The aqueous environment in a paper mill tends to be almost ideal for the growth of slime. There is usually a lot of food in the form of starch products, various sugars from the wood, and nitrogen and phosphorous from some of the additives.
In some parts of the system there may be a lot of aeration, but in other parts of the system there may be a lack of oxygen, favoring the growth of anaerobic bacteria.
Though some bacteria have distinct preferences for higher or lower temperatures or pH values, the general consensus among single-celled organisms is that a paper machine system is a fine place to live.
But there is just one problem with life as a slime organism in a paper machine system. The operators like to use slimacides and other strategies to minimize slime growth.
Papermakers tend to be especially interested in using slimacides chemicals in cases where slime growth involves sessile organisms, those that remain attached to surfaces.
To start from the beginning, it is usually recommended to suitably treat each incoming stream to the paper machine system to avoid inoculation of the process water with bacteria or fungi.
For example, fresh surface water needs to be chlorinated or otherwise disinfected before it is brought into the system. Various proprietary biocides can be added to starch formulations, filler slurries, and chemical mixtures to prevent slime growth.
The kind of biocide strategy for addition to the wet-end furnish as a whole depends on the type of furnish. In the case of bleached kraft furnish it makes sense to add chlorine dioxide, a strong oxidizing agent often used as a bleach.
Trials are needed to find the right dosage that will give a residual concentration of the order of magnitude of 1 ppm so that the slime problems are controlled without causing excessive degradation of dyes, starch, and other polymers.
In the case of high-yield or unbleached kraft pulps it makes more sense to rely completely on toxic biocides.
Supplier recommendations and trials can be used to select a product and dosage that has the needed effect on both bacteria and fungi at the prevailing conditions of temperature and pH.
To save costs it often makes sense to use intermittent treatments with biocides. The idea is to add enough material to exceed the threshold concentration needed to kill the organisms.
Then the treatment can be cut off during the time it takes for the population to begin to recover. The downside of this approach is that it sometimes causes cycling of the performance of other additives such as retention aids or sizing agents.
Some compromises in the kinds of additives, the dosages, and the cycling frequencies may be needed in order to avoid such interferences.
The development of anaerobic bacteria becomes much more likely if there are significant amounts of deposited materials in areas such as white water chests or in the headbox. The accumulation of deposits can be slowed down by the effective use of retention aids to keep the fine materials bound to fibers. Dead spaces in chests sometimes can be avoided by proper agitation.Some indications that the system is becoming loaded with an unacceptable level of bacterial slime include
- unpleasant odors, especially if there are sulfur-type odors,
- a slippery feeling of wetted surfaces in the system,
- holes in the product, especially if they occur at random locations in the web,
- high plate counts of bacteria when diluted samples of process water are placed on growth medium in petri dishes and allowed to incubate, and
- high levels of adenosine triphosphate (ATP) indicated by tests that are sensitive to this material.