amine source solution from reservoir 6 into the water from source 8 in proportions which are predetermined for optimal formation of the biofilm inhibiting substance these two chemicals are mixed in mixer 21 and react with each other in mixer 21 which feeds into outlet pipe 16 so that the reaction product namely the biofilm inhibiting substance produced by the reaction of these two chemicals is introduced into the liquid 3 as it is produced in situ the two branch lines 12 14 for the two venturi tubes 18 20 include control valves 22 24 which enable the flow rate of the water to be controlled via the two venturi tubes 18 20 lines 26 28 connecting the two reservoirs 4 6 to their respective venturi tubes 18 20 also include valves shown at 30 32 for controlling the dosage of the chemicals into the water passing through the venturi tubes the latter valves also enable the supply of chemicals to be terminated at the end of the introduction of the biofilm inhibiting substance so that continued flow of the water via the branch lines 12 14 mixer 21 and the outlet line 16 will wash away any residue of these chemicals or their decomposition products and thereby avoid accumulation of decomposition products which may form at the end of each biofilm inhibiting substance production cycle in the outlet line 16 or in mixer 21 the control of the foregoing valves is done by a control system schematically illustrated by block 40 the ph of the biofilm inhibiting substance decreases as the biofilm inhibiting substance decomposes outlet line 16 therefore may also and preferably does include a ph sensor 47 for sensing the ph of the biofilm inhibiting substance and controlling the control system 40 in response.