A mobile water treatment plant in the form of a container of cuboidal shape with the dimensions of an ISO standard freight container, and thus having a length greater than its width and having sides, ends, a top and a bottom, the container having corner fittings placed at positions to allow the container to be engaged by standard load handling equipment for handling freight containers, wherein at least the following are provided within the container: - a plurality of tanks (32, 34, 36, 38), the plurality of tanks comprising a clarifying tank fitted with a multiplicity of inclined plates defining a multiplicity of channels there between, an upstream tank from which water is arranged to be fed to the clarifying tank and a downstream tank to which water is arranged to be fed from the clarifying tank, wherein the tanks are arranged in a row along the length of the container with adjacent tanks sharing a common wall, and wherein the tanks are arranged adjacent to a second side of the container; - means for introducing a neutralising agent into water in the upstream tank; - means for introducing a neutralising agent into water in the downstream tank; - at least one first storage space within the interior space of the container, said at least one storage space including space for accommodating the neutralising agent, characterised in that - the tanks are spaced from a first side of the container by an internal longitudinal wall that extends from one end to the other and divides the container into two sections, one wider than the other; - the narrower section has a closed top extending from one end of the container along a major part of the length of the container above the first storage space which adjoins the first side of the container; and - wherein at the end of the top that terminates partway along the length of the container, a vertical transverse wall is provided, between the transverse wall, the adjacent side of the container and the adjacent end of the container, an open-topped recess, accessible from the end of the container is defined, and steps are fixed to provide access to a user from the ground to the top which defines a walkway along the top of the container. - a plurality of tanks (32, 34, 36, 38), the plurality of tanks comprising a clarifying tank fitted with a multiplicity of inclined plates defining a multiplicity of channels there between, an upstream tank from which water is arranged to be fed to the clarifying tank and a downstream tank to which water is arranged to be fed from the clarifying tank, wherein the tanks are arranged in a row along the length of the container with adjacent tanks sharing a common wall, and wherein the tanks are arranged adjacent to a second side of the container; - means for introducing a neutralising agent into water in the upstream tank; - means for introducing a neutralising agent into water in the downstream tank; - at least one first storage space within the interior space of the container, said at least one storage space including space for accommodating the neutralising agent, characterised in that - the tanks are spaced from a first side of the container by an internal longitudinal wall that extends from one end to the other and divides the container into two sections, one wider than the other; - the narrower section has a closed top extending from one end of the container along a major part of the length of the container above the first storage space which adjoins the first side of the container; and - wherein at the end of the top that terminates partway along the length of the container, a vertical transverse wall is provided, between the transverse wall, the adjacent side of the container and the adjacent end of the container, an open-topped recess, accessible from the end of the container is defined, and steps are fixed to provide access to a user from the ground to the top which defines a walkway along the top of the container.