A system for cleaning and sterilizing a water flow, comprising: a main treatment line, comprising an inlet with a pump to pump water from a water storage, such as a ballast water tank of a vessel, through a main filter to a sterilizing station with one or more UV lamps to sterilize the water flow, and a system outlet, wherein the main filter is provided with a first filter element and a backflush member with a first backflush outlet to rinse the first filter element with backflush water, and a secondary treatment line, comprising an inlet being in fluid communication to the first backflush outlet, a backflush filter (31; 39; 47) with a backflush filter element that has filter openings with a largest dimension between 50 micrometer and 20 micrometer to remove floating organisms or silt from the backflush water, and a secondary outlet being in fluid communication with the main treatment line upstream relative to the sterilizing station, to lead filtered backflush water from the secondary treatment line to the main treatment line, wherein the backflush filter is designed such that a backflush pressure difference over the backflush filter element is less than 0.05 bar (5 kPa), characterized in that the main filter is designed such that the pressure difference over the first filter element is at least 0.1 bar (10 kPa) or at least 0.2 bar (20 kPa) and wherein the largest dimensions of openings in the first filter element are smaller than half or one third of the largest dimension of the openings in the backflush filter element, wherein the backflush filter (31; 39; 47) comprises a filter rinsing system (36, 37, 50) to rinse the silt from the backflush filter element with rinsing water, wherein an outlet of the filter rinsing system is connected to a silt tank to collect the rinsing water with the silt rinsed from the backflush filter, wherein the silt tank is connected to a centrifuge, to dewater the silt, and wherein a first centrifuge outlet is connected to the main treatment line or the secondary treatment line such that water flowing from the centrifuge is re-circulated upstream to the main filter or backflush filter and a second centrifuge outlet is connected to a storage area or to a storage tank to store the dewatered silt. a main treatment line, comprising an inlet with a pump to pump water from a water storage, such as a ballast water tank of a vessel, through a main filter to a sterilizing station with one or more UV lamps to sterilize the water flow, and a system outlet, wherein the main filter is provided with a first filter element and a backflush member with a first backflush outlet to rinse the first filter element with backflush water, and a secondary treatment line, comprising an inlet being in fluid communication to the first backflush outlet, a backflush filter (31; 39; 47) with a backflush filter element that has filter openings with a largest dimension between 50 micrometer and 20 micrometer to remove floating organisms or silt from the backflush water, and a secondary outlet being in fluid communication with the main treatment line upstream relative to the sterilizing station, to lead filtered backflush water from the secondary treatment line to the main treatment line, wherein the backflush filter is designed such that a backflush pressure difference over the backflush filter element is less than 0.05 bar (5 kPa),