Additive injection assembly (100; 200) configured to supply a predetermined amount of an additive to a fluid in a pipe, the additive injection assembly comprising: - a reservoir configured to contain the additive; - a pipe portion comprising a first end and an opposite second end ; - an injection pump comprising: ∘ an electromagnetic linear motor comprising a stator and a movable armature that is configured to be driven reciprocatingly with respect to the stator , and ∘ a pump portion comprising a piston and an additive inlet chamber , wherein the piston is coupled to the armature , and is configured to reciprocate in said additive inlet chamber , thereby, in use, alternatingly compressing and decompressing a volume of said additive inlet chamber with a compression stroke (CP) and decompression stroke (DP); and - an additive outlet chamber arranged or arrangable in direct fluid communication with the pipe portion and arranged in fluid communication with the additive inlet chamber ; wherein, in use, a first decompression stroke (DP) of said piston discharges a predetermined amount of said additive from said reservoir into said additive inlet chamber , a consecutive compression stroke (CP) of said piston causes said additive to flow from the additive inlet chamber into the additive outlet chamber , and a consecutive second decompression stroke (DP) of said piston causes the additive to be directly injected into the pipe portion from the additive outlet chamber , characterized in that the additive injection assembly further comprises: - two fluid-tight couplings (2, 2"), arranged to respectively couple the first end and the second end of the pipe portion in line to the pipe in a fluid-tight manner; and - a flow sensor provided on the pipe portion and a controller (C), wherein the flow sensor is configured to measure the mass flow of the fluid inside the pipe portion , and wherein the controller (C) is configured to control the reciprocating frequency and/or the stroke length of the armature based on at least a measurement of the flow sensor . - a reservoir configured to contain the additive; - a pipe portion comprising a first end and an opposite second end ; - an injection pump comprising: ∘ an electromagnetic linear motor comprising a stator and a movable armature that is configured to be driven reciprocatingly with respect to the stator , and ∘ a pump portion comprising a piston and an additive inlet chamber , wherein the piston is coupled to the armature , and is configured to reciprocate in said additive inlet chamber , thereby, in use, alternatingly compressing and decompressing a volume of said additive inlet chamber with a compression stroke (CP) and decompression stroke (DP); and ∘ an electromagnetic linear motor comprising a stator and a movable armature that is configured to be driven reciprocatingly with respect to the stator , and ∘ a pump portion comprising a piston and an additive inlet chamber , wherein the piston is coupled to the armature , and is configured to reciprocate in said additive inlet chamber , thereby, in use, alternatingly compressing and decompressing a volume of said additive inlet chamber with a compression stroke (CP) and decompression stroke (DP); and - an additive outlet chamber arranged or arrangable in direct fluid communication with the pipe portion and arranged in fluid communication with the additive inlet chamber ; - two fluid-tight couplings (2, 2"), arranged to respectively couple the first end and the second end of the pipe portion in line to the pipe in a fluid-tight manner; and - a flow sensor provided on the pipe portion and a controller (C), wherein the flow sensor is configured to measure the mass flow of the fluid inside the pipe portion , and wherein the controller (C) is configured to control the reciprocating frequency and/or the stroke length of the armature based on at least a measurement of the flow sensor . <EOS>