all five biocides naocl naobr mcdmh bcdmh meh and dcdmh were prepared as 1000 ppm fresh cl 2 master solutions treatment concentrations for all biocides were calculated from the measurement of free and total residual halogen as measured by the dpd cl 2 test conducted immediately before treatment tests incorporating repeated slug plus continuous treatments at increasing initial concentrations 10 15 and 20 ppm were performed for three consecutive days on naocl naobr mcdmh bcdmh meh and dcdmh heat transfer rate and dissolved oxygen levels in the system were automatically monitored and their dynamics were analyzed based on obtained parameters the following conclusions were reached naocl naobr and bcdmh meh were not able to remove biofilm at any of the tested concentrations biofilm recovery was observed 24 hours after the start of each treatment and htr values were higher than values observed at the start of each treatment as shown in fig1 2 and 3 dissolved oxygen response to biocide treatment was the strongest in the case of dcdmh and the weakest in the case of naocl through analysis of curve patterns fig1 fig 5 it was concluded that biofilm regrowth control could be achieved by a slug plus continuous treatment of 15 ppm bcdmh meh or 20 ppm of naobr as shown in fig2 and fig3 however neither of these biocides was able to initiate biofilm removal testing of chlorinated hydantoins mcdmh and dcdmh demonstrated a unique effect biofilm sloughing occurred soon after addition of 20 ppm of either mcdmh or.