| |
|
Historically, papermakers’ concern with microbial contamination has focused on visible machine deposits that form holes or sheet defects when they slough into the furnish (Figure 1). However, machine deposits are only part of the problems caused by microbes in the paper mill. While microbial deposits are easily seen, it is the less visible spoilage that causes additive and end product problems.
Traditional methods of control have included the use of biocides to inhibit or kill the organisms before they form slime or non-toxic chemistries that block the initial attachment of microbes to machine surfaces. Specialty chemicals are also used to prevent microbial spoilage of additives, reduce the formation of hazardous gases, and to keep foul odors out of the finished products or from being released into the community. The pursuit of improved control programs has prompted the development of targeted monitoring tools and treatment programs that maximize machine performance while minimizing or eliminating adverse environmental impacts.
FIGURE 1: Examples of sheet defects and holes caused by sloughed deposits.
Problems caused by microbes can be reduced by the proper application of biocides. However, the traditional methods for assessing microbes are of limited value in rapidly solving these problems. Aerobic plate counts give limited information about microbial growth on surfaces. Typically, a 48-hour incubation period is needed to detect visible colonies, which means a two-day production lapse before the results are available. In addition, many organisms that can be seen microscopically cannot grow on the traditional media or within the incubation period.
Although phase-contrast microscopy provides immediate information about the deposit composition, it is not quantitative nor can it provide information about the status of microbes in mill fluids. Fluorescent microscopy can provide direct counts; however, the techniques require specialized equipment and are difficult to perform at the mill.
This article discusses data collection with two instruments that provide immediate information about the status of microbes in the papermaking system. The instruments include the optical fouling monitor (OFM) and a specialized photometer unit. The OFM determines the rate of fouling on machine surfaces and is used to indicate the need for deposit control program adjustments and to predict boilouts. The specialized photometer unit allows rapid assessment of microbial metabolism by monitoring adenosine triphosphate (ATP). The unit’s diagnostics are also used to monitor relative levels of biocides in mill fluids. Collection of data is possible within minutes with these tools, rather than days. This allows rapid changes to the control program.
MONITORING DEPOSITS. When a mono-layer of bacteria colonize the surface of the paper machine, the deposit can further increase in size as it entraps the wood fibers, carbonates, clays, and other particles used in making paper.1 Bacteria and fungi bind these materials to form slime deposits (Figure 2). The deposits can grow in size to several centimeters in thickness. If they slough away from the surface they can form holes, defects, or sheet breaks. When this happens, mills have to stop production to wash up or take several hours to do a thorough boilout with harsh chemicals that can disrupt the wastewater treatment system.
Microbes attached to a surface behave much differently than free-swimming organisms.2 Attached microbes are not washed away. Fluid flow patterns within the biofilm increases nutrient availability. The variations in oxygen gradients within the biofilm permit growth of microbes with different oxygen requirements. In addition, microbes are much less susceptible to biocides in the attached form.
Figure 2:200X phase-contrast photomicrograph of a deposit containing fungi and bacteria. Amorphous debris particles are also present.
Methods for monitoring deposits. Judging the efficacy of a deposit control program can take days or even weeks. For most microbial control programs, the common assessment method is plate counts3 performed on agar or Petrifilms, a trademarked Monsanto product. These report the number of microorganisms or colony forming units (CFU)/mL of fluid using a standard microbiological medium. Unfortunately, measurement of microbes in the fluids is only an indirect indicator of the degree of deposit formation. At best, a plate count will indicate the potential for fouling or warn of changes in the overall population. Another weakness of these counts is the fact that non-toxic chemistries such as deposit control polymers or biodispersants may dramatically reduce surface deposition without affecting the microbial plate counts.
Several methods have been used to monitor deposition. The simplest fouling monitor is the machine itself. By examining the same area of the machine at regular intervals, it is possible to get an idea of the rate of fouling and predict the need for a boilout. This simple method can be effective, although it is highly subjective unless hole counts and wet end breaks are documented. Taking pictures of the same spot on the machine over time is also useful. The photos can record gross differences in program efficacy.
Coupons have been used to measure the deposition rate. Wet weights are subject to errors caused by sloughing of the biofilm on the coupon edges as well as the large error inherent in a wet weight. By using a template and dry weights it is possible to reduce some of the error; however, this method is cumbersome and requires destructive offline testing.
| Uncontrolled growth of bacteria and fungi in the papermaking process adversely affects machine runnability, but it also has many negative impacts on the finished sheet and sheet print properties. Many of the materials used in making paper coatings are excellent microbial nutrients.
Water, starch, alum, carbonates, polymers, proteins, clays, latexes, or other materials used to aid in paper formation or to produce the coatings that form the printing surface can support the growth of a wide variety of microorganisms. Microbial degradation problems in paper machines can be roughly grouped into two broad types: spoilage and biofilm formation.
The following sections discuss the different ways spoilage from papermaking additives affects finished sheet quality. Microbial contamination of additives and coatings can come from a number of sources. These sources may include poorly treated make-up or quench water, incoming product containing a heavy loading of spoilage microbes, and/or the heel from a previous batch or shipment. Although there are differences between the additives, there are many similarities in the way they spoil.
Starch. Starch is an ideal food. Microbes enzymatically convert the starch components into sugar for growth. Spoiled starch will not give optimum performance as either a strength additive or as a binder for coating formulations. An indication of a microbial problem may be a drop in pH accompanied by viscosity loss. Many mills reduce or eliminate preservative treatment of starch in the mistaken belief that the cooking process will sterilize the slurry, or they assume that the incoming starch is sterile. Neither is true.
Recycling of starch from the machine back to the run tank from a size press application can contaminate the cooked starch by bringing actively growing bacteria into a fresh food source. As the bacteria grow, they produce acidic by-products that reduce the pH. Papermakers may try to “recover” the starch by adding caustic to increase the pH. Although biocide can be added to stop further microbiological growth, the damage has been done. To eliminate a poor sizing response, the batch must be discarded.
Proteins. Protein binders are often used with other binders such as starch in coating formulations. Microbes readily degrade the large protein molecules. A very large drop in viscosity is normally accompanied by only a slight decrease in pH. Not long after the initial pH drop, the precipitation of protein occurs. This is accompanied by the production of malodors and product discoloration.
Synthetic Binders. Styrene-butadiene, vinyl acetates, and acrylates are synthetic binders often used in conjunction with starch and proteins. Organisms can break down the dispersing and stabilizing components in latex formulations. It is more difficult to detect this degradation since it is usually not accompanied by pH or viscosity decreases. Instead, poor adhesion of the coating to base sheet, low pigment holding capability, and undesirable supercalendering properties may be the only indicators of spoilage.
Dyes. Dye metering can be impeded due to contamination of the dye by microbes. The ensuing biofilm can cause plugging of feed lines and uneven addition of tinting dyes.
Fillers. Unlike starch and proteins, fillers—clays, precipitated and ground calcium carbonate, and titanium dioxide — do not serve as nutrients in their own right. However, they contain dispersants critical to performance. If the dispersants are degraded, viscosity changes may result. Fine scratches in coating surfaces have been traced to microagglomeration of the particles caused by degradation of dispersants in the fillers.
Anaerobes, such as sulfate reducing bacteria, can even cause a darkening of the filler and production of hydrogen sulfide. This is often seen with improperly preserved clays that must then be returned to the manufacturer for re-bleaching and treatment.
Coating Formulations. Each of the materials listed above may be in a coating formulation, and spoilage of individual chemicals can impact the final formulation. The formulations present a particular challenge because carbon and nitrogen are present in ratios that enhance microbial growth. Often, the separate components are preserved with biocides that are antagonistic. When the coating components are mixed, the preservatives may actually counteract each other. Furthermore, common coating practices increase the likelihood of spoilage. In a manner similar to cooked size press starch, coatings are recirculated from the machine to back to run tanks.
|
Online monitoring. Wetegrove and Banks developed a useful optical fouling monitor (OFM) that provides online monitoring of paper machine surface fouling.4 With the OFM, it is possible to determine the rate of fouling on surfaces. The OFM can be used to indicate the need for adjustments to the deposit control program and predict the need for boilouts.
FIGURE 3: Fouling index from an OFM at a fine paper mill documenting the change to a new deposit control program. Figure 3 shows data collected to compare program changes. In data collected before the start of a new program, the fouling index climbed rapidly to 1,200. The machine averaged 2.7 breaks/day and had 30 boilouts during the year, many of which were unscheduled. The new program was started in mid-February and the fouling index stayed below 400. The new program averaged 1.6 breaks/day and the number of boilouts during a 12-month period was reduced to 12. All boilouts were scheduled.
The on-line OFM is an excellent tool for demonstrating the activity of non-toxic or less toxic programs. These substances will not change the plate counts or ATP levels in bulk fluids if they are non-toxic. However, they can be very effective in blocking microbial adhesion.5Without tools to accurately measure the effect of the chemicals, it is difficult to assess the difference between effective non-toxic chemistries and what might be wishful thinking.
SPOILAGE MONITORING. Many additives used in making paper serve as nutrients to support microbial growth, which impacts paper quality (see sidebar, “Additive Spoilage Impacts Finished Sheet Quality”). Papermakers understand the economic benefits associated with deposit control; unfortunately, they tend to overlook spoilage of additives and fibers because it can be difficult to detect visually. Ignoring spoilage can be costly, environmentally risky and a direct safety hazard when toxic or explosive gases such as H2 or H2S are generated by anaerobic bacteria.6
Spoilage negatively impacts paper quality in a variety of ways. For example, uncoated free sheet paper used in copy machines can give off an unpleasant odor when it is heated through a copier. The problem has been traced to improperly preserved fiber chests that allow anaerobic bacteria to flourish and produce volatile fatty acids such as butyric and proprionic.
The normal plating process allows enumeration of many spoilage microbes. However, problem solving is hindered by the one to two days it takes to form visible colonies in the growth medium. Furthermore, microbiologists estimate that less than 0.1% to 1% of bacteria present are recovered on agar. Even with these limitations, plating methods do provide valuable information on the general type of microbes causing the problem.
The extended incubation period necessary with plating techniques makes it difficult to expediently solve a problem. For example, if a mill only makes salable paper intermittently because of on-going sizing problems, the two-day time period needed for incubation can result in the loss of significant paper.
Measuring ATP. One useful technique for measuring the metabolic activity is ATP analysis. When ATP is extracted from cells it can be quickly evaluated by using a luciferin:luciferase reaction. The light emitted by the reaction can be measured using a photometer such as the Tra-Cide unit, a trademarked product from Nalco Chemical. In general, a high ATP reading means that numerous, active cells are present, while a low value indicates that either few cells are present or that the cells are at a relatively low metabolic state. ATP does not give a plate count, nor does it differentiate between the causative organisms. However, this test does give test results in minutes, which allows immediate corrective actions.
Careful interpretation of the test is critical. For example, low ATP levels can accompany severe problems caused by anaerobic bacteria. Anaerobic fermentation produces low levels of ATP in comparison to the high levels of ATP produced in aerobic respiration. Some biocides appear to cause a temporary accumulation in ATP, perhaps because the biocide makes the cell membranes leaky, allowing ATP to be released from non-viable cells. A second hypothesis suggests that the accumulation in ATP is thought to be caused, in part, by the mode of action of the biocide. This may temporarily allow ATP to accumulate prior to cell death.
FIGURE 4: Headbox toxicity and ATP readings conducted over a biocide feed cycle.
MONITORING BIOCIDES. It is possible to improve the deposit control program without increasing the amount of biocide by targeting correct feed points. Correct feed points can reduce overall biocide use, which is both environmentally and fiscally friendly.
While ATP can be used to determine if a biocide has an effect against the population, it is not a direct indicator of the presence of a toxicant. Gas chromatographic analysis is accurate, but rarely available at a mill site. The toxicity mode (TOX) of the Tra-Cide photometer can be used to monitor levels of anti-microbial substances. The instrument is used to measure the initial light output of a bioluminescent bacterial suspension. An aliquot of process water is mixed with the cells and incubated for a few minutes. A second light reading is taken. Any decrease in light output is measured and converted to relative toxicity units (RTU).
Figure 4 shows the ATP relative light unit (RLU) levels and TOX RTU in a headbox over a 5-hour cycle on a paper machine in Asia. When the biocide is slug-fed to the blend chest, the RLU go down and the RTU increase. By 5:30 PM, the biocide completely disappears and the ATP climbs to the pre-dosing levels.
FIGURE 5: Example of extreme over-feeding of biocides. TOX values are exceptionally high and the ATP levels are quite low.
The graph in Figure 5 is from data collected in a Central European board mill. Multiple biocides were fed at numerous dosing points at the wet end of the machine at extremely high levels. ATP values ranged from 56 RLU to 210 RLU. This program was neither economical nor was it environmentally responsible.
Linda R. Robertson is a senior consultant for paper microbiology with Nalco Chemical Co. in Naperville, Ill., and Jayne Walker is a senior research microbiologist with Nalco Europe Oegstgeest, The Netherlands.
REFERENCES
1. Robertson, L. R. “The influence of microbial contamination on the quality of printing paper,” Proceedings: 1st International Symposium of Interactive Paper, Guadalajara, Mexico, October (1996).
2. Characklis W. G. and K. C. Marshall, “Biofilms: A Basis for an Interdisciplinary Approach,” in Characklis and Marshall, (Eds.), Biofilms, John Wiley & Sons Inc., 3-16 (1990).
3. TAPPI Method T 631 om-89 “Microbiological examination of process water and slush pulp,” 1989 Revision. In: TAPPI Methods TAPPI Press (1994).
4. Wetegrove, R. L. and R. H. Banks, “Monitoring of film formers,” US Patent #5,155,555. (1992).
5. Robertson, L. R. and N. R. Taylor, “Biofilms and dispersants: a less toxic approach to deposit control,” TAPPI Journal 77:99-103 (1990).
6. Rowbottom, R. S. “Bacteria cause fatal explosion at corrugating medium mill,” Pulp and Paper Canada. 90(4) 75-81 (1989).
|
