Microbiological control: search and destroy on a tight budget; papermakers have been battling the wet end's unseen enemies--bacteria and fungi--for years. In addition to tools like the trusted microscope, some new weapons may help as well.

There are good bugs and there are bad bugs in papermaking. Some of the good bugs can be found in the waste treatment plant, chewing up harmful substances into sludge or gas. Some of the bad actors can be found in the wet end, growing fast and gumming up the papermaking process.

As papermakers try to stay one step ahead of these critters, they turn to the marvels of modern chemistry to identify and eliminate bacterial and fungal growth in the warm confines of the wet end. Solutions! asked two experts in this field to identify the best current technology for effectively detecting and deleting bacterial and fungi contamination.

According to Janet Woodward, technical specialist-microbiology, Buckman Laboratories, Memphis, Tennessee, USA, there will always be a microbial population in a wet end because paper mills do not run under sterile conditions. That is why early detection of an increase in the microbial population or a population shift is so vital.

"Traditional plate counts are still one of the most effective methods for the enumeration of aerobic, planktonic organisms in chests, headbox, white water, etc.," she said. "Although all aerobes will not grow on any one specific growth medium, the trending of these numbers will indicate whether there is a difference in biological activity."

Woodward added that adenosine triphosphate (ATP) determination is also a convenient and rapid procedure for an indirect enumeration. However, she noted that results may vary depending upon the amount of fiber in the test sample. The ATP results will only give indications of whether there is a difference in overall biological activity.

"To detect a population shift, a thorough microbiological audit is needed," she asserted. "Again, basic enumeration of different groups, such as general anaerobes, SRBs, and fungi, is needed." Woodward cited a variety of tools that can accomplish this, including the basic light microscope with simple stains--still the best way for the detection of fungi and bacteria in deposits. "Analysis of deposits should be performed on a regular basis and documented between boilout cycles in order to detect changes in microbial population," she concluded.

Jim Anderson, applications team manager-microbiological control, Hercules Pulp and Paper Division, Jacksonville, Florida, noted that, in the search for "bad actors" there is no substitute for old-fashioned hard work.

"Detection of fungal or bacterial growth in the paper machine wet end is still very' much a hands-on management process," he said. "In-depth machine inspections with deposit analysis should be done on a regular basis. A microscope, various plating methods, and mill experience will help to rapidly determine biological components. All these tools help keep bioactivity within the established control range, which in turn will keep the machine surfaces at the desired cleanliness."

Anderson agreed that ATP is an excellent tool and added that Petri film and pour-plate methods are also useful--though slower--tools for routine monitoring of bacterial contamination. Like Woodward, he noted that ATP is a relative measure of the bioactivity in the process and usually can be correlated to the overall effectiveness of the microbiological control program.

"ATP offers a huge advantage over pour-plates or Petri film since no incubation time is required; therefore the test results are immediately available and adjustments can be made based on real-time information," said Anderson.

He added that ATP measurement does have some limitations and cannot be used for every process or system. In the cases where ATP is not representative, then pour-plates or Petri film (48-hour incubation) are commonly used for the routine monitoring of bacteria. Routine monitoring of fungal contamination is most effective using the pour-plate or Petri film method and incubating the samples for 5 days, said Anderson.

New techniques currently being evaluated may offer advances in microbiological detection. "An on-line deposit detection device has been extensively evaluated in Europe and the initial evaluations in North America show promise," said Anderson. "This patented device takes a side-stream of white water and measures/records the deposition weight via a sensitive balance. Operators can then enter the data into a spreadsheet and develop trends."

NEW PROBLEMS

Like all organisms, bacteria and fungi change with the environment they live in. As papermakers use new processes, they unwittingly create new opportunities for biological contamination. Solutions! asked the experts what they thought was the most important emerging biological control problem facing papermakers. Anderson pointed to the growth of alkaline printing and writing grades as one challenge. "These grades have used oxidizing biocides as an effective and inexpensive method of controlling biological growth for some time," he said. "However, with strong oxidizing biocides it is important to consistently feed the appropriate amount of biocide. Excessive use of strong oxidizing biocides is common, and this can lead to operational problems such as corrosion, additive degradation, and felt issues, among other problems. Likewise, underfeed situations can lead to uncontrolled biological growth."

Anderson noted that a new, patented biocide has recently been approved by the U.S. Environmental Protection Agency that effectively controls microorganisms in alkaline systems without the adverse side effects associated with strong oxidizing biocides. The new biocide, which quickly degrades into inert compounds before effluent discharge, is produced onsite by blending an ammonium bromide solution with sodium hypochlorite and mill Fresh water. According to Anderson, dedicated blending and dosing equipment ensures safe, consistent application of the biocide at the mill site.

When it comes to emerging problems, Buckman's Woodward noted that the nose knows. "Odor control is becoming the most important biological control problem for two reasons," she stated. "First, more recycled fiber is being used. This fiber source, especially coated paper and board, is a great source of nutrients for microbial growth. It also has the potential of being contaminated with odor-causing microorganisms, such as fungi.

"Secondly, many mills are closing up water systems in the mill," she continued. "As more process water is re-used, there is a greater tendency for the system to become anaerobic. Many anaerobes produce acids and gases that are extremely odiferous."

THE PRICE IS RIGHT

Of course, good biological control at a mill site comes at a price. The key is to keep biological program costs to a minimum while still maintaining effective control. How can mills maintain this delicate balancing act?

The best approach, according to Woodward, is to nip the problem in the bud. "If at all possible, treat a biological issue at the source of the problem," she said. "Any other approach is just a 'band-aid' and will probably be more costly. Also, routine housekeeping practices are always an integral part of an effective control program."

Hercules' Anderson urges mills to partner with their biocide supplier to develop an effective plan that won't break the bank. "Working together, the mill and supplier should understand all the variables that affect the microbiological control program," he said. "These include various incoming additives, process streams, and the wet-end system design."

Once the variables are understood, the mill and supplier should agree on the desired outcome of the control program (such as boilout frequency, machine cleanliness, odors, and/or "Dairyman's Standard"). "With an understanding of the process and the desired outcome, a total system approach program can be designed to control bacterial and fungal growth," said Anderson. "This typically includes treating all the potential sources of incoming contamination, such as fresh water, additives, and fiber; using a stock biocide treatment program that keeps the stock, especially the broke and recycled Furnishes, in check; plus, if necessary, an oxidizing treatment for the white water."

All microbiological programs need a routine monitoring regime that will ensure that biological growth is managed along with program costs, he concluded.

IN THIS ARTICLE, YOU WILL LEARN:

* What is the best current technology for detecting bacterial and fungi contamination in the wet end?

* What is the most important emerging biological control problem facing papermakers?

* What is the best strategy for mills to keep biological program costs to a minimum while still maintaining effective control?

ADDITIONAL RESOURCES:

* "Taking wet end chemistry to the next level," Janice Bottiglieri, Solutions!, June 2003, p. 48.

* "Wet end chemistry: Paring down, pairing up," Janice Bottiglieri, Solutions!, Dec. 2002, p. 26.

* To view previous articles on this topic, search for "wet end chemistry" in the category "all journals" at www.tappi.org