A method of monitoring and optionally controlling removal of microplastics from microplastic containing raw water, drinking water, storm water, water originating from melted snow, surface water, effluent of industrial wastewater treatment plants, effluent of municipal wastewater treatment plants, industrial process water, said method comprising the steps of: admixing at least one coagulant and/or polymer to the microplastic containing water, and allowing the at least one coagulant and/or polymer to interact with the microplastic containing water to form flocs and/or aggregates containing microplastic particles; and removing the formed flocs and/or aggregates in a separation process to provide treated water with decreased microplastic content; wherein the addition amount of said at least one coagulant and/or polymer is controlled in relation to a predetermined reference input value; wherein the number of microplastic particles of the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer is determined by using an optical measurement measuring light scattering and fluorescence of particles in a predetermined volume of the microplastic containing water, by: a) measuring the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer, b) measuring a reference sample comprising the microplastic containing water, before and/or after addition of said at least one coagulant and/or polymer, doped with predetermined microplastics; c) combining the measured light scattering intensities against fluorescence intensities of each measurement in a) and b), d) identifying particle populations measured which represents microplastics particle populations, e) determining the number of microplastic particles of the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer. admixing at least one coagulant and/or polymer to the microplastic containing water, and allowing the at least one coagulant and/or polymer to interact with the microplastic containing water to form flocs and/or aggregates containing microplastic particles; and removing the formed flocs and/or aggregates in a separation process to provide treated water with decreased microplastic content; wherein the addition amount of said at least one coagulant and/or polymer is controlled in relation to a predetermined reference input value; wherein the number of microplastic particles of the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer is determined by using an optical measurement measuring light scattering and fluorescence of particles in a predetermined volume of the microplastic containing water, by: a) measuring the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer, b) measuring a reference sample comprising the microplastic containing water, before and/or after addition of said at least one coagulant and/or polymer, doped with predetermined microplastics; c) combining the measured light scattering intensities against fluorescence intensities of each measurement in a) and b), d) identifying particle populations measured which represents microplastics particle populations, e) determining the number of microplastic particles of the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer. a) measuring the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer, b) measuring a reference sample comprising the microplastic containing water, before and/or after addition of said at least one coagulant and/or polymer, doped with predetermined microplastics; c) combining the measured light scattering intensities against fluorescence intensities of each measurement in a) and b), d) identifying particle populations measured which represents microplastics particle populations, e) determining the number of microplastic particles of the microplastic containing water before and/or after addition of said at least one coagulant and/or polymer.