Introduction
In the late 1970's papers were published indicating that the mutagenicity of pulp mill effluents
decreased significantly as chlorine dioxide was substituted for chlorine. No other technique can
match the high quality that is achievable with chlorine dioxide – it produces clean, strong and very
bright paper – and over 2,000 tons per day are now consumed in this application. Chlorine dioxide
is effective in the control of microbiological growths in paper mills under conditions unfavourable to
chlorine. It is particularly effective in systems having a high pH, ammonia-nitrogen contamination,
persistent slime problems, or where the microbial contamination is aggravated by contamination
with vegetable or mineral oils and phenols. Chlorine dioxide does not react with organic materials to
form trihalomethanes.
Application Description
Some of the major operational problems are caused by proliferation of microbiological organisms.
As microorganisms grow, they form slime deposits on screens, wires and other equipment,
necessitating frequent cleanup. If not removed, these deposits can appear in the paper as spots,
holes or tears. The microbes can also produce undesirable properties to the finished paper, such
as unacceptably high spore counts or offensive odors. Microorganisms can also accelerate the
corrosion of metal surfaces and parts, leading to premature replacement of equipment.
Treatment Alternatives
The use of non-oxidizing biocides may not be effective on sessile bacteria and may be ineffective
at preventing the formation of slime. Many non-oxidizers lack the penetrating ability to control the
biological growth within the slime layer and/or effectively remove it.
Chlorine Dioxide
As a broad-spectrum, oxidizing biocide, chlorine dioxide generated from sodium chlorite is effective
for use in controlling microbiological growth in white water paper mill systems. Although chlorine
dioxide is non-reactive with ammonia-nitrogen, it may oxidize some sheet additives such as wet
strength resins or retention aids.
Since chlorine dioxide is used for potable water disinfection, it is appropriate to use this versatile
disinfectant in food grade paper applications. Food grade paper is required to meet higher microbial
standards than fine paper. Therefore, the cost of microbiological control is considerably higher
than for fine paper. This is because it is difficult to inactivate bacterial spores, particularly the genus
Bacillus, which survive the extreme temperatures of the dryers in the papermaking process. Chlorine
dioxide has been found to be a very effective sporicide in food grade paper applications1-2, in
potable water applications3-4, and in some food processing applications5-6.
Unlike chlorine, chlorine dioxide is relatively non-reactive with most of the organics found in alkaline
whitewater. As a result, a large portion of the chlorine dioxide fed will be available for disinfection.
Thus the bacterial activity can be effectively reduced to almost any desired level by controlling the
chlorine dioxide feedrate.
Advantages of Chlorine Dioxide
The use of chlorine dioxide for alkaline whitewater microbiological control is not new. Only recently
have some of the additional benefits been recognized. A summary of the important benefits of
chlorine dioxide is given below:
Chlorine dioxide is a very effective slime control agent.
Chlorine dioxide reacts rapidly and can be applied at a site immediately before the problem
area, unlike many conventional antimicrobials, which are generally slow acting.
Chlorine dioxide remains relatively non-reactive with the vast majority of organics, reducing
the dose rate necessary to achieve effective control.
Low dose rates result in typically low corrosion rates when compared to other oxidizers.
In addition, minimizing or eliminating the slime layer reduces microbiologically influenced
corrosion on equipment.
The chlorite ion (chlorine dioxide byproduct) keeps working as both a bacteriostatic and
slime control agent, even after the chlorine dioxide has reacted.
By effectively controlling slime growth, the frequency of boil outs can be reduced and the
potential for unscheduled downtime due to paper breaks can be minimized.
Effectively controlling slime growth minimizes the hole count, maintaining the quality of the
finished sheet.
Odors resulting from bacterial fermentation, phenols, sulfides, or mercaptans are virtually
eliminated by use of chlorine dioxide.
