A fluid treatment system comprising: a mobile device (510; 610); one or more treatment vessels (520; 612; 614; 712; 1010), the or each treatment vessel (520; 612; 614; 712; 1010) being surrounded by a frame and comprising a treatment material disposed inside the treatment vessel, at least one fluid inlet (616; 1116), and at least one fluid outlet (618; 1118); an input conduit (540; 620) that receives a fluid to be treated, the input conduit (540; 620) in fluid communication with the fluid inlet (616; 1116) on the treatment vessel (520; 612; 614; 712; 1010); and an output conduit (550, 622) in fluid communication with the fluid outlet (618; 1118) on the treatment vessel (520; 612; 614; 712; 1010), the output conduit (550; 622) being arranged to receive treated fluid from the treatment vessel via the fluid outlet (618; 1118); characterised in that: the or each frame comprises a plurality of legs and one or more horizontally oriented feet; the fluid treatment system further comprises a guide and locking device providing releasable connection between the mobile device (510, 610) and the or each treatment vessel (520; 612;712;1010), the guide and locking device comprising: two or more elongated rails, the elongated rails being substantially parallel and the feet being spaced just less than the distance the elongated rails are spaced apart; one or more cross members (736, 936; 1210), the cross member(s) (736; 936; 1210) extending between the elongated rails in a substantially perpendicular configuration, the cross member(s) (736; 936; 1210) being removably attached to the elongated rails; one or more locking devices (1230, 1240), the locking device(s) (1230, 1240) removably attached to the elongated rails or the cross member(s) (736; 936; 1210) and configured to receive the frame(s) in order to releasably lock the or each treatment vessel (520; 612; 712; 1010) into a location along the elongated rails. a mobile device (510; 610); one or more treatment vessels (520; 612; 614; 712; 1010), the or each treatment vessel (520; 612; 614; 712; 1010) being surrounded by a frame and comprising a treatment material disposed inside the treatment vessel, at least one fluid inlet (616; 1116), and at least one fluid outlet (618; 1118); an input conduit (540; 620) that receives a fluid to be treated, the input conduit (540; 620) in fluid communication with the fluid inlet (616; 1116) on the treatment vessel (520; 612; 614; 712; 1010); and an output conduit (550, 622) in fluid communication with the fluid outlet (618; 1118) on the treatment vessel (520; 612; 614; 712; 1010), the output conduit (550; 622) being arranged to receive treated fluid from the treatment vessel via the fluid outlet (618; 1118); characterised in that: the or each frame comprises a plurality of legs and one or more horizontally oriented feet; the fluid treatment system further comprises a guide and locking device providing releasable connection between the mobile device (510, 610) and the or each treatment vessel (520; 612;712;1010), the guide and locking device comprising: two or more elongated rails, the elongated rails being substantially parallel and the feet being spaced just less than the distance the elongated rails are spaced apart; one or more cross members (736, 936; 1210), the cross member(s) (736; 936; 1210) extending between the elongated rails in a substantially perpendicular configuration, the cross member(s) (736; 936; 1210) being removably attached to the elongated rails; one or more locking devices (1230, 1240), the locking device(s) (1230, 1240) removably attached to the elongated rails or the cross member(s) (736; 936; 1210) and configured to receive the frame(s) in order to releasably lock the or each treatment vessel (520; 612; 712; 1010) into a location along the elongated rails. the or each frame comprises a plurality of legs and one or more horizontally oriented feet; the fluid treatment system further comprises a guide and locking device providing releasable connection between the mobile device (510, 610) and the or each treatment vessel (520; 612;712;1010), the guide and locking device comprising: two or more elongated rails, the elongated rails being substantially parallel and the feet being spaced just less than the distance the elongated rails are spaced apart; one or more cross members (736, 936; 1210), the cross member(s) (736; 936; 1210) extending between the elongated rails in a substantially perpendicular configuration, the cross member(s) (736; 936; 1210) being removably attached to the elongated rails; one or more locking devices (1230, 1240), the locking device(s) (1230, 1240) removably attached to the elongated rails or the cross member(s) (736; 936; 1210) and configured to receive the frame(s) in order to releasably lock the or each treatment vessel (520; 612; 712; 1010) into a location along the elongated rails. two or more elongated rails, the elongated rails being substantially parallel and the feet being spaced just less than the distance the elongated rails are spaced apart; one or more cross members (736, 936; 1210), the cross member(s) (736; 936; 1210) extending between the elongated rails in a substantially perpendicular configuration, the cross member(s) (736; 936; 1210) being removably attached to the elongated rails; one or more locking devices (1230, 1240), the locking device(s) (1230, 1240) removably attached to the elongated rails or the cross member(s) (736; 936; 1210) and configured to receive the frame(s) in order to releasably lock the or each treatment vessel (520; 612; 712; 1010) into a location along the elongated rails.