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 and decompression stroke; 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 of said piston discharges a predetermined amount of said additive from said reservoir into said additive inlet chamber, a consecutive compression stroke of said piston causes said additive to flow from the additive inlet chamber into the additive outlet chamber, and a consecutive second decompression stroke 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, wherein the flow sensor is configured to measure the mass flow of the fluid inside the pipe portion, and wherein the controller 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 and decompression stroke; 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 and decompression stroke; 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, wherein the flow sensor is configured to measure the mass flow of the fluid inside the pipe portion, and wherein the controller 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.