Therefore it is convenient for facilities employing these bleaching agents to add small amounts to the paper machine system as well. The oxidizing action either kills the bacteria and fungi outright, or it weakens the cell walls so that they are more susceptible to the other main class of biocides.

The other class involves highly toxic organic chemicals. Subclasses of toxic biocides go by such names as thiazoles, thiocyanates, isothiazolins, cyanobutane, dithiocarbamate, thione, and bromo-compounds. As the names imply, many of them contain sulfur ("thio"-). A possible third category consists of materials that have an ability to inhibit biological film formation, e.g. surfactants such as alkylsulfosuccinates.

Function: One has to keep in mind that the main goal is to limit the growth of sessile bacteria, i.e. those that are attached to surfaces. These are the bacteria that tend to build up, cause slime deposits and holes, hurt productivity, and hurt product quality.

Strategies for Use: Grades composed of bleached pulps very often involve a combination of treatments with oxidizing biocides, supplemented by toxic organic biocides. The usual recommendation is to treat each of the incoming streams, including the fresh water, filler slurries, chemical additives and makedown streams, etc.

Special attention has to be paid to the starch preparation area, since starch is such a favored food for slime growth. The level of oxidizing agent (e.g. chlorine dioxide) has to be maintained at a sufficiently low level that there are not any problems with bleaching of dyes or decomposition of starch, etc.

A possible starting level to consider is a residual of 1 ppm of active oxidizing agent in the paper machine system. Hydrogen peroxide, when used as a biocide, is slower to act but longer lasting. As a consequence it needs to be controlled at a higher level of residual activity in the system. The toxic organic biocides can be selected based on the temperature of the system and on the relative needs to control either bacteria or fungal growth

It is common to cycle the toxic biocide on and off over periods from several minutes to several hours; by this means it is possible to reach a required threshold of activity and also minimize the cost of chemicals. Such practices of intermittent addition need to be checked to make sure that they do not cause excessive swings in first-pass retention or other problems. Certain biocides are known to contain anionic dispersants that interfere with retention.

The residual level of oxidizing chemicals is most conveniently estimated by measuring the redox potential of the furnish with a Pt electrode relative to a standard reference electrode. The effectiveness of a biocide program is best evaluated with a combination of measurements, including petri-dish cultures of water, tests for the presence of biological deposits on surfaces, the slipperiness of wetted surfaces, and the level of smells within the facility.

Cautions: Biocides are probably the most toxic materials present in a paper mill and they need to be treated with great respect. Biocide addition systems should be installed and adjusted only by highly qualified and authorized people. Anaerobic conditions under deposits can cause release of hydrogen sulfide, which can build up in such locations as tanks. Never enter a tank until procedures are followed to make sure that the air is safe.

Illustration of bacteria and fungal cells likely to be found in a slime deposit