of an, optionally second, sprocket cluster that rests on a bush that freely rotates on the hub and comprises at least one further sprocket wheel or a pulley that is connected to a sprocket or a pulley of said chain gear or belt drive via a further chain or a belt and intended to drive the displacement pump; and at least one sprocket wheel (6a-6c), optionally being part of a sprocket cluster, is firmly connected to a hub of the bicycle's rear wheel and intended to drive the bicycle's rear wheel, and at least one sprocket wheel (7b-7d) which is part of an, optionally second, sprocket cluster that rests on a bush that freely rotates on the hub and comprises at least one further sprocket wheel or a pulley that is connected to a sprocket or a pulley of said chain gear or belt drive via a further chain or a belt and intended to drive the displacement pump; and iv) said filtration unit is firmly, but preferably releasably, connected to said bicycle and movable therewith, which renders the entire water filtration device mobile.