A system for monitoring quality of ballast water comprising: a central data hub comprising a data hub computer adapted for generating a set of acceptance criteria for ballast water quality parameters at one or more geographic positions based upon uploaded ballast water data from on-board computers of at least two vessels (101 a, 101 b, 101 c); wherein the uploaded ballast water data indicates where, and possibly when, a volume of ballast water was loaded into a ballast water tank of each of the at least two vessels and the respective values of each of the ballast water quality parameters that are measured on each of the volumes of ballast water; and the at least two vessels (101 a, 101 b, 101 c), such as at least two ships, that each comprises an on-board ballast water system comprising: an on-board computer with a monitor, a data logger, a data storage for storage of a set of acceptance criteria for a number of the ballast water quality parameters corresponding to a geographical position and at least one geographical position detection means adapted for logging into the data logger the geographical position where the volume of ballast water is loaded into the ballast water tank, a number of ballast water quality sensors each being adapted for measuring at least one of the ballast water quality parameters of the ballast water in the ballast water piping or in ballast water tank, and adapted for logging ballast water data comprising a value of each of the ballast water quality parameters into the data logger, the on-board computer being further adapted for downloading the set of acceptance criteria from the central data hub and up-loading the ballast water data and the corresponding geographical position to the central data hub, wherein the on-board computer is adapted to perform a comparison of the values of the ballast water quality parameters with corresponding acceptance criteria corresponding to said geographical position, and to display information on the monitor depending on said comparison. a central data hub comprising a data hub computer adapted for generating a set of acceptance criteria for ballast water quality parameters at one or more geographic positions based upon uploaded ballast water data from on-board computers of at least two vessels (101 a, 101 b, 101 c); wherein the uploaded ballast water data indicates where, and possibly when, a volume of ballast water was loaded into a ballast water tank of each of the at least two vessels and the respective values of each of the ballast water quality parameters that are measured on each of the volumes of ballast water; and the at least two vessels (101 a, 101 b, 101 c), such as at least two ships, that each comprises an on-board ballast water system comprising: an on-board computer with a monitor, a data logger, a data storage for storage of a set of acceptance criteria for a number of the ballast water quality parameters corresponding to a geographical position and at least one geographical position detection means adapted for logging into the data logger the geographical position where the volume of ballast water is loaded into the ballast water tank, a number of ballast water quality sensors each being adapted for measuring at least one of the ballast water quality parameters of the ballast water in the ballast water piping or in ballast water tank, and adapted for logging ballast water data comprising a value of each of the ballast water quality parameters into the data logger, the on-board computer being further adapted for downloading the set of acceptance criteria from the central data hub and up-loading the ballast water data and the corresponding geographical position to the central data hub, wherein the on-board computer is adapted to perform a comparison of the values of the ballast water quality parameters with corresponding acceptance criteria corresponding to said geographical position, and to display information on the monitor depending on said comparison. an on-board computer with a monitor, a data logger, a data storage for storage of a set of acceptance criteria for a number of the ballast water quality parameters corresponding to a geographical position and at least one geographical position detection means adapted for logging into the data logger the geographical position where the volume of ballast water is loaded into the ballast water tank, a number of ballast water quality sensors each being adapted for measuring at least one of the ballast water quality parameters of the ballast water in the ballast water piping or in ballast water tank, and adapted for logging ballast water data comprising a value of each of the ballast water quality parameters into the data logger, the on-board computer being further adapted for downloading the set of acceptance criteria from the central data hub and up-loading the ballast water data and the corresponding geographical position to the central data hub, wherein the on-board computer is adapted to perform a comparison of the values of the ballast water quality parameters with corresponding acceptance criteria corresponding to said geographical position, and to display information on the monitor depending on said comparison.