R-657.pdf - Nalco

Managing microbio control
programs Reprint R-657
By Tim Nahley, Nalco Company
Microbiological control programs play a key role in
maintaining machine runnability and product quality.
In the December 1995 issue of PIMA Magazine, we
discussed how performance and economics can vary
from mill to mill—even between mills that produce
the same grades. Five key elements influence your results
and bottom line. The elements listed in Figure 1
affect either the growth of microorganisms in paper
machine systems or the development of microbiological
deposits, or both. In the first article, system inoculation—the
first element—was discussed. In this
article, the effects of the remaining four elements will
be reviewed.
Microbiological Growth Requirements—Uncontrolled
paper systems provide an excellent environment
for microorganisms to thrive. These systems
provide the right nutrients, pH and temperature
ranges to support microbiological activity. These
growth requirements differ from mill to mill—even
on identical grades, depending on the way the paper
is made and the machine is operated. Operational
changes can often affect microbiological growth
requirements and ultimately, the overall performance
and economics of the microbiological deposit control
program.
Nutrient loading—The nutrients available in any
particular machine system are the result of several
variables, one being grade structure. The fiber source
will affect microbiological activity through the introduction
of nutrients, particularly in post-consumer
fiber grades. Coated versus uncoated grades can
affect the degree of nutrients, since many coating formulations
contain ingredients bacteria use as food.
Starch is a well known nutrient for increasing microbiological
growth. Changes in the amounts of starch
used for internal or surface sizing or in coating formulations
often influence the results and costs of a
microbiological deposit control program.
The retention program also plays a key role in machine
system nutrient loading. Additives not retained
in the sheet build up in the water and stock systems
of a paper machine. Once there, they become an excellent
media for microbiological growth. Differences
in the performance of a retention program from
Reprinted by permission from PIMA Magazine, December 1995
Key Elements Influencing Performance and Economics
of Microbiological Deposit Control Programs
■ System Inoculation
■ Microbiological Growth Requirements
■ Control Philosophy
■ Mill Design
■ Stock Management & Housekeeping
Figure 1
machine to machine, or fluctuations in retention on
the same machine, will have a major impact on the
microbiological control program.
An increasingly important cause of nutrient loading
in paper mills is closure. As less water is discharged
from paper mills, contaminants in the water begin to
cycle up. Again, nutrients build up as closure increases,
making microbiological control more challenging.
System pH—pH is a critical factor affecting the
growth of microbiological organisms in paper mill
systems. Although microorganisms can live within the
pH range of most papermaking processes (pH 4-8),
the degree of growth will be different within this
range. As pH increases from acid to neutral to alkaline,
the environment becomes more favorable for
vigorous microbiological activity.
System temperature—Temperature also influences
the performance of a microbiological control program.
Generally, as machine temperature increases past
130°F, the activity of many microorganisms begins
to decrease. There are, however, thermophilic bacteria
that can thrive under these conditions.
Oxygen requirements—Oxygen levels not only
affect the metabolism of microorganisms in a machine
system, but also affect the type of organisms present.
This results in the existence of a variety of organisms
over a range of oxygen levels, all requiring a
different approach to treatment. Changes in the

oxygen levels due to factors such as temperature or
machine closure will change the microbial population
of the machine and require different control methods.
Control Philosophy—The method a mill chooses
for microbiological deposit control plays a large role
in the performance and economics of the program.
Whether a mill chooses to use an on-machine oxidizer
(such as chlorine), non-oxidizing biocides, or controls
deposits primarily thorough boilouts and washups
greatly affects results and costs. There are tradeoffs
for each of these methods.
The use of microbiological control chemicals will provide
advantages in machine runnability. Line item
chemical costs, however, can run into dollars per ton.
Minimal use of these agents in conjunction with frequent
boilouts and washups will keep chemical costs
low but at the expense of machine downtime. Total
machine downtime and product quality should be considered
relative to deposit control costs.
Mill Design—One factor seldom considered for its
impact on the performance and economics of a microbiological
deposit control program is mill design.
A survey of 12 alkaline fine paper mills showed a wide
range of results and costs. When analyzed, the mills
with larger machines typically had fewer microbiological
related problems and higher chemical costs
per ton than those with smaller machines. A combination
of the following conditions may be the reason.
The larger mills have larger chests with longer turnover
times. The longer turnover times, particularly
in coated broke silos, can require more biological control
agents. These larger machines were also newer
and had less maintenance-related downtime than the
smaller, older machines. Consequently, the smaller
machines had more opportunities for boilouts and
wash-ups. Finally, the machine time of the larger
machine was more costly, so the machines were kept
cleaner to maximize time between boilouts.
Mill design, particularly cross connections between
machines, is an important consideration. A two
machine mill, one producing food grade products and
one producing boxboard, with a common white water
system, must consider how the performance of the
program on one machine will affect the other.
Headbox and approach piping design and the type of
construction materials can also affect the rate at
which deposition occurs. For instance, open and air
padded headbox designs will typically develop
biofilms faster than a hydraulic headbox design.
Machine speed also plays a role but in this case, it is
Printed in U.S.A. 3-04
the ability of the machine to tolerate deposits that
make it to the sheet that will affect runnability. A
faster machine is less tolerant of sheet deposits and
will develop more breaks than a slower machine.
Stock Management and Housekeeping Practices—Stock
management and housekeeping practices
will affect the efficiency of the microbiological
control program. The way in which stock is stored as
well as housekeeping procedures, such as boilouts and
wash-ups, can differ between mills.
Stock chests can be an area of concern. The consistency
of stock streams in conjunction with chest and
silo design constraints can result in channeling and
dead zones where stock can stagnate, which promotes
microbiological activity. Biological control agents
never reach these areas due to the channeling. As
chest levels rise and fall, this highly contaminated
stock will be transferred to the machine where the
machine microbiological control program can be overwhelmed.
Regarding housekeeping, effective wash-ups at each
opportunity will reduce deposit-related runnability
problems and also the frequency required for boilout.
Proper boilouts will help to minimize chemical
requirements by completely eliminating microbiological
deposits that would serve to reinoculate the system
if not properly removed.
The performance and economics of a microbiological
control program are affected by a
variety of factors. Many of these are in turn
affected by the variables of each papermaking operation.
The variables that have the most significant
impact on the control program include:
• fresh water quality
• fiber source
• additives
• grade structure
• retention program
• mill closure
• operating pH
• operating temperatures
• system & machine design
• stock management practices
• boilout & wash-up practices
To prevent unplanned microbiological-related problems
resulting from operational changes, consult with
your in-house and supplier experts. Taking time to
discuss potential changes in any of the areas on the
potential impact of the microbiological control program
will be time well spent.
Nalco Company 1601 West Diehl Road • Naperville, Illinois 60563-1198
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