Biocide-free slime control in pulp and paper mills
Mariana Björklund, Ph.D
Most micro-organisms in the process waters are harmless, but some cause problems as they colonise surfaces, so-called biofouling, leading to the growth of biofilm.
The activities of micro-organisms in the pulp and paper mills lead to frequent cleaning and maintenance costs, lower quality of the paper or board due to holes and spots, and breaks during production.
Conventionally, the papermaker has used biocides to control growth of micro-organisms. However, increased public consciousness regarding environmental issues has led to strict regulation on the use of biocides (including slimicides) in western countries. This has led to a limited number of biocides being available for the papermaker to use.
It is for this reason that biocide-free slime control appears to be an attractive part of the present and future, but actually it is an almost twenty-year-old concept, which is today well proven and effective when the right product is used.
BIM Kemi AB has marketed biocide-free slime control since 1981, and its own product, Bimogard, since 1991. This paper will describe practical experiences of the mode of action of this slime control.
Mode of Action
It is not toxic to micro-organisms at the concentrations used in the mills.
Biofilm and polysaccharides
Table 1 shows the effect of Bimogard on the amount of EPS after introduction to a mill previously using biocides. The mill uses 100 % DIP. DIP, on the contrary to TMP, does not itself contain measurable amounts of EPS.
Table 1. The amount of EPS decreases after introduction of biocide-free slime control in a paper mill using 100 % DIP
Table 2. Endospore formation of Bacillus subtilis (colony forming unit/ml)
Figure 1 shows laboratory tests on growth curves for the spore forming bacteria Bacillus subtilis with or without Bimogard. As can be seen, depending on the dosage, the treatment delays or totally inhibits the growth of Bacillus subtilis.
Yet, the bacteria remain as vegetative cells and are thus killed in the drying section. The data in Table 2 and Figure 1 have been confirmed in mill cases.
Figure 1. Bimogard delays the growth curve of Bacillus subtilis. The bacteria remain vegetative; they do not form spores
UNLIKE BIOCIDES, BIOCIDE-FREE SLIME CONTROL DOES NOT TRIGGER BACTERIAL DEFENSE MECHANISMS
Both EPS-production and spore formation are bacterial defence mechanisms which can be triggered by dehydration, starvation, heat and toxins. As mentioned above, Bimogard is not toxic to micro-organisms and therefore does not trigger these mechanisms, unlike biocides. Further, this biocide-free slime control is dosed continuously, which is not economically feasible when using biocides. Yet, the incoming flow of bacteria is continuous, and so should the slime control be.
The bacteria count is usually 105/ml to 107/ml in mills using Bimogard. Most of these bacteria are harmless and die in the drying section without causing slime-problems.
No limitations in pulp or paper qualities
Also, this slime control has been successfully used in low as well as neutral to high pH conditions, and at small, old as well as large, modern, fast-moving paper machines. Bimogard is also used at intermittent board machines in pulp mills.
Table 3. Different pulp and paper qualities in mills presently using Bimogard.
· Keeps surfaces clean and smooth thereby preventing biofouling.
· Decreases the production of EPS ("slime"), thereby preventing growth of biofilm.
· Delays the growth of bacteria and decreases the formation of spores, thereby preventing bacteria in ready-made paper or carton.
· Is pH-stable
· Is temperature-stable
· Is environmentally friendly.
· Does not endanger the health of the staff.
· Is proven to be a satisfactory or even better alternative to biocides.
· Is cost effective.