Biocide-free slime control in pulp and paper mills

Mariana Björklund, Ph.D

Abstract
The tendency in modern pulp and paper mills is both to close the circulation of process waters and to get closer to neutral pH. Both trends favour the growth of micro-organisms, especially bacteria. In some mills, the use of recycled fibres at moderate temperatures adds further to the presence and activity of micro-organisms.

Most micro-organisms in the process waters are harmless, but some cause problems as they colonise surfaces, so-called biofouling, leading to the growth of biofilm.

The activities of micro-organisms in the pulp and paper mills lead to frequent cleaning and maintenance costs, lower quality of the paper or board due to holes and spots, and breaks during production.

Conventionally, the papermaker has used biocides to control growth of micro-organisms. However, increased public consciousness regarding environmental issues has led to strict regulation on the use of biocides (including slimicides) in western countries. This has led to a limited number of biocides being available for the papermaker to use.

It is for this reason that biocide-free slime control appears to be an attractive part of the present and future, but actually it is an almost twenty-year-old concept, which is today well proven and effective when the right product is used.

BIM Kemi AB has marketed biocide-free slime control since 1981, and its own product, Bimogard, since 1991. This paper will describe practical experiences of the mode of action of this slime control.

Mode of Action
Bimogard is a multi-functional multi-component product consisting of chemically modified ligno-sulphonates and is fortified with various surfactants.

It is not toxic to micro-organisms at the concentrations used in the mills.

Biofilm and polysaccharides
Large amounts of exopolysaccharides (EPS) are produced in biofilm. EPS is the main chemical component of biological slime, and functions as a binder to keep the biofilm together as well as a shield for protection. Nutrients are caught and filtered by the EPS surrounding the cells. Different strains of bacteria produce different amounts of EPS, and any one bacterium can produce various sorts of EPS at different times. For example, it has been shown how a strain of Pseudomonas uses one sort of EPS to attach to a surface, and another to later detach from the same surface. So, to control the production of EPS is the same as controlling the consistency and the growth of the biofilm.

Table 1 shows the effect of Bimogard on the amount of EPS after introduction to a mill previously using biocides. The mill uses 100 % DIP. DIP, on the contrary to TMP, does not itself contain measurable amounts of EPS.

EPS (mg/g)

Sample

The day before Bimogard introduction

2 weeks after Bimogard introduction

4 weeks after Bimogard introduction

4 months after Bimogard introduction

Deposit from section under forming roll

13,0

6,1

5,5

0,0

Backwater

6,3

3,1

1,2

0,0

Process-water from wire pit

3,8

1,3

0,0

0,0

Fresh water

0,0

0,0

0,0

0,0

Table 1. The amount of EPS decreases after introduction of biocide-free slime control in a paper mill using 100 % DIP

Spore information
Almost all vegetative bacteria die off in the drying section of the paper machine, and are not transported to the paper or carton. However, most gram-positive bacteria that can form so-called endospores survive the drying section and are encapsulated in the paper or the carton. It is of great interest in certain mills producing carton for food or hygiene tissue to minimise endospore content. Table 2 compares the endospore formation in the presence or absence of the slime control agent.

Bimogard concentration

Time (h)

0 ppm

500 ppm

1600 ppm

9

1,6 x 104

-

-

13

1,5 x 105

-

-

17

1,6 x 105

1,6 x 105

-

21

3,4 x 105

3,0 x 105

-

24

5,3 x 105

4,9 x 105

-

Table 2. Endospore formation of Bacillus subtilis (colony forming unit/ml)

Figure 1 shows laboratory tests on growth curves for the spore forming bacteria Bacillus subtilis with or without Bimogard. As can be seen, depending on the dosage, the treatment delays or totally inhibits the growth of Bacillus subtilis.

Yet, the bacteria remain as vegetative cells and are thus killed in the drying section. The data in Table 2 and Figure 1 have been confirmed in mill cases.

Figure 1. Bimogard delays the growth curve of Bacillus subtilis. The bacteria remain vegetative; they do not form spores

UNLIKE BIOCIDES, BIOCIDE-FREE SLIME CONTROL DOES NOT TRIGGER BACTERIAL DEFENSE MECHANISMS

Both EPS-production and spore formation are bacterial defence mechanisms which can be triggered by dehydration, starvation, heat and toxins. As mentioned above, Bimogard is not toxic to micro-organisms and therefore does not trigger these mechanisms, unlike biocides. Further, this biocide-free slime control is dosed continuously, which is not economically feasible when using biocides. Yet, the incoming flow of bacteria is continuous, and so should the slime control be.

The bacteria count is usually 105/ml to 107/ml in mills using Bimogard. Most of these bacteria are harmless and die in the drying section without causing slime-problems.

No limitations in pulp or paper qualities
Bimogard is presently run on some 30 paper machines in Scandinavia using a variety of different pulps for production of tissue, newsprint and carton seen in table 3. In Sweden, this has decreased the use of toxic slimicides with 20 – 25 %.

Also, this slime control has been successfully used in low as well as neutral to high pH conditions, and at small, old as well as large, modern, fast-moving paper machines. Bimogard is also used at intermittent board machines in pulp mills.

PAPER GRADE

TYPE OF PULP

Newsprint

DIP, TMP

Newsprint

DIP, TMP

Newsprint

DIP, TMP

newsprint

DIP, TMP

Wood containing printing paper

Sulphite, SGW-pulp

Folding box board

SGW-pulp, sulphate

Printing & writing paper

Sulphite

Printing & writing paper

Sulphite

Tissue

Sulphate

Tissue

DIP

Tissue

DIP

Tissue

DIP

Tissue

Sulphate

Tissue

Sulphite, sulphate

Printing & writing paper

Sulphite, sulphate

Printing & writing paper

Sulphite, sulphate

Greaseproof

Sulphate

Newsprint

DIP, TMP

White-lined chipboard

Waste paper

Tissue

Sulphate

Table 3. Different pulp and paper qualities in mills presently using Bimogard.

Staff Health
Human dermatological tests performed in Canada on tissue produced in the presence of Bimogard show that it is not a skin irritant. Unlike biocides, it does not pose a hazard to the staff in the mill.

Costs
The cost for using Bimogard for slime control is the same or, most often, less than using biocides.

Conclusions
Bimogard, the biocide-free slime control described in this paper

· Keeps surfaces clean and smooth thereby preventing biofouling.

· Decreases the production of EPS ("slime"), thereby preventing growth of biofilm.

· Delays the growth of bacteria and decreases the formation of spores, thereby preventing bacteria in ready-made paper or carton.

· Is pH-stable

· Is temperature-stable

· Is environmentally friendly.

· Does not endanger the health of the staff.

· Is proven to be a satisfactory or even better alternative to biocides.

· Is cost effective.