A device for regulating a content (X) of microorganisms (M) in a liquid , comprising: a measuring unit for measuring said microorganism content (X), a regulation unit , which is connected to the measuring unit and which is configured to determine a dosage (D) of a biocide (B) based on the measured microorganism content (X) in order to achieve a predefined microorganism content (X*), and a supply unit , which is connected to the regulation unit and which is configured to supply the biocide (B) to the liquid in the dosage (D) determined by the regulation unit , characterised by a computing unit connected to the measuring unit and the regulation unit , wherein the regulation unit is configured to determine the dosage (D) using a model with at least one parameter (k; k 1 , k 2 ; n) of the relationship between an amount (P) of biocide (B) supplied in a time interval (Δt) and the change in the microorganism content (X) caused thereby in this time interval (Δt), and wherein the computing unit is configured to calculate the at least one parameter (k; k 1 , k 2 ; n) for use by the regulation unit in at least one later time interval (Δt) from a recording of the microorganism content (X) measured by the measuring unit over at least one past time interval (Δt) and the amount (P) of biocide (B) supplied in this time interval, wherein said model is given either by the equation dX dt = − kB n X or by the equation dX dt = − k 1 B 0 n e − k 2 nt X with: dX dt change in microorganism content (X) in a time interval (Δt), k, k 1 , k 2 parameter (k; k 1 , k 2 ) of the model, B amount (P) of biocide (B) supplied in this time interval (Δt), B 0 amount (P) of biocide (B) supplied in this time interval (Δt) at the time of its supply, n dilution coefficient (n) of the biocide (B), and X measured microorganism content (X). a measuring unit for measuring said microorganism content (X), a regulation unit , which is connected to the measuring unit and which is configured to determine a dosage (D) of a biocide (B) based on the measured microorganism content (X) in order to achieve a predefined microorganism content (X*), and a supply unit , which is connected to the regulation unit and which is configured to supply the biocide (B) to the liquid in the dosage (D) determined by the regulation unit , characterised by a computing unit connected to the measuring unit and the regulation unit , wherein the regulation unit is configured to determine the dosage (D) using a model with at least one parameter (k; k 1 , k 2 ; n) of the relationship between an amount (P) of biocide (B) supplied in a time interval (Δt) and the change in the microorganism content (X) caused thereby in this time interval (Δt), and wherein the computing unit is configured to calculate the at least one parameter (k; k 1 , k 2 ; n) for use by the regulation unit in at least one later time interval (Δt) from a recording of the microorganism content (X) measured by the measuring unit over at least one past time interval (Δt) and the amount (P) of biocide (B) supplied in this time interval, wherein said model is given either by the equation dX dt = − kB n X or by the equation dX dt = − k 1 B 0 n e − k 2 nt X with: dX dt change in microorganism content (X) in a time interval (Δt), k, k 1 , k 2 parameter (k; k 1 , k 2 ) of the model, B amount (P) of biocide (B) supplied in this time interval (Δt), B 0 amount (P) of biocide (B) supplied in this time interval (Δt) at the time of its supply, n dilution coefficient (n) of the biocide (B), and X measured microorganism content (X). dX dt change in microorganism content (X) in a time interval (Δt), k, k 1 , k 2 parameter (k; k 1 , k 2 ) of the model, B amount (P) of biocide (B) supplied in this time interval (Δt), B 0 amount (P) of biocide (B) supplied in this time interval (Δt) at the time of its supply, n dilution coefficient (n) of the biocide (B), and X measured microorganism content (X). <EOS>