AbstractIn this study a rapid at-line ATP (adenosine triphosphate) analysis is applied in papermaking. This ATP analysis takes less than a minute, and the information can be utilized instantly to adapt the biocide program. The study shows the effect of different biocide strategies at paper mills. Comparison is made between oxidative and reductive biocides on the one hand, and on the other hand between continuous vs. batch additions of biocide. Continuous biocide addition keeps the microbial activity at a constant level. However, a long production period without a boil-out might result in accumulation of resistant bacteria, which cannot be eliminated without changing the biocide strategy. Batch addition of biocide creates a high temporary concentration of biocide in the process. This causes lower temporary microbial activity in the process, but between the doses the microbial activity may rise to an intolerable level. Batch addition causes chemical variation to the wet end of a paper machine more easily than continuous addition. This can affect the performance of papermaking chemicals and cause problems with retention, fixing, etc. Both biocide addition strategies can be used if they are monitored and optimized properly. Rapid ATP analysis is a suitable tool for both purposes.